air pollution in metropolitan cities essay

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• Published: 29 October 2020

Urban and air pollution: a multi-city study of long-term effects of urban landscape patterns on air quality trends

• Lu Liang 1 &
• Peng Gong 2 , 3 , 4  

Scientific Reports volume  10 , Article number:  18618 ( 2020 ) Cite this article

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Most air pollution research has focused on assessing the urban landscape effects of pollutants in megacities, little is known about their associations in small- to mid-sized cities. Considering that the biggest urban growth is projected to occur in these smaller-scale cities, this empirical study identifies the key urban form determinants of decadal-long fine particulate matter (PM 2.5 ) trends in all 626 Chinese cities at the county level and above. As the first study of its kind, this study comprehensively examines the urban form effects on air quality in cities of different population sizes, at different development levels, and in different spatial-autocorrelation positions. Results demonstrate that the urban form evolution has long-term effects on PM 2.5 level, but the dominant factors shift over the urbanization stages: area metrics play a role in PM 2.5 trends of small-sized cities at the early urban development stage, whereas aggregation metrics determine such trends mostly in mid-sized cities. For large cities exhibiting a higher degree of urbanization, the spatial connectedness of urban patches is positively associated with long-term PM 2.5 level increases. We suggest that, depending on the city’s developmental stage, different aspects of the urban form should be emphasized to achieve long-term clean air goals.

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Introduction.

Air pollution represents a prominent threat to global society by causing cascading effects on individuals 1 , medical systems 2 , ecosystem health 3 , and economies 4 in both developing and developed countries 5 , 6 , 7 , 8 . About 90% of global citizens lived in areas that exceed the safe level in the World Health Organization (WHO) air quality guidelines 9 . Among all types of ecosystems, urban produce roughly 78% of carbon emissions and substantial airborne pollutants that adversely affect over 50% of the world’s population living in them 5 , 10 . While air pollution affects all regions, there exhibits substantial regional variation in air pollution levels 11 . For instance, the annual mean concentration of fine particulate matter with an aerodynamic diameter of less than 2.5  \(\upmu\mathrm{m}\) (PM 2.5 ) in the most polluted cities is nearly 20 times higher than the cleanest city according to a survey of 499 global cities 12 . Many factors can influence the regional air quality, including emissions, meteorology, and physicochemical transformations. Another non-negligible driver is urbanization—a process that alters the size, structure, and growth of cities in response to the population explosion and further leads to lasting air quality challenges 13 , 14 , 15 .

With the global trend of urbanization 16 , the spatial composition, configuration, and density of urban land uses (refer to as urban form) will continue to evolve 13 . The investigation of urban form impacts on air quality has been emerging in both empirical 17 and theoretical 18 research. While the area and density of artificial surface areas have well documented positive relationship with air pollution 19 , 20 , 21 , the effects of urban fragmentation on air quality have been controversial. In theory, compact cities promote high residential density with mixed land uses and thus reduce auto dependence and increase the usage of public transit and walking 21 , 22 . The compact urban development has been proved effective in mitigating air pollution in some cities 23 , 24 . A survey of 83 global urban areas also found that those with highly contiguous built-up areas emitted less NO 2 22 . In contrast, dispersed urban form can decentralize industrial polluters, improve fuel efficiency with less traffic congestion, and alleviate street canyon effects 25 , 26 , 27 , 28 . Polycentric and dispersed cities support the decentralization of jobs that lead to less pollution emission than compact and monocentric cities 29 . The more open spaces in a dispersed city support air dilution 30 . In contrast, compact cities are typically associated with stronger urban heat island effects 31 , which influence the availability and the advection of primary and secondary pollutants 32 .

The mixed evidence demonstrates the complex interplay between urban form and air pollution, which further implies that the inconsistent relationship may exist in cities at different urbanization levels and over different periods 33 . Few studies have attempted to investigate the urban form–air pollution relationship with cross-sectional and time series data 34 , 35 , 36 , 37 . Most studies were conducted in one city or metropolitan region 38 , 39 or even at the country level 40 . Furthermore, large cities or metropolitan areas draw the most attention in relevant studies 5 , 41 , 42 , and the small- and mid-sized cities, especially those in developing countries, are heavily underemphasized. However, virtually all world population growth 43 , 44 and most global economic growth 45 , 46 are expected to occur in those cities over the next several decades. Thus, an overlooked yet essential task is to account for various levels of cities, ranging from large metropolitan areas to less extensive urban area, in the analysis.

This study aims to improve the understanding of how the urban form evolution explains the decadal-long changes of the annual mean PM 2.5 concentrations in 626 cities at the county-level and above in China. China has undergone unprecedented urbanization over the past few decades and manifested a high degree of heterogeneity in urban development 47 . Thus, Chinese cities serve as a good model for addressing the following questions: (1) whether the changes in urban landscape patterns affect trends in PM 2.5 levels? And (2) if so, do the determinants vary by cities?

City boundaries

Our study period spans from the year 2000 to 2014 to keep the data completeness among all data sources. After excluding cities with invalid or missing PM 2.5 or sociodemographic value, a total of 626 cities, with 278 prefecture-level cities and 348 county-level cities, were selected. City boundaries are primarily based on the Global Rural–Urban Mapping Project (GRUMP) urban extent polygons that were defined by the extent of the nighttime lights 48 , 49 . Few adjustments were made. First, in the GRUMP dataset, large agglomerations that include several cities were often described in one big polygon. We manually split those polygons into individual cities based on the China Administrative Regions GIS Data at 1:1 million scales 50 . Second, since the 1978 economic reforms, China has significantly restructured its urban administrative/spatial system. Noticeable changes are the abolishment of several prefectures and the promotion of many former county-level cities to prefecture-level cities 51 . Thus, all city names were cross-checked between the year 2000 and 2014, and the mismatched records were replaced with the latest names.

PM 2.5 concentration data

The annual mean PM 2.5 surface concentration (micrograms per cubic meter) for each city over the study period was calculated from the Global Annual PM 2.5 Grids at 0.01° resolution 52 . This data set combines Aerosol Optical Depth retrievals from multiple satellite instruments including the NASA Moderate Resolution Imaging Spectroradiometer (MODIS), Multi-angle Imaging SpectroRadiometer (MISR), and the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS). The global 3-D chemical transport model GEOS-Chem is further applied to relate this total column measure of aerosol to near-surface PM 2.5 concentration, and geographically weighted regression is finally used with global ground-based measurements to predict and adjust for the residual PM 2.5 bias per grid cell in the initial satellite-derived values.

Human settlement layer

The urban forms were quantified with the 40-year (1978–2017) record of annual impervious surface maps for both rural and urban areas in China 47 , 53 . This state-of-art product provides substantial spatial–temporal details on China’s human settlement changes. The annual impervious surface maps covering our study period were generated from 30-m resolution Landsat images acquired onboard Landsat 5, 7, and 8 using an automatic “Exclusion/Inclusion” mapping framework 54 , 55 . The output used here was the binary impervious surface mask, with the value of one indicating the presence of human settlement and the value of zero identifying non-residential areas. The product assessment concluded good performance. The cross-comparison against 2356 city or town locations in GeoNames proved an overall high agreement (88%) and approximately 80% agreement was achieved when compared against visually interpreted 650 urban extent areas in the year 1990, 2000, and 2010.

Control variables

To provide a holistic assessment of the urban form effects, we included control variables that are regarded as important in influencing air quality to account for the confounding effects.

Four variables, separately population size, population density, and two economic measures, were acquired from the China City Statistical Yearbook 56 (National Bureau of Statistics 2000–2014). Population size is used to control for the absolute level of pollution emissions 41 . Larger populations are associated with increased vehicle usage and vehicle-kilometers travels, and consequently boost tailpipes emissions 5 . Population density is a useful reflector of transportation demand and the fraction of emissions inhaled by people 57 . We also included gross regional product (GRP) and the proportion of GRP generated from the secondary sector (GRP2). The impact of economic development on air quality is significant but in a dynamic way 58 . The rising per capita income due to the concentration of manufacturing industrial activities can deteriorate air quality and vice versa if the stronger economy is the outcome of the concentration of less polluting high-tech industries. Meteorological conditions also have short- and long-term effects on the occurrence, transport, and dispersion of air pollutants 59 , 60 , 61 . Temperature affects chemical reactions and atmospheric turbulence that determine the formation and diffusion of particles 62 . Low air humidity can lead to the accumulation of air pollutants due to it is conducive to the adhesion of atmospheric particulate matter on water vapor 63 . Whereas high humidity can lead to wet deposition processes that can remove air pollutants by rainfall. Wind speed is a crucial indicator of atmospheric activity by greatly affect air pollutant transport and dispersion. All meteorological variables were calculated based on China 1 km raster layers of monthly relative humidity, temperature, and wind speed that are interpolated from over 800 ground monitoring stations 64 . Based on the monthly layer, we calculated the annual mean of each variable for each year. Finally, all pixels falling inside of the city boundary were averaged to represent the overall meteorological condition of each city.

Considering the dynamic urban form-air pollution relationship evidenced from the literature review, our hypothesis is: the determinants of PM 2.5 level trends are not the same for cities undergoing different levels of development or in different geographic regions. To test this hypothesis, we first categorized city groups following (1) social-economic development level, (2) spatial autocorrelation relationship, and (3) population size. We then assessed the relationship between urban form and PM 2.5 level trends by city groups. Finally, we applied the panel data models to different city groups for hypothesis testing and key determinant identification (Fig.  1 ).

Methodology workflow.

Calculation of urban form metrics

Based on the previous knowledge 65 , 66 , 67 , fifteen landscape metrics falling into three categories, separately area, shape, and aggregation, were selected. Those metrics quantify the compositional and configurational characteristics of the urban landscape, as represented by urban expansion, urban shape complexity, and compactness (Table 1 ).

Area metrics gives an overview of the urban extent and the size of urban patches that are correlated with PM 2.5 20 . As an indicator of the urbanization degree, total area (TA) typically increases constantly or remains stable, because the urbanization process is irreversible. Number of patches (NP) refers to the number of discrete parcels of urban settlement within a given urban extent and Mean Patch Size (AREA_MN) measures the average patch size. Patch density (PD) indicates the urbanization stages. It usually increases with urban diffusion until coalescence starts, after which decreases in number 66 . Largest Patch Index (LPI) measures the percentage of the landscape encompassed by the largest urban patch.

The shape complexity of urban patches was represented by Mean Patch Shape Index (SHAPE_MN), Mean Patch Fractal Dimension (FRAC_MN), and Mean Contiguity Index (CONTIG_MN). The greater irregularity the landscape shape, the larger the value of SHAPE_MN and FRAC_MN. CONTIG_MN is another method of assessing patch shape based on the spatial connectedness or contiguity of cells within a patch. Larger contiguous patches will result in larger CONTIG_MN.

Aggregation metrics measure the spatial compactness of urban land, which affects pollutant diffusion and dilution. Mean Euclidean nearest-neighbor distance (ENN_MN) quantifies the average distance between two patches within a landscape. It decreases as patches grow together and increases as the urban areas expand. Landscape Shape Index (LSI) indicates the divergence of the shape of a landscape patch that increases as the landscape becomes increasingly disaggregated 68 . Patch Cohesion Index (COHESION) is suggestive of the connectedness degree of patches 69 . Splitting Index (SPLIT) and Landscape Division Index (DIVISION) increase as the separation of urban patches rises, whereas, Mesh Size (MESH) decreases as the landscape becomes more fragmented. Aggregation Index (AI) measures the degree of aggregation or clumping of urban patches. Higher values of continuity indicate higher building densities, which may have a stronger effect on pollution diffusion.

The detailed descriptions of these indices are given by the FRAGSTATS user’s guide 70 . The calculation input is a layer of binary grids of urban/nonurban. The resulting output is a table containing one row for each city and multiple columns representing the individual metrics.

Division of cities

Division based on the socioeconomic development level.

The socioeconomic development level in China is uneven. The unequal development of the transportation system, descending in topography from the west to the east, combined with variations in the availability of natural and human resources and industrial infrastructure, has produced significantly wide gaps in the regional economies of China. By taking both the economic development level and natural geography into account, China can be loosely classified into Eastern, Central, and Western regions. Eastern China is generally wealthier than the interior, resulting from closeness to coastlines and the Open-Door Policy favoring coastal regions. Western China is historically behind in economic development because of its high elevation and rugged topography, which creates barriers in the transportation infrastructure construction and scarcity of arable lands. Central China, echoing its name, is in the process of economic development. This region neither benefited from geographic convenience to the coast nor benefited from any preferential policies, such as the Western Development Campaign.

Division based on spatial autocorrelation relationship

The second type of division follows the fact that adjacent cities are likely to form air pollution clusters due to the mixing and diluting nature of air pollutants 71 , i.e., cities share similar pollution levels as its neighbors. The underlying processes driving the formation of pollution hot spots and cold spots may differ. Thus, we further divided the city into groups based on the spatial clusters of PM 2.5 level changes.

Local indicators of spatial autocorrelation (LISA) was used to determine the local patterns of PM 2.5 distribution by clustering cities with a significant association. In the presence of global spatial autocorrelation, LISA indicates whether a variable exhibits significant spatial dependence and heterogeneity at a given scale 72 . Practically, LISA relates each observation to its neighbors and assigns a value of significance level and degree of spatial autocorrelation, which is calculated by the similarity in variable \(z\) between observation \(i\) and observation \(j\) in the neighborhood of \(i\) defined by a matrix of weights \({w}_{ij}\) 7 , 73 :

where \({I}_{i}\) is the Moran’s I value for location \(i\) ; \({\sigma }^{2}\) is the variance of variable \(z\) ; \(\bar{z}\) is the average value of \(z\) with the sample number of \(n\) . The weight matrix \({w}_{ij}\) is defined by the k-nearest neighbors distance measure, i.e., each object’s neighborhood consists of four closest cites.

The computation of Moran’s I enables the identification of hot spots and cold spots. The hot spots are high-high clusters where the increase in the PM 2.5 level is higher than the surrounding areas, whereas cold spots are low-low clusters with the presence of low values in a low-value neighborhood. A Moran scatterplot, with x-axis as the original variable and y-axis as the spatially lagged variable, reflects the spatial association pattern. The slope of the linear fit to the scatter plot is an estimation of the global Moran's I 72 (Fig.  2 ). The plot consists of four quadrants, each defining the relationship between an observation 74 . The upper right quadrant indicates hot spots and the lower left quadrant displays cold spots 75 .

Moran’s I scatterplot. Figure was produced by R 3.4.3 76 .

Division based on population size

The last division was based on population size, which is a proven factor in changing per capita emissions in a wide selection of global cities, even outperformed land urbanization rate 77 , 78 , 79 . We used the 2014 urban population to classify the cities into four groups based on United Nations definitions 80 : (1) large agglomerations with a total population larger than 1 million; (2) mid-sized cities, 500,000–1 million; (3) small cities, 250,000–500,000, and (4) very small cities, 100,000–250,000.

Panel data analysis

The panel data analysis is an analytical method that deals with observations from multiple entities over multiple periods. Its capacity in analyzing the characteristics and changes from both the time-series and cross-section dimensions of data surpasses conventional models that purely focus on one dimension 81 , 82 . The estimation equation for the panel data model in this study is given as:

where the subscript \(i\) and \(t\) refer to city and year respectively. \(\upbeta _{{0}}\) is the intercept parameter and \(\upbeta _{{1}} - { }\upbeta _{{{18}}}\) are the estimates of slope coefficients. \(\varepsilon \) is the random error. All variables are transformed into natural logarithms.

Two methods can be used to obtain model estimates, separately fixed effects estimator and random effects estimator. The fixed effects estimator assumes that each subject has its specific characteristics due to inherent individual characteristic effects in the error term, thereby allowing differences to be intercepted between subjects. The random effects estimator assumes that the individual characteristic effect changes stochastically, and the differences in subjects are not fixed in time and are independent between subjects. To choose the right estimator, we run both models for each group of cities based on the Hausman specification test 83 . The null hypothesis is that random effects model yields consistent and efficient estimates 84 : \({H}_{0}{:}\,E\left({\varepsilon }_{i}|{X}_{it}\right)=0\) . If the null hypothesis is rejected, the fixed effects model will be selected for further inferences. Once the better estimator was determined for each model, one optimal panel data model was fit to each city group of one division type. In total, six, four, and eight runs were conducted for socioeconomic, spatial autocorrelation, and population division separately and three, two, and four panel data models were finally selected.

Spatial patterns of PM 2.5 level changes

During the period from 2000 to 2014, the annual mean PM 2.5 concentration of all cities increases from 27.78 to 42.34 µg/m 3 , both of which exceed the World Health Organization recommended annual mean standard (10 µg/m 3 ). It is worth noting that the PM 2.5 level in the year 2014 also exceeds China’s air quality Class 2 standard (35 µg/m 3 ) that applies to non-national park places, including urban and industrial areas. The standard deviation of annual mean PM 2.5 values for all cities increases from 12.34 to 16.71 µg/m 3 , which shows a higher variability of inter-urban PM 2.5 pollution after a decadal period. The least and most heavily polluted cities in China are Delingha, Qinghai (3.01 µg/m 3 ) and Jizhou, Hubei (64.15 µg/m 3 ) in 2000 and Hami, Xinjiang (6.86 µg/m 3 ) and Baoding, Hubei (86.72 µg/m 3 ) in 2014.

Spatially, the changes in PM 2.5 levels exhibit heterogeneous patterns across cities (Fig.  3 b). According to the socioeconomic level division (Fig.  3 a), the Eastern, Central, and Western region experienced a 38.6, 35.3, and 25.5 µg/m 3 increase in annual PM 2.5 mean , separately, and the difference among regions is significant according to the analysis of variance (ANOVA) results (Fig.  4 a). When stratified by spatial autocorrelation relationship (Fig.  3 c), the differences in PM 2.5 changes among the spatial clusters are even more dramatic. The average PM 2.5 increase in cities belonging to the high-high cluster is approximately 25 µg/m 3 , as compared to 5 µg/m 3 in the low-low clusters (Fig.  4 b). Finally, cities at four different population levels have significant differences in the changes of PM 2.5 concentration (Fig.  3 d), except for the mid-sized cities and large city agglomeration (Fig.  4 c).

( a ) Division of cities in China by socioeconomic development level and the locations of provincial capitals; ( b ) Changes in annual mean PM 2.5 concentrations between the year 2000 and 2014; ( c ) LISA cluster maps for PM 2.5 changes at the city level; High-high indicates a statistically significant cluster of high PM 2.5 level changes over the study period. Low-low indicates a cluster of low PM 2.5 inter-annual variation; No high-low cluster is reported; Low–high represents cities with high PM 2.5 inter-annual variation surrounded by cities with low variation; ( d ) Population level by cities in the year 2014. Maps were produced by ArcGIS 10.7.1 85 .

Boxplots of PM 2.5 concentration changes between 2000 and 2014 for city groups that are formed according to ( a ) socioeconomic development level division, ( b ) LISA clusters, and ( c ) population level. Asterisk marks represent the p value of ANOVA significant test between the corresponding pair of groups. Note ns not significant; * p value < 0.05; ** p value < 0.01; *** p value < 0.001; H–H high-high cluster, L–H low–high cluster, L–L denotes low–low cluster.

The effects of urban forms on PM 2.5 changes

The Hausman specification test for fixed versus random effects yields a p value less than 0.05, suggesting that the fixed effects model has better performance. We fit one panel data model to each city group and built nine models in total. All models are statistically significant at the p  < 0.05 level and have moderate to high predictive power with the R 2 values ranging from 0.63 to 0.95, which implies that 63–95% of the variation in the PM 2.5 concentration changes can be explained by the explanatory variables (Table 2 ).

The urban form—PM 2.5 relationships differ distinctly in Eastern, Central, and Western China. All models reach high R 2 values. Model for Eastern China (refer to hereafter as Eastern model) achieves the highest R 2 (0.90), and the model for the Western China (refer to hereafter as Western model) reaches the lowest R 2 (0.83). The shape metrics FRAC and CONTIG are correlated with PM 2.5 changes in the Eastern model, whereas the area metrics AREA demonstrates a positive effect in the Western model. In contrast to the significant associations between shape, area metrics and PM 2.5 level changes in both Eastern and Western models, no such association was detected in the Central model. Nonetheless, two aggregation metrics, LSI and AI, play positive roles in determining the PM 2.5 trends in the Central model.

For models built upon the LISA clusters, the H–H model (R 2  = 0.95) reaches a higher fitting degree than the L–L model (R 2  = 0.63). The estimated coefficients vary substantially. In the H–H model, the coefficient of CONTIG is positive, which indicates that an increase in CONTIG would increase PM 2.5 pollution. In contrast, no shape metrics but one area metrics AREA is significant in the L–L model.

The results of the regression models built for cities at different population levels exhibit a distinct pattern. No urban form metrics was identified to have a significant relationship with the PM 2.5 level changes in groups of very small and mid-sized cities. For small size cities, the aggregation metrics COHESION was positively associated whereas AI was negatively related. For mid-sized cities and large agglomerations, CONTIG is the only significant variable that is positively related to PM 2.5 level changes.

Urban form is an effective measure of long-term PM 2.5 trends

All panel data models are statistically significant regardless of the data group they are built on, suggesting that the associations between urban form and ambient PM 2.5 level changes are discernible at all city levels. Importantly, these relationships are found to hold when controlling for population size and gross domestic product, implying that the urban landscape patterns have effects on long-term PM 2.5 trends that are independent of regional economic performance. These findings echo with the local, regional, and global evidence of urban form effect on various air pollution types 5 , 14 , 21 , 22 , 24 , 39 , 78 .

Although all models demonstrate moderate to high predictive power, the way how different urban form metrics respond to the dependent variable varies. Of all the metrics tested, shape metrics, especially CONTIG has the strongest effect on PM 2.5 trends in cities belonging to the high-high cluster, Eastern, and large urban agglomerations. All those regions have a strong economy and higher population density 86 . In the group of cities that are moderately developed, such as the Central region, as well as small- and mid-sized cities, aggregation metrics play a dominant negative role in PM 2.5 level changes. In contrast, in the least developed cities belonging to the low-low cluster regions and Western China, the metrics describing size and number of urban patches are the strongest predictors. AREA and NP are positively related whereas TA is negatively associated.

The impacts of urban form metrics on air quality vary by urbanization degree

Based on the above observations, how urban form affects within-city PM 2.5 level changes may differ over the urbanization stages. We conceptually summarized the pattern in Fig.  5 : area metrics have the most substantial influence on air pollution changes at the early urban development stage, and aggregation metrics emerge at the transition stage, whereas shape metrics affect the air quality trends at the terminal stage. The relationship between urban form and air pollution has rarely been explored with such a wide range of city selections. Most prior studies were focused on large urban agglomeration areas, and thus their conclusions are not representative towards small cities at the early or transition stage of urbanization.

The most influential metric of urban form in affecting PM 2.5 level changes at different urbanization stages.

Not surprisingly, the area metrics, which describe spatial grain of the landscape, exert a significant effect on PM 2.5 level changes in small-sized cities. This could be explained by the unusual urbanization speed of small-sized cities in the Chinese context. Their thriving mostly benefited from the urbanization policy in the 1980s, which emphasized industrialization of rural, small- and mid-sized cities 87 . With the large rural-to-urban migration and growing public interest in investing real estate market, a side effect is that the massive housing construction that sometimes exceeds market demand. Residential activities decline in newly built areas of smaller cities in China, leading to what are known as ghost cities 88 . Although ghost cities do not exist for all cities, high rate of unoccupied dwellings is commonly seen in cities under the prefectural level. This partly explained the negative impacts of TA on PM 2.5 level changes, as an expanded while unoccupied or non-industrialized urban zones may lower the average PM 2.5 concentration within the city boundary, but it doesn’t necessarily mean that the air quality got improved in the city cores.

Aggregation metrics at the landscape scale is often referred to as landscape texture that quantifies the tendency of patch types to be spatially aggregated; i.e., broadly speaking, aggregated or “contagious” distributions. This group of metrics is most effective in capturing the PM 2.5 trends in mid-sized cities (population range 25–50 k) and Central China, where the urbanization process is still undergoing. The three significant variables that reflect the spatial property of dispersion, separately landscape shape index, patch cohesion index, and aggregation index, consistently indicate that more aggregated landscape results in a higher degree of PM 2.5 level changes. Theoretically, the more compact urban form typically leads to less auto dependence and heavier reliance on the usage of public transit and walking, which contributes to air pollution mitigation 89 . This phenomenon has also been observed in China, as the vehicle-use intensity (kilometers traveled per vehicle per year, VKT) has been declining over recent years 90 . However, VKT only represents the travel intensity of one car and does not reflect the total distance traveled that cumulatively contribute to the local pollution. It should be noted that the private light-duty vehicle ownership in China has increased exponentially and is forecast to reach 23–42 million by 2050, with the share of new-growth purchases representing 16–28% 90 . In this case, considering the increased total distance traveled, the less dispersed urban form can exert negative effects on air quality by concentrating vehicle pollution emissions in a limited space.

Finally, urban contiguity, observed as the most effective shape metric in indicating PM 2.5 level changes, provides an assessment of spatial connectedness across all urban patches. Urban contiguity is found to have a positive effect on the long-term PM 2.5 pollution changes in large cities. Urban contiguity reflects to which degree the urban landscape is fragmented. Large contiguous patches result in large CONTIG_MN values. Among the 626 cities, only 11% of cities experience negative changes in urban contiguity. For example, Qingyang, Gansu is one of the cities-featuring leapfrogs and scattered development separated by vacant land that may later be filled in as the development continues (Fig.  6 ). Most Chinese cities experienced increased urban contiguity, with less fragmented and compacted landscape. A typical example is Shenzhou, Hebei, where CONTIG_MN rose from 0.27 to 0.45 within the 14 years. Although the 13 counties in Shenzhou are very far scattered from each other, each county is growing intensively internally rather than sprawling further outside. And its urban layout is thus more compact (Fig.  6 ). The positive association revealed in this study contradicts a global study indicating that cities with highly contiguous built-up areas have lower NO 2 pollution 22 . We noticed that the principal emission sources of NO 2 differ from that of PM 2.5. NO 2 is primarily emitted with the combustion of fossil fuels (e.g., industrial processes and power generation) 6 , whereas road traffic attributes more to PM 2.5 emissions. Highly connected urban form is likely to cause traffic congestion and trap pollution inside the street canyon, which accumulates higher PM 2.5 concentration. Computer simulation results also indicate that more compact cities improve urban air quality but are under the premise that mixed land use should be presented 18 . With more connected impervious surfaces, it is merely impossible to expect increasing urban green spaces. If compact urban development does not contribute to a rising proportion of green areas, then such a development does not help mitigating air pollution 41 .

