air pollution research question

REVIEW article

Environmental and health impacts of air pollution: a review.

• 1 Delphis S.A., Kifisia, Greece
• 2 Laboratory of Hygiene and Environmental Protection, Faculty of Medicine, Democritus University of Thrace, Alexandroupolis, Greece
• 3 Centre Hospitalier Universitaire Vaudois (CHUV), Service de Médicine Interne, Lausanne, Switzerland
• 4 School of Social and Political Sciences, University of Glasgow, Glasgow, United Kingdom

One of our era's greatest scourges is air pollution, on account not only of its impact on climate change but also its impact on public and individual health due to increasing morbidity and mortality. There are many pollutants that are major factors in disease in humans. Among them, Particulate Matter (PM), particles of variable but very small diameter, penetrate the respiratory system via inhalation, causing respiratory and cardiovascular diseases, reproductive and central nervous system dysfunctions, and cancer. Despite the fact that ozone in the stratosphere plays a protective role against ultraviolet irradiation, it is harmful when in high concentration at ground level, also affecting the respiratory and cardiovascular system. Furthermore, nitrogen oxide, sulfur dioxide, Volatile Organic Compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) are all considered air pollutants that are harmful to humans. Carbon monoxide can even provoke direct poisoning when breathed in at high levels. Heavy metals such as lead, when absorbed into the human body, can lead to direct poisoning or chronic intoxication, depending on exposure. Diseases occurring from the aforementioned substances include principally respiratory problems such as Chronic Obstructive Pulmonary Disease (COPD), asthma, bronchiolitis, and also lung cancer, cardiovascular events, central nervous system dysfunctions, and cutaneous diseases. Last but not least, climate change resulting from environmental pollution affects the geographical distribution of many infectious diseases, as do natural disasters. The only way to tackle this problem is through public awareness coupled with a multidisciplinary approach by scientific experts; national and international organizations must address the emergence of this threat and propose sustainable solutions.

Approach to the Problem

The interactions between humans and their physical surroundings have been extensively studied, as multiple human activities influence the environment. The environment is a coupling of the biotic (living organisms and microorganisms) and the abiotic (hydrosphere, lithosphere, and atmosphere).

Pollution is defined as the introduction into the environment of substances harmful to humans and other living organisms. Pollutants are harmful solids, liquids, or gases produced in higher than usual concentrations that reduce the quality of our environment.

Human activities have an adverse effect on the environment by polluting the water we drink, the air we breathe, and the soil in which plants grow. Although the industrial revolution was a great success in terms of technology, society, and the provision of multiple services, it also introduced the production of huge quantities of pollutants emitted into the air that are harmful to human health. Without any doubt, the global environmental pollution is considered an international public health issue with multiple facets. Social, economic, and legislative concerns and lifestyle habits are related to this major problem. Clearly, urbanization and industrialization are reaching unprecedented and upsetting proportions worldwide in our era. Anthropogenic air pollution is one of the biggest public health hazards worldwide, given that it accounts for about 9 million deaths per year ( 1 ).

Without a doubt, all of the aforementioned are closely associated with climate change, and in the event of danger, the consequences can be severe for mankind ( 2 ). Climate changes and the effects of global planetary warming seriously affect multiple ecosystems, causing problems such as food safety issues, ice and iceberg melting, animal extinction, and damage to plants ( 3 , 4 ).

Air pollution has various health effects. The health of susceptible and sensitive individuals can be impacted even on low air pollution days. Short-term exposure to air pollutants is closely related to COPD (Chronic Obstructive Pulmonary Disease), cough, shortness of breath, wheezing, asthma, respiratory disease, and high rates of hospitalization (a measurement of morbidity).

The long-term effects associated with air pollution are chronic asthma, pulmonary insufficiency, cardiovascular diseases, and cardiovascular mortality. According to a Swedish cohort study, diabetes seems to be induced after long-term air pollution exposure ( 5 ). Moreover, air pollution seems to have various malign health effects in early human life, such as respiratory, cardiovascular, mental, and perinatal disorders ( 3 ), leading to infant mortality or chronic disease in adult age ( 6 ).

National reports have mentioned the increased risk of morbidity and mortality ( 1 ). These studies were conducted in many places around the world and show a correlation between daily ranges of particulate matter (PM) concentration and daily mortality. Climate shifts and global planetary warming ( 3 ) could aggravate the situation. Besides, increased hospitalization (an index of morbidity) has been registered among the elderly and susceptible individuals for specific reasons. Fine and ultrafine particulate matter seems to be associated with more serious illnesses ( 6 ), as it can invade the deepest parts of the airways and more easily reach the bloodstream.

Air pollution mainly affects those living in large urban areas, where road emissions contribute the most to the degradation of air quality. There is also a danger of industrial accidents, where the spread of a toxic fog can be fatal to the populations of the surrounding areas. The dispersion of pollutants is determined by many parameters, most notably atmospheric stability and wind ( 6 ).

In developing countries ( 7 ), the problem is more serious due to overpopulation and uncontrolled urbanization along with the development of industrialization. This leads to poor air quality, especially in countries with social disparities and a lack of information on sustainable management of the environment. The use of fuels such as wood fuel or solid fuel for domestic needs due to low incomes exposes people to bad-quality, polluted air at home. It is of note that three billion people around the world are using the above sources of energy for their daily heating and cooking needs ( 8 ). In developing countries, the women of the household seem to carry the highest risk for disease development due to their longer duration exposure to the indoor air pollution ( 8 , 9 ). Due to its fast industrial development and overpopulation, China is one of the Asian countries confronting serious air pollution problems ( 10 , 11 ). The lung cancer mortality observed in China is associated with fine particles ( 12 ). As stated already, long-term exposure is associated with deleterious effects on the cardiovascular system ( 3 , 5 ). However, it is interesting to note that cardiovascular diseases have mostly been observed in developed and high-income countries rather than in the developing low-income countries exposed highly to air pollution ( 13 ). Extreme air pollution is recorded in India, where the air quality reaches hazardous levels. New Delhi is one of the more polluted cities in India. Flights in and out of New Delhi International Airport are often canceled due to the reduced visibility associated with air pollution. Pollution is occurring both in urban and rural areas in India due to the fast industrialization, urbanization, and rise in use of motorcycle transportation. Nevertheless, biomass combustion associated with heating and cooking needs and practices is a major source of household air pollution in India and in Nepal ( 14 , 15 ). There is spatial heterogeneity in India, as areas with diverse climatological conditions and population and education levels generate different indoor air qualities, with higher PM 2.5 observed in North Indian states (557–601 μg/m 3 ) compared to the Southern States (183–214 μg/m 3 ) ( 16 , 17 ). The cold climate of the North Indian areas may be the main reason for this, as longer periods at home and more heating are necessary compared to in the tropical climate of Southern India. Household air pollution in India is associated with major health effects, especially in women and young children, who stay indoors for longer periods. Chronic obstructive respiratory disease (CORD) and lung cancer are mostly observed in women, while acute lower respiratory disease is seen in young children under 5 years of age ( 18 ).

Accumulation of air pollution, especially sulfur dioxide and smoke, reaching 1,500 mg/m3, resulted in an increase in the number of deaths (4,000 deaths) in December 1952 in London and in 1963 in New York City (400 deaths) ( 19 ). An association of pollution with mortality was reported on the basis of monitoring of outdoor pollution in six US metropolitan cities ( 20 ). In every case, it seems that mortality was closely related to the levels of fine, inhalable, and sulfate particles more than with the levels of total particulate pollution, aerosol acidity, sulfur dioxide, or nitrogen dioxide ( 20 ).

Furthermore, extremely high levels of pollution are reported in Mexico City and Rio de Janeiro, followed by Milan, Ankara, Melbourne, Tokyo, and Moscow ( 19 ).

Based on the magnitude of the public health impact, it is certain that different kinds of interventions should be taken into account. Success and effectiveness in controlling air pollution, specifically at the local level, have been reported. Adequate technological means are applied considering the source and the nature of the emission as well as its impact on health and the environment. The importance of point sources and non-point sources of air pollution control is reported by Schwela and Köth-Jahr ( 21 ). Without a doubt, a detailed emission inventory must record all sources in a given area. Beyond considering the above sources and their nature, topography and meteorology should also be considered, as stated previously. Assessment of the control policies and methods is often extrapolated from the local to the regional and then to the global scale. Air pollution may be dispersed and transported from one region to another area located far away. Air pollution management means the reduction to acceptable levels or possible elimination of air pollutants whose presence in the air affects our health or the environmental ecosystem. Private and governmental entities and authorities implement actions to ensure the air quality ( 22 ). Air quality standards and guidelines were adopted for the different pollutants by the WHO and EPA as a tool for the management of air quality ( 1 , 23 ). These standards have to be compared to the emissions inventory standards by causal analysis and dispersion modeling in order to reveal the problematic areas ( 24 ). Inventories are generally based on a combination of direct measurements and emissions modeling ( 24 ).

As an example, we state here the control measures at the source through the use of catalytic converters in cars. These are devices that turn the pollutants and toxic gases produced from combustion engines into less-toxic pollutants by catalysis through redox reactions ( 25 ). In Greece, the use of private cars was restricted by tracking their license plates in order to reduce traffic congestion during rush hour ( 25 ).

Concerning industrial emissions, collectors and closed systems can keep the air pollution to the minimal standards imposed by legislation ( 26 ).

Current strategies to improve air quality require an estimation of the economic value of the benefits gained from proposed programs. These proposed programs by public authorities, and directives are issued with guidelines to be respected.

In Europe, air quality limit values AQLVs (Air Quality Limit Values) are issued for setting off planning claims ( 27 ). In the USA, the NAAQS (National Ambient Air Quality Standards) establish the national air quality limit values ( 27 ). While both standards and directives are based on different mechanisms, significant success has been achieved in the reduction of overall emissions and associated health and environmental effects ( 27 ). The European Directive identifies geographical areas of risk exposure as monitoring/assessment zones to record the emission sources and levels of air pollution ( 27 ), whereas the USA establishes global geographical air quality criteria according to the severity of their air quality problem and records all sources of the pollutants and their precursors ( 27 ).

In this vein, funds have been financing, directly or indirectly, projects related to air quality along with the technical infrastructure to maintain good air quality. These plans focus on an inventory of databases from air quality environmental planning awareness campaigns. Moreover, pollution measures of air emissions may be taken for vehicles, machines, and industries in urban areas.

Technological innovation can only be successful if it is able to meet the needs of society. In this sense, technology must reflect the decision-making practices and procedures of those involved in risk assessment and evaluation and act as a facilitator in providing information and assessments to enable decision makers to make the best decisions possible. Summarizing the aforementioned in order to design an effective air quality control strategy, several aspects must be considered: environmental factors and ambient air quality conditions, engineering factors and air pollutant characteristics, and finally, economic operating costs for technological improvement and administrative and legal costs. Considering the economic factor, competitiveness through neoliberal concepts is offering a solution to environmental problems ( 22 ).

The development of environmental governance, along with technological progress, has initiated the deployment of a dialogue. Environmental politics has created objections and points of opposition between different political parties, scientists, media, and governmental and non-governmental organizations ( 22 ). Radical environmental activism actions and movements have been created ( 22 ). The rise of the new information and communication technologies (ICTs) are many times examined as to whether and in which way they have influenced means of communication and social movements such as activism ( 28 ). Since the 1990s, the term “digital activism” has been used increasingly and in many different disciplines ( 29 ). Nowadays, multiple digital technologies can be used to produce a digital activism outcome on environmental issues. More specifically, devices with online capabilities such as computers or mobile phones are being used as a way to pursue change in political and social affairs ( 30 ).

In the present paper, we focus on the sources of environmental pollution in relation to public health and propose some solutions and interventions that may be of interest to environmental legislators and decision makers.

Sources of Exposure

It is known that the majority of environmental pollutants are emitted through large-scale human activities such as the use of industrial machinery, power-producing stations, combustion engines, and cars. Because these activities are performed at such a large scale, they are by far the major contributors to air pollution, with cars estimated to be responsible for approximately 80% of today's pollution ( 31 ). Some other human activities are also influencing our environment to a lesser extent, such as field cultivation techniques, gas stations, fuel tanks heaters, and cleaning procedures ( 32 ), as well as several natural sources, such as volcanic and soil eruptions and forest fires.

The classification of air pollutants is based mainly on the sources producing pollution. Therefore, it is worth mentioning the four main sources, following the classification system: Major sources, Area sources, Mobile sources, and Natural sources.

Major sources include the emission of pollutants from power stations, refineries, and petrochemicals, the chemical and fertilizer industries, metallurgical and other industrial plants, and, finally, municipal incineration.

Indoor area sources include domestic cleaning activities, dry cleaners, printing shops, and petrol stations.

Mobile sources include automobiles, cars, railways, airways, and other types of vehicles.

Finally, natural sources include, as stated previously, physical disasters ( 33 ) such as forest fire, volcanic erosion, dust storms, and agricultural burning.

However, many classification systems have been proposed. Another type of classification is a grouping according to the recipient of the pollution, as follows:

Air pollution is determined as the presence of pollutants in the air in large quantities for long periods. Air pollutants are dispersed particles, hydrocarbons, CO, CO 2 , NO, NO 2 , SO 3 , etc.

Water pollution is organic and inorganic charge and biological charge ( 10 ) at high levels that affect the water quality ( 34 , 35 ).

Soil pollution occurs through the release of chemicals or the disposal of wastes, such as heavy metals, hydrocarbons, and pesticides.

Air pollution can influence the quality of soil and water bodies by polluting precipitation, falling into water and soil environments ( 34 , 36 ). Notably, the chemistry of the soil can be amended due to acid precipitation by affecting plants, cultures, and water quality ( 37 ). Moreover, movement of heavy metals is favored by soil acidity, and metals are so then moving into the watery environment. It is known that heavy metals such as aluminum are noxious to wildlife and fishes. Soil quality seems to be of importance, as soils with low calcium carbonate levels are at increased jeopardy from acid rain. Over and above rain, snow and particulate matter drip into watery ' bodies ( 36 , 38 ).

Lastly, pollution is classified following type of origin:

Radioactive and nuclear pollution , releasing radioactive and nuclear pollutants into water, air, and soil during nuclear explosions and accidents, from nuclear weapons, and through handling or disposal of radioactive sewage.

Radioactive materials can contaminate surface water bodies and, being noxious to the environment, plants, animals, and humans. It is known that several radioactive substances such as radium and uranium concentrate in the bones and can cause cancers ( 38 , 39 ).

Noise pollution is produced by machines, vehicles, traffic noises, and musical installations that are harmful to our hearing.

The World Health Organization introduced the term DALYs. The DALYs for a disease or health condition is defined as the sum of the Years of Life Lost (YLL) due to premature mortality in the population and the Years Lost due to Disability (YLD) for people living with the health condition or its consequences ( 39 ). In Europe, air pollution is the main cause of disability-adjusted life years lost (DALYs), followed by noise pollution. The potential relationships of noise and air pollution with health have been studied ( 40 ). The study found that DALYs related to noise were more important than those related to air pollution, as the effects of environmental noise on cardiovascular disease were independent of air pollution ( 40 ). Environmental noise should be counted as an independent public health risk ( 40 ).

Environmental pollution occurs when changes in the physical, chemical, or biological constituents of the environment (air masses, temperature, climate, etc.) are produced.

Pollutants harm our environment either by increasing levels above normal or by introducing harmful toxic substances. Primary pollutants are directly produced from the above sources, and secondary pollutants are emitted as by-products of the primary ones. Pollutants can be biodegradable or non-biodegradable and of natural origin or anthropogenic, as stated previously. Moreover, their origin can be a unique source (point-source) or dispersed sources.

Pollutants have differences in physical and chemical properties, explaining the discrepancy in their capacity for producing toxic effects. As an example, we state here that aerosol compounds ( 41 – 43 ) have a greater toxicity than gaseous compounds due to their tiny size (solid or liquid) in the atmosphere; they have a greater penetration capacity. Gaseous compounds are eliminated more easily by our respiratory system ( 41 ). These particles are able to damage lungs and can even enter the bloodstream ( 41 ), leading to the premature deaths of millions of people yearly. Moreover, the aerosol acidity ([H+]) seems to considerably enhance the production of secondary organic aerosols (SOA), but this last aspect is not supported by other scientific teams ( 38 ).

Climate and Pollution

Air pollution and climate change are closely related. Climate is the other side of the same coin that reduces the quality of our Earth ( 44 ). Pollutants such as black carbon, methane, tropospheric ozone, and aerosols affect the amount of incoming sunlight. As a result, the temperature of the Earth is increasing, resulting in the melting of ice, icebergs, and glaciers.

In this vein, climatic changes will affect the incidence and prevalence of both residual and imported infections in Europe. Climate and weather affect the duration, timing, and intensity of outbreaks strongly and change the map of infectious diseases in the globe ( 45 ). Mosquito-transmitted parasitic or viral diseases are extremely climate-sensitive, as warming firstly shortens the pathogen incubation period and secondly shifts the geographic map of the vector. Similarly, water-warming following climate changes leads to a high incidence of waterborne infections. Recently, in Europe, eradicated diseases seem to be emerging due to the migration of population, for example, cholera, poliomyelitis, tick-borne encephalitis, and malaria ( 46 ).

The spread of epidemics is associated with natural climate disasters and storms, which seem to occur more frequently nowadays ( 47 ). Malnutrition and disequilibration of the immune system are also associated with the emerging infections affecting public health ( 48 ).

The Chikungunya virus “took the airplane” from the Indian Ocean to Europe, as outbreaks of the disease were registered in Italy ( 49 ) as well as autochthonous cases in France ( 50 ).

An increase in cryptosporidiosis in the United Kingdom and in the Czech Republic seems to have occurred following flooding ( 36 , 51 ).

As stated previously, aerosols compounds are tiny in size and considerably affect the climate. They are able to dissipate sunlight (the albedo phenomenon) by dispersing a quarter of the sun's rays back to space and have cooled the global temperature over the last 30 years ( 52 ).

Air Pollutants

The World Health Organization (WHO) reports on six major air pollutants, namely particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead. Air pollution can have a disastrous effect on all components of the environment, including groundwater, soil, and air. Additionally, it poses a serious threat to living organisms. In this vein, our interest is mainly to focus on these pollutants, as they are related to more extensive and severe problems in human health and environmental impact. Acid rain, global warming, the greenhouse effect, and climate changes have an important ecological impact on air pollution ( 53 ).

Particulate Matter (PM) and Health

Studies have shown a relationship between particulate matter (PM) and adverse health effects, focusing on either short-term (acute) or long-term (chronic) PM exposure.

Particulate matter (PM) is usually formed in the atmosphere as a result of chemical reactions between the different pollutants. The penetration of particles is closely dependent on their size ( 53 ). Particulate Matter (PM) was defined as a term for particles by the United States Environmental Protection Agency ( 54 ). Particulate matter (PM) pollution includes particles with diameters of 10 micrometers (μm) or smaller, called PM 10 , and extremely fine particles with diameters that are generally 2.5 micrometers (μm) and smaller.

Particulate matter contains tiny liquid or solid droplets that can be inhaled and cause serious health effects ( 55 ). Particles <10 μm in diameter (PM 10 ) after inhalation can invade the lungs and even reach the bloodstream. Fine particles, PM 2.5 , pose a greater risk to health ( 6 , 56 ) ( Table 1 ).

Table 1 . Penetrability according to particle size.

Multiple epidemiological studies have been performed on the health effects of PM. A positive relation was shown between both short-term and long-term exposures of PM 2.5 and acute nasopharyngitis ( 56 ). In addition, long-term exposure to PM for years was found to be related to cardiovascular diseases and infant mortality.

Those studies depend on PM 2.5 monitors and are restricted in terms of study area or city area due to a lack of spatially resolved daily PM 2.5 concentration data and, in this way, are not representative of the entire population. Following a recent epidemiological study by the Department of Environmental Health at Harvard School of Public Health (Boston, MA) ( 57 ), it was reported that, as PM 2.5 concentrations vary spatially, an exposure error (Berkson error) seems to be produced, and the relative magnitudes of the short- and long-term effects are not yet completely elucidated. The team developed a PM 2.5 exposure model based on remote sensing data for assessing short- and long-term human exposures ( 57 ). This model permits spatial resolution in short-term effects plus the assessment of long-term effects in the whole population.

Moreover, respiratory diseases and affection of the immune system are registered as long-term chronic effects ( 58 ). It is worth noting that people with asthma, pneumonia, diabetes, and respiratory and cardiovascular diseases are especially susceptible and vulnerable to the effects of PM. PM 2.5 , followed by PM 10 , are strongly associated with diverse respiratory system diseases ( 59 ), as their size permits them to pierce interior spaces ( 60 ). The particles produce toxic effects according to their chemical and physical properties. The components of PM 10 and PM 2.5 can be organic (polycyclic aromatic hydrocarbons, dioxins, benzene, 1-3 butadiene) or inorganic (carbon, chlorides, nitrates, sulfates, metals) in nature ( 55 ).

Particulate Matter (PM) is divided into four main categories according to type and size ( 61 ) ( Table 2 ).

Table 2 . Types and sizes of particulate Matter (PM).

Gas contaminants include PM in aerial masses.

Particulate contaminants include contaminants such as smog, soot, tobacco smoke, oil smoke, fly ash, and cement dust.

Biological Contaminants are microorganisms (bacteria, viruses, fungi, mold, and bacterial spores), cat allergens, house dust and allergens, and pollen.

Types of Dust include suspended atmospheric dust, settling dust, and heavy dust.

Finally, another fact is that the half-lives of PM 10 and PM 2.5 particles in the atmosphere is extended due to their tiny dimensions; this permits their long-lasting suspension in the atmosphere and even their transfer and spread to distant destinations where people and the environment may be exposed to the same magnitude of pollution ( 53 ). They are able to change the nutrient balance in watery ecosystems, damage forests and crops, and acidify water bodies.

As stated, PM 2.5 , due to their tiny size, are causing more serious health effects. These aforementioned fine particles are the main cause of the “haze” formation in different metropolitan areas ( 12 , 13 , 61 ).

Ozone Impact in the Atmosphere

Ozone (O 3 ) is a gas formed from oxygen under high voltage electric discharge ( 62 ). It is a strong oxidant, 52% stronger than chlorine. It arises in the stratosphere, but it could also arise following chain reactions of photochemical smog in the troposphere ( 63 ).

Ozone can travel to distant areas from its initial source, moving with air masses ( 64 ). It is surprising that ozone levels over cities are low in contrast to the increased amounts occuring in urban areas, which could become harmful for cultures, forests, and vegetation ( 65 ) as it is reducing carbon assimilation ( 66 ). Ozone reduces growth and yield ( 47 , 48 ) and affects the plant microflora due to its antimicrobial capacity ( 67 , 68 ). In this regard, ozone acts upon other natural ecosystems, with microflora ( 69 , 70 ) and animal species changing their species composition ( 71 ). Ozone increases DNA damage in epidermal keratinocytes and leads to impaired cellular function ( 72 ).

Ground-level ozone (GLO) is generated through a chemical reaction between oxides of nitrogen and VOCs emitted from natural sources and/or following anthropogenic activities.

Ozone uptake usually occurs by inhalation. Ozone affects the upper layers of the skin and the tear ducts ( 73 ). A study of short-term exposure of mice to high levels of ozone showed malondialdehyde formation in the upper skin (epidermis) but also depletion in vitamins C and E. It is likely that ozone levels are not interfering with the skin barrier function and integrity to predispose to skin disease ( 74 ).

Due to the low water-solubility of ozone, inhaled ozone has the capacity to penetrate deeply into the lungs ( 75 ).

Toxic effects induced by ozone are registered in urban areas all over the world, causing biochemical, morphologic, functional, and immunological disorders ( 76 ).

The European project (APHEA2) focuses on the acute effects of ambient ozone concentrations on mortality ( 77 ). Daily ozone concentrations compared to the daily number of deaths were reported from different European cities for a 3-year period. During the warm period of the year, an observed increase in ozone concentration was associated with an increase in the daily number of deaths (0.33%), in the number of respiratory deaths (1.13%), and in the number of cardiovascular deaths (0.45%). No effect was observed during wintertime.

Carbon Monoxide (CO)

Carbon monoxide is produced by fossil fuel when combustion is incomplete. The symptoms of poisoning due to inhaling carbon monoxide include headache, dizziness, weakness, nausea, vomiting, and, finally, loss of consciousness.

The affinity of carbon monoxide to hemoglobin is much greater than that of oxygen. In this vein, serious poisoning may occur in people exposed to high levels of carbon monoxide for a long period of time. Due to the loss of oxygen as a result of the competitive binding of carbon monoxide, hypoxia, ischemia, and cardiovascular disease are observed.

Carbon monoxide affects the greenhouses gases that are tightly connected to global warming and climate. This should lead to an increase in soil and water temperatures, and extreme weather conditions or storms may occur ( 68 ).

However, in laboratory and field experiments, it has been seen to produce increased plant growth ( 78 ).

Nitrogen Oxide (NO 2 )

Nitrogen oxide is a traffic-related pollutant, as it is emitted from automobile motor engines ( 79 , 80 ). It is an irritant of the respiratory system as it penetrates deep in the lung, inducing respiratory diseases, coughing, wheezing, dyspnea, bronchospasm, and even pulmonary edema when inhaled at high levels. It seems that concentrations over 0.2 ppm produce these adverse effects in humans, while concentrations higher than 2.0 ppm affect T-lymphocytes, particularly the CD8+ cells and NK cells that produce our immune response ( 81 ).It is reported that long-term exposure to high levels of nitrogen dioxide can be responsible for chronic lung disease. Long-term exposure to NO 2 can impair the sense of smell ( 81 ).

However, systems other than respiratory ones can be involved, as symptoms such as eye, throat, and nose irritation have been registered ( 81 ).

High levels of nitrogen dioxide are deleterious to crops and vegetation, as they have been observed to reduce crop yield and plant growth efficiency. Moreover, NO 2 can reduce visibility and discolor fabrics ( 81 ).

Sulfur Dioxide (SO 2 )

Sulfur dioxide is a harmful gas that is emitted mainly from fossil fuel consumption or industrial activities. The annual standard for SO 2 is 0.03 ppm ( 82 ). It affects human, animal, and plant life. Susceptible people as those with lung disease, old people, and children, who present a higher risk of damage. The major health problems associated with sulfur dioxide emissions in industrialized areas are respiratory irritation, bronchitis, mucus production, and bronchospasm, as it is a sensory irritant and penetrates deep into the lung converted into bisulfite and interacting with sensory receptors, causing bronchoconstriction. Moreover, skin redness, damage to the eyes (lacrimation and corneal opacity) and mucous membranes, and worsening of pre-existing cardiovascular disease have been observed ( 81 ).

Environmental adverse effects, such as acidification of soil and acid rain, seem to be associated with sulfur dioxide emissions ( 83 ).

Lead is a heavy metal used in different industrial plants and emitted from some petrol motor engines, batteries, radiators, waste incinerators, and waste waters ( 84 ).

Moreover, major sources of lead pollution in the air are metals, ore, and piston-engine aircraft. Lead poisoning is a threat to public health due to its deleterious effects upon humans, animals, and the environment, especially in the developing countries.

Exposure to lead can occur through inhalation, ingestion, and dermal absorption. Trans- placental transport of lead was also reported, as lead passes through the placenta unencumbered ( 85 ). The younger the fetus is, the more harmful the toxic effects. Lead toxicity affects the fetal nervous system; edema or swelling of the brain is observed ( 86 ). Lead, when inhaled, accumulates in the blood, soft tissue, liver, lung, bones, and cardiovascular, nervous, and reproductive systems. Moreover, loss of concentration and memory, as well as muscle and joint pain, were observed in adults ( 85 , 86 ).

Children and newborns ( 87 ) are extremely susceptible even to minimal doses of lead, as it is a neurotoxicant and causes learning disabilities, impairment of memory, hyperactivity, and even mental retardation.

Elevated amounts of lead in the environment are harmful to plants and crop growth. Neurological effects are observed in vertebrates and animals in association with high lead levels ( 88 ).

Polycyclic Aromatic Hydrocarbons(PAHs)

The distribution of PAHs is ubiquitous in the environment, as the atmosphere is the most important means of their dispersal. They are found in coal and in tar sediments. Moreover, they are generated through incomplete combustion of organic matter as in the cases of forest fires, incineration, and engines ( 89 ). PAH compounds, such as benzopyrene, acenaphthylene, anthracene, and fluoranthene are recognized as toxic, mutagenic, and carcinogenic substances. They are an important risk factor for lung cancer ( 89 ).

Volatile Organic Compounds(VOCs)

Volatile organic compounds (VOCs), such as toluene, benzene, ethylbenzene, and xylene ( 90 ), have been found to be associated with cancer in humans ( 91 ). The use of new products and materials has actually resulted in increased concentrations of VOCs. VOCs pollute indoor air ( 90 ) and may have adverse effects on human health ( 91 ). Short-term and long-term adverse effects on human health are observed. VOCs are responsible for indoor air smells. Short-term exposure is found to cause irritation of eyes, nose, throat, and mucosal membranes, while those of long duration exposure include toxic reactions ( 92 ). Predictable assessment of the toxic effects of complex VOC mixtures is difficult to estimate, as these pollutants can have synergic, antagonistic, or indifferent effects ( 91 , 93 ).

Dioxins originate from industrial processes but also come from natural processes, such as forest fires and volcanic eruptions. They accumulate in foods such as meat and dairy products, fish and shellfish, and especially in the fatty tissue of animals ( 94 ).

Short-period exhibition to high dioxin concentrations may result in dark spots and lesions on the skin ( 94 ). Long-term exposure to dioxins can cause developmental problems, impairment of the immune, endocrine and nervous systems, reproductive infertility, and cancer ( 94 ).

Without any doubt, fossil fuel consumption is responsible for a sizeable part of air contamination. This contamination may be anthropogenic, as in agricultural and industrial processes or transportation, while contamination from natural sources is also possible. Interestingly, it is of note that the air quality standards established through the European Air Quality Directive are somewhat looser than the WHO guidelines, which are stricter ( 95 ).

Effect of Air Pollution on Health

The most common air pollutants are ground-level ozone and Particulates Matter (PM). Air pollution is distinguished into two main types:

Outdoor pollution is the ambient air pollution.

Indoor pollution is the pollution generated by household combustion of fuels.

People exposed to high concentrations of air pollutants experience disease symptoms and states of greater and lesser seriousness. These effects are grouped into short- and long-term effects affecting health.

Susceptible populations that need to be aware of health protection measures include old people, children, and people with diabetes and predisposing heart or lung disease, especially asthma.

As extensively stated previously, according to a recent epidemiological study from Harvard School of Public Health, the relative magnitudes of the short- and long-term effects have not been completely clarified ( 57 ) due to the different epidemiological methodologies and to the exposure errors. New models are proposed for assessing short- and long-term human exposure data more successfully ( 57 ). Thus, in the present section, we report the more common short- and long-term health effects but also general concerns for both types of effects, as these effects are often dependent on environmental conditions, dose, and individual susceptibility.

Short-term effects are temporary and range from simple discomfort, such as irritation of the eyes, nose, skin, throat, wheezing, coughing and chest tightness, and breathing difficulties, to more serious states, such as asthma, pneumonia, bronchitis, and lung and heart problems. Short-term exposure to air pollution can also cause headaches, nausea, and dizziness.

These problems can be aggravated by extended long-term exposure to the pollutants, which is harmful to the neurological, reproductive, and respiratory systems and causes cancer and even, rarely, deaths.

The long-term effects are chronic, lasting for years or the whole life and can even lead to death. Furthermore, the toxicity of several air pollutants may also induce a variety of cancers in the long term ( 96 ).

As stated already, respiratory disorders are closely associated with the inhalation of air pollutants. These pollutants will invade through the airways and will accumulate at the cells. Damage to target cells should be related to the pollutant component involved and its source and dose. Health effects are also closely dependent on country, area, season, and time. An extended exposure duration to the pollutant should incline to long-term health effects in relation also to the above factors.

Particulate Matter (PMs), dust, benzene, and O 3 cause serious damage to the respiratory system ( 97 ). Moreover, there is a supplementary risk in case of existing respiratory disease such as asthma ( 98 ). Long-term effects are more frequent in people with a predisposing disease state. When the trachea is contaminated by pollutants, voice alterations may be remarked after acute exposure. Chronic obstructive pulmonary disease (COPD) may be induced following air pollution, increasing morbidity and mortality ( 99 ). Long-term effects from traffic, industrial air pollution, and combustion of fuels are the major factors for COPD risk ( 99 ).

Multiple cardiovascular effects have been observed after exposure to air pollutants ( 100 ). Changes occurred in blood cells after long-term exposure may affect cardiac functionality. Coronary arteriosclerosis was reported following long-term exposure to traffic emissions ( 101 ), while short-term exposure is related to hypertension, stroke, myocardial infracts, and heart insufficiency. Ventricle hypertrophy is reported to occur in humans after long-time exposure to nitrogen oxide (NO 2 ) ( 102 , 103 ).

