fluoride in drinking water research paper

Is Fluoridated Drinking Water Safe?

Countries that do not fluoridate their water have also seen big drops in the rate of cavities..

Since the mid-1940s, compounds containing the mineral fluoride have been added to community water supplies throughout the U.S. to prevent tooth decay. Health concerns expressed by opponents have largely been dismissed until recently. Now, evidence is mounting that in an era of fluoridated toothpastes and other consumer products that boost dental health, the potential risks from consuming fluoridated water may outweigh the benefits for some individuals. Last summer, for the first time in 53 years, the U.S. Public Health Service lowered its recommended levels of fluoride in drinking water.

The Evidence

Beginning in the early 20th century, scientists linked high levels of naturally occurring fluoride in certain community water supplies to low levels of tooth decay. In 1945, Grand Rapids, Michigan, became the first community in the world to add fluoride to tap water. When subsequent studies showed a significantly lower rate of cavities in schoolchildren, water fluoridation spread to other towns and cities. U.S. Centers for Disease Control and Prevention named community water fluoridation one of the 10 great public health achievements of the 20th century.

But many experts now question the scientific basis for the intervention. In June 2015, the Cochrane Collaboration—a global independent network of researchers and health care professionals known for rigorous scientific reviews of public health policies—published an analysis of 20 key studies on water fluoridation.  They found that while water fluoridation is effective at reducing tooth decay among children, “no studies that aimed to determine the effectiveness of water fluoridation for preventing caries [cavities] in adults met the review’s inclusion criteria.”  *

The Cochrane report also concluded that early scientific investigations on water fluoridation (most were conducted before 1975) were deeply flawed. “We had concerns about the methods used, or the reporting of the results, in … 97 percent of the studies,” the authors noted. One problem: The early studies didn’t take into account the subsequent widespread use of fluoride-containing toothpastes and other dental fluoride supplements, which also prevent cavities. This may explain why countries that do not fluoridate their water have also seen big drops in cavity rates (see chart).

Source: OECD.Stat/Dental Health

Chart updated on June 15, 2016. An earlier version of this chart incorrectly listed Australia and Chile as having non-fluoridated water. The water in both countries is fluoridated.

Moreover, fluoride itself may be dangerous at high levels. Excessive fluoride causes fluorosis—changes in tooth enamel that range from barely noticeable white spots to staining and pitting. Fluoride can also become concentrated in bone—stimulating bone cell growth, altering the tissue’s structure, and weakening the skeleton.

Perhaps most worrisome is preliminary research in laboratory animals suggesting that high levels of fluoride may be toxic to brain and nerve cells. And human epidemiological studies have identified possible links to learning, memory, and cognition deficits, though most of these studies have focused on populations with fluoride exposures higher than those typically provided by U.S. water supplies.

The Bottom Line

Comments by Philippe Grandjean, adjunct professor of environmental health, Harvard T.H. Chan School of Public Health:

“We should recognize that fluoride has beneficial effects on dental development and protection against cavities. But do we need to add it to drinking water so it gets into the bloodstream and potentially into the brain? To answer this, we must establish three research priorities.

“First, since dental cavities have decreased in countries both with and without water fluoridation, we need to make sure we are dosing our water with the proper amount of fluoride for dental medicine purposes, but no more.

“Second, we need to make sure fluoridation doesn’t raise the risk of adverse health effects. In particular, we need basic research on animals that would help us understand the mechanisms by which fluoride may be toxic to the developing brain.

“Third, we need to find out if there are populations highly vulnerable to fluoride in drinking water—bottle-fed infants whose formula is made with tap water, for example, or patients undergoing dialysis. If these individuals are at risk, their water must come from a source that is lower in fluoride.”

* This description of the Cochrane Collaboration’s findings in relation to water fluoridation and adult cavities is a clarification of the text in the print edition of the Spring 2016 Harvard Public Health , where this article originally appeared.

Nicole Davis is a science writer and communications consultant specializing in biomedicine and biotechnology. She holds a PhD in genetics from Harvard University.

News from the School

Red meat and diabetes

How for-profit medicine is harming health care

A tradition of mentoring

Promising HIV treatment

Fluoride in Drinking Water: A Scientific Review of EPA's Standards (2006)

Chapter: 1 introduction, 1 introduction.

Under the Safe Drinking Water Act, the U.S. Environmental Protection Agency (EPA) is required to establish the concentrations of contaminants that are permitted in public drinking-water systems. A public water system is defined by EPA as a “system for the provision to the public of water for human consumption through pipes or other constructed conveyances, if such system has at least fifteen service connections or regularly serves at least twenty-five individuals” (63 Fed. Reg. 41940 [1998]). Section 1412 of the act, as amended in 1986, requires EPA to publish maximum-contaminant-level goals (MCLGs) and promulgate national primary drinking-water regulations (maximum contaminant levels [MCLs]) for contaminants in drinking water that might cause any adverse effect on human health and that are known or expected to occur in public water systems. MCLGs are health goals set at concentrations at which no known or expected adverse health effects occur and the margins of safety are adequate. MCLGs are not regulatory requirements but are used by EPA as a basis for establishing MCLs. MCLs are enforceable standards to be set as close as possible to the MCLG with use of the best technology available. For some contaminants, EPA also establishes secondary maximum contaminant levels (SMCLs), which are nonenforceable guidelines for managing drinking water for aesthetic, cosmetic, or technical effects related to public acceptance of drinking water.

Fluoride is one of the natural contaminants found in public drinking water supplies regulated by EPA. In 1986, an MCLG of 4 milligrams per liter (mg/L) and an SMCL of 2 mg/L were established for fluoride, and an MCL of 4 mg/L was promulgated. It is important to make the distinction that EPA’s standards are guidelines for restricting the amount of naturally

occurring fluoride in drinking water; they are not recommendations about the practice of adding fluoride to public drinking-water systems (see below). In this report, the National Research Council’s (NRC’s) Committee on Fluoride in Drinking Water reviews the nature of the human health risks from fluoride, estimates exposures to the general public from drinking water and other sources, and provides an assessment of the adequacy of the MCLG for protecting public health from adverse health effects from fluoride and of the SMCL for protecting against cosmetic effects. Assessing the efficacy of fluoride in preventing dental caries is not covered in this report.

This chapter briefly reviews the sources of fluoride in drinking water, states the task the committee addressed, sets forth the committee’s activities and deliberative process in developing the report, and describes the organization of the report.

FLUORIDE IN DRINKING WATER

Fluoride may be found in drinking water as a natural contaminant or as an additive intended to provide public health protection from dental caries (artificial water fluoridation). EPA’s drinking water standards are restrictions on the amount of naturally occurring fluoride allowed in public water systems, and are not recommendations about the practice of water fluoridation. Recommendations for water fluoridation were established by the U.S. Public Health Service, and different considerations were factored into how those guidelines were established.

Fluoride occurs naturally in public water systems as a result of runoff from weathering of fluoride-containing rocks and soils and leaching from soil into groundwater. Atmospheric deposition of fluoride-containing emissions from coal-fired power plants and other industrial sources also contributes to amounts found in water, either by direct deposition or by deposition to soil and subsequent runoff into water. Of the approximately 10 million people with naturally fluoridated public water supplies in 1992, around 6.7 million had fluoride concentrations less than or equal to 1.2 mg/L (CDC 1993). Approximately 1.4 million had natural fluoride concentrations between 1.3 and 1.9 mg/L, 1.4 million had between 2.0 and 3.9 mg/L, and 200,000 had concentrations equal to or exceeding 4.0 mg/L. Exceptionally high concentrations of fluoride in drinking water are found in areas of Colorado (11.2 mg/L), Oklahoma (12.0 mg/L), New Mexico (13.0 mg/L), and Idaho (15.9 mg/L).

Areas of the United States with concentrations of fluoride in drinking water greater than 1.3 mg/L are all naturally contaminated. As discussed

below, a narrow concentration range of 0.7 to 1.2 mg/L is recommended when decisions are made to intentionally add fluoride into water systems. This lower range also occurs naturally in some areas of the United States. Information on the fluoride content of public water supplies is available from local water suppliers and local, county, or state health departments.

Since 1945, fluoride has been added to many public drinking-water supplies as a public-health practice to control dental caries. The “optimal” concentration of fluoride in drinking water for the United States for the prevention of dental caries has been set at 0.7 to 1.2 mg/L, depending on the mean temperature of the locality (0.7 mg/L for areas with warm climates, where water consumption is expected to be high, and 1.2 mg/L for cool climates, where water consumption is low) (PHS 1991). The optimal range was determined by selecting concentrations that would maximize caries prevention and limit enamel fluorosis, a dose-related mottling of teeth that can range from mild discoloration of the surface to severe staining and pitting. Decisions about fluoridating a public drinking-water supply are made by state or local authorities. CDC (2002a) estimates that approximately 162 million people (65.8% of the population served by public water systems) received optimally fluoridated water in 2000.

The practice of fluoridating water supplies has been the subject of controversy since it began (see reviews by Nesin 1956; Wollan 1968; McClure 1970; Marier 1977; Hileman 1988). Opponents have questioned the motivation for and the safety of the practice; some object to it because it is viewed as being imposed on them by the states and as an infringement on their freedom of choice (Hileman 1988; Cross and Carton 2003). Others claim that fluoride causes various adverse health effects and question whether the dental benefits outweigh the risks (Colquhoun 1997). Another issue of controversy is the safety of the chemicals used to fluoridate water. The most commonly used additives are silicofluorides, not the fluoride salts used in dental products (such as sodium fluoride and stannous fluoride). Silicofluorides are one of the by-products from the manufacture of phosphate fertilizers. The toxicity database on silicofluorides is sparse and questions have been raised about the assumption that they completely dissociate in water and, therefore, have toxicity similar to the fluoride salts tested in laboratory studies and used in consumer products (Coplan and Masters 2001).

It also has been maintained that, because of individual variations in exposure to fluoride, it is difficult to ensure that the right individual dose to protect against dental caries is provided through large-scale water fluoridation. In addition, a body of information has developed that indicates

the major anticaries benefit of fluoride is topical and not systemic (Zero et al. 1992; Rölla and Ekstrand 1996; Featherstone 1999; Limeback 1999a; Clarkson and McLoughlin 2000; CDC 2001; Fejerskov 2004). Thus, it has been argued that water fluoridation might not be the most effective way to protect the public from dental caries.

Public health agencies have long disputed these claims. Dental caries is a common childhood disease. It is caused by bacteria that colonize on tooth surfaces, where they ferment sugars and other carbohydrates, generating lactic acid and other acids that decay tooth enamel and form a cavity. If the cavity penetrates to the dentin (the tooth component under the enamel), the dental pulp can become infected, causing toothaches. If left untreated, pulp infection can lead to abscess, destruction of bone, and systemic infection (Cawson et al. 1982; USDHHS 2000). Various sources have concluded that water fluoridation has been an effective method for preventing dental decay (Newbrun 1989; Ripa 1993; Horowitz 1996; CDC 2001; Truman et al. 2002). Water fluoridation is supported by the Centers for Disease Control and Prevention (CDC) as one of the 10 great public health achievements in the United States, because of its role in reducing tooth decay in children and tooth loss in adults (CDC 1999). Each U.S. Surgeon General has endorsed water fluoridation over the decades it has been practiced, emphasizing that “[a] significant advantage of water fluoridation is that all residents of a community can enjoy its protective benefit…. A person’s income level or ability to receive dental care is not a barrier to receiving fluoridation’s health benefits” (Carmona 2004).

As noted earlier, this report does not evaluate nor make judgments about the benefits, safety, or efficacy of artificial water fluoridation. That practice is reviewed only in terms of being a source of exposure to fluoride.