Six cities illustrating negative to positive changes in CONTIG_MN and AREA_MN. Pixels in black show the urban areas in the year 2000 and pixels in red are the expanded urban areas from the year 2000 to 2014. Figure was produced by ArcGIS 10.7.1 85 .

Conclusions

This study explores the regional land-use patterns and air quality in a country with an extraordinarily heterogeneous urbanization pattern. Our study is the first of its kind in investigating such a wide range selection of cities ranging from small-sized ones to large metropolitan areas spanning a long time frame, to gain a comprehensive insight into the varying effects of urban form on air quality trends. And the primary insight yielded from this study is the validation of the hypothesis that the determinants of PM 2.5 level trends are not the same for cities at various developmental levels or in different geographic regions. Certain measures of urban form are robust predictors of air quality trends for a certain group of cities. Therefore, any planning strategy aimed at reducing air pollution should consider its current development status and based upon which, design its future plan. To this end, it is also important to emphasize the main shortcoming of this analysis, which is generally centered around the selection of control variables. This is largely constrained by the available information from the City Statistical Yearbook. It will be beneficial to further polish this study by including other important controlling factors, such as vehicle possession.

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Acknowledgements

Lu Liang received intramural research funding support from the UNT Office of Research and Innovation. Peng Gong is partially supported by the National Research Program of the Ministry of Science and Technology of the People’s Republic of China (2016YFA0600104), and donations from Delos Living LLC and the Cyrus Tang Foundation to Tsinghua University.

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Liang, L., Gong, P. Urban and air pollution: a multi-city study of long-term effects of urban landscape patterns on air quality trends. Sci Rep 10 , 18618 (2020). https://doi.org/10.1038/s41598-020-74524-9

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Air Pollution in Cities

How it works

• 1 Background:
• 5 Analysis of Data:
• 6 Solutions:

Background:

Air pollution is particle matter in the earth’s atmosphere, and it gradually harms humans, animals, plants, and the earth itself. Air pollution arose side to side with the industrial revolution; the rise to modern manufacturing processes. Air pollution is a problem because eventually at this rate the air is being polluted, the sky will be filled with smog and completely black in only a matter of decades. This problem ties in with the needs and wants of society because we constantly purchase and use everyday items such as vehicles, plastic, burn fuel, and to buy those vehicles in the first place, they must be assembled at an industrial factory, which emit massive amounts of pollution.

Air pollution in cities is caused by a variety of reasons, both natural and caused by humans. Contributors to air pollution include fossil fuels (coal, oil, gasoline) being burned in industrial factories, cars, airplanes, helicopters, etc., crop-dusting and farming chemicals, household sprays like insect repellant, hair spray, and other chemical sprays, forest fires, volcanic eruption, and high quantity of decaying plant/animal matter are all examples of natural and unnatural air pollution causes.

There are various effects of air pollution. Air pollution effects humans, the biosphere, atmosphere, lithosphere, animals, and plants. In humans and animals, the particle matter and other harmful chemicals in the air can make it hard to breathe, and even damage the respiratory and circulatory system. Skies can be covered in thick smog, reducing sunlight in the area, reducing the vitamin D we absorb and the energy that plants absorb. Air pollution affects the lithosphere by causing plants to die, thus not being able to start photosynthesis.

Air pollution is a major problem in today’s society, contributed to by humans and nature itself.

Analysis of Data:

The graph above represents carbon dioxide levels in Mauna Loa from 1960 to 2010. The rise in carbon dioxide in this area is likely due to a mixture of mostly volcanic activity and humans inhabiting it with modern manufacturing, vehicles, etc..

Since burning fossil fuels are the main contributor to air pollution through carbon dioxide, switching cars to hybrid or electric would be the most obvious step. If more vehicles were hybrid or electric, significantly less exhaust fumes would enter the atmosphere, thus reducing the rate of air pollution.

Most industrial factories manufacture products while burning fossil fuels. Factories could be converted to electric, or reduce the amount of fumes they let out into the air entirely with a new bill.

Although forest fires are generally natural, they emit massive amounts of carbon dioxide. Working harder to prevent forest fires without use of chemicals which ALSO pollute the atmosphere would be detrimental to fight air pollution.

Chemical sprays are an overlooked but significant portion of air pollution. These include sprays for crop-dusting, any beauty product such as hair sprays, perfume, cologne, etc.. Humans should try and use these less, and instead use a natural replacement for such purposes, or using them less in general.

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Introductory lecture: air quality in megacities

First published on 30th October 2020

Urbanization is an ongoing global phenomenon as more and more people are moving from rural to urban areas for better employment opportunities and a higher standard of living, leading to the growth of megacities, broadly defined as urban agglomeration with more than 10 million inhabitants. Intense activities in megacities induce high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence air quality in regions far from the megacity sources, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been made in improving the scientific understanding of air pollution and in developing emissions reduction technologies. However, much remains to be understood about the complex processes of atmospheric oxidation mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. While air quality has substantially improved in megacities in developed regions and some in the developing regions, many still suffer from severe air pollution. Strong regional and international collaboration in data collection and assessment will be beneficial in strengthening the capacity. This article provides an overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management in a few megacities, and the impacts on health and climate. The challenges and opportunities facing megacities due to lockdown during the COVID-19 pandemic is also discussed.

Introduction

In 1970, only 37% of the world’s population lived in urban areas; this increased to 55% by 2018 and is projected to increase to 68% by 2030, with almost 90% of the growth happening in Asia and Africa, 3,4 as shown in Fig. 1 . In 2018, about 23% of the world’s population lived in 550 cities with at least 1 million inhabitants, of which 50 cities had populations between 5 million and 10 million, and 33 cities had more than 10 million inhabitants (loosely defined as megacities). The world is projected to have 43 megacities (representing 8.8% of the global population of 8.5 billion) by 2030, with most of them located in developing countries, 3 many are facing the challenge of growing their economies and managing the environment to provide better quality of life and cleaner and breathable air for the population.

Activities from megacities are responsible for the emissions of primary pollutants, including gaseous species such as volatile organic compounds (VOCs), nitrogen oxides (NO x ), carbon monoxide (CO), sulfur dioxide (SO 2 ), ammonia (NH 3 ) and air toxins ( e.g. , benzene, 1,3-butadiene), some of which contribute to the formation of secondary pollutants such as ozone (O 3 ) and secondary aerosols; semi-volatile species ( e.g. , polycyclic aromatics, dioxins, furans); particulate matter ( e.g. , combustion soot, dust); and metals ( e.g. , lead, mercury). Despite the large amount of research that has been conducted, including air quality monitoring, field measurements and modeling studies, the sources and processes of emissions generated in megacities that lead to high concentrations of major pollutants such as ozone and secondary particulate matter, are still not well understood, thus limiting our ability to mitigate air pollution and make air quality forecasts to alert the residents of potentially unhealthy air pollution episodes.

This article will provide an overview on the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management programs in a few megacities, and the impacts on health and climate change. The final section describes the challenges and opportunities facing the megacities due to lockdown during the COVID-19 pandemic. There is a very large volume of literature articles and topics related to megacities, this article will not be able to cover all of them, but will present the main ideas and include references where more information is available.

Sources of emissions in megacities

The following sections provide an overview of some of the major sources and control strategies.

Transportation

Several megacities have implemented measures to reduce emissions from on-road vehicles, including improvement of vehicle technology and fuel quality, implementation of strict international vehicle emission standards, replacement of diesel with natural gas, and introduction of hybrid and electric vehicles. Some megacities have introduced more efficient mobility through a number of strategies, such as improving the efficiency and security of the public transportation network to encourage use of public transit, expanding infrastructure for non-motorized transportation (walking and cycling); reducing traffic congestion by limiting the circulation of vehicles ( e.g. , “No drive day” in Mexico and many megacities) (SEDEMA, http://www.aire.cdmx.gob.mx) and road pricing ( e.g. , London) (https://tfl.gov.uk/modes/driving/congestion-charge).

In the last decades, technological advances have been responsible for significant changes observed in the energy performance, fuel efficiency, and emissions reductions of vehicle fleets (https://ww2.arb.ca.gov/). Few other polluting sources have been so dramatically affected by improvements in emission control technology as motor vehicles. As a result of the technological and regulatory measures implemented, many megacities with high vehicle turnover or retrofit rates have experienced overall vehicle emission reductions despite large increases in fleet size, particularly for gasoline-powered vehicles. However, reductions of emissions from some megacities in developing countries have been more difficult to achieve, partly due to limited access to economic instruments that promote the acquisition of emission control technologies and fleet turnover programs. Reducing transport-related emissions will continue to present a challenge with growing urban population and transportation demand for both people and freight. It is important to integrate land-use planning, infrastructure development and transportation management in the design of policy options.

In addition to on-road vehicles, non-road vehicles such as agricultural and construction equipment can substantially contribute to emissions of particulate matter, CO 2 , CO, NO x and VOCs. In contrast to on-road vehicles, there is no regulation on the emissions levels for in-use non-road vehicles and they are often kept in service for several decades. Their relative emissions contributions increase over time as emissions from on-road vehicles continue to be reduced by advanced technologies. 6–8

Area sources

Municipal solid waste (MSW) is the third largest source of global anthropogenic methane emissions, generating about 800 million tons of CO 2eq annually; 10 it is also a significant source of black carbon and CO 2 . Many cities have recycling and waste separation programmes – some are converting organic waste into compost. Landfill is the most widespread method for final waste disposal, followed by capturing of the landfill gas for power generation as part of the integrated waste management program. Many megacities, with large urban populations, are host to some of the world’s largest landfills. Along with urban population growth, the amount of solid waste generated in megacities is expected to continue to increase, which will pose a major challenge for MSW management.

The use of solid fuels for cooking and heating continues to be a major source of emissions with adverse health effects. Some cities are promoting the use of cleaner fuel, such as liquefied petroleum gas (LPG). However, emissions from LPG leakage in homes, businesses and during distribution can contribute to substantial emissions of VOCs (mainly propane and butane), as shown in Mexico City. 11,12 Many cities are promoting energy efficiency programs for public and private buildings, including incentives for using renewable-energy technologies, such as solar heating systems and solar water heaters.

As the emissions of urban VOCs from transport-related sources have decreased due to technological advances and regulatory measures, volatile chemical products (VCPs) from sources such as consumer products (personal care and household products), aerosol coating, painting, solvent use and pesticides have gained in importance. A study by McDonald et al. 13 found that VCPs have emerged as the largest photochemical source of urban organic emissions, highlighting the need for regulatory actions to control the sources.

In some megacities, especially in developing regions, the agricultural sector is a large source of emissions, generated from raising domestic animals, operation of heavy-duty farming machinery, application of nitrogen based fertilizers and chemical pesticides, and burning of crop residues. Agriculture has evolved as a major source of global ammonia emissions. Enteric fermentation from ruminant livestock is one of the largest sources of methane; numerous studies are underway to mitigate the enteric methane production as well as livestock manure management. 10

Biomass burning

There are many sources and fire types related to biomass burning emissions; some are natural sources such as forest fires, while others, such as emissions from burning of crop residue, municipal solid waste, residential wood burning for cooking and heating, and biofuel for brick production, are the result of human activities. Different approaches have been used to estimate emission factors for biomass burning, including direct measurements over fires in field experiments, 18 aircraft measurements, 19–22 and laboratory measurements. 23 Andreae and Merlet 24 compiled emission factors for about 100 trace-gases and aerosols emitted from burning of savannas and grasslands, tropical forest, extratropical forest, domestic biofuel, and agricultural waste burning. The compilation was updated by Andreae 14 to include the number of species and the burning types. Akagi et al. 25 have also compiled emission factors for open and domestic biomass burning for use in atmospheric models.

In spite of the significant progress in emission factor measurements, detection and quantification of fires, there is still a need to improve the accuracy in the activity estimates, both for open burning and biofuel use. Once released, the gas and particle emissions undergo substantial chemical processing in the atmosphere. In some cases, this processing may lead to compounds that are more detrimental to human health. In the case of wildfires, some of the large number of compounds from fire smoke are not found in a typical urban atmosphere; more research is needed to better understand the chemical processes forming secondary pollutants, 26 especially as smoke plumes are transported into urban population centers. 27

Agricultural residue burning has been a common practice in many regions around the world to control pests and weeds and to prepare the land for the next crop, which releases a large amount of aerosols and trace gases to the atmosphere. 14,24,25 In some countries in South America, e.g. , Argentina and Brazil, the burning of stubble has decreased substantially due to investment in direct drilling, known as no-till, which seeds into untilled soil without removing stubble; restrictions on burning; and the use of machinery for harvesting. 10 Prescribed burning is an important forest management tool to reduce fuel loading and improve ecosystem health. Most of the burning tends to occur in the non-summer months and is a major source of combustion products to the atmosphere. Wildland fires are a natural occurrence (but often caused by arson and deliberate clearing of rain forest); they are a critical part of ecosystems. However, the area burned in recent years has markedly increased, as demonstrated by the fires in the Amazon, Indonesia, and the western USA. Several studies have documented the importance of climate change on the increasing frequency and size of fires in the western USA, especially in California. A warmer and drier climate is expected to lead to more frequent and more intense fires near or within populated areas. 27–29

Many megacities in developing countries still use biomass and fossil fuels (wood, agricultural wastes, charcoal, coal and dung) for residential and industrial cooking and heating; these are a major source of several gases and fine particles in developing countries, and in the wintertime in developed regions. 18,25,30 Another important source is open garbage burning, which occurs not only in rural, but also in urban areas, especially in cities that do not have adequate solid waste disposal facilities such as landfills. 10 In some cities, small scale brick production is an important source of urban pollution because of the burning of high polluting fuels such as wood, coal and dung, emitting significant levels of black carbon, organic carbon and other pollutants. 31–33

The chemical composition of biomass burning particles shows a wide variety of organic compounds, fragments, and functional groups, 26,34,35 in addition to the classic tracer levoglucosan. 36 Recent work shows the important contribution of secondary organic aerosol to biomass emissions. 37 Health effects of biomass burning, similar to hydrocarbon burning, has been shown to include carcinogenic compounds in varying amounts depending on fuel, burning conditions, and secondary contributions.

Atmospheric physicochemical processes in megacities

In the early 1950s, Arie Haagen-Smit and his coworkers discovered the nature and causes of Los Angeles smog: principally that a major component of smog is O 3 formed by NO x (produced by combustion sources, cars, heaters, etc. ) and VOCs (from evaporation of gasoline and solvents, etc. ) in a complex series of chemical reactions that also produce other oxidants and secondary aerosols. 43 They also reported that synthetic polluted air exposed to sunlight could cause plant damage observed by Middleton et al. 44 Since then, high O 3 levels have been observed in many urban areas throughout the world, and photochemical smog – induced primarily from transport and industrial activities – is now recognized as a major persistent environmental problem and a priority research area for atmospheric scientists.

Fig. 2 shows an overview of our current understanding of the complex physicochemical processes taking place in the atmosphere.

Gas phase chemistry

In urban environments, there are many VOC sources of anthropogenic and biogenic origin. VOC oxidation initiated by OH during the daytime produces a number of organic products of oxygenated functional groups, such as aldehyde, ketone, alcohol, carboxylic acid, hydroperoxide, percarboxylic acid, and peroxyacyl nitrate groups. 51,52 The relative abundance of these products depends on the VOC structure, the NO x level, temperature, relative humidity (RH), and the solar intensity. Some compounds formed in the first oxidation step undergo additional oxidation reactions to yield multi-functional groups, and the resulting multi-generations of products are of lower volatility and higher solubility in comparison with their parent compounds.

NO x plays an important role in determining the fate of peroxy radical intermediates (HO 2 and RO 2 ). Under relatively clean (low-NO) conditions, peroxy radicals will react with other peroxy radicals or (in the case of RO 2 ) will isomerize; under polluted urban conditions, peroxy radicals will react with NO, forming NO 2 , which rapidly photolyzes in the daytime, producing O 3 . 53 However, the ambient levels of OH, HO 2 , and RO 2 radicals not only depend on NO x but also simultaneously on the VOC abundance and VOC reactivity. This dual NO x and VOC–reactivity dependency ultimately controls the chemical regimes of O 3 production. Therefore, successful emission control policies strongly depend on determining the chemical regimes of O 3 production.

The sensitivity of O 3 production to changes in concentrations of precursor VOCs and NO x is complex and nonlinear. 53–55 Under high VOC concentrations and low NO x concentrations, O 3 production rates increase with increasing NO concentrations (NO x limited) due to increases of NO 2 through reactions of NO and peroxy radicals. But at higher NO x , O 3 production rates decrease with increasing NO x (NO x saturated) due to OH and NO x termination reactions that form HNO 3 and alkyl nitrates. The additional NO x also serves as a sink for OH radicals, slowing down the oxidation of VOCs and suppressing O 3 production. NO x can also sequester O 3 in temporary reservoirs such as NO 2 and N 2 O 5 ; in these conditions, lower NO x emissions can lead to higher O 3 concentrations. This result could suggest that O 3 production in polluted urban areas, as is the case in many megacities, may be in the NO x saturated regime. 56–59 However, recent studies indicate that O 3 production can have marked spatially different chemical regimes within megacities due to the heterogeneous distribution of VOC and NO x sources, VOC reactivity, and meteorological conditions. 60,61

During the night-time, the oxidation of NO via O 3 and organic radicals has two main effects: it depletes night-time O 3 levels and accumulates NO 2 and NO 3 radicals that subsequently form N 2 O 5 and HNO 3 through heterogeneous reactions. 62 This condition increases the early morning NO 2 /NO ratios and affects O 3 production during the next day by increasing the contribution of excited oxygen atoms via NO 2 photolysis. The accumulated NO 2 and nitrate can also form HONO through surface-catalyzed reactions, 63 further impacting the accumulation of free radicals.

Particulate matter

The abundance and chemical constituents of PM 2.5 vary considerably in urban cities, depending on the complex interplay between meteorology, emissions, and chemical processes. 67,68 Fig. 3 shows the average measured chemical composition of submicron PM (PM 1 ), which typically comprises most of the PM 2.5 for various megacities, urban areas, and outflow regions around the world. 69 A substantial fraction of urban PM 1 is organic aerosol (OA), which is composed of primary OA (POA, organic compounds emitted directly in the particle phase) and secondary OA (SOA, formed from chemical reactions of precursor organic gases). SOA is typically a factor of 2 to 3 higher than POA for these locations.

More recently, ultrafine particles (UFP, particles with diameter 0.1 μm or less) have become increasingly important in urban air because they are produced predominantly from local combustion processes with major contributions from vehicular exhaust and new particle formation (NPF) in cities. 70–73 Particles that are smaller than 1 μm have both longer lifetimes and higher probability of penetration into alveolar sacs in the lungs, and even smaller “nanoparticles” (<100 nm in diameter) have been shown to have some of the most toxic exposures. Recent evidence suggests that nanoparticles and transition metals, which are also associated with fossil fuel combustion, may play an important role. 74–79

Currently, particle mass concentration has been used for regulatory air quality standards. However, this metric accounts mainly for larger particles with larger mass, while particle number concentration (PNC) has been used as a metric for UFP, which are smaller with little mass. de Jesus et al. 80 evaluated the hourly average PNC and PM 2.5 from 10 cities over a 12 month period and observed a relatively weak relationship between the two metrics, suggesting that control measures aiming to reduce PM 2.5 do not necessarily reduce PNC. It is important to monitor both PM 2.5 and UFP for health impact assessment.

For developing effective pollution control strategies and exposure risk assessment, it is necessary to know the contribution of the various sources of pollutants. Several techniques have been used in source apportionment studies of PM, including chemical mass balance (CMB) and positive matrix factorization (PMF) analysis on filter-based chemical speciation data, carbon mass balance modeling of filter-based radiocarbon ( 14 C) data, aerosol mass spectrometry or aerosol chemical speciation monitoring coupled with PMF. While significant progress has been made in evaluating the sources of pollutants, some sources remain poorly characterized, such as food cooking and open trash burning (see e.g. , Molina et al. 81 for Mexico City).

Pandis et al. 82 investigated the PM pollution in five cities (Athens, Paris, Pittsburg, Los Angeles and Mexico City) and found that reductions of emissions from industrial and transportation related sources have led to significant improvements in air quality in all five cities; however, other sources such as cooking, residential and agricultural biomass burning contribute an increasing share of the PM concentrations. These changes highlight the importance of secondary PM and the role of atmospheric chemical processes, which complicate the source apportionment analysis. Xu et al. (DOI: 10.1039/D0FD00095G ) evaluated the fine OC and PM 2.5 in Beijing using different methods (CMB, PMF and AMS/ACSM-PMF) and found that the fine particles were mainly secondary inorganic aerosols, primary coal combustion and biomass burning emissions. Although there are some consistencies, modeled contributions for several sources differed significantly between the different methods, particularly for cooking aerosols.

New particle formation

Extensive efforts have been made to elucidate the fundamental mechanism relevant to atmospheric NPF from field measurements, laboratory experiments, and theoretical calculations. Previous field studies include measurements of ultrafine particles down to approximately 1 nm in size, gaseous concentrations of nucleating precursors (such as H 2 SO 4 , NH 3 , and amines), and pre-nucleation clusters. 95,96 Numerous laboratory experiments have been conducted to understand aerosol nucleation. 92,97–99 In addition, theoretical investigations of aerosol nucleation have been carried out to determine the stability and dynamics of pre-nucleation clusters using thermodynamic data from quantum chemical calculations. 100–102

NPF events occur with a frequency of 50, 20, 35, and 45% in spring, summer, fall, and winter, respectively, in Beijing. 102–104 NPF events have been occasionally measured in Houston during several campaigns. 105,106 In addition to the correlation with elevated SO 2 , 107 the contribution of secondary condensable organics to NPF is implicated in Houston. 108 On the other hand, NPF events are rarely measured in Los Angeles. 109 One plausible explanation is that the heavy accumulation of pre-existing particles and low levels of SO 2 lead to unfavorable conditions for aerosol nucleation in the Los Angeles basin. 110 NPF events are frequently observed during field campaigns in Mexico City 81,111,112 and are usually accompanied with a high level of SO 2 , 113 indicating that the oxidation of SO 2 contributes to the formation and growth of freshly nucleated particles. The polluted layer substantially ventilated from the Mexico City basin represents another potential factor in driving NPF in the afternoon, which is characterized by a decrease in pre-existing particle concentrations preceding the NPF events. 114

A recent study shows the striking formation of NPF in urban air by combining ambient and chamber measurements. 91 By replicating the ambient conditions ( i.e. , temperature, relative humidity, sunlight, and the types and abundance of chemical species), the existing particles, photochemistry, and synergy of multi-pollutants play a key role in NPF. In particular, NPF is dependent on preexisting particles and photochemistry, both of which impact the formation and growth rates of freshly nucleated nanoparticles. Synergetic photooxidation of vehicular exhaust provides abundant precursors, and organics, rather than sulfuric acid or base species, dominating NPF in the urban environment.

Another laboratory chamber study by Wang et al. 115 reported that airborne particles can grow rapidly through the condensation of ammonium nitrate (NH 4 NO 3 ) under conditions typical of many urban environments in wintertime, such as Beijing and Delhi. NH 4 NO 3 exists in a temperature-dependent equilibrium with gaseous NH 3 and HNO 3 , but NH 4 NO 3 can quickly condense onto newly formed clusters at temperatures below 5 °C, allowing the clusters to reach stable particle sizes before they are scavenged by other existing particles in the atmosphere. Moreover, at temperatures below −15 °C, NH 3 and HNO 3 can nucleate directly to form NH 4 NO 3 particles. The formation of new particles through NH 4 NO 3 condensation could become increasingly important as the SO 2 emissions continue to reduce due to pollution controls implemented in many cities. This may in turn imply the importance of controlling NO x and NH 3 emissions.

Based on the observations in Beijing over a period 14 months, Kulmala et al. (DOI: 10.1039/D0FD00078G ) found that almost all present-day haze episodes in Beijing originate from NPF, suggesting that air quality can be improved by reducing the gas phase precursors for NPF, such as dimethyl amine, NH 3 and further reductions of SO 2 emissions, as well as anthropogenic organic and inorganic gas-phase precursor emissions.

Secondary organic aerosol

Atmospheric models typically underestimate the SOA mass measured in field studies if only traditional SOA precursors are considered. 122,129–131 Inclusion of non-traditional SOA precursors, such as organic gases from POA evaporation and di-carbonyls, has helped to bring better closure between models and observations. 132–136 However, there are still inconsistencies between modeled and measured SOA yields, which can be explained by several factors, including incorrect emission inventories, missing precursors, and unaccounted processes of gas-to-particle conversion. 13,68

Nault et al. 69 investigated the production of anthropogenic SOA (ASOA) in urban areas across three continents (see Fig. 3 ) and observed that it is strongly correlated with the reactivity of specific VOCs; the differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds (IVOC and SVOC) influence the ASOA production across different cities. Emissions from fossil fuel sources ( e.g. , gasoline, diesel, kerosene, etc. ) and volatile chemical products (VCPs, such as personal care products, cleaning agents, coatings, etc. ) contribute nearly similar amounts to estimated ASOA, further supporting the important role of VCPs in urban air quality. 13

Sulfate formation

The aqueous-phase conversion of dissolved SO 2 to sulfate driven by O 3 and hydrogen peroxide (H 2 O 2 ) is an important chemical formation pathway in cloud/fog water, but the two SO 2 oxidation pathways still cannot close the gap between field observations and modeling studies. 139 Aqueous SO 2 oxidation by O 2 catalyzed by transition metal ions (TMI) in models has improved sulfate simulations, 140 and recent studies have further revealed the enhanced effect of TMI during in-cloud oxidation of SO 2 ( ref. 141 ). However, during wintertime haze days free of cloud or fog in North China, rapid sulfate production has been observed 142,143 showing that the sulfate formation mechanism is still not well understood.