Neurological effects have been observed in adults and children after extended-term exposure to air pollutants.

Psychological complications, autism, retinopathy, fetal growth, and low birth weight seem to be related to long-term air pollution ( 83 ). The etiologic agent of the neurodegenerative diseases (Alzheimer's and Parkinson's) is not yet known, although it is believed that extended exposure to air pollution seems to be a factor. Specifically, pesticides and metals are cited as etiological factors, together with diet. The mechanisms in the development of neurodegenerative disease include oxidative stress, protein aggregation, inflammation, and mitochondrial impairment in neurons ( 104 ) ( Figure 1 ).

Figure 1 . Impact of air pollutants on the brain.

Brain inflammation was observed in dogs living in a highly polluted area in Mexico for a long period ( 105 ). In human adults, markers of systemic inflammation (IL-6 and fibrinogen) were found to be increased as an immediate response to PNC on the IL-6 level, possibly leading to the production of acute-phase proteins ( 106 ). The progression of atherosclerosis and oxidative stress seem to be the mechanisms involved in the neurological disturbances caused by long-term air pollution. Inflammation comes secondary to the oxidative stress and seems to be involved in the impairment of developmental maturation, affecting multiple organs ( 105 , 107 ). Similarly, other factors seem to be involved in the developmental maturation, which define the vulnerability to long-term air pollution. These include birthweight, maternal smoking, genetic background and socioeconomic environment, as well as education level.

However, diet, starting from breast-feeding, is another determinant factor. Diet is the main source of antioxidants, which play a key role in our protection against air pollutants ( 108 ). Antioxidants are free radical scavengers and limit the interaction of free radicals in the brain ( 108 ). Similarly, genetic background may result in a differential susceptibility toward the oxidative stress pathway ( 60 ). For example, antioxidant supplementation with vitamins C and E appears to modulate the effect of ozone in asthmatic children homozygous for the GSTM1 null allele ( 61 ). Inflammatory cytokines released in the periphery (e.g., respiratory epithelia) upregulate the innate immune Toll-like receptor 2. Such activation and the subsequent events leading to neurodegeneration have recently been observed in lung lavage in mice exposed to ambient Los Angeles (CA, USA) particulate matter ( 61 ). In children, neurodevelopmental morbidities were observed after lead exposure. These children developed aggressive and delinquent behavior, reduced intelligence, learning difficulties, and hyperactivity ( 109 ). No level of lead exposure seems to be “safe,” and the scientific community has asked the Centers for Disease Control and Prevention (CDC) to reduce the current screening guideline of 10 μg/dl ( 109 ).

It is important to state that impact on the immune system, causing dysfunction and neuroinflammation ( 104 ), is related to poor air quality. Yet, increases in serum levels of immunoglobulins (IgA, IgM) and the complement component C3 are observed ( 106 ). Another issue is that antigen presentation is affected by air pollutants, as there is an upregulation of costimulatory molecules such as CD80 and CD86 on macrophages ( 110 ).

As is known, skin is our shield against ultraviolet radiation (UVR) and other pollutants, as it is the most exterior layer of our body. Traffic-related pollutants, such as PAHs, VOCs, oxides, and PM, may cause pigmented spots on our skin ( 111 ). On the one hand, as already stated, when pollutants penetrate through the skin or are inhaled, damage to the organs is observed, as some of these pollutants are mutagenic and carcinogenic, and, specifically, they affect the liver and lung. On the other hand, air pollutants (and those in the troposphere) reduce the adverse effects of ultraviolet radiation UVR in polluted urban areas ( 111 ). Air pollutants absorbed by the human skin may contribute to skin aging, psoriasis, acne, urticaria, eczema, and atopic dermatitis ( 111 ), usually caused by exposure to oxides and photochemical smoke ( 111 ). Exposure to PM and cigarette smoking act as skin-aging agents, causing spots, dyschromia, and wrinkles. Lastly, pollutants have been associated with skin cancer ( 111 ).

Higher morbidity is reported to fetuses and children when exposed to the above dangers. Impairment in fetal growth, low birth weight, and autism have been reported ( 112 ).

Another exterior organ that may be affected is the eye. Contamination usually comes from suspended pollutants and may result in asymptomatic eye outcomes, irritation ( 112 ), retinopathy, or dry eye syndrome ( 113 , 114 ).

Environmental Impact of Air Pollution

Air pollution is harming not only human health but also the environment ( 115 ) in which we live. The most important environmental effects are as follows.

Acid rain is wet (rain, fog, snow) or dry (particulates and gas) precipitation containing toxic amounts of nitric and sulfuric acids. They are able to acidify the water and soil environments, damage trees and plantations, and even damage buildings and outdoor sculptures, constructions, and statues.

Haze is produced when fine particles are dispersed in the air and reduce the transparency of the atmosphere. It is caused by gas emissions in the air coming from industrial facilities, power plants, automobiles, and trucks.

Ozone , as discussed previously, occurs both at ground level and in the upper level (stratosphere) of the Earth's atmosphere. Stratospheric ozone is protecting us from the Sun's harmful ultraviolet (UV) rays. In contrast, ground-level ozone is harmful to human health and is a pollutant. Unfortunately, stratospheric ozone is gradually damaged by ozone-depleting substances (i.e., chemicals, pesticides, and aerosols). If this protecting stratospheric ozone layer is thinned, then UV radiation can reach our Earth, with harmful effects for human life (skin cancer) ( 116 ) and crops ( 117 ). In plants, ozone penetrates through the stomata, inducing them to close, which blocks CO 2 transfer and induces a reduction in photosynthesis ( 118 ).

Global climate change is an important issue that concerns mankind. As is known, the “greenhouse effect” keeps the Earth's temperature stable. Unhappily, anthropogenic activities have destroyed this protecting temperature effect by producing large amounts of greenhouse gases, and global warming is mounting, with harmful effects on human health, animals, forests, wildlife, agriculture, and the water environment. A report states that global warming is adding to the health risks of poor people ( 119 ).

People living in poorly constructed buildings in warm-climate countries are at high risk for heat-related health problems as temperatures mount ( 119 ).

Wildlife is burdened by toxic pollutants coming from the air, soil, or the water ecosystem and, in this way, animals can develop health problems when exposed to high levels of pollutants. Reproductive failure and birth effects have been reported.

Eutrophication is occurring when elevated concentrations of nutrients (especially nitrogen) stimulate the blooming of aquatic algae, which can cause a disequilibration in the diversity of fish and their deaths.

Without a doubt, there is a critical concentration of pollution that an ecosystem can tolerate without being destroyed, which is associated with the ecosystem's capacity to neutralize acidity. The Canada Acid Rain Program established this load at 20 kg/ha/yr ( 120 ).

Hence, air pollution has deleterious effects on both soil and water ( 121 ). Concerning PM as an air pollutant, its impact on crop yield and food productivity has been reported. Its impact on watery bodies is associated with the survival of living organisms and fishes and their productivity potential ( 121 ).

An impairment in photosynthetic rhythm and metabolism is observed in plants exposed to the effects of ozone ( 121 ).

Sulfur and nitrogen oxides are involved in the formation of acid rain and are harmful to plants and marine organisms.

Last but not least, as mentioned above, the toxicity associated with lead and other metals is the main threat to our ecosystems (air, water, and soil) and living creatures ( 121 ).

In 2018, during the first WHO Global Conference on Air Pollution and Health, the WHO's General Director, Dr. Tedros Adhanom Ghebreyesus, called air pollution a “silent public health emergency” and “the new tobacco” ( 122 ).

Undoubtedly, children are particularly vulnerable to air pollution, especially during their development. Air pollution has adverse effects on our lives in many different respects.

Diseases associated with air pollution have not only an important economic impact but also a societal impact due to absences from productive work and school.

Despite the difficulty of eradicating the problem of anthropogenic environmental pollution, a successful solution could be envisaged as a tight collaboration of authorities, bodies, and doctors to regularize the situation. Governments should spread sufficient information and educate people and should involve professionals in these issues so as to control the emergence of the problem successfully.

Technologies to reduce air pollution at the source must be established and should be used in all industries and power plants. The Kyoto Protocol of 1997 set as a major target the reduction of GHG emissions to below 5% by 2012 ( 123 ). This was followed by the Copenhagen summit, 2009 ( 124 ), and then the Durban summit of 2011 ( 125 ), where it was decided to keep to the same line of action. The Kyoto protocol and the subsequent ones were ratified by many countries. Among the pioneers who adopted this important protocol for the world's environmental and climate “health” was China ( 3 ). As is known, China is a fast-developing economy and its GDP (Gross Domestic Product) is expected to be very high by 2050, which is defined as the year of dissolution of the protocol for the decrease in gas emissions.

A more recent international agreement of crucial importance for climate change is the Paris Agreement of 2015, issued by the UNFCCC (United Nations Climate Change Committee). This latest agreement was ratified by a plethora of UN (United Nations) countries as well as the countries of the European Union ( 126 ). In this vein, parties should promote actions and measures to enhance numerous aspects around the subject. Boosting education, training, public awareness, and public participation are some of the relevant actions for maximizing the opportunities to achieve the targets and goals on the crucial matter of climate change and environmental pollution ( 126 ). Without any doubt, technological improvements makes our world easier and it seems difficult to reduce the harmful impact caused by gas emissions, we could limit its use by seeking reliable approaches.

Synopsizing, a global prevention policy should be designed in order to combat anthropogenic air pollution as a complement to the correct handling of the adverse health effects associated with air pollution. Sustainable development practices should be applied, together with information coming from research in order to handle the problem effectively.

At this point, international cooperation in terms of research, development, administration policy, monitoring, and politics is vital for effective pollution control. Legislation concerning air pollution must be aligned and updated, and policy makers should propose the design of a powerful tool of environmental and health protection. As a result, the main proposal of this essay is that we should focus on fostering local structures to promote experience and practice and extrapolate these to the international level through developing effective policies for sustainable management of ecosystems.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

IM is employed by the company Delphis S.A.

The remaining authors declare that the present review paper was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Keywords: air pollution, environment, health, public health, gas emission, policy

Citation: Manisalidis I, Stavropoulou E, Stavropoulos A and Bezirtzoglou E (2020) Environmental and Health Impacts of Air Pollution: A Review. Front. Public Health 8:14. doi: 10.3389/fpubh.2020.00014

Received: 17 October 2019; Accepted: 17 January 2020; Published: 20 February 2020.

Reviewed by:

Copyright © 2020 Manisalidis, Stavropoulou, Stavropoulos and Bezirtzoglou. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Ioannis Manisalidis, giannismanisal@gmail.com ; Elisavet Stavropoulou, elisabeth.stavropoulou@gmail.com

† These authors have contributed equally to this work

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• Published: 22 July 2023

Global air pollution exposure and poverty

• Jun Rentschler   ORCID: orcid.org/0000-0002-2014-2124 1 &
• Nadezda Leonova   ORCID: orcid.org/0009-0001-6968-1794 1  

Nature Communications volume  14 , Article number:  4432 ( 2023 ) Cite this article

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Air pollution is one of the leading causes of health complications and mortality worldwide, especially affecting lower-income groups, who tend to be more exposed and vulnerable. This study documents the relationship between ambient air pollution exposure and poverty in 211 countries and territories. Using the World Health Organization’s (WHO) 2021 revised fine particulate matter (PM2.5) thresholds, we show that globally, 7.3 billion people are directly exposed to unsafe average annual PM2.5 concentrations, 80 percent of whom live in low- and middle-income countries. Moreover, 716 million of the world’s lowest income people (living on less than $1.90 per day) live in areas with unsafe levels of air pollution, especially in Sub-Saharan Africa. Air pollution levels are particularly high in lower-middle-income countries, where economies tend to rely more heavily on polluting industries and technologies. These findings are based on high-resolution air pollution and population maps with global coverage, as well as subnational poverty estimates based on harmonized household surveys.

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Introduction.

Air pollution has wide-ranging and profound impacts on human health and well-being. Poor air quality has been shown to be responsible for over 4 million deaths each year from outdoor pollutants, 2.3 million from indoor air pollution 1 , and a wide range of cardiovascular, respiratory, and neurological diseases 2 , 3 , 4 , 5 , 6 . It also impacts productivity, exacerbates inequalities 2 , and reduces cognitive abilities 3 .

Studies show that the vast majority of the world’s population faces unsafe air pollution levels 4 , 5 . Exposure is especially high in major urban centers, where 86 percent of people live in areas that exceed the WHO’s 2005 guideline threshold of 10 μg/m 3 6 . Yet, our understanding of what constitutes unsafe levels of air pollution levels is still evolving. Based on the latest medical evidence, the WHO updated its air quality guidelines in 2021, revising the threshold down to 5 μg/m 3 and significantly increasing the stringency of its 2005 guidelines.

A growing evidence base also highlights the unequal distribution of exposure to and impact of air pollution, with the burden falling disproportionately on lower-income and more marginalized communities 7 , 8 . The evidence is in strong agreement that air pollution—predominantly the result of human activities—is one of the leading causes of death in low- and middle-income countries 9 , where less stringent air quality regulations, the prevalence of older, more polluting machinery and vehicles, fossil fuel subsidies, congested urban transport systems, rapidly developing industrial sectors, and cut-and-burn practices in agriculture all contribute to heightened concentration levels 10 .

As health and productivity suffer, evidence from the United States has shown that air pollution reinforces socioeconomic inequalities—with ethnic minorities and low-income populations often exposed to higher pollution levels 11 , 12 , 13 , 14 , 15 , 16 , 17 —and that these disparities have increased over time 7 . These groups also tend to be more vulnerable to the impacts of pollution 8 , as low-paying jobs are more likely to require physical and outdoor labor, increasing people’s exposure. With industrial plants, transport corridors, and other pollution sources disproportionately placed in low-income neighborhoods, air pollution is higher in these areas 7 , 17 , 18 , driving down housing prices and reinforcing their status as low-income neighborhoods 19 , 20 . Finally, constraints on healthcare accessibility, availability, and quality further increase air pollution-related mortality among low-income groups 9 , 21 .

Substantial evidence from the United States illustrates how socioeconomic marginalization can increase people’s exposure and vulnerability to air pollution, and there are many documented individual cases of environmental inequalities 22 . But there is limited evidence at the global scale on how people’s exposure to harmful air pollution interacts with poverty and how this pollution burden is distributed across and within low- and middle-income countries. This is often due to a lack of socioeconomic data with high spatial disaggregation.

A better understanding of the interplay between air pollution and poverty could be crucial for several reasons 23 . Studies from high-income countries on the mortality and morbidity associated with air pollution may not be directly transferable to low-income countries and communities, where the nature of occupations and health care differ substantially 24 . The health and productivity implications of unsafe air pollution will also impact low- and middle-income countries’ socioeconomic development prospects. This is especially pertinent in low-income countries, which—as this study shows—still tend to have relatively low pollution levels compared to more industrialized, middle-income countries. In these countries, it is important to ensure that future development progress does not intensify air pollution, with its associated adverse effects.

Against this context, this study explores the global prevalence of unsafe outdoor air pollution and the way it interacts with poverty (defined as daily expenditure below $1.90, $3.20, and $5.50, respectively, as defined by the World Bank; see Methods). Reflecting 2018 and 2020 conditions, we use global high-resolution data on ambient air pollution (outdoor PM2.5 concentrations), population distribution, and poverty to provide aggregate exposure estimates (see Methods). We show that pollution levels are most hazardous in middle-income countries, where economies tend to rely more heavily on polluting industries and technologies.

Overall, this study contributes to the literature in two ways by offering global estimates of (i) population PM2.5 exposure, based on the WHO’s revised air pollution guidelines 25 , with detailed national and subnational estimates and (ii) how these interact with national and subnational poverty levels.

Global and regional air pollution exposure

Our estimates show that, globally, 7.3 billion people face air pollution levels that are considered unsafe by the WHO—that is, they are exposed to annual average PM2.5 concentrations over 5 μg/m 3 , which increases mortality rates by 4 percent compared to safe areas. Of these, 6.2 billion are directly exposed to at least moderate (over 10 μg/m 3 ) air pollution levels and an 8 percent increase in mortality risk, and 2.8 billion are exposed to hazardous (over 35 μg/m 3 ) pollution levels and a 24 percent increase in mortality risk. Globally, only 462 million people are exposed to PM2.5 concentrations that are lower than 5 μg/m 3 , the WHO’s “safe” threshold (Fig.  1a ). Considering a global population of 7.7 billion, this means that approximately 94 percent of the world’s population is exposed to unsafe levels of PM2.5 concentration.

a Global population headcounts exposed to different levels of air pollution risk. b Number of people and share of population exposed to air pollution, by region. c Top ten countries with highest population exposure to unsafe PM2.5 levels. Hazard categories are defined based on estimated average annual PM2.5 concentration levels. “Unsafe” refers to PM2.5 concentrations over 5 μg/m 3 . “Hazardous” refers to PM2.5 concentrations over 35 μg/m 3 .

Regionally disaggregating global exposure headcounts show that air pollution risks are particularly prevalent in some regions. At 2.2 billion people, East Asia and Pacific (EAP) has the highest number of people exposed to unsafe PM2.5 concentrations, corresponding to about 95 percent of its total population. In South Asia (SAR), about 1.8 billion people (99 percent) are exposed to unsafe air pollution levels. In all other regions, the share of the overall population exposed to unsafe PM2.5 concentrations is smaller, at 92–94 percent in the Middle East and North Africa (MENA), Sub-Saharan Africa (SSA), Europe and Central Asia (ECA), and the United States and Canada (USA & CAN), and 84 percent in Latin America and the Caribbean (Fig.  1b ).

Countries with the largest air pollution-exposed populations

Estimates confirm that several countries stand out with particularly large populations directly exposed to unsafe air pollution levels 26 . The world’s two most populous countries—China and India—have the highest absolute population exposure to unsafe air pollution and are home to about 38 percent of all people exposed to unsafe concentrations of PM2.5. In India, 1.36 billion people (99 percent of the population) are exposed to unsafe PM2.5 concentrations (over 5 μg/m 3 ); and 1.33 billion (96 percent) to hazardous levels (over 35 μg/m 3 ). In China, 1.41 billion people (99 percent of the population) face unsafe PM2.5 concentrations (over 5 μg/m 3 ), and 0.765 billion (53 percent) face hazardous levels (Fig. 1c ).

Presenting relative exposure estimates for all countries, Fig.  2 demonstrates that in large parts of the world and across all regions, the vast majority of the population is exposed to PM2.5 levels over 5 μg/m 3 . Unlike flood hazards, which are highly localized, air pollution tends to cover and move across large areas, blanketing entire cities or regions. So, if large proportions of a population live in densely populated areas, they tend to be collectively exposed to unsafe pollution levels. Considering a higher pollution threshold of 15 μg/m 3 shows that populations in low- and middle-income countries—in parts of Central and South America, across Western and Middle Africa, Eastern Europe, Middle East, and Central, South, and East Asia—face high exposure levels (Fig.  2b ), while in Eastern China, the Indian subcontinent, and parts of West Africa, large parts of the population face hazardous PM2.5 concentrations (Fig.  2c ).

a Percentage of the population exposed to PM2.5 over 5 μg/m. b Percentage of population exposed to PM2.5 over 15 μg/m 3 . c Percentage of population exposed to PM2.5 over 35 μg/m 3 .

Poverty and air pollution

Evidence suggests that low-income communities tend to be both disproportionately exposed to unsafe air pollution levels and more vulnerable to serious health impacts 3 , 27 . Low-income groups tend to be more exposed to air pollution because they are more likely to depend on jobs that require outdoor physical labor, and when affected by pollution-related diseases, they tend to have more limited access to adequate and affordable health care, increasing mortality rates. Low-income countries often also have less developed healthcare systems. So, considering the interplay between pollution, exposure, and poverty can shed light on the vulnerability of affected populations.

Combining air pollution exposure estimates with survey-based subnational poverty data allows us to estimate exposure of the global population living in poverty (Table  1 ). Our estimates show that 716 million people living on less than $1.90 per day are directly exposed to unsafe PM2.5 concentrations—405 million (57 percent) of them in Sub-Saharan Africa (Fig.  3 )—and 275 million are exposed to hazardous PM2.5 concentrations. Countries where poverty and unsafe air pollution coincide also score poorly in terms of health care access and quality, thus exacerbating vulnerabilities (Fig.  3c ). Approximately one in every 10 people exposed to unsafe levels of air pollution lives in extreme poverty.

a Number of people living in poverty and facing unsafe air pollution exposure, at different poverty thresholds and by region. b Top ten countries—percentage of people living on $1.90/day and exposed to hazardous PM2.5 levels. c Health care access and quality in countries with high air pollution and poverty. The Healthcare Access & Quality (HAQ) index is by GBD 2019 Healthcare Access and Quality Collaborators (2022) 38 , 39 , 40 , 41 .

When we use less extreme (i.e., higher) poverty thresholds, the number of air pollution and poverty-exposed people increases significantly. We estimate that around 1.8 billion people living on less than $3.20 a day and 2.9 billion people living on less than $5.50 a day live in unsafe air pollution areas. In Sub-Saharan Africa, increasing the poverty threshold from $1.90 to $5.50 doubles the number of people living in poverty and exposed to unsafe PM2.5 levels from 405 to 877 million (In Sub-Saharan Africa, 39.3 percent of the region’s total population lives in extreme poverty ($1.90), and 91.82 percent of the region’s total population faces unsafe PM2.5 levels (over 5 μg/m3)). In South Asia and East Asia, it increases more than six-fold, from 220 million to 1.4 billion and 38 to 229 million, respectively. Overall, four in 10 people exposed to unsafe PM2.5 levels live on less than $5.50 a day.

Of the 716 million people living in extreme poverty and exposed to unsafe levels of air pollution, almost half (48.6 percent) are in India, Nigeria, and the Democratic Republic of Congo. With over 202 million, India has the highest number of people living in extreme poverty and exposed to unsafe PM2.5 levels, corresponding to 14.7 percent of its overall population. The 10 countries with the most people who are both living on less than $1.90 a day and exposed to unsafe PM2.5 levels account for 67.8 percent of the world’s people exposed to poverty and unsafe PM2.5 concentrations; and seven of the top ten are in Sub-Saharan Africa (Fig.  3b ). Although extreme poverty and exposure to unsafe PM2.5 concentrations coincide most acutely in Sub-Saharan Africa, when considering higher poverty thresholds, exposure is also high in the Middle East, South and East Asia, and Latin America (Fig.  4 ).

a Share of the population exposed to unsafe PM2.5 levels and living on less than $1.90/day. b Share of the population exposed to unsafe PM2.5 levels and living on less than $3.20/day. c Share of the population exposed to unsafe PM2.5 levels and living on less than $5.50/day.

Income and air pollution concentrations

Our estimates on the geographic distribution of PM2.5 exposure suggest that pollution levels differ according to a country’s stage of economic development and industrialization. Most of the people breathing unsafe air live in middle-income countries (Fig.  5 ). Of the 7.3 billion exposed to unsafe concentrations of PM2.5, 3.4 billion (47.3 percent) live in low- or lower-middle-income countries. Of the 2.8 billion worldwide exposed to hazardous PM2.5 levels, 98.6 percent live in middle-income countries, compared to just 1.4 percent (40.5 million) in low- and high-income countries combined.

a Over 5 μg/m 3 (4% increased mortality rate). b Over 10 μg/m 3 (8% increased mortality rate). c Over 35 μg/m 3 (>24% increased mortality rate). d Regional distribution of mean PM2.5 concentrations. Concentration thresholds and estimated mortality rates are based on the WHO Global Air Quality Guidelines 3 , which provide details on estimation methods. LIC are low-income countries, LMIC are lower-middle-income countries, UMIC are upper-middle-income countries, HIC are high-income countries.

As a share of the overall population, PM2.5 exposure is also highest in lower-middle-income countries (Fig.  5 ), with about 64.5 percent of people exposed to PM2.5 levels over 35 μg/m 3 , compared to just 4.4 percent in low-income countries and 0.9 percent in high-income countries. The pattern holds regardless of which concentration threshold we consider (Fig.  5 ). The regional distribution of PM2.5 concentrations (Fig.  5d ) suggests that these high ambient air pollution levels in middle-income counties are located to a large extent in the countries of South and East Asia, which have experienced rapid economic growth and industrialization in recent decades 6 . Computing spatially averaged PM2.5 concentrations for each of the 2,183 subnational areas in this study and statistically examining their relationship with population and income data also suggests that areas with larger populations tend to have higher pollution levels, and average pollution levels appear particularly high for areas in the middle-income category (Supplementary Fig.  3.1 ).

This study offers a comprehensive account of the relationship between outdoor air pollution exposure, economic development, and poverty in 211 countries and territories. Its global exposure estimates highlight that unsafe air quality poses significant health risks to a vast majority of the global population. We find that 7.3 billion people—that is, 94 percent of the world’s population—live in areas that are exposed to PM2.5 concentrations over 5 μg/m 3 , which increases mortality rates by 4 percent. About 2.8 billion people, or 36 percent of the world population, are directly exposed to concentrations above 35 μg/m 3 , which increases mortality rates by over 24 percent.

Our study also shows that pollution levels are particularly high in middle-income countries, where a wide range of factors contribute to increased concentration levels. These include less stringent air quality regulations, the prevalence of older, more polluting machinery and vehicles, fossil fuel subsidies, congested urban transport systems, coal-based residential heating, rapidly developing industrial sectors, and cut-and-burn agricultural practices 6 , 10 . Of the 7.3 billion people exposed to unsafe PM2.5 levels, 80 percent live in low- and middle-income countries. The rapidly growing economies in South and East Asia stand out in terms of absolute exposure, driven by decades of rapid economic growth and industrialization. China (1.41 billion people) and India (1.36 billion) alone account for 38 percent of global exposure to PM2.5 concentrations above WHO guidelines.

This pattern is broadly consistent with the notion of an environmental Kuznets curve, which suggests that air pollution levels would be highest in middle-income countries, where polluting activities, such as manufacturing, dominate the economy while productive capital, such as technology, and regulations tend not to prioritize environmental quality 28 , 29 . In low-income countries, air pollution concentrations would be relatively low, as economic activities, such as agriculture, tend to rely less on fossil fuels, and the consumption of polluting goods—such as high electricity use or private car ownership—is limited to small population groups. In high-income countries, pollution would be low, as economic activity tends to be focused on less polluting sectors, such as services, polluting activities tend to be offshored, and clean technologies are widely available and mandated by regulation.

Yet these results also imply that the pollution intensity along the economic development path is not set in stone. Whether today’s low-income countries indeed witness intensifying pollution as a byproduct of development depends on the availability and affordability of clean technologies, and the incentive structure for adopting them. For example, subsidizing fossil fuel consumption undermines the uptake of clean technologies, entrenching high pollution levels in low- and middle-income countries, where such subsidies are common 30 . Stricter regulations on the embodied pollution content of traded goods can address the offshoring of polluting activities and technologies.

Our study also estimates that 716 million people live in extreme poverty (under $1.90 per day) while facing unsafe air pollution. At least 405 million of them live in Sub-Saharan Africa. Low-income population groups are more likely to perform physical and outdoor labor, and therefore face higher exposure and intake of pollutants. They are particularly vulnerable to prolonged adverse impacts on livelihoods and well-being: with lower access to, and availability and quality of, health care provision, the health risks of exposure to air pollution are probably more severe—and air pollution-related mortality higher—for them than for higher-income households exposed to the same levels. One study on air pollution and infant mortality, for example, suggests that mortality risks in India are two to three times larger than in high-income countries 3 . And, although not covered in this study, exposure to indoor air pollution also affects low-income groups disproportionately, as they tend to be more dependent on polluting, low-cost fuels such as charcoal, kerosene, or firewood for cooking and lighting.

Air pollution is one of the world’s leading causes of death, especially affecting lower-income communities, who tend to be more exposed and more vulnerable. Our estimates affirm the case for implementing targeted measures to reduce the pollution intensity of economic growth—for example, by supporting the uptake of less polluting technologies in industry and infrastructure, or facilitating the transition towards cleaner fuels, particularly electrification.

Measures are also warranted to directly address the disproportionate exposure of low-income communities highlighted in this study. Expanding the provision of affordable and adequate healthcare in large urban centers in low- and middle-income countries can help reduce mortality, bringing it closer to levels experienced in higher-income countries. Mandating transparent accounting for environmental and health externalities in planning decisions can help steer pollution sources—such as industrial zones or power plants—away from low-income communities. Finally, removing incentives that perpetuate the over-consumption of fossil fuels can yield a double dividend for lower-income groups. For example, while fossil fuel subsidies confer disproportionate monetary benefits to richer households, the air pollution externalities associated with subsidized fossil fuel consumption are disproportionately borne by low-income households. Addressing such policy distortions can benefit low-income groups in terms of both fiscal and health benefits.

This section details the datasets used in this study to calculate global population exposure to high concentrations of air pollution.

Air pollution data (PM2.5)

Rather than consider the cumulative load of all pollutants, this study looks at the differentiated exposure to anthropogenic PM2.5 pollution across countries. Particulate matter (PM) is one of the most common pollutants, primarily caused by fossil fuel combustion, such as car engines and coal or gas power plants 10 . Airborne PM is commonly categorized by the diameter of particles—PM2.5 for particles of up to 2.5 µm in diameter, and PM10 for those up to 10 µm in diameter—as this determines aerial transport, removal processes, and impacts within the respiratory tract 3 . This study focuses on PM2.5, for two main reasons. First, as one of the most pervasive and harmful pollutants, which can pass through the lungs into the bloodstream and affect other organs, PM2.5 is responsible for the vast majority of air pollution-related deaths, and its impacts are on the rise. It is estimated that 4.5 million people died in 2019 from adverse health effects related to long-term exposure to ambient air pollution, and that 4.1 million of these deaths were caused by PM2.5 (IHME 2020) 31 . And between 2000 and 2019, PM2.5-attributable deaths increased in all regions except Europe, Latin America, and North America 6 . Second, unlike many other pollutant types, datasets on PM2.5 spatial distribution and concentration levels are available with global coverage. Due to data limitations, this study does not cover indoor air pollution, another pervasive risk to health and well-being, especially in low- and middle-income countries.

We use the gridded dataset of ground-level fine particulate matter (PM2.5) concentrations provided by ref. 32 , which offers both annual and monthly mean concentrations for 1998–2019, with global coverage and at 0.01-degree resolution (Fig.  6 ). The dataset is constructed by combining Aerosol Optical Depth satellite retrievals from the NASA MODIS, MISR, and SeaWIFS instruments with the GEOS-Chem chemical transport model, and subsequently calibrating to global ground-based observations using a geographically weighted regression. The 0.01-degree resolution (equivalent to about 1.1 km at the equator) is well suited for capturing regional variation in concentrations, but not granular local variations.

Estimates represent annual average concentrations in 2018, constructed based on satellite-based remote sensing data, global chemical transport modeling, and ground measurements. (Source: data by van Donkelaar et al. 2021).

As a globally modeled dataset, some uncertainty is to be expected, though sensitivity tests suggest good agreement with ground measurement 32 . More spatially nuanced analysis—for example, at a neighborhood or street level—would require alternative data based on local measures. It should also be noted that the chemical composition of PM2.5 particles can differ by pollution source 33 , and those associated with fossil fuel combustion are more toxic due to higher acidity levels (for example, sulfuric PM from coal burning). The global PM2.5 dataset can inform on total particle concentration, but not on acidity.

Population data

To estimate the location of people, we use the WorldPop Global High-Resolution Population dataset, produced by the University of Southampton, the World Bank, and other partners, which offers global coverage and is available yearly from 2000–20. WorldPop provides several datasets, including poverty, demographics, and urban change mapping. This study uses the population count map, a dataset in a raster format, that provides the number of inhabitants per cell, with a 3-arcsecond resolution, thus specifying the distribution of population. This information is based on administrative or census-based population data, disaggregated to grid cells based on distribution and density of built-up area, which is derived from satellite imagery 34 .

The choice of a population density map is important for estimating people’s exposure to natural hazards. Smith et al. 35 provide a sensitivity analysis for flood exposure assessments using different population density maps, including WorldPop. They show that high-resolution population density maps perform best in capturing local exposure distribution, particularly the High-Resolution Settlement Layer (HRSL), jointly produced by Facebook, Columbia University, and the World Bank, which has 1-arcsecond or ~30-m resolution. But HRSL is only available for a limited number of countries, and WorldPop is shown to perform better than alternatives with global coverage, such as LandScan data (30-arcsecond, ~900-m resolution) 36 .