HISTORY OF EPA’S REGULATION OF FLUORIDE

In 1975, EPA proposed an interim primary drinking-water regulation for fluoride of 1.4-2.4 mg/L. That range was twice the “optimal” range of 0.7-1.2 mg/L recommended by the U.S. Public Health Service for water fluoridation. EPA’s interim guideline was selected to prevent the occurrence of objectionable enamel fluorosis, mottling of teeth that can be classified as mild, moderate, or severe. In general, mild cases involve the development of white opaque areas in the enamel of the teeth, moderate cases involve visible brown staining, and severe cases include yellow to brown staining and pitting and cracking of the enamel (NRC 1993). EPA considered objectionable enamel fluorosis to involve moderate to severe cases with dark stains and pitting of the teeth.

The history of EPA’s regulation of fluoride is documented in 50 Fed. Reg. 20164 (1985). In 1981, the state of South Carolina petitioned EPA

to exclude fluoride from the primary drinking-water regulations and to set only an SMCL. South Carolina contended that enamel fluorosis should be considered a cosmetic effect and not an adverse health effect. The American Medical Association, the American Dental Association, the Association of State and Territorial Dental Directors, and the Association of State and Territorial Health Officials supported the petition. After reviewing the issue, the U.S. Public Health Service concluded there was no evidence that fluoride in public water supplies has any adverse effects on dental health, as measured by loss of teeth or tooth function. U.S. Surgeon General C. Everett Koop supported that position. The National Drinking Water Advisory Council (NDWAC) recommended that enamel fluorosis should be the basis for a secondary drinking-water regulation. Of the health effects considered to be adverse, NDWAC found osteosclerosis (increased bone density) to be the most relevant end point for establishing a primary regulation.

EPA asked the U.S. Surgeon General to review the available data on the nondental effects of fluoride and to determine the concentrations at which adverse health effects would occur and an appropriate margin of safety to protect public health. A scientific committee convened by the surgeon general concluded that exposure to fluoride at 5.0 to 8.0 mg/L was associated with radiologic evidence of osteosclerosis. Osteosclerosis was considered to be not an adverse health effect but an indication of osseous changes that would be prevented if the maximum content of fluoride in drinking water did not exceed 4 mg/L. The committee further concluded that there was no scientific documentation of adverse health effects at 8 mg/L and lower; thus, 4 mg/L would provide a margin of safety. In 1984, the surgeon general concluded that osteosclerosis is not an adverse health effect and that crippling skeletal fluorosis was the most relevant adverse health effect when considering exposure to fluoride from public drinking-water supplies. He continued to support limiting fluoride concentrations to 2 mg/L to avoid objectionable enamel fluorosis (50 Fed. Reg. 20164 [1985]).

In 1984, NDWAC took up the issue of whether psychological and behavioral effects from objectionable enamel fluorosis should be considered adverse. The council concluded that the cosmetic effects of enamel fluorosis could lead to psychological and behavioral problems that affect the over-all well-being of the individual. EPA and the National Institute of Mental Health convened an ad hoc panel of behavioral scientists to further evaluate the potential psychological effects of objectionable enamel fluorosis. The panel concluded that “individuals who have suffered impaired dental appearance as a result of moderate or severe fluorosis are probably at increased risk for psychological and behavioral problems or difficulties” (R. E. Kleck, unpublished report, Nov. 17, 1984, as cited in 50 Fed. Reg. 20164 [1985]). NDWAC recommended that the primary drinking-water guideline for fluoride be set at 2 mg/L (50 Fed. Reg. 20164 [1985]).

On the basis of its review of the available data and consideration of the recommendations of various advisory bodies, EPA set an MCLG of 4 mg/L on the basis of crippling skeletal fluorosis (50 Fed. Reg. 47,142 [1985]). That value was calculated from an estimated lowest-observed-adverse-effect level of 20 mg/day for crippling skeletal fluorosis, the assumption that adult water intake is 2 L per day, and the application of a safety factor of 2.5. This factor was selected by EPA to establish an MCLG that was in agreement with a recommendation from the U.S. Surgeon General. In 1986, the MCL for fluoride was promulgated to be the same as the MCLG of 4 mg/L (51 Fed. Reg. 11,396 [1986]).

EPA also established an SMCL for fluoride of 2 mg/L to prevent objectionable enamel fluorosis in a significant portion of the population (51 Fed. Reg. 11,396 [1986]). To set that guideline, EPA reviewed data on the incidence of moderate and severe enamel fluorosis and found that, at a fluoride concentration of 2 mg/L, the incidence of moderate fluorosis ranged from 0% to 15%. Severe cases appeared to be observed only at concentrations above 2.5 mg/L. Thus, 2 mg/L was considered adequate for preventing enamel fluorosis that would be cosmetically objectionable. EPA established the SMCL as an upper boundary guideline for areas that have high concentrations of naturally occurring fluoride. EPA does not regulate or promote the addition of fluoride to drinking water. If fluoride in a community water system exceeds the SMCL but not the MCL, a notice about potential risk of enamel fluorosis must be sent to all customers served by the system (40 CFR 141.208[2005]).

In the early 1990s, the NRC was asked to independently review the health effects of ingested fluoride and EPA’s MCL. The NRC (1993) found EPA’s MCL of 4 mg/L to be an appropriate interim standard. Its report identified inconsistencies in the fluoride toxicity database and gaps in knowledge. Accordingly, the NRC recommended research in the areas of fluoride intake, enamel fluorosis, bone strength and fractures, and carcinogenicity. A list of the specific recommendations from that report is provided in Box 1-1 .

COMMITTEE’S TASK

The Safe Drinking Water Act requires that EPA periodically review existing standards for water contaminants. Because of that requirement and new research on fluoride, EPA’s Office of Water requested that the NRC reevaluate the adequacy of the MCLG and SMCL for fluoride to protect public health. The NRC assigned this task to the standing Committee on Toxicology, and convened the Committee on Fluoride in Drinking Water. The committee was asked to review toxicologic, epidemiologic, and clinical data, particularly data published since 1993, and exposure data on orally ingested fluoride from drinking water and other sources (e.g., food, tooth-

paste, dental rinses). On the basis of those reviews, the committee was asked to evaluate independently the scientific basis of EPA’s MCLG of 4 mg/L and SMCL of 2 mg/L in drinking water and the adequacy of those guidelines to protect children and others from adverse health effects. The committee was asked to consider the relative contribution of various fluoride sources (e.g., food, dental-hygiene products) to total exposure. The committee also was asked to identify data gaps and make recommendations for future research relevant to setting the MCLG and SMCL for fluoride. Addressing questions of economics, risk-benefit assessment, and water-treatment technology was not part of the committee’s charge.

The committee is aware that some readers expect this report to make a determination about whether public drinking-water supplies should be fluoridated. That expectation goes beyond the committee’s charge. As noted above, the MCLG and SMCL are guidelines for areas where fluoride con-

centrations are naturally high. They are designed with the intent to protect the public from adverse health effects related to fluoride exposure and not as guidelines to provide health benefits.

COMMITTEE’S APPROACH

To accomplish its task, the committee held six meetings between August 2003 and June 2005. The first two meetings involved data-gathering sessions that were open to the public. The committee heard presentations from EPA, CDC, individuals involved in fluoride research, fluoridation supporters, and antifluoridation proponents. The committee also reviewed a large body of written material on fluoride, primarily focusing on research that was completed after publication of the 1993 NRC report. The available data included numerous research articles, literature reviews, position papers, and unpublished data submitted by various sources, including the public. Each paper and submission was evaluated case by case on its own merits.

Unless otherwise noted, the term fluoride is used in this report to refer to the inorganic, ionic form. Most of the nonepidemiologic studies reviewed involved exposure to a specified fluoride compound, usually sodium fluoride. Various units of measure are used to express exposure to fluoride in terms of exposure concentrations and internal dose (see Table 1-1 and Chapter 3 ). To the extent possible, the committee has tried to use units that allow for easy comparisons.

In this report, the committee updates information on the issues considered in the 1993 review—namely, data on pharmacokinetics; dental effects; skeletal effects; reproductive and developmental effects; neurological and behavioral effects; endocrine effects; gastrointestinal, renal, hepatic, and immune effects; genotoxicity; and carcinogenicity. More inclusive reviews are provided on effects to the endocrine and central nervous systems, because the previous NRC review did not give those effects as much attention. The committee used a general weight-of-evidence approach to evaluate the literature, which involved assessing whether multiple lines of evidence

TABLE 1-1 Units Commonly Used for Measuring Fluoride

indicate a human health risk. This included an evaluation of in vitro assays, animal research, and human studies (conducted in the United States and other countries). Positive and negative results were considered, as well as mechanistic and nonmechanistic information. The collective evidence was considered in perspective with exposures likely to occur from fluoride in drinking water at the MCLG or SMCL.

In evaluating the effects of fluoride, consideration is given to the exposure associated with the effects in terms of dose and time. Dose is a simple variable (such as mg/kg/day), and time is a complex variable because it involves not only the frequency and duration of exposure but also the persistence of the agent in the system (kinetics) and the effect produced by the agent (dynamics). Whether the key rate-limiting events responsible for the adverse effect are occurring in the kinetic or in the dynamic pathway is important in understanding the toxicity of a chemical and in directing future research (see Rozman and Doull 2000). The committee also attempts to characterize fluoride exposures from various sources to different subgroups within the general population and to identify subpopulations that might be particularly susceptible to the effects of fluoride.

STRUCTURE OF THE REPORT

The remainder of this report is organized into 10 chapters. Chapter 2 characterizes the general public’s exposure to fluoride from drinking water and other sources. Chapter 3 provides a description of the chemistry of fluoride and pharmacokinetic information that was considered in evaluating the toxicity data on fluoride. In Chapters 4 - 9 , the committee evaluates the scientific literature on adverse effects of fluoride on teeth, the musculoskeletal system, reproduction and development, the nervous system, the endocrine system, the gastrointestinal system, the kidneys, the liver, and the immune system. Chapter 10 evaluates the genotoxic and carcinogenic potential of fluoride. Finally, Chapter 11 provides an assessment of the most significant health risks from fluoride in drinking water and its implications for the adequacy of EPA’s MCLG and SMCL for protecting the public.

Most people associate fluoride with the practice of intentionally adding fluoride to public drinking water supplies for the prevention of tooth decay. However, fluoride can also enter public water systems from natural sources, including runoff from the weathering of fluoride-containing rocks and soils and leaching from soil into groundwater. Fluoride pollution from various industrial emissions can also contaminate water supplies. In a few areas of the United States fluoride concentrations in water are much higher than normal, mostly from natural sources. Fluoride is one of the drinking water contaminants regulated by the U.S. Environmental Protection Agency (EPA) because it can occur at these toxic levels. In 1986, the EPA established a maximum allowable concentration for fluoride in drinking water of 4 milligrams per liter, a guideline designed to prevent the public from being exposed to harmful levels of fluoride. Fluoride in Drinking Water reviews research on various health effects from exposure to fluoride, including studies conducted in the last 10 years.

READ FREE ONLINE

Welcome to OpenBook!

You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

Do you want to take a quick tour of the OpenBook's features?

Show this book's table of contents , where you can jump to any chapter by name.

...or use these buttons to go back to the previous chapter or skip to the next one.

Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

Switch between the Original Pages , where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

To search the entire text of this book, type in your search term here and press Enter .

Share a link to this book page on your preferred social network or via email.

View our suggested citation for this chapter.

Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

Get Email Updates

Do you enjoy reading reports from the Academies online for free ? Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released.

• - Google Chrome

Intended for healthcare professionals

• My email alerts
• BMA member login
• Username * Password * Forgot your log in details? Need to activate BMA Member Log In Log in via OpenAthens Log in via your institution

Search form

• Advanced search
• Search responses
• Search blogs
• Systematic review of...