A laboratory/field study of wintertime haze events in Beijing and Xi’an has indicated that the aqueous oxidation of SO 2 by NO 2 is key to efficient sulfate formation under the conditions of high RH and NH 3 neutralization, 143,144 Li et al. 145 proposed a SO 2 heterogeneous formation pathway, in which the SO 2 oxidation in aerosol water by O 2 catalyzed by Fe 3+ , is limited by mass resistance in the gas-phase and gas–particle interface, and closes the gap between model and observation. A recent experimental study has highlighted that the oxidation of SO 2 by H 2 O 2 in hygroscopic, pH-buffered aerosol particles occurs more efficiently than under cloud water conditions, because of high solute strength. 146 Furthermore, another recent study 147 has unraveled a novel sulfate formation mechanism, showing that SO 2 oxidation is efficiently catalyzed by black carbon (BC) in the presence of NO 2 and NH 3 , even at low SO 2 levels (down to a few ppb) and an intermediate RH range (30–70%). The sulfate formation mechanism during wintertime haze days in China is still controversial considering the uncertainties of the aerosol pH value, rather low oxidants level, and possible loss of active sites in BC.

Nitrate formation

Aerosol radiative effect.

Aerosol radiation interaction has also significantly contributed to the PM pollution during haze days. 68 It is well established that ARI cools the surface but heats the air aloft, increases the atmospheric stability, enhances accumulation and formation of PM 2.5 in the planetary boundary layer (PBL), and eventually deteriorates the air quality during haze days. 155–157 Wu et al. 157 have revealed that the ARI contribution to near-surface PM 2.5 concentrations increased from 12% to 20% when PM 2.5 concentrations increased from 250 to 500 μg m −3 during a persistent and severe PM pollution episode in the North China Plain. However, modification of photolysis caused by aerosol absorbing and/or scattering solar radiation (referred to as aerosol–photolysis interaction or API) changes the atmospheric oxidizing capability and influences secondary aerosol formation. Coatings also affect the ratio of absorption to scattering, and these control changes in radiative forcing. 158 Simulations have revealed that API hinders secondary aerosol formation and substantially mitigates the PM pollution caused by ARI. 159

It is worth noting that ARI or API is highly sensitive to the single scattering albedo (SSA) that is dependent on aerosol composition, particularly regarding absorbing aerosols including BC and brown carbon (BrC). 159 Primary BC and BrC aerosols undergo chemical transformation in the atmosphere, by coating with organic and inorganic constituents, commonly referred to as the aging process. 158 Aging of primary aerosols not only changes the particle mixing state ( i.e. , from externally to internally), but also alters the particle properties, including the morphology, hygroscopicity, and optical properties, further enhancing aerosol absorption capability. 117,160–162

Air quality management in megacities

Since 1987, the WHO has produced air quality guidelines designed to inform policy makers and to provide appropriate targets in reducing the impacts of air pollution on public health. Currently, many countries have established ambient air quality standards to protect the public from exposure to harmful level of air pollutants and are an important component of national risk management and environmental policies. National standards vary according to the approach adopted for balancing health risks, technological feasibility, economic, political and social considerations, as well as national capability in air quality management. Some countries set additional standards for lead and CO. Table 1 presents the current air quality standards for O 3 , PM 10 , PM 2.5 , SO 2 , CO, Pb and NO 2 for China, India, Mexico and the USA, together with the WHO guidelines. 163

The WHO estimated that about 90% of the world’s population breathe polluted air, many of the world’s megacities exceed WHO’s guideline levels for air quality by more than 5 times. 164 Fig. 4 shows the annual average of PM 2.5 for a three-year period (2017, 2018, and 2019) for the megacities where data is available; this includes London, Seoul and Chengdu.

The data were compiled by IQAir 165 from real-time, hourly data from government monitoring stations, validated PM 2.5 monitors operated by private individuals and organizations. Ideally, the monitoring data used to calculate the average annual PM concentrations should be collected throughout the year, for several years, to reduce bias owing to seasonal fluctuations or to a non-representative year. However, data for most cities are not available for trend analysis. Some of the megacities were not included in the report; the data were taken from WHO 166 for a single year reported in 2015 or 2016.

The PM 2.5 levels for all the megacities shown in Fig. 4 , with the exception of New York, are above the WHO guideline value of 10 μg m −3 . The megacities with the highest PM 2.5 concentrations are located in South Asia; however, comparison of the three-year data show reduction in the PM 2.5 levels in the cities from 2018 to 2019. Much of this can be attributed to increased monitoring data, economic slowdown, favorable meteorological conditions and government actions. For example, 2019 marked the launch of India’s first National Clean Air Program, which set PM 2.5 and PM 10 targets and outlined new strategies for tackling air pollution. However, India still has a relatively limited air quality monitoring network, with many communities lacking access to real-time information. 167

The data shown in Fig. 4 are the annual average; however, there is a large seasonal variation for some cities (Delhi, Lahore, Dhaka, Kolkata), as shown in Fig. 5 , due to geographical location and prevailing meteorology. The PM 2.5 concentrations are the highest in November to January, and the lowest from July to September, as monsoon rains wash out airborne particulates, leading to cleaner air. During the winter, emissions from residential heating, burning of crop residues, and intensive brick production lead to higher PM 2.5 concentrations in these cities. The landlocked geography of Delhi and the coastal location of Mumbai influence the distribution of air pollutants in the two cities. 168

Lahore ranks as one of the megacities with the highest annual PM 2.5 concentrations, weighted by city population. Until recently, there was no government monitoring in Pakistan. The data provided in the IQAir 165 report (2019) comes from low-cost sensors operated by individuals and non-governmental organizations. Recently the Pakistan government cited air pollution as a key priority and has reinstated the monitoring infrastructure in Lahore. Current anti-smog measures include stricter emission standards on factories and penalties for high-polluting vehicles and farmers burning crop stubble.

Although more countries are taking action and more cities are now included in the air quality database, there are still many cities that do not have ambient monitoring and their residents do not have access to air quality information where pollution levels may be high. For example, South America is the most urbanized region of the world; five of the megacities are located in this continent: Bogotá (Colombia), Buenos Aires (Argentina), Rio de Janeiro (Brazil), São Paulo (Brazil) and the metropolitan area of Lima-Callao (Peru). Recently, Gómez Peláez et al. 169 reviewed the air quality trends of the criteria pollutants collected by the automatic monitoring networks of 11 metropolitan areas in South America, including four megacities (Rio de Janeiro, São Paulo, Buenos Aires, and Lima). Despite concerted efforts to monitor air quality, the data provided by environmental authorities in some cities are of poor quality, making it difficult to assess the air quality trends and take action for critical air pollution episodes. Integration of the emission from the whole continent and their application in an air quality model are essential to investigate the effect of long-range transport and to construct air quality and emission control strategies for the entire region. Integrated coordination due to transboundary pollution transport, mainly from the biomass burning in the Amazon basin, is essential, especially considering the record-breaking number of Amazon fires in 2019 and again in 2020. Analysis of an aerosol particles’ chemical composition and optical properties during the biomass burning season in 2014 showed that, depending on the wind direction, smoke plumes from central Brazil and southern regions of the Amazon basin can be transported over São Paulo. 170

In February 2020, the United Nations Environment Programme (UNEP), together with the UN-Habitat and IQAir, launched the world’s largest air quality platform, bringing together real-time air pollution data from over 4000 contributors, including governments, citizens, communities, and private sectors. 171 This partnership covers more than 7000 cities worldwide and aims to empower governments to take action to improve air quality, allowing citizens to make informed health choices, and businesses to make investment decisions promoting a cleaner and greener environment.

The following describe the air quality trends and air quality management programs for Los Angeles, the Mexico City metropolitan area and four Chinese megacities. While the differences in the governance, economics, and culture of the megacities greatly influence the decision-making process, all have overcome severe air pollution and have made significant progress in reducing concentrations of harmful pollutants by implementing comprehensive integrated air quality management programs. The experience can be valuable for other megacities.

Air quality in the Los Angeles basin

Following the recognition of Los Angeles photochemical smog as a severe environmental problem in the 1940s, comprehensive emissions control efforts have been implemented by the air quality management authorities, the California Air Resources Board (CARB) and the South Coast Air Quality Management District (SCAQMD) principally, particularly in the transportation sector, which plays a major role in the air pollution problem. The urban center is decentralized; major commercial, financial and cultural institutions are geographically dispersed, relying on a vast network of interconnected freeways. The emissions control measures included the introduction of unleaded gasoline and an eventual complete ban of lead in gasoline, three-way catalytic converters, stringent NO x control for ozone and PM 2.5 , low-sulfur fuels, and diesel particle filters. Other regulations such as controls on power plants and boilers have reduced smog-forming oxides of nitrogen emissions, rules on consumer products such as paints and solvents have limited volatile organic compounds, and other controls on gasoline components, chrome platers, dry cleaners, and other sources have reduced levels of airborne toxics. 6,172

Other emission sources include goods movement sources, such as railroads, ocean-going vessels, commercial harbor craft, cargo handling equipment, drayage trucks, and transport refrigeration units. California adopted the first-in-the-nation regulation requiring ocean-going vessels to use cleaner fuel when near the California coast in 2008, which has been effective in reducing SO 2 emission from ships. 173 Emissions from ports have also been reduced by making shore power available to docked ships that previously idled their engines, while the more polluting drayage trucks are either removed from service or retrofitted. 6 The Advanced Clean Car Regulation (https://ww2.arb.ca.gov/our-work/programs/advanced-clean-cars-program) is the latest of a series of technology-forcing standards aimed at limiting passenger vehicle emissions and reducing smog as well as mitigating climate change. 174 As a result of the stringent emissions reduction measures, peak ozone levels and PM 2.5 concentrations in Los Angeles today are about one third of their level in 1970. Nevertheless, the ozone concentration is frequently still above the current USA ambient 8 h ozone standard of 70 ppb (see Fig. 6 ).

One of the main challenges is that a substantial fraction of the ozone in Southern California is transported into the region from outside its border, which is not subject to local control. This includes the baseline ozone concentrations, which are not affected by continental influences, such as ozone transported from the Pacific 175 and the background ozone (the ozone concentration that would be present if anthropogenic precursor emissions were reduced to zero), which are affected by continental influences such as deposition to continental surfaces, vegetation, production from natural ozone precursors ( e.g. from trees, soils and lightning). 176 This could be as high as 89% of the USA NAAQS (62.0 ± 1.9 ppb) and that about 35 years of additional emission control efforts will be needed to meet the NAAQS.

Altuwayjiri et al. (DOI: 10.1039/D0FD00074D ) investigated the long-term variations in the contribution of emission sources to ambient PM 2.5 organic carbon (OC) in the Los Angeles basin and the effect of the regulations targeted tailpipe emissions during 2005–2015. They found a significant reduction in the absolute and relative contribution of tailpipe emissions to the ambient OC level, while the relative contribution of non-tailpipe emissions (road dust resuspension, tire dust, and brake wear particles) increases over the same period, suggesting that the regulations were effective but also underscore the importance of regulating non-tailpipe emissions.

Recent wildfires in California have markedly increased, worsening air quality in much of the region. A warmer and drier climate is expected to lead to more frequent and more intense fires near or within the populated areas, threatening to undo the significant improvement in air quality after decades of implementing the Clean Air Act. 27–29 Long-term monitoring and reevaluation of forest management strategies will be needed to address the wildfire problem as climate change continues to bring hotter and drier conditions conducive to wildfire activity. 177

Air quality in the Mexico City metropolitan area

Field measurements studies conducted in the MCMA during MCMA-2003 ( ref. 112 ) and MILAGRO-2006 ( ref. 81 ) showed that ozone formation was generally VOC-limited within the urban core, while mostly NO x -limited in the surrounding area depending on the prevailing meteorology, 56–58 and that O 3 production might continue in the outflow for several days due to the formation of peroxyacetyl nitrate (PAN), which could regenerate NO x and contribute to regional O 3 formation. 181 A recent study by Zavala et al. 60 shows that there is an overall reduction in the VOC–OH reactivity during the morning hours in the urban area with large spatial variability, implying a large spatial variability in O 3 production, which in turns suggests spatially different O 3 sensitivity regimes to precursor gases. While alkanes (from leakage and unburned LPG used for cooking and water heating) are still key contributors to VOC–OH reactivity, the contribution from aromatic and alkene species has decreased, consistent with reduction of VOCs from mobile sources. Changes in ozone production suggest that increases in the relative contributions from highly oxygenated volatile chemical products, such as consumer and personal care and solvent use, are responsible for sustained high O 3 levels in recent years. The study also found a significant increase in NO 2 /NO ratios, suggesting changes in the night-time accumulation of radicals that could impact the morning photochemistry. The results suggest a need for a new field study of radical budgets in the MCMA, expanding measurements of VOCs to include oxygenated species, and using the data to support modeling studies in the design of new air quality improvement programs.

The MCMA faces additional challenges from regional contributions. Urban expansion of the MCMA has produced the megalopolis, consisting of Mexico City and contiguous municipalities of six surrounding states. The Megalopolis Environmental Commission (CAMe, https://www.gob.mx/comisionambiental) was created in 2013 to coordinate the regional policies and programs; however, the different administrative and legislative jurisdictions and the available resources have created an ongoing challenge. With the exception of the MCMA, there is limited air quality monitoring and air pollution studies in the other states, making it difficult to evaluate the regional air quality and the impacts of pollutants in the region. 9

Burning of regional biomass is a major contributor of fine particles to the MCMA during air pollution episodes (http://www.aire.cdmx.gob.mx). During the dry season, agricultural and forest fires in the surrounding areas are frequent, the wind transports air masses enriched with organic aerosols, VOCs, as well as other reactive gases to the MCMA, severely impacting the air quality. 20 Lei et al. 182 evaluated the impact of biomass burning in the MCMA and found that biomass burning contributed significantly to primary organic aerosol (POA), secondary organic aerosol (SOA), and elemental carbon (EC) locally and regionally but has relatively little effect on O 3 . Despite the important contribution of biomass burning to local and regional air quality, the authorities did not include fire mitigation in the air quality management strategies. In May 2019, following a severe air pollution episode caused by regional wildfires, the authorities announced new action contingencies, adding PM 2.5 threshold level, in addition to O 3 and PM 10 , to the contingency plan.

Air quality in Chinese megacities

As a global hub for manufacturing, the heavy industries, including production of iron and steel, other nonmetal materials and chemical products, play an important role in the Chinese economy, resulting in huge consumption of energy and large emissions of air pollutants. In addition, more and more people are moving from rural to urban areas, leading to a fast expansion of cities and an increasing demand for vehicles, contributing to severe air pollution.

Faced with increasing pressure on the environment in urban development, the Chinese government launched the Action Plan for Air Pollution Prevention and Control (Action Plan) in September 2013 (http://www.gov.cn/zwgk/2013-09/12/content_2486773.htm), which stated the development targets and roadmap for 2013–2017. The Action Plan provides the framework for air pollution control measures in cities, covering capacity building, emission reduction measures and supporting measures. The implementation of a series of control measures, including coal combustion pollution control, vehicle emission control and VOCs control, have resulted in the reduction of most pollutants and a large decrease in PM 2.5 concentrations. 167 However, according to a government report, 183 74.3% of 74 key cities exceeded the NAAQS of annual mean PM 2.5 concentrations (35 μg m −3 ) in 2017.

As shown in Fig. 7 , while the concentration of most pollutants have decreased for each city, O 3 was not effectively controlled (red line). Also, the annual mean PM 2.5 concentrations still exceeded the NAAQS of China, except Shenzhen, the first city that met the PM 2.5 standard. The exceeding of the PM daily average concentration often occurred during the cold winter from November to February, while the maximum daily 8 h average concentration of O 3 is more likely to exceed in the summer during the afternoon, according to the local monitoring data (China National Environmental Monitoring Center, http://www.cnemc.cn/en/).

Although the major sources of emissions differ among the four cities, in general, vehicle emissions remain the primary source of air pollution and contribute significantly to VOC, NO x , CO, and PM 2.5 (including BC). The primary source of SO 2 is fossil fuel combustion from industry and mobile vehicles; while road and construction dust is the main source of PM 10 , and agriculture is the primary source of NH 3 .

Vehicle emission control has been a priority of air quality management, and the cities have continuously tightened emission standards for new gasoline and diesel vehicles; promoting the use of electric vehicles through subsidies. 167 Shenzhen is the first megacity in China and in the world to adopt electric vehicles for the entire public transportation system. However, as a coastal city, diesel trucks carrying large amounts of cargo is the primary source of local emissions and ocean-going vessels, contributing a large portion of SO 2 due to the use of low-quality heavy fuel oil. Emission control policies for the port area and the ocean-going vessels are areas also being implemented. In addition to local sources, pollutants transported from the outskirts have contributed to the pollution levels of the cities. 167

Although the PM levels have decreased significantly due to the stringent measures implemented by the government authorities, 184–186 summertime O 3 concentrations have increased in the megacity clusters in China. 187,188 Several studies have investigated the anthropogenic and meteorological factors that are responsible for the O 3 pollution in China. 189–194 Flat VOCs emissions and reduced NO x emissions have slightly increased the O 3 concentration in most urban areas of eastern China. A significant anthropogenic driver for the O 3 enhancement is the over 40% reduction of PM 2.5 in the North China Plain (NCP), which slows down the aerosol sink of hydroperoxyl radicals, thus stimulates the O 3 production. 61,193 However, meteorological influences have been thought to be comparable to or even more important than the impact of changes in anthropogenic emissions. Increased solar radiation reaching the surface level due to the decrease of cloud cover, cloud optical thickness as well as the aerosol optical depth has promoted photochemical reactions and resulted in O 3 enhancement. Higher temperature, as a result of enhanced solar radiation, has been recognized as an important factor corresponding to the increasingly serious O 3 pollution for enhancing biogenic emissions and decreasing O 3 dry deposition. 195

The dominant cause of increasing O 3 due to changes in anthropogenic emissions was found to vary geographically. In Beijing, NO x and PM emission reductions were the two main causes of O 3 increase; in Shanghai, NO x reduction and VOC increase were the major causes; in Guangzhou, NO x reduction was the primary cause; and in Chengdu, the PM and SO 2 emission reductions contributed most to the O 3 increase. 195,196 While NO x reduction in recent years has helped to contain the total O 3 production in China, VOC emission controls should be added to the current NO x –SO 2 –PM policy in order to reduce O 3 levels in major urban and industrial areas.

In addition to O 3 , there have been several extreme haze events in China during wintertime in recent years, as a consequence of diverse, high emissions of primary pollutants ( e.g. , from residential heating) and efficient production of secondary pollutants. 68,197–201 In particular, the North China Plain (Beijing–Tianjin–Hebei) and Chengdu–Chongqing region have suffered from severe haze pollution. 68,197,198 Unfavorable meteorological conditions enhancing the air static stability and shallow planetary boundary layer due to aerosol–radiation and aerosol–cloud interactions, could also aggravate severe haze formation. 68,157,159

Atmospheric NH 3 plays an important role in fine particle pollution, acid rain, and nitrogen deposition. In contrast to those in developed countries, agricultural NH 3 emissions largely overlap with the industrial emissions of SO 2 and NO 2 in northern China. A model study showed that the average contribution of the agricultural NH 3 emissions in the NCP was ∼30% of the PM 2.5 mass during a severe haze event in December 2015. 68 Control of NH 3 would mitigate PM pollution and nitrogen deposition. However, another study 202 found that NH 3 control would significantly enhance acid rain pollution and offset the benefit from reducing PM pollution and nitrogen deposition.

The examples of the four Chinese and two North American megacities illustrate the complexity of managing urban pollution. In spite of significant progress in cleaning the air, there are still remaining challenges. While each city has its own unique circumstances – geographical location, meteorology, emission sources, financial and human resources, the need for an integrated, multidisciplinary assessment of the complex urban air pollution problem is the same. In light of the multicomponent nature of air pollution, application of integrated control strategies that address multiple pollutants, supported by ambient monitoring, emissions characterization, air quality modeling, and comprehensive rather than separate strategies for each single pollutant, would be more cost-effective. 203

Impacts of degraded air quality in megacities

Health effects.

Over the past few decades, data on air quality has become increasingly available and the science underlying the related health impacts is also evolving rapidly. Effects of air pollution on human health have been investigated with epidemiology, animal studies, and human exposure studies. Populations at greater risk include children and the elderly and those that have pre-existing conditions such as diabetes, or cardiovascular and respiratory diseases. While many countries have established air quality standards for criteria pollutants, or follow WHO guidelines ( Table 1 ), there is an ongoing debate as to the maximum permissible limit of a particular pollutant concentration. As more information becomes available, the standards have been strengthened to protect public health.

It is evidenced from epidemiological and clinical studies that exposure to particulate matter, especially PM 2.5 , is linked to cardiorespiratory disease and adverse birth outcomes. 208–211 Although there is a large volume of research on the adverse effects of PM exposure, composition of the particles and the mechanisms causing such association are still not well understood. The physicochemical characteristics of PM vary according to emission sources, secondary atmospheric chemical reactions and meteorological conditions. Other factors can also affect the toxicity of PM, such as the metal content of the particles and their reactivity. For example, some physiological studies of health effects have shown that the causes of cell degradation from exposure to fine particles are most likely from specific toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs) and black carbon.

More recently, ultrafine particles were found to possibly exert higher toxicity than larger particles due to their small size; they generally enter the body through the lungs but are translocated to essentially all organs. 212,213 Nanoparticles and transition metals, which are also associated with fossil fuel combustion, may also play an important role. 74–79 Although exposure to UFP is commonly attributed to vehicular exhaust, monitoring in Ghana showed higher exposure from trash burning and domestic cooking. 213

Several approaches have been used to elucidate the mechanism of toxicity, one is the use of in vivo experimental models to evaluate the effects of PM on the respiratory, cardiovascular and nervous system, another one is in vitro models, which has proven useful for investigating mechanistic responses, such as inflammatory/immune alterations and genotoxicity. 214 Besides the well-documented impacts on respiratory and cardiovascular health, the evidence is accumulating around exposures during pregnancy and adverse birth outcomes, cancer, brain alterations and interactions between infectious agents and air pollution. 215,216 H. Bové et al. 217 reported the presence of black carbon particles as part of combustion-derived PM in human placenta, suggesting that ambient particulates could be transported towards the fetus, representing a potential mechanism for the adverse health effects of pollution from early life onwards.

Many studies have investigated the association between oxidative potential of air pollutants with adverse health outcomes; however, there are some contradictory results. For example, Quintana et al. 218 reported the oxidative potential correlated with PM 10 Cu/Zn content but not with the in vitro biological effects from samples collected in Mexico City during the MILAGRO field campaign. Weichenthal et al. 219 examined the relationship between PM 2.5 oxidation burden and cause-specific mortality; the results suggest that glutathione-related oxidative burden may be more strongly associated with lung cancer than the mass concentrations. Strak et al. 220 examined the role of particle size, composition and oxidative potential; the results suggest that changes in particle number concentrations (PNC), NO 2 and NO x were associated with acute airway inflammation and impaired lung function, while PM mass concentration and PM 10 oxidative potential were not predictive. A study conducted in central London also indicated the association of PNC with cardiovascular effects, while non-primary PM components (nitrate, sulfate, chloride and organic carbon) were associated with adverse respiratory outcomes. 221

Results from the various studies and epidemiological evidence suggest that each megacity will have contributing factors that create different air pollution impacts on health, 222 among those could be specific chemical mixtures in the atmosphere, meteorology, socioeconomic conditions/disparities. An important research topic is the health impact related to exposure from smoke from biomass burning (wildfires, burning of agricultural residues and trash, etc. ). PM 2.5 , O 3 , and other compounds in smoke have clearly demonstrated human health impacts; however, the episodic nature of smoke exposure and the large and variable mix of compounds make health studies even more challenging than traditional air pollution episodes. It is important to better understand the long-term consequences, such as birth outcomes, neurological and cognitive effects, and progression and incidence of chronic disease related to smoke exposure and to establish exposure guidelines. 27

To estimate the health impact to be expected from measures affecting air quality, it is important to conduct health risk assessment. An important step is the exposure–response function, such as the exposure–mortality model (EMM) which is based on total concentration of PM 2.5 and does not consider the unequal toxicity of different components of PM 2.5 . Xue et al. (DOI: 10.1039/D0FD00093K ) developed a component-specific EMM (CS-EMM) using the census data, the concentration of ambient PM 2.5 and satellite-based concentrations simulated by a chemical transport model. The CS-EMM was found to perform better than the EMM. Among the components, although BC contributed only 6.4% of PM 2.5 , it corresponded to a 46.7% increase in PM 2.5 -associated deaths. This new approach will allow policy makers to target the toxic source of air pollution and design cost-effective control strategies.

Recently Apte et al. 66 estimated the population-weighted median decrement in life expectancy from PM 2.5 (ΔLE). If PM 2.5 concentrations worldwide were limited to the WHO air quality guideline concentration of 10 μg m −3 , global life expectancy would be on average 0.59 years longer. This benefit would be especially large in countries with the highest current levels of pollution, with approximately 0.8–1.4 years of additional survival in countries such as Egypt, India, Pakistan, Bangladesh, China, and Nigeria. In contrast, many high income countries already nearly meet the WHO guideline and would have much smaller LE benefits; for example, the ΔLE of 0.38 years for the USA is about 3 times lower than that of countries with higher PM 2.5 concentrations. The result of this study illustrates that reducing air pollution at all levels of economic development could lead to substantial gains in life expectancy, a benefit similar in magnitude to that of eradicating lung and breast cancer.