Subnational poverty rates

For 1755 of the 2183 subnational units, the World Bank’s Global Subnational Poverty Atlas offers poverty estimates, derived from the latest available Living Standards Measurement Survey for the respective country 3 . This harmonized inventory of household surveys offers ground-up empirical poverty estimates. Areas, where no poverty estimates are available tend to be high-income countries and small island states. This study uses the standard World Bank definitions of poverty—that is, daily expenditure thresholds of $1.90, $3.20, and $5.50—to determine the number of people living in poverty in a given subnational administrative unit.

Administrative boundaries

The definition of national administrative boundaries follows the standard World Bank global administrative map. However, national boundaries are further disaggregated into subnational units for all countries where World Bank household surveys are available with subnational representativeness. These subnational units are typically provinces or states but can also include custom groupings of subnational regions determined by the sampling strategy of household surveys. Overall, this study covers 211 countries, disaggregated into 2183 subnational units.

Methodology and stepwise computational process

To estimate the number of people exposed to unsafe air pollution levels, this study follows a computational process in four main steps, outlined here.

Step 1. Resample the PM2.5 data: First, we resample the air pollution map to ensure that pixels align with the gridded population density map to identify average annual PM2.5 concentration levels along a continuous scale.

Step 2. Define air pollution risk categories: Second, we aggregate the values into six risk categories (Table  2 ), defined in line with the WHO’s Air Quality Guidelines 3 , which recommend an annual PM2.5 level of up to 5 µg/m 3 . For countries that exceed this threshold, it recommends interim targets at 10, 15, 25, and 35 μg/m 3 , corresponding to a linearly increasing mortality rate (Table  2 ). At higher concentrations, the concentration-response function of mortality may not be linear 37 . For each country, we assign each 1-degree cell one of the six risk categories, repeating this process for the world’s landmass of 149 million square kilometers, processing about 300 million data points.

Step 3. Assign air pollution risk categories to population headcounts at the pixel level and aggregate to the administrative unit: As the air pollution and population density maps are converted into the same spatial resolution, we assign each population map cell a unique air pollution risk classification and aggregated them to the administrative unit (such as province or district) level. This allows us to calculate population headcounts for each risk category and for each (sub)national administrative unit, yielding an estimate of the number and share of people exposed to no, low, moderate, high, very high, and hazardous air pollution concentrations throughout the year. Finally, we aggregate these into administrative units—including country and subnational units—to yield regional and global estimates.

Step 4. Compute the number of people living in poverty and exposed to air pollution risk: In this final step, we multiply poverty shares with the estimated population headcount exposed to unsafe air pollution, to obtain an estimate of the number of people in each administrative unit living in poverty and exposed to air pollution risk. In the absence of pixel-level poverty share data, we use the World Bank’s Global Subnational Poverty Atlas for these calculations, which provide subnational-level data for at least 153 countries.

Reporting summary

Further information on research design is available in the  Nature Portfolio Reporting Summary linked to this article.

Data availability

Global population count data are provided by WorldPop and publicly available for download at https://hub.worldpop.org/geodata/listing?id=69 . Global PM2.5 concentration maps are provided by van Donkelaar et al. (2021) and are publicly available for download at https://sites.wustl.edu/acag/datasets/surface-pm2-5/ . Global subnational poverty rate estimates are provided by the World Bank and are publicly available for download at https://datacatalog.worldbank.org/search/dataset/0042041 .

Code availability

The Python source code for this study is available at https://doi.org/10.5281/zenodo.8016653

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Acknowledgements

This study has benefited from helpful comments, feedback, and inputs by Mattia Amadio, Esteban Balseca, Samira Barzin, Lander Bosch, Richard Damania, Ira Dorband, Xinming Du, Bramka Jafino, Kichan Kim, Christoph Klaiber, Helena Naber, Jason Russ, Melda Salhab, Ernesto Sanchez-Triana, Lucy Southwood, Margaret Triyana, and Esha Zaveri. The study was supported by the Korea Green Growth Trust Fund.

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Peer-reviewed

Research Article

Exposure to outdoor air pollution and its human health outcomes: A scoping review

Contributed equally to this work with: Zhuanlan Sun, Demi Zhu

Roles Writing – original draft

Affiliation Department of Management Science and Engineering, School of Economics and Management, Tongji University, Shanghai, China

Roles Writing – review & editing

* E-mail: [email protected]

Affiliation Department of Comparative Politics, School of International and Public Affairs, Shanghai Jiaotong University, Shanghai, China

• Zhuanlan Sun, 

• Published: May 16, 2019
• https://doi.org/10.1371/journal.pone.0216550
• Reader Comments

Despite considerable air pollution prevention and control measures that have been put into practice in recent years, outdoor air pollution remains one of the most important risk factors for health outcomes. To identify the potential research gaps, we conducted a scoping review focused on health outcomes affected by outdoor air pollution across the broad research area. Of the 5759 potentially relevant studies, 799 were included in the final analysis. The included studies showed an increasing publication trend from 1992 to 2008, and most of the studies were conducted in Asia, Europe, and North America. Among the eight categorized health outcomes, asthma (category: respiratory diseases) and mortality (category: health records) were the most common ones. Adverse health outcomes involving respiratory diseases among children accounted for the largest group. Out of the total included studies, 95.2% reported at least one statistically positive result, and only 0.4% showed ambiguous results. Based on our study, we suggest that the time frame of the included studies, their disease definitions, and the measurement of personal exposure to outdoor air pollution should be taken into consideration in any future research. The main limitation of this study is its potential language bias, since only English publications were included. In conclusion, this scoping review provides researchers and policy decision makers with evidence taken from multiple disciplines to show the increasing prevalence of outdoor air pollution and its adverse effects on health outcomes.

Citation: Sun Z, Zhu D (2019) Exposure to outdoor air pollution and its human health outcomes: A scoping review. PLoS ONE 14(5): e0216550. https://doi.org/10.1371/journal.pone.0216550

Editor: Mathilde Body-Malapel, University of Lille, FRANCE

Received: December 15, 2018; Accepted: April 10, 2019; Published: May 16, 2019

Copyright: © 2019 Sun, Zhu. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: This work received support from Major projects of the National Social Science Fund of China, Award Number: 13&ZD176, Grant recipient: Demi Zhu.

Competing interests: The authors have declared that no competing interests exist.

Introduction

In recent years, despite considerable improvements in air pollution prevention and control, outdoor air pollution has remained a major environmental health hazard to human beings. In some developing countries, the concentrations of air quality far exceed the upper limit announced in the World Health Organization guidelines [ 1 ]. Moreover, it is widely acknowledged that outdoor air pollution increases the incidence rates of multiple diseases, such as cardiovascular disease, lung cancer, respiratory symptoms, asthma, negatively affected pregnancy, and poor birth outcomes [ 2 – 6 ].

The influence of outdoor air pollution exposure and its mechanisms continue to be hotly debated [ 7 – 11 ]. Some causal inference studies have been conducted to examine these situations [ 12 ]; these have indicated that an increase in outdoor air exposure affects people’s health outcomes both directly and indirectly [ 13 ]. However, few studies in the existing literature have examined the extent, range, and nature of the influence of outdoor air pollution with regard to human health outcomes. Thus, such research gaps need to be identified, and related fields of study need to be mapped.

Systematic reviews and meta-analyses, the most commonly used traditional approach to synthesize knowledge, use quantified data from relevant published studies in order to aggregate findings on a specific topic [ 14 ]; furthermore, they formally assesses the quality of these studies to generate precise conclusions related to the focused research question [ 15 ]. In comparison, scoping review is a more narrative type of knowledge synthesis, and it focuses on a broader area [ 16 ] of the evidence pertaining to a given topic. It is often used to systematically summarize the evidence available (main sources, types, and research characteristics), and it tends to be more comprehensive and helpful to policymakers at all levels.

Scoping reviews have already been used to examine a variety of health related issues [ 17 ]. As an evidence synthesis approach that is still in the midst of development, the methodology framework for scoping reviews faces some controversy with regard to its conceptual clarification and definition [ 18 , 19 ], the necessity of quality assessment [ 20 – 22 ], and the time required for completion [ 19 , 21 , 23 ]. Comparing this approach with other knowledge synthesis methods, such as evidence gap map and rapid review, the scoping review has become increasingly influential for efficient evidence-based decision-making because it offers a very broad topic scope [ 15 ].

To our knowledge, few studies have systematically reviewed the literature in the broad field of outdoor air pollution exposure research, especially with regard to related health outcomes. To fill this gap, we conducted a comprehensive scoping review of the literature with a focus on health outcomes affected by outdoor air pollution. The purposes of this study were as follows: 1) provide a systematic overview of relevant studies; 2) identify the different types of outdoor air pollution and related health outcomes; and 3) summarize the publication characteristics and explore related research gaps.

Materials and methods

The methodology framework used in this study was initially outlined by Arksey and O’Malley [ 23 ] and further advanced by Levac et al. [ 20 ], Daudt et al. [ 21 ], and the Joanna Briggs Institute [ 24 ]. The framework was divided into six stages: identifying the research question; identifying relevant studies; study selection; charting the data; collating, summarizing and reporting the results; and consulting exercise.

Stage one: Research question identification

As recommended, we combined broader research questions with a clearly articulated scope of inquiry [ 20 ]; this included defining the concept, target population, and outcomes of interest in order to disseminate an effective search strategy. Thus, an adaptation of the “PCC” (participants, concept, context) strategy was used to guide the construction of research questions and inclusion criteria [ 24 ].

Types of participants.

There were no strict restrictions on ages, genders, ethnicity, or regions of participants. Everyone, including newborns, children, adults, pregnant women, and the elderly, suffer from health outcomes related to exposure to outdoor air pollution; hence, all groups were included in the study to ensure that the inquiry was sufficiently comprehensive.

The core concept was clearly articulated in order to guide the scope and breadth of the inquiry [ 24 ]. A list of outdoor air pollution and health outcome related terms were compiled by reviewing potential text words in the titles or abstracts of the most pertinent articles [ 25 – 33 ]; we also read the most cited literature reviews on air pollution related health outcomes. To classify the types of air pollution and health outcomes, we consulted researchers from different air pollution related disciplines. The classified results are shown in Table 1 .

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https://doi.org/10.1371/journal.pone.0216550.t001

Our scoping review included studies from peer-reviewed journals. There were no restrictions in terms of the research field, time period, and geographical coverage. The intended audiences of our scoping review were researchers, physicians, and public policymakers.

Stage two: Relevant studies identification

We followed Joanna Briggs Institute’s instructions [ 24 ] to launch three-step search strategies to identify all relevant published and unpublished studies (grey literature) across the multi-disciplinary topic in an iterative way. The first step included a limited search of the entire database using keywords relevant to the topic and conducting an abstract and indexing categorizations analysis. The second step was a further search of all included databases based on the newly identified keywords and index terms. The final step was to search the reference list of the identified reports and literatures.

Electronic databases.

We conducted comprehensive literature searches by consulting with an information specialist. We searched the following three electronic databases from their inception until now: PubMed, Web of Science, and Scopus. The language of the studies included in our sample was restricted to English.

Search terms.

The search terms we used were broad enough to uncover any related literature and prevent chances of relevant information being overlooked. This process was conducted iteratively with different search item combinations to ensure that all relevant literature was captured ( S1 Table ).

The search used combinations of the following terms: 1) outdoor air pollution (ozone, sulfur dioxide, carbon monoxide, nitrogen dioxide, PM 2.5 , PM 10 , total suspended particle, suspended particulate matter, toxic air pollutant, volatile organic pollutant, nitrogen oxide) and 2) health outcomes (asthma, lung cancer, respiratory infection, respiratory disorder, diabetes, chronic respiratory disease, chronic obstructive pulmonary disease, hypertension, heart rate variability, heart attack, cardiopulmonary disease, ischemic heart disease, blood coagulation, deep vein thrombosis, stroke, morbidity, hospital admission, outpatient visit, emergency room visit, mortality, DNA methylation change, neurobehavioral function, inflammatory disease, skin disease, abortion, Alzheimer’s disease, disability, cognitive function, Parkinson’s disease).

Additional studies search.

Key, important, and top journals were read manually, reference lists and citation tracing were used to collect studies and related materials, and suggestions from specialists were considered to guarantee that the research was as comprehensive as possible.

Bibliographies Management Software (Mendeley) was used to remove duplicated literatures and manage thousands of bibliographic references that needed to be appraised to check whether they should be included in the final study selection.

Our literature retrieval generated a total of 5759 references; the majority of these (3567) were found on the Scopus electronic database, which emphasized the importance of collecting the findings on this broad topic.

Stage three: Studies selection

Our study identification picked up a large number of irrelevant studies; we needed a mechanism to include only the studies that best fit the research question. The study selection stage should be an iterative process of searching the literature, refining the search strategy, and reviewing articles for inclusion. Study inclusion and exclusion criteria were discussed by the team members at the beginning of the process, then two inter-professional researchers applied the criteria to independently review the titles and abstracts of all studies [ 21 ]. If there were any ambiguities, the full article was read to make decision about whether it should be chosen for inclusion. When disagreements on study inclusion occurred, a third specialist reviewer made the final decision. This process should be iterative to guarantee the inclusion of all relevant studies.

Inclusion and exclusion criteria.

The inclusion criteria used in our scoping study ensured that the articles were considered only if they were: 1) long-term and short-term exposure, perspective or prospective studies; 2) epidemiological time series studies; 3) meta-analysis and systematic review articles rather than the primary studies that contained the main parameters we were concerned with; 4) economic research studies using causality inference with observational data; and 5) etiology research studies on respiratory disease, cancer, and cardiovascular disease.

Articles were removed if they 1) focused exclusively on indoor air pollution exposure and 2) did not belong to peer-reviewed journals or conference papers (such as policy documents, proposals, and editorials).

Stage four: Data charting

The data extracted from the final articles were entered into a “data charting form” using the database, programmed Excel, so that the following relevant data could be recorded and charted according to the variables of interest ( Table 2 ).

https://doi.org/10.1371/journal.pone.0216550.t002

Stage five: Results collection, summarization, and report

The extracted data were categorized into topics such as people’s health types and regions of diseases caused by outdoor air exposure. Each reported topic should be provided with a clear explanation to enable future research. Finally, the scoping review results were tabulated in order to find research gaps to either enable meaningful research or obtain good pointers for policymaking.

Stage six: Consultation exercise

Our scoping review took into account the consultation phase of sharing preliminary findings with experts, all of whom are members of the Committee on Public Health and Urban Environment Management in China. This enabled us to identify additional emerging issues related to health outcomes.

The original search was conducted in May of 2018; the Web of Science, PubMed, and Scopus databases were searched, resulting in a total of 5759 potentially relevant studies. After a de-duplication of 1451 studies and the application of the inclusion criteria, 3027 studies were assessed as being irrelevant and excluded based on readings of the titles and the abstracts. In the end, 1281 studies were assessed for in-depth full-text screening. To prevent overlooking potentially relevant papers, we manually screened the top five impact factor periodicals in the database we were searching. We traced the reference lists and the cited literatures of the included studies, and then we reviewed the newly collected literatures to generate more relevant studies. Further, after preliminary consultation with experts, we included studies on two additional health outcome categories, pregnancy and children and mental disorders. Hence, 214 more potential studies were included during this process. Besides, 379 original studies of the inclusive meta-analysis and systematic review studies were removed for duplication. In total, 1116 studies were included for in-depth full-text screening analysis and 799 eligible studies were included in the end. The detailed articles selection process was shown in Fig 1 .

https://doi.org/10.1371/journal.pone.0216550.g001

The included air pollution related health outcome studies increased between 1992 and 2018, as shown in Fig 2 . Most studies were published in the last decade and more than 75% of studies (614/76.9%) were published after 2011.

The included studies increased during this period, more than 75% of studies were published after 2011.

https://doi.org/10.1371/journal.pone.0216550.g002

The general characteristics summary of all included studies are shown in Table 3 . Most studies were carried out in Asia, Europe and North America (280/35.0%, 261/32.7% and 219/27.4%, respectively). According to the category system of journal citation reports (JCR) in the Web of Science, 323/40.4% of all studies on health outcomes came from environmental science, 213/26.7% came from the field of medicine, and 24/3.0% were from economics. The top three research designs of the included studies were cohort studies, systematic reviews and meta-analyses, and time series studies (116/14.5%, 107/13.4% and 76/9.5%, respectively). Almost all included studies were published in journals (794/99.4%). The lengths of the included studies ranged from four pages [ 34 ] to over thirty-nine pages [ 35 ].

https://doi.org/10.1371/journal.pone.0216550.t003

Regions of studies

Table 4 outlines the locations in which health outcomes were affected by outdoor air pollution. The continents of Asia (277/34.7%), Europe (219/27.4%), and North America (168/21.0%) account for most of these studies. As the word cloud in Fig 3 illustrates, most of the included studies had been mainly conducted in the United States and China. About 62.8% of the studies (502) had been especially conducted in developed countries.

https://doi.org/10.1371/journal.pone.0216550.t004

Word cloud representing the country of included studies, the size of each term is in proportion to its frequency.

https://doi.org/10.1371/journal.pone.0216550.g003

Most authors (573/799) evaluated the air pollution health outcomes of their own continent, at a proportion of 71.7%.

Types of air pollution and related health outcomes

We categorized the health outcomes, by consulting with experts, into respiratory diseases, chronic diseases, cardiovascular diseases, health records, cancer, mental disorders, pregnancy and children, and other diseases ( Table 5 ). We also divided the outdoor air pollution into general air pollution gas, fine particulate matter, other hazardous substances, and a mixture of them.

https://doi.org/10.1371/journal.pone.0216550.t005

Most of the health records showed that mortality (163/286; 57.0%) was the most common health outcome related to outdoor air pollution, as is visually represented in Fig 4 . Respiratory diseases (e.g., asthma and respiratory symptoms) and cardiovascular diseases (e.g., heart disease) that resulted from exposure to outdoor air pollution were also common (69/199, 63/199 and 23/90; or 34.7%, 31.7% and 25.6%; respectively).

Word cloud representing the health outcomes of included studies, the size of each term is in proportion to its frequency.

https://doi.org/10.1371/journal.pone.0216550.g004

Types of affected groups

The population of included studies was categorized into seven subgroups: birth and infant, children, women and pregnancy, adults, elderly, all ages and not specified ( Table 6 ). The largest air pollution proportion fell under the groups of all ages and children (261/799; 32.7% and 165/799; 20.7%), health outcomes of respiratory diseases in children account for the largest groups (114/199; 57.3%).

https://doi.org/10.1371/journal.pone.0216550.t006

There were 121 research studies in the “Not Specified” group. As shown in Table 6 , the “Birth & Infant,” “Women & Pregnancy,” “Children,” and “elderly” groups occupied the subject areas of more than half of the total included studies, which means that air pollution affected these population groups more acutely. Moreover, age is a confounding factor for the prevalence of cancer and cardiovascular diseases. However, there were only 2 studies (2/38, 5.3%) on cancer and 14 studies (14/90, 15.6%) on cardiovascular diseases in the elderly group.

Summary of results

Of all included studies, 95.2% reported at least one statistically positive result, 4.4% were convincingly negative, and only 0.4% showed ambiguous results ( Table 7 ).

https://doi.org/10.1371/journal.pone.0216550.t007

There were 27 primary studies that showed no association between air pollution and disease, including cancer (n = 1), chronic diseases (n = 1), cardiovascular diseases (n = 6), health records (n = 7), pregnancy and children (n = 2), respiratory diseases (n = 6), mental disorders (n = 1), and other diseases (n = 3). Moreover, eight meta-analyses showed no evidence for any association between air pollution and disease prevalence (childhood asthma, chronic bronchitis, asthma, cardio-respiratory mortality, acute respiratory distress syndrome and acute lung injury, mental disorder, cardiovascular disease, and daily respiratory death). Three meta-analyses showed ambiguous results for mental health, venous thromboembolism, and hypertension.

Our scoping review provided an overview on the subject of outdoor air pollution and health outcomes. We adhered to the methodology outlined for publishing guidelines and used the six steps outlined by the scoping review protocol. The guiding principle ensured that our methods were transparent and free from potential bias. The strengths of the included studies are that they tend to focus on large sample sizes and broad geographical coverage. This research helped us to identify research gaps and disseminate research findings [ 23 ] to policymakers, practitioners, and consumers for further missing or potentially valuable investigations.

Principal findings

Among the included studies, we identified various health outcomes of outdoor air pollution, including respiratory diseases, chronic diseases, cardiovascular diseases, health records, cancer, mental disorders, pregnancy and children, and other diseases. Among them, asthma in respiratory diseases and mortality in health records were the most common ones. The study designs contained cohort, meta-analysis, time series, crossover, cross-sectional, and other qualitative methods. In addition, we included economically relevant studies [ 12 , 36 , 37 ] to investigate the causal inference of outdoor air pollution on health outcomes. Further, pregnancy and children, mental disorders, and other diseases are health outcomes that might have uncertain or inconsistent effects. For example, Kirrane et al. [ 38 ] reported that PM 2.5 had positive associations with Parkinson’s disease; however, some studies report that there is no statistically significant overall association between PM exposure and such diseases [ 39 ]. Overall, the majority of these studies suggested a potential positive association between outdoor air pollution and health outcomes, although several recent studies revealed no significant correlations [ 40 – 42 ].

Time frame of included studies

The time frame of included studies is one of the most important characteristics of air pollution research. Even in the same country or region, industrialization and modernization caused by air pollution is distinguished between different time periods [ 43 , 44 ]. In addition, the more the public understands environment science, the more people will take preventative measures to protect themselves. This is also influenced by time. Although air pollution should not be seen as an inevitable side effect of economic growth, time period should be considered in future studies. The publication trends with regard to air pollution related health outcome research increased sharply after 2010. In recent times, published studies have begun to pay more attention to controlling confounding factors such as socioeconomic factors and human behavior.

Population and country

More than 50% of the studies on the relationship between air pollution and health outcomes originated from high income countries. There was less research (<25%) from developing countries and poor countries [ 45 – 48 ], which may result from inadequate environmental monitoring systems and public health surveillance systems. Less cohesive policies and inadequate scientific research may be another reason. In this regard, stratified analysis by regional income will be helpful for exploring the real estimates. It is reported that the stroke incidence is largely associated with low and middle income countries rather than with high income countries [ 49 ]. More studies are urgently needed in highly populated regions, such as Eastern Asia and North and Central Africa.

It is worth noting that rural and urban differences in air pollution research have been neglected. There are only eight studies focused on the difference of spatial variability of air pollution [ 50 – 57 ]. Variation is common even across relatively small areas due to geographical, topographical, and meteorological factors. For example, an increase in PM 2.5 in Northern China was predominantly from abundant coal combustion used for heating in the winter months [ 58 ]. These differences should be considered with caution by urbanization and by region. Data analysis adjustment for spatial autocorrelation will provide a more accurate estimate of the differences in air. What’s more, in some countries such as China, migrants are not able to access healthcare within the cities; this has resulted in misleading conclusions about a “healthier” population and null based bias was introduced [ 59 ].

Other studies (including systematic reviews and economic studies) on outdoor air pollution

Our scoping review included a large number of systematic reviews and meta-analyses. Of the included 107 systematic review and meta-analyses, the most discussed topics were respiratory diseases influenced by mixed outdoor air pollution [ 60 – 62 ]. Little systematic review research focused on chronic diseases, cancer, and mental disorders, which are current research gaps and potential research directions. A large overlap remains between the primary studies included in the systematic reviews. However, some systematic reviews that focused on the same topic have conflicting results, which were mainly caused by different inclusion criteria and subgroup analyses [ 63 , 64 ]. To solve this problem, it is critical that reporting of systematic reviews should retrieve all related published systematic reviews and meta-analyses.

As for the 24 included economic studies, two kinds of health outcomes—morbidity [ 65 ] and economic cost [ 66 ]—were discussed separately using regression approaches. The economic methods were different from those used in the epidemiology; the study focused on causal inference and provided a new perspective for examining the relevant environmental health problems. Furthermore, meta-regression methodology, an economic synthesis approach, proved to be very effective for evaluating the outcomes in a comprehensive way [ 67 ].

Diagnostic criteria for diseases

The diagnostic criteria for diseases forms an important aspect of health-related outcomes. The diagnostic criteria for stroke and mental disorders might be less reliable than those for cancer, mobility, and cardiovascular diseases [ 68 ]. Few studies provided detailed disease diagnostic information on how the disease was measured. Thus, the overall effect estimation of outdoor air pollution might be overestimated. It is recommended that ICD-10 or ICD-11 classification should be adopted as the health outcome classification criterion to ensure consistency among studies in different disciplines considered in future research [ 69 ].

In spite of these broad disease definitions, studies in healthy people or individuals with chronic diseases were not conducted separately. People with chronic diseases were more susceptible to air pollution [ 70 ]. It is obvious that air pollution related population mobility might be underestimated. However, the obvious association of long-term exposure to air pollution with chronic disease related mortality has been reported by prospective cohort studies [ 71 ]. It should be translated to other diverse air pollution related effect research. The population with pre-existing diseases should be analyzed as subgroups.

Except for the overall population, subgroups of people with outdoor occupations and athletes [ 72 , 73 ], sensitive groups such as infants and children, older adults [ 74 , 75 ], and people with respiratory or cardiovascular diseases, should be analyzed separately.

Measurement of personal exposure

The measurement of personal exposure to air pollutants (e.g., measurement of errors associated with the monitoring instruments, heterogeneity in the amount of time spent outdoors, and geographic variation) was lacking in terms of accurate determination. There is a need for clear reporting of these measurements. The key criterion to determine if there is causal relationship between air pollution and negative health outcomes was that at least one aspect of these could be measured in an unbiased manner.

Pollutant dispersion factor

It is well known that the association between air pollution and stroke, and respiratory and cardiovascular disease subtype might be caused by many other factors such as temperature, humidity, season, barometric pressure, and even wind speed and rain [ 76 – 78 ]. These confounding factors related to aspects of energy, transportation, and socioeconomic status, may explain the varying effect size of the association between air pollution and diseases.

While the associations reported in epidemiological studies were significant, proving a causal relationship between the different air pollutants affected by any other factors and adverse effects has been more challenging. To avoid bias, these modifier effects should be compared with previous localized studies. In fact, how the confounding variables account for the heterogeneity should be explored by case-controlled study design or other causal interference research designs.

Study limitations

The following limitations should not be overlooked. First, scoping reviews are based on a knowledge synthesis approach that allows for the mapping of gaps in the existing literature; however, they lack quality assessment for the included studies, which may be an obstacle for precise interpretation. Some improvements have been made by adding a quality assessment [ 15 , 22 , 79 ] to increase the reliability of the findings, and other included studies control for quality by including only peer-reviewed publications [ 80 ]; however, this is not a requirement for scoping reviews. While our paper aimed to comprehensively present a broader range of global-level current published literatures related to outdoor air pollution health outcomes, we did not assess the quality of the analyzed literature. The conclusions of this scoping review were based on the existence of the selected studies rather than their intrinsic qualities.

Second, bias is an inevitable problem from the perspectives of languages, disciplines, and literatures in knowledge synthesis. We included literatures from electronic databases, key journals, and reference lists to avoid “selection bias” and then included unpublished literature to avoid “publication bias”; further, we also conscientiously sampled among the studies to ensure that there was a safeguard against “researcher bias.” We only took English language articles into account because of the cost and time involved in translating the material, which might have led to a potential language bias [ 23 ]. However, in scoping reviews, language restriction does not have the importance that it does in meta-analysis [ 81 ].

Conclusions

In all, the topic of outdoor air pollution exposure related health outcomes is discussed across multiple-disciplines. The various characteristics and contexts of different disciplines suggest different underlying mechanisms worth of the attention of researchers and policymakers. The presentation of the diversity of health outcomes and its relationship to outdoor exposure air pollution is the purpose of this scoping review for new findings in future investigations.

Supporting information

S1 table. literature search strategies..

https://doi.org/10.1371/journal.pone.0216550.s001

S2 Table. PRISMA-ScR checklist.

https://doi.org/10.1371/journal.pone.0216550.s002

Acknowledgments

The authors would like to thank Miaomiao Liu, an assistant professor in School of the Environment of Nanjing University, for her valuable advice with regard to this article.

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• Published: 06 November 2008

Health effects of ambient air pollution – recent research development and contemporary methodological challenges

• Cizao Ren 1 , 2 &
• Shilu Tong 1  

Environmental Health volume  7 , Article number:  56 ( 2008 ) Cite this article

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Exposure to high levels of air pollution can cause a variety of adverse health outcomes. Air quality in developed countries has been generally improved over the last three decades. However, many recent epidemiological studies have consistently shown positive associations between low-level exposure to air pollution and health outcomes. Thus, adverse health effects of air pollution, even at relatively low levels, remain a public concern. This paper aims to provide an overview of recent research development and contemporary methodological challenges in this field and to identify future research directions for air pollution epidemiological studies.

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Introduction

It is well known that exposure to high levels of air pollution can adversely affect human health. A number of air pollution catastrophes occurred in industrial countries between 1950s and 1970s, such as the London smog of 1952 [ 1 ]. Air quality in western countries has significantly improved since the 1970s. However, adverse health effects of exposure to relatively low level of air pollution remain a public concern, motivated largely by a number of recent epidemiological studies that have shown the positive associations between air pollution and health outcomes using sophisticated time-series and other designs [ 2 ].

This review highlights the key findings from major epidemiological study designs (including time-series, case-crossover, panel, cohort, and birth outcome studies) in estimating the associations of exposure to ambient air pollution with health outcomes over the last two decades, and identifies future research opportunities. We do not intend for this to be a formal systematic literature review or meta-analysis, but to discuss issues we feel are vitally important based on the recent literature and our own experience. This paper is divided into two parts: firstly to summarize recent findings from major epidemiological studies, and secondly to discuss key methodological challenges in this field and to identify research opportunities for future air pollution epidemiological studies.

Health effects of ambient air pollution

Time-series studies.

There are a large number of time-series studies on the short-term health effects of air pollution, with the emphasis on mortality and hospital admissions by means of fitting Poisson regression models at a community level or ecological level. This type of time-series design is a major approach to estimating short-term health effects of air pollution in epidemiological studies for the last two decades. Many studies have found associations between daily changes in ambient particulate air pollution and increased cardiorespiratory hospital admissions [ 3 – 6 ], along with cardiorespiratory mortality [ 7 – 9 ] and all cause mortality [ 10 ]. Because numerous air pollution time-series studies show that exposure to air pollution is associated with different kinds of human health outcomes, it is impossible to list results from all studies. Table 1 only selects major time-series studies on short-term health effects of particulate matter (PM) and ozone from different countries around the world published over the last two decades because these two air pollutants are important toxic agents and widely explored by the majority of air pollution epidemiological studies. Early findings have been systematically and thoroughly reviewed by other authors [ 11 , 12 ].

Single-site time-series studies have been criticized because of exposure measurement errors, substantial variation of the air pollution effects and the heterogeneity of the statistical approaches used in different studies [ 13 ]. Recently, several multi-site time-series studies have been conducted in Europe and the United States. Two large collaborative air pollution projects in Europe and U.S. are summarised below.

In Europe, the APHEA (Air Pollution and Health: a European Approach) studies have provided many new insights. Initial studies were based on older data (APHEA-1) [ 14 ] and a new series of studies (APHEA-2) used data of the PM 10 fraction since the late 1990s [ 15 ]. The APHEA-2 mortality studies covered over 43 million people and 29 European cities, which were all studied for more than 5 years in the 1990s. The combined effect estimate showed that all-cause daily mortality increased by 0.6% (95% CI: 0.4%, 0.8%) for each 10 μg/m 3 increase in PM 10 from data involving 21 cities. It was found that there was heterogeneity between cities with different levels of NO 2 . The estimated increase in daily mortality for an increase of 10 μg/m 3 in PM 10 were 0.2% (95% CI: 0.0%, 0.4%), and 0.8% (95% CI: 0.7%, 0.9%) in cities with low and high average NO 2 , respectively [ 16 ]. The APHEA-2 hospital admission study involved 38 million people living in eight European cities. Hospital admissions for asthma and chronic obstructive pulmonary disease (COPD) increased by 1.0% (95% CI: 0.4%, 1.5%) per 10 μg/m 3 PM 10 increment among people older than 65 years [ 15 ].