Systematic review of water fluoridation

• Related content
• Peer review
• Marian S McDonagh ( msm7{at}york.ac.uk ) , research fellow a ,
• Penny F Whiting , research fellow a ,
• Paul M Wilson , research fellow a ,
• Alex J Sutton , lecturer in medical statistics c ,
• Ivor Chestnutt , senior lecturer b ,
• Jan Cooper , research fellow b ,
• Kate Misso , information officer a ,
• Matthew Bradley , research fellow a ,
• Elizabeth Treasure , professor b ,
• Jos Kleijnen , professor a
• a NHS Centre for Reviews and Dissemination, University of York, York YO10 5DD
• b Dental Public Health Unit, Dental School, University of Wales College of Medicine, Cardiff CF14 4XY,
• c Department of Epidemiology and Public Health, University of Leicester, Leicester LE1 6TP
• Correspondence to: M McDonagh
• Accepted 12 September 2000

Objective: To review the safety and efficacy of fluoridation of drinking water.

Design: Search of 25 electronic databases and world wide web. Relevant journals hand searched; further information requested from authors. Inclusion criteria were a predefined hierarchy of evidence and objectives. Study validity was assessed with checklists. Two reviewers independently screened sources, extracted data, and assessed validity.

Main outcome measures: Decayed, missing, and filled primary/permanent teeth. Proportion of children without caries. Measure of effect was the difference in change in prevalence of caries from baseline to final examination in fluoridated compared with control areas. For potential adverse effects, all outcomes reported were used.

Results: 214 studies were included. The quality of studies was low to moderate. Water fluoridation was associated with an increased proportion of children without caries and a reduction in the number of teeth affected by caries. The range (median) of mean differences in the proportion of children without caries was −5.0% to 64% (14.6%). The range (median) of mean change in decayed, missing, and filled primary/permanent teeth was 0.5 to 4.4 (2.25) teeth. A dose-dependent increase in dental fluorosis was found. At a fluoride level of 1 ppm an estimated 12.5% (95% confidence interval 7.0% to 21.5%) of exposed people would have fluorosis that they would find aesthetically concerning.

Conclusions: The evidence of a beneficial reduction in caries should be considered together with the increased prevalence of dental fluorosis. There was no clear evidence of other potential adverse effects.

Editorial by Hausen

Introduction

In the white paper, Saving Lives: Our Healthier Nation , the UK government highlighted the commonly held belief that there is strong evidence that water fluoridation improves and considerably reduces inequality in dental health. 1 The government also acknowledged that “the extensive research linking water fluoridation to improved dental health was mostly undertaken a few years ago,” and as a result this study was commissioned to provide a comprehensive systematic review of the safety and efficacy of fluoridation of the public water supply.

We focused on the two main objectives: the effects of fluoridation of drinking water supplies on the incidence of caries and whether fluoridation has negative effects. The full report is available elsewhere. 2

Search strategy

We searched 25 specialist databases, including Medline, Embase, TOXLINE, and Current Contents (Science Citation Index) from inception of the database to February 2000. In addition, we hand searched Index Medicus (1945-63) and Excerpta Medica (1955-73). Further searches included the world wide web and bibliographies of all included studies. We sought additional references from individuals and organisations through a dedicated web site for this review ( www.york.ac.uk/inst/crd/fluorid.htm includes the full report) and through members of a specifically designated advisory panel. Published and unpublished studies in any language were included. Full details of the search strategy are reported elsewhere. 2

Inclusion criteria

We applied two types of inclusion criteria. The first was the level of evidence, based on the risk of bias. Studies were classified into the levels of evidence. Evidence rated below level B (moderate quality evidence, moderate risk of bias) was not considered in the evaluation of efficacy. In the assessment of safety all levels of evidence were considered. If a study met only one or two of three criteria for a given level of evidence, it was assigned the next level down. Details of both types of inclusion criteria can be found on the BMJ ‘s website.

Data extraction and assessment of study quality

Inclusion criteria were assessed independently by at least two reviewers. Extraction of data from studies and assessment of validity was independently performed by two reviewers and checked by a third reviewer. Disagreements were resolved through consensus. We assessed study validity formally using a published checklist modified for this review. 3 Each item on the checklist was given one point, with a total of eight points possible for all study designs except case-control studies, which could attain a total of nine points. 2

Outcome measures

Studies that estimated the effect of fluoridation on caries investigated two main outcomes at baseline and at the final examination. These were decayed, missing, and filled primary/permanent teeth and the proportion of children without caries. The measure of effect used for the analysis was the difference of the change in prevalence of caries from baseline to the final examination in the fluoridated area compared with the control area in children of the same age.

Change in proportion (%) of children without caries in fluoridated compared with non-fluoridated areas (mean difference and 95% confidence interval)

• Download figure
• Open in new tab
• Download powerpoint

Change in decayed, missing, and filled teeth for primary/permanent teeth (mean difference and 95% confidence interval)

To allow investigation of the effect of baseline levels of caries, we took the outcome measure from the final survey data for the meta-regressions of caries studies. The outcomes used were the data on effect size (mean difference) for decayed, missing, and filled primary/permanent teeth and the data on difference in risk for the proportion of children without caries. This was done because correlation between the mean difference of the change in incidence of caries and baseline caries may lead to a spurious association. The median risk difference was used to calculate the number needed to treat for the proportion of children without caries.

Several indices are used to classify enamel opacities, including fluorosis. Dental fluorosis was defined here as any score other than normal on each index used. As the importance of a fluorosis score at the lowest level of each index is debatable, a second method was selected. This method describes the number of people who have dental fluorosis that may cause “aesthetic concern to the patient.” The level at which fluorosis was judged to cause aesthetic concern was taken from a survey of 12 year old children in the United Kingdom w10 and corresponded to a tooth surface index of fluorosis score of two or more, a Thylstrup and Fejerskov index score of three or more, or Dean's classification of “mild” or worse. Studies that used other indices could not be included in this analysis. Full details of indices can be found elsewhere. 2

Where the data were in a suitable format we plotted measures of effect and 95% confidence intervals. Heterogeneity was investigated by visual examination of plots and statistically with the Q statistic. 4 If we found significant heterogeneity we conducted meta-regression. Random effects models were adopted throughout to combine study results. 5 Meta-regression was used to explore the influence of study characteristics on outcome in an attempt to try to explain any heterogeneity between studies. 4 Stata version 6.0 (Stata Corporation, US) was used for this analysis. 6

We used multi-level regression analysis to combine studies and investigate the association of water fluoride concentration with the prevalence of dental fluorosis (the analysis was conducted separately for all fluorosis and fluorosis of aesthetic concern) and used a multilevel model to combine studies. Each area with a different fluoride concentration under observation within a study was included separately in the model. The log (odds) of having fluorosis was modelled as a function of fluoride concentration. The analysis was carried out with the MIXED procedure within SAS (SAS Institute, US). Full details of methods used in the analyses, including all factors investigated in meta-regressions can be found elsewhere. 2

We included 214 studies; none was of evidence level A (high quality, bias unlikely). The study designs used included 45 controlled before-after studies, 102 cross sectional studies, 47 ecological studies, 13 cohort (prospective or retrospective) studies, and seven case-control studies. Summaries of individual study designs and full details on findings are available elsewhere. 2

Positive effects

Twenty six studies of the effect of water fluoridation on dental caries met the inclusion criteria. All but three of the studies included were controlled before-after studies. Of the three remaining, two used prospective cohort designs and the other a retrospective cohort design. The controlled before-after studies assessed different groups of children of the same age (12 years) at the baseline (before fluoridation) and final (after fluoridation) surveys. All studies were of evidence level B (moderate), and the mean validity score was 5 (range 3.5 to 6.8) out of 8.

Figures 1 and 2 show estimates of the effect of fluoridation on the change in decayed, missing, and filled teeth and on the change in children without caries compared with control children for studies in which fluoridation was initiated after the baseline survey. w1-9 Individual studies contributed more than one age group to the results. There was significant heterogeneity among the included studies (P<0.001).

The range (median) of the mean difference in the proportion (%) of children without caries was −5.0% to 64% (14.6%; interquartile range 5.05-22.1%). In the fluoridated areas there was a significant increase in the proportion of children without caries in 19 of 30 analyses. Only one analysis found a significant decrease in the proportion of children without caries in the fluoridated area. We estimate that that a median of six people would need to receive fluoridated water for one extra person to be free from caries (interquartile range of the distribution of number needed to treat was 4 to 9 people).

Fifteen of 16 analyses found a significantly greater mean change in decayed, missing, and filled primary/permanent teeth in the fluoridated areas than the non-fluoridated areas (fig 2 ). The range (median) of mean change in decayed, missing, and filled primary/permanent teeth was 0.5-4.4 (2.25) teeth (interquartile range 1.28-3.63 teeth).

Meta-regression showed that the proportion of children without caries at baseline, the setting, and the validity score show a significant association with the difference in risk in the proportion of children without caries. A table of the results of the meta-regression can be found on the BMJ 's website. Baseline decayed, missing, and filled primary/permanent teeth, age, setting, and duration of study show a significant association with the mean difference in decayed, missing, and filled primary/permanent teeth.

Negative effects

A total of 175 included studies examined possible negative effects of water fluoridation.

Dental fluorosis

We included 88 studies of dental fluorosis. These were largely cross sectional designs, with only four controlled before-after designs. The mean (range) validity score for fluorosis was only 2.8 (1.3-5.8) out of 8. All of the studies were of evidence level C (lowest quality), except one level B study. A full list of citations is available elsewhere. 2

Regression analysis showed a significant dose-response relation for both methods of measuring the prevalence of fluorosis (figs 3 and 4 ). From these models, the pooled estimate of the prevalence of fluorosis at a water fluoride concentration of 1.0 ppm was 48% (95% confidence interval 40% to 57%) and for fluorosis of aesthetic concern 12.5% (7.0% to 21.5%). There was, however, considerable heterogeneity between results of individual studies.

Proportion of population with dental fluorosis by water fluoride concentration with 95% confidence interval for proportion. Fluoride concentration is plotted on log scale because of linear association between this and log (odds) of fluorosis. Each circle represents a study area in which the proportion of people with fluorosis is estimated—the larger the circle, the higher the precision of the estimate

Proportion of population with fluorosis of aesthetic concern by water fluoride concentration (plotted on untransformed scale because of linear association between this and log (odds) of “aesthetic fluorosis”). Each circle represents a study area in which the proportion of people with fluorosis is estimated—the larger the circle, the higher the precision of the estimate

These results show a strong association between water fluoride concentration and the proportion of the population with dental fluorosis. We estimate that six people (95% confidence interval 4 to 21) would have to be exposed to water fluoride concentrations of 1.0 ppm for one additional person to develop fluorosis of any degree, compared with a theoretical low fluoride concentration of 0.4 ppm. Of these, about one quarter will have fluorosis of aesthetic concern (number needed to treat 22, 95% confidence interval 13.6 to ∞). These estimates apply only to the comparison of 1.0 ppm with 0.4 ppm. The model may not fit data at the extreme ends (low or high concentrations) well because of the small numbers of data points at these concentrations. Though many areas in Britain may have water fluoride concentrations lower than 0.4 ppm, this concentration was chosen as the comparator (low fluoride) to ensure that the results were as reliable as possible.

Bone fracture and problems with bone development

Twenty nine studies were included on the association with bone fracture or problems with bone development and water fluoride. These studies had a mean (range) validity score of 3.4 (1.5-6.0) out of 8. All but one study was evidence level C (the other being level B).