Regional and global climate impacts

The contributions of megacities to global anthropogenic emissions have been estimated to be 12%, 7% and 4% for CO 2 , CH 4 and N 2 O respectively for the base year (2005), and are projected to increase significantly in 2050, 224 while the contribution to BC, OC, CO, NO x and SO 2 are relatively small (3 to 5%). With the exception of CO 2 , all the estimated emissions are disproportionately smaller compared to the population. This could be due in part to some of the energy production taking place outside the cities. However, there is a large uncertainty in estimating the emissions and their geographic distributions; further research is needed to better understand the role of megacities in the Earth’s environment.

Recently, short-lived climate forcers (SLCFs), also known as short-lived climate pollutants (SLCPs), have received increasing attention due to their relatively short residence time in the atmosphere and the multiple benefits of reducing them using existing available technologies. 10,225–227 The major SLCFs with lifetimes under a few decades are BC (∼days to weeks), CH 4 (∼a decade), tropospheric O 3 (weeks to months) and some hydrofluorocarbons (HFCs, average 15 years). Due to their nature, these substances can be rapidly controlled, providing near-time climate benefits and air quality improvement. It is important to emphasize that despite these near-term benefits, reducing warming in the longer term will also require action now to reduce current and future CO 2 emissions.

Anthropogenic CH 4 is emitted into the atmosphere from ruminant livestock, rice cultivation, microbial waste processing (landfills, manure, and waste water), coal mining, and oil and natural gas systems. Methane has about 34 times the Global Warming Potential (GWP) of CO 2 (100 year horizon). Due to its shorter lifetime, it is even more effective over a 20 year time horizon. Methane is included as one of the six greenhouse gases (CO 2 , CH 4 , N 2 O, HFCs, perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6 )) controlled under the Kyoto Protocol. Black carbon is emitted directly into the atmosphere in the form of PM 2.5 ; from diesel engines, industrial sources, residential coal and solid biofuels for cooking and heating, and agricultural and forest fires and open burning of solid waste. Black carbon could be the second largest contributor to global warming after CO 2 . 228–229 Although there are large uncertainties about the magnitude of BC climate impacts, it is very likely that mitigating sources with a high proportion of BC, such as diesel engines, will have positive climate benefits, in addition to significant improvement in public health. Many countries have included or are in the process of including BC reduction in their national determined contributions (NDC) to the United Nations Framework Convention for Climate Change (UNFCCC, https://unfccc.int/); Mexico was the first country to commit to reducing black carbon. HFCs are synthetic chemicals produced for use as substitutes for ozone-depleting substances in refrigeration, air-conditioning, insulating foams, aerosols, solvents, and fire protection. However, most HFCs currently in use have high GWP. In 2016, the Parties to the Montreal Protocol agreed to the Kigali Amendment to phase down the production and consumption of HFCs (Ozone Secretariat, https://ozone.unep.org).

Many megacities are located along coastal areas, on floodplain and in dry areas, and are increasingly experiencing the effects of extreme weather and climate-related events, such as heat waves, hurricanes, heavy downpours, flooding, droughts, and more frequent and intense wildfires. Despite these risks, many cities have not yet incorporated climate action plans to existing urban planning due to a lack of resources to prepare for the extreme events as well as public awareness on climate change and its impacts. The continued urban expansion and infrastructure development provide an opportunity for cities to manage risks and develop strategies for climate mitigation and adaptation at the local level while at the same time improving the air quality. Organizations such as C40 Cities, a network of the world’s megacities, is supporting cities to collaborate and share knowledge to drive measurable and sustainable action on climate change (http://c40.org).

Megacities not only influence the environment as large sources of pollutants, but also change the urban landscape and meteorological conditions by replacing vegetation and green areas with asphalt and concrete for roads, buildings and other structures to accommodate the growing population, creating the urban heat island effects. 230,231 The temperature difference between the urban area and the rural surroundings is usually larger at night than during the day and most apparent when winds are weak. The increased demand for air conditioning to cool buildings and homes relies on power plants to meet the demand, leading to an increase in air pollution and greenhouse gas emissions. Higher air pollution, reduced night-time cooling and increased daytime temperature can adversely affect human health and comfort. Some megacities, such as Mexico City, are using green roofs and vertical gardens to help reduce urban heat island effects by shading building surfaces.

Impact of COVID-19 on megacities’ air quality

Transmission of SARS-CoV-2 is considered to be predominantly by respiratory droplets produced when an infected person coughs, sneezes, or talks. Most public health guidelines have focused on social distancing measures, regular hand-washing, and other precautions to avoid large respiratory droplets. 238 Several studies have implicated airborne transmission of SARS-CoV-2 via respiratory microdroplets as a probable route for the spreading of the disease. 239–244 Following an open letter from more than 200 scientists appealing to international and national bodies to consider the risk of airborne transmission, 245 the WHO revised its guidelines and also recognized the threat of airborne transmission, particularly in inadequately ventilated indoor spaces. 246

While national responses to the unprecedented COVID-19 pandemic have been varied, most countries have enacted strict measures to contain the spread of the disease to protect lives and preserve health systems, including lockdowns, quarantines, and travel restrictions, bringing global economic activity, particularly that of developing economies, to a major pause. Thus, in addition to the enormous human toll, the pandemic has led to a deep global recession. 247,248 The stress from the pandemic and the resulting economic recession have negatively affected the mental health and well-being of people all over the world. 249

The response of the scientific community to COVID-19 has resulted in the publication of a large volume of articles at extraordinary speed; with many studies made available in parallel to peer review. In light of the global health emergency of the pandemic, rapid publication ensures that new evidence is shared in a timely manner. However, this also poses a challenge, especially the publicly released preprints that have not been fully evaluated for scientific quality. As noted by Palayew et al. , 250 it is important for the scientific community to take measures to safeguard the integrity of scientific evidence and avoid the risk of misinterpretation and misleading application in public policy.

The drastic measures implemented around the world to contain the spread of COVID-19 have led to significant reductions in the emissions of air pollutants, notably NO x and CO 2 emissions from fossil fuel combustion. Many cities have seen dramatic improvement in air quality. Delhi, one of the most polluted megacities, experienced the clearest skies in years as pollution dropped to its lowest level in three decades (https://earthobservatory.nasa.gov/images/146596/airborne-particle-levels-plummet-in-northern-india).

The dramatic reduction in air pollution associated with COVID-19 lockdowns and other restrictions imposed by governments in cities around the world has provided an opportunity for atmospheric scientists to conduct a unique natural experiment to gain a better understanding of the complex interactions between emissions, meteorology, and atmospheric processes, as well as the efficiency of control measure surrogates ( e.g. , reduced gasoline- and diesel-fueled vehicle traffic as a stand-in for large-scale zero emission vehicle deployment, reduced fossil fuel-derived electricity demand for renewable energy) that could lead to long-term emission reductions. Most of the studies have been conducted in China since the first stringent lockdown was enacted by the Chinese authorities in response to the initial outbreak of SARS-CoV-2 in Wuhan. 251–259 A similar reduction in air pollution levels has been reported in other megacities, for example, Sao Paulo, 260,261 Barcelona, 262 Rio de Janeiro, 263,264 and Delhi. 265,266 Sharma et al. 267 analyzed the air quality of 22 cities in India, including Delhi, Kolkata, Mumbai and Chennai, and found that the concentrations of PM 2.5 decreased while O 3 increased in most regions during the lockdown period. Jain and Sharma 266 assessed the impact of nationwide lockdowns on the air quality in five megacities of India: Delhi, Mumbai, Chennai, Kolkata, and Bangalore. The study evaluated the criteria pollutants PM 2.5 , PM 10 , NO 2 , CO and O 3 before and during the lockdown period (March–April 2020) and compared them with air quality in the same period of the previous year. The results showed a statistically significant decline in all the pollutant concentrations except for O 3 . The increase in O 3 levels during lockdown may be attributed to more favorable conditions for photochemical reactions due to increased solar insolation (due to the reduced primary pollutant levels) and a decrease in NO 2 , which is consistent with the VOC-limited regime of India for O 3 production. 268

While the COVID-19 lockdown improved air quality in many regions across the world, 269 secondary air pollutant levels in some megacities has not improved due to the complex interplay among emissions, meteorology, and atmospheric chemistry, as illustrated in the following example. 45,253 Le et al. 253 examined the changes in emissions during the COVID-19 lockdown in four megacities in China: Wuhan, Shanghai, Guangzhou, and Beijing. Satellite and ground-based observations revealed up to 90% reductions of NO 2 and SO 2 concentrations. PM 2.5 concentrations were also reduced in Wuhan, Shanghai and Guangzhou. In contrast, PM 2.5 concentrations in Beijing–Tianjin–Hebei (BTH) increased substantially during lockdown; the region experienced several severe haze episodes. Ozone followed similar trends to that of PM 2.5 . Synergistic observation analyses and model simulations show that anomalously high humidity during this period promoted aerosol heterogeneous chemistry, along with stagnant airflow and uninterrupted emissions from power plants and petrochemical facilities, contributing to severe haze formation. Due to nonlinear O 3 production chemistry, reduced NO x resulted in O 3 enhancement in urban areas, increasing the atmospheric oxidizing capacity and facilitating secondary aerosol formation. The results of this study suggest that it is not sufficient to control emissions from vehicular traffic and manufacturing activities, a comprehensive regulation of precursor gases from all possible emission sources, such as power plants and heavy industries, must be considered for long-term improvement of air quality. The study also highlights the importance of meteorological factors when planning short-term stringent emission controls. Sun et al. 257 analyzed the responses of primary and secondary aerosols to the changes in emissions during the outbreak in Beijing, along with the effects of emissions reductions during the Chinese New Year holiday of the previous years. The results showed substantial reductions in primary aerosols associated with traffic, cooking, and coal combustion emissions but much smaller decreases in secondary aerosols, suggesting the need for better understanding of the mechanism driving the chemical responses of secondary aerosols to emissions changes under complex meteorological conditions. Zhu et al. (DOI: 10.1039/D0FD00091D ) conducted hourly measurements of PM 2.5 and chemical speciation at an urban site in Shanghai before and during the restriction. They observed an overall reduction in PM 2.5 , with a similar amount from OC, while nitrate accounts for most of the decrease. The reduction was due mostly from the decrease in vehicle traffic volume and fuel consumption; however, this was partially offset by an increase in secondary sources during lockdown, indicating the challenge of predicting PM 2.5 improvement based on emissions reduction from primary sources.

As noted by the WHO, 204 while more countries are taking action to improve the air quality, air pollution levels still remain dangerously high in many regions of the world. Several studies have found that air pollution substantially increases the risk of infection and the severity of COVID-19 symptoms. 270 Furthermore, people with pre-existing conditions from past air pollution exposure are more vulnerable to COVID-19. A study in the USA reported an increase in COVID-19 death rates in areas with higher long-term average PM 2.5 pollution levels, emphasizing the importance of enforcing existing air pollution regulations during and after the COVID-19 crisis. 271 High levels of pollution have also been found to be a co-factor in the high lethality risk of COVID-19 disease in Northern Italy. 272

An important consequence of the improvement in air quality during the COVID-19 pandemic is the health benefits in non-COVID-19 illnesses. Chen et al. 273 reviewed the daily concentrations of NO 2 and PM 2.5 in 367 Chinese cities, and estimated that the improved air quality led to substantial cases of avoided death from cardiovascular diseases.

While the unintended consequences of the COVID-19 crisis have brought some temporary non-COVID-19 related health benefits, the drastic measures of shutting down the global economy to clean the air are not sustainable. In fact, as some of the restrictions were lifted and the recovery began, satellite images from NASA show that much of the air pollution has returned (https://earthobservatory.nasa.gov/images/146741/nitrogen-dioxide-levels-rebound-in-china). Nevertheless, the unprecedented global pandemic demonstrates that it is possible to achieve better air quality by implementing emission reduction strategies that have been proven to be effective; furthermore, it raises public awareness about the benefits of cleaner air and calls for governments to take actions for the longer term.

Conclusions

Pollutant emissions from vehicles and industrial activities have reduced in many megacities by applying technology-forcing policies. However, establishing stringent regulations and their enforcement is more difficult in megacities with limited economic and human resources. International collaboration and cooperation are strongly encouraged, including strengthening local capacity in air quality monitoring and emissions inventory development, so that megacities confronting severe air pollution challenges will have the opportunity to learn from the experience of those cities that have successfully addressed them.

Conflicts of interest

Acknowledgements.

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Air pollution and human health in kolkata, india: a case study.

1. Introduction

2. study area, 2.1 sources of air pollution in kolkata, 3. database and methodology, 3.1. monitoring stations and criteria pollutants, 3.2. air quality assessment.

• Critical pollution (C): when EF is more than 1.5;
• High pollution (H): when the EF is between 1.0–1.5;
• Moderate pollution (M): when the EF between 0.5–1.0; and
• Low pollution (L): when the EF is less than 0.5.

3.3. Health Assessment

3.4. data analysis, 4. results and discussion, 4.1. concentration and trends of ambient air quality, 4.2. interpreting health outcomes of surveyed dispensaries in kolkata, 4.3. outdoor pollution-averting activities, 4.4. diseases analysis, 5. conclusions, acknowledgments, author contributions, conflicts of interest.

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Haque, M.S.; Singh, R.B. Air Pollution and Human Health in Kolkata, India: A Case Study. Climate 2017 , 5 , 77. https://doi.org/10.3390/cli5040077

Haque MS, Singh RB. Air Pollution and Human Health in Kolkata, India: A Case Study. Climate . 2017; 5(4):77. https://doi.org/10.3390/cli5040077

Haque, Md. Senaul, and R. B. Singh. 2017. "Air Pollution and Human Health in Kolkata, India: A Case Study" Climate 5, no. 4: 77. https://doi.org/10.3390/cli5040077

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Clear the Air: 11 Solutions to Air Pollution in Cities

By Annie Granger Categories: Environment & Nature August 11, 2023, 3:21 PM

From small lifestyle changes to large-scale policy interventions, discover 11 actionable solutions to air pollution that can make a huge difference.

Air pollution in cities is a growing concern that poses a serious threat to both human health and the environment. The World Health Organization (WHO) estimates that 90% of the global population is exposed to air pollution levels that exceed safe limits , and the situation is particularly dire in urban areas. Considering the many human activities that contribute to air pollution , solutions are seriously needed.

Exposure to air pollution has been linked to health problems like respiratory illness, heart disease and cancer. Furthermore, air pollution contributes to climate change and damages ecosystems. Fortunately, there are many simple remedies that individuals, communities and policymakers can implement to reduce air pollution in cities. From improving public transportation to creating urban forests and encouraging plant-based eating, embracing these 11 solutions to air pollution can reduce the pollutants in the air and create a cleaner, greener world for generations to come.

Learn more about different types of pollution and how to combat them:

• What Is Noise Pollution? 5 Examples and Solutions
• Drowning in Plastic: Ocean Pollution & How to Stop It
• What Is Light Pollution? Definition, Causes and Impact
• 10 Solutions to Water Pollution in Our Daily Lives
• The Top 5 Human Activities That Contribute to Air Pollution
• Thermal Pollution: What Is It and What’s the Damage?
• What Causes Ocean Acidification? Can It Be Reversed?

1. Bicycle-Friendly Infrastructure

The problem with traditional transportation methods — like cars — is that they rely on fossil fuels, releasing toxic fumes into the air. Bicycles, however, need nothing more than human power to get you from point A to point B. The wind in your hair, the sun on your face and the freedom of the open road are all yours to enjoy when you travel by bike . Learn how to bike in the winter  and the basics of cycling in the rain , and nothing will be able to stop you.

Trends show that more and more people are riding bicycles in cities . By encouraging bicycle-friendly infrastructure , cities can reduce air pollution and improve the health of their citizens. Bicycle-friendly infrastructure includes dedicated bike lanes, bike racks and other amenities, all of which make cycling a more appealing alternative to cars.

Real-life example: Portland, Oregon, has an extensive network of bike lanes and paths  and has implemented policies that promote cycling as a viable means of transportation. While more work is certainly needed, this city is a shining example of how a commitment to cycling can make a positive impact on air pollution and create a more liveable, sustainable city.

2. Green Spaces and Urban Forests: Natural Solutions to Air Pollution

In the middle of a bustling concrete jungle, green spaces and urban forests are havens of nature — places where trees, plants and other living things thrive. Additionally, green spaces and urban forests provide shade and create spaces for recreation and relaxation. They’re also great spots for urban camping .

But green spaces are more than just nice to look at. They also serve as a simple solution to air pollution in big cities. Trees and plants purify the air by absorbing carbon dioxide and releasing oxygen. The more green spaces and urban forests a city has, the more these natural air purifiers work around the clock to clear smog.

For more information about the many benefits of woods and forests, check out: What Is a Forest? Describing Our Most Important Ecosystems .

Real-life example: New York City has implemented an ambitious program to increase the number of trees in the city by one million over the next decade . The city has also created a network of green spaces and parks, including the High Line, which repurposes an old elevated railway into a public park, and the Brooklyn Bridge Park, which features over 500 trees and other greenery.

3. Public Transportation

Promises of speed, convenience and freedom keep us attached to our cars. But the truth is that cars simply can’t live up to any of those promises. Among the many important reasons not to own a car is that they are leading contributors to air pollution. Public transportation is a simple solution . The fewer cars on the road, the less air pollution is created.

Public transportation eases traffic congestion, improves traffic flow and shortens commute times. Furthermore, public transit is more efficient in terms of carbon emissions per passenger and is a simple, inexpensive way to reduce your carbon footprint .

Real-life examples: Cities like New York, Chicago and LA have established public transportation systems that are heavily used by residents and visitors alike. In fact, New York’s famed public transport system is the largest in North America and one of the largest worldwide. Accessible and convenient public transportation is one of the most widely beneficial solutions to air pollution.

You might also enjoy: The 15-Minute-City: How Real Can It Be? and LA’s Public Transportation: How to Use It & 6 Cool Places to Go .

4. Carpooling and Ride-Sharing: Budget-Friendly Solutions to Air Pollution

If public transportation and cycling don’t work for you, join a carpool or ride-share to minimize your contribution to air pollution. Carpooling and sharing allow travelers to share a ride to a common destination.

Similar to other options for driving in the city, this approach reduces the number of cars on the roads and, thus, traffic congestion and idling time. Carpooling is also associated with many other social benefits , including tighter-knit communities, saving on fuel costs and reduced demand for parking infrastructure.

Real-life examples: New York City, San Francisco  and Seattle have implemented carpooling and sharing programs in efforts to find solutions to air pollution. These programs are fantastic for the environment and for our communities and pocketbooks.

The worst places to visit are those that are already inundated with tourists, putting a strain on locals and the…

5. Plant-based Eating

What we eat significantly affects our health and the environment. One significant source of pollution is the release of greenhouse gases like methane and carbon dioxide during the process of animal farming. These gases are produced during the digestion process of livestock and are released into the atmosphere through manure storage and application. Consuming more plant-based foods greatly reduces emissions , ultimately leading to cleaner air.

The production of meat and animal products also contributes to air pollution through energy consumption, chemical use and waste disposal.

Every journey begins with a single step, and every change starts with a single person. By adopting a plant-based diet or reducing your meat intake a bit and eating a flexitarian diet , you make a conscious decision to reduce your own carbon footprint and inspire others, creating a ripple effect of positive change. You can also support local farmers and businesses that prioritize sustainable practices, further lowering air pollution in your part of the world. Plus, who wants to support the horrors of factory farming ?

Try going vegan or looking into the many benefits of going vegetarian — as long as you’re mindful to eat a balanced diet with a variety of foods, your health and the Earth will thank you. Want to make an even bigger difference? Buy organic produce whenever time and money allow.

Real-life examples: The plant-based movement is gaining traction worldwide. Some cities, like San Francisco and New York , have started Meat-Free Monday programs, which ask residents to go meat-free for just one day a week. It’s not just about reducing emissions. A plant-based diet can also help people be happier and healthier. Change can grow into a movement that transforms the world, even if it starts with one person.

Precision fermentation may have the potential to transform our current food system. Could a new cellular agriculture really turn microbes…

6. Alternative Fuel Vehicles: Futuristic Solutions to Air Pollution

The transportation of the future is here now, and it is powered by electricity, hydrogen and other clean fuels . These vehicles produce less — or no — harmful emissions compared to traditional gasoline and diesel-powered vehicles. Learn about the pros and cons of electric vehicles in our guides, How Do Electric Cars Work? The Inner Workings Explained and E-Mobility Pros and Cons: The Benefits and Challenges of Electric Vehicles .

Using alternative fuel vehicles for personal and public transportation can significantly lower air pollution, harmful airborne particulates and smog.

Real-life examples: Several cities in the US have already taken steps to increase the number of alternative fuel vehicles on their roads. For example, California has set a goal of having 5 million zero-emission vehicles on the road by 2030. As of 2021, California has the highest number of alternative fuel vehicles in the US, with over 700,000 electric and hybrid vehicles registered in the state.

7. Solar and Wind Energy

Cars and transportation aren’t the only culprits when it comes to air pollution and smog. According to the WHO, “unclean” energy sources , such as coal, gas and oil, are major contributors to air pollution in urban areas, releasing harmful pollutants like sulfur dioxide, nitrogen oxides and particulate matter when burned and creating toxic breathing conditions.

The sun’s rays and strong winds can be powerful sources of clean energy used to power our cities and homes. These renewable energy sources produce little to no harmful emissions, making them an excellent alternative to fossil fuels . By harnessing the power of the wind and sun, cities can reduce their dependence on fossil fuels and improve air quality, taking a step towards a sustainable future.

Real-life examples: Several cities in the US have made significant progress in promoting different types of renewable energies like solar and wind energy. Las Vegas has even set a goal to power the city entirely with renewable energy sources by 2050 and has already started installing solar panels to help it reach that goal. The many benefits of solar panels include the long-term generation of renewable energy and reduced reliance on fossil fuels.

Solar energy is a renewable form of energy. We’ll discuss the advantages and disadvantages of solar energy and how it…

8. Waste Reduction and Recycling: Easy Solutions to Air Pollution

Waste reduction and recycling can reduce air pollution in cities by lowering emissions from landfills, incineration, production and transportation. Do you know where your trash goes?

By reducing the amount of waste we produce, we can reduce the amount of air pollution created and help shape a cleaner, healthier environment. On an individual level, that means:

• learning what you can compost and what you can’t to reduce food waste
• avoiding basic recycling mistakes and using less plastic, or living a life without plastic
• implementing ways to be more sustainable and getting closer to a zero-waste lifestyle
• supporting local recycling programs, donating or reselling gently-used items and buying second-hand whenever possible — read 6 Great Places to Buy and Sell Second-hand Clothes Online for ideas on how to start

Real-life examples: Many cities in the United States have embraced this solution, implementing recycling programs and encouraging residents to reduce their waste. For instance, Seattle achieved their goal of recycling 70% of its waste by 2022 by implementing programs like composting and curbside recycling.

9. Sustainable Building Practices: Long-term Solutions to Air Pollution

The buildings we live in reflect our values and aspirations. Sustainable building practices and eco-friendly housing can create beautiful, efficient, clean cities. But how exactly can it provide solutions to air pollution?

The answer is simple. The need for power plants to burn fossil fuels to generate electricity can be reduced if we design and construct buildings to minimize their energy consumption and maximize their use of renewable energy sources and sustainable building materials . This leads to a reduction in the harmful pollutants getting released into the air.

Real-life examples: A number of cities have taken steps to reduce air pollution by implementing sustainable building practices. In San Francisco, all new buildings must meet strict environmental standards , including installing solar panels or green roofs. Meanwhile, in Seattle, the municipal government has established incentives for developers to build green buildings. The city also runs a program to recognize environmentally friendly buildings with certification.

Interested in green design? Take a look at these 12 Awesome Examples of Green Architecture in the US.

10. Green Roofs

Cities trap heat and pollutants in the air, leading to smog and health problems for their inhabitants. However, every roof is a potential oasis of clean air and greenery when they are covered with vegetation , grasses and even small trees to help mitigate the effects of urbanization.

Installing green roofs creates pockets of natural beauty that can help to reduce the urban heat island effect, which occurs when cities become significantly hotter than their surrounding rural areas due to the lack of greenery and overabundance of heat-absorbing material . Green roofs cool the surrounding air and provide natural insulation, subsequently also reducing the need for energy-intensive air conditioning.

Real-life examples: New York, Chicago and Portland have implemented green roofs as a solution to air pollution. As part of its sustainability efforts, New York has even mandated that certain new buildings must incorporate green roofs or solar panels.

11. Education and Awareness About the Environment

Education and awareness are crucial in the fight against air pollution and climate change. Education gives individuals and corporations the chance to act now to establish effective, long-term solutions to air pollution. While we need to be mindful to avoid supporting greenwashing , there are many companies and environmental organizations trying to make a difference — or at least avoid contributing to the problem.

Awareness campaigns encourage companies to adopt environmentally friendly business practices, such as investing in cleaner energy sources or reducing waste in their production processes. An informed public can also put pressure on their elected officials to enact laws and regulations that protect air quality.

Education and awareness won’t solve the problem of air pollution entirely. Still, they are a key part of the solution, inspiring individuals and companies to act and pushing governments to enact necessary laws and regulations.

Here are some resources to get you started:

• Environmental Activism: How to Get Involved
• The 16 Best Movies & Documentaries About Sustainability & the Environment
• 11 Educational YouTube Channels That Change Your Perspective
• The Top 16 Universities With Sustainability Programs
• The 8 Best Books About Climate Change

The world’s greenest cities can serve as examples for urban planners. As the climate crisis looms, it’s more important than…

Read more :

• 15 Everyday Ways to Prevent Climate Change
• Are You Guilty of Performative Activism?
• Renewable Energy Certificate: How ‘Green’ Are They, Really?