In the United States, the National Morbidity, Mortality and Air Pollution Studies (NMMAPS) focused on the 20 largest metropolitan areas in the USA, involving 50 million inhabitants, during 1987–94 [ 2 ]. All-cause mortality was increased by 0.5% (95% CI: 0.1%, 0.9%) for each increase of 10 μg/m 3 in PM 10 . The estimated increase in the relative rate of death from cardiovascular and respiratory disease was 0.7% (95% CI: 0.2%, 1.2%). Effects on hospital admissions were studied in ten cities with a combined population of 1 843 000 individuals older than 65 years [ 17 ]. The model used considered simultaneously the effects of PM 10 up to the lag of 5 days and effects of PM 10 on chronic obstructive pulmonary disease admissions to be 2.5% (95% CI: 1.8%, 3.3%) and on cardiovascular disease admissions to be 1.3% (95% CI: 1.0%, 1.5%) for an increase of 10 μg/m 3 in PM 10 . Bell et al. [ 18 ] analysed 95 NMMAPS community data to examine the association between ozone concentration and mortality, showing that a 10-ppb increase in the previous week's ozone was associated with a 0.5% (95% posterior interval (PI), 0.3%, 0.8%) increase in daily mortality and a 0.64% (95% PI, 0.31%, 0.98%) increase in cardiovascular and respiratory mortality. The effect estimates of the exposure over the previous week were larger than those considering only a single day's exposure. Recently, Dominici et al. [ 13 ] examined the short-term association between fine particulate air pollution and hospital admissions and found that exposure to PM 2.5 was associated with different health outcomes. The largest association was observed for heart failure, and a 10 μg/m 3 increase in PM 2.5 was found to be associated with a 1.3% (95% PI: 0.8%, 1.8%) increase in hospital admissions from heart failure on the same day.

Although time-series studies have shown that day-to-day variations in air pollutant concentrations are associated with daily deaths and hospital admissions, it is still unclear how many days, weeks or months of air pollution have brought such events forward. Harvesting or mortality/morbidity displacement means that some cases are occurring only in those to whom it would have happened in a few days anyway [ 19 ]. If so, the increase in cases immediately after exposure would be offset by a deficit in daily deaths a few days later [ 19 , 20 ]. If air pollution has harvesting effects, normal time-series models are unable to estimate the effects due to the issues of collinearity and statistical power. The polynomial distributed lag (PDL) model [ 21 ] and the time-scale model [ 19 ] have been adopted to explore whether air pollution has harvesting or displacement effects on daily deaths or hospital admissions. A few studies suggested potential harvesting effects of ambient air pollution while other studies have shown that there is no evidence for harvesting effects [ 19 , 22 , 23 ]. Although one study shows that potential bias might occur in PDL model [ 24 ], the estimated effects of ambient air pollutants seem to increase when longer lags of air pollution are included [ 19 , 20 ].

Case-crossover studies

Case-crossover study design is an alternative approach to estimating short-term health effects of air pollution in epidemiological studies. In the last two decades, the case-crossover design has been applied in a large number of studies of air pollution and health [ 25 – 28 ]. For example, Neas et al. [ 27 ] used a case-crossover study design to estimate the association between air pollution and mortality in Philadelphia and found a 100 μg/m 3 increment in the 48 hours mean level of TSP was associated with increased all-cause mortality (odds ratio (OR) = 1.06; 95% CI: 1.03, 1.09). A similar association was observed for deaths in individuals over 65 years of age (OR: 1.07; 95% CI: 1.04, 1.11). Levy et al. [ 28 ] estimated the effect of short-term changes in exposure to particulate matter on the rate of sudden cardiac arrest. The cases were obtained from a previously conducted population-based case-control study and were combined with ambient air monitoring data. The results did not show any evidence of a short-term effect of particulate air pollution on the risk of sudden cardiac arrest in people without previously recognised heart disease. Schwartz [ 26 ] conducted a case-crossover study to examine the sensitivity of the association between ozone and mortality when adjusted for temperature and found that 10-ppb increase of maximum hourly ozone was associated with 0.23% (95% CI: 0.01% ~0.44%) increase in daily deaths after adjusting for temperature in 14 US cities. Barnett et al. [ 25 ] examined the association between air pollution and cardio-respiratory hospital admissions in Australia and New Zealand cities. The results show that air pollution arising from common emission sources was significantly associated with cardiovascular health outcomes in the elderly. For example, for a 0.9-ppm increase in CO, there were significant increases in elderly hospital admissions for 2.2% (95% CI: 0.9%, 3.4%) increase of total cardiovascular disease and 2.8% (95% CI: 1.3%, 4.4%) increase of all cardiac disease.

Panel studies

Many air pollution panel studies have been conducted, including several large longitudinal studies of air pollution and health effects such as the Southern California Children's Health Study [ 29 , 30 ], in which children from grades 4, 7, and 10 residing in twelve communities near Los Angeles were followed annually. The results indicated that exposure to ambient particles, NO 2 , and inorganic acid vapour was associated with reduced lung function in children. Another large panel study, the Pollution Effects on Asthmatic Children in Europe (PEACE), was designed to examine the relationship between short-term changes in air pollution and lung function, respiratory symptoms and medication use [ 31 ]. This project was conducted in 14 centres using a common protocol in the winter of 1993–1994. Each PEACE centre involved an urban and a rural panel of symptomatic children and followed at least seventy-five 6–12 year old children [ 31 ]. The pooled estimates of two literature reviews which were separately conducted about the PEACE study and showed that no clear relation could be established for changes in PM 10 , black smoke, SO 2 and NO 2 and changes in respiratory health. The non-significant effects were thought to be possibly due to the short observation period. Ward and Ayres [ 32 ] reviewed 22 panel studies published in the 1990s to estimate the overall effects of ambient particles on children. Results show that the majority of identified panel studies indicated an adverse effect of particulate air pollution. Several recent panel studies also show that particulate air pollution is associated with human health [ 33 – 37 ].

Cohort studies

Compared to time-series and case-crossover studies, there are only a few large cohort studies. About a dozen cohort studies have been conducted in the United States [ 38 – 44 ], Europe [ 45 – 48 ] and Australia [ 49 ]. A cohort study conducted by Dockery et al. [ 39 ] in six U.S. cities shows that there was a statistically significant and robust association between air pollution and mortality. The adjusted mortality rate ratio for the most polluted city was 1.26 (95% CI: 1.08–1.47) compared with the least polluted city. Air pollution was also associated with deaths from lung cancer and cardiopulmonary diseases. Abbey et al. [ 38 ] conducted a cohort study during 1973–1992 to estimate effect of exposure to long-term ambient concentrations of PM 10 and other air pollutants, and show that PM 10 was strongly associated with mortality from respiratory disease for both sexes adjusting for a wide range of potentially confounding factors. The relative risk (RR) for an interquartile range (IQR) difference of PM 10 was 1.18 (95% CI: 1.42, 3.97). Ozone was strongly associated with lung cancer mortality for males for the IQR difference (RR: 4.19; 95% CI: 1.81, 9.69). Sulphur dioxide was also strongly associated with lung cancer mortality for both sexes. Pope et al. [ 44 ] conducted one cohort study in the US to examine the long-term effect of exposure to fine particulate air pollution. They found that fine particulate and sulphur oxide-related pollution were associated with all-cause, lung cancer and cardiopulmonary mortality. A 10 μg/m 3 increase in fine particulate air pollution was associated with an increase of 4%, 6%, and 8% for all-cause, cardiopulmonary, and lung cancer mortality, respectively. Hoek et al. [ 48 ] investigated a random sample of 5000 people and 489 of 4492 (11%) died during 1986–1994 in the Netherlands fining that cardiopulmonary mortality was associated with living near a major road with relative risk of 1.95 (95% CI: 1.09–3.52). A cohort study conducted by Filleul et al. [ 46 ] in France found that urban air pollution to be associated with increased mortality over 25 years in France. Frostad et al. [ 47 ], in a 30-year follow-up cohort study in Norway, found that respiratory symptoms were a significant predictor of mortality from all causes. In Australia, Jalaludin et al.[ 49 ] enrolled a cohort of primary school children with a history of wheeze (n = 148) in an 11-month longitudinal study to examine the association between ambient air pollution and respiratory morbidity. They found that PM 10 and NO 2 , but not ozone, were significantly associated with doctor visits for asthma.

Birth outcome studies

Even though effects of exposure to ambient air pollution on mortality and hospital admissions have been increasingly demonstrated over the past 30 years, exploring its adverse impact on pregnant outcomes has only begun since the last decade [ 50 ]. Because pregnancy is a period of human development particularly susceptible to the influence of many environmental factors due to high cell proliferation, organ develop and the changes of capabilities of fetal metabolism, the relative short-term period provides a unique opportunity to study the adverse effects of ambient toxins on human health [ 51 ]. The majority of birth outcome studies are based on large datasets routinely collected from air pollution monitoring systems and birth registration processes, and therefore, in general, the statistical power is strong [ 52 – 59 ]. Logistic regression models or linear regression models at the individual level are usually adopted to assess the effects of ambient air pollution on adverse birth outcomes adjusting for potential confounders including maternal age, maternal race, parity, fetal gender, season, gestational period, etc. Birth outcomes usually include low birth weight, preterm delivery and other biomarkers such as birth defect and ultrasound measures of head circumference. Personal exposures are often estimated at different terms, including the full gestation, trimesters, month after the pregnancy or before the time of delivery, etc.

Many studies have shown that there are significant associations between exposure to ambient air pollutants and adverse birth outcomes [ 52 – 60 ]. For example, Liu et al. [ 53 ] found that 5-ppb increase of sulfur dioxide was associated with an 11% (95% CI: 1%, 22%) increase of low birth weight (< 2500 grams) during the first month gestation and with a 9% increase of preterm delivery in Vancouver, Canada. A 1.0 ppm increase of carbon monoxide during the last month of pregnancy was associated with an 8% increase of preterm delivery. Parker et al. [ 60 ] selected population within 5 miles of over 40 air pollution monitoring sites across 28 California counties to estimate the adverse effects of air pollution and found that per 10 μg/m 3 PM increase was associated with 13 g (95% CI: 7.6 g, 18.3 g) decrease of birth weight. Similarly, Ritz et al. [ 59 ] conducted a population-nested case-control study to examine associations between air pollution and birth outcomes in Los Angeles and found that air pollution exposure was associated with preterm birth. Hansen et al. [ 58 ] examined the associations of exposure to ambient air pollution during early pregnancy with fetal ultrasonic measurements during mid-pregnancy in Australia. They found that a reduction in fetal abdominal circumference was associated with exposure to O 3 during the days 31–60 of pregnancy (-1.42 mm, 95% CI: -2.74, -0.09), SO2 during the days 61–90 (-1.67 mm, 95% CI: -2.94, -0.40), and PM 10 during the days 90–120 (-0.78 mm, 95% CI: -1.49, -0.08).

Implications of weak health effects

Even though the association of air pollution with health outcomes is weak, it still has strong public health implications. One reason is that air pollution is ubiquitous and affects the whole population in most metropolitan cities. Another reason is that residents are continuously and permanently exposed to air pollution, which may have both short- and long-term effects on health outcomes. Some intervention studies have shown that the reduction in air pollution has resulted in an improvement in population health [ 55 , 61 ]. For example, Hedley et al. [ 61 ] reported that cardiovascular, respiratory and all cause mortality reduced by 2.0% (p < 0.05), 3.9% (p < 0.05) and 2.1% (p < 0.05) respectively in the first 12 months after an introduction of the restrictions on sulphur content of fuel in Hong Kong.

Contemporary methodological challenges

Air pollution epidemiologic research is challenged by the complexity of human exposure to environmental agents and by the difficulty of accurately measuring exposure. Residents are usually ubiquitously exposed to air pollution. In order to detect small effects of air pollution, both high statistical power and sophisticated study design are required. In addition, the characteristics of air pollutants vary and their concentrations change both spatially and temporally. Although everyone is susceptible to high concentration of pollution, its concentrations are not evenly distributed across populations. Due to such complexities, there are still many research questions to be addressed by future air pollution epidemiological studies. The following section discusses these issues.

Shape of exposure response curve

The shape of the exposure and response curve is very important. A key research question to be addressed is whether a threshold exists below which a certain air pollutant has no effect on population health. If such a threshold could be identified, public health benefits would be expected from bringing the pollutant below this level. Both theoretical and empirical works have been done to shed light on this issue [ 62 , 63 ]. In the analysis of NMMAPS data, no threshold evidence was found for the relationship between PM 10 and daily all-cause and cardiorespiratory mortality [ 63 ]. By contrast, a threshold of about 50 μg/m 3 was indicated for non-cardiorespiratory causes of death – viz, below this point, PM 10 had little influence on non-cardiorespiratory mortality. These issues remain to be clarified.

Model uncertainty and bias

The process of model selection includes how to select covariates (eg, meteorological variables and co-pollutants), lag structure for air pollutants and the number of degrees of freedom for smoothing functions to adjust for long-term trend, short fluctuation, seasonality, other covariates and the determination of referent in case-crossover design. Studies have shown that the model choice will impact on estimates of relative risk [ 64 ]. As a result, many authors attempted to estimate the effects using the best single lag or combination of lags for meteorological factors and/or air pollutants and to identify the best degree of freedom for smoothing to adjust for different potential confounders. Some types of data can use several different models. Some authors do not clearly state why they select models and how they conduct data analyses. For example, when we estimate associations between exposure to air pollution and recurrent asthma episodes, based on different assumptions, at least five survival Cox models could be applied to estimate the associations between exposure to air pollution and asthma episodes [ 65 ]. Different assumptions or models may result in different estimates, and sometimes the difference is considerable. The choice of software options may cause this kind of uncertainty as well [ 65 ].

Results presented by the "best" final models are likely to cause publication bias because stronger and positive estimates tend to be published but negative results are usually difficult to be published. Multi-site time series design in which all data are analysed using the same model is one way to solve this problem. However, model uncertainty still exists in a multi-site study to some extent due to the model choice. Some studies have used Bayesian Model Averaging (BMA) to take into account uncertainties in model choice when making an inference [ 64 ]. BMA uses hierarchical models. The predictions and inferences are based on multiple models rather than a single model. Predictions are obtained by forming weighted averages of predictions over the different models where weights depend on the degree to which the data support each model.

Measurement errors of exposure to air pollution and potential confounders usually exist in air pollution epidemiological studies and it is impossible to be solved in most air pollution studies [ 66 ]. Due to spatial and temporal variations, data obtained in air pollution central monitors are not well representative of individual exposures. Some models are used to assess individual exposure to air pollution [ 66 , 67 ], but they could not efficiently adjust for measurement errors. Therefore, potential misclassification bias of exposure is one of the main concerns in air pollution studies.

There are both spatial and temporal variations for exposure and outcomes in air pollution studies [ 68 ]. Both times-series and case-crossover designs at a community level can efficiently adjust for some measured and unmeasured time-invariant characteristics of the subjects (such as gender, age, smoking status and spatial characteristics) via matching, and therefore, the potential confounding from these measured and unmeasured characteristics is minimised [ 69 , 70 ]. The key concern for these designs is how to control for temporal confounding and meteorological variables, such as seasonality, short-term variations and weather conditions (eg, temperature and humidity). In a prospective cohort study design, a major issue is how to identify a cohort with a sufficient variation in cumulative exposures, particularly when data recorded in central monitoring stations are used to measure ambient air pollution levels [ 44 ]. However, in maximizing the geographical variability of exposure the relative risk estimates from cohort studies are likely to be confounded by area-specific characteristics [ 68 ]. Due to collection of relatively detailed individual characters and sufficient adjustment for potential individual social and economic status, such confounding might be efficiently adjusted for.

Birth outcome studies are mainly based on routinely collected data, including exposure, outcome and potential confounders [ 52 – 60 ]. Most studies use pollution data obtained from the different monitors and the closest residential monitoring data are used as exposure proxies [ 58 , 60 , 71 , 72 ]. In general, information related to birth outcomes is well recorded in birth registration systems. However, the data may not include complete and accurate information on other potential confounders, such as maternal social and economic status and life styles. Birth outcome data analyses are usually conducted at an individual level. Therefore, this design is inherently vulnerable to some potential biases, including both temporal and spatial misclassification bias. Ritz and Wilhelm [ 73 ] has discussed the methodological issues of birth outcome designs in detail, and this review would not repeat these issues but rather than focus on potential bias in relation to spatial variation, which was ignored in their review, in the following section.

In birth outcome studies, both exposure and outcome data include temporal and spatial variations to some extent. The majority of birth outcome studies have adjusted for temporal and other confounders which are related to delivery information, including season, maternal age and race, fetal gender, parity, and maternal education attainment [ 52 – 60 , 71 , 72 ]. However, so far, few studies have paid much attention to the potential spatial confounding. Unlike time-series or case-crossover studies, most birth outcome studies lack the ability for automatic adjustment for measured and unmeasured time-invariant spatial variations. Unlike cohort studies, birth outcome study designs also lack the ability to efficiently adjust for personal life styles and social and economic status due to the lack of the detailed information available in routinely collected data. Because both exposure to air pollution and birth outcomes are influenced by some geographic characteristics, such as land use, forest, public infrastructure, and residential social and economic status, etc, the previous birth outcome studies might introduce bias to some extent due to the failure to consider spatial variation. In general, these spatial-related factors are favourable to links between air pollution exposure and birth outcomes. Therefore, we presume that the stronger associations reported in the previous birth outcome studies might partially attribute to this kind of bias. The simple way to adjust for the spatial variation is to add a categorical variable for individual residential areas to fit a fixed effect model or to include the residential areas to fit a mixed model or a random effect model.

Interaction of temperature and air pollution

In many locations, patterns of air pollution are driven by weather. Therefore, concentrations of air pollutants may be associated with temperature. Therefore, it may be possible that temperature and air pollution interact to affect health outcomes. Although effect modification has important public health implications [ 74 ], this issue has so far received limited attention, probably because of methodological complexity and the difficulty in data interpretation. Several studies examined whether or not ambient air pollution and temperature interact to affect human health outcomes, but they produced conflicting results [ 75 – 78 ]). For example, Samet et al. [ 78 ] investigated the sensitivity of the particulate air pollution mortality effect estimate to alternative methods of controlling weather and did not find any evidence that weather conditions modified the associations of particulate air pollution and sulphur dioxide with mortality, regardless of approaches of synoptic weather conditions. Katsouyanni et al. [ 75 ] used a multiple linear regression to investigate the interaction between air pollution and high temperature during a heat wave in Athens in July 1987. They found that while the main effects of air pollution index were not statistically significant, there was statistically significant synergistic effect between high levels of sulphur dioxide and high temperature (P < 0.05). Roberts [ 77 ] found evidence that the effect of particulate air pollution on mortality might depend on temperature but the synergistic effect was sensitive to the number of degrees of freedom used in confounder adjustments. Recently, we found that temperature and particulate matter symmetrically enhanced the effect [ 76 ]. Since then, several multiple-site studies have found evidence that temperature and air pollutants interacted to impact human health but the nature and magnitude of such an interaction varied with geographic area [ 79 – 82 ]. Thus, further research is needed to examine the interactive effects between air pollutants and temperature on mortality and morbidity, especially in different spatial settings.

Many time-series, case-crossover and panel studies have shown that there are consistent short-term effects of air pollution on health outcomes (hospital admissions or deaths). Some cohort studies have also shown long-term health effects of air pollution. In spite of the weak associations of air pollution with human morbidity or mortality, its public health implications are strong because exposure to air pollution is ubiquitous and widespread. However, there are several key methodological challenges in the estimation of the health effects of low-level exposure to air pollution, such as the shape of the exposure response curve, threshold of air pollution, interactive effects of air pollution and weather conditions, and model uncertainty and potential bias. Future research efforts should focus on these important issues.

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Acknowledgements

We thank Prof. Gail Williams, School of Population Health, University of Queensland for the comments on the earlier version of the manuscript. We also thank two reviewers for their insightful and constructive comments.

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CR conceived of the study, participated in its design, and is responsible for the draft of the manuscript. ST participated in the study design and revised the draft of the manuscript.

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Ren, C., Tong, S. Health effects of ambient air pollution – recent research development and contemporary methodological challenges. Environ Health 7 , 56 (2008). https://doi.org/10.1186/1476-069X-7-56

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Received : 29 January 2008

Accepted : 06 November 2008

Published : 06 November 2008

DOI : https://doi.org/10.1186/1476-069X-7-56

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Air Pollution

Our overview of indoor and outdoor air pollution.

By Hannah Ritchie and Max Roser

This article was first published in October 2017 and last revised in February 2024.

Air pollution is one of the world's largest health and environmental problems. It develops in two contexts: indoor (household) air pollution and outdoor air pollution.

In this topic page, we look at the aggregate picture of air pollution – both indoor and outdoor. We also have dedicated topic pages that look in more depth at these subjects:

Indoor Air Pollution

Look in detail at the data and research on the health impacts of Indoor Air Pollution, attributed deaths, and its causes across the world

Outdoor Air Pollution

Look in detail at the data and research on exposure to Outdoor Air Pollution, its health impacts, and attributed deaths across the world

Look in detail at the data and research on energy consumption, its impacts around the world today, and how this has changed over time

See all interactive charts on Air Pollution ↓

Other research and writing on air pollution on Our World in Data:

• Air pollution: does it get worse before it gets better?
• Data Review: How many people die from air pollution?
• Energy poverty and indoor air pollution: a problem as old as humanity that we can end within our lifetime
• How many people do not have access to clean fuels for cooking?
• What are the safest and cleanest sources of energy?
• What the history of London’s air pollution can tell us about the future of today’s growing megacities
• When will countries phase out coal power?

Air pollution is one of the world's leading risk factors for death

Air pollution is responsible for millions of deaths each year.

Air pollution – the combination of outdoor and indoor particulate matter and ozone – is a risk factor for many of the leading causes of death, including heart disease, stroke, lower respiratory infections, lung cancer, diabetes, and chronic obstructive pulmonary disease (COPD).

The Institute for Health Metrics and Evaluation (IHME), in its Global Burden of Disease study, provides estimates of the number of deaths attributed to the range of risk factors for disease. 1

In the visualization, we see the number of deaths per year attributed to each risk factor. This chart shows the global total but can be explored for any country or region using the "change country" toggle.

Air pollution is one of the leading risk factors for death. In low-income countries, it is often very near the top of the list (or is the leading risk factor).

Air pollution contributes to one in ten deaths globally

In recent years, air pollution has contributed to one in ten deaths globally. 2

In the map shown here, we see the share of deaths attributed to air pollution across the world.

Air pollution is one of the leading risk factors for disease burden

Air pollution is one of the leading risk factors for death. But its impacts go even further; it is also one of the main contributors to the global disease burden.

Global disease burden takes into account not only years of life lost to early death but also the number of years lived in poor health.

In the visualization, we see risk factors ranked in order of DALYs – disability-adjusted life years – the metric used to assess disease burden. Again, air pollution is near the top of the list, making it one of the leading risk factors for poor health across the world.

Air pollution not only takes years from people's lives but also has a large effect on the quality of life while they're still living.

Who is most affected by air pollution?

Death rates from air pollution are highest in low-to-middle-income countries.

Air pollution is a health and environmental issue across all countries of the world but with large differences in severity.

In the interactive map, we show death rates from air pollution across the world, measured as the number of deaths per 100,000 people in a given country or region.

The burden of air pollution tends to be greater across both low and middle-income countries for two reasons: indoor pollution rates tend to be high in low-income countries due to a reliance on solid fuels for cooking, and outdoor air pollution tends to increase as countries industrialize and shift from low to middle incomes.

A map of the number of deaths from air pollution by country can be found here .

How are death rates from air pollution changing?

Death rates from air pollution are falling – mainly due to improvements in indoor pollution.

In the visualization, we show global death rates from air pollution over time – shown as the total air pollution – in addition to the individual contributions from outdoor and indoor pollution.

Globally, we see that in recent decades, the death rates from total air pollution have declined: since 1990, death rates have nearly halved. But, as we see from the breakdown, this decline has been primarily driven by improvements in indoor air pollution.

Death rates from indoor air pollution have seen an impressive decline, while improvements in outdoor pollution have been much more modest.

You can explore this data for any country or region using the "change country" toggle on the interactive chart.

Interactive charts on air pollution

Murray, C. J., Aravkin, A. Y., Zheng, P., Abbafati, C., Abbas, K. M., Abbasi-Kangevari, M., ... & Borzouei, S. (2020). Global burden of 87 risk factors in 204 countries and territories, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019 .  The Lancet ,  396 (10258), 1223-1249.

Here, we use the term 'contributes,' meaning it was one of the attributed risk factors for a given disease or cause of death. There can be multiple risk factors for a given disease that can amplify one another. This means that in some cases, air pollution was not the only risk factor but one of several.

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The Public’s Perceptions of Air Pollution. What’s in a Name?

Air pollution is a major global health threat. There is growing evidence for a negative effect of air pollution on health and well-being. Relationships between air pollution and health are mediated by health risk perceptions and play a crucial role in public response to it. Air pollution in the public’s mind is often different from air pollution defined by the scientific community. Therefore, in order to develop successful prevention and alleviation strategies, an understanding of public risk perceptions is key. The central question of this paper is: ‘How does “the public” (in Brussels) perceive air pollution?’ This research is an attempt to enrich the limited body of qualitative research in the field, approaching the topic of perception from 4 different, complementary angles: definition, association, categorisation and problematisation. About 51 interviews were conducted in the Brussels-Capital Region. Consistent with earlier research, this research illustrates that perceptions of air pollution are diverse, subjective, context-dependent and often deviate from conceptualisations and definitions in the scientific community. Respondents underestimate the potential harm and problematisation depends on comparative strategies and perceived avoidability. The novel aspect of this paper is the identification of 5 mental schemes by which specific elements are categorised as being air pollution: (1) the source of the element, (2) its health impact, (3) its climate impact, (4) its functionality and (5) sensory perceptions. The insights gained from this research contribute to the field of environmental epidemiology through a better understanding of how ‘the public’ perceives air pollution and in what way this may deviate from how it is perceived by experts. We hope to raise the awareness among experts and policy makers that air pollution perceptions are far from universal and consensual but on the contrary individual and contested.

Introduction

Air pollution is alongside climate change one of the biggest environmental threats to human health. 1 According to the WHO, 91% of the world’s population lives in places where ambient air pollution levels exceed WHO guideline limits. 2 Despite improvements in air quality over the past 3 decades, exposure to air pollution is estimated to cause 7 million premature deaths, and results in the loss of millions more healthy years of life. 1

There is indeed growing evidence for a negative effect of air pollution on health and well-being. Many studies provide solid evidence of an association between high concentrations of air pollution and mortality 3 or other health outcomes, such as increased ischaemic heart disease, strokes, infections of the lower respiratory tract, asthma or chronic obstructive pulmonary disease 4 and mental health indicators, such as psychological stress, symptoms of depression or suicide. 5 - 9 Brain damage caused by air pollution seems to be associated with dementia and with weakened cognitive functioning throughout the life course. 10 , 11 Exposure to air pollutants has potentially harmful effects from the earliest stages of life with negative effects on pregnancies as well as long-term effects that affect susceptibility to disease later in life. 12

Given this growing evidence of a negative impact on health and quality of life, there is generally an increasing interest in fighting air pollution at the global, regional and local level. It is therefore important to figure out what air pollution is exactly about.

Air pollution obviously has an ontologically objective existence, but the way in which people come to know and make sense of it, is highly contextual, subjective and therefore far from universal. 13 Air pollution in the public’s mind is often different from air pollution as defined by the scientific community. Truth claims of scientists are evidence-based and therefore more convincing for policy makers. However, from a policy perspective, definitions and perceptions of the public need to be considered as well as they define the margins for possible policy action to a large extent. Perceptions being influenced by the social, economic and political context, by knowledge and evolving insights, they will differ between people and contexts. The ambition of this paper is to make a taxonomy of definitions, perceptions and associations that go along with air pollution among Brussels’ citizens.

Why does one’s perception about air pollution matter? Relationships between environmental exposure (eg, air pollution) and physical and mental health (eg, respiratory effects) are mediated by perceptions of the ‘exposure’ (eg, air quality). 14 Risk perceptions – or more exactly the there out resulting attitudes – play thus a crucial role in the public’s response to environmental exposure 15 and in its response to the sources of the exposure. These attitudes impact health both in a direct and an indirect way. In a direct way, high risk perceptions might constitute a cognitive antecedent of a stress reaction negatively impacting upon mental health 16 , 17 or on the other hand, when risks are underestimated, people might not take appropriate measures to protect themselves which impacts on their physical health. Attitudes resulting from risk perceptions also mediate the potentially harmful human health effects of air pollution in a more indirect way since they might result in behavioural changes and support measures aiming to decrease air pollution thereby mitigating air pollution and its negative health impact. Public awareness and realistic perceptions of the health risks associated with air pollution are therefore key in improving public health and in creating public support for policy measures aimed at reducing air pollution.

In order to develop successful prevention and alleviation strategies, understanding risk perceptions is key. Risk perceptions can be defined as involving ‘ people’s beliefs, attitudes, judgements and feelings, as well as the wider cultural and social dispositions they adopt towards hazards and their benefits ’. 18 Key in shaping a health risk perception, is the definition and identification of air pollution. Indeed, if air pollution is not recognised as such, one will not act upon it. 14 These reactions might consist of (individual) behavioural changes, impacting heath directly through protective measures or indirectly through behaviours that reduce levels of air pollution at a personal level (eg, changes in car use). Risk awareness is also crucial for citizens to engage in collective action (eg, through different forms or degrees of activism and to support/call for policy initiatives initiated by local, regional or national governments). 7 , 13 Therefore, understanding how individuals perceive air pollution, is crucial for combating it and to improve public health.

From a review of qualitative research on air pollution perceptions we learn that qualitative research about the topic remains fairly scarce and most often neglects how air pollution is defined by the public and which mental schemes are employed to categorise an element as being air pollution or not. 13

What we learn from the existing body of research on the topic, is that the public and scientists define air pollution differently. The scientific community focuses on specific pollutants derived from multiple sources; the public rarely refers to specific pollutants and rather emphasises the sources of air pollution. In their study on pupils’ knowledge of air pollution in Greece, Dimitriou and Christidou 19 observed that the majority of respondents referred to specific air pollutants as ‘smoke’, ‘exhaust-gases’ or ‘harmful substances’, without making any distinction between the different substances found in the air.

Knowledge about air pollution sources differs between experts and the public. The public often associates air pollution sources with odour. In the Nairobi slums, for instance, smelly drainage channels and toilets were frequently cited as a source of air pollution. 20 Similarly, respondents in Beijing 21 mentioned garbage as a source of air pollution thereby considering odour as the clue connecting garbage with air pollution.

What people categorise as being air pollution is very much culturally defined. In a community in California, smoke caused by wildland fire was perceived as air pollution. 22 On the contrary, in a study on open burning of municipal solid waste (MSW-burning) in India, respondents expressed the belief that smoke from ceremonial fire is a purifier when good fuel is used. 23 When asked explicitly if smoke from MSW-burning also purifies, there was consensus that it was not purifying, but polluting. The ‘pure’ character of ceremonial fire smoke relative to MSW-burning smoke was explained through the fuel used for the burning. In a community in Australia, the presentation of wood smoke as natural and the idea that wood heating is a traditional source of warmth counteracts the strong association of pollution with modernity and ‘artificial’ sources of energy (Reeve, Scott, Hine and Bhullar, 2013). 24 Obviously, the classification of elements as contributing to air pollution is context-dependent. People refer to sources of pollution that are part of their daily lives and the society they live in. Respondents from a London study for instance indicated cars, buses, heavy goods vehicles and pollen as the most significant causes of air pollution, 25 while respondents in a poor neighbourhood in Nairobi mostly pointed to road dust, industrial areas and burning trash. 26 In sum, definitions of air pollution and elements identified as air pollution are not universal: they differ between experts and the public and between different populations in different contexts.

Study aim and research questions

In this qualitative study, we aim at identifying the beliefs, attitudes, judgements and feelings that the public in the Brussels-Capital Region has about ambient air pollution. Our main research question is: How does the public (in Brussels) perceive air pollution?

This question (see Figure 1 ) crystallises into 4 sub-questions that approach the topic of perception each from a different but complementary angle:

Schematic overview of the research questions and how they relate to the main research question.

• How does the public define air pollution? (cognition)
• Which associations does air pollution evoke in the public? (intuition)
• Which elements are perceived by the public as being air pollution and why? (mental schemes)
• Is perceived air pollution also seen as problematic by the public? (ethic)

We consider it relevant to investigate whether air pollution is problematized by the respondents as people can only be mobilised or stimulated to fight air pollution if they recognise it to be problematic. This research aims to enrich the limited body of qualitative research on health risk perception of air pollution. It is to our knowledge the first research investigating how air pollution is defined and identified by the public. The richness of the research lies in the different angles from which the study of ‘perception’ is approached and the detailed and complementary information that results thereout.

First, we present the results related to the definitions of air pollution given by our respondents. The definition question was designed to come to the essence of air pollution through a cognitive way of thinking stimulating the respondent to be concise, to the point and synthetic. To explore the sentimental dimension of respondent’s perceptions in a more intuitive way, an association exercise was done to invite the respondent to think in an open-minded way. The categorisations exercise intended to explore the symbolic dimension of air pollution through mental schemes handed by the respondent. Mental schemas are cognitive structures or mental representations that allows people to categorise knowledge about the world. These schemas help to simplify interactions with the world. In this paper, we focus more specifically on object schemas. 27 , 28 Expressing claims about the problematisation of air pollution encompasses a more ethical dimension through the values expressed towards the problematic character of air pollution. Both ‘feeling’ and ‘thinking’ are touched upon through these 4 questions by means of conscious and more unconscious processes. The insights gained from this research should contribute to the field of environmental epidemiology through a better understanding of how ‘the public’ perceives air pollution.