Figure 5 shows the estimate of effect of water fluoridation compared with control for studies that provided sufficient information. w11-30 The estimates are distributed evenly around the line of no effect (1.0). There were four analyses that indicated a significant increase in risk of fracture w12 w13 w20 w21 and five that indicated a significant decrease in risk at the 5% significance level. w14 w16 w19 w24 w28 Significant heterogeneity was found (P<0.001) among studies. There were no definite patterns of association for fractures of the hip or “other sites” taken as a group.

Incidence of bone fracture (estimate of effect and 95% confidence interval). See the BMJ ‘s website for further details

Meta-regression showed that the only variable associated with the summary measure was duration of study, with studies that were 10 years or longer in duration associated with a protective effect of water fluoridation (fewer fractures).

Cancer studies

We included 26 of the association of water fluoridation and cancer. Eighteen of these studies were of evidence level C and eight of level B. The mean (range) validity score was 3.8 (2.8-4.8). Incidence of all cause cancer and mortality was considered as an outcome in 10 studies, and 22 analyses were made. w31-40 Of these, only two studies found a significant association: one found a negative association (more cancers) in one of eight subgroups, w32 the other found a significant positive effect (fewer cancers). w31 Of nine studies comprising 20 analyses of bone cancers, w41-49 one found a significant negative effect in both men and boys (more cancers). w41 Because of the varying outcome measures we could not formally pool results.

Other possible adverse effects

We included 32 studies of the association of water fluoridation with other possible negative effects. These studies examined various different outcomes, including Down's syndrome, mortality, senile dementia, goitre, and IQ. The quality of these studies was low; all studies were of evidence level C, and the average validity checklist score was 2.7 (range 1.5-4.5) out of 8. None of the studies had a prospective follow up or incorporated any form of blinding. While 22 studies mentioned potential confounding factors, only six used an analysis that controlled for them.

Three of the 33 studies found significant effects. One found a significant negative effect of water fluoride on Alzheimer's disease (increased incidence) and a significant positive effect on impaired mental functioning (decreased incidence). w49 The other found a significant positive association with congenital malformations in one of two sets of data. w50 A third study found that the combination of low iodine and high fluoride concentrations was associated with goitre and learning difficulties. w53 Because of the varying outcome measures we could not formally pool results.

The most serious defect of the studies of possible beneficial effects of water fluoridation was the lack of appropriate design and analysis. Many studies did not present an analysis at all, while others did not attempt to control for potentially confounding factors. Age, sex, social class, ethnicity, country, tooth type (primary or permanent), mean daily regional temperature, use of fluoride, total fluoride consumption, method of measurement (clinical exam or radiographs, or both), and training of examiners are all possible confounding factors in the assessment of development of dental caries.

While some of these studies were conducted in the 1940s and 50s, before the common use of such analyses, later studies also failed to use methods that were then commonplace. Many studies lacked any measure of variance for the estimates of caries presented. While most of the studies evaluating the proportion of children without caries contained sufficient data to calculate standard errors, only four of the eight studies that reported decayed, missing, and filled primary/permanent teeth provided any estimate of variance.

Outcomes measured and bias

The outcome of fluorosis was the most studied of all the adverse effects considered. Observer bias may be of particular importance in studies that assess fluorosis. Because assessment is subjective, unless the observer is blinded to the exposure status of the person being evaluated, bias can be introduced. Efforts to reduce potential observer bias were rarely undertaken in the included studies. The prevalence of fluorosis is overestimated by the indices used in the included studies because enamel opacities not caused by fluoride may be included. The degree to which the estimated 48% prevalence of fluorosis at a water fluoride concentration of 1 ppm overestimates the true prevalence is unknown. Figures 3 and 4 do not originate at 0% fluorosis because all areas included in the studies had at least a small amount of fluoride in the water. In addition, the effects of fluoride from other sources may also be playing a part.

Many studies of other potential negative effects also did not take steps to reduce bias or use analytic techniques to control for potential confounding factors. Interpretation of the results of these studies is difficult because few met inclusion criteria on each specific outcome and studies were generally of poor quality.

Statistical heterogeneity among studies may explain why individual studies report differing estimates of effect. Significant heterogeneity was found among studies of caries, fluorosis, and bone fracture and was also apparent among studies of cancer and other negative effects but could not be tested for. In addition, methodological and clinical diversity was present among these studies.

Publication bias is defined as the failure to publish research on the basis of the nature and directional significance of the results. Because of this, systematic reviews that fail to include unpublished studies may overestimate the true effect of an intervention. Because of the nature of the outcomes and study designs that we examined in this review we considered that the standard methods developed to investigate publication bias were not practical or appropriate. It is thus difficult to estimate whether publication bias is having an effect. As we took such a broad approach in searching for studies, any missed studies would have to be large and different from those that were included to overturn the overall result.

Conclusions

Given the level of interest surrounding the issue of public water fluoridation, it is surprising to find that little high quality research has been undertaken. As such, this review should provide both researchers and commissioners of research with an overview of the methodological limitations of previous research.

The evidence of a reduction in caries should be considered together with the increased prevalence of dental fluorosis. No clear evidence of other potential negative effects was found. This evidence on positive and negative effects needs to be considered along with the ethical, environmental, ecological, financial, and legal issues that surround any decisions about water fluoridation. Any future research into the safety and efficacy of water fluoridation should be carried out with appropriate methodology to improve the quality of the existing evidence base.

What is already known on this topic

Dental caries cause morbidity and suffering and incur costs

Artificial water fluoridation has been used as a community intervention to reduce the prevalence of dental caries for decades in some communities, but its use remains controversial

What this study adds

A systematic review of water fluoridation reveals that the quality of the evidence is low

Overall, reductions in the incidence of caries were found, but they were smaller than previously reported

The prevalence of fluorosis (mottled teeth) is highly associated with the concentration of fluoride in drinking water

An association of water fluoride with other adverse effects was not found

Acknowledgments

We thank Dr Keith Abrams, University of Leicester, for contributions to the analysis; Vanda Castle, NHS Centre for Reviews and Dissemination, University of York, for secretarial support; Dr Alan Glanz, Department of Health, for coordination and organisation with the Department of Health; and Marijke van Gestel, University of Maastricht, for technical assistance early in the review process. Details of the members of the advisory panel can be found on the BMJ 's website.

Contributors: All authors contributed to the design of the protocol, execution of the review and content of the paper. JK led the project and provided methodological skill to the review. MSM was lead reviewer. KM designed and implemented the electronic search strategies and assisted in locating authors. PFW, JC, MM, and MB pilot tested data extraction forms, screened studies, and extracted data. ET and PFW assessed study validity. RT and IC provided clinical interpretation of included dental trials and terminology. PFW and AJS conducted analysis of results. PMW contributed to the interpretation of the results. The advisory panel provided peer review and advice regarding the protocol, analysis, and interpretation. MSM, JK, PFW, and ET are guarantors of the paper.

Funding This review was commissioned and funded by the Department of Health. The views expressed in this review are those of the authors and not necessarily those of the Department of Health.

Competing interests None declared.

• Secretary of State for Health
• Thompson SG ,
• Dersimonian R ,

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings
• My Bibliography
• Collections
• Citation manager

Save citation to file

Email citation, add to collections.

• Create a new collection
• Add to an existing collection

Add to My Bibliography

Your saved search, create a file for external citation management software, your rss feed.

• Search in PubMed
• Search in NLM Catalog
• Add to Search

Fluoride in Drinking Water and Skeletal Fluorosis: a Review of the Global Impact

Affiliations.

• 1 National Institute of Pharmaceutical Education and Research, Near CRPF Base Camp, Bijnor-Sisendi Road, Post Office Mati, Lucknow, U.P., 226 002, India.
• 2 National Institute of Pharmaceutical Education and Research, Near CRPF Base Camp, Bijnor-Sisendi Road, Post Office Mati, Lucknow, U.P., 226 002, India. [email protected].
• PMID: 32207100
• DOI: 10.1007/s40572-020-00270-9

When safe and adequate exposure of an essential trace element is exceeded it becomes potentially toxic. Fluoride is one classic example of such a double edged sword which both plays a fundamental role in the normal growth and development of the body for example the consumption of levels between 0.5-1.0 ppm via drinking water is beneficial for prevention of dental caries but its excessive consumption leads to development of fluorosis. PURPOSE OF REVIEW: The abundance of fluorine in the environment as well as in drinking water sources are the major contributors to fluorosis. It is a serious public health concern as it is a noteworthy medical problem in 24 nations including India yet the threat of fluorosis has not been rooted out. The review focuses on recent findings related to skeletal fluorosis and role of oxidative stress in its development. The fluoride mitigation strategies adopted in recent years are also discussed. RECENT FINDINGS BASED ON CASE STUDIES: Recent findings revealed that consumption of fluoride at concentrations of 1.5 ppm is majorly responsible for skeletal fluorosis. The sampling from rural areas showed that 80% villages are having fluoride concentrations more than the WHO permissible limits and people residing in such areas are affected by the skeletal fluorosis and also in the regions of Africa and Asia endemic fluorosis have been accounted in the majority of the region affecting approximately 100 million people. Various mitigation programmes and strategies have been conducted all over the world using defluoridation. Fluorosis is a slow and progressive malady affecting our body and a serious concern to be taken into consideration and to be dealt with effectively. The fluoride toxicity although reversible, is a slow process and the side effects lack treatment options. The treatment options available are either not approachable or affordable in the rural areas commonly suffering from the fluoride toxicity. No specific treatments are available to date to treat skeletal fluorosis affectively; therefore, prevention is one of most safest and best approach to fight fluorosis. The current review lays emphasis on the skeletal fluorosis and its prevalence in recent years. It also includes the recent findings as well as the current strategies related to combat skeletal fluorosis and provides findings that might be helpful to promote the research in the field of effective treatment for fluorosis as well as development of easy and affordable methods of fluoride removal from water.

Keywords: Defluoridation; Drinking water; Fluorosis; Mitigation; Skeletal fluorosis; Treatment.