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Air Pollution and Health in Cities

Population-weighted annual average pollutant concentrations and associated health burden in cities in 2019. Toggle to select pollutant type/health estimates.

Population-weighted annual average pollutant concentrations and associated health burden in cities, in 2019.

Cities are not only at the front line for air pollution impacts, but also for progress and interventions.

Cities are often hotspots for poor air quality. As rapid urbanization increases the number of people breathing dangerously polluted air, city-level data can help inform targeted efforts to curb urban air pollution and improve public health.

Explore air quality and health data for your city using our new interactive app here .

Read the full report:

POLLUTION PRIORITIES

Most cities have polluted air, but the type of pollution varies from place to place.

DIVERGENT TRENDS

Local policies have improved air quality in some cities, while pollution has worsened in others.

Some of the most polluted cities lack air quality monitoring stations and health data.

It’s a simple fact: Most urban residents around the world are breathing unhealthy levels of pollution. While there are many forms of air pollution, two main pollutants are particularly important in urban environments: ambient (outdoor) fine particle air pollution (PM 2.5 ) and nitrogen dioxide (NO 2 ).

Ambient PM 2.5 comes from vehicle emissions, coal-burning power plants, industrial emissions, and other sources. Because of their size – 2.5 micrograms or smaller – these tiny particles can easily get into the lungs, and in some cases, the bloodstream and impact our health in various ways . Nitrogen dioxide comes from many of these same sources, with vehicle traffic being a main source of NO 2 in urban areas.

Research suggests NO 2 exposure is not only linked to aggravation of asthma symptoms but is also linked to the development of asthma in children.

Comparing levels of these pollutants in cities around the world reveals strikingly different geographic patterns. PM 2.5 pollution tends to be highest in low- and middle-income countries, whereas NO 2 levels are high across countries of all income levels.

Population-weighted annual average pollutant concentrations in the five most populous cities in each region in 2019.

PM 2.5 exposures are highest in populous cities located in South Asia, East Asia, Southeast Asia, West Sub-Saharan Africa, and Andean and Central Latin America. Cities in high-income regions see significantly lower levels of PM 2.5 pollution.

Almost all people living in large cities are breathing high levels of NO 2 .

• The most populous cities across all 21 regions (81 out of 103 cities) reported NO 2 exposures higher than the global average of 15.5 µg/m 3 ; the only exceptions were cities in Oceania, Australia, and Central and East sub-Saharan Africa.
• Four large cities — Beirut, Lebanon; Shenyang, China; Shanghai, China; and Moscow, Russia, collectively home to over 53 million people — had NO 2 levels that exceeded even the least stringent WHO guideline (40 µg/m 3 ).

Overall, many cities have seen persistently high — and even rising — levels of air pollution over the past decade. PM 2.5 exposures remained stagnant in many cities from 2010 to 2019. In 2019, 41% of the cities still experience PM 2.5 levels that exceed even the least-stringent WHO PM 2.5 interim target of 35 µg/m 3 , compared to 43% in 2010.

NO 2 exposures have been falling in many cities, particularly in high-income regions and in East Asia. Globally, NO 2 exposures are heading in an encouraging direction as 211 more cities met the WHO guideline of 10 µg/m 3 in 2019 compared to 2010. However, NO 2 pollution is worsening in some other regions.

Percentage of cities by population-weighted annual average pollutant concentration in 2010 and 2019.

However, interventions targeting pollution at the local scale have successfully improved air quality in some cities. For example,

• Beijing, China, reduced its PM 2.5 levels by 36% in just five years thanks to controls on power plant and industrial emissions along with new fuel quality and emission standards for vehicles. MORE
• London’s Ultra Low Emission Zone initiative delivered a 36% reduction in NO 2 in the first six months after its launch in 2019. MORE
• Promisingly, mayors from more than 45 cities around the world have signed the C40 Clean Air Accelerator and made a commitment to provide healthy air for everyone and implement substantive clean air policies by 2025. MORE

Urban mobility and air pollution at the neighbourhood scale in the Megacity of São Paulo, Brazil

• Open access
• Published: 20 August 2024
• Volume 1 , article number  13 , ( 2024 )

Cite this article

You have full access to this open access article

• Carolina Girotti 1 ,
• Maria Carla Queiroz Diniz Oliveira 2 ,
• André Eiji Sato 1 ,
• Júlio B. Chiquetto 3 ,
• Alexandre Pereira Santos 4 , 5 ,
• Regina Maura de Miranda 2 ,
• Roberta Consentino Kronka Mülfarth 1 ,
• Alessandra Rodrigues Prata Shimomura 1 &
• Juan Miguel Rodriguez Lopez 4  

Developing countries’ megacities, characterised by dense populations and socioeconomic disparities, often face high levels of pollutants, mainly from vehicle emissions. Poor air quality can lead to a range of public health problems which in turn, need to be addressed by public policies. The main goal of this article is to examine the impact of public policies and the influence of transport modes on air pollution of three districts from the Metropolitan Region of São Paulo, in Brazil: Pinheiros, Parque Dom Pedro II and Taboão da Serra. The method was held through a comparative analysis, which in turn, took into account urban indicators, urban mobility data and pollutants levels (CO and NOx). These data were collected from the 2007 and 2017 São Paulo Metrô Origin and Destination Surveys and the Air Quality Database of the Environmental Company of the State of São Paulo. As key findings, the three study areas’ pollutant concentrations presented a downward trend from 2007 to 2017 as the same time there was an increase in the public transport and non-motorized transport modes. However, it is important to highlight the confluence of the state and federal public policies occuring at the same period such as PRONCOVE, Rodoanel and the Yellow Subway Line. The identified socio-environmental disparities in the urban realm highlight the importance of localised analyses in order to reveal problems and opportunities to get a better response in terms of urban mobility and air quality. Thus, the perpetuation of constant policy’s updates and interdisciplinary collaboration is crucial.

Avoid common mistakes on your manuscript.

1 Introduction

Air pollution has become a significant menace, exerting detrimental effects on public health and climate and is a severe problem in urban areas, particularly in global megacities, which in turn, establish significant flows of finance, production, and population [ 1 ]. They are also central nodes to political, socioeconomic and technological systems, concentrating decision-making institutions, significant economic activities and political power [ 2 , 3 , 4 ]. This article focuses on the Metropolitan Region of São Paulo (MRSP), the largest urban area in South America, and aims to analyse the impact of public policies and the influence of transport modes on air pollution at neighbourhood-scale level, allowing a better understanding on the air quality of the MRSP.

Poor air quality can lead to a range of public health problems, including respiratory complications and cardiovascular diseases [ 5 ], such as asthma [ 6 ], diabetes [ 7 ] and several types of cancers [ 8 ]. The most damaging air pollutants for human health in urban environments are fine particulate matter (PM) (with less than 2.5 μm diameter - PM 2.5 ), nitrogen oxides (NOx), ground-level ozone (O 3 ) [ 9 ], inhalable particulate matter (PM 10 ), sulphur dioxide (SO 2 ), carbon monoxide (CO) [ 10 ] and methane (CH 4 ) [ 11 ]. Among its complexities, the issue of air pollution should focus not only on which pollutants people are exposed to but also on what level they are exposed [ 12 ].

Denser cities may have a greater potential for concentrating air pollution due to their configuration of buildings and lack of vegetation [ 10 ], among other reasons. In developing countries’ megacities, motorised vehicles are the main source of air pollution as these urban areas are majorly dependent on this transportation mode and present a greater density of outdoor human activities [ 13 ]. Over this way, the level of pollutants’ exposure plays an important role especially for the health of cyclists and pedestrians [ 12 ]. While some exposure and duration factors may be controlled by the user (such as the location of the person or the travel mode choice), others are not (the pollutants’ concentration, for example). Developing countries, where most megacities are located, will host approximately 95% of the world’s urban population growth for the next 40 years [ 13 ]. Thus, megacities in developing countries should pay greater attention to the problems related to air pollution at the scale of its own inhabitants.

The MRSP in Brazil is a major example of a megacity in the developing world, being considered the main financial and commercial centre in South America [ 14 ] and one of the largest innovation hubs in the world with an estimated population of 20,743,587 inhabitants [ 15 ]. The region was officially created in 1973 by Federal Complementary Law Nº 14 [ 16 ], being reorganised in 2011 through Complementary Law Nº 1,139 [ 17 ]. It occupies an area of more than 7946.96 km 2 with a high level of human agglomeration, as it brings together 39 municipalities and represents more than 47.54% of the population of the State of São Paulo [ 18 ]. As such, its urbanisation is hierarchical and fragmented and still lacks a more sustainable transport system, leading to several urban mobility problems and intense exposure to atmospheric pollutants.

Even though there have been public policies regarding air pollution and urban air quality since 1990, the MRSP’s concentrations of pollutants still often exceed the levels suggested by the World Health Organization (WHO) [ 19 , 20 , 21 ]. Studies reported that about ten thousand deaths per year are associated with air pollution in the MRSP [ 22 , 23 ], with the road transport’s sector being the main responsible for the degradation of air quality in the urban areas [ 24 ]. The intense economic activity of the region (i.e., industrial output and advanced services) and the lack of alternatives to road transportation (e.g., railways and subways) contribute significantly to intensify emissions from motorised transportation [ 25 ].

In Brazil, there are public policies at the municipal, state and federal levels that aim to improve air quality. At the municipal level (the City of São Paulo - CSP), there are public policies that encourage the update of the current bus fleet with less polluting vehicles such as COMFROTA (acronym in Portuguese meaning “Steering Committee of the Fleet Replacement Monitoring Program with Cleaner Alternatives'') [ 26 ]. Additionally, between the years 2010 and 2013, the municipal government implemented the Environmental Vehicle Inspection Program [ 27 ] to minimise the emission of pollutants and in 2014, the city had its Master Plan revised [ 28 ].

At the state level, there was the implementation of the Rodoanel, which consists of a ring road of approximately 180 kilometres that aimed at reducing vehicle’s traffic, especially of trucks, across the CSP area. The state government also implemented new subway lines, which reduced the need for individual transport in the MRSP [ 18 ].

At the national level, the main public policy was the implementation of PROCONVE (Vehicular Emissions Control Program, in Portuguese), a policy designed to mitigate instances of air quality deterioration in major Brazilian cities like the CSP. Since the 1990s, PROCONVE has focused on the reduction of vehicle emissions and the widespread adoption of biofuels on a large scale [ 19 ].

As in other cities in the world, urban mobility directly influences air quality in the CSP. Since 1990, vehicles are the main source of air pollutants, accounting for 95.5% of CO emissions (generally related to light vehicles’ emissions) and 60.4% of NOx emissions (generally related to heavy vehicles’ emissions) [ 29 , 30 ]. Therefore, urban mobility plays an important role in the formation of the atmospheric chemistry of the region and, consequently, in the quality of life of the population. Another important aspect to be considered in megacities is the spatial spread variation of air pollution in the intraurban scale. Depending on factors such as proximity to the roads, urban geometry and the reactivity of air pollutants, concentrations may vary drastically from one block to another, resulting in different exposure profiles [ 31 ]. Some works have focused on measuring air pollutants in this spatial scale, but very few have tried to analyse it in conjunction with urban mobility data and public policies.

Over this way, there are significant factors for promoting sustainable urban mobility and combating air pollution in the MRSP. They include monitoring vehicular emissions, updating vehicle fleets (e.g. public buses), improving traffic conditions and distribution of road infrastructure (e.g. through ring roads), as well as offering mass-transportation modes such as the subway. It shows that policy at different levels have sought to tackle air pollution from the transportation perspective with varying goals and claims of effectiveness. A research gap still exists, though, in understanding how these different policies interact to the urban mobility pattern in order to determine the pollutant concentration levels (e.g. NOx or CO) and the exposure degree affecting the region’s inhabitants, considering its acute socioeconomic segregation.

Thus, this article has as its main goal to analyse the impact of public policies (federal, state and municipal levels) and the influence of transport modes on air pollution at neighbourhood-scale level of the MRSP. Therefore, three districts within this territory were chosen as study areas, varying in terms of socioeconomic and urban characteristics. So, in order to achieve this main goal, several specific objectives were established such as (1) an urban and mobility analysis comparing each chosen area; (2) a mapping of the CO and NOx concentration levels in each selected district through a historical series; (3) a comparison of the pollutant levels with the changes in transport modes; (4) a comparison of the pollutant levels with the main actions of the implemented public policies.

Since this article approaches a relevant and current problem that many megacities of developing countries face, it is expected that this work contributes to the air quality discussion, approaching methods that help to better understand how the pollutant levels are behaviouring according to changes in transportation modes within the implementation of public policies. So, this empirical study provides nuanced insights to guide future urban policy formulations aiming at improving not only the air quality, but also the quality of life of the inhabitants of the MRSP megacity.

2 Study area

The MRSP is similar to other Latin American cities, presenting a fragmented urbanisation process where discontinuities and segregations persist [ 32 ]. Initially settled according to the colonial economic drivers (e.g., commodity exports), these cities rapidly developed contrasts between their affluent cores and lacking peripheries. In the 20th Century, Latin American cities had the fastest urbanisation rate in the world, especially between 1950 and 1990, and thus, the region became mostly urban in the 1980s [ 33 ]. These massive transfers did not occur without a cost, though. Urban planning was unable to keep pace with such development and a mix of private interests and political wrangling led to a hybrid urban form that included formal and informal urbanisation at its core [ 33 ]. The CSP was not an exception to these phenomena. In fact, it set the pace for most of it as the largest city in the region. Its position as a global city [ 34 ] and the inequality of Brazilian society often clashed and the resulting urbe was one of selective concentration of urban common goods (e.g. infrastructure, jobs, and services). These conditions made the CSP earn the name of “city for the few'' [ 34 ], highlighting its exclusionary character. The overall pattern has also been called “Comp-Fused” [ 35 ], meaning it combines extreme concentration (and congestion) in the metropolitan core, with large swathes of diffuse growth in the periphery.

In the last decades this pattern persists, even if with greater complexity. The legacy of these processes is that the population was ineffectively distributed in the MRSP [ 36 ], the infrastructure planning never had the conditions to anticipate or drive growth [ 37 ] and that a strong mis-match between employment and housing supply persists [ 38 ]. The two former phenomena guarantee strong qualitative differences between rich and poor people in urban environment and infrastructure, with significant impacts in health [ 39 ], well-being, and for sustainable development. They also incidentally push people towards less-efficient individual transportation [ 37 ], especially at the periphery where little efficiency is possible given its fragmented nature [ 40 ]. Figure 1 provides a socioeconomic map delineating the comparative analysis of population, employment distribution and per capita income in the MRSP for the years 2007 and 2017. The database used for this figure was the São Paulo Metro Origin and Destination Surveys (ODS) [ 41 , 42 ].

Socioeconomic maps of the MRSP in terms of population, employment distribution and per capita income for the years of 2007 and 2017

Examining Fig. 1 , it is possible to note that several high population density sectors in the MRSP is located in the peripheral region (e.g. Capão Redondo - south zone, Carapicuíba - west zone, Brasilândia - north zone and Itaim Paulista - east zone). Population density also grew between 2007 and 2017 in many peripheral sectors, while the high-density sectors in the core region (e.g. Paraíso), remained constant. On the other hand, job density is predominantly concentrated in the central region of the CSP, a trend consistently observed in both 2007 and 2017 and observed in the literature [ 36 ].

The highest concentration of per capita income is observed in the central region of São Paulo, while households in the outskirts of the CSP and in other municipalities in the MRSP presented modest incomes.

The detailed study area involves three districts of the ODS at the MRSP. These study areas were selected for relating public policies to the concentration of atmospheric pollutants. The three locations studied are Pinheiros, Parque Dom Pedro II (PDP) and Taboão da Serra (TBS). These locations were chosen because their transport mode supply changed between 2007 and 2017 and they represent different socioeconomic profiles. Also, there are meteorological and air quality monitoring stations in these districts, and they match the districts in the ODS. Table 1 presents the comparison between the districts.

Pinheiros, a high-income urban area located in the West Zone of São Paulo, was selected for this study due to its dynamic evolution in land use and the presence of the Pinheiros Metro station. Operating since 2011 as part of Line 4/ Yellow, Pinheiros Station became a fundamental link integrating the São Paulo Metro (METRÔ, in Portuguese) and the metropolitan railways (CPTM, in Portuguese). Adjacent to the Pinheiros district is Marginal Pinheiros, a road of great importance for the CSP, which intersects with important arterial roads in the region. Attracting a substantial flow of vehicles, including cars and trucks, these arterial roads in CSP are essential for local traffic and conduits that cross the city.

PDP is located in the centre of the CSP, has a metro station and a bus terminal (opened in 1967). The bus terminal is one of the largest in CSP, serving as a hub for comprehensive transit with bus routes stretching across the entire city. This gives the PDP, a primary importance in the urban mobility of CSP and encompasses a commercial enclave, playing a key role in the broader socioeconomic fabric of São Paulo.

TBS constitutes a peripheral enclave heavily reliant on individual motorised transport. Functioning as a gateway to São Paulo, it is located near the Regis Bittencourt Highway. As a peripheral district, it lacks significant transportation infrastructure that is not based on vehicles.

3 Data collection and analysis method

The methodology used for this study was designed to provide a comprehensive understanding of the relationships between urban indicators, transportation characteristics and air quality in the MRSP. Through the integration of diverse data sets and analytical approaches, this article aims to unravel the complexities of the urban environment, examining the impact of public policies and the influence of transport modes on air pollution, allowing a detailed examination of air quality in the MRSP.

3.1 Study design and research question

The study was designed to address the following research questions:

How have public policy measures implemented between 2007 and 2017 affected air quality in different neighbourhoods of the MRSP?

What is the relationship between transportation characteristics and air quality over this period?

To answer these questions, the study employs a comparative approach to analyse the effects of public policy measures on air quality in three strategically chosen neighbourhoods within the MRSP (Pinheiros, PDP, and TBS). The analysis began with an urban analysis of these regions, followed by their transport characterization through the analysis of the transport modes’ supply. This was done to understand the impact of transportation modes on air quality.

Finally, the study investigated how the concentration of CO and NOx changed before and after the implementation of Brazilian public policies, considering their impact at municipal, state, and national scales. Recognizing the influence of meteorological factors such as precipitation on air quality, climate data were incorporated into the analysis to account for potential discrepancies caused by seasonal variations.

3.2 Data collection and processing

A detailed exploration of urban dynamics was carried out in the three chosen regions – Pinheiros, PDP and TBS. Using datasets from the 2007 and 2017 São Paulo Metro ODS [ 41 , 42 ], a comprehensive set of urban indicators was examined. These indicators cover households, families, population, school enrolment, employment, private automobiles, produced and attracted trips, total area, total income, average family income, per capita income, median family income and average travel time of trips produced by mode and by district of origin.

A comparative assessment of the impact of public policies on air quality was performed examining the trends of CO and NOX levels between 2007 and 2017. The data came from the Air Quality Database (QUALAR) of the Environmental Company of the State of São Paulo (CETESB) [ 43 ]. A public policy analysis was carried out through reports provided by both CETESB and the National Environment Council (CONAMA) [ 44 ]. Thus, the qualitative analysis of NOx and CO annual hours aimed to provide insights into the effectiveness of various policies from municipal, state and national levels in the mitigation of air pollution in the MRSP.

Furthermore, it is important to highlight that the CO and NOx concentration data from weather stations are subject to numerical methods to discern patterns and variations in air pollutant levels. This quantitative analysis provided a detailed understanding of the temporal evolution of these pollutants and their correlation with meteorological factors. Precipitation data, extracted from the National Water and Sanitation Agency (ANA) database [ 45 ] and meteorological data from the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo [ 46 ], were incorporated into the analysis to further enrich the understanding of precipitation dynamics in the MRSP.

3.3 GIS analysis

Spatial data were processed using Geographic Information Systems (GIS) to map and analyse the spatial distribution of urban indicators and transportation characteristics through QGIS, by comparing data on population density, employment density, and per capita income. This initial analysis provides a comparative socio-environmental territorial view between 2007 and 2017 for the metropolitan region as a whole.

3.4 Data analysis

To address the research question on the relationship between transportation characteristics and air quality, the initial step involves visualising the data to understand the distribution and trends. This is accomplished using bar plots to compare the percentage change in various categories across the three neighbourhoods (Pinheiros, PDP, TBS). Data analysis and graphical visualisation were conducted using the Python library Matplotlib to create bar graphs that compare the percentage change in pollutants (CO and NOx) and modes of transport in the studied neighbourhoods. The NumPy library was used for efficient data manipulation.

To understand the relationship between different modes of transport and air quality, a comparison was made between modes of transport within the MRSP and air quality. This comparative analysis aimed to uncover how the availability of transport modes influenced the dynamics of air quality between 2007 and 2017 in the MRSP.

The relationship between public policy and air quality was examined by overlaying the validity periods of public policies on graphs of pollutant concentration data. This approach helps to understand the temporal relationship between policy implementation and changes in air quality.

3.5 Advantages and limitations of the method

The applied method offers graphical visualisation that makes complex data more accessible and comprehensible, facilitating the communication of results and the identification of patterns. This visual approach helps stakeholders and policymakers to better understand the spatial and temporal dynamics of urban indicators, transportation characteristics, and air quality. Due to its simplicity, this method can be applied in any city that has data on origin and destination of transportation and concentrations of atmospheric pollutants.

However, the method also has limitations. The temporal and territorial constraints of the data limit the amount of data analysed and the application of more robust statistical methods. Additionally, the visualisation approach does not necessarily capture all the nuances and complex interactions between the different factors analysed. This can result in an oversimplification of the relationships and potential overlooking of critical subtleties in the data.

4 Results and discussion

The results are presented in two parts: (1) urban analysis of regions, their transport characteristics and the analysis of the transport modes’ supply within them; (2) an analysis of relevant public policies in Brazil and CO and NOx concentration data from meteorological stations.

4.1 Urban analysis of regions and their transport characteristics and analysis of the supply of transport modes

The depiction of various socio-environmental indicators from 2007 to 2017 in Fig. 2 provides a percentage-based comparison for Pinheiros, PDP, and TBS districts.

Comparison between the 2007 and the 2017 socio-environmental data for Pinheiros, PDP and TBS

Substantial changes were observed in Pinheiros, PDP, and TBS neighbourhoods over the decade from 2007 to 2017, evident in key socio-environmental indicators. In Pinheiros, the analysis reveals the growth in households (51.02%), families (51.02%), and population (33.79%). School enrolments experienced a considerable uptick (88.40%). Employment rose by 24.38%, while private car ownership increased by 13.67%. Produced trips grew by 34.89% and attracted trips by 36.07%.

In PDP, a similar pattern emerged with a substantial rise in households (48.51%), families (48.51%), and population (33.31%), School enrolment, however, declined by 31.83%. Employment showed a slight decrease (− 4.13%), while private car ownership skyrocketed by 141.19%, emphasising a significant change in transportation preferences. Produced trips increased by 15.22% and attracted trips by 14.29%.

In TBS, there was a notable increase in households (68.07%), families (67.88%), and population (41.59%), while school enrolments rose by 41.68%. There was a modest decline in employment (− 5.14%), and the number of private cars surged by 131.11%, indicating changing mobility patterns similarly to PDP. Travel activities, both produced and attracted, experienced single-digit increases, with produced trips rising by 9.72%, and attracted trips by 9.10%.

Urban planning policies, as articulated in the São Paulo Master Plan [ 28 ], emphasise higher-density construction in central areas and proximity to public transportation hubs. However, demographic trends reveal urban segregation, with pronounced population growth in the distant TBS district. This spatial disparity, coupled with job concentration in affluent regions like Pinheiros, underscores the complex interplay between urban development initiatives and socio-economic factors.

A notable observation is the relatively low increase in the use of private cars in the Pinheiros district, attributed to the influential role of the Yellow Line-4 of the subway system, shaping transportation preferences. The interaction of infrastructure development and transport modalities emphasises the imperative need for holistic urban planning.

Figure 3 illustrates the comparison of transport mode data from the 2007 and 2017 ODS. The modes experiencing the most growth in 2017 include unconventional taxis, rail transport, motorcycles, and bicycles, across collective, individual, walking, and cycling modes.

Variables between 2007 and 2017 modes of transport and transit time in percentage for Pinheiros, PDP and TBS districts

Figure 3 shows that Pinheiros experienced a decrease in bus usage (-14%) but witnessed notable growth in cycling (100%), motorbike (800%), taxi apps (561%) and metro (146%). PDP witnessed a decrease in bus utilisation (− 28%) but notable increases in metro (115%), bicycle usage experienced a 93% surge. TBS saw a decline in bus usage (-15%), with substantial increases in unconventional taxis (400%).

In 2017, bus, car, and conventional taxi trips remained generally similar to 2007, while metro and train trips increased in Pinheiros and PDP. The primary increase in 2017 occurred in unconventional taxis and motorcycles, linked to the implementation of the companies Uber in 2014 and iFood in 2011. Travel time analysis indicated an increase in public transport time in TBS, signifying social inequality growth. Peripheral district experienced a more substantial population increase and public transport deficit, particularly those farthest from downtown areas, leading to heightened commute times.

Regarding walking travel time, studies show that socio-environmental inequality influences walkability, so those zones that have high urban density, with easy access to sidewalks, connectivity and mixed-use encourage walkability [ 47 , 48 ]. On the other hand, zones that are not very stimulating for walking are those that are unsafe and that lack infrastructure and have little access to services and commerce. In this study, this question can be seen through the data, considering that the zone with the highest walking rate is the richest zone, Pinheiros, and on the other hand, the zone with the lowest walking rate is the most peripheral zone, TBS.

Regarding cycling, the ODS only considers trips with origin and destination’s displacements, so the data does not represent leisure trips. The zone with the best infrastructure for bicycle riders is the zone of Pinheiros, which had no change in bicycle travel time between 2007 and 2017.