Methodology

This study adheres to a symbolic interactionist perspective, viewing social interaction in terms of the meaning that social actors attach to action and things. 29 In line with this perspective, we use a qualitative research methodology. We use individual face-to-face in-depth interviews with 51 respondents. The duration of the semi-structured interviews was 90 minutes on average.

Two themes were explored during the interview, perceptions about public green spaces within the Brussels-Capital Region (BCR) and perceptions about air pollution. During the recruitment, respondents were not informed that the subject of air pollution was going to be discussed to avoid that they would inform themselves about the topic as a preparation to the interview, thereby creating potential bias.

Recruitment and fieldwork

Respondents were recruited through the distribution of flyers, a call in a Brussels Facebook group, civil society organisations and schools and snowball-recruitment. The interviews were conducted between October 2019 and March 2020. Interviews were in Dutch or French. An incentive of 15 euros was granted to the respondents after participation to the interview.

Respondents were interviewed by the first and second author, at different places: the Vrije Universiteit Brussel, the respondent’s home, a place of preference of the respondent and in the civil society organisations or schools that helped with the recruitment.

The interviews were audio-recorded, transcribed, analysed and manually coded according to the pre-determined research questions.

The data used for this paper were generated through 3 questions in the interview guide.

The first one (to answer research question 2) consisted of a free association question at the start of the interview asking: ‘What does the word “air pollution” spontaneously make you think of? Which words, images, ideas, impressions, associations or feelings come to mind?’ In the second question (to answer research question 1) respondents were asked how they define air pollution: ‘Suppose I don’t know anything about air pollution and I ask you, what is air pollution, how would you define it?’ The third question (to answer research question 3) was a fairly comprehensive one that examined which specific elements 1 were categorised as air pollution and why: ‘I am going to list a few elements. For each item, I will ask whether you consider this to be air pollution or not. Then I am going to ask you why you think it is or is not air pollution. This question is not intended as a test. It does not matter whether your answer is right or wrong. All I want to understand is the reasoning behind your answer’ . With this question, it was our intention to get an idea of the mental schemes employed by the respondent to perceive a specific element as being air pollution or not.

We also investigated the extent to which identified air pollution is polemised. This issue did not belong to our initial research aim, but appeared to be important during discussions with our respondents resulting from the 3 aforementioned questions.

The analysis is supplemented with quotations of respondents characterised through an anonymous identification code that refers to some main characteristics such as age, gender, migration background and socioeconomic situation (see Appendix 1 ).

We recruited a diverse group of people in terms of age, gender, sociocultural background and socio-economic position (see details in Table 1 ). In what follows we report on the perceptions about air pollution through (1) definition, (2) association, (3) categorisation and (4) problematisation.

Overview of the respondents.

How does the public define air pollution? (Cognition)

To understand how air pollution is perceived, it is first important to know how it is defined. Analysis of the definitions of air pollution indicated that respondents most often referred to the sources of air pollution (n = 38) and also to its consequences (n = 26) while defining air pollution. In doing so, they exclusively denoted anthropogenic sources (no one mentioned natural sources).

‘Air pollution is actually what man makes, it’s what we produce more than the earth can handle. What the earth can convert into good air for us’. (R34)

Many respondents (n = 13) referred exclusively to cars as a source of air pollution in their definition.

‘They are the exhaust fumes from the cars and there is something in them that gets into the lungs and makes you cough and is not very healthy. So the less we drive, the better. Although public transport could be better too’. (R33)

Other sources mentioned by the respondents were boats, trucks, infrastructure works, fire, machinery, factories and aircraft. Less conventional sources such as cigarettes and barbecues were brought up as well.

Respondents rarely (n = 5) referred to specific pollutants (PM, NO 2 , soot. . .) but rather to vague terms such as particles, things, elements, small particles of carbon dioxide and small dust.

‘Air pollution is dirty particles in the air. I don’t know exactly what that is’. (R45)

In addition, they often (n = 4) referred to sources perceptible through the senses, such as the water vapour from nuclear power plants, which was perceived as polluting smoke, or to sensory manifestations of air pollution such as particulate matter on windows or white doors.

‘Air pollutants are particles that are in the air and go everywhere. On the plants, in the respiratory system. I also see it on the windows. The windows are dirty very fast. There are small dust particles on them’. (R46)

Next to sources, respondents also cited the negative health impact of air pollution (n = 24). No specific diseases or disorders were mentioned, instead reference was made to health problems in general, especially for vulnerable groups such as the elderly, people with respiratory problems and children. Some respondents also referred to the impact that air pollution has on the climate (n = 8).

‘They are tiny particles in the air that are created by man himself and caused by the way he lives. This results in the air becoming polluted causing atmospheric warming’. (R39)

In sum, our respondents saw air pollution solely as the consequence of human activity, perceived cars as the main source of air pollution without referring to specific pollutants. They mainly referred to sensory sources and manifestations of air pollution. The impact of air pollution on health and to a lesser extent on climate change seemed central in their definitions. Air pollution was perceived as ‘negative for health’ in general terms especially for ‘weaker’ persons in society.

What associations does air pollution evoke in the public? (Intuition)

To gain insight into the emotive and associative dimension of air pollution perception, we asked respondents which associations ‘air pollution’ evoked in them.

Air pollution seemed to be a vague concept for our respondents. They mainly associated it with sources such as cars and other motorised traffic (n = 28), to a lesser extent with general sources such as exhaust gases, chemical products, petrol or smog (=11) and to a limited extent (n = 4) with the specific pollutants PM and CO 2 . Besides the car and other motorised transport, other conventional sources of air pollution such as industry, agriculture and wood combustion were mentioned sporadically (n = 3). Air pollution was also associated with less conventional sources such as rubbish, barbecues, the smell of snacks, dustbins, smoking people, the smell of food, the smell of cigarettes, dog fouling and smelly places such as underground and train stations. The link with air pollution was made by the smell that these elements emit and, in the case of the barbeque, also the dust that thereout results.

Air pollution was also often associated with the word ‘disease’ (n = 16), mainly referring to health problems in general and to breathing, lung problems or asthma in particular. Respondents rarely referred to their own situation or concrete personal experience (n = 2). Connections with other health problems such as cancer, cardiovascular diseases, inflammation of the airways, worse mental health, brain damage, low birth weight or mortality were not made by our respondents, in contrast to expert knowledge. Air pollution was emotionally exclusively associated with negative feelings (n = 3) such as sadness, anger, concern and disappointment.

Respondents made less obvious or, from a scientific point of view, even incorrect associations. Certain visually perceptible elements were incorrectly labelled as air pollution, such as clouds for example.

Again, it appeared that respondents often associated air pollution with sources that can be sensed through smell, sight and sound. Consequently, air pollution is geographically associated with places with many perceived sources of air pollution, especially cars. The city as a place with high levels of air pollution was contrasted with places outside the city that were associated with cleaner air. Air pollution was thus mainly perceived as an urban phenomenon.

‘ . . . There is more air pollution in the city than in the countryside because there are many cars and many people. Houses are close together, people go around the city more by car than by public transport’. (R21) ‘On the countryside you don’t see smog. I feel much healthier when I am in the countryside’. (R24)

In addition, air pollution was associated with elements that are part of the local context in which it occurs and that contribute to it in a direct or indirect way, such as ‘Flemish people who come to Brussels by car’ and ‘a lack of bicycle lanes’. Thus, there is a tension between residents of the Brussels Region – about half of the inhabitants of Brussels do not own a car 30 – and the Flemish who commute to Brussels by car. This creates the image that it is the Flemish who come to pollute the Brussels’ air. The results also mirror the existing field of tension between those who travel mainly by bicycle and those who travel mainly by car within the BCR. Those who cycle complain that there is not enough (safe) space for cyclists and that it is ‘King Car’ that dominates the public space.

In sum, respondents associated air pollution with vague non-specific pollutants and were rather partial in their identification of air pollution sources. They associated air pollution with ‘negative health’ in general or with lung diseases that have a negative impact on breathing specifically. Furthermore, it appeared again that respondents associate air pollution with sensory perceptions, that they blur the distinction between climate and environmental problems, and make ‘erroneous’ associations from a scientific point of view. Air pollution is mainly perceived as an urban phenomenon for which the ‘other’ is blamed. Respondents made associations linked to the local context that reveal fields of tension between different actors.

What elements are perceived by the public as being air pollution and why? (Mental schemes)

We asked respondents if they would categorise specific elements as air pollution or not and why. The elements discussed in the interview were: particulate matter caused by forest fires, cigarette smoke (secondary smoke), pollen, particulate matter caused by wood burning (stove), ammonia from manure, methane caused by the intestinal system of livestock, water vapour, particulate matter caused by traffic and particulate matter caused by volcanic eruptions (see Appendix 2 for more information about this question).

Firstly, data showed that there was no unanimity in the categorisation of elements as air pollution or not (see Appendix 3 ). The only exception concerned ‘particulates caused by traffic’, which was unanimously categorised as air pollution.

Secondly, the categorisation of an element as air pollution was only to a limited extent based on the element itself. For example, relatively few respondents categorised ‘particulate matter’ independently of its context. There was unanimity on the categorisation of ‘particulate matter caused by traffic’ as air pollution, but no unanimity regarding ‘particulate matter caused by forest fires’, ‘particulate matter caused by wood burning’ and ‘particulate matter caused by volcanic eruptions’, notwithstanding the fact that these all refer to the same element: of ‘particulate matter’.

In this case, when respondents have no explicit knowledge of a certain element being air pollution or not through education or media, people fall back on different mental schemes while categorising elements as air pollution. Based on the data, we were able to identify 5 different mental schemes: the origin of the element, the health impact of the element, the climate impact of the element, sensory perceptions of the element and functionality of the element. An overview of the mental schemes used per element can be found in Appendix 4 .

Mental scheme 1: The origin of the element

The most common mental scheme related to the origin of the element. It mainly evaluates whether the element has an anthropogenic or a natural origin. Elements of anthropogenic origin were usually perceived as air pollution, whereas elements of natural origin were generally not perceived as air pollution. The origin of an element was however not unambiguously determined by different respondents and involved different dimensions: the element itself, the source from which the element raised and the origin of the source.

An element itself could be associated by its name or by its origin with positive, natural things or, on the contrary, with negative, artificial things. For example, according to this mental scheme, ‘pollen’ was not perceived as air pollution because it is a ‘natural’ element.

‘Pollen? No. That’s natural, isn’t it?’ (R4)

Ammonia and methane, on the other hand, were perceived by many respondents as chemical non-natural elements and therefore as air pollution.

‘Ammonia from manure is, I think, definitely air pollution because chemicals are then released into the air and that causes bad air quality . . . Methane, again, is chemical so bad’. (R5)

The same logic was applied to the element ‘particulate matter caused by traffic’.

‘Yes, it is air pollution, because it is not natural’. (R27)

The second dimension is the source from which the element originates . For example, some respondents did not categorise particulate matter from untreated, natural wood as air pollution, whereas particulate matter resulting from the combustion of treated wood or from petrol was categorised as air pollution because the source was perceived as non-natural or no longer natural.

‘I don’t think “particulate matter caused by wood burning” is air pollution because to me, anything natural has no negative impact on health. Unless the wood was processed of course’. (R9)

Similarly, the element ‘water vapour’ was generally not seen as air pollution since the raw material from which water vapour is formed – water – is perceived as natural. If, on the other hand, the water is not or no longer natural, it was categorised as air pollution.

‘It depends on what kind of water one evaporates. If you evaporate water from a clean river, it is not air pollution at all, but if you evaporate water from the toilets, that is pollution. Water from factories is polluted’. (R27)

The origin of the source from which the element originates is the third dimension of the origin and can be natural or in contrast human initiated. Several respondents did not perceive particulate matter from a volcano as air pollution, whereas particulate matter from wood combustion was perceived as air pollution. Although both cases involve the same element (particulate matter), the ‘ignition mechanism’ behind them is different. After all, a volcanic eruption is a natural phenomenon where burning wood is initiated by human activity.

‘I don’t think particulate matter from a volcano is air pollution because it is something natural’. (R9)

Another example relates to the element ‘ammonia from manure’. Those who perceived the source of the element ammonia – livestock, animal husbandry or manure – as natural, did not categorise it as air pollution. Accordingly, one respondent perceived ‘farts’ as the source of ‘methane caused by the intestinal system of livestock’. Since farts were perceived as natural, the resulting methane was also perceived as such and therefore not categorised as air pollution.

‘No, methane caused by the intestinal system of cattle is not air pollution because everyone farts, that’s human, that’s natural’. (R3)

For other respondents, the categorisation of this element as air pollution depended on the scale at which animals are farmed. Methane caused by the intestinal system of cattle’ farmed on a large scale was perceived as unnatural and was therefore categorised as air pollution.

‘It is natural, but in nature you never find such concentrations of cows together, producing so much. So actually it is not natural but human’. (R33)

Mental scheme 2: The health impact of the element

The second most common mental schema concerned the perceived health impact of the element, evaluating the extent to which the element has a negative health impact. Elements that were perceived as harmful, were categorised as air pollution.

‘Actually, from the moment there is a harmfulness, I think there is pollution. So, I think we have to define air pollution from the point of view of harmfulness. Because otherwise you shouldn’t call it pollution, then it’s just an aspect of the air like pollen’. (R32)

The health impact of an element was not unambiguously determined by different respondents and includes different dimensions: a time dimension, a spatial dimension and the experience dimension.

The time dimension refers to both the duration of exposure and the duration of the health impact. For example, ‘pollen’ was often not considered to be air pollution because there is no continuous exposure. Pollen exposure and any resulting health problems were considered as of temporary, seasonal nature.

‘I think that pollen is a natural phenomenon that is not harmful to health but that can trigger allergic reactions but that it is not harmful to health in the long term in the way that air pollution is. I think pollen can just cause an annual allergic reaction that also stops and that also has no long-term effects on health or on nature. I would not call it pollution. Pollution is really something that is harmful. Pollen is more of an element that is in the air and that can be inhaled and that can cause temporary irritation that is not harmful to overall health, but only irritation. Just like the sound of small children causes irritation, just like other natural things can cause irritation but are not harmful’. (R32)

Similarly, particulate matter caused by burning wood is limited in duration, as it is only in the cold evenings that stoves are lit. The exposure to and impact of ‘particulate matter caused by traffic’, on the other hand, was not perceived as being seasonal but continuous.

‘Yes cars have a very big impact on air pollution. We can say that cars circulate in the streets 24/7’. (R41)

Similarly, ‘particulate matter caused by volcanic eruptions’ was not categorised as air pollution because of its perceived short duration.

‘A volcanic eruption does not last very long. The particulate matter goes out of the air. Because it is short, it is not air pollution’. (R6)

The spatial dimension also determines how respondents assess the health impact of elements. This spatial dimension consists of several aspects: the scale of the health impact, the concentration of the source and the distance to the source.

The scale at which people experience a health impact determined if an element is categorised as air pollution or not. For example, the proportion of the population that suffers from the effects of ‘pollen’ is perceived to be limited compared to the proportion of the population that suffers from the effects of exposure to ‘particulate matter caused by traffic’.

‘There are people who are allergic to pollen but I don’t think that is air pollution. It is the reproduction of the plants. It can also cause health problems for some people but those are more exceptions. It is not dangerous for everyone. Smoking is dangerous for everyone’. (R46)

Some respondents stated that ‘pollen’ is air pollution for people who are sensitive to it, but not for people who are not affected by it. They ‘individualise’ the phenomenon of air pollution.

‘Yes, pollen is air pollution for people who suffer from it’. (R2)

Similarly, the concentration of the source influences the extent to which resulting elements were perceived as negative for health. The health impact of particulate matter caused by wood combustion for instance was perceived as relatively limited due to its perceived low concentration. Particulate matter caused by forest fires, on the other hand, was perceived as having a negative impact on health since the resulting concentration of particulate matter is perceived as being much higher.

‘Forest fires are pollution because it is so very massive but wood burning, yes, if everyone is doing that now, then yes. Then I think it can be very polluting. But it remains fairly minimal compared to your air. But it will have an impact on air quality for a while. You’re going to smell that strongly for a while’. (R5)

The distance to the source also influences the perception of the health impact. For example, the distance to traffic and the resulting particulate matter was perceived as relatively small. Particulate matter from wood combustion was perceived as remaining far away because it is emitted at a height via the chimney, as a result of which its impact on health was estimated to be more limited.

‘That’s not good for your health. I do think that’s something because it’s above the roofs, that that dissipates faster so that’s less harmful to you as an individual anyway if you’re downstairs’. (R13)

In addition, personal experiences played a role in evaluating the negative health impact of a certain element. A respondent who experienced a direct, severe and perceived as with air pollution related physical reaction to ‘particulate matter from wood burning’ said:

‘Yes, that is air pollution. Although I only became aware of it later in life, namely when there was an awareness campaign by the Flemish government and I worked for the Flemish government. And I couldn’t believe it at first, but I’ve experienced it first-hand because I was at my father’s house the other day. He has a wood-burning stove and the heating was broken. I lit the stove. I know it’s not good for the environment but I felt like burning wood for once and the next day I had a severe asthma attack. I’m not used to doing that so maybe I didn’t do it right, maybe I didn’t let it soak in and I really felt it’. (R32)

Although many respondents fell back on either the single mental schema of origin or the mental schema of health impact, there were also respondents who combined both and perceived elements of natural origin as harmless in terms of health.

‘I think everything that is natural, that is not man-made, is good, that it does not have a negative impact on us’. (R4)

Mental scheme 3: The impact of the element on the climate

The third frequently used mental scheme related to the perceived impact of the element on the climate. A perceived negative impact on the climate usually resulted in the element being categorised as air pollution. For example, methane was associated with global warming and was therefore categorised as air pollution by some respondents.

‘Methane is a gas that is certainly one of the causes of global warming, so yes. It is also in the air. So that pollutes the climate, but on the other hand, it doesn’t affect us as much, but it’s still bad for the climate. So, yes’. (R14)

The element ‘cigarette smoke’ was also categorised as air pollution due to its perceived negative impact on the climate.

‘A cigarette is something that is on fire and also puts CO 2 into the air I think’. (R11)

With regard to ‘particulate matter caused by traffic’, one respondent commented:

‘Yes, that is simply the biggest contributor to greenhouse gases in the atmosphere’. (R15)

Mental scheme 4: Sensory perceptions

The fourth less frequently used mental scheme related to the sensory. When elements were associated with negative sensory perceptions, they were categorised as air pollution on that basis. For example, particulate matter caused by traffic was associated with a perceptible odour.

‘I think you notice that when you come outside. Then you smell it and feel it and it doesn’t feel good’. (R30)

Besides odour, sight proved to be also important in the perception of air pollution. For example, one respondent remarked in relation to particulate matter caused by traffic:

‘Yes, you notice it when it rains. Then the sky is grey and if you look at the raindrops, you can see that they are not entirely clear. That there are particles in them’. (R37)

Mental scheme 5: Functionality of the element

A final least frequently used mental scheme, related to the perceived functionality of the element. Certain elements were not categorised as air pollution because of their function within a particular ecosystem. For example, water vapour was not perceived as air pollution as it is part of a natural cycle.

‘Water vapour, no, that’s just rain. That falls down and that is part of the cycle’. (R21)

Methane caused by the intestinal system of cattle’ was not perceived as air pollution by some respondents as it was associated with manure and manure was perceived as good for the soil.

‘No, methane is not an air pollutant. It is an energy that we can use. It is a raw material’. (R46)

The element ‘pollen’ was not categorised as air pollution because of its link with green elements in the neighbourhood.

‘For me it is, because I have hay fever. But is that air pollution? Not ultimately, except for people. That’s not actually air pollution but I would still prefer, no I don’t want less pollen because that means even less green, so yes. I don’t see that as air pollution because I think that if there is pollen in the neighbourhood, there is also greenery in the neighbourhood. And because I don’t think that’s really bad for your health because ultimately that’s just the, the fact that there are flowers growing or grass living. That seems to me to be rather a positive thing’. (R23)

We conclude that different mental schemes with different dimensions were used to categorise or not elements as air pollution. We did not observe unanimity in the use of these mental schemes: different mental schemes were used by different respondents, but also within one and the same respondent. Frequently, different mental pictures were weighed against each other about an element, whereby respondents nuanced or questioned the categorisations they had made.

‘Yes, in the strictest sense of the word, I think that is air pollution. It causes the air quality in the immediate vicinity of the volcano to drop drastically and become very unhealthy. On the other hand, I am now contradicting myself because I just said that it is man-made and a volcanic eruption is not man-made but I would still classify it as air pollution’. (R29) ‘Ok, health is important to me, but to me climate is still ahead of health in the sense that if our climate is all fucked up, which we are doing well, then we have nothing to worry about in terms of our health because we are not going to be here anyway. So for me, climate is ahead of health’. (R14)

Is perceived air pollution perceived as problematic by the public? (Ethic)

If people are to be mobilised to reduce air pollution, it is important that elements categorised as air pollution are problematized. Our data showed that categorised elements were not always problematized to the same extent by our respondents. We therefore examined why specific forms of air pollution are problematized, while others are not. We identified 2 mechanisms that influence the extent to which air pollution is relativized or problematized: comparative problematisation and perceived avoidability of emissions.

Comparative problematisation

Comparative problematisation resulted in the problematisation or conversely in the relativisation of perceived air pollution. Comparative problematisation contained several dimensions: a spatial dimension, a time dimension and a source dimension.

In a spatial perspective, respondents relativized air pollution by claiming that air quality was worse elsewhere than in their own environment. Other respondents, on the other hand, polemised the existing air quality by comparing it with places where they felt air quality was better.

Some respondents put air pollution into perspective by comparing pollution levels with a past in which there was much more air pollution or by stating that it has always existed.

‘Yes, particulate matter is pollution. I don’t think we should attach the importance to it that we do now. That is something completely different. Because we can’t say that the world has just become civilised from five years ago. Before five years ago there was no talk about that. There are many things that are now suddenly very important, but that is the way the media works. The way that influence works from all sides. Particulate matter has always existed. There is relatively less particulate matter than there used to be, despite the honking, because there used to be a lot of people who burned coal, or burned wood. A few hundred years ago there was a lot of particulate pollution. But now all of a sudden that’s a hot topic. . . .And pollution from burning wood has always been there. It has been going on for six million years, when they were roasting mammoth legs on a wood fire’. (R2) ‘And then there are the combustion residues of all kinds. Like CO emissions, particulates and so on. These have always been present. The stove in the Middle Ages, the open fire in the castles that was’. (R8)

Other respondents, on the other hand, problematized the current air quality by comparing it with a past in which, according to them, air pollution was much more limited.

Several respondents balanced different forms or sources of air pollution against each other. Certain forms or sources were seen as worse and therefore more problematic. For example, particulate matter caused by forest fires was generally perceived to be less problematic than air pollution from cars. Factories and cars were perceived as not natural while forest fires were generally considered as natural.

‘I would say yes and no. Yes, that (particulate matter caused dose fires) makes the air dirty but that is not as bad as factories or cars because it comes from nature. The others are manufactured substances and they are worse than fire from nature’. (R50)

Perceived avoidability

Another mechanism influencing the problematisation of air pollution was its perceived avoidability. Air pollution was often problematized when it was perceived as avoidable. When it was perceived as inevitable, it was often relativised.

‘So what can be mitigated as a negative effect from human action, I do think is pollutant and so I also think that due attention should be paid to mitigating it’. (R8)

For example, ‘particulate matter caused by traffic’ was perceived as problematic air pollution when it was considered avoidable, whereas ‘particulate matter caused by forest fires’ was not considered as problematic when forest fires were seen as a natural phenomenon making them unavoidable. As a result, particulate matter from an ignited forest fire was problematized where particulate matter from an ignited forest fire was relativised.

‘It is the smoke from the fire that remains in the air but it is not done intentionally. It’s just a natural disaster and no one can really do anything about it’. (R20)

Similarly, categorised air pollution consisting of ‘particulate matter caused by volcanic eruptions’ was never perceived as problematic as respondents considered it as unavoidable.

‘From a purely material point of view, yes. If that is a natural phenomenon. Radioactive radiation is also there as a natural phenomenon. Is that positive or negative, no, it is there. That is a fact. . .The forest fires in Brazil are an example of a forest fire that is not a natural phenomenon. It is malicious. But if pollution arises from a natural phenomenon, there is no way around it. Then you have to accept that natural phenomenon and its consequences’. (R8) ‘That’s like those forest fires. That’s not intentional but it’s bad. It’s still bad but just the fact that it’s not intentional, it’s ok’. (R20)

Another example relates to ‘particulate matter caused by burning wood’. When wood was burnt for fun and therefore perceived as avoidable, it was problematized; when wood was burnt for survival to heat or cook on, the resulting air pollution was put into perspective as it was considered unavoidable.

‘I keep playing with the tension you can find between the causing or the natural process. A wood-burning stove is effectively polluting. Can it be replaced? If that wood-burning stove is to serve only to see a bit of atmospheric pleasant flames, then I would say, maybe not so much. If it is necessary to heat you, then yes’. (R8)

The same reasoning was applied to the categorisation of ‘cigarette smoke’ as air pollution. For the following respondent, the ‘avoidability’ of cigarette smoke determined the extent to which it was problematized.

‘Smoking is disturbing to the environment, it is polluting. It is air pollution because it is a negative consequence of an action that can be avoided’. (R8)

Discussion, Limitations, Further Research and Conclusion

The aim of this research was to understand the perception of air pollution by the public in the BCR. We investigated this perception through 4 sub-questions that approached the topic of perception each from a different but complementary angle: definition, association, categorisation and problematisation.

The first dimension investigated how the public defines air pollution. Data showed that respondents depict air pollution solely as a consequence of human activity, thereby portraying the car as the main source of air pollution without referring to specific pollutants such as NO 2 , O 3 or PM. This observation aligns with the findings of Dimitriou and Christidou. 19

A recurrent theme in definitions is the negative health impact of air pollution. Respondents refer to health in general terms and tend to link this negative impact to ‘vulnerable’ groups in society rather than to their own health. Furthermore, in line with earlier research, respondents refer in their definitions to sensory sources and manifestations of air pollution. The most frequent mentioned pollutant was PM, the most tangible of all. Intangible pollutants such as NO 2 , or SO 2 were not noticed at all.

The second dimension that we studied concerned the associations that air pollution evokes in the public. The gathered data partially overlapped with the data gathered on definitions but were complementary and gave more detail. Respondents seemed partial in identifying sources of air pollution and in identifying negative health outcomes derived from these sources. They also made associations that deviate from scientific knowledge and that demonstrated ambiguities and misunderstandings about air pollution. Finally, respondents seemed to perceive air pollution as an exclusively urban phenomenon caused by ‘the other’.

Related to the perception of the public about the health impact of air pollution, 3 insights are relevant. First, nevertheless the negative impact of air pollution was central in the definitions of, the associations with and the categorisation of air pollution, it was not mentioned by all respondents. In line with former research, a relatively big share of people does not consequently link air pollution to health problems. 19 , 26

Second, when reference is made to health impacts of air pollution this is done in a general and often partial way. Former research stated that people’s perceptions tend to be influenced less by scientifically derived information and more by local and personal experiences. 31 These experiences are more acute and relate more easily to respiratory complaints than other long-term impacts that are less obvious from the perspective of the public (eg, cognition and depressive symptoms). Third, respondents tend to link the negative health impact of air pollution to vulnerable groups rather than to their own health.

These insights contrast with the established scientific body of knowledge showing a diverse set of serious, often long lasting negative impacts of air pollution on health for all. These scientific insights obviously do not reach the public, which results in an under-estimation of the health impact associated with air pollution. Respondents clearly underestimate the probability and the severity of the harm resulting from air pollution. This has implications for their health risk perception. A study on the relationship between perceived likelihood of a threat, perceived severity of a threat and the motivation to act, established an interaction between likelihood and severity. The motivation to take precautions essentially vanished when either probability or severity was perceived as zero. 32 Also Bickerstaff and Walker 33 found that their respondents related air pollution to poor health at a general level and that only few identified health problems directly affecting themselves. People might not deny the health risk of air pollution but its personal effect as a psychological reaction to avoid psychic anxiety.

Our results also emphasise that perceptions of air pollution are context dependent. Respondents refer to pollution sources that are part of their daily lives and the society they are living in. The emphasis that Brussels respondents lay on the car as the most important source of air pollution and the agreement among them that PM resulting from traffic is air pollution is an illustration of this. Indeed, at the time of the interviews, there was a lot of debate about the polluting impact of cars and civil society (Filter Café Filtré) was protesting against car traffic near schools in different neighbourhoods across the city. It is illustrated that local actions, media and social networks can impact public perception about air pollution. 34 This partial focus on cars as the main source of air pollution might however result in an underestimation of the actual exposure to air pollution from other sources. For NO 2 , 44% of the concentrations in the BCR originate from traffic 35 but for PM 10 , 59% of the emissions is caused by the heating of buildings and (only) 38% by the transport sector. 36

The third dimension related to the categorisation of elements as being air pollution and the reasons behind this categorisation. Our research led to the identification of 5 mental schemes present during the categorisation and identification processes related to air pollutants. These schemes allow for a better understanding of the hidden, partly unconscious rationales behind such processes. However, there was no unanimity about the categorisation of elements as being air pollutants or not, except for PM caused by traffic.

This categorisation of elements as being air pollution did not happen in a vacuum but in a specific context that influenced this process through 5 mental schemes: the origin of the element, its health impact, its impact on the climate, sensory perceptions and functionality of the element.

Our respondents – especially the younger ones – seemed very concerned about the climate. At the time of the interviews, weekly manifestations were organised and frequented by many students to stress the importance of climate action. However, respondents blur the distinction between climate problems and environmental problems. This distinction made by the scientific community seems absent among the public. And indeed, nevertheless different problems, air pollution and climate change are intertwined. 37 Air quality is closely linked to the earth’s climate and ecosystems globally. Many of the drivers of air pollution (ie, combustion of fossil fuels) are also sources of greenhouse gas emissions. Policies to reduce air pollution, therefore, can for many pollutants offer a ‘win-win’ strategy for both climate and health, lowering the burden of diseases attributable to air pollution and contributing to the near- and long-term mitigation of climate change. 38 - 40 Linking the topic of air pollution to climate change in sensitising communications might thus increase the motivation of the public to support specific measures aimed at limiting air pollution.

Saksena 14 already stated that if air pollution is not recognised as such, one will not act upon it. We agree with this statement but argue that an extra step is required for action to be undertaken once air pollution has been ‘identified’ or recognised: problematisation. Therefore, a fourth dimension of the perception about air pollution that we studied was its problematisation.

Respondents tended to problematize or on the contrary to relativize the identified air pollution through comparative problematizing or through the perceived avoidability of the identified air pollution. We identified 3 dimensions of comparative problematisation, a spatial dimension, a time dimension and a source dimension. Related to the avoidability of the identified air pollution, it is often problematized when it is perceived as avoidable. Whereas when it is perceived as inevitable, it is often relativised.

The observed relativisation of the problematic character of (identified) air pollution through comparative problematizing aligns with a disassociation strategy that has been labelled by others as ‘othering’. 41

The observation that the avoidability of the identified air pollution is linked to its problematisation, aligns partly with earlier research. Xu et al. 21 found in this respect that when people feel powerless about an issue which they have to bear with, that they tend to allocate little concern to it.

Our research contributed to a better understanding of how the public in Brussels perceive air pollution.

This research illustrates that the notion of air pollution is difficult for the public to conceptualise. The public’s perceptions are diverse, subjective and often deviate from the way in which air pollution is conceptualised by the scientific community.

It should increase the awareness among experts and policy makers that perceptions about air pollution are far from universal and consensual but on the contrary individual and contested. These insights are highly relevant: to fight air pollution, it is key that all actors communicate at the same conceptual level. Important is that health promoters are/become aware that there might be a communication bias because of different perceptions about air pollution. There is indeed room and need for communication/information/sensitisation about the negative impacts of air pollution on health taking the severity and the probability of its impact into account, the different sources of air pollution, and the different ways to combat it.

To develop successful health campaigns and sensitisation strategies and to find carrying capacity for the implementation of policy measures to fight air pollution, an understanding of the perceptions of the ‘target group’ is key.