PubMed Disclaimer

Similar articles

• Health impact of supplying safe drinking water containing fluoride below permissible level on flourosis patients in a fluoride-endemic rural area of West Bengal. Majumdar KK. Majumdar KK. Indian J Public Health. 2011 Oct-Dec;55(4):303-8. doi: 10.4103/0019-557X.92411. Indian J Public Health. 2011. PMID: 22298140
• Exposure to high-fluoride drinking water and risk of dental caries and dental fluorosis in Haryana, India. Marya CM, Ashokkumar BR, Dhingra S, Dahiya V, Gupta A. Marya CM, et al. Asia Pac J Public Health. 2014 May;26(3):295-303. doi: 10.1177/1010539512460270. Epub 2012 Oct 15. Asia Pac J Public Health. 2014. PMID: 23070755
• Prevalence of dental fluorosis & dental caries in association with high levels of drinking water fluoride content in a district of Gujarat, India. Kotecha PV, Patel SV, Bhalani KD, Shah D, Shah VS, Mehta KG. Kotecha PV, et al. Indian J Med Res. 2012 Jun;135(6):873-7. Indian J Med Res. 2012. PMID: 22825606 Free PMC article.
• A review on the potential sources and health implications of fluoride in groundwater of Sub-Saharan Africa. Onipe T, Edokpayi JN, Odiyo JO. Onipe T, et al. J Environ Sci Health A Tox Hazard Subst Environ Eng. 2020;55(9):1078-1093. doi: 10.1080/10934529.2020.1770516. Epub 2020 Jun 11. J Environ Sci Health A Tox Hazard Subst Environ Eng. 2020. PMID: 32525728 Review.
• A brief and critical review on hydrofluorosis in diverse species of domestic animals in India. Choubisa SL. Choubisa SL. Environ Geochem Health. 2018 Feb;40(1):99-114. doi: 10.1007/s10653-017-9913-x. Epub 2017 Jan 31. Environ Geochem Health. 2018. PMID: 28144801 Review.
• Fluoride-Induced Alterations in the Pancreas of Mammals: A Meta-analysis. Rana S, Thakur N, Thakur R. Rana S, et al. Biol Trace Elem Res. 2024 Aug 20. doi: 10.1007/s12011-024-04344-9. Online ahead of print. Biol Trace Elem Res. 2024. PMID: 39162920 Review.
• Development and Psychometric Validation of Population-Based Knowledge Attitudes and Practices-Questionnaire on Fluoride (PBKAP-QF). Bahekar T, Meenakshi S, Narapaka PK, Kumar N, Prakash V, Murti K. Bahekar T, et al. Biol Trace Elem Res. 2024 Aug 20. doi: 10.1007/s12011-024-04334-x. Online ahead of print. Biol Trace Elem Res. 2024. PMID: 39162919
• VMT/ACP/Dextran composite nanosheets against dental caries through promoting mineralization of dentin tubules, pH buffering, and antibacterial. Xu Y, Mou J, Dai J. Xu Y, et al. J Nanobiotechnology. 2024 Aug 17;22(1):490. doi: 10.1186/s12951-024-02709-9. J Nanobiotechnology. 2024. PMID: 39153990 Free PMC article.
• Waste bamboo-derived magnetically separable bamboo-activated carbon: from characterization to effective remediation of fluoride (F - ) ions from water. Bagbi Y, Yomgam P, Libang E, Boruah B, Kaur J, Jayanthi S, Kumar S, Dhania NK. Bagbi Y, et al. RSC Adv. 2024 Aug 9;14(34):24952-24968. doi: 10.1039/d4ra03752a. eCollection 2024 Aug 5. RSC Adv. 2024. PMID: 39131494 Free PMC article.
• Apoptosis and Inflammation Involved with Fluoride-Induced Bone Injuries. Wang M, Luo K, Sha T, Li Q, Dong Z, Dou Y, Zhang H, Zhou G, Ba Y, Yu F. Wang M, et al. Nutrients. 2024 Jul 31;16(15):2500. doi: 10.3390/nu16152500. Nutrients. 2024. PMID: 39125380 Free PMC article.

Publication types

• Search in MeSH

Related information

• PubChem Compound (MeSH Keyword)

LinkOut - more resources

Full text sources.

• MedlinePlus Health Information

Research Materials

• NCI CPTC Antibody Characterization Program

Miscellaneous

• NCI CPTAC Assay Portal
• Citation Manager

NCBI Literature Resources

MeSH PMC Bookshelf Disclaimer

The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited.

Fluoride in Drinking Water and Skeletal Fluorosis: a Review of the Global Impact

• Water and Health (T Wade, Section Editor)
• Published: 23 March 2020
• Volume 7 , pages 140–146, ( 2020 )

Cite this article

• Sakshi Srivastava 1 &
• S.J.S. Flora 1  

4474 Accesses

220 Citations

2 Altmetric

Explore all metrics

When safe and adequate exposure of an essential trace element is exceeded it becomes potentially toxic . Fluoride is one classic example of such a double edged sword which both plays a fundamental role in the normal growth and development of the body for example the consumption of levels between 0.5–1.0 ppm via drinking water is beneficial for prevention of dental caries but its excessive consumption leads to development of fluorosis.

Purpose of Review

The abundance of fluorine in the environment as well as in drinking water sources are the major contributors to fluorosis. It is a serious public health concern as it is a noteworthy medical problem in 24 nations including India yet the threat of fluorosis has not been rooted out. The review focuses on recent findings related to skeletal fluorosis and role of oxidative stress in its development. The fluoride mitigation strategies adopted in recent years are also discussed.

Recent Findings Based on Case Studies

Recent findings revealed that consumption of fluoride at concentrations of 1.5 ppm is majorly responsible for skeletal fluorosis. The sampling from rural areas showed that 80% villages are having fluoride concentrations more than the WHO permissible limits and people residing in such areas are affected by the skeletal fluorosis and also in the regions of Africa and Asia endemic fluorosis have been accounted in the majority of the region affecting approximately 100 million people. Various mitigation programmes and strategies have been conducted all over the world using defluoridation.

Fluorosis is a slow and progressive malady affecting our body and a serious concern to be taken into consideration and to be dealt with effectively. The fluoride toxicity although reversible, is a slow process and the side effects lack treatment options. The treatment options available are either not approachable or affordable in the rural areas commonly suffering from the fluoride toxicity. No specific treatments are available to date to treat skeletal fluorosis affectively; therefore, prevention is one of most safest and best approach to fight fluorosis. The current review lays emphasis on the skeletal fluorosis and its prevalence in recent years. It also includes the recent findings as well as the current strategies related to combat skeletal fluorosis and provides findings that might be helpful to promote the research in the field of effective treatment for fluorosis as well as development of easy and affordable methods of fluoride removal from water.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Subscribe and save.

• Get 10 units per month
• Download Article/Chapter or eBook
• 1 Unit = 1 Article or 1 Chapter
• Cancel anytime

Price includes VAT (Russian Federation)

Instant access to the full article PDF.

Rent this article via DeepDyve

Institutional subscriptions

Similar content being viewed by others

Fluoride Contamination and Health Effects: An Indian Scenario

Fluoride Toxicity in the Fluoride Endemic Villages of Gaya District, Bihar, India

Fluoride and Fluorosis Mitigation: Indian Contributions and Its Impact

Papers of particular interest, published recently, have been highlighted as: • of importance •• of major importance.

Swapnila C, Flora SJ. Arsenic and fluoride: two major ground water pollutants. Indian J Exp Biol. 2010;48:666–78.

Google Scholar  

• Pranesh MB, Arjundas G, Kalyanaraman S, Bharati RS. Autopsy study of a case of skeletal fluorosis. Neurology India . 2019 May 1. 1977;67(3):643. The paper presents a detailed autopsy report of a patient with crippling skeletal and neurological fluorosis.

•• Mohammadi AA, Yousefi M, Yaseri M, Jalilzadeh M, Mahvi AH. Skeletal fluorosis in relation to drinking water in rural areas of West Azerbaijan, Iran. Scientific reports. 2017;7(1):17300. The study demonstrates a significant relationship between the fluoride concentrations in the water and the prevalence of skeletal fluorosis in an endemic fluorosis area.

Kurdi MS. Chronic fluorosis: the disease and its anaesthetic implications. Indian journal of anaesthesia. 2016;60(3):157–62.

CAS   PubMed   PubMed Central   Google Scholar  

Joshi S, Hlaing T, Whitford GM, Compston JE. Skeletal fluorosis due to excessive tea and toothpaste consumption. Osteoporos Int. 2011;22(9):2557–60.

CAS   PubMed   Google Scholar  

Misra UK. Endemic skeletal fluorosis. Neurol India. 2010;58(2):338.

Krishnamachari KA. Skeletal fluorosis in humans: a review of recent progress in the understanding of the disease. Prog Food Nutr Sci. 1986;10(3–4):279–314.

Kleerekoper M. Fluoride and the skeleton. Crit Rev Clin Lab Sci. 1996;33(2):139–61.

Cohn PD. A brief report on the association of drinking water fluoridation and the incidence of osteosarcoma among young males. New Jersey Department of Health. Environ . Health Service. 1992;8:1–17.

Richards A, Moskilder L, Sogaard CH. Normal age-related changes in fluoride content of vertebral trabecular bone-relation to bone quality. Bone . 1994;15(1):21–6.

Barbier O, Arreola-Mendoza L, Del Razo LM. Molecular mechanisms of fluoride toxicity. ChemBiol Interact . 2010;188(2):319–33.

CAS   Google Scholar  

Flora SJ, Pachauri V, Mittal M, Kumar D. Interactive effect of arsenic and fluoride on cardio-respiratory disorders in male rats: possible role of reactive oxygen species. Biometals. 2011;24(4):615–28.

Chouhan S, Lomash V, Flora SJ. Fluoride-induced changes in haem biosynthesis pathway, neurological variables and tissue histopathology of rats. J Appl Toxicol . 2010;30(1):63–73.

Reddy GB, Khandare AL, Reddy PY, Rao GS, Balakrishna N, Srivalli I. Antioxidant defense system and lipid peroxidation in patients with skeletal fluorosis and in fluoride-intoxicated rabbits. Toxicol Sci. 2003;72(2):363–8.

Shivarajashankara YM, Shivashankara AR, Rao SH, Bhat PG. Oxidative stress in children with endemic skeletal fluorosis. Fluoride. 2001;34(2):103–7.

Chouhan S, Tuteja U, Flora SJ. Isolation, identification and characterization of fluoride resistant bacteria: possible role in bioremediation. Appl Biochem Microbiol. 2012;48(1):43–50.

WHO. Guidelines for Drinking-water Quality. Health criteria and other supporting information, vol. 2. 2nd ed. Geneva: World Health Organization; 1996. p. 796–803.

Meena KS, Meena K, Gunsaria RK. Skeletal fluorosis in populace of Deoli Tehsil (Tonk District), Rajasthan, India. Journal of Water Pollution & Purification Research . 2018;4(3):24–8.

•• Haritash AK, Aggarwal A, Soni J, Sharma K, Sapra M, Singh B. Assessment of fluoride in groundwater and urine, and prevalence of fluorosis among school children in Haryana, India. Applied water science . 2018;8(2):52. The study suggests a correlation between the concentration of fluoride in groundwater, and urine of school children.

•• Khairnar MR, Dodamani AS, Jadhav HC, Naik RG, Deshmukh MA. Mitigation of fluorosis-a review. J Clin Diagn Res . 2015;9(6):ZE05–9. This review provides information about the available treatment/ modalities for fluorosis, available technologies for fluoride removal from water and the recent development for fluorosis mitigation program.

Dahi, E., Mtalo, F., Njau, B. and Bregnhj, H., 1996. Defluoridation using the Nalgonda technique in Tanzania.

NEERI. Defluoridation. Technology mission on drinking water in villages and related water management. Nagpur: National Environment Engineering Research Institute; 1987.

Lathaa SS, Ambika SR, Prasad SJ. Fluoride contamination status of groundwater in Karnataka. Curr Sci . 1999;76(6):730–4.

Rawlani S, Rawlani S, Rawlani S. Assessment of skeletal and non-skeletal fluorosis in endemic fluoridated areas of Vidharbha region, India: a survey. Indian journal of community medicine: official publication of Indian Association of Preventive & Social Medicine . 2010; 35 (2):298.

Flourides H.F. Toxicological Profile for Fluorides, Hydrogen Fluoride, and Fluorine. Atlanta, GA: U.S.Department of Health and Human Services, Public Health Service. 2003.

Kohn W.G, Maas WR, Malvitz DM, Presson S.M, Shaddix K.K. Recommendations for Using fluoride to prevent and control dental caries in the United States. 2001 ;50.

Download references

Author information

Authors and affiliations.

National Institute of Pharmaceutical Education and Research, Near CRPF Base Camp, Bijnor-Sisendi Road, Post Office Mati, Lucknow, U.P., 226 002, India

Sakshi Srivastava & S.J.S. Flora

You can also search for this author in PubMed   Google Scholar

Corresponding author

Correspondence to S.J.S. Flora .

Ethics declarations

Conflict of interest.

The authors declare no conflict of interest.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.