4.2 Analysis of public pollutant emission policies in Brazil and CO and NOx concentration data from meteorological stations

CO emissions primarily stem from light vehicles, while NOx emissions are predominantly associated with heavy vehicles. In the surveyed areas, buses and trucks are the primary sources of NOx in Pinheiros, buses in PDP and trucks in TBS [ 29 , 30 ].

To analyse pollutant behaviour, historical series of average annual CO and NOx concentrations from 2007 to 2017 were conducted. Despite an increase in road vehicles, there was a continuous drop in pollutants. The year 2016 showed a significant reduction, possibly influenced by increased precipitation and variations in fossil fuel use, unique to Brazil's biofuel-centric atmospheric chemistry.

Comparing 2007 and 2017 Origin and Destination modal data with CO and NOx concentrations, Fig. 4 illustrates the downward trend in pollutant concentrations, correlating with changes in transport modes in the three study districts.

Relationship between 2007 and 2017 types of transport and pollutants

In Pinheiros, there was a reduction of 50% in CO concentrations and 49% in NOx concentrations besides an increase of 75% in the use of collective transportation, while individual motorised transportation experienced a decrease of 19%, non-motorized transportation mode saw an increase of 116%.

In PDP there was a decrease of 56% in CO concentrations and 34% in NOx concentrations, the collective transportation showed a modest increase of 4%, while individual motorised and non-motorized transportation modes increased by 3% and 96%, respectively.

In TBS there were reductions of 57% in CO concentrations and 42% in NOx concentrations, the collective transportation decreased by 7%, while individual motorised and non-motorized transportation modes increased by 8% and 38%, respectively.

The impact of public transport on air quality becomes apparent, with Pinheiros experiencing a 0.75% increase, suggesting a potential correlation with the reduction in CO and NOx levels. In contrast, PDP and TBS recorded marginal increases (0.04% and 0.07%, respectively), highlighting nuances in the relationship between public transport and improving air quality. Regarding the impact of individual motorised transport on air quality, Pinheiros showed a reduction in the levels of individual motorised transport by 0.19%, while PDP and TBS showed an increase in the use of private cars (1.41% and 1.31%, respectively), which appears to be strongly correlated with recorded CO values.

Despite increased road vehicles, a reduction in NOx and CO concentrations occurred, attributed to Brazilian legislation, particularly PROCONVE [ 44 ], and increased public transport use. Unique regional characteristics were observed, such as a notable drop in NOx in Pinheiros from 2015, possibly due to heavy vehicle restrictions. Figure 5 depicts the relationship between CO and NOx concentrations and public policies, including the Environmental Vehicle Inspection Program, Yellow Metro Line, Rodoanel implementation, and climate information.

Relationship between the concentration of CO and NOx in the three districts analysed and the public policies Environmental Vehicle Inspection Program (Brazil, 1986), Yellow subway line and Rodoanel implementation

Figure 5 shows that 2010 was a year of major changes in public policies on a municipal scale, with the implementation of the Vehicle Environmental Inspection Program on a national scale and the implementation of the Yellow subway line and Rodoanel, on a state scale. These changes appear to have had an effect on reducing the concentration of CO and NOx in the three districts studied, taking into account that in 2010 there was a decrease in the concentration of CO and NOx at different levels. Upon closer examination of the municipal scale, it becomes apparent that these changes had a limited impact on diminishing the concentrations of CO and NOx. This is evident as the downward trend persisted even after the conclusion of the Vehicle Environmental Inspection Program in 2013 [ 27 ].

In 2015 there was an increase in the concentrations of both CO and NOx at the TBS station, which may be related to weather conditions, as between 2014 and 2015 there was a critical episode of drought during the summer season [ 49 ]. In 2016, there was a significant decrease in the annual concentration of NOx at the three stations analysed, while the decrease in CO concentration occurred at the TBS and PDP stations. This can be explained by meteorological factors, such as the accumulated rainfall that year than the climatological average for the CSP [ 50 ] These rainfall conditions, characterised by heavy downpours, were driven by the influence of El Nino in that year, which possibly influenced the decrease in pollutant concentrations.

This shows the complexity of air quality degradation events, which are based on various sets of factors, including meteorology, human activities such as urban traffic, the need to implement public policies and update them frequently, as is the case with PROCONVE, and urban mobility planning and the extension of metro lines. The importance of interdisciplinary work is observed, between scientists, public, environmental and urban managers and architects for the development of policies and instruments that can improve the quality of urban life of the population of large urban centres, such as the megacity of São Paulo.

5 Conclusions

This study explored the intricate dynamics governing the nexus of urban mobility, atmospheric pollutant concentrations and the efficacy of public policies within the context of the megacity of São Paulo. The nuanced examination of shifts in transport modes and transit times from 2007 to 2017 unearthed noteworthy transformations in commuting patterns. Unconventional modes such as ride-hailing services, rail transport, motorcycles and bicycles experienced substantial growth, particularly catalysed by the advent of private transport applications in the region in 2014.

From the comparison between the pollutant levels and the transport modes, the results show that the increased use of public transport can be correlated with a more pronounced decrease in CO levels, as observed in Pinheiros. This finding can strongly reinforce the positive environmental impact of promoting more public collective transportation. Conversely, higher usage of private cars’ service can be correlated with a less pronounced decrease in CO levels, as evidenced in TBS. This emphasises the latent environmental challenges associated with the dependence on individual motorised transportation, common in peripheral districts as a consequence of the intense urban sprawl. Along that, areas experiencing more significant population growth, such as TBS, tend to show smaller reductions in CO levels. This dynamic suggests a complex interplay between demographic factors, urban development and air quality.

The empirical analysis highlighted the dichotomy between central districts, exemplified by Pinheiros and PDP, and peripheral locations like TBS. While the former boasted enhanced access to infrastructure and public transportation, the latter grappled with socio-economic disparities manifested in prolonged travel times and limited access to efficient transportation modalities. Thus, this research also underscored the pivotal role of urban density and socio-environmental factors in shaping pedestrian and cyclist behaviours, highlighting the imperative for holistic urban planning strategies.

The study of atmospheric pollutant concentrations spanning the period from 2007 to 2017 revealed an overarching decrease. This reduction has been attributed to a confluence of legislative measures, notably the PROCONVE emissions control program, as well as changes in local and regional infrastructure which prompted consequential shifts in transportation behaviours. Significant reductions in both CO and NOx concentrations were particularly evident in areas where public transport use was high.

The comparison between the pollutant concentrations and the implemented public policies (the Vehicle Environmental Inspection Program, the Yellow Metro Line opening and the introduction of the Rodoanel) showcased the discernible impact of strategic interventions such as a pollutant emission’s control program and urban mobility alternatives. Nevertheless, the study acknowledges the intricate interplay of multifactorial urban determinants, necessitating the perpetuation of constant policy updates and an interdisciplinary collaboration.

Still, this study presented two major limitations: the data availability and compatibility. Regarding the data availability, the ODS provides the mobility data every 10 years. This can be an issue because several changes in the mobility pattern can occur in this time gap and these changes may not be presented in the data (e.g. the impact of COVID in urban mobility). The QUALAR data is available instantly. and this can represent a difficulty since it is necessary to get a high volume of 10 years data in order to compare to the ODS. Regarding the data compatibility and besides the time compatibility, the ODS and the QUALAR data do not use the same territorial limits. The ODS uses the Origin and Destination Zones while the QUALAR data refer to a single meteorological station in a given area. The problem is that not every OD Zone has its correspondent air quality station and thus, the proposed analysis of this paper cannot be applied to all of the MRSP. Another difficulty is within the QUALAR data, since they do not have all the pollutant levels for all of the stations. Thus, this study presents these two major limitations that impact directly not only on the replicability for all of the MRSP’s areas, but also on the accuracy of the comparative analysis.

Besides that, in a comprehensive synthesis, this article highlights the significant impact of state and federal policy frameworks, exemplified by initiatives such as Rodoanel and PROCONVE, in effectively reducing atmospheric pollutants. These initiatives overshadow the influence of municipal efforts, though. The prioritisation of public transport emerges as a pivotal strategy for emissions reduction, with noticeable benefits observed from investments in subway expansions and non-motorized transport infrastructure (e.g. better sidewalks and bike lanes). Additionally, this study brings attention to socio-environmental disparities, where central areas appreciate superior urban amenities compared to economically disadvantaged regions in the peripheral ones. This stresses the importance of localised analyses in order to reveal problems and opportunities to get a better response in terms of urban mobility and air quality, unravelling even more the intricate urban fabric of megacities like São Paulo. In conclusion, this empirical study provides nuanced insights to guide future urban policy formulations aimed at improving not only the air quality, but majorly, the quality of life in sprawling megacities of the developing countries.

Data availability

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Acknowledgements

Grant #2021/14533-7, São Paulo Research Foundation (FAPESP). Grant #2022/02365-5, São Paulo Research Foundation (FAPESP). Grant #88887.667919/2022-00, Coordination for the Improvement of Higher Education Personnel (CAPES). Grant FAUUSP PROAP 2023 financial support. Grant #309739/2022-5, National Council for Scientific and Technological Development (CNPq). Grant Summer Winter School-Hamburg Universität 2023 edition. Grant USP Cities Research Center.

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Carolina Girotti, André Eiji Sato, Roberta Consentino Kronka Mülfarth & Alessandra Rodrigues Prata Shimomura

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Girotti, C., Oliveira, M.C.Q.D., Sato, A.E. et al. Urban mobility and air pollution at the neighbourhood scale in the Megacity of São Paulo, Brazil. Discov Cities 1 , 13 (2024). https://doi.org/10.1007/s44327-024-00016-4

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• Pollution Due to Urbanisation Essay

Essay on Pollution Due to Urbanisation

Below, you will find an essay on pollution due to urbanisation (long) and also a short essay on pollution due to urbanisation. While urbanisation has its positives, it is imperative to look at every object according to its pros and cons. Here are two essays on pollution due to urbanisation of 400-500 words and 100-200 words, respectively. We will discuss the importance of urbanisation for countries, and how urbanisation is polluting the world.

Long Essay on Pollution Due to Urbanisation

Urbanisation is a great concept which is required to develop any country. It refers to the concept of urbanising remote areas by building infrastructure which then brings about development. Infrastructure refers to all the buildings and institutions which are necessary for economic development to take place in an area. For example, educational institutions like schools, colleges, vocational learning centres are part of the infrastructure. Healthcare facilities such as hospitals and clinics, employment opportunities, food security, etc. are also part of the infrastructure of a country.

It is seen very often that a big corporation sets up shop in a rural area, and around this, infrastructure is built, and development and urbanisation take place. Jamshedpur is an example of such a place, where Tata Industries set up shop many years ago and made the area highly developed. Thus, urbanisation definitely encourages the people of a place to have a better life by giving them more opportunities to achieve good life through education, jobs, etc.

On the other hand, it must be duly noted that urbanisation is one of the leading causes of pollution in today’s world. There are several different kinds of pollution, such as air pollution, water pollution, soil pollution and noise pollution. The facets of urbanisation contribute to each one of these types of pollution in one way or another. Factories and mines contribute to air pollution through the fumes that each of them emits into the air. The damage done to the water and soil around factories because of their flowing septic is harmful to both humans as well as aquatic life. Additionally, the noises that come from mines, the whirring of machinery in factories, etc. contribute to noise pollution.

Additionally, it is not only big industries that contribute to pollution due to urbanisation. Part of urbanisation is also the development of roads, which means more cars, buses, two-wheelers, three-wheelers, trucks, etc. on the road. These all contribute to noise pollution because of the incessant honking, and also to air pollution, because of the fumes that all motor vehicles emit. Even when we are stuck in traffic in an auto, it becomes difficult to breathe because of the fumes which surround us on the roads. If we are finding it difficult to breathe, imagine what so many fumes are doing to our planet.

Short Essay on Pollution Due to Urbanisation

150 Words Paragraph On Pollution Due to Urbanisation

Pollution takes place when air, water or soil becomes contaminated with unwanted substances. Air pollution takes place because of the fumes of factories and motor vehicles on th e road. Soil pollution and water pollution take place due to the septic waste being released into soil or water that surrounds a factory. Even oil spills are a major reason for water pollution, and all kinds of pollution can be very dangerous for living beings. Another type of pollution is noise pollution, which comes from the honking of cars, loud sounds in factories, the passing of aeroplanes and trains, etc.

Urbanisation is a result of the need to achieve economic development. It refers to when a relatively rural or remote area is made more urban by constructing roads, hospitals, schools, offices, etc. In this way, development is a result of urbanisation, which is extremely good for all countries.

However, all the great factors that urbanisation brings in, such as factories to work in, motor vehicles to drive, and so much more, all of these contribute to pollution more and more. Even though urbanisation is very important for a country, it is important to address all the kinds of pollution

Pollution is one of the most pressing concerns confronting our civilization today. When their environment deteriorates on a daily basis, humans face major challenges. The mixing of any toxic element or contaminants in our natural environment is referred to as pollution. Many contaminants are introduced into the natural environment as a result of human activities, contaminating it too dangerous proportions. Pollution is caused by a variety of factors, one of which is urbanisation.

The negative aspect of urbanisation is the manufacturers, which emit a great deal of pollution. Their equipment emits smoke into the environment, pollutes water streams and the surrounding land, and makes a lot of noise. As a result, there is a lot of pollution as a result of urbanisation, and it is extremely destructive to the environment when it first begins.

The majority of the pollution in our environment is due to urbanisation. It's because factories are springing up all over the place, there are a lot more cars on the road now, and so on.

Pollution Due to Urbanisation

Our mother planet is choking, and we are unable to do anything about it. Today, we confront several issues, one of which is pollution. Pollution occurs when a contaminating substance is introduced into our environment and pollutes our natural resources. There are numerous causes of pollution, most of which are caused by humans. Natural resources and habitats have been depleted as a result of our activities.

Urbanisation is one of the primary causes of human pollution. Pollution levels began to rise when humans began to construct cities and industrialization developed. Human needs continue to expand, and we loot our mother planet to meet them. As a result of development, many beautiful valleys, mountains, hilltop stations, and woods have become pollution carriers. Trees have been felled, rivers and lakes have been poisoned, and natural reserves have been exploited.

As a result, we now live in severely polluted cities where daily life has become increasingly challenging. As a result of urban pollution, we are experiencing a variety of health issues, the worst part of which is that we are fully unconscious of it. It is past time for us to take steps to reduce pollution and make the world a better place for future generations.

Urbanisation is a really great step forward for any country, and it is and should be the main aim of all countries. All people around the world should have access to proper healthcare, education, sanitation, nourishment and safety, and urbanisation is how we can help achieve this goal. However, in the process of meeting this goal, we cannot forget that pollution due to urbanisation does take place, and is very dangerous for the planet and, therefore, all species living on earth in the long run.

FAQs on Pollution Due to Urbanisation Essay

1. What are the pros and cons of urbanisation according to the essay on pollution due to urbanisation?

The essay on pollution due to urbanisation says that urbanisation is good and is vital for a country, but can also be harmful for the environment. Urbanisation brings in better education, better healthcare facilities, better roads, and better infrastructure in general. However, it improves the lifestyles of human beings at the cost of hurting the environment by putting more contaminants into air, water and soil in the form of toxic fumes and septic waste. Thus, urbanisation is important, but it has to be brought about in a more sustainable manner.

2. How can we reduce pollution due to urbanisation?

At the individual level, there are some very simple ways to reduce pollution due to urbanisation. To reduce air pollution, we can choose to walk, carpool, or use public transport instead of taking a taxi. Garbage should not be thrown on roads and in water bodies, in order for us to stop soil and water pollution. We should also not honk on roads unnecessarily, to curb noise pollution. Unless the big companies and industries do not decide to take a stand and do what’s good for the environment, we will have to keep relying only on individual measures.

3. What are the different types of pollution and their causes?

Pollution in Cities: Types and Causes

Air Pollution: The air in metropolitan places is constantly polluted with harmful compounds, making breathing increasingly dangerous. The air in cities is suffocating. The air is polluted by smoke from autos, factories, and power plants. There are also other contaminants in the air, such as chemical spills and other harmful substances.

Water Pollution: Natural water supplies are becoming increasingly scarce in metropolitan areas, and those that do exist are becoming progressively contaminated. There is a lot of waste dumping in lakes and rivers, such as residential and industrial waste. A lot of trash is washed into the rivers when it rains.

Soil Pollution: Toxic mixtures in the soil are causing ecosystem disruption.

Noise Pollution: Cities are among the noisiest places on the planet. Noise pollution is caused by a variety of sources, including traffic noises, loudspeakers, and other undesirable noises, which cause a variety of health problems.

Radioactive Pollution:   Nuclear power facilities' unintentional leaks represent a serious concern.

Visual Pollution: Signs, billboards, screens, high-intensity lights, and other forms of overexposure to sights in cities can also be highly unsettling.

There is also ' Thermal pollution ,' which is created by an excess of heat trapped in the earth's atmosphere.

4. How can pollution due to urbanisation be controlled?

One can implement the following methods to reduce pollution caused by urbanisation: 

Conserve Energy: People in urban areas always use more energy than people in rural areas. The use of energy results in numerous types of pollution. One of the most effective strategies to reduce pollution is to conserve energy wherever possible. When you are not using an electrical appliance, turn it off. This tiny step can make a tremendous difference.

Reduce water waste: We waste a lot of water on a daily basis, which might have negative implications. We must make every effort to utilize as little water as possible.

Plant more trees: Urban areas are the ones with the least amount of greenery. It's a good idea to have a kitchen garden and a little lawn near your house.

Green belts: The government can assist by declaring specific sections in each city as green belts, allowing trees and other plants to flourish freely.

Use fewer loudspeakers: Using fewer loudspeakers can significantly minimise noise pollution. It's also a good idea to turn down the music level at functions after a specific amount of time has passed.

Indoors: In cities, home interiors are likewise heavily contaminated. We must also have some plants inside our homes to filter the polluted indoor air.

Industrial trash: Factory owners must make every effort to avoid dumping industrial waste in lakes or rivers. The government can also enact legislation in this regard.

5.  What problems are caused due to Urbanization?

The necessity for open space to develop roads, buildings, and bridges, among other things, resulted in widespread deforestation. To accommodate the ever-increasing population, trees were cut down, fields were cleared, and built new space. It goes without saying that tree cutting is a major source of pollution. The high population density resulted in a scarcity of everything, including space and natural resources such as water and coal.

A number of serious challenges have arisen as a result of the interaction of the urban population with the environment. The spending habits and lifestyles of the urban people had a significant impact on the environment. Consumption of food, energy, and water is all higher in cities. Cities have much more filthy air than rural areas. This is mainly due to the increased use of automobiles and the expansion of industries and factories that pollute the air.  We utilise electricity to power almost all of our equipment.

6. What is urbanisation, and how is it caused?

The population shift from rural to urban regions, the resulting decline in the number of people living in rural areas, and the methods in which societies adjust to this transition are all referred to as urbanisation. It is basically the process by which towns and cities evolve and grow as more people choose to live and work in central locations.

Individual, community and state activity result in either organic or planned urbanisation. Living in a city can be culturally and economically advantageous since it can provide more options for access to the labour market, better education, housing, and safety conditions, as well as lower commute and transit time and costs. A healthy urban environment is characterised by density, proximity, diversity, and marketplace rivalry. However, there are also negative social consequences associated with urban living, such as alienation, stress, higher living costs, and mass marginalisation. Suburbanization, which is occurring in the greatest developing countries' cities, can be seen as an attempt to balance these negative aspects of city living while still giving access to a huge number of shared resources.

7. What is the Impact of Urbanisation in Indian Cities?

The following are the main effects of urbanisation on environmental quality in Indian cities:

According to the entire slum population in India in 1991, 41 per cent of the overall slum population lived in cities with populations of one million or more, which account for 27 percent of the country's total population.

According to the current situation of municipal solid trash creation and collection situation in Indian metropolitan cities, Maharashtra creates the most municipal solid garbage (11,000 tonnes per day), followed by Delhi (8700 tonnes per day) in 2019, both of which are expected to rise in the near future.

In India and other Metropolitan Cities, the number of automobiles on the road is increasing.

In India and other metropolitan cities, the number of automobiles on the road has increased. The usage of vehicles has increased by 10% or more on average, posing a significant threat to air pollution.

Water resources are dwindling day by day as a result of rising population, wasteful usage, and a lack of conservation. Huge amounts of wastewater enter rivers as cities and industries grow, contaminating river streams that are used for drinking and other reasons.

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Progress Cleaning the Air and Improving People's Health

The Clean Air Act has a proven record of public health and environmental protection since 1970.

EPA uses voluntary partnership programs in tandem with regulatory programs to protect public health and the environment. Clean Air Act partnership programs reduce conventional air pollution and greenhouse gas emissions, improve energy efficiency, reduce oil imports, and save money. <Learn more about voluntary partnerships>

More information on the national progress toward clean air

Air Quality Trends

Progress Reports on Market-Based Air Programs for Power Plants and Industry

Impact of Five Major Rules for Vehicles and Engines

• History of Reducing Air Pollution from Transportation in the United States

Quick Overview

For more than forty-five years the clean air act has cut pollution as the u.s. economy has grown..

Americans breathe less pollution and face lower risks of premature death and other serious health effects.

Environmental damage from air pollution is reduced.

The value of clean air act health benefits far exceeds the costs of reducing pollution., new cars, trucks, and nonroad engines use state-of-the-art emission control technologies..

New plants and factories install modern pollution control technology.

Power plants have cut emissions that cause acid rain and harm public health.

Interstate air pollution has been reduced..

Mobile and industrial pollution sources release much less toxic pollution to the air than in 1990.

Actions to protect the ozone layer are saving millions of people from skin cancers and cataracts.

The scenic vistas in our national parks are clearer due to reductions in pollution-caused haze.

EPA has taken initial steps to limit emissions that cause climate change and ocean acidification.

The Act has prompted deployment of clean technologies, and has helped provide impetus for technology innovations that reduce emissions and control costs.

Detailed summary: clean air act results.

• Experience with the Clean Air Act since 1970 has shown that protecting public health and building the economy can go hand in hand.
• Clean Air Act programs have lowered levels of six common pollutants -- particles, ozone, lead, carbon monoxide, nitrogen dioxide and sulfur dioxide -- as well as numerous toxic pollutants.

Between 1970 and 2020, the combined emissions of the six common pollutants (PM2.5 and PM10, SO2, NOx, VOCs, CO and Pb) dropped by 78 percent. This progress occurred while U.S. economic indicators remain strong.

The emissions reductions have led to dramatic improvements in the quality of the air that we breathe. Between 1990 and 2020, national concentrations of air pollutants improved 73 percent for carbon monoxide, 86 percent for lead (from 2010), 61 percent for annual nitrogen dioxide, 25 percent for ozone, 26 percent for 24-hour coarse particle concentrations, 41 percent for annual fine particles (from 2000), and 91 percent for sulfur dioxide. (For more trends information, see EPA's  Air Trends  site.)

• These air quality improvements have enabled many areas of the country to meet national air quality standards set to protect public health and the environment. For example, all of the 41 areas that had unhealthy levels of carbon monoxide in 1991 now have levels that meet the health-based national air quality standard. A key reason is that the motor vehicle fleet is much cleaner because of Clean Air Act emissions standards for new motor vehicles.
• Airborne lead pollution, a widespread health concern before EPA phased out lead in motor vehicle gasoline under Clean Air Act authority, now meets national air quality standards in most areas of the country.
• State emission control measures to implement the Act, as well as EPA's national emissions standards, have contributed to air quality improvements.

Because of the Act, Americans breathe less pollution and face lower risks of premature death and other serious health effects.

A peer-reviewed EPA study issued in March 2011 found that the Clean Air Act Amendments of 1990 are achieving large health benefits that will grow further over time as programs take full effect.

This chart shows the health benefits of Clean Air Act programs that reduce levels of fine particles and ozone.

• The 2011 report did not include the large benefits of the pre-1990 Clean Air Act. A peer-reviewed 1997 EPA Report to Congress reviewed the benefits of the Act from 1970 to 1990, and concluded that in 1990 alone, pollution reductions under the Act prevented 205,000 early deaths, 10.4 million lost I.Q. points in children due to lead exposure, and millions of other cases of health effects.
• Independent scientific research shows that reductions in air pollution are associated with widespread public health benefits. For example, one study found that reductions in fine particle pollution between 1980 and 2000 in U.S. cities led to improvements in average life expectancy at birth of approximately seven months. 1
• Lower air pollution levels mean less damage to the health of ecosystems.
• Environmental effects of air pollution include damage to plants and long-term forest health, soil nutrient deterioration, accumulation of toxics in the food chain, damage to fish and other aquatic life in lakes and streams, and nitrogen enrichment of coastal estuaries causing oxygen depletion and resulting harm to fish and other aquatic animal populations.
• Reducing air pollution also improves crop and timber yields, a benefit worth an estimated $5.5 billion to those industries' welfare in 2010, according to the peer-reviewed March 2011 EPA study .  Better visibility conditions in 2010 from improved air quality in selected national parks and metropolitan areas had an estimated value of $34 billion.
• EPA’s peer-reviewed 2011 study found that clean air programs established by the 1990 CAA amendments are expected to yield direct benefits to the American people which vastly exceed compliance costs.
• The study's central benefits estimate of $2 trillion in 2020 exceeds costs by a factor of more than 30-to-1, and the high benefits estimate exceeds costs by 90 times. Even the low benefits estimate exceeds costs by about 3-to-1.
• In addition to direct benefits vastly exceeding direct costs, economy-wide modeling conducted for the study found that the economic welfare of American households is better with post-1990 clean air programs than without them. 
• Economic welfare and economic growth rates are improved because cleaner air means fewer air-pollution-related illnesses, which in turn means less money spent on medical treatments and lower absenteeism among American workers. The study projects that the beneficial economic effects of these two improvements alone more than offset the expenditures for pollution control.
• The EPA report received extensive review and input from the Council on Clean Air Compliance Analysis, an independent panel of distinguished economists, scientists and public health experts established by Congress in 1991.