There is no room to elaborate on how these health campaigns and sensitisation strategies should look like concretely, but we think that it is worthwhile to give some relevant suggestions that were touched upon by our respondents during the interview. It is important to take into account the trustworthiness of the information sources, 31 , 41 the scale of the information, 33 , 42 , 43 the comprehensiveness of information 31 , 33 , 44 and the degree of affect in information. 45 , 46

Limitations and further research

This study has several limitations.

Firstly, a bias might have occurred resulting from the recruitment of the respondents. Those willing to do an interview – knowing that it was about public green spaces (they didn’t know in advance that another important part of the interview was on air pollution) – might have been more ‘nature-minded’ resulting in the recruitment of profiles that were more against the perceived main contributor of air pollution ‘the car’. However, since we were aware of this potential bias during the recruitment phase, we decided to provide an incentive of 15 euros in cash in order to also attract a diverse mix of people, some ecology-minded, some not. Some of the respondents motivated by this incentive to participate in the interview might have not been intrinsically motivated to participate but this was seen as an advantage to increase the diversity of profiles and perceptions in the research.

Secondly, concerning the problematisation of identified air pollution, our results were only partial since this topic was initially not the focus of our research and questions did not explicitly focus on this topic. However, since it appeared relevant, we dedicated attention to and reported about it. Other research explicitly focussing on the topic, identifies more factors that influence the concern related to air pollution such as personal health experiences, uncontrollability or powerlessness, crowding-out effects, perceived benefits, perceived fairness, delays of health effects and habituation. 21

The identification of the different mental schemes to categorise elements as being air pollution or not, is novel. Further research could further finetune and compare these results. First, it would be interesting to investigate through quantitative research methods, whether different social groups – in terms of age, sex, socioeconomic situation or socio-cultural background – tend to rely on specific mental schemes to further finetune understandings about how perceptions develop and their implications for targeted health campaigns and sensitisation strategies. Another interesting research project could investigate whether different social groups have different associations with air pollution. Secondly, if perceptions are context-dependent, it would be interesting to repeat this research in a totally different social, cultural or political context. In this respect Douglas 47 developed a ‘cultural theory’ of risk in which she considers dirt – and pollution – as a ‘matter out of place’ in terms of the range of powers and dangers symbolically constructed in a cultural universe. She states that dirt is not a unique and isolated phenomenon. Where there is dirt, there is a symbolic system. Dirt is the by-product of an organisation and of a classification of matters that causes the rejection of non-appropriate elements. These elements are not on the right place according to a dominating or ruling symbolic system. Repeating this research in other places, in another cultural universes with other symbolic systems might be interesting to pinpoint differences and analogies between them.

This qualitative research investigated how the public in Brussels perceives air pollution and is an attempt to enrich the limited body of qualitative research in the field. We studied this perception from 4 different, complementary angles: definition, association, categorisation and problematisation.

This research illustrates that the notion of air pollution is difficult for the public to conceptualise and that their perceptions are diverse, subjective, context dependent and often deviate from conceptualisations and definitions by the scientific community.

Respondents underestimate the probability and severity of the harm involved and its problematisation depends on comparative strategies and its perceived avoidability. We identified 5 mental schemes by means of which elements are categorised, or not categorised by respondents as being air pollution: (1) the source of the element, (2) the health impact of the element, (3) the impact of the element on the climate, (4) sensory perceptions and (5) the functionality of the element.

We hope to have contributed to a better understanding of how the public in Brussels perceives air pollution and to an increased awareness among experts and policy makers that perceptions about air pollution are far from universal and consensual but on the contrary individual and contested. After all, these understandings and awareness are key in order to fight air pollution in a successful way through the development of effective and targeted health campaigns, sensitisation strategies in order to create common ground for the implementation of measures to fight air pollution successfully.

Appendix 1.

Characteristics respondents.

Additional information on which elements were perceived as air pollution and why?

In order to answer research questions 3 and 4, the answers to the following question were analysed: ‘I am going to list some elements. I am going to ask you each time, is this element according to you air pollution yes or no? Then I am going to ask you why you think this element is or is not air pollution. This question is not intended as a test. It does not matter whether your answer is right or wrong. All I want to understand is the reasoning behind your answer’.

This question was not meant to gauge the respondent’s knowledge, but to get a picture on the basis of which mental schemes the respondent does or does not perceive a specific element as being air pollution. The following elements were discussed (always in the same order):

• Particulate matter caused by forest fires
• Cigarette smoke (secondary smoke)
• Particulate matter caused by wood burning (stove)
• Ammonia from manure
• Methane caused by the intestinal system of livestock
• Water vapour
• Particulate matter caused by traffic
• Particulate matter caused by volcanic eruptions

The different elements were chosen in such a way that there was variation in different pollutants. We also created a variation of sources within the same element ‘particulate matter’ and a variation of sources within the particulate matter pollutant (forest fires, wood burning, traffic, volcanic eruptions). We also provided variation in elements from natural sources and from anthropogenic sources. We also included elements that are by definition not air pollution (pollen, water vapour) but are potentially perceived as such.

In order to avoid guessing and meaningless reflections, the respondent was clearly told that if he or she really had no idea to what extent the element was or was not air pollution, or even did not know the element, this was no problem and would not be further asked. For example, there were several respondents who could not say anything about ammonia from manure and methane caused by the intestinal system of cattle. Also, not everyone was familiar with the element particulate matter.

Furthermore, it should be specified that respondents did not find this an easy exercise and their answers were often formulated in the form of ‘I think’ rather than ‘I am sure’.

Within the framework of this exercise, respondents were also confronted with inconsistencies of their own answers in order to obtain a deeper reflection and more refined answers.

Appendix 3. Frequency table for categorising elements as air pollution or not

Appendix 4. an overview of the mental schemes used per element (an ‘x’ indicates that for the elements in the left column, the classification of an element as air pollution happened or through ‘knowledge’ that an element is air pollution [first column] or through the application of different mental schemes).

1. Elements here refer to particulate matter from different sources, smoke, pollen, ammonia, methane and water vapour. These elements form the basis for our analysis. Elements should thus not be understood as defined from a chemistry or physics point of view as substances that cannot be broken down into simpler components by any non-nuclear chemical reactions.

Funding: The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: from Innoviris, the regional institute for research and innovation of the Brussels-Capital Region for the research reported in this article.

Declaration of conflicting interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ENCYCLOPEDIC ENTRY

Air pollution.

Air pollution consists of chemicals or particles in the air that can harm the health of humans, animals, and plants. It also damages buildings.

Biology, Ecology, Earth Science, Geography

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Air pollution consists of chemicals or particles in the air that can harm the health of humans, animals, and plants. It also damages buildings. Pollutants in the air take many forms. They can be gases , solid particles, or liquid droplets. Sources of Air Pollution Pollution enters the Earth's atmosphere in many different ways. Most air pollution is created by people, taking the form of emissions from factories, cars, planes, or aerosol cans . Second-hand cigarette smoke is also considered air pollution. These man-made sources of pollution are called anthropogenic sources . Some types of air pollution, such as smoke from wildfires or ash from volcanoes , occur naturally. These are called natural sources . Air pollution is most common in large cities where emissions from many different sources are concentrated . Sometimes, mountains or tall buildings prevent air pollution from spreading out. This air pollution often appears as a cloud making the air murky. It is called smog . The word "smog" comes from combining the words "smoke" and " fog ." Large cities in poor and developing nations tend to have more air pollution than cities in developed nations. According to the World Health Organization (WHO) , some of the worlds most polluted cities are Karachi, Pakistan; New Delhi, India; Beijing, China; Lima, Peru; and Cairo, Egypt. However, many developed nations also have air pollution problems. Los Angeles, California, is nicknamed Smog City. Indoor Air Pollution Air pollution is usually thought of as smoke from large factories or exhaust from vehicles. But there are many types of indoor air pollution as well. Heating a house by burning substances such as kerosene , wood, and coal can contaminate the air inside the house. Ash and smoke make breathing difficult, and they can stick to walls, food, and clothing. Naturally-occurring radon gas, a cancer -causing material, can also build up in homes. Radon is released through the surface of the Earth. Inexpensive systems installed by professionals can reduce radon levels. Some construction materials, including insulation , are also dangerous to people's health. In addition, ventilation , or air movement, in homes and rooms can lead to the spread of toxic mold . A single colony of mold may exist in a damp, cool place in a house, such as between walls. The mold's spores enter the air and spread throughout the house. People can become sick from breathing in the spores. Effects On Humans People experience a wide range of health effects from being exposed to air pollution. Effects can be broken down into short-term effects and long-term effects . Short-term effects, which are temporary , include illnesses such as pneumonia or bronchitis . They also include discomfort such as irritation to the nose, throat, eyes, or skin. Air pollution can also cause headaches, dizziness, and nausea . Bad smells made by factories, garbage , or sewer systems are considered air pollution, too. These odors are less serious but still unpleasant . Long-term effects of air pollution can last for years or for an entire lifetime. They can even lead to a person's death. Long-term health effects from air pollution include heart disease , lung cancer, and respiratory diseases such as emphysema . Air pollution can also cause long-term damage to people's nerves , brain, kidneys , liver , and other organs. Some scientists suspect air pollutants cause birth defects . Nearly 2.5 million people die worldwide each year from the effects of outdoor or indoor air pollution. People react differently to different types of air pollution. Young children and older adults, whose immune systems tend to be weaker, are often more sensitive to pollution. Conditions such as asthma , heart disease, and lung disease can be made worse by exposure to air pollution. The length of exposure and amount and type of pollutants are also factors. Effects On The Environment Like people, animals, and plants, entire ecosystems can suffer effects from air pollution. Haze , like smog, is a visible type of air pollution that obscures shapes and colors. Hazy air pollution can even muffle sounds. Air pollution particles eventually fall back to Earth. Air pollution can directly contaminate the surface of bodies of water and soil . This can kill crops or reduce their yield . It can kill young trees and other plants. Sulfur dioxide and nitrogen oxide particles in the air, can create acid rain when they mix with water and oxygen in the atmosphere. These air pollutants come mostly from coal-fired power plants and motor vehicles . When acid rain falls to Earth, it damages plants by changing soil composition ; degrades water quality in rivers, lakes and streams; damages crops; and can cause buildings and monuments to decay . Like humans, animals can suffer health effects from exposure to air pollution. Birth defects, diseases, and lower reproductive rates have all been attributed to air pollution. Global Warming Global warming is an environmental phenomenon caused by natural and anthropogenic air pollution. It refers to rising air and ocean temperatures around the world. This temperature rise is at least partially caused by an increase in the amount of greenhouse gases in the atmosphere. Greenhouse gases trap heat energy in the Earths atmosphere. (Usually, more of Earths heat escapes into space.) Carbon dioxide is a greenhouse gas that has had the biggest effect on global warming. Carbon dioxide is emitted into the atmosphere by burning fossil fuels (coal, gasoline , and natural gas ). Humans have come to rely on fossil fuels to power cars and planes, heat homes, and run factories. Doing these things pollutes the air with carbon dioxide. Other greenhouse gases emitted by natural and artificial sources also include methane , nitrous oxide , and fluorinated gases. Methane is a major emission from coal plants and agricultural processes. Nitrous oxide is a common emission from industrial factories, agriculture, and the burning of fossil fuels in cars. Fluorinated gases, such as hydrofluorocarbons , are emitted by industry. Fluorinated gases are often used instead of gases such as chlorofluorocarbons (CFCs). CFCs have been outlawed in many places because they deplete the ozone layer . Worldwide, many countries have taken steps to reduce or limit greenhouse gas emissions to combat global warming. The Kyoto Protocol , first adopted in Kyoto, Japan, in 1997, is an agreement between 183 countries that they will work to reduce their carbon dioxide emissions. The United States has not signed that treaty . Regulation In addition to the international Kyoto Protocol, most developed nations have adopted laws to regulate emissions and reduce air pollution. In the United States, debate is under way about a system called cap and trade to limit emissions. This system would cap, or place a limit, on the amount of pollution a company is allowed. Companies that exceeded their cap would have to pay. Companies that polluted less than their cap could trade or sell their remaining pollution allowance to other companies. Cap and trade would essentially pay companies to limit pollution. In 2006 the World Health Organization issued new Air Quality Guidelines. The WHOs guidelines are tougher than most individual countries existing guidelines. The WHO guidelines aim to reduce air pollution-related deaths by 15 percent a year. Reduction Anybody can take steps to reduce air pollution. Millions of people every day make simple changes in their lives to do this. Taking public transportation instead of driving a car, or riding a bike instead of traveling in carbon dioxide-emitting vehicles are a couple of ways to reduce air pollution. Avoiding aerosol cans, recycling yard trimmings instead of burning them, and not smoking cigarettes are others.

Downwinders The United States conducted tests of nuclear weapons at the Nevada Test Site in southern Nevada in the 1950s. These tests sent invisible radioactive particles into the atmosphere. These air pollution particles traveled with wind currents, eventually falling to Earth, sometimes hundreds of miles away in states including Idaho, Utah, Arizona, and Washington. These areas were considered to be "downwind" from the Nevada Test Site. Decades later, people living in those downwind areascalled "downwinders"began developing cancer at above-normal rates. In 1990, the U.S. government passed the Radiation Exposure Compensation Act. This law entitles some downwinders to payments of $50,000.

Greenhouse Gases There are five major greenhouse gases in Earth's atmosphere.

• water vapor
• carbon dioxide
• nitrous oxide

London Smog What has come to be known as the London Smog of 1952, or the Great Smog of 1952, was a four-day incident that sickened 100,000 people and caused as many as 12,000 deaths. Very cold weather in December 1952 led residents of London, England, to burn more coal to keep warm. Smoke and other pollutants became trapped by a thick fog that settled over the city. The polluted fog became so thick that people could only see a few meters in front of them.

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Related Resources

• Open access
• Published: 31 August 2021

A national survey of ambient air pollution health literacy among adult residents of Taiwan

• Wen-Hsuan Hou 1 , 2 , 3 ,
• Yi-Chin Huang 4 ,
• Chien-Yeh Lu 4 ,
• I-Chen Chen 4 , 5 ,
• Pei-Chen Lee 6 ,
• Ming-Yeng Lin 7 ,
• Yu-Chen Wang 8 ,
• Lilis Sulistyorini 9 &
• Chung-Yi Li 4 , 9 , 10 , 11  

BMC Public Health volume  21 , Article number:  1604 ( 2021 ) Cite this article

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Metrics details

To investigate the level of and covariates associated with ambient air pollution health literacy (AAPHL) among adult residents of Taiwan.

With a cross-sectional study design, we conducted telephone interviews using a Chinese version AAPHL scale, which consisted of 24 items assessing 12 subdomains of AAPHL formed by 4 information processing competence matrices (i.e., access , understand , appraise , and apply ) and 3 health contexts (i.e., healthcare , disease prevention , and health promotion ). The AAPHL was with the lowest and highest score at 1 to 4, respectively. Between September and November 2020, a sample of 1017 and 280 adults was successfully interviewed via home phones and mobile phones, respectively. We employed multiple linear regression models to identify covariates significantly associated with overall and 4 matric-specific AAPHL scores.

The mean and standard deviation (±SD) of overall AAPHL score was considered as moderate at 2.90 (±0.56), with the highest and lowest metric-specific score for “ apply ” (3.07 ± 0.59) and “ appraise ” (2.75 ± 0.66). Lower education was significantly associated with a lower overall score; and living with children < 12 years and single were both significantly associated with higher overall scores. We also noted a significant geographic variation in overall score in which people living in the east/remote islands had highest scores.

Conclusions

People in Taiwan had only moderate level of AAPHL; and covariates including education, living arrangement, marital status, and area of living were significantly associated with AAPHL. These covariates should be considered in future educational interventions aiming to improve the AAPHL in the community.

Peer Review reports

Introduction

Ambient air pollution is a major environmental health problem affecting everyone in low-, middle-, and high-income countries; and representing a considerable threat to health worldwide [ 1 ]. According to the definition form WHO, ambient air pollution is a broader term used to describe air pollution in outdoor environments, while urban outdoor air pollution is a more specific term referring to the ambient air pollution experienced by populations living in urban areas, typically in or around cities [ 1 , 2 ]. According to the 2015 Global Burden of Disease Study [ 3 ], exposure to ambient fine particulate matter PM 2.5 is the fifth leading cause of death worldwide, accounting for 4.2 million deaths and 103.1 million disability-adjusted life-years in 2015 globally. Epidemiological studies reported links between air pollution and certain diseases of public health importance such as cardiovascular diseases, cancers, and respiratory diseases [ 4 ]. Recent studies also revealed potential influence of air pollution on psychiatric disorders [ 5 , 6 ]. Taiwan is no exception [ 7 ]. Estimated in 2014, PM 2.5 accounted for 6,282 deaths from ischemic heart disease, stroke, lung cancer, and chronic obstructive pulmonary disease, representing a population attributable mortality fraction of 18.6% associated with the four disease causes [ 7 ].

Ambient air pollution is a function of complex systems, and solutions to the problem also require multilevel intervention [ 8 ]; and education of people involved in the air pollution control strategies, including scientists, negotiators, decision makers and the public, to raise the environmental awareness is essential for reducing the air pollution [ 9 ]. Previous studies have demonstrated that environmental health education interventions at formal education or in the community could significantly have knowledge gains related to environmental health, individual behavior changes, and collective action for community change [ 10 , 11 ]. Effective educational interventions were found to increase prevalence and effects of so-called avoidance behaviors in lowering the adverse effects of air pollution on health [ 12 ]. Such activities include purchasing preventive pharmaceuticals and reducing time spent in polluted environments [ 13 ].

The knowledge of environmental health and the behavior of environmental protection, which is so called environmental health literacy (EHL), with a root of both health literacy [ 14 ] and risk communication [ 10 , 11 ], is an emerging area of study that incorporates content and strategies from environmental, health, and social sciences to promote understanding of the ways environmental contaminants affect health [ 15 ], which can be used as a tool for evaluating the effectiveness of environmental education. The earlier conceptual model of EHL was adapted from Bloom (1956), representing the stepwise progression of six distinct educational stages (i.e, create, evaluate, analyze, apply, understand, and recognize) to approach the development of targeted interventions for different levels of EHL [ 16 ]. The EHL on ambient air pollution can be enhanced by empowering individuals and communities to use appropriate communication for controlling air pollution exposures. However, to the best of our knowledge, previous air pollution assessments were focused on the air pollutants concentration or air quality monitoring [ 17 , 18 ], and our study is the first one that assessed the level of ambient air pollution health literacy (AAPHL) in the general population. In fact, there is still a lack of easy and self-administrated checklist items like the one that we used in the current study for assessing the level of knowledge, competence, and motivation in dealing with ambient air pollution and human health. AAPHL which is defined as individual’s competencies to access, understand, appraise, and apply the ambient air pollution health information to make judgments and decisions concerning healthcare, disease prevention, and health promotion contexts to maintain or improve their quality of life and to protect the environment in urban and non-urban community [ 19 ]. The questionnaire was developed based on three rounds of consensus meetings including 5 experts of public health, environmental science, medical physicians, and air pollutant researchers in order to reflect the synergized subjective opinions of AAPHL. The content of the AAPHL scale included questions related to both urban and non-urban ambient air pollutant categories, air quality detection, pollutant sources, control strategies, law regulations, application of health-related information according to the above health literacy subdomains. Therefore, we developed the AAPHL questionnaire on the basis of an existing conceptual framework of health literacy proposed by the European Health Literacy Consortium, which composed of 12 subdomains of health literacy formed by 4 information processing competencies of individuals (ie, accessing, understanding, appraising, and applying) and 3 health contexts (ie, healthcare, disease prevention, and health promotion) [ 19 ]. Despite that, only very few studies have been conducted to address the issue of EHL on ambient air pollution and health threats [ 20 , 21 ], and no survey data on ambient air pollution health literacy available at a population-based level. We therefore conducted this national population-based survey on level of AAPHL in adult residents of Taiwan.

The study was ethnically approved by the National Chung Kung University Governance Framework for Human Research Ethics (No. 109–385).

Study design and participants

This was a population-based cross-sectional study design. The target population was all Taiwanese residents aged 20 years and older. There are 22 cities/counties in Taiwan. By the end of 2019, a total of 19,338,629 adult (> = 20 years) residents including 9,486,379 men and 9,852,250 women were registered in the Household Registration [ 22 ]. These adults were inhabitants of 6,956,341 households all over the country. Seven covariates which are potential predictors (i.e., gender, age, education, and occupation as the personal determinants; living arrangement and marital status for situational determinants; living area as a socio-environmental determinants) for AAPHL scores were tested in this study on the basis of the integrated model of health literacy proposed by the European Health Literacy Survey Consortium [ 19 ].

The sample size required for this survey was calculated based on the multiple linear regression that identified factors significantly associated with AAPHL. Given that there were 7 potential covariates assessed for their associations with AAPHL, a sample size of 107 may achieve 90% power to detect a partial ρ 2 of at least 0 (null hypothesis) attributed to 7 independent variable(s) when the significance level (alpha) is 0.050 and the actual value of ρ 2 is 0.1 (alternative hypothesis). The corresponding sample size for a smaller actual value of ρ 2  = 0.05 and ρ 2  = 0.01 was 212 and 1,017, respectively (NCSS, LLC, Utah USA). Because the 7 covariates included in the models were potentially associated with health literacy (see the Covariate section below), we believe that they may have at least a partial ρ 2 of 0.01, and as such a minimum of 1017 participants is needed for this study. The main purpose of cell phone survey was to increase the representativeness of those who do not have home phones. The number of 280 cell phone interviews was arbitrarily determined based on the availability of time and funding.

Instrument and measurements of AAPHL

The Chinese version AAPHL scale was developed on the basis of The European Health Literacy Survey Questionnaire (HLS-EU-Q) [ 23 ]. The AAPHL was designed to be an integrated model of EHL, which comprised 24 items assessing 12 subdomains of EHL formed by 4 information processing competencies of individuals (i.e., accessing/obtaining information, understanding information, appraising/processing information, and applying/using information) and 3 health contexts (i.e., healthcare, disease prevention, and health promotion) to maintain or improve their quality of life and protect the environment in the community. The questionnaire was developed based on three rounds of consensus meetings including 5 experts of public health, environmental science, medical physicians, and air pollutant researchers in order to reflect the synergized subjective opinions of AAPHL. The content of the AAPHL scale included questions related to both urban and non-urban outdoor air pollutant categories, air quality detection, pollutant sources, control strategies, law regulations, application of health-related information according to the above health literacy subdomains. The HLS-EU-Q is highly recommended because it is founded on a testable conceptual framework, captures multiple conceptual domains of health literacy, and covers a diverse range of health contexts [ 24 , 25 ]. Contents of the Chinese version AAPHL scale were displayed in Supplementary Table 1 .

Two environmental epidemiologists and one environmental health scientist who have expertise on air pollution and human health were asked to perform content validity that assesses whether our AAPHL scale is representative of all aspects of the construct, namely the three health contexts: (1) healthcare; (2) disease prevention; (3) health promotion, and each context explored four health information processing competences: accessing/obtaining information; understanding information; processing/appraising information and applying/using information. The 4-point Liker’s scale was used to indicate the level of appropriateness for 3 categories in “ relevance ”, “ importance ”, and “ unambiguity ”, respectively of the AAPHL scale. The mean score for “ relevance ”, “ importance ”, and “ unambiguity ” of the scale was 3.90, 3.97, and 3.91, respectively; and the corresponding figures of Content Validity Index (CVI) were 0.97, 0.99, and 0.94, which were calculated from the method proposed by Aiken [ 26 ].

Apart from the content validity, we also examined the construct validity to evaluate whether the Chinese version AAPHL scale really represents the concept (i.e., construct) we are interested in measuring. Information of construct validity is central to establishing the overall validity of a method [ 27 ]. Based on the data of our study, we performed confirmatory factor analysis (CFA) to assess the construct validity. We performed a first-order CFA to verify the 12-subdomain factor structure of the Chinese version AAPHL scale. The first-order model was considered valid if the CFA demonstrated acceptable fit between the overall model and data on the basis of the following absolute and relative fit indices: (1) the χ 2 test: it indicates the difference between observed and expected covariance matrices. Values closer to zero indicate a better fit; (2) the root mean squared error of approximation (RMSEA) of 0.08 or less [ 28 ]; (3) the standardized root mean square residual (SRMR) of 0.08 or less [ 29 ]; (4) the adjusted goodness of fit index (AGFI) of 0.80 or over [ 30 ]; and (5) both the normed fit index (NFI) and non-normed fit index (NNFI) of 0.90 or greater [ 29 ].

Although the χ 2 test indicates a significant difference ( p  < 0.001) between observed and expected covariance matrices in our sample, the other fit indices tended to support the factorial validity of the 12-subdomain factor structure of the Chinese version AAPHL: RMSEA = 0.067; SRMR = 0.039; AGFI = 0.864; and NFI/NNFI = 0.9138/0.9021. It was thus recommended that the 12 subdomain scores be summed up to represent overall AAPHL. In addition, the factor loading for the 24 items of AAPHL ranged from 0.609 to 0.881, also suggesting an acceptable level [ 31 ].

The reliability of Chinese version AAPHL scale was determined by two internal consistence indicators. Based on our study sample, the Cronbach’s alpha was calculated at 0.934. Moreover, the composite reliability (i.e., construct reliability), a measure of internal consistency in scale items, much like Cronbach’s alpha [ 32 ] and can be thought of as being equal to the total amount of true score variance relative to the total scale score variance [ 33 ] showed that the composite reliability coefficient for the “access”, “understand”, “appraise”, and “apply” matrices was 0.852, 0.839, 0.845, and 0.798, respectively, which were all greater than an acceptable reliability level of 0.60 [ 32 ].

The possible responses and their scores for the AAPHL scale were as follows: very difficult = 1, fairly difficult = 2, fairly easy = 3, very easy = 4, and a “5” was indicated when participants did not answer or did not have a definite answer, coded as a missing value. The overall AAPHL score was calculated as the mean of all items applicable, scored from 1 to 4. Higher scores indicate better AAPHL. In addition to overall AAPHL score, we also calculated matric-specific score to indicate the information processing competencies of individuals, namely accessing, understanding, appraising, and applying matrices. Contents of the 24-item Chinese version AAPHL scale were displayed in Supplementary Table 1 .

We collected the following covariates also via telephone interview to assess their associations with AAPHL level. The covariates included gender, age (20–34, 35–44, 45–54, 55–64, and > =65 years), education (Junior high school and lower, high school, college, and graduate studies), current occupation, living arrangement (living alone, living with children < 12 years, living with older [> = 12 years] students, or living with elderly [> = 65 years] people), marital status (single, married, others), and geographic area of living (north, central, south, and east/remote islands).

The currently held job was classified into one of 10 occupational categories: legislators, government administrators, business executives, and managers; professionals; technicians and associate professionals; clerks; service workers and shop and market sales workers; technology professionals; construction workers, agricultural, animal husbandry, forestry, and fishing workers; transportation and communication workers; teachers, athletes, and art performers; healthcare and social workers; legislators, government administrators, business executives and managers; and others (including housekeeper, retirees, and students). The occupational classification was based on the Standard Occupational Classification System (SOCS) of Taiwan for which interrater reliability has been shown to be good [ 34 , 35 ].

Studies have shown that a number of factors may influence an individual’s health literacy, including living in poverty, education, race/ethnicity, age, and disability [ 36 , 37 ]. In addition to socioeconomic status and co-morbidity, a recent study by Cho [ 38 ] indicated an association between work environment and level of health literacy. Because the above-mentioned covariates were reported to be associated with heath literacy, rather than with EHL or specifically with AAPHL, we examined in this study whether these sociodemographic and work characteristics also influence the AAPHL level.

Data collection

The sampling method used in this study comprises two steps:. First, based on a predetermined total number of 1017 participants to be collected, we used the probability proportional to size (PPS) technique to determine the number of home phone to be called for each city/county according to the city/county specific population size [ 39 , 40 ]. The PPS resulted in the number of call to be made for each city/county ranging from 37 to 385. Second, once the sample size was determined for each city/county we further employed a quota sampling by setting the age-specific sample size needed for each of the 7 age-specific populations (i.e., 20–29, 30–39, … ..80+) bases on the underlying age distributions of that particular city/county. The area codes and the first four digits of home phone numbers are unique for households in each city/county. The phone interviews continued until the predetermined sample size in each specific age group of specific city/county is met.

Considering an increasing number of residents in Taiwan only subscribed to mobile phones and some residents are usually not available during the period of making the home phone call, we additionally selected a supplemental sample via mobile phone. The mobile phone number is not city/county specific; therefore, the sample was selected from all mobile phone numbers of the entire country.

We employed the Computer-Assisted-Telephone Interview (CATI) to perform the interview. To achieve the predetermined age-specific sample size of sample for each city/county, we continuously performed the random digital dialing (RDD) procedure until the predetermined age-specific sample size was achieved. For each home phone called, only one eligible person best available in that household was invited. The CATI procedure reached a total of 4084 eligible adults via home phones, and 1017 adults successfully completed the home phone interviews, with a response rate of 24.90%. Reasons for unsuccessful home phone interviews were mainly due to the interviewees felt that the interview consumed more time than he/she expected and decided not to continue (19.9%), or the interviewees declined to be interviewed before the interview began (80.1%). The response rate (280/835 or 33.53%) was somewhat higher for the mobile phone interview as compared to the home phone.

The interview took around 15–20 min. The CATI was performed between September and November 2020 by five interviewers standardized to conduct telephone interviews. All phone calls were made between 5:00 PM and 10:00 PM to maximize the chance of reaching eligible subjects and to increase the likelihood of acceptance to be interviewed. Once the phone call reached a home phone/mobile phone, the people who answered the call were asked to determine his/her age eligibility. If the person who answered the call was eligible, he/she was invited. If not eligible or declined the invitation, any other eligible subject next him/her and available was invited.

Table 1 shows that the mobile phone interviewees tended to be female dominance, younger, more educated, living alone, single, and living in the north. Despite significant differences in socio-demographic characteristics between home phone and mobile phone interviewees, there were only slight differences in overall AAPHL and matric-specific scores, we therefore combined the home phone and mobile phone samples in the subsequent analyses to increase the representativeness of our study sample (Supplementary Table 2 ).

Statistical analysis

We first presented characteristics of the study participants with numbers and percentages. The AAPHL score was presented with mean and standard deviation (SD), and comparison of the differences among the 4 matric-specific AAPHL scores was made with repeated measurement analysis of variance, which considers the inter-correlation of the matric-specific scores made by the same interviewee. Multiple linear regression analysis was performed to investigate the covariates significantly associated with overall and matric-specific AAPHL score separately. We checked the assumptions of linearity, normality, and homoscedasticity for linear regression model by examining the residual plots, and found no violation of the above assumptions.

All statistical analyses were performed using SAS statistical software (SAS System for Windows, Version 9.4, SAS Institute Inc., Cary, NC, USA). Results with two-sided P values less than .05 were considered statistically significant.

Figure 1 shows the mean and SD of overall and 4 matric-specific AAPHL scores. Supplementary Table 2 further shows distributions of overall and metric-specific scores of AAPHL according to phone type. The mean and SD of overall AAPHL score was considered as moderate at 2.90 (0.56), with significant variation in metric-specific AAPHL. The highest and lowest metric-specific mean ± SD score was noted for “apply” (3.07 ± 0.59) and “appraise” (2.75 ± 0.66), respectively. Each metric-specific AAPHL score showed a high correlation with overall AAPHL score, with a Spearman’s correlation coefficient ranging from 0.83 (“apply” and overall) to 0.90 (“understand” and overall). However, the inter-correlation between the 4 metric-specific AAPHL scores was only moderate at 0.61 (“access” and “apply”) to 0.74 (“access” and “understand”) (Table 2 ).

Overall and matric-specific scores of the Ambient Air Pollution Health Literacy (AAPHL)

Table 3 shows the results of multiple linear regression analysis. Covariates significantly associated with overall AAPHL score included education, living arrangement, marital status, and area of living. Compared to those with college education, the participants with education levels of high school (adjusted β  = − 0.09) and junior high school and lower (adjusted β  = − 0.15) had significantly lower overall scores. The participants living with children < 12 years (adjusted β  = 0.15, with reference to those living alone) and those who were single (adjusted β  = 0.10, with reference to those married) had significantly higher AAPHL scores. Compared to living in the north, living in central (adjusted β  = 0.09) and east/remote islands (adjusted β  = 0.25) were both associated with significantly higher overall AAPHL scores.

Supplementary Tables 3 to 6 showed the covariates in association with the 4 metric-specific AAPHL scores individually. Lower education was significantly associated with lower scores of all matrices except “appraise” metric. In addition, adults living with children < 12 years had significantly higher scores in both “appraisal” and “apply” metrices; and single was significantly associated with higher scores of all matrices except “appraisal”. A significant geographic variation in AAPHL score was observed for all matrices, with participants from east/remote islands had consistently higher scores. Some occupations were found to be sporadically associated with certain metric-specific AAPHL scores.