Additional information

Publisher’s note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

This article is part of the Topical Collection on on Water and Health

Rights and permissions

Reprints and permissions

About this article

Srivastava, S., Flora, S. Fluoride in Drinking Water and Skeletal Fluorosis: a Review of the Global Impact. Curr Envir Health Rpt 7 , 140–146 (2020). https://doi.org/10.1007/s40572-020-00270-9

Download citation

Published : 23 March 2020

Issue Date : June 2020

DOI : https://doi.org/10.1007/s40572-020-00270-9

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Skeletal fluorosis
• Defluoridation
• Drinking water

Advertisement

• Find a journal
• Publish with us
• Track your research

Fluoride detection in drinking water using evanescent fiber cavity ring down spectroscopy

• Conference: Optical Fibers and Sensors for Medical Diagnostics, Treatment and Environmental Applications XXI

• Lahore University of Management Sciences

• Universiti Malaysia Pahang
• This person is not on ResearchGate, or hasn't claimed this research yet.

Abstract and Figures

Discover the world's research

• 25+ million members
• 160+ million publication pages
• 2.3+ billion citations
• INT J BIOL MACROMOL

• J DRUG DELIV SCI TEC
• EUR POLYM J

• Daniyal Ghauri

• M. Imran Cheema
• M. Daniyal Ghauri
• OPT EXPRESS

• J OPT SOC AM B

• Yigit Uysalli

• Ravi Chavali
• Naga Siva Kumar Gunda

• Sushanta K. Mitra
• SENSORS-BASEL

• Jessica Litman
• Adrienne H Cheung

• Chengjie Wang
• Giel Berden

• ANAL CHIM ACTA

• Mark E. Meyerhoff

• Guoqiang Yang
• Michelle M. Patterson

• Richard E. Jackson
• Recruit researchers
• Join for free
• Login Email Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google Welcome back! Please log in. Email · Hint Tip: Most researchers use their institutional email address as their ResearchGate login Password Forgot password? Keep me logged in Log in or Continue with Google No account? Sign up

Maintenance work is planned from 22:00 BST on Monday 16th September 2024 to 22:00 BST on Tuesday 17th September 2024.

During this time the performance of our website may be affected - searches may run slowly, some pages may be temporarily unavailable, and you may be unable to access content. If this happens, please try refreshing your web browser or try waiting two to three minutes before trying again.

We apologise for any inconvenience this might cause and thank you for your patience.

Environmental Science: Advances

Fluoride contamination, consequences and removal techniques in water: a review †.

* Corresponding authors

a Department of Civil Engineering, National Institute of Technology Patna, Patna - 800005, Bihar, India E-mail: [email protected]

b Hyderabad Zonal Centre, CSIR-National Environmental Engineering Research Institute (NEERI), IICT Campus, Tarnaka, Hyderabad, Telangana, India

Fluoride contamination has created a drinking water crisis globally. At low concentrations, its presence is essential; however, it becomes toxic to human beings upon consumption of more than 1.5 mg L −1 in mainly contaminated drinking water due to geochemical reactions and geological or anthropogenic factors. To better understand the toxicity of fluoride, in this study, we examine the recent research on the possible negative consequences of excess fluoride on diverse species. A high fluoride concentration in drinking water cause skeletal fluorosis and long-term kidney, brain, thyroid, and liver damages. This review also focuses on the different techniques for the defluoridation of water, such as electro-coagulation, adsorption, membrane processes, etc. , and compares their adsorption capabilities under various situations, while their changes in the literature are reviewed. Furthermore, we present the advantages and disadvantages of different methods and conclude that each technique has shortcomings, with no single approach fitting all aspects. The condition of water pollution with fluoride and recently created technology to remove fluoride from water is evaluated, although research on fluoride contamination of water resources has been reviewed in the literature. Alternatively, this study also examines fluorosis mitigation strategies in the global and Indian settings and existing physicochemical and biological mitigation approaches. Also, the research and development results in fluoride clean-up are reviewed. Specifically, the following topics will be covered in this review: (1) fluoride contamination status, (2) consequences of fluoride contamination in drinking water on human health, and (3) current defluoridation technology.

• This article is part of the themed collections: Environmental Science Advances Recent Review Articles , Protecting Our Water Collection and Topic Collection: Drinking Water Treatment

Supplementary files

• Supplementary information PDF (867K)

Article information

Download Citation

Permissions.

Fluoride contamination, consequences and removal techniques in water: a review

S. Ahmad, R. Singh, T. Arfin and K. Neeti, Environ. Sci.: Adv. , 2022,  1 , 620 DOI: 10.1039/D1VA00039J

This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence . You can use material from this article in other publications, without requesting further permission from the RSC, provided that the correct acknowledgement is given and it is not used for commercial purposes.

To request permission to reproduce material from this article in a commercial publication , please go to the Copyright Clearance Center request page .

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party commercial publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page .

Read more about how to correctly acknowledge RSC content .

Social activity

Search articles by author, advertisements.

Welcome to FAN’s new website! If you encounter any technical issues, email us the specifics so we can resolve them.

Scientists Find Link Between Fluoride in Water, Lower Intelligence

Chalk another win for the conspiracy theorists…

In new report from the U.S. Department of Health and Human Services (HHS) found that high levels of fluoride in drinking water leads to decreased intelligence among children.

The federal report , released on Wednesday, is a confirmation of sorts of a theory long espoused by independent journalists and media commentators — a theory that has historically been scoffed at as a baseless conspiracy.

“Since 1945, the use of fluoride has been a successful public health initiative for reducing dental cavities and improving general oral health of adults and children,” the National Toxicology Program (NTP) said in the report. “The NTP monograph concluded that higher levels of fluoride exposure, such as drinking water containing more than 1.5 milligrams of fluoride per liter, are associated with lower IQ in children.”

[RELATED: Fauci Admits: No Evidence for Social Distancing, and Can’t Remember Studies Favoring Child Masking… ]

By examining data from Canada, China, India, Iran, Pakistan, and Mexico, the federal study discovered that levels of fluoride exposure above the World Health Organization’s (WHO) recommended 1.5 mg of fluoride per liter of water are associated with lower IQ in children.

The United States, which has intentionally added fluoride to drinking water federally since 1950 for its benefits to dental health, recommends fluoride levels no greater than 0.7 mg/L.

The study did not determine whether negative cognitive effects are also caused by levels recommended in the U.S., citing “insufficient data.”

The NTP did claim, however, that it found no negative effects of fluoride exposure on adults, although it is unclear if the harm done by fluoride in childhood will persist throughout adulthood.

American children are, however, potentially exposed to far higher levels of fluoride than even those recommended by the WHO.

In the U.S., the standard level of fluoride in public water supplies is usually lower than 0.7 milligrams per liter; however, there have been multiple reported instances of fluoride concentrations arriving to end users in much higher concentrations in Texas, Colorado, New Mexico, the Dakotas, Arizona, Nevada, Florida, and South Carolina — in most cases because of naturally present fluoride.

Currently, the Environmental Protection Agency (EPA) only requires that the level of fluoride in the water system be below 4mg/L, which is significantly higher than the 1.5mg/L levels above which cognitive harm was discovered.

Despite its findings, the NTP made no recommendation on potential changes to the U.S. policy of putting fluoride in water.

In Maine, the state recommends a concentration of 0.5 to 1.2 milligrams per liter for the 65 public utilities in the state that fluoridate drinking water for customers. The “Maximum Contaminant Level (MCL) the state sets for fluoride is 0.4 milligrams per liter.

The Maine CDC’s website on the health impacts of fluoride does not currently list any concerns about the impact of high levels on child intelligence.

The study comes after years of claims by the mainstream media that anyone questioning the safety and wisdom of intentionally pouring fluoride into the supply of drinking water is an unhinged conspiracy theorist.

In February, independent presidential candidate Robert Kennedy Jr. announced on X that he would instruct the Centers for Disease Control to remove fluoride from American drinking water if he were elected president.

“As president. I’m going to order the CDC to take every step necessary to remove neurotoxic fluoride from American drinking water,” said Kennedy on X.

Although Kennedy cited the testimony of a Harvard professor as proof of his claims that fluoride is a neurotoxin, he was nevertheless derided by FactCheck.org , which denied his claims and argued that the chemical additive is perfectly safe and beneficial.

Mainstream outlets have been trying for years to convince the public that there is no reason to question the widespread use of fluoride, with a 2017 report from fact-checking organization Snopes outright denying that fluoride causes lower IQ.

Original article online at: https://www.themainewire.com/2024/08/scientists-find-link-between-fluoride-in-water-lower-intelligence/

Related Articles

Welcome to the soft launch of the new FAN website! In the coming weeks the site will be updated to restore all old links, content, and functionality. We appreciate your patience.

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• Open access
• Published: 08 September 2024

Efficient and simultaneous immobilization of fluoride and lead in water and tea garden soil by bayberry tannin foam loaded zirconium

• Xiaolu Huang 1 , 2 ,
• Mei Zhang 1 , 2 ,
• Minghui Wang 1 , 2 ,
• Zhuoyu Wen 1 , 2 ,
• Yamei Jiang 1 , 2 ,
• Yunhao Sui 1 , 2 ,
• Jun Ma 1 , 3 ,
• Yang Liao 1 , 2 , 3 &
• Xiaoting Li 1 , 2 , 3  

Scientific Reports volume  14 , Article number:  20901 ( 2024 ) Cite this article

88 Accesses

1 Altmetric

Metrics details

• Environmental chemistry
• Environmental sciences
• Pollution remediation

Nowadays, human activities intensified the combined pollution of fluoride and lead in acidic tea garden soil. The key to eliminating this combined pollution is to immobilize pollutants simultaneously, thus preventing their migration from tea garden soil to tea trees. In this paper, the natural product bayberry tannin was employed as raw material to fabricate functional materials (TF-Zr) for simultaneous adsorption of fluorine (F) and lead (Pb) in water and soil by the reactivity of tannin with Pb 2+ and the affinity of Zr with F. SEM-Mapping, EDS, FT-IR, XPS were utilized to probe the immobilization mechanisms. The results showed that TF-Zr could simultaneously and efficiently adsorb F – and Pb 2+ from water with the adsorption capacity of 5.02 mg/g (Pb) and 4.55 mg/g (F). The adsorption processes were both in accordance with the proposed secondary kinetic adsorption model. Besides, the presence of F – promoted the adsorption of Pb 2+ by TF-Zr. The materials were applied into tea garden soil to explore its effect on the variation of F and Pb forms in the soil. It was found that the proportion of water-soluble fluorine, exchangeable fluorine and exchangeable lead in the tea garden soil decreased significantly, while the proportion of residual fluorine and lead increased evidently, illustrating TF-Zr possessed eximious fixation effect on the highly reactive fluorine and lead in the soil and facilitated their conversion to the more stable residue state. Therefore, TF-Zr can be used for the efficient and simultaneous immobilization of fluorine and lead in water and tea garden soil.

Similar content being viewed by others

Study on improvement of copper sulfide acid soil properties and mechanism of metal ion fixation based on Fe-biochar composite

Acidified biochar improves lead tolerance and enhances morphological and biochemical attributes of mint in saline soil

Heavy metals immobilization and bioavailability in multi-metal contaminated soil under ryegrass cultivation as affected by ZnO and MnO 2 nanoparticle-modified biochar

Introduction.

With the intensification of human activities, various pollutants have entered the environment one after another, causing serious environmental pollution in water and soil 1 . More seriously, the pollution has changed to combined pollution, which is mainly manifested by heavy metals, other inorganic and organic pollutants 2 , 3 . Recently, the combined pollution of fluorine and lead in tea garden soil is particularly prominent in China. Qin et al. found that the average value of total fluorine content in a tea garden soil in China was 945 mg/kg, which was higher than the national average value of total soil fluorine 4 . Guo et al. tested 150 tea gardens in Fujian Province, China, for soil heavy metals and found that 70% of the tea gardens had soil lead levels exceeding the organic tea garden limit (NY 5199-2002) (50 mg/kg) 5 . As the tea tree grows, fluorine and lead in soil can be enriched into tea leaves 6 , 7 . Various diseases in human bodies may caused by drinking tea polluted by lead and fluorine 8 , 9 . Research has shown that the absorption of heavy metals and fluorine by tea trees comes from water in soil 10 , 11 , 12 . Meanwhile, the bioavailability of fluoride and lead is greatly increased in acidic tea plantation soils 13 , 14 , 15 , which also potentially increases the migration of fluoride and lead from soil to tea tree and into human body through the food chain 16 , 17 . Therefore, achieving the immobilization of lead and fluorine in water and soil is the key to solving the combined pollution of fluorine and lead in tea garden soil, which is of great significance for soil ecology and human health and safety.