EPA has required dramatic reductions in emissions from new motor vehicles and non-road engines - such as those used in construction, agriculture, industry, trains and marine vessels -- through standards that require a combination of cleaner engine technologies and cleaner fuels. In 2013, EPA estimated the benefits of  five key standards to cut emissions from vehicles, engines and fuel to 2030.

• Compared to 1970 vehicle models, new cars, SUVs and pickup trucks are roughly 99 percent cleaner for common pollutants (hydrocarbons, carbon monoxide, nitrogen oxides and particle emissions), while Annual Vehicle Miles Traveled has dramatically increased.
• New heavy-duty trucks and buses are roughly 99 percent cleaner than 1970 models. In August 2016, EPA and the U.S. Department of Transportation’s National Highway Traffic Safety Administration (NHTSA) jointly finalized standards for medium- and heavy-duty vehicles that will improve fuel efficiency and cut carbon pollution, while bolstering energy security and spurring manufacturing innovation.
• Starting in the 2014 model year,  locomotives are 90 percent cleaner than pre-regulation locomotives. In March 2008, EPA finalized a three part program that dramatically reduces emissions from diesel locomotives of all types -- line-haul, switch, and passenger rail. The rule cuts particulate emissions from these engines by as much as 90 percent and nitrogen oxides emissions by as much as 80 percent when fully implemented. <Learn more about regulations for emissions from locomotives.>
• New commercial marine vessels (non-ocean-going) are 90 percent cleaner for particle emissions than in 1970. Clean Air Act and international standards for ocean-going vessel emissions and fuels are reducing emissions from ocean-going vessels as well. <Learn more about ocean vessels and large ships.>
• EPA is taking action to reduce emissions caused by Aircraft. In 2016, EPA finalized findings that GHG emissions from certain classes of engines used in aircraft contribute to the air pollution that causes climate change endangering public health and welfare under section 231(a) of the Clean Air Act. <Learn more about Regulations for Greenhouse Gas Emissions from Aircraft>
• Sulfur in gasoline has been reduced by 90 percent, and sulfur in diesel fuel has been reduced by 99 percent, from pre-regulation levels.

New power plants and factories use modern pollution control technology.

• In areas not meeting air quality standards, to avoid making pollution worse, new and modified large plants and factories must meet the lowest achievable emission rate and obtain offsetting emissions reductions from other sources.
• In areas that meet air quality standards, new and modified large plants and factories must apply the best available technology considering cost and avoid causing significant degradation of air quality or visibility impairment in national parks.
• For example, new coal-fired power plants typically install control devices that capture up to 98 percent of the sulfur dioxide and in many cases 90 percent of the nitrogen oxide emissions, relative to uncontrolled levels.
• These requirements are applied through pre-construction permitting programs that are administered by state, local, tribal, or EPA permitting authorities, depending on the location.
• State and local permitting authorities usually administer the pre-construction permit programs that determine how to apply these requirements to facilities.

• A national system of marketable pollution allowances has dramatically cut power plant emissions of sulfur dioxide, reducing acid rain as well as secondary formation of fine particle pollution that contributes to premature death. Acid rain , which includes wet and dry deposition of acidic compounds from the atmosphere, results from emissions of sulfur dioxide and nitrogen oxides.
• Between the 1989 to 1991 and 2009 to 2011 observation periods, wet deposition of sulfate (which causes acidification) decreased by more than 55 percent on average across the eastern United States.
• The dramatic emissions reductions achieved by the acid rain program have helped to reduce atmospheric levels of fine particle pollution, avoiding numerous premature deaths.
• Government and independent analyses have concluded that the benefits of the program far outweigh the costs, as detailed in the U.S. government's National Acid Precipitation Assessment Program (NAPAP) 2011 Report to Congress (PDF) . (132 pp, 14.5 MB, About PDF )
• Multiple analyses show that the cost of the fully implemented program is a fraction of the originally estimated cost - between $1-2 billion annually rather than the $6 billion EPA originally estimated in 1990, according to the NAPAP report.

Further reductions in power plant pollution have been achieved by state and EPA efforts to cut interstate air pollution, achieving additional public health benefits and helping downwind states meet health-based air quality standards for fine particles and ozone.

• Twelve New England and mid-Atlantic states and the District of Columbia -- the Ozone Transport Region created by the 1990 Amendments -- worked together to create a nitrogen oxides (NO x ) Budget Program and to adopt other controls that help improve ozone levels throughout the region.
• Building on that success, EPA issued a broader “NOx SIP Call” Rule creating a similar NOx Budget Trading Program for much of the eastern United States, which ran from 2003 to 2008. As of 2008, the program cut summertime NOx emissions from power plants by 62 percent from 2000 levels. These reductions, along with the NOx reductions from federal motor vehicle standards, are responsible for substantial improvement in ozone levels across the eastern United States.
• CAIR, which had initial compliance deadlines in 2009 and 2010, is a major reason that almost all areas in the East have met the 1997 and 2006 air quality standards for fine particles. In the 2005 CAIR Regulatory Impact Analysis, EPA estimated that the reductions from the CAIR requirements would avoid 13,000 premature deaths a year in 2010.
• CAIR was replaced by the Cross-State Air Pollution Rule , as of January 1, 2015 to address the 1997 ozone National Ambient Air Quality Standards (NAAQS).
• On September 7, 2016, the EPA revised the CSAPR by finalizing an update for the 2008 ozone NAAQS, known as the CSAPR Update . CSAPR Update will further reduce summertime NO X emissions from power plants in the eastern U.S. and help downwind states to meet the new ozone standards.

Mobile and industrial pollution sources release far less toxic air pollution than in 1990.

• EPA has issued emissions standards to control toxic emissions from all 174 categories of major sources (e.g., chemical plants, oil refineries, aerospace manufacturing facilities, etc.), as well as from 68 categories of small “area” sources that represent 90 percent of the worst urban toxic pollutants. 3 States have elected to administer or enforce many of these federal standards.
• These emissions are projected to be reduced by 80 percent by 2030 from 1990 levels.
• Onroad and nonroad diesel particulate matter emissions decreased by about 27 percent from 1990 to 2005 and are projected to be reduced an additional 90 percent from 2005 to 2030.
• Airborne levels of benzene, a carcinogen found in gasoline, declined by 66 percent from 1994 to 2009 based on available air quality monitoring information. 5
• Mercury emissions fell by about 80 percent between 1990 and 2014. 6 EPA regulations for several large sources of mercury such as municipal waste combustion and medical waste incineration played a significant role.
• Power plants remain the largest man-made source of mercury emissions in the United States, emitting more than half of all emissions of certain air toxics.
• The pollutants reduced under MATS are associated with harm to the developing nervous systems of unborn babies and children, cancer, and with contributing to asthma and other respiratory diseases.
• MATS was estimated to prevent up to 11,000 premature deaths, 4,700 heart attacks and 130,000 asthma attacks annually beginning in 2016.
• The value of the quantified air quality improvements from MATS for people's health alone totals $37 billion to $90 billion each year. That means that for every dollar spent to reduce this pollution, Americans get $3-9 in health benefits. These significant health benefits do not include the benefits associated with reducing air toxics emitted from power plants because EPA does not at this time have the ability to quantify such benefits. Thus, the Agency is likely underestimating the benefits of the rule.
• The benefits of MATS are widely distributed and are especially important to minority and low income populations who are disproportionately impacted by asthma and other debilitating health conditions.
• Up to 540,000 missed work or "sick" days were estimated to be avoided each year beginning in 2016, enhancing productivity and lowering health care costs for American families.

Actions to protect the ozone layer are saving millions of people from fatal skin cancers and eye cataracts.

• Actions to protect the stratospheric ozone layer will save millions of American lives from skin cancer between 1990 and 2165. The actions also will avoid hundreds of millions of non-fatal skin cancers and tens of millions of cases of eye cataracts for Americans born between 1985 and 2100, according to a peer-reviewed 1999 EPA study .
• The United States is one of 197 countries that are parties to the Montreal Protocol , an international treaty to protect the ozone layer. In 2012 the treaty marked its 25th anniversary. Helping developing countries comply through mechanisms like the Montreal Protocol's Multilateral Fund (MLF) will help assure success in restoring the ozone layer. Scientists estimate recovery by the middle of the 21st century. <Learn more about the Montreal Protocol.>
• Consistent with the Montreal Protocol, the Clean Air Act requires that EPA develop and implement regulations for the responsible management of ozone-depleting substances in the United States to help restore the ozone layer. The law uses multiple tools including the phase-out of certain chemicals, bans on nonessential products containing or made with such chemicals, and prohibition of the release of ozone-depleting refrigerants during the service, maintenance, and disposal of air conditioners and other refrigeration equipment.
• The United States already has phased out the ozone depleting substances that Congress identified as "most damaging," such as CFCs and halons.
• The phase-out for Class I substances was implemented 4-6 years faster, included 13 more chemicals, and cost 30 percent less than was predicted at the time the 1990 Clean Air Act Amendments were enacted. EPA's peer-reviewed 1999 study found that under the primary estimate, every dollar invested in ozone layer protection provides $20 of societal health benefits in the United States, and that after accounting for uncertainties, the benefits still far outweigh the costs.

• The acid rain program , interstate air pollution rules , motor vehicle rules and diesel sulfur rules have dramatically cut sulfur dioxide and nitrogen oxide emissions that contribute to fine particle pollution. This has improved visibility over broad regions, including many of our national parks. <Learn more about  Protecting our Nation's Treasured Vistas .>
• Further visibility improvements are anticipated from the regional haze program mandated by Congress. As of January 15, 2013, EPA has taken more than 100 proposed or final actions on Regional Haze State Implementation Plans. Out of 52 required plans, there are 45 plans in place to ensure control of emissions that impair visibility in national parks and wilderness areas.
• States, tribes, and five multi-jurisdictional regional planning organizations worked together to develop the technical basis for these plans. Comprehensive periodic revisions to these initial plans are currently due in 2018, 2028, and every 10 years thereafter.

EPA has taken initial steps under the Act to limit emissions that cause climate change and ocean acidification.

• Consistent with a 2007 Supreme Court decision , EPA in 2009 completed a scientific determination that greenhouse gases in the atmosphere are reasonably anticipated to endanger the public health and welfare of current and future generations and that emissions of greenhouse gases from new motor vehicles contributes to this air pollution.
• EPA and the National Highway and Traffic Safety Administration worked together to set greenhouse gas and fuel economy standards for passenger vehicles in model years 2012-2016 and 2017-2025.
• Over the life of these vehicles, the standards will save an estimated $1.7 trillion for consumers and businesses and cut America's oil consumption by 12 billion barrels, while reducing greenhouse gas emissions by 6 billion metric tons.
• EPA's and NHTSA's standards for heavy-duty trucks and buses , which were issued in August 2011, present large similar benefits. The final phase two program, finalized in August of 2016, promotes a new generation of cleaner, more fuel-efficient trucks by encouraging the wider application of currently available technologies and the development of new and advanced cost-effective technologies through model year 2027.
• In January 2011, states and EPA initiated Clean Air Act permitting of greenhouse gas pollution from the largest new and modified stationary sources. In the first year of permitting, dozens of large sources such as power plants, cement plants, refineries and steel mills received pre-construction permits for greenhouse gas emissions.
• On August 3, 2015, President Obama and the EPA unveiled the Clean Power Plan -- a historic and important step in reducing carbon pollution from power plants. 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 May 12, 2016 EPA issued three final rules that together will curb emissions of methane, smog-forming volatile organic compounds (VOCs) and toxic air pollutants such as benzene from new, reconstructed and modified oil and gas sources.

On October 15, 2016, with the United States’ leadership, 197 countries adopted an amendment to phase down HFCs under the Montreal Protocol  in Kigali, Rwanda. HFCs are greenhouse gases which can have warming impacts hundreds to thousands of times more potent than carbon dioxide. Under the amendment, countries committed to cut the production and consumption of HFCs by more than 80 percent over the next 30 years.

• Selective catalytic reduction (SCR) and ultra-low NOx burners for NOx emissions
• Scrubbers which achieve 95 percent and even greater SO2 control on boilers
• Sophisticated new valve seals and leak detection equipment, including cameras that can see leaks, for refineries and chemical plans
• Low or zero VOC paints, consumer products and cleaning processes
• Chlorofluorocarbon (CFC) and hydrochlorofluorocarbons (HCFC)-free air conditioners, refrigerators, aerosol sprays and cleaning solvents
• Water and powder-based coatings to replace petroleum-based formulations
• Vehicles far cleaner than believed possible in the late 1980s due to improvements in evaporative controls, catalyst design and fuel control systems for light-duty vehicles; and treatment devices and retrofit technologies for heavy-duty engines
• Idle-reduction technologies for engines, including truck stop electrification efforts
• Market penetration of gas-electric hybrid vehicles, and clean fuels
• Routine use of continuous monitoring technology that provides data more quickly
• Multi-pollutant monitors that helps us to better understand the complex nature of air pollution

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1 Pope, C.A. III, E. Majid, and D. Dockery, 2009. “Fine Particle Air Pollution and Life Expectancy in the United States,” New England Journal of Medicine, 360: 376-386.

2 EPA, Air Toxics Web Site, About Air Toxics . (For the latest information about reducing air toxics, see the webpage, Reducing Emissions of Hazardous Air Pollutants .

3 EPA,  Air Toxics Web Site, Rules and Implementation .

4 Mobile emissions estimates are based on modeling runs conducted using the MOVES2010 highway vehicle emissions modeling system and the NONROAD2008 emissions model for nonroad sources , as well as historical and projected activity and emission rate data for aircraft, marine vessels and locomotives.

5 Estimates of the change in national benzene emissions are based on benzene ambient air monitoring data in EPA's Air Quality System (U.S. EPA, 2010), using the subset of benzene monitoring stations that have sufficient data to assess trends since 1994.

6 Mercury emissions data for 1990, 2005, and 2008 featured in table 7 in the EPA 2008 National Emissions Inventory, Version 2  Technical Support Document,  June 2012 draft .

7 EPA, (April 2012) Inventory of U.S. Greenhouse Gas Emissions and Sinks; 1990-2010 .

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Air pollution in cities is growing at an alarming rate. What measures could be taken to address this problem?

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Include an introduction and conclusion

A conclusion is essential for IELTS writing task 2. It is more important than most people realise. You will be penalised for missing a conclusion in your IELTS essay.

The easiest paragraph to write in an essay is the conclusion paragraph. This is because the paragraph mostly contains information that has already been presented in the essay – it is just the repetition of some information written in the introduction paragraph and supporting paragraphs.

The conclusion paragraph only has 3 sentences:

• Restatement of thesis
• Prediction or recommendation

To summarize, a robotic teacher does not have the necessary disciple to properly give instructions to students and actually works to retard the ability of a student to comprehend new lessons. Therefore, it is clear that the idea of running a classroom completely by a machine cannot be supported. After thorough analysis on this subject, it is predicted that the adverse effects of the debate over technology-driven teaching will always be greater than the positive effects, and because of this, classroom teachers will never be substituted for technology.

Start your conclusion with a linking phrase. Here are some examples:

• In conclusion
• To conclude
• To summarize
• In a nutshell

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Some experts say for road safety cyclists should pass a test before being allowed on public roads. To what extend do you agree or disagree?

The health benefits tof physical excercise are well-known. despite this, a lot of people do not exercise regularly. what are the reasons for this what could be done to encourage them to excercise more often, you have a full time job and also doing a part time evening course. you now find that you can not continue this course. -describe the situation -explain why you can not continue at this time -say what action you would like to take, some people suggest that setting up more gyms will encourage people to remain active. what problems are associated with this proposal what solutions can you offer for good health, you have seen an advertisement in an english newspaper for a job working in the city museum shop during the holidays. you decide to apply for the job. write a letter to the director of the museum. in your letter: introduce yourself explain what experience and special skills you have explain why you are interested in the job.

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“Air pollution in Delhi: Its Magnitude and Effects on Health”

Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi, India

Baridalyne Nongkynrih

Sanjeev kumar gupta.

Air pollution is responsible for many health problems in the urban areas. Of late, the air pollution status in Delhi has undergone many changes in terms of the levels of pollutants and the control measures taken to reduce them. This paper provides an evidence-based insight into the status of air pollution in Delhi and its effects on health and control measures instituted. The urban air database released by the World Health Organization in September 2011 reported that Delhi has exceeded the maximum PM10 limit by almost 10-times at 198 μ g/m3. Vehicular emissions and industrial activities were found to be associated with indoor as well as outdoor air pollution in Delhi. Studies on air pollution and mortality from Delhi found that all-natural-cause mortality and morbidity increased with increased air pollution. Delhi has taken several steps to reduce the level of air pollution in the city during the last 10 years. However, more still needs to be done to further reduce the levels of air pollution.

Pollution refers to the contamination of the earth's environment with materials that interfere with human health, quality of life or the natural functioning of the ecosystems. The major forms of pollution include water pollution, air pollution, noise pollution and soil contamination. Other less-recognised forms include thermal pollution and radioactive hazards. It is difficult to hold any one particular form responsible for maximum risk to health; however, air and water pollution appear to be responsible for a large proportion of pollution related health problems.

Of late, the air pollution status in Delhi has undergone many changes in terms of the levels of pollutants and the control measures taken to reduce them. This paper provides an evidence-based insight into the status of air pollution in Delhi and its effects on health and control measures instituted.

Status of Air Pollution in Delhi

Delhi (or the National Capital Territory of Delhi), is jointly administered by the central and state governments. It accommodates nearly 167.5 lakh people (2011 Census of India).( 1 )

Metros across the world bear the major brunt of environmental pollution; likewise, Delhi is at the receiving end in India.

A study funded by the World Bank Development Research Group was carried out in 1991-1994 to study the effects of air pollution.( 2 ) During the study period, the average total suspended particulate (TSP) level in Delhi was approximately five-times the World Health Organization's annual average standard. Furthermore, the total suspended particulate levels in Delhi during this time period exceeded the World Health Organization's 24-h standard on 97% of all days on which readings were taken. The study concluded that the impact of particulate matter on total non-trauma deaths in Delhi was smaller than the effects found in the United States of America, but found that a death associated with air pollution in Delhi caused more life-years to be lost because these deaths were occurring at a younger age.

A report by the Ministry of Environment and Forests, India, in 1997 reviewed the environmental situation in Delhi over concerns of deteriorating conditions.( 3 ) Air pollution was one of the areas of concern identified in this study. It was estimated that about 3000 metric tons of air pollutants were emitted every day in Delhi, with a major contribution from vehicular pollution (67%), followed by coal-based thermal power plants (12%). There was a rising trend from 1989 to 1997 as monitored by the Central Pollution Control Board (CPCB). The concentrations of carbon monoxide from vehicular emissions in 1996 showed an increase of 92% over the values observed in 1989, consequent upon the increase in vehicular population. The particulate lead concentrations appeared to be in control; this was attributable to the de-leading of petrol and restrictions on lead-handling industrial units. Delhi has the highest cluster of small-scale industries in India that contribute to 12% of air pollutants along with other industrial units.

Vehicular pollution is an important contributor to air pollution in Delhi. According to the Department of Transport, Government of National Capital Territory of Delhi, vehicular population is estimated at more than 3.4 million, reaching here at a growth rate of 7% per annum. Although this segment contributes to two-thirds of the air pollution, there has been a palpable decline compared to the 1995-1996 levels.

The PM 10 standard is generally used to measure air quality. The PM 10 standard includes particles with a diameter of 10 μm or less (0.0004 inches or one-seventh the width of a human hair). These small particles are likely to be responsible for adverse health effects because of their ability to reach the lower regions of the respiratory tract. According to the Air Quality Guideline by the World Health Organization, the annual mean concentration recommended for PM 10 was 20 μg/m 3 , beyond which the risk for cardiopulmonary health effects are seen to increase.( 4 ) Major concerns for human health from exposure to PM 10 include effects on breathing and respiratory systems, damage to lung tissue, cancer and premature death. Elderly persons, children and people with chronic lung disease, influenza or asthma are especially sensitive to the effects of particulate matter. The urban air database released by the World Health Organization in September 2011 reported that Delhi has exceeded the maximum PM 10 limit by almost 10-times at 198 μg/m 3 , trailing in the third position after Ludhiana and Kanpur.( 5 ) Vehicular emissions and industrial activities were found to be associated with indoor as well as outdoor air pollution in Delhi [ Table 1 ].( 6 – 9 )

Air pollutants in Delhi

Effects of Air Pollution on Health

A large number of studies in Delhi have examined the effect of air pollution on respiratory functions and the associated morbidity. The most comprehensive study among them was the one conducted by the Central Pollution Control Board in 2008, which identified significant associations with all relevant adverse health outcomes.( 10 ) The findings were compared with a rural control population in West Bengal. It was found that Delhi had 1.7-times higher prevalence of respiratory symptoms (in the past 3 months) compared with rural controls ( P < 0.001); the odds ratio of upper respiratory symptoms in the past 3 months in Delhi was 1.59 (95% CI 1.32-1.91) and for lower respiratory symptoms (dry cough,wheeze, breathlessness, chest discomfort) was 1.67 (95% CI 1.32-1.93). Prevalence of current asthma (in the last 12 months) and physician-diagnosed asthma among the participants of Delhi was significantly higher than in controls. Lung function was reduced in 40.3% individuals of Delhi compared with 20.1% in the control group. Delhi showed a statistically significant ( P < 0.05) increased prevalence of restrictive (22.5% vs. 11.4% in control), obstructive (10.7% vs. 6.6%) as well as combined (both obstructive and restrictive) type of lung functions deficits (7.1% vs. 2.0%). Metaplasia and dysplasia of airway epithelial cells were more frequent in Delhi, and Delhi had the greater prevalence of several cytological changes in sputum. Besides these, non-respiratory effects were also seen to be more in Delhi than in rural controls. The prevalence of hypertension was 36% in Delhi against 9.5% in the controls, which was found to be positively correlated with respirable suspended particulate matter (PM 10 ) level in ambient air. Delhi had significantly higher levels of chronic headache, eye irritation and skin irritation.

Several other community-based studies have found that air pollution is associated with respiratory morbidity.( 11 – 13 ) Numerous studies have reported an association between indoor air pollution and respiratory morbidity.( 14 – 19 )Some of these studies have concentrated on children's respiratory morbidity.( 15 , 17 , 19 ) Other studies in children have found similar correlations between particulate matter in ambient air and attention-deficit hyperactivity disorder( 20 ) between vehicular air pollution and increased blood levels of lead (a potential risk factor for abnormal mental development in children)( 21 ) and between decreased serum concentration of vitamin D metabolites and lower mean haze score (a proxy measure for ultraviolet-B radiation reaching the ground).( 22 )

Studies that have examined the compounding effect of meteorological conditions on air pollution found that winter worsened the air quality of both indoor air and outdoor air. They also found a positive correlation between the winter weather and rise in the number of patients with chronic obstructive airway disease in hospitals.( 12 , 16 )

There was a relative paucity of studies that measured outdoor air pollutant levels first hand and then tried to objectively correlate them to adverse health effects. However, some studies measured air pollutant levels and found a correlation with health-related events.( 17 , 19 )

A time-series study on air pollution and mortality from Delhi found that all-natural-cause mortality increased with increased air pollution.( 23 ) In another study, gaseous pollutants, in spite of being at a level lower than the permissible level, showed more consistent association with respiratory admissions.( 24 ) In a hospital-based study, an increase in emergency room visits for asthma, chronic obstructive airway disease and acute coronary events was reported with an increase in air pollutant levels.( 25 ) These studies are summarized in Table 2 .

Effects of air pollution in Delhi on health

Control Measures Instituted by the Government of Delhi

The nodal ministry for protecting the environment is the Ministry of Environment and Forests at the Centre and the Department of Environment of the Government of National Capital Territory of Delhi. The Central Pollution Control Board set up in 1974 under the Water Act is the principal watchdog for carrying out the functions stated in the environmental acts, implementation of National Air Quality Monitoring Programme and other activities. The Delhi Pollution Control Board is the body responsible at the state level.

From time to time, the judiciary has taken strong note of the deteriorating environmental conditions in Delhi in response to public litigations. One of the earliest such instances was the judgement passed by the Supreme Court of India to deal with the acute problem of vehicular pollution in Delhi in response to a writ petition filed in 1985. Subsequently, it ordered the shutdown of hazardous, noxious industries and hot-mix plants and brick kilns operating in Delhi.

Vehicular Policy

Control measures so far instituted include introduction of unleaded petrol (1998), catalytic converter in passenger cars (1995), reduction of sulfur content in diesel (2000) and reduction of benzene content in fuels (2000). Others include construction of flyovers and subways for smooth traffic flow, introduction of Metro rail and CNG for commercial transport vehicles (buses, taxis, auto rickshaws), phasing out of very old commercial vehicles, introduction of mandatory “Pollution Under Control” certificate with 3-month validity and stringent enforcement of emission norms complying with Bharat Stage II/Euro-II or higher emission norms. Introduction of The Air Ambience Fund levied from diesel sales and setting up of stringent emission norms for industries and thermal power stations are the other measures. Environmental awareness campaigns are also carried out at regular intervals. The Delhi Pollution Control Board conducts monthly Ambient Air Quality Monitoring at 40 locations in Delhi, and takes corrective action wherever necessary.

Industrial Policy

The first Industrial Policy for Delhi was introduced in 1982. Subsequently, a second Industrial policy (2010–2021) was issued by the Department of Industries, Government of Delhi. It is a comprehensive document envisioning higher industrial development in Delhi, with one of its mandates being to develop clean and non-polluting industries and details of steps to be undertaken in this direction have been described.

There are many other organizations that work synergistically with the government efforts to reduce air pollution. These include the Centre for Science and Environment and The Energy and Resources Institute, and the Indian Association for Air Pollution Control. Representatives of the industries include Confederation of Indian Industry and Society of Indian Automobile Manufacturers. Government agencies like Factories Inspectorate are also involved in the control of pollution. Research and academic institutions include National Environmental Engineering Research Institute, Indian Institute of Technology, Council of Scientific and Industrial Research institutions, Indian Agricultural Research Institute and various other academic institutions in and around Delhi. Professional organizations like the Indian National Science Academy, the Indian Institute of Chemical Engineers and the Indian Institute of Engineers are also involved in pollution control.