To the best of our knowledge, this is the first study developing the measurement tool of EHL in ambient air pollution, exploring the level of AAPHL nationally, and identifying covariates significantly associated with AAPHL score. For the AAPHL assessment, we employed the conceptual framework proposed by European Health Literacy Consortium. The comprehensive questionnaire of HLS-EU-Q47 not only assesses the concept of HL in terms of 3 health contexts (health care, disease prevention, and health promotion) and four competencies regarding health information (“access”, “understanding”, “appraisal”, and “apply”), but also encompasses the antecedents and consequences of HL [ 19 ]. People with chronic diseases (particularly cardiorespiratory illnesses) [ 41 , 42 ], little social support, and poor access to medical services are most at risk from air pollution [ 43 ]. Therefore, exploring the level of ambient air pollution health-related skills and identifying key covariates associated with these skills may help guide policies to improve the understanding of the link between air pollution exposures and health.

EHL is a natural outgrowth concept incorporating health literacy, public health, environmental health science, and environmental literacy to develop the wide range of skills and competencies linking between environmental exposures and health [ 10 ]. Therefore, it is a must to assess the ability of population to seek out, comprehend, evaluate, and use the health information regarding to air pollution, as well as to make informed choices, reduce health risks, improve quality of life and protect the environment [ 11 ]. Both overall and matric-specific AAPHL scores in our survey revealed a sufficient (2.7–3.1 out of 4) level of health literacy which is similar as our previous study [ 44 ]. Our study showed the self-reported difficulty of AAPHL competency from the easiest to the most difficult is “apply”, “understand”, “access”, and “appraisal”. This finding is similar to the result from the previous survey conducted in eight Europeans countries proposing that tasks relating to “appreciation” or “accessing” of information are perceived as more difficult than “understanding” or “applying” information [ 45 ]. In addition, our study revealed that all four information processing competence measures have high correlation with the overall score of AAPHL. Among the four competences of “access”, “understand”, “appraise”, and “apply” the ambient air pollution health information, the matric of “understand” correlates most to the total AAPHL while the matric of “apply” correlates least with the other three competence matrices. This implied that our current self-reported AAPHL questionnaire measures more air pollution health literacy related domains of knowledge level than skill or behavior concepts. This barrier in measuring health literacy had been noticed by a Canadian study which emphasized the “two-sided” nature of health literacy, both the knowledge and skill that improve the ability of people to act on information in order to live healthier lives [ 46 ].

Recently, a model integrating public health and health care views of general health literacy has been proposed by the European Health Literacy Survey Consortium, which encompasses the 12 matrices concept of health literacy, determinants, and consequences of health literacy [ 19 ]. Among the determinants of health literacy proposed by EU, distinction factors include personal, situational, and socio-environmental determinants [ 19 ]. This is consistent with our study results indicating that the determinants of AAPHL include education attainment (personal determinants), living arrangement (situational determinants), marital status (situational determinants), and area of living (socio-environmental determinants). As for the personal determinants of the AAPHL score, our study disclosed that lower educational level is a significant influencing factor of AAPHL score, which is similar to those from a previous systematic review for heart failure population [ 47 ] and cross-sectional survey of community dwelling older adults [ 48 ].

Among the situational determinants, our results demonstrated both living with children below 12 years old and single status were factors significantly associated with higher levels of AAPHL. Previous epidemiological studies strongly suggested that air pollution damages vulnerable children’s health and its toxic effects not only occurring at the air-tissue interface of the lung but also affecting on other organs [ 49 ]. The government of Taiwan currently implements a health policy in which kindergartens and elementary school should raise colored flag (i.e., which green, yellow, orange, red, purple, and brown respectively) [ 50 ] according to the air quality index (AQI) announced by the Taiwan Environmental Protection Administration [ 51 ]. This may explain our study result showed a better AAPHL score among participants living with children below 12 years old. As for the other situational determinants of health literacy, our results revealed that participant who were single had better AAPHL scores as compared to those married ones. Individuals living alone were likely to have higher exposures to ambient air pollutants monitoring as they have to go outside often for work or shopping, which in turns may raise their awareness of ambient air pollution [ 52 ]. One previous study proposed that single breast cancer survivors would have better generic health literacy and involvement of medical decisions [ 52 , 53 ].

Since AAPHL is a measure related to EHL, certain socio-environmental determinants such as living environment is also a critical factor affecting the awareness of ambient air pollution and health. Our study showed that people living in east and remote islands has higher AAPHL scores than living the other areas. A previous systematic review has mentioned that an association between air quality index and pollution risk perception is existed through the influence of behavior, experience, socioeconomic factors, and information/communication [ 54 ]. This might explain our study result because most of the time in Taiwan. According to the Air Quality Monitoring Network of Taiwan Environmental Protection Administration, the air quality is better in the east and remote islands. We speculated the observation that where people from the areas with better air quality tended to have higher awareness of air pollution and health could be related to the frequent self-rescue campaign for ecological conservation and environmental protection against the build-up of petrochemical plants in the neighborhood initiated by local indigenous residents, which might also lead to a success of certain local ambient air pollution control programs and activities.

This study has several limitations. First, due to the cross-sectional study design, causal relationships cannot be established. Second, we cannot rule out potential selection bias because the telephone interview survey would exclude people without home phones or mobile phones. In addition, people who were willing to respond to interview might be different from those who were not, which might have entails at least in some extent certain degrees of selection bias. Although the home phone participants were sampled using PPS method, and a cell phone sample was collected to increase the representativeness of the study sample, low response rates for home phone (24.90%) and cell phone (33.5%) surveys might compromise the truthfulness about some sensitive issues, such as education and occupation. Third, the AAPHL were self-reported in our study, which is subject to information bias. However, some studies have found no differences between self-reported and performance-based health literacy measures [ 55 ]. Last, although we have included some socioeconomic status (e.g., occupation, education, living status) as covariates which might influence the level of AAPHL, we did not collect information of medical conditions so that we were unable to further identify certain subgroups (e.g., existing chronic diseases, social support, medical access, etc.) potentially vulnerable to lower level of AAPHL. Additionally, owing to incomplete adjustment for the aforementioned known risk factors for health literacy of ambient air pollution, a potential for residual confounding cannot be entirely excluded. The generalisability of the results may also be limited as our sample was comprised of a larger portion of participants with higher socioeconomic status (i.e., 58% graduated from college or above) than the general population.

Adult residents in Taiwan had only moderate level of AAPHL; and education, living arrangement, marital status, and area of living were significantly associated with AAPHL. Because ambient air pollution is local and air quality varies seasonally and throughout the day, our study results may provide evidence-based health polity for researchers to consider tailored educational intervention programs with the consideration of personal, situational, and socio-environmental determinants to improve the AAPHL in the community; as well as for practitioners to provide an effective risk communication about air quality with local needs and real-time information across each community.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Abbreviations

Ambient Air Pollution Health Literacy

Environmental Health Literacy

European Health Literacy Survey Questionnaire

Content Validity Index

Confirmatory Factor Analysis

Root Mean Squared Error of Approximation

Standardized Root Mean Square Residual

Adjusted Goodness of Fit Index

Normed Fit Index

Non-Normed Fit Index

Probability Proportional to Size

Computer-Assisted-Telephone Interview

Random Digital Dialing

Standard Deviation

Air Quality Index

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Acknowledgments

The authors are grateful for grants from Health Promotion Administration, Ministry of Health and Welfare B1090205-109(109-0331-02-18-04) and the Ministry of Science and Technology (grant number MOST-109-0331-02-18-04). The funder has no role in conducting and submitting this work. The guarantor is CY Li who takes full responsibility for the work as a whole, including the study design, access to data, and the decision to submit and publish the manuscript.

Health Promotion Administration, Ministry of Health and Welfare (B1090205–109). The funder has no role in conducting and submitting this work.

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WH Hou, YC Huang, CY Lu, IC Chen, PC Lee, MY Lin, YC Wang, and CY Li designed the study, contributed to the interpretation of results, and drafted the initial manuscript. WH Hou, CY Lu, PC Lee, MY Lin, and CY Li developed the questionnaire and conducted the survey. YC Huang, IC Chen, and CY Lu performed the statistical analyses. WH Hou, PC Lee, Lilis Sulistyorini and CY Li revised the manuscript. All authors reviewed the manuscript. The author(s) read and approved the final manuscript.

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A telephone consent script that included the following information was concisely described to the study subjects before the telephone interview: purposes of the study, duration of the subject’s participation, description of the procedures, description of risks/discomforts, description of benefits, confidentiality, whom (and how) to contact for questions regarding the study (researcher) and their rights as participants, voluntary nature of participation in the research and her/his ability to withdraw without any penalty, and the approximate number of subjects . The informed verbal consent process was considered complete when the study subject raised no more questions and agreed to initiation of the telephone interview. This current study was approved by the National Chung Kung University Governance Framework for Human Research Ethics (No. 109–385) that also approved the content of telephone consent script as well as the procedure for verbal consent used in this study. All authors confirm that all methods were performed in accordance with the relevant guidelines and regulations.

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Hou, WH., Huang, YC., Lu, CY. et al. A national survey of ambient air pollution health literacy among adult residents of Taiwan. BMC Public Health 21 , 1604 (2021). https://doi.org/10.1186/s12889-021-11658-z

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What is air pollution?

What causes air pollution, effects of air pollution, air pollution in the united states, air pollution and environmental justice, controlling air pollution, how to help reduce air pollution, how to protect your health.

Air pollution  refers to the release of pollutants into the air—pollutants that are detrimental to human health and the planet as a whole. According to the  World Health Organization (WHO) , each year, indoor and outdoor air pollution is responsible for nearly seven million deaths around the globe. Ninety-nine percent of human beings currently breathe air that exceeds the WHO’s guideline limits for pollutants, with those living in low- and middle-income countries suffering the most. In the United States, the  Clean Air Act , established in 1970, authorizes the U.S. Environmental Protection Agency (EPA) to safeguard public health by regulating the emissions of these harmful air pollutants.

“Most air pollution comes from energy use and production,” says  John Walke , director of the Clean Air team at NRDC. Driving a car on gasoline, heating a home with oil, running a power plant on  fracked gas : In each case, a fossil fuel is burned and harmful chemicals and gases are released into the air.

“We’ve made progress over the last 50 years in improving air quality in the United States, thanks to the Clean Air Act. But climate change will make it harder in the future to meet pollution standards, which are designed to  protect health ,” says Walke.

Air pollution is now the world’s fourth-largest risk factor for early death. According to the 2020  State of Global Air  report —which summarizes the latest scientific understanding of air pollution around the world—4.5 million deaths were linked to outdoor air pollution exposures in 2019, and another 2.2 million deaths were caused by indoor air pollution. The world’s most populous countries, China and India, continue to bear the highest burdens of disease.

“Despite improvements in reducing global average mortality rates from air pollution, this report also serves as a sobering reminder that the climate crisis threatens to worsen air pollution problems significantly,” explains  Vijay Limaye , senior scientist in NRDC’s Science Office. Smog, for instance, is intensified by increased heat, forming when the weather is warmer and there’s more ultraviolet radiation. In addition, climate change increases the production of allergenic air pollutants, including mold (thanks to damp conditions caused by extreme weather and increased flooding) and pollen (due to a longer pollen season). “Climate change–fueled droughts and dry conditions are also setting the stage for dangerous wildfires,” adds Limaye. “ Wildfire smoke can linger for days and pollute the air with particulate matter hundreds of miles downwind.”

The effects of air pollution on the human body vary, depending on the type of pollutant, the length and level of exposure, and other factors, including a person’s individual health risks and the cumulative impacts of multiple pollutants or stressors.

Smog and soot

These are the two most prevalent types of air pollution. Smog (sometimes referred to as ground-level ozone) occurs when emissions from combusting fossil fuels react with sunlight. Soot—a type of  particulate matter —is made up of tiny particles of chemicals, soil, smoke, dust, or allergens that are carried in the air. The sources of smog and soot are similar. “Both come from cars and trucks, factories, power plants, incinerators, engines, generally anything that combusts fossil fuels such as coal, gasoline, or natural gas,” Walke says.

Smog can irritate the eyes and throat and also damage the lungs, especially those of children, senior citizens, and people who work or exercise outdoors. It’s even worse for people who have asthma or allergies; these extra pollutants can intensify their symptoms and trigger asthma attacks. The tiniest airborne particles in soot are especially dangerous because they can penetrate the lungs and bloodstream and worsen bronchitis, lead to heart attacks, and even hasten death. In  2020, a report from Harvard’s T.H. Chan School of Public Health showed that COVID-19 mortality rates were higher in areas with more particulate matter pollution than in areas with even slightly less, showing a correlation between the virus’s deadliness and long-term exposure to air pollution. 

These findings also illuminate an important  environmental justice issue . Because highways and polluting facilities have historically been sited in or next to low-income neighborhoods and communities of color, the negative effects of this pollution have been  disproportionately experienced by the people who live in these communities.

Hazardous air pollutants

A number of air pollutants pose severe health risks and can sometimes be fatal, even in small amounts. Almost 200 of them are regulated by law; some of the most common are mercury,  lead , dioxins, and benzene. “These are also most often emitted during gas or coal combustion, incineration, or—in the case of benzene—found in gasoline,” Walke says. Benzene, classified as a carcinogen by the EPA, can cause eye, skin, and lung irritation in the short term and blood disorders in the long term. Dioxins, more typically found in food but also present in small amounts in the air, is another carcinogen that can affect the liver in the short term and harm the immune, nervous, and endocrine systems, as well as reproductive functions.  Mercury  attacks the central nervous system. In large amounts, lead can damage children’s brains and kidneys, and even minimal exposure can affect children’s IQ and ability to learn.

Another category of toxic compounds, polycyclic aromatic hydrocarbons (PAHs), are by-products of traffic exhaust and wildfire smoke. In large amounts, they have been linked to eye and lung irritation, blood and liver issues, and even cancer.  In one study , the children of mothers exposed to PAHs during pregnancy showed slower brain-processing speeds and more pronounced symptoms of ADHD.

Greenhouse gases

While these climate pollutants don’t have the direct or immediate impacts on the human body associated with other air pollutants, like smog or hazardous chemicals, they are still harmful to our health. By trapping the earth’s heat in the atmosphere, greenhouse gases lead to warmer temperatures, which in turn lead to the hallmarks of climate change: rising sea levels, more extreme weather, heat-related deaths, and the increased transmission of infectious diseases. In 2021, carbon dioxide accounted for roughly 79 percent of the country’s total greenhouse gas emissions, and methane made up more than 11 percent. “Carbon dioxide comes from combusting fossil fuels, and methane comes from natural and industrial sources, including large amounts that are released during oil and gas drilling,” Walke says. “We emit far larger amounts of carbon dioxide, but methane is significantly more potent, so it’s also very destructive.” 

Another class of greenhouse gases,  hydrofluorocarbons (HFCs) , are thousands of times more powerful than carbon dioxide in their ability to trap heat. In October 2016, more than 140 countries signed the Kigali Agreement to reduce the use of these chemicals—which are found in air conditioners and refrigerators—and develop greener alternatives over time. (The United States officially signed onto the  Kigali Agreement in 2022.)

Pollen and mold

Mold and allergens from trees, weeds, and grass are also carried in the air, are exacerbated by climate change, and can be hazardous to health. Though they aren’t regulated, they can be considered a form of air pollution. “When homes, schools, or businesses get water damage, mold can grow and produce allergenic airborne pollutants,” says Kim Knowlton, professor of environmental health sciences at Columbia University and a former NRDC scientist. “ Mold exposure can precipitate asthma attacks  or an allergic response, and some molds can even produce toxins that would be dangerous for anyone to inhale.”

Pollen allergies are worsening  because of climate change . “Lab and field studies are showing that pollen-producing plants—especially ragweed—grow larger and produce more pollen when you increase the amount of carbon dioxide that they grow in,” Knowlton says. “Climate change also extends the pollen production season, and some studies are beginning to suggest that ragweed pollen itself might be becoming a more potent allergen.” If so, more people will suffer runny noses, fevers, itchy eyes, and other symptoms. “And for people with allergies and asthma, pollen peaks can precipitate asthma attacks, which are far more serious and can be life-threatening.”

More than one in three U.S. residents—120 million people—live in counties with unhealthy levels of air pollution, according to the  2023  State of the Air  report by the American Lung Association (ALA). Since the annual report was first published, in 2000, its findings have shown how the Clean Air Act has been able to reduce harmful emissions from transportation, power plants, and manufacturing.

Recent findings, however, reflect how climate change–fueled wildfires and extreme heat are adding to the challenges of protecting public health. The latest report—which focuses on ozone, year-round particle pollution, and short-term particle pollution—also finds that people of color are 61 percent more likely than white people to live in a county with a failing grade in at least one of those categories, and three times more likely to live in a county that fails in all three.

In rankings for each of the three pollution categories covered by the ALA report, California cities occupy the top three slots (i.e., were highest in pollution), despite progress that the Golden State has made in reducing air pollution emissions in the past half century. At the other end of the spectrum, these cities consistently rank among the country’s best for air quality: Burlington, Vermont; Honolulu; and Wilmington, North Carolina. 

No one wants to live next door to an incinerator, oil refinery, port, toxic waste dump, or other polluting site. Yet millions of people around the world do, and this puts them at a much higher risk for respiratory disease, cardiovascular disease, neurological damage, cancer, and death. In the United States, people of color are 1.5 times more likely than whites to live in areas with poor air quality, according to the ALA.

Historically, racist zoning policies and discriminatory lending practices known as  redlining  have combined to keep polluting industries and car-choked highways away from white neighborhoods and have turned communities of color—especially low-income and working-class communities of color—into sacrifice zones, where residents are forced to breathe dirty air and suffer the many health problems associated with it. In addition to the increased health risks that come from living in such places, the polluted air can economically harm residents in the form of missed workdays and higher medical costs.

Environmental racism isn't limited to cities and industrial areas. Outdoor laborers, including the estimated three million migrant and seasonal farmworkers in the United States, are among the most vulnerable to air pollution—and they’re also among the least equipped, politically, to pressure employers and lawmakers to affirm their right to breathe clean air.

Recently,  cumulative impact mapping , which uses data on environmental conditions and demographics, has been able to show how some communities are overburdened with layers of issues, like high levels of poverty, unemployment, and pollution. Tools like the  Environmental Justice Screening Method  and the EPA’s  EJScreen  provide evidence of what many environmental justice communities have been explaining for decades: that we need land use and public health reforms to ensure that vulnerable areas are not overburdened and that the people who need resources the most are receiving them.

In the United States, the  Clean Air Act  has been a crucial tool for reducing air pollution since its passage in 1970, although fossil fuel interests aided by industry-friendly lawmakers have frequently attempted to  weaken its many protections. Ensuring that this bedrock environmental law remains intact and properly enforced will always be key to maintaining and improving our air quality.

But the best, most effective way to control air pollution is to speed up our transition to cleaner fuels and industrial processes. By switching over to renewable energy sources (such as wind and solar power), maximizing fuel efficiency in our vehicles, and replacing more and more of our gasoline-powered cars and trucks with electric versions, we'll be limiting air pollution at its source while also curbing the global warming that heightens so many of its worst health impacts.

And what about the economic costs of controlling air pollution? According to a report on the Clean Air Act commissioned by NRDC, the annual  benefits of cleaner air  are up to 32 times greater than the cost of clean air regulations. Those benefits include up to 370,000 avoided premature deaths, 189,000 fewer hospital admissions for cardiac and respiratory illnesses, and net economic benefits of up to $3.8 trillion for the U.S. economy every year.

“The less gasoline we burn, the better we’re doing to reduce air pollution and the harmful effects of climate change,” Walke explains. “Make good choices about transportation. When you can, ride a bike, walk, or take public transportation. For driving, choose a car that gets better miles per gallon of gas or  buy an electric car .” You can also investigate your power provider options—you may be able to request that your electricity be supplied by wind or solar. Buying your food locally cuts down on the fossil fuels burned in trucking or flying food in from across the world. And most important: “Support leaders who push for clean air and water and responsible steps on climate change,” Walke says.

• “When you see in the news or hear on the weather report that pollution levels are high, it may be useful to limit the time when children go outside or you go for a jog,” Walke says. Generally, ozone levels tend to be lower in the morning.
• If you exercise outside, stay as far as you can from heavily trafficked roads. Then shower and wash your clothes to remove fine particles.
• The air may look clear, but that doesn’t mean it’s pollution free. Utilize tools like the EPA’s air pollution monitor,  AirNow , to get the latest conditions. If the air quality is bad, stay inside with the windows closed.
• If you live or work in an area that’s prone to wildfires,  stay away from the harmful smoke  as much as you’re able. Consider keeping a small stock of masks to wear when conditions are poor. The most ideal masks for smoke particles will be labelled “NIOSH” (which stands for National Institute for Occupational Safety and Health) and have either “N95” or “P100” printed on it.
• If you’re using an air conditioner while outdoor pollution conditions are bad, use the recirculating setting to limit the amount of polluted air that gets inside. 

This story was originally published on November 1, 2016, and has been updated with new information and links.

This NRDC.org story is available for online republication by news media outlets or nonprofits under these conditions: The writer(s) must be credited with a byline; you must note prominently that the story was originally published by NRDC.org and link to the original; the story cannot be edited (beyond simple things such as grammar); you can’t resell the story in any form or grant republishing rights to other outlets; you can’t republish our material wholesale or automatically—you need to select stories individually; you can’t republish the photos or graphics on our site without specific permission; you should drop us a note to let us know when you’ve used one of our stories.

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Air Pollution: Current and Future Challenges

Despite dramatic progress cleaning the air since 1970, air pollution in the United States continues to harm people’s health and the environment. Under the Clean Air Act, EPA continues to work with state, local and tribal governments, other federal agencies, and stakeholders to reduce air pollution and the damage that it causes.

• Learn about more about air pollution, air pollution programs, and what you can do.

Outdoor air pollution challenges facing the United States today include:

• Meeting health-based standards for common air pollutants
• Limiting climate change
• Reducing risks from toxic air pollutants
• Protecting the stratospheric ozone layer against degradation

Indoor air pollution, which arises from a variety of causes, also can cause health problems. For more information on indoor air pollution, which is not regulated under the Clean Air Act, see EPA’s indoor air web site .

Air Pollution Challenges: Common Pollutants

Great progress has been made in achieving national air quality standards, which EPA originally established in 1971 and updates periodically based on the latest science. One sign of this progress is that visible air pollution is less frequent and widespread than it was in the 1970s.

However, air pollution can be harmful even when it is not visible. Newer scientific studies have shown that some pollutants can harm public health and welfare even at very low levels. EPA in recent years revised standards for five of the six common pollutants subject to national air quality standards. EPA made the standards more protective because new, peer-reviewed scientific studies showed that existing standards were not adequate to protect public health and the environment.

Status of common pollutant problems in brief

Today, pollution levels in many areas of the United States exceed national air quality standards for at least one of the six common pollutants:

• Although levels of particle pollution and ground-level ozone pollution are substantially lower than in the past, levels are unhealthy in numerous areas of the country. Both pollutants are the result of emissions from diverse sources, and travel long distances and across state lines. An extensive body of scientific evidence shows that long- and short-term exposures to fine particle pollution, also known as fine particulate matter (PM 2.5 ), can cause premature death and harmful effects on the cardiovascular system, including increased hospital admissions and emergency department visits for heart attacks and strokes. Scientific evidence also links PM to harmful respiratory effects, including asthma attacks. Ozone can increase the frequency of asthma attacks, cause shortness of breath, aggravate lung diseases, and cause permanent damage to lungs through long-term exposure. Elevated ozone levels are linked to increases in hospitalizations, emergency room visits and premature death. Both pollutants cause environmental damage, and fine particles impair visibility. Fine particles can be emitted directly or formed from gaseous emissions including sulfur dioxide or nitrogen oxides. Ozone, a colorless gas, is created when emissions of nitrogen oxides and volatile organic compounds react.  
• For unhealthy peak levels of sulfur dioxide and nitrogen dioxide , EPA is working with states and others on ways to determine where and how often unhealthy peaks occur. Both pollutants cause multiple adverse respiratory effects including increased asthma symptoms, and are associated with increased emergency department visits and hospital admissions for respiratory illness. Both pollutants cause environmental damage, and are byproducts of fossil fuel combustion.  
• Airborne lead pollution, a nationwide health concern before EPA phased out lead in motor vehicle gasoline under Clean Air Act authority, now meets national air quality standards except in areas near certain large lead-emitting industrial facilities. Lead is associated with neurological effects in children, such as behavioral problems, learning deficits and lowered IQ, and high blood pressure and heart disease in adults.  
• The entire nation meets the carbon monoxide air quality standards, largely because of emissions standards for new motor vehicles under the Clean Air Act.

In Brief: How EPA is working with states and tribes to limit common air pollutants

• EPA's air research provides the critical science to develop and implement outdoor air regulations under the Clean Air Act and puts new tools and information in the hands of air quality managers and regulators to protect the air we breathe.  
• To reflect new scientific studies, EPA revised the national air quality standards for fine particles (2006, 2012), ground-level ozone (2008, 2015), sulfur dioxide (2010), nitrogen dioxide (2010), and lead (2008). After the scientific review, EPA decided to retain the existing standards for carbon monoxide.  EPA strengthened the air quality standards for ground-level ozone in October 2015 based on extensive scientific evidence about ozone’s effects.

EPA has designated areas meeting and not meeting the air quality standards for the 2006 and 2012 PM standards and the 2008 ozone standard, and has completed an initial round of area designations for the 2010 sulfur dioxide standard. The agency also issues rules or guidance for state implementation of the various ambient air quality standards – for example, in March 2015, proposing requirements for implementation of current and future fine particle standards. EPA is working with states to improve data to support implementation of the 2010 sulfur dioxide and nitrogen dioxide standards.

For areas not meeting the national air quality standards, states are required to adopt state implementation plan revisions containing measures needed to meet the standards as expeditiously as practicable and within time periods specified in the Clean Air Act (except that plans are not required for areas with “marginal” ozone levels).

• EPA is helping states to meet standards for common pollutants by issuing federal emissions standards for new motor vehicles and non-road engines, national emissions standards for categories of new industrial equipment (e.g., power plants, industrial boilers, cement manufacturing, secondary lead smelting), and technical and policy guidance for state implementation plans. EPA and state rules already on the books are projected to help 99 percent of counties with monitors meet the revised fine particle standards by 2020. The Mercury and Air Toxics Standards for new and existing power plants issued in December 2011 are achieving reductions in fine particles and sulfur dioxide as a byproduct of controls required to cut toxic emissions.  
• Vehicles and their fuels continue to be an important contributor to air pollution. EPA in 2014 issued standards commonly known as Tier 3, which consider the vehicle and its fuel as an integrated system, setting new vehicle emissions standards and a new gasoline sulfur standard beginning in 2017. The vehicle emissions standards will reduce both tailpipe and evaporative emissions from passenger cars, light-duty trucks, medium-duty passenger vehicles, and some heavy-duty vehicles. The gasoline sulfur standard will enable more stringent vehicle emissions standards and will make emissions control systems more effective. These rules further cut the sulfur content of gasoline. Cleaner fuel makes possible the use of new vehicle emission control technologies and cuts harmful emissions in existing vehicles. The standards will reduce atmospheric levels of ozone, fine particles, nitrogen dioxide, and toxic pollution.

Learn more about common pollutants, health effects, standards and implementation:

• fine particles
• ground-level ozone
• sulfur dioxide
• nitrogen dioxide
• carbon monoxide

Air Pollution Challenges: Climate Change

EPA determined in 2009 that emissions of carbon dioxide and other long-lived greenhouse gases that build up in the atmosphere endanger the health and welfare of current and future generations by causing climate change and ocean acidification. Long-lived greenhouse gases , which trap heat in the atmosphere, include carbon dioxide, methane, nitrous oxide, and fluorinated gases. These gases are produced by a numerous and diverse human activities.

In May 2010, the National Research Council, the operating arm of the National Academy of Sciences, published an assessment which concluded that “climate change is occurring, is caused largely by human activities, and poses significant risks for - and in many cases is already affecting - a broad range of human and natural systems.” 1 The NRC stated that this conclusion is based on findings that are consistent with several other major assessments of the state of scientific knowledge on climate change. 2

Climate change impacts on public health and welfare

The risks to public health and the environment from climate change are substantial and far-reaching. Scientists warn that carbon pollution and resulting climate change are expected to lead to more intense hurricanes and storms, heavier and more frequent flooding, increased drought, and more severe wildfires - events that can cause deaths, injuries, and billions of dollars of damage to property and the nation’s infrastructure.

Carbon dioxide and other greenhouse gas pollution leads to more frequent and intense heat waves that increase mortality, especially among the poor and elderly. 3 Other climate change public health concerns raised in the scientific literature include anticipated increases in ground-level ozone pollution 4 , the potential for enhanced spread of some waterborne and pest-related diseases 5 , and evidence for increased production or dispersion of airborne allergens. 6

Other effects of greenhouse gas pollution noted in the scientific literature include ocean acidification, sea level rise and increased storm surge, harm to agriculture and forests, species extinctions and ecosystem damage. 7 Climate change impacts in certain regions of the world (potentially leading, for example, to food scarcity, conflicts or mass migration) may exacerbate problems that raise humanitarian, trade and national security issues for the United States. 8

The U.S. government's May 2014 National Climate Assessment concluded that climate change impacts are already manifesting themselves and imposing losses and costs. 9 The report documents increases in extreme weather and climate events in recent decades, with resulting damage and disruption to human well-being, infrastructure, ecosystems, and agriculture, and projects continued increases in impacts across a wide range of communities, sectors, and ecosystems.

Those most vulnerable to climate related health effects - such as children, the elderly, the poor, and future generations - face disproportionate risks. 10 Recent studies also find that certain communities, including low-income communities and some communities of color (more specifically, populations defined jointly by ethnic/racial characteristics and geographic location), are disproportionately affected by certain climate-change-related impacts - including heat waves, degraded air quality, and extreme weather events - which are associated with increased deaths, illnesses, and economic challenges. Studies also find that climate change poses particular threats to the health, well-being, and ways of life of indigenous peoples in the U.S.

The National Research Council (NRC) and other scientific bodies have emphasized that it is important to take initial steps to reduce greenhouse gases without delay because, once emitted, greenhouse gases persist in the atmosphere for long time periods. As the NRC explained in a recent report, “The sooner that serious efforts to reduce greenhouse gas emissions proceed, the lower the risks posed by climate change, and the less pressure there will be to make larger, more rapid, and potentially more expensive reductions later.” 11

In brief: What EPA is doing about climate change

Under the Clean Air Act, EPA is taking initial common sense steps to limit greenhouse gas pollution from large sources:

EPA and the National Highway and Traffic Safety Administration between 2010 and 2012 issued the first national greenhouse gas emission standards and fuel economy standards for cars and light trucks for model years 2012-2025, and for medium- and heavy-duty trucks for 2014-2018.  Proposed truck standards for 2018 and beyond were announced in June 2015.  EPA is also responsible for developing and implementing regulations to ensure that transportation fuel sold in the United States contains a minimum volume of renewable fuel. Learn more about clean vehicles

EPA and states in 2011 began requiring preconstruction permits that limit greenhouse gas emissions from large new stationary sources - such as power plants, refineries, cement plants, and steel mills - when they are built or undergo major modification. Learn more about GHG permitting

• On August 3, 2015, President Obama and EPA announced the Clean Power Plan – a historic and important step in reducing carbon pollution from power plants that takes real action on climate change. Shaped by years of unprecedented outreach and public engagement, the final Clean Power Plan is fair, flexible and designed to strengthen the fast-growing trend toward cleaner and lower-polluting American energy. With strong but achievable standards for power plants, and customized goals for states to cut the carbon pollution that is driving climate change, the Clean Power Plan provides national consistency, accountability and a level playing field while reflecting each state’s energy mix. It also shows the world that the United States is committed to leading global efforts to address climate change. Learn more about the Clean Power Plan, the Carbon Pollution Standards, the Federal Plan, and model rule for states

The Clean Power Plan will reduce carbon pollution from existing power plants, the nation’s largest source, while maintaining energy reliability and affordability.  The Clean Air Act creates a partnership between EPA, states, tribes and U.S. territories – with EPA setting a goal, and states and tribes choosing how they will meet it.  This partnership is laid out in the Clean Power Plan.

Also on August 3, 2015, EPA issued final Carbon Pollution Standards for new, modified, and constructed power plants, and proposed a Federal Plan and model rules to assist states in implementing the Clean Power Plan.

On February 9, 2016, the Supreme Court stayed implementation of the Clean Power Plan pending judicial review. The Court’s decision was not on the merits of the rule. EPA firmly believes the Clean Power Plan will be upheld when the merits are considered because the rule rests on strong scientific and legal foundations.