The addition of chemical stabilizers is one of the most common means of reducing the bio-availability of lead and fluorine in soil. Currently, a number of absorptive materials obtained from natural sources or from waste products have been reported to immobilize lead and fluorine in soil, such as chitosan, sludge and diatomite 18 , 19 , 20 . However, these absorbents have complex compositions and limited adsorption capacity. Therefore, more materials have been developed to increase the immobilization capacity for lead and fluorine in soil. For example, aluminum humate prepared by Huang et al. was able to significantly reduce the water-soluble fluorine content in tea plantation soil 21 . Different charcoals reported by Gao could also effectively reduce the content of water-soluble and exchangeable fluorine in soil, which in turn significantly reduced the accumulation of fluorine in tea plants. Biochar loaded with iron lanthanides (BC/Fe-La) and aluminum lanthanides (BC/Al-La) prepared by Fan et al. were able to reduce water-soluble fluorine in soil by 87.58% and 90.17%, respectively 22 . Chemical stabilizers can also change the morphology of heavy metal Pb and reduce the ionic mobility and bioavailability of Pb. For example, Zhang and Xia et al. achieved in situ immobilization of Pb in soil by preparing a chemical stabilizer 23 , 24 . The MgO-loaded fish scale biochar (MgO-FB) prepared by Qi et al. was effective in immobilizing Pb and other heavy metals in soil 25 . Although a series of chemical stabilizers have been developed for controlling fluorine or heavy metal Pb contamination in soils, few efficient chemical stabilizers have been reported for using in the combined contamination of fluorine and Pb in acidic tea garden soil, meanwhile reducing the biological effectiveness of fluorine and Pb. Plant tannins are natural and easily extractable polyphenols that can chelate with various heavy metal ions 26 . However, tannin molecules are water-soluble and need to be modified when used as heavy metal adsorbents. In addition, Zr (IV) is highly electropositive and it shows excellent affinity for the highly electronegative F – 27 . Both Singh et al. and Liu et al. reported that Zr (IV) is able to complex with F 28 , 29 . Cho et al. prepared hydrated zirconia chitosan bead composites, which were able to achieve simultaneous adsorption of fluorine and lead in aqueous solutions, but the adsorption efficiency needs to be improved, and the effect of its application in soil has not been explored 30 .

Therefore, in this paper, the natural product bayberry tannin was used as raw material to prepare functional materials for simultaneous immobilization of fluorine (F) and lead (Pb) by using the efficient chelation of tannin with Pb and the strong affinity of Zr with F. Firstly, the adsorption properties of the prepared materials on F – and Pb 2+ and their influencing factors were investigated in aqueous solution, and the mechanism of the adsorption of F – and Pb 2+ by the materials was revealed. Then, the material was applied to tea garden soil to investigate its stabilization effect on F and Pb in tea garden soil. This study may provide new ideas for the management of the combined pollution of fluorine and heavy metal Pb in water and soil, and provide guidance for reducing the biological effectiveness of F and Pb in tea garden soil.

Experimental

Bayberry tannin was purchased from Guangxi Rubber Factory. Zr(SO 4 ) 2 ·4H 2 O, NaF, hexamine, toluene-4-sulfonic acid, Tween80, NaHCO 3 , HNO 3 , urea, trisodium citrate, potassium dihydrogen phosphate, potassium chloride, lead nitrate, magnesium chloride, sodium acetate, hydroxylamine hydrochloride, hydrogen peroxide, ammonium oxalate, lead standard solution and fluorine standard solution were obtained from China Kolon Chemical, of which lead standard solution and fluorine standard solution are national standard substance reagents, and the rest of the drugs are analytical grade.

Preparation of tannin foam (TF)

In a typical synthesis process, 40.00 g of tannin powder was dissolved in 60 mL of distilled water with 2 h of stirring at ambient temperature. Then, 2.84 g of hexamine was added as the crosslinker while 1.12 g of toluene-4-sulfonic acid was as the catalyst. After stirring for 10 min, 1 mL of Tween80 was appended and the mixture was consequently stirred at 1800 r·min –1 for 1 h. In the end, the obtained mixture was cured at 85 ℃ for 24 h to gain brown tannin foam (TF).

Preparation of TF-Zr

Firstly, 2.00 g of Zr (SO 4 ) 2 ·4H 2 O was dissolved in 100 mL of distilled water. 1.00 g of as-prepared TF material was consequently added following with 4 h of stirring at 30 ℃. Then 7% NaHCO 3 was added dropwise to adjust the solution pH to 2–3 within 2 h, followed by continuous reaction at 40 ℃ for 1–2 h until the solution pH changed to 1.28–1.30. The obtained material was centrifuged at 4000 r / min for 10 min, filtered, and the solid material was collected. It was washed repeatedly with deionized water and dried overnight at 40 ℃ under vacuum to obtain TF-Zr. The dosage of TF-Zr of 0, 0.8, 1.4 and 2 g is represented with 0, 0.79, 1.38 and 1.96% in the experiment.

Characteristics of the materials

SEM (FEI/quanta250) was used to observe the surface morphology of the materials, XPS (Shimadzu 119 ESCA-850) was utilized to characterize the electron binding energy of elements in materials. FTIR spectra (VERTEX 70) was applied to analyze the functional groups of the materials. The surface atomic composition of the adsorbent was examined by scanning electron microscopy (S-3400N Hitachi) and EDS techniques.

Adsorption of Pb 2+ and F –

To investigate the adsorption capacity of TF-Zr on F – and Pb 2+ , different batches of TF-Zr dosage (0.02–0.12 g), the pH of solution (pH = 2.5–7), temperature (25–45 ℃) and different concentration ratios of Pb 2+ :F – (10:2, 10:5, 10:10, 10:15, 10:20, 10:30, 10:40, 10:50) were all studied, which explored the effect of different parameters on the adsorption process. The procedure is as follows: A 150 mL conical flask was applied with a certain amount of adsorbents, placed in a constant temperature water bath shaker and shaken at 120 rpm until equilibrium. The initial pH of the solution was adjusted to the desired value with dilute sodium hydroxide or hydrochloric acid, and samples were taken at certain time intervals. After filtering through a 0.45 μm membrane, the concentration of F - in the solution was determined by the fluoride ion selective electrode method. Besides, the concentration of Pb 2+ in the solution was measured by ICP-OES.

Effect of TF-Zr on the changes of fluorine and lead forms in tea garden soil

Preparation of simulated contaminated soil.

Soil samples were collected from a tea garden soil in Ya'an and followed by cleaning, drying and passing through 60 mesh. Consequently, 2 kg of soil sample was prepared, 4 g of lead nitrate was dissolved in 400 mL of distilled water and poured into the soil to be simulated, allowing uniform mixing just to wet the soil, aged for 40 days under natural conditions, air-dried, ground, sieved and set aside 31 . The basic physicochemical properties of the soil are shown in Table S1 .

Soil experimental design

The effect of the TF-Zr on the F and Pb forms in the soil was investigated in the planted tea garden soil according to the rainy weather condition in Ya'an summer. 100 g of tea garden soil was added in a 250 mL beaker and different amounts of TF-Zr (0.0 g, 0.8 g, 1.4 g, 2.0 g) was also appended in, after uniform mixing, 50 mL of distilled water was added to keep the water content at 50%. Consequently, 0.0428 g of urea, 0.0575 g of potassium dihydrogen phosphate, and 0.0161 g of potassium chloride were added to the obtained mixture respectively. Taking into account that the growth process of the tea tree needs nitrogen fertilizer, potassium fertilizer and phosphorus fertilizer and other nutrients, thus, in order to simulate the real growth environment of the tea tree in the design of the experiment, these nutrients are added in the experiment. The mixtures were placed and distilled water was added in every one week to ensure the moisture content of the soil. 30 g of soil was sampled every 20 days, air dried, ground and sieved through 60 mesh. Lastly, the pH of the soil was tested, and the different forms of lead and fluorine in soil was also examined by Tessier continuous extraction method and continuous extraction technique 32 , 33 .

Results and discussion

Adsorption performance of tf-zr on pb 2+ and f –, the effect of ph value.

Generally, the pH of the solution is one of the most crucial factors affecting adsorption. The effect of pH on the adsorption of the prepared TF-Zr on single-component Pb 2+ at a concentration of 10 mg/L and two-component Pb 2+ and F - at a concentration of 10 mg/L, respectively, was examined at the pH of 2.5–7.0, The results are shown in Fig.  1 a. Pb 2+ adsorption experiments were not performed at pH over 7 because Pb 2+ would form metal precipitates under alkaline conditions, leading to an overestimation of the adsorption capacity of Pb 2+ . Accordingly, the adsorbed Pb 2+ increased significantly as the pH increased from 2.5 to 7.0 and remained almost constant thereafter, which was due to the reduced competition between Pb 2+ and H + at the same adsorption sites of the adsorbent, and this intense competition occurs at lower pH values.

The effect of pH ( a ) and TF-Zr dosage ( b ) (the volume of the sample is 50 mL) on adsorption of single-component Pb 2+ (10 mg/L) and two-component Pb 2+ (10 mg/L) and F- (10 mg/L) (pH = 4.5 for 90 min).

The existence of F – could greatly influence the adsorption of Pb 2+ by TF-Zr material. It was found that for the adsorption of single component Pb 2+ by TF-Zr, the adsorption capacity was 2.08 mg/g and the adsorption equilibrium was reached at pH = 4.5. However, when 10 mg/L F – was added in the solution with 10 mg/L Pb + , the adsorption capacity and the pH at adsorption equilibrium was significantly increased to 4.71 mg/g and 5.5 respectively, which may be attributed to the strong affinity of Zr (IV) on TF-Zr for F – 34 , resulting in the enhanced electrostatic attraction between TF-Zr and Pb 2+ . In addition, the effect of pH on the adsorption of F - by TF-Zr is tiny 35 .

The effect of TF-Zr dosage

In Fig.  1 b, it was found that with the increase of TF-Zr dosage, the adsorption capacity of TF-Zr on Pb 2+ increased from 1.91 to 2.65 mg/g in the single-component system with only Pb 2+ and from 3.50 to 4.51 mg/g in the co-existence of 10 mg/L F – and 10 mg/L Pb 2+ , both of which reached the adsorption equilibrium at the dosage of 0.1 g TF-Zr. The higher TF-Zr dosage provided a large number of adsorption active sites for the whole reaction and increased the surface contact area of the solid–liquid phase in the system. At the same time, the presence of F - had a facilitating effect on the adsorption of Pb 2+ by TF-Zr, which was attributed to enhanced electrostatic attraction between TF-Zr and Pb 2+ , similar as the effect of pH.

Pb 2+ initial concentration and adsorption kinetics

Commonly, the initial concentration of Pb 2+ in the solution is an important factor affecting the mass transfer resistance between the aqueous solution and the adsorbent 36 . According to Fig.  2 a, in the initial adsorption stage, the adsorption capacity increased with the increase of initial concentration of Pb 2+ . Besides, the adsorption reaction of TF-Zr on Pb 2+ mainly occurred within 20 min, after which the adsorption efficiency gradually declined and slowly tended to equilibrium. At lower initial concentration of Pb 2+ (3–8 mg/L), there were sufficient number of active sites on the surface of 0.1 g TF-Zr for Pb 2+ adsorption, thus TF-Zr had higher adsorption efficiency for Pb 2+ . While, at higher initial concentration of Pb 2+ (12 mg/L), the adsorption efficiency was declined by nearly 4.35%. It was possible that the number of metal ions was more than that of active adsorption sites, leading to the lower adsorption of Pb 2+ by TF-Zr. As a whole, the adsorption of Pb 2+ by TF-Zr was limited by the adsorption time and the initial concentration of Pb 2+ .