Benefits Accrued as a Result of Control Measures

Since the first act on pollution was instituted, huge progress has been made in terms of human resource, infrastructure development and research capability. Some studies tried to gather evidence for the effectiveness of control measures by comparing pre- and post-intervention health status. The study conducted by the Central Pollution Control Board demonstrated that spending 8-10 h in clean indoor environment can reduce health effects of exposure to chronic air pollution.( 10 ) A recent study found significant improvement in the respiratory health following large-scale government initiatives to control air pollution.( 26 ) It was reported that use of lower-emission motor vehicles resulted in a significant gain in disability-adjusted life-years in Delhi.( 27 ) Another study found significant evidence for reduction in respiratory illness following introduction of control measures.( 24 )

Most of the studies were ecological correlation studies, which are severely limited in their ability to draw causal inferences. But, considering the context that demanded the research, these were probably the best available designs to produce preliminary and,sometimes, policy-influencing evidences, as any other methodology would be unethical or operationally impossible.

The Government of National Capital Territory of Delhi has taken several steps to reduce the level of air pollution in the city during the last 10 years. The benefits of air pollution control measures are showing in the readings. However, more still needs to be done to further reduce the levels of air pollution. The already existing measures need to be strengthened and magnified to a larger scale. The governmental efforts alone are not enough. Participation of the community is crucial in order to make a palpable effect in the reduction of pollution. The use of public transport needs to be promoted. The use of Metro rail can be encouraged by provision of an adequate number of feeder buses at Metro stations that ply with the desired frequency. More frequent checking of Pollution Under Control Certificates needs to be undertaken by the civic authorities to ensure that vehicles are emitting gases within permissible norms. People need to be educated to switch-off their vehicles when waiting at traffic intersections. Moreover, the “upstream” factors responsible for pollution also need to be addressed. The ever-increasing influx of migrants can be reduced by developing and creating job opportunities in the peripheral and suburban areas, and thus prevent further congestion of the already-choked capital city of Delhi.

Health, as we all know, is an all-pervasive subject, lying not only within the domains of the health department but with all those involved in human development. Many great scholars from Charaka to Hippocrates have stressed the importance of environment in the health of the individual. Therefore, all those who play a role in modifying the environment in any way, for whatever reason, need to contribute to safeguard people's health by controlling all those factors which affect it.

Source of Support: Nil

Conflict of Interest: None declared.

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Up to ‘very unhealthy’ air still detected in NCR — DENR

The air quality in some areas in the National Capital Region (NCR) remained up to “very unhealthy” levels on Tuesday morning, the Department of Environment and Natural Resources - Environmental Management Bureau (DENR - EMB) said.

As of 8 a.m., the DENR - EMB's Real-Time Ambient Air Quality Monitoring said the air quality in the following areas reached “very unhealthy” and “unhealthy for sensitive groups” levels:

• Pateros - very unhealthy with 167 air quality index index (AQI)
• Makati - unhealthy for sensitive groups with 139 AQI
• Caloocan - unhealthy for sensitive groups with 128 AQI

Most of the stations for air quality monitoring remained offline.

“Very unhealthy” level ranges from 151 to 200 AQI, which means people with heart or respiratory diseases such as asthma should stay indoors and rest as much as possible. Unnecessary trips should be postponed.

“Unhealthy for sensitive groups” level ranges from 101 to 150 AQI, which means people with respiratory diseases such as asthma shall limit outdoor exertion.

The AQI represents air pollution concentrations, providing an indication of the quality of the air and its health effects on the public.

In this case, DENR - EMB monitored air pollutants under Particulate Matter 2.5 (PM2.5) category which have diameters less than 2.5 micrometers.

PM2.5 can penetrate deep into the lungs and could lead to difficulty in breathing and lung tissue damage as well as aggravate existing cardiovascular diseases and lung problems.

“Good” level of air quality meanwhile ranges from 0 to 50 AQI, which means air quality is satisfactory and air pollution poses little or no risk.

The haze observed in Metro Manila on Monday was most likely due to local pollutants rather than vog , the Philippine Institute of Volcanology and Seismology (PHIVOLCS) said Tuesday.

PHIVOLCS Director Teresito Bacolcol said the local pollutants in Metro Manila stayed at the lower levels of the air because of the weak winds in the past three days.

“The haze that we observed yesterday is most likely due to local pollutants rather than vog. Katulad po ng nangyari sa Batangas, hindi rin po makaangat 'yung pollutants dahil mabagal po 'yung hangin causing these pollutants to remain at the lower levels and created the haze we saw yesterday,” he said.

(The haze that we observed yesterday is most likely due to local pollutants rather than vog. Just like what happened in Batangas, the pollutants could not rise because the wind was slow, causing these pollutants to remain at the lower levels and created the haze we saw yesterday.)

This "very unhealthy" air should be a wakeup call, especially with the impending harm climate change can cause to humans. On its website, the United Nations Environment Programme said air pollution is closely linked to climate change because "pollutants have a major impact on climate." 

Mainly derived from burning fossil fuels for electricity and transportation, greenhouse gas emissions are a form pollution that harms not just the environment but humans as well. Here's a good visualization of how polluted  Metro Manila air is .

The Paris Agreement of 2015 has set to limit earth's warming to 1.5C to 2C above pre-industrial times. At the moment, the world has warmed 1.1C. 

— RSJ/LA, GMA Integrated News

50+ Recent IELTS Writing Topics with Answers: Essays & Letters

14 min read

Updated On Aug 21, 2024

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This article lists recent IELTS Writing topics for Academic and General Training exams, covering Task 1 visual data and essays on themes like health, education, environment, and more. It also offers sample questions to aid in effective exam preparation.

Table of Contents

Ielts writing topics for academic writing task 1, ielts writing topics for general writing task 1, common ielts writing topics for writing task 2.

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IELTS Writing topics are one of the most essential study resources for IELTS exam preparation. There are two reasons for this: firstly, topics are often repeated in the IELTS exam and secondly, practising these IELTS Writing questions will help test-takers familiarise themselves with the format and requirements of the exam.

While the first task for the IELTS Writing exam has different versions of IELTS Academic and IELTS General , the second task is essay-writing for both. Even with differences in format or difficulty levels, both these tasks revolve around common IELTS writing topics like health, environment, education, travel, family and children, etc.

In this blog, we have compiled a list of the most popular and recent IELTS Writing topics based on the different tasks in this section and recurrent themes. Also, get hold of the IELTS writing questions and answers PDF that will help you practice at your own pace.

In the IELTS Writing Task 1 of the Academic exam, candidates have to summarize important visual information presented in graphs, charts, tables, maps, or diagrams in at least 150 words within 20 minutes.

Below are some IELTS Writing Task 1 topics with answers for each type of graphs and diagrams in IELTS Academic.

Line Graphs

Check out the list of IELTS Writing Task 1 - Line graph with IELTS writing questions and answers. Make sure to use appropriate IELTS Writing Task 1 Line Graph Vocabulary to write effective answers.

• IELTS Academic Writing Task 1 - Shops that Closed
• IELTS Academic Writing Task 1 Topic: Different sources of air pollutants - Line Graph
• IELTS Writing Task 1 - The Graph Below Shows Different Sources of Air Pollutants in the UK Sample Answers
• IELTS Academic Writing Task 1 Topic : Price changes for fresh fruits and vegetables - Line Graph
• The Percentage Of The Population In Four Asian Countries - IELTS Writing Task 1
• The Changes In Ownership Of Electrical Appliances And Amount Of Time Spent Doing Housework In Households - IELTS Writing Task 1
• IELTS Academic Writing Task 1 Topic 38: Paris Metro station passengers - Line Graph
• Projected Population Growth of China and India- Line Graph
• IELTS Academic Writing Task 1 Topic : Percentage of Car Ownership in Great Britain - Line Graph
• Waste Recycling Rates in the US From 1960 to 2011- Line Graph
• Weekday Volume of Passenger Activity on the Toronto Metro system- Line Graph
• US Consumers' Average Annual Expenditures on Cell Phone- Line Graph
• Consumption of Fish and Different kinds of Meat in a European Country- Line Graph
• Demographic Trends in Scotland- Line Graph

Here is a list of IELTS Writing topics with answers on the IELTS bar chart .

• People Who Ate Five Portions of Fruits and Vegetables Per Day in the UK - IELTS Writing Task 1
• IELTS Academic Writing Task 1 Topic : People affected by four types of noise pollution - Bar graph
• How Families in One Country Spent their Weekly Income - IELTS Writing Task 1
• Division of Household Tasks by Gender in Great Britain- Bar Graph
• Annual Pay for Doctors and Other Workers - IELTS Academic Writing Task 1 Bar Chart
• Estimated World Illiteracy Rates by Region and by Gender - IELTS Writing Task 1
• Southland’s Main Exports in 2000 and Future Projections For 2025 - IELTS Writing Task 1
• Carbon Emissions in Different Countries - IELTS Writing Task 1
• IELTS Academic Writing Task 1 Topic 22: Railway system in six cities in Europe – Bar Chart
• IELTS Writing Task 1 Test On 28th July With Band 8.0-9.0 Sample
• IELTS Academic Writing Task 1 Topic: Percentage of people living alone in 5 different age groups in the US - Bar Chart
• Amount of Leisure Time Enjoyed by Men and Women of Different Employment Statuses – Bar Chart
• USA Marriage and Divorce Rates Between 1970 and 2000 and the Marital Status of Adult Americans- Bar Graph
• Top Ten Rice-Producing Countries in the World in 2015- Bar Graph
• Rural Households that Had Internet Access Between 1999 and 2004- Bar Graph
• Information About Underground Railway Systems in Six Cities - IELTS Writing Task 1

Explore the list of IELTS writing topics related to pie charts and solve them with the help of pie chart vocabulary for IELTS preparation.

• IELTS Academic Writing Task 1 Topic : Survey conducted by a university library - Pie chart
• Methods of Transportation for People Traveling to a University - Pie Chart
• IELTS Academic Writing Task 1 Topic 13: Percentage of housing owned and rented in the UK – Pie Chart
• IELTS Academic Writing Task 1 Topic : The percentage of water used by different sectors - Pie chart
• Online shopping sales for retail sectors in Canada - IELTS Writing Task 1 Pie chart
• Percentage of Water Used for Different Purposes in Six Areas of the World- Pie Chart
• IELTS Academic Writing Task 1 Topic 18: Average Consumption of food in the world – Pie Chart
• Main Reasons Why Students Chose to Study at a Particular UK University - IELTS Writing Task 1 Academic Pie Chart
• Composition Of Household Rubbish In The United Kingdom - IELTS Writing Task 1

Here is a list of IELTS Writing topics with answers on the IELTS table chart .

• Fishing Industry in a European Country - IELTS Writing Task 1 Academic
• IELTS Academic Writing Task 1: Social and economic indicators for four countries - Table
• The Situation of Marriage and Age from 1960 to 2000 in Australia - IELTS Writing Task 1
• Past And Projected Population Figures In Various Countries - IELTS Writing Task 1
• IELTS Academic Writing Task 1 Topic 35: Number of travelers using three major German airports - Table
• IELTS Academic Writing Task 1 Topic 05: Size of US households over a number of years
• Changes in Modes of Travel in England Between 1985 and 2000- IELTS Writing Task 1 (Table)
• IELTS Academic Writing Task 1 Topic 12: Internet use in six categories by age group – Table
• Cinema Viewing Figures for Films by Country, in Millions- Table
• Number of Medals Won by the Top Ten Countries in the London 2012 Olympic Games- Table
• Sales at a Small Restaurant in a Downtown Business District- Table

Here is a list of IELTS Writing topics 2024 with answers on the IELTS Map Diagram .

• Paradise Island Map – IELTS Academic Writing Task 1 Answers
• Floor Plan of a Public Library 20 years ago and now - IELTS Writing Task 1
• A School in 1985 and the School Now - IELTS Writing Task 1
• Village of Stokeford in 1930 and 2010 - IELTS Writing Task 1 Map
• Map of the Centre of a Small Town Before and After - IELTS Writing Task 1
• Plan A & B shows a Health Centre in 2005 and in Present Day - IELTS Writing Task 1
• IELTS Academic Writing Task 1 Example 9 : Chorleywood is a village near London whose population has increased steadily - Map
• Two possible sites for the supermarket Sample Answers
• IELTS Academic Writing Task 1 Topic : Cross-sections of two tunnels
• IELTS Academic Writing Task 1: Local industrial village in England called Stamdorf - Map
• IELTS Academic Writing Task 1 : Hawaiian island chain in the centre of the Pacific Ocean - Map

Process Diagrams

Here is a list of IELTS Writing topics with answers on the IELTS Process diagram .

• Process of Making Soft Cheese - IELTS Writing Task 1
• Growing and Preparing Pineapples and Pineapple Products – IELTS Writing Task 1 Diagram
• Ceramic Pots Process - IELTS Academic Writing Task 1 Diagram
• How Orange Juice is Produced - IELTS Academic Writing Task 1
• IELTS Academic Writing Task 1 Topic 09 : Consequence of deforestation
• The Diagram Shows the Manufacturing Process of Sugar- IELTS Writing Task 1
• IELTS Academic Writing Task 1 Topic 10: How apple is canned - Diagram
• Life Cycle of the Salmon - IELTS Writing Task 1
• Academic IELTS Writing Task 1 Recycling process of wasted glass bottles Sample Answers
• Production of Potato Chips - IELTS Writing Task 1
• The Process of Milk Production - IELTS Writing Task 1
• Process of Making Pulp and Paper - IELTS Writing Task 1 Diagram
• Stages of Processing Cocoa Beans - IELTS Writing Task 1

Mixed/Combination Diagrams

The following is a list of IELTS Writing topics 2024 with answers on IELTS mixed or combination diagrams, practising which will aid in mastering these visual presentations for a top IELTS band score .

• Anthropology Graduates From One University - IELTS Writing Task 1
• Water use Worldwide and Water Consumption- Line Graph and Table
• Transport and Car Use in Edmonton- Pie Chart + Table
• Demand for Electricity in England- Line Graph and Pie Chart
• IELTS Academic Writing Task 1 Topic : Newly graduated students in the UK and their proportions - Multiple Graphs
• The table and charts below give information on the police budget - IELTS Writing Task 1

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In the IELTS General Writing Task 1 , test-takers are required to write a letter in response to a given situation. The letters are of three types depending on the context, namely formal, semi-formal and informal.

Below are some common IELTS Letter Writing topics that cover all the 3 ielts writing questions types of letters.

Formal Letters

Have a look at the list of IELTS General Writing Task 1 Sample Formal Letters that will help IELTS candidates prepare for the IELTS Writing questions for the actual exam.

• An Article in an International Travel Magazine - IELTS Writing Task 1
• A Magazine Wants to Include Contributions from its Readers - IELTS Writing Task 1
• Recently Booked a Part-Time Course at a College Now Need to Cancel Your Booking - IELTS Writing Task 1 General Formal Letter
• Advertisement From a Couple Who Live in Australia - IELTS Writing Task 1 General Formal Letter
• You Found You had Left Some Important Papers at the Hotel – IELTS General Writing Task 1
• Advertisement for a Training Course which will be Useful – IELTS Writing Task 1
• Write a Letter to Your Manager about a Party that You Want to Organize at the Office – IELTS General Writing Task 1
• A Feedback for a Short Cookery Course – IELTS General Writing Task 1
• Letter to the Local Authority about Construction of an Airport - IELTS Writing Task 1
• You Are Soon Going to Spend Three Months Doing Work Experience in an Organisation - IELTS Writing Task 1

Semi - formal Letters

The following is a list of IELTS General Writing Task 1 Sample Semi-Formal Letters with answers.

• A Friend Of Yours Is Thinking About Applying For The Same Course - IELTS Writing Task 1 General Semi-Formal Letter
• Letter to Neighbour About Barking Dog - IELTS Writing Task 1
• A Letter to Your Friend Who Lives in Another Town and Invite - IELTS Writing Task 1
• Letter to a Singer about His/Her Performance – IELTS General Writing Task 1
• You Have a Full-time Job and Doing a Part-time Evening Course - IELTS Writing Task 1
• Letter to Neighbor About the Damaged Car While Parking - IELTS Writing Task 1
• You Work for an International Company- Semi-formal letter
• You and Your Family are Living in Rented Accommodation- Semiformal Letter

Informal Letters

Here is a list of IELTS Writing topics with answers on the IELTS General Writing Task 1 Informal Letters that will help you to learn how to write an IELTS informal letter and brush up your writing skills.

• A Friend is Thinking of Going on a Camping Holiday - IELTS Writing Task 1
• Advice about Learning a New Sport – IELTS Writing Task 1 (Informal Letter)
• Help with a College Project - IELTS Writing Task 1 from Cambridge IELTS General 18
• Write a Letter to Your Friend Planning a Weekend Trip - IELTS General Writing Task 1
• Your Parents will be Celebrating their 50th Anniversary Next Month- Informal letter
• You are Studying English at a Private Language School- Informal Letter
• You Have a Friend Who has always Liked the Car you Currently Drive- Informal Letter
• You Have Recently Started Work in a New Company- Informal letter
• A friend Asking for Advice About a Problem at Work- Informal letter
• A Friend has Agreed to Look After your House- Informal Letter

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IELTS Writing Task 2 is similar for both IELTS Academic and IELTS General Training with minor differences in the difficulty level. Therefore, let us have a look at the compilation of IELTS writing topics with answers for different IELTS Writing Task 2 sample essays based on the common common IELTS Writing topics 2024.

Business, Work & Talent

Work-related topics often cover issues such as work-life balance, the gig economy, and the impact of automation on employment. Also, business topics may include discussions on corporate responsibility, entrepreneurship, and the impact of globalization on local businesses.

• Some people are born with certain talents - IELTS Writing Task 2
• Women Should be Allowed to Join the Army, the Navy and the Air Force just like Men - IELTS Writing Task 2
• IELTS Writing Task 2: Until What Age Do You Think People Should be Encouraged to Remain in Paid Employment?
• IELTS Writing Task 2 - Top Level Authorities Should Take Suggestions From Employees
• How Realistic is the Expectation of Job Satisfaction for all Workers - IELTS Writing Task 2
• Men and Women Can Be Equally Suited to Do Any Type of Work - IELTS Writing Task 2
• People Work Long Hours Leaving Little Time for Leisure - IELTS Writing task 2
• Some People Say that it is Better to Work for a Larger Company than a Small One - IELTS Writing Task 2

Education topics often focus on the role of technology in education, the importance of higher education, and the debate over traditional vs. modern teaching methods.

• IELTS Writing Task 2: Nowadays it is More Difficult for Children to Concentrate to Pay Attention in School
• Placing Advertisements in Schools is a Great Resource for Public Schools - IELTS Writing Task 2
• IELTS Writing Task 2: Giving Homework Daily to School Children Works Well
• Very Few School Children Learn About the Value of Money: IELTS Writing Task 2
• Traditional Examination Are Not Often True to Students Ability - IELTS Writing Task 2
• Secondary School Children Should Study International News - IELTS Writing Task 2

Environment

Environmental issues are increasingly prominent in IELTS Writing, with topics covering pollution, climate change, and the conservation of natural resources.

• IELTS Writing Task 2 - Some people say domestic animals, like cats, should not be reared in cities
• We No Longer Need to have Animals Kept in Zoos - IELTS Writing Task 2
• The Importance of Biodiversity is Being More Widely Recognised - IELTS Writing Task 2
• People Should Use Public Transport to Support Pollution Control Initiatives - IELTS Writing Task 2
• International Community Must Act Immediately to Reduce Consumption of Fossil Fuels - IELTS Writing Task 2

Family and Children

IELTS Writing questions related to family and children often explore the changing dynamics of family life, parenting styles, and the impact of technology on children.

• IELTS Writing Task 2 - Young Single People No Longer Stay With Their Parents Until They Are Married
• Is it Better to Rear Children in Joint Family or in Nuclear Family - IELTS Writing Task 2
• IELTS Writing Task 2: Majority of Children are Raised by their Grandparents Due to the Fact that their Parents are Busy
• IELTS Writing Task 2: In Some Countries Children Have Very Strict Rules of Behaviour
• Some People Spend Their Lives Living Close to Where They Were Born - IELTS Writing Task 2
• Should Parents Read or Tell Stories to Their Children - IELTS Writing Task 2
• Women Make Better Parents than Men - IELTS Writing Task 2
• The Older Generations Tend to Have very Traditional Ideas - IELTS Writing Task 2

Food, Lifestyle and Entertainment

Food and entertainment related IELTS writing topics often discuss issues related to diet, the global food industry, and cultural food practices.

• Explain Why the Movies are As Popular As a Means of Entertainment - IELTS Writing Task 2
• IELTS Writing Task 2: Popular Hobbies and Interests Change Over Time
• IELTS Writing Task 2 - Which Do You Prefer Planning or Not Planning For Leisure Time?
• IELTS Writing Task 2: People Always Throw the Old Things Away When they Buy New Things
• Food Can Be Produced Much More Cheaply Today | IELTS Writing Task 2
• IELTS Writing Task 2: The Era of the Silver Screen is Coming to an End
• Why is Music Important for Many People - IELTS Writing Task 2
• IELTS Writing Task 2: Why is the Circus Still a Popular Form of Entertainment
• Crime Novels and TV Crime Dramas are Becoming Popular - IELTS Writing Task 2

Health-related topics are a staple in the IELTS Writing section, focusing on public health issues, diet, and the impact of modern lifestyles on health.

• Discuss the cause and effects of widespread drug abuse by young people - IELTS Writing Task 2
• Obesity is a Major Disease Prevalent among Children - IELTS Writing Task 2
• Exercise is the Key to Health while Others Feel that Having a Balanced Diet is More Important - IELTS Writing Task 2
• Advantages and Disadvantages of Government Providing Free Healthcare - IELTS Writing Task 2
• Tobacco and Alcohol are Drugs that Cause Addiction and Health Problems - IELTS Writing Task 2
• Many People Complain that They Have Difficulties Getting Enough Sleep - IELTS Writing Task 2
• and More People are Hiring a Personal Fitness Trainer - IELTS Writing Task 2

Language and Culture

Topics related to language and literature often explore the importance of preserving cultural heritage, language learning, and the impact of globalization on languages.

• Many Old Cities Around the World are Going Through a Major Process of Modernization - IELTS Writing Task 2
• Reading for Pleasure Develops Imagination and Better Language Skills - IELTS Writing Task 2
• IELTS Writing Task 2 - Traditional Festivals and Celebrations Have Disappeared
• Globalization will Inevitably Lead to the Total Loss of Cultural Identity - IELTS Writing Task 2

Societal issues such as violence, social inequality, and media influence are common in IELTS Writing topics.

• Individual Greed and Selfishness Have Been the Basis of Modern Society - IELTS Writing Task 2
• IELTS Writing Task 2 - Individuals Should Not Be Allowed To Carry Guns
• Nowadays We are Living in a Throwaway Society - IELTS Writing Task 2
• Different People Have Different Approaches to Life - IELTS Writing Task 2
• Violence and Conflict were more Evident under Male Leadership than under Female Leadership - IELTS Writing Task 2
• What Changes Do You Think this New Century Will Bring - IELTS Writing Task 2
• People Remember Special Gifts or Presents they Receive - IELTS Writing Task 2

Sports topics in IELTS Writing often cover the role of sports in education, the impact of professional sports on society, and issues related to sportsmanship.

• Many People Think Olympic Games and World Cup are an Enormous Waste of Money- IELTS Writing Task 2
• Cricket has Become More Popular than the National Sports - IELTS Writing Task 2
• Sports Today is Turning into a Business - IELTS Writing Task 2

Technology & Science

Technology is a rapidly evolving field, and its impact on society, work, and communication is a common topic in IELTS Writing. Media-related topics also come under this section and often focus on the influence of mass media, the ethics of journalism, and the role of the internet in modern communication.

• Internet and Computers Will Ever Replace the Book or the Written Word - IELTS Writing Task 2
• IELTS Writing Task 2: More and More People are Choosing to Read Ebooks Rather than Paper Books
• Some People Think That Mobile Phones Should Be Banned in Public Places - IELTS Writing Task 2
• Persuade More People to Embrace Electric Cars – IELTS Writing
• The most important aim of science should be to improve people's lives - IELTS Writing Task 2
• People May No Longer Be Able to Pay for Things Using Cash - IELTS Writing Task 2
• Mobile Phones and the Internet could have Many Benefits for Old People - IELTS Writing Task 2

Tourism and Travel

Tourism and travel topics may include discussions on the impact of tourism on local cultures, the environment, and the global economy.

• Foreign Visitors Should Pay More Than Local Visitors for Cultural and Historical Attractions - IELTS Writing Task 2
• IELTS Writing Task 2: What Do You Think are the Benefits of Going Away on Holidays?
• In the Future More People Will Go On Holiday in Their Own Country - IELTS Writing Task 2

Download the IELTS writing topics PDF that contain all the IELTS writing topics with answers to fasttrack your IELTS preparation!

Being familiar with these IELTS Writing topics and practicing your writing skills within these themes can help you prepare more effectively for the IELTS Writing test. Moreover, understanding the issues and arguments related to each topic will enable you to write well-rounded essays that meet the IELTS criteria. So, if you need further guidance through a free demo session or sign up for free IELTS webinars .

Additional Reads

• IELTS Academic Writing Task 1: Useful Tips and Vocabulary to Describe a Graph or Chart
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• IELTS Band 9 Essay Samples: Writing Task 2 Insights for IELTS Learners
• Visuals: Writing About Graphs, Tables and Diagrams for IELTS Writing Task 1 (Ebook)
• How to Plan an IELTS Writing Task 2 Essay (Best Strategy)
• IELTS Writing Task 2 Preparation Tips/Tricks

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