On October 16, 2017, EPA  proposed to repeal the CPP and rescind the accompanying legal memorandum.

EPA is implementing its Strategy to Reduce Methane Emissions released in March 2014. In January 2015 EPA announced a new goal to cut methane emissions from the oil and gas sector by 40 – 45 percent from 2012 levels by 2025, and a set of actions by EPA and other agencies to put the U.S. on a path to achieve this ambitious goal. In August 2015, EPA proposed new common-sense measures to cut methane emissions, reduce smog-forming air pollution and provide certainty for industry through proposed rules for the oil and gas industry . The agency also proposed to further reduce emissions of methane-rich gas from municipal solid waste landfills . In March 2016 EPA launched the National Gas STAR Methane Challenge Program under which oil and gas companies can make, track and showcase ambitious commitments to reduce methane emissions.

EPA in July 2015 finalized a rule to prohibit certain uses of hydrofluorocarbons -- a class of potent greenhouse gases used in air conditioning, refrigeration and other equipment -- in favor of safer alternatives. The U.S. also has proposed amendments to the Montreal Protocol to achieve reductions in HFCs internationally.

Learn more about climate science, control efforts, and adaptation on EPA’s climate change web site

Air Pollution Challenges: Toxic Pollutants

While overall emissions of air toxics have declined significantly since 1990, substantial quantities of toxic pollutants continue to be released into the air. Elevated risks can occur in urban areas, near industrial facilities, and in areas with high transportation emissions.

Numerous toxic pollutants from diverse sources

Hazardous air pollutants, also called air toxics, include 187 pollutants listed in the Clean Air Act. EPA can add pollutants that are known or suspected to cause cancer or other serious health effects, such as reproductive effects or birth defects, or to cause adverse environmental effects.

Examples of air toxics include benzene, which is found in gasoline; perchloroethylene, which is emitted from some dry cleaning facilities; and methylene chloride, which is used as a solvent and paint stripper by a number of industries. Other examples of air toxics include dioxin, asbestos, and metals such as cadmium, mercury, chromium, and lead compounds.

Most air toxics originate from manmade sources, including mobile sources such as motor vehicles, industrial facilities and small “area” sources. Numerous categories of stationary sources emit air toxics, including power plants, chemical manufacturing, aerospace manufacturing and steel mills. Some air toxics are released in large amounts from natural sources such as forest fires.

Health risks from air toxics

EPA’s most recent national assessment of inhalation risks from air toxics 12 estimated that the whole nation experiences lifetime cancer risks above ten in a million, and that almost 14 million people in more than 60 urban locations have lifetime cancer risks greater than 100 in a million. Since that 2005 assessment, EPA standards have required significant further reductions in toxic emissions.

Elevated risks are often found in the largest urban areas where there are multiple emission sources, communities near industrial facilities, and/or areas near large roadways or transportation facilities. Benzene and formaldehyde are two of the biggest cancer risk drivers, and acrolein tends to dominate non-cancer risks.

In brief: How EPA is working with states and communities to reduce toxic air pollution

EPA standards based on technology performance have been successful in achieving large reductions in national emissions of air toxics. As directed by Congress, EPA has completed emissions standards for all 174 major source categories, and 68 categories of small area sources representing 90 percent of emissions of 30 priority pollutants for urban areas. In addition, EPA has reduced the benzene content in gasoline, and has established stringent emission standards for on-road and nonroad diesel and gasoline engine emissions that significantly reduce emissions of mobile source air toxics. As required by the Act, EPA has completed residual risk assessments and technology reviews covering numerous regulated source categories to assess whether more protective air toxics standards are warranted. EPA has updated standards as appropriate. Additional residual risk assessments and technology reviews are currently underway.

EPA also encourages and supports area-wide air toxics strategies of state, tribal and local agencies through national, regional and community-based initiatives. Among these initiatives are the National Clean Diesel Campaign , which through partnerships and grants reduces diesel emissions for existing engines that EPA does not regulate; Clean School Bus USA , a national partnership to minimize pollution from school buses; the SmartWay Transport Partnership to promote efficient goods movement; wood smoke reduction initiatives; a collision repair campaign involving autobody shops; community-scale air toxics ambient monitoring grants ; and other programs including Community Action for a Renewed Environment (CARE). The CARE program helps communities develop broad-based local partnerships (that include business and local government) and conduct community-driven problem solving as they build capacity to understand and take effective actions on addressing environmental problems.

Learn more about air toxics, stationary sources of emissions, and control efforts Learn more about mobile source air toxics and control efforts

Air Pollution Challenges: Protecting the Stratospheric Ozone Layer

The  ozone (O 3 ) layer  in the stratosphere protects life on earth by filtering out harmful ultraviolet radiation (UV) from the sun. When chlorofluorocarbons (CFCs) and other ozone-degrading chemicals  are emitted, they mix with the atmosphere and eventually rise to the stratosphere. There, the chlorine and the bromine they contain initiate chemical reactions that destroy ozone. This destruction has occurred at a more rapid rate than ozone can be created through natural processes, depleting the ozone layer.

The toll on public health and the environment

Higher levels of  ultraviolet radiation  reaching Earth's surface lead to health and environmental effects such as a greater incidence of skin cancer, cataracts, and impaired immune systems. Higher levels of ultraviolet radiation also reduce crop yields, diminish the productivity of the oceans, and possibly contribute to the decline of amphibious populations that is occurring around the world.

In brief: What’s being done to protect the ozone layer

Countries around the world are phasing out the production of chemicals that destroy ozone in the Earth's upper atmosphere under an international treaty known as the Montreal Protocol . Using a flexible and innovative regulatory approach, the United States already has phased out production of those substances having the greatest potential to deplete the ozone layer under Clean Air Act provisions enacted to implement the Montreal Protocol. These chemicals include CFCs, halons, methyl chloroform and carbon tetrachloride. The United States and other countries are currently phasing out production of hydrochlorofluorocarbons (HCFCs), chemicals being used globally in refrigeration and air-conditioning equipment and in making foams. Phasing out CFCs and HCFCs is also beneficial in protecting the earth's climate, as these substances are also very damaging greenhouse gases.

Also under the Clean Air Act, EPA implements regulatory programs to:

Ensure that refrigerants and halon fire extinguishing agents are recycled properly.

Ensure that alternatives to ozone-depleting substances (ODS) are evaluated for their impacts on human health and the environment.

Ban the release of ozone-depleting refrigerants during the service, maintenance, and disposal of air conditioners and other refrigeration equipment.

Require that manufacturers label products either containing or made with the most harmful ODS.

These vital measures are helping to protect human health and the global environment.

The work of protecting the ozone layer is not finished. EPA plans to complete the phase-out of ozone-depleting substances that continue to be produced, and continue efforts to minimize releases of chemicals in use. Since ozone-depleting substances persist in the air for long periods of time, the past use of these substances continues to affect the ozone layer today. In our work to expedite the recovery of the ozone layer, EPA plans to augment CAA implementation by:

Continuing to provide forecasts of the expected risk of overexposure to UV radiation from the sun through the UV Index, and to educate the public on how to protect themselves from over exposure to UV radiation.

Continuing to foster domestic and international partnerships to protect the ozone layer.

Encouraging the development of products, technologies, and initiatives that reap co-benefits in climate change and energy efficiency.

Learn more About EPA’s Ozone Layer Protection Programs

Some of the following links exit the site

1 National Research Council (2010), Advancing the Science of Climate Change , National Academy Press, Washington, D.C., p. 3.

2 National Research Council (2010), Advancing the Science of Climate Change , National Academy Press, Washington, D.C., p. 286.

3 USGCRP (2009).  Global Climate Change Impacts in the United States . Karl, T.R., J.M. Melillo, and T.C. Peterson (eds.). United States Global Change Research Program. Cambridge University Press, New York, NY, USA.

4 CCSP (2008).  Analyses of the effects of global change on human health and welfare and human systems . A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Gamble, J.L. (ed.), K.L. Ebi, F.G. Sussman, T.J. Wilbanks, (Authors). U.S. Environmental Protection Agency, Washington, DC, USA.

5 Confalonieri, U., B. Menne, R. Akhtar, K.L. Ebi, M. Hauengue, R.S. Kovats, B. Revich and A. Woodward (2007). Human health. In:  Climate Change 2007: Impacts, Adaptation and Vulnerability  .  Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change  Parry, M.L., O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson, (eds.), Cambridge University Press, Cambridge, United Kingdom.

7 An explanation of observed and projected climate change and its associated impacts on health, society, and the environment is included in the EPA’s Endangerment Finding and associated technical support document (TSD). See EPA, “ Endangerment and Cause or Contribute Findings for Greenhouse Gases under Section 202(a) of the Clean Air Act ,” 74 FR 66496, Dec. 15, 2009. Both the Federal Register Notice and the Technical Support Document (TSD) for Endangerment and Cause or Contribute Findings are found in the public docket, Docket No. EPA-OAR-2009-0171.

8 EPA, Endangerment Finding , 74 FR 66535.

9 . U.S. Global Change Research Program, Climate Change Impacts in the United States: The Third National Climate Assessment , May 2014.

10 EPA, Endangerment Finding , 74 FR 66498.

11 National Research Council (2011) America’s Climate Choices: Report in Brief , Committee on America’s Climate Choices, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., p. 2.

12 EPA, 2005 National-Scale Air Toxics Assessment (2011).

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• Developing Programs Through Dialogue
• Flexibility with Accountability
• The Clean Air Act and the Economy

Air Pollution Research Paper Topics

This comprehensive guide to air pollution research paper topics is designed to assist students studying environmental science in selecting a suitable topic for their research paper. The guide provides a broad range of topics divided into ten categories, each containing ten unique research topics. Additionally, the guide offers expert advice on how to choose a topic from the multitude of air pollution research paper topics and how to write a compelling research paper on air pollution. The guide also introduces iResearchNet’s writing services, which offer students the opportunity to order a custom air pollution research paper on any topic. The services include a range of features designed to ensure the delivery of high-quality, custom-written papers.

100 Air Pollution Research Paper Topics

Air pollution is a critical environmental issue that affects every living being on the planet. It is a topic that requires in-depth understanding and research. To aid students in their quest for knowledge and to help them in their academic pursuits, we have compiled a comprehensive list of air pollution research paper topics. These topics are categorized into ten different sections, each containing ten unique topics.

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Causes of Air Pollution

• The role of industrialization in air pollution.
• The impact of transportation on air pollution.
• The effect of agriculture on air pollution.
• The influence of waste disposal on air pollution.
• The role of deforestation in air pollution.
• The impact of urbanization on air pollution.
• The effect of household activities on air pollution.
• The influence of natural disasters on air pollution.
• The role of power generation in air pollution.
• The impact of mining activities on air pollution.

Effects of Air Pollution

• The impact of air pollution on human health.
• The effect of air pollution on the environment.
• The influence of air pollution on climate change.
• The role of air pollution in biodiversity loss.
• The impact of air pollution on agriculture.
• The effect of air pollution on water bodies.
• The influence of air pollution on the ozone layer.
• The role of air pollution in acid rain.
• The impact of air pollution on urban heat islands.
• The effect of air pollution on mental health.

Air Pollution and Climate Change

• The role of air pollution in global warming.
• The impact of air pollution on weather patterns.
• The influence of air pollution on greenhouse gas emissions.
• The role of air pollution in climate change mitigation.
• The impact of air pollution on climate change adaptation.
• The effect of air pollution on carbon sequestration.
• The influence of air pollution on climate change policies.
• The role of air pollution in climate change communication.
• The impact of air pollution on climate change denial.
• The effect of air pollution on climate change education.

Air Pollution Policies

• The effectiveness of the Clean Air Act in addressing air pollution.
• The impact of the Paris Agreement on air pollution.
• The role of national policies in mitigating air pollution.
• The influence of international cooperation on air pollution.
• The effectiveness of emission standards in addressing air pollution.
• The role of renewable energy policies in mitigating air pollution.
• The impact of transportation policies on air pollution.
• The influence of waste management policies on air pollution.
• The effectiveness of urban planning policies in addressing air pollution.
• The role of education policies in mitigating air pollution.

Air Pollution Solutions

• The role of renewable energy in mitigating air pollution.
• The impact of energy efficiency on air pollution.
• The influence of green building on air pollution.
• The effectiveness of public transportation in addressing air pollution.
• The role of waste management in mitigating air pollution.
• The impact of urban green spaces on air pollution.
• The influence of sustainable agriculture on air pollution.
• The effectiveness of carbon capture and storage in addressing air pollution.
• The role of education in mitigating air pollution.
• The impact of individual actions on air pollution.

Air Pollution and Society

• The social impacts of air pollution.
• The role of media in shaping perceptions of air pollution.
• The influence of air pollution on social inequality.
• The impact of air pollution on social movements.
• The role of community engagement in addressing air pollution.
• The influence of air pollution on public health policies.
• The impact of air pollution on economic development.
• The role of air pollution in urban planning.
• The influence of air pollution on migration patterns.
• The impact of air pollution on cultural practices.

Air Pollution and Health

• The impact of air pollution on respiratory diseases.
• The role of air pollution in cardiovascular diseases.
• The influence of air pollution on allergies.
• The impact of air pollution on mental health.
• The role of air pollution in premature deaths.
• The influence of air pollution on children’s health.
• The impact of air pollution on elderly health.
• The role of air pollution in health inequalities.
• The influence of air pollution on public health interventions.
• The impact of air pollution on health care costs.

Air Pollution and Technology

• The role of technology in monitoring air pollution.
• The impact of technology on reducing air pollution.
• The influence of technology on air pollution modeling.
• The role of technology in air pollution forecasting.
• The impact of technology on air pollution communication.
• The influence of technology on air pollution policies.
• The role of technology in air pollution education.
• The impact of technology on air pollution mitigation.
• The influence of technology on air pollution adaptation.
• The role of technology in air pollution research.

Air Pollution and Economy

• The economic impacts of air pollution.
• The role of air pollution in economic inequality.
• The influence of air pollution on economic development.
• The impact of air pollution on economic policies.
• The role of air pollution in economic planning.
• The influence of air pollution on economic growth.
• The impact of air pollution on economic sustainability.
• The role of air pollution in economic transitions.
• The influence of air pollution on economic resilience.
• The impact of air pollution on economic sectors.

Air Pollution and Ethics

• The ethical implications of air pollution.
• The role of ethics in air pollution policies.
• The influence of ethics on air pollution communication.
• The impact of ethics on air pollution mitigation.
• The role of ethics in air pollution adaptation.
• The influence of ethics on air pollution research.
• The impact of ethics on air pollution education.
• The role of ethics in air pollution decision-making.
• The influence of ethics on air pollution justice.
• The impact of ethics on air pollution futures.

This comprehensive list of topics is designed to inspire and guide students in their quest for knowledge about air pollution. Each topic is a doorway to a vast field of research and understanding. As you embark on your academic journey, remember that the goal is not just to write a research paper but to contribute to the global understanding of air pollution and its impacts. Your research could be the key to solving one of the most pressing environmental issues of our time.

Air Pollution Research Guide

Air pollution is a pressing global issue that poses significant threats to human health and the environment. As students studying environmental science, it is essential to delve into the complexities of air pollution and understand its causes, impacts, and potential solutions. Writing research papers on air pollution topics not only enhances our knowledge but also contributes to the collective effort in combating this environmental challenge. In this comprehensive guide, we will explore a wide range of air pollution research paper topics to inspire and assist you in your academic journey.

The field of environmental science has increasingly focused on air pollution due to its detrimental effects on air quality, climate change, and public health. As the world grapples with the consequences of human activities and industrialization, it becomes crucial to investigate the different dimensions of air pollution and develop innovative approaches to mitigate its impact. Research papers serve as a valuable tool for investigating and disseminating knowledge about air pollution, making them an integral part of environmental science education.

The primary aim of this page is to provide students like you with an extensive array of air pollution research paper topics. By exploring diverse and engaging topics, you can gain a deeper understanding of the various aspects related to air pollution, ranging from its sources and consequences to policy interventions and sustainable solutions. Whether you are just starting your research journey or seeking inspiration for a specific area of interest, this comprehensive list will serve as a valuable resource to guide your exploration and empower you to contribute meaningfully to the field.

Moreover, this page offers expert advice on how to choose the most suitable air pollution research paper topics. With the abundance of available topics, it is important to select a research question that aligns with your interests, academic goals, and the current needs of the field. Our expert tips will help you navigate through the vast landscape of air pollution research and enable you to select a topic that is both relevant and impactful.

In addition to topic selection, we will also provide guidance on how to write an effective air pollution research paper. Writing a research paper requires a systematic approach, from conducting a literature review and collecting data to analyzing findings and presenting a coherent argument. By following our step-by-step instructions and incorporating our writing tips, you can enhance the quality and rigor of your research paper, ensuring that your work makes a valuable contribution to the field of environmental science.

Furthermore, to support your academic journey, we introduce our writing services, offering you the opportunity to order a custom air pollution research paper tailored to your specific requirements. Our team of expert degree-holding writers specializes in environmental science and is equipped with the knowledge and skills to deliver top-quality research papers. With a commitment to in-depth research, customized solutions, and timely delivery, our writing services provide a convenient and reliable option for students seeking assistance in their academic endeavors.

Choosing an Air Topic for Research

Choosing the right air pollution research paper topic is a crucial step in the research process. It sets the foundation for your study and determines the direction of your research. With the vast scope of air pollution issues, it can be challenging to narrow down your focus and select a topic that is both relevant and compelling. In this section, we provide expert advice and 10 valuable tips to help you navigate the process of choosing air pollution research paper topics effectively.

• Identify your research interests : Start by reflecting on your personal interests within the field of air pollution. What aspects of air pollution intrigue you the most? Are you interested in studying the health effects, the impact on ecosystems, policy interventions, or technological solutions? Identifying your research interests will guide you towards topics that resonate with your passion and motivation.
• Consider current issues and debates : Stay informed about the latest developments and ongoing debates in the field of air pollution. Read scientific journals, news articles, and policy reports to understand the pressing issues and emerging trends. By choosing a topic that addresses current concerns, you contribute to the existing knowledge and engage in the relevant conversations.
• Conduct preliminary research : Before finalizing a topic, conduct preliminary research to familiarize yourself with the existing literature and identify research gaps. This will help you refine your research question and ensure that your topic contributes to the existing knowledge base. Look for recent studies, key theories, and seminal works that can provide a solid foundation for your research.
• Define the scope of your study : Determine the scope and boundaries of your research. Are you focusing on a specific geographic region, a particular pollutant, or a certain population group? Clarifying the scope of your study will help you narrow down your topic and ensure that it is manageable within the given time and resources.
• Consider interdisciplinary approaches : Air pollution is a complex issue that requires interdisciplinary perspectives. Consider integrating concepts and methods from various fields such as environmental science, public health, engineering, sociology, and policy studies. This interdisciplinary approach can lead to innovative research and contribute to a holistic understanding of air pollution.
• Engage with stakeholders : Air pollution affects various stakeholders, including communities, policymakers, industry professionals, and advocacy groups. Engaging with these stakeholders can provide valuable insights and enhance the relevance of your research. Consider topics that address the concerns and needs of different stakeholders, ensuring that your research has practical implications and can make a meaningful impact.
• Seek guidance from your professors and mentors : Consult with your professors and mentors who have expertise in the field of air pollution. They can provide valuable guidance, suggest potential research topics, and help you refine your research question. Utilize their knowledge and experience to ensure that your topic aligns with current research trends and academic standards.
• Consider the availability of data : Before finalizing your research topic, consider the availability of data and resources. Ensure that you have access to reliable and relevant data sources that will support your research objectives. Assess the feasibility of data collection and analysis, considering factors such as time constraints, cost, and ethical considerations.
• Aim for a balance between novelty and significance : While it is important to choose a topic that is unique and novel, also consider its significance within the broader field of air pollution research. Balance your desire to explore new avenues with the need for topics that contribute to the existing body of knowledge and have real-world implications.
• Think critically and creatively : Finally, approach the topic selection process with a critical and creative mindset. Think beyond the conventional boundaries and explore unconventional ideas. Consider innovative research methodologies, alternative perspectives, and emerging trends in air pollution research. By thinking critically and creatively, you can identify research topics that are both intellectually stimulating and have the potential for significant contributions.

By following these expert tips, you can navigate the process of choosing air pollution research paper topics with confidence and clarity. Remember that the topic you choose will shape the entire research process, so take the time to select a topic that aligns with your interests, expertise, and aspirations. Now, let’s move on to the next section, where we will provide you with valuable insights on how to write an impactful air pollution research paper.

How to Write an Air Pollution Research Paper

Writing an air pollution research paper requires careful planning, systematic research, and effective organization. In this section, we will guide you through the essential steps and provide you with 10 tips to help you write a well-structured and compelling research paper on air pollution.

• Understand the research question : Start by clearly understanding the research question or objective of your study. Define the specific aspect of air pollution that you intend to investigate and the key research aims. This will provide you with a focused direction and ensure that your paper addresses the core issues related to air pollution.
• Conduct a comprehensive literature review : Before diving into your research, conduct a thorough literature review to familiarize yourself with the existing body of knowledge on air pollution. Identify key theories, concepts, and empirical studies relevant to your topic. The literature review will help you identify research gaps and build a strong theoretical foundation for your study.
• Develop a clear research methodology : Determine the research methodology and data collection techniques that align with your research objectives. Will you conduct experiments, surveys, interviews, or analyze existing datasets? Clearly define your research design, sampling strategy, and data analysis methods to ensure the rigor and validity of your findings.
• Collect and analyze data : If your research involves primary data collection, carefully collect and organize your data using appropriate methods. If you are analyzing secondary data, ensure that the datasets are reliable and relevant to your research objectives. Apply appropriate statistical or qualitative analysis techniques to derive meaningful insights from your data.
• Structure your paper effectively : Organize your research paper using a clear and logical structure. Typically, a research paper includes an introduction, literature review, methodology, results, discussion, and conclusion. Ensure smooth transitions between sections and maintain a coherent flow of ideas throughout your paper.
• Write a compelling introduction : Begin your paper with an engaging introduction that provides context and background information on air pollution. Clearly state the research question, explain the significance of your study, and highlight the objectives and expected outcomes. Grab the reader’s attention and set the tone for the rest of the paper.
• Present your findings accurately : In the results section, present your findings in a clear and concise manner. Use appropriate tables, graphs, and figures to present data effectively. Provide relevant statistical measures and interpret the results objectively. Ensure that your findings directly address the research question and support your hypotheses or research objectives.
• Analyze and discuss your results : In the discussion section, analyze and interpret your findings in light of the existing literature. Compare your results with previous studies, identify similarities and differences, and explain any discrepancies. Discuss the implications of your findings and their significance for understanding air pollution and its effects.
• Address limitations and future research : Acknowledge the limitations of your study, such as sample size constraints, data limitations, or potential biases. Suggest avenues for future research to address these limitations and further advance knowledge in the field of air pollution. This demonstrates your critical thinking and opens up opportunities for future research contributions.
• Craft a strong conclusion : Conclude your research paper by summarizing the key findings, emphasizing their significance, and restating the research question and objectives. Discuss the implications of your study for theory, practice, and policy-making in the context of air pollution. Avoid introducing new information in the conclusion and leave the reader with a lasting impression of your research.

By following these tips, you can effectively structure and write an air pollution research paper that contributes to the existing knowledge, addresses key research questions, and provides valuable insights into this critical environmental issue. In the next section, we will introduce you to the writing services offered by iResearchNet, where you can order a custom research paper on any air pollution topic.

Custom Research Paper Writing Services

At iResearchNet, we understand the challenges faced by students in conducting research and writing a high-quality research paper on air pollution. That’s why we offer custom writing services tailored to meet your specific needs. Our team of expert writers, who hold advanced degrees in environmental science, are dedicated to delivering top-notch research papers that showcase your knowledge and understanding of air pollution. When you choose our writing services, you can expect the following:

• Expert degree-holding writers : Our team consists of skilled writers with expertise in environmental science and air pollution research. They have the knowledge and experience to tackle complex topics and deliver well-researched and insightful papers.
• Custom written works : We understand the importance of originality and uniqueness in academic writing. Our writers craft each research paper from scratch, ensuring that it is tailored to your specific requirements and adheres to the highest standards of quality.
• In-depth research : Our writers conduct thorough research using credible sources to gather the most relevant and up-to-date information on air pollution. They critically analyze the literature and integrate it seamlessly into your research paper to support your arguments and strengthen your findings.
• Custom formatting : We are well-versed in various formatting styles, including APA, MLA, Chicago/Turabian, and Harvard. Our writers will format your research paper according to the specified guidelines, ensuring consistency and professionalism throughout.
• Top quality : Quality is our utmost priority. We strive to deliver research papers that meet the highest academic standards. Our writers pay attention to detail, ensure accurate referencing, and use clear and concise language to convey your ideas effectively.
• Customized solutions : We understand that every research paper is unique. Our writers take a personalized approach, tailoring their writing to your specific research objectives, methodology, and findings. They adapt their writing style and tone to match your requirements and ensure a seamless integration of your ideas.
• Flexible pricing : We offer competitive and flexible pricing options to accommodate your budget. Our pricing is transparent, and there are no hidden fees or additional charges. You can select the pricing plan that suits your needs, whether it’s for a comprehensive research paper or a specific section.
• Short deadlines : We understand that time is of the essence when it comes to academic assignments. Our writers are capable of working under tight deadlines and can deliver your custom research paper within short timeframes, even as little as 3 hours.
• Timely delivery : We are committed to delivering your research paper on time. We understand the importance of meeting deadlines, and our writers work diligently to ensure that your paper is delivered within the agreed-upon timeframe.
• 24/7 support : Our dedicated support team is available 24/7 to assist you with any questions or concerns you may have. Whether you need updates on your order or have inquiries about our services, our friendly support staff is here to provide prompt and helpful assistance.
• Absolute Privacy : We prioritize the privacy and confidentiality of our clients. Your personal information and order details are kept secure and protected. We adhere to strict privacy policies to ensure that your information remains confidential.
• Easy order tracking : Our user-friendly platform allows you to easily track the progress of your order. You can stay updated on the status of your research paper, communicate with your writer, and receive notifications throughout the writing process.
• Money back guarantee : We are confident in the quality of our services and the expertise of our writers. If, for any reason, you are not satisfied with the final product, we offer a money-back guarantee. Your satisfaction is our priority, and we strive to ensure that you are fully content with the research paper you receive.

When you choose iResearchNet, you can be confident in receiving a well-written and thoroughly researched custom air pollution research paper that meets your academic requirements. We value your privacy and guarantee absolute confidentiality throughout the entire process. Our easy order tracking system allows you to stay updated on the progress of your paper, ensuring a seamless experience from start to finish. If, for any reason, you are not satisfied with the final product, we offer a money-back guarantee.

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4 Questions About Air Pollution and Your Health

The Conversation

The Conversation US launched as a pilot project in October 2014. It is an independent source of news and views from the academic and research community, delivered direct to the public.

What you need to know

Air pollution knows no boundaries and is a leading cause of early death worldwide.

By Richard E. Peltier

Not a day seems to go by without a story of an “airpocalypse,” usually somewhere in a developing nation. It’s hard not to empathize with the people in the smoggy images of New Delhi or Ulaanbataar or Kathmandu, often wearing masks , walking to school or work though soupy cloudiness.

Last year, a study found that more than 8 million people per year die early from air pollution exposure. This amounts to more deaths than diarrheal disease, tuberculosis and HIV/AIDS combined.

As a researcher in air pollution and its health effects, I know that even if you don’t live in these places, air pollution likely still affects your quality of life. Here’s what you need to know.

1. What exactly is air pollution?

Air pollution is a general term that usually describes a mixture of different chemicals that circulate in the air.

Invisible gases, like ozone or carbon monoxide, and tiny particles or droplets of liquids mix together in the atmosphere. Each molecule is impossible to see with the naked eye, but when trillions gather together, you can see them as haze.

Reuters / TPG

These chemicals are almost always mixed together in varied amounts. Scientists do not yet understand how these different mixtures affect us. Each person responds differently to air pollution exposure – some people have few effects, while others, such as kids with asthma, might become very ill.

What’s more, air pollution mixtures in a given location change over time. Changes can occur quickly over a few hours or gradually over months.

Short-term increases in air pollution from, for example, heavy traffic in rush hour, can make us sick. Such pollution occurs year-round. But seasonal pollutants, such as ozone, usually occur only in the warmest and sunniest parts of year. What’s more, the amount of ozone in air also goes up and down through the day – generally highest in the afternoons and lowest in the early mornings.

These variations can make it quite difficult for environmental health scientists and epidemiologists to know precisely how air pollution can affect humans.

2. Where does air pollution come from?

You might imagine air pollution as smoke pouring out of a factory chimney or the tailpipe of a car.

While these are important sources of air pollution, there are many others. Air pollution includes chemicals humans put into the atmosphere and chemicals released by natural events. For example, forest fires are a large source of air pollutants that affect many communities. Dust that’s picked up by wind can also contribute to poor air quality.

[One short ton is 907 kilograms]

Ronald Reagan famously said that “trees cause more pollution than automobiles do.” While this myth has been debunked, he was right in at least some ways. Trees do release certain gases, such as volatile organic carbon, that are ingredients in air pollution chemistry. This, when mixed together with emissions from cars and industry, leads to increases in other types of pollution, such as ozone.

There isn’t much that scientists can, or should, do about tree emissions. Public health researchers like myself focus most on the ingredients from human activities – from burning petroleum to emissions controls on industrial facilities – because these are sources located close to where people live and work.

There are also many chemical reactions that occur in the air itself. These reactions create what are known as secondary pollutants, some of which are quite toxic .

Finally, it’s important to realize that air pollution knows no boundaries. If a pollutant is emitted in one location, it very easily moves across borders – both regional and national – to different places. New Delhi, for example, experiences seasonal pollution, thanks to extensive burning of agricultural fields some 200 miles away.

New Delhi is an extreme example. But, even if you live in a less polluted environment, pollutants emitted elsewhere often travel to where other people live and work, as seen in recent wildfires in California.

3. How do we know that air pollution causes problems?

This is a tricky question, because air pollution is a hidden problem that acts as a trigger for many health problems. Plenty of people suffer from asthma and lung diseases, heart attacks and cancer, and all of these are linked to particulate matter exposure . The best evidence to date suggests that the higher the dose of air pollution, the worse our response will be.

Unfortunately, there are many other things that lead to these diseases, too: poor diet, your inherited genes, or whether you have access to high quality medical care or you smoke cigarettes, for example. This makes figuring out the cause of a specific illness attributed to air pollution exposure much more difficult.

Every health study provides a slightly different result, because each study observes a different group of people and usually different types of air pollution. Scientists usually report their results based on any change in risk of developing a disease from air pollution, or based on whether your odds of developing a certain disease might change.

For example, a study in Taiwan looked at concentrations of particulate matter averaged over two years. The researchers found that, for every 10 micrograms per cubic meter increase in particulate matter, the odds of developing high blood pressure increased by about 3 percent. This could suggest that if an increase of particulate matter concentration in any community might lead to an increase in high blood pressure.

Conversely, scientists usually assume that decreases in air pollution lead to decreases in diseases.

4. Why does this matter to you?

A typical adult takes around 20,000 breaths per day. Whether or not you become sick from air pollution depends on the amount and type of chemicals you inhale, and whether you might be susceptible to these diseases.

A typical adult takes around 20,000 breaths per day.

For someone living in polluted New Delhi, for example, those 20,000 breaths include the equivalent of around 20 grains of table salt worth of particulate matter deposited in their lungs each day. While this may not seem like much, keep in mind that this particulate matter isn’t harmless table salt – it’s a mixture of chemicals that come from burning materials, unburned oils, metals and even biological material. And this doesn’t include any of the pollutants that are gases, like ozone or carbon monoxide or oxides of nitrogen.

The U.S. and Europe have made excellent progress in reducing air pollution concentrations over the past couple of decades, largely by crafting effective air quality regulation.

Reuter / TPG

However, in the U.S. today, where environmental laws are being methodically dismantled , there is a bigger worry that policymakers are simply choosing to ignore science. One new member of the Environmental Protection Agency’s science advisory board is Robert Phalen of the University of California, Irvine, who has suggested that “modern air is too clean for optimum health” .

This goes against thousands of research papers and is certainly not true. While some components of air pollution have little effect on human health, this should not be used to muddy our understanding of air pollution exposure. This is a common tactic to confuse the public with unimportant statistics in order to sow confusion, presumably with an underlying intent to influence policy.

The evidence is clear: Air pollution exposure is lethal and causes death across the world. That should be important to all of us.

Read Next: What Will It Take to Improve Taiwan's Air?

Richard E. Peltier is an associate professor of environmental health sciences at the University of Massachusetts Amherst

This article was originally published on The Conversation . Read the original article .

TNL Editor: Morley J Weston

• air pollution
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