( a ) The effect of initial Pb 2+ concentration on the adsorption of Pb 2+ by 0.1 g TF-Zr; The curves of ( b ) proposed primary dynamic and ( c ) proposed secondary dynamic; ( d ) Adsorption capacity of TF-Zr on Pb 2+ and F – under different concentration ratios of F – and Pb 2+ .

The proposed the pseudo-first-order and pseudo-second-order kinetic models were employed to investigate the kinetics of TF-Zr adsorption and analyze the removal mechanism of TF-Zr during the adsorption process. The proposed kinetic models were shown in the following mathematical expressions ( 1 ) and ( 2 ), respectively:

where q e and q t are the amount of Pb 2+ adsorbed at equilibrium and at any time t, k 1 and k 2 are the equilibrium rate constants, and t is the reaction time (min).

The linear fitting of TF-Zr for single-component Pb 2+ and two-components F – and Pb 2+ adsorption are shown in Fig.  2 b and c. Accordingly, the proposed secondary kinetic results showed a q e value of 2.364 mg/g for Pb 2+ adsorption in the absence of F – . While the q e value of TF-Zr for Pb 2+ was 4.608 mg/g in the existence of F – , which was elevated by 2.224 mg/g in comparison with that without F - addition. The fitted parameters are shown in Table 1 . It was found that the adsorption of Pb 2+ on TF-Zr fitted the second-order kinetic model well (R 2  = 0.998) in the absence of F – , demonstrating the adsorption of Pb 2+ on TF-Zr was mainly dominated by chemisorption. Similar results were obtained even if F – existed. In the meantime, the adsorption of F – on TF-Zr was consistent with the proposed the pseudo-second-order kinetic model (R 2  = 0.997) and was predominant by chemisorption as well. The positive effect of F - on the adsorption of Pb 2+ on TF-Zr was also quantified by fitting the q e values obtained from the the pseudo-first-order and pseudo-second-order kinetic models, where the q e values in the system containing F - were higher than the q e values in the system without F – .

The effect of different F – and Pb 2+ concentration ratios

As the results mentioned above, the existence of F – in the adsorption system could significantly improve the adsorption performance of Pb 2+ by the adsorbent TF-Zr. If the adsorbent would apply in actual soils for simultaneous adsorbing F – and Pb 2+ , the concentration ratios of F – and Pb 2+ could be an important factor affecting the adsorption performance of TF-Zr. It has been identified that lead and fluorine are present in various forms in actual soli and the concentration of both is a factor that impacts their forms 37 . Therefore, the effect of the concentration ratio of F – and Pb 2+ in aqueous solution on the adsorption characteristics of the adsorbent TF-Zr was investigated. At ambient temperature, F – and Pb 2+ can easily react and form PbF 2 sediment. The solubility product (k sp ) of PbF 2 refers to 3.3 × 10 –8 , and ion product (Q c ) is calculated by the following Eq. ( 3 ):

In this study, it was calculated that Q c  < k sp existed in both F – and Pb 2+ coexisting solutions, which indicated that there was no precipitation of PbF 2 . Meanwhile, the concentrations of Pb 2+ and F – in the solutions were examined by ICP-OES and fluorine ion-selective electrode techniques, respectively. As shown in Fig. S1 . both F – and Pb 2+ existed in the ionic form.

The influence of TF-Zr on Pb 2+ adsorption under different F – concentrations (different ratio of F – and Pb 2+ ) was shown in Fig.  2 d. The adsorption efficiency of TF-Zr on Pb 2+ increased with the increase of F - concentration, and the adsorption of Pb 2+ by the adsorbent tended to equilibrium when the concentration ratio of F – :Pb 2+ was 30:10, at which time the removal efficiency of lead was 99.99%. Under this condition, the corresponding adsorption capacity was 5.02 mg/g, which was 2.37 mg/g higher than that in the absence of F - . it was mainly due to that on the one hand, Pb 2+ could coordinate with the neighboring phenolic hydroxyl groups on TF-Zr, on the other hand, the electrostatic adsorption of Pb 2+ could possibly happen on the TF-Zr surface after F – adsorption 38 . It was also found that the adsorption efficiency of TF-Zr on F – increased and then decreased with the increase of F – concentration, which was attributed to the facts that when the amounts of TF-Zr adsorbent were constant, the active sites were limited. At this time, too much F – cannot interact with the limited adsorption sites (mainly Zr ions). In conclusion, the presence of F - favored the adsorption of Pb 2+ by TF-Zr, which was consistent with the above obtained results.

The exploration of adsorption mechanism

The microscopic morphology of TF-Zr was obtained by SEM scanning, finding that the surface of TF-Zr was a non-uniform porous structure (Fig.  3 a). It was observed that there was nearly no changes in the microscopic morphology of TF-Zr before and after adsorption (Figs.  3 b and 4 c). The results of EDS demonstrated that Zr was successfully loaded onto the TF surface (Fig. S2 a). The signal peaks of Pb and F were observed in Fig. S2 b and c, respectively, indicating that F and Pb were successfully adsorbed by TF-Zr. Besides, the amount of Pb increased from 1.68 to 1.78 with the addition of F - , reflecting that the existence of F - had a facilitating effect on the adsorption of Pb 2+ by TF-Zr. The Mapping element analysis of TF-Zr was shown in Fig.  3 d–f, revealing that Zr (IV) was uniformly dispersed on the TF surface. In addition, the presence of lead and fluorine elements on the surface of TF-Zr also proved that Pb 2+ and F – reacted with the function groups on the surface of TF-Zr.

SEM of TF-Zr before and after adsorption ( a ) TF-Zr, ( b ) TF-Zr adsorbed lead, ( c ) TF-Zr adsorbed both fluorine and lead and Mapping of TF-Zr adsorbed both fluorine and lead, ( d ) Zr, ( e ) Pd ( f ) F.

( a ) FT-IR; ( b ) XPS spectra of TF and TF-Zr before and after adsorption; ( c ) XPS O 1 s of TF and TF-Zr before and after TF-Zr adsorption; ( d ) XPS Zr 3d before and after TF-Zr adsorption.

As shown in Fig.  4 a, the peak at 3400–3500 cm –1 is caused by OH stretching vibration, and the peak at 1398 cm –1 is caused by OH bending vibration 39 , indicating that there are a large number of hydroxyl groups on TF surface 40 . The peaks near 1620 cm –1 and 1454 cm –1 are attributed to the characteristic peaks of the carbon skeleton structure of the benzene ring, and the peaks near 1040 cm –1 are caused by the C–O stretching vibration on the benzene ring. When Zr is loaded onto TF, the peak at 624 cm –1 is caused by the tensile vibration of Zr–O–C indicating that Zr is successfully loaded onto TF surface 41 . After the adsorption of Pb 2+ , the peak of OH bending vibration at 1398 cm –1 shifted to 1381 cm –1 . Simultaneously, the peaks at 3461 cm -1 belonging to OH stretching vibration shifted to 3429 cm –1 . These results indicated that the material TF-Zr was able to adsorb Pb 2+ , which was mainly because Pb 2+ could react with the hydroxyl groups on the surface of tannins to form stable chelates. Besides, when F - was added, the shift of OH peak at 1398 cm –1 was stronger to 1377 cm –1 in comparison with that without F – , which was possible that the absorption of Pb 2+ was intensified by the addition of F – .

XPS was utilized to determine the surface elemental composition and atomic binding energy of TF and TF-Zr before and after adsorption, through which could reveal the elemental changes and chemical reactions on the surface of the materials TF and TF-Zr 42 . As shown in Fig.  4 b, for TF-Zr, two peaks of Zr 3d 3/2 and 3d 5/2 appeared at 185.54 eV and 183.19 eV, respectively, indicating that Zr (IV) was successfully loaded on the TF surface. The peak of Pb 4f appeared after the adsorption of Pb 2+ by TF-Zr (Fig.  4 b) and the Pb 4f 5/2 and Pb 4f 7/2 peaks with 144.1 eV and 139.2 eV were found in the high-resolution spectrum of Pb (Fig. S3 ), which confirmed the successful adsorption of Pb 2+ onto the TF-Zr surface. In addition, the TF-Zr with simultaneous adsorption of Pb 2+ and F – also showed the Pb 4f. peak and F 1 s peak (Fig. S3 ), confirming that Pb 2+ and F – could be adsorbed onto the TF-Zr surface. All of these were in accordance with the results of EDS.

To reveal the mechanism of Pb 2+ adsorbing on TF-Zr, the high-resolution spectrum of O element was analyzed (Fig.  4 c). It was found that three different types of O-linked bonds were existed on TF-Zr at 533.13 eV, 532.42 eV and 530.85 eV, corresponding to C–O, C=O and Zr–O, respectively before adsorption of Pb 2+ . However, after adsorption of Pb 2+ , two new peaks appeared at 532.86 eV and 531.82 eV, corresponding to the C–O–Pb and C=O-Pb bond, respectively, while the Zr–O peak was unvaried. Moreover, the increase of the C–O–Pb peak was larger than that of C=O–Pb, indicating that Pb 2+ was mainly coordinated to the hydroxyl group on the TF-Zr surface during the adsorption. Similar results were observed after the simultaneous adsorption of Pb 2+ and F – , which indicated that the addition of F would not change the adsorption mechanism of Pb 2+ by TF-Zr, but enhance the adsorption capacity of Pb 2+ by intensifying the electrostatic adsorption between TF-Zr and Pb 2+ .

It was found from the high-resolution spectrum of Zr element (Fig.  4 d) that the binding energy of Zr had no change before and after adsorption of Pb 2+ , while the binding energies corresponding to the Zr 3d 3/2 and Zr 3d 5/2 peaks increased from 185.54 eV and 183.19 eV to 185.64 eV and 183.28 eV, respectively with F addition. These results indicated the adsorption process of Pb and F by TF-Zr was independent. Besides, the adsorption of F by TF-Zr was mainly due to the chemical reaction of Zr and F, which decreased the electron cloud density of Zr, resulting in an increase in the binding energy. Similar results were also reported by Wolter and Dou 43 , 44 . Meanwhile, the binding energy of the F1s peak was 685.2 eV after the adsorption of F and Pb by TF-Zr, which was closed to that of ZrF 4 with 685.1 eV, also indicating the reaction of F - with Zr (IV).

The application of TF-Zr in tea garden soil

Changes of soil ph.

It is reported that soil pH may affect the forms of Pb and F in soil, which can further affect the uptake of F and Pb by tea plants. Thus, in this study, the prepared material TF-Zr with different dosages were added in the tea garden soil to investigate its effect on the soil pH. Importantly, a wet experimental environment was simulated as that of tea garden soil in Ya'an city. The experimental results were showed in Fig.  5 . The addition of TF-Zr could significantly reduce the soil pH at the initial period (20 days). When different amount of TF-Zr (0, 0.8, 1.4, 2.0 g) were added after 20 days, the soil pH was CK (control) > 0.8 > 1.4 > 2.0 g. Specifically, the soil pH with 2.0 g of TF-Zr addition was significantly reduced by 0.57 compared with the control group (P < 0.05), which was due to the release of H + caused by the weak acidity of bayberry tannins. Subsequently, the soil pH gradually increased in the treatment groups with different amount of TF-Zr addition after 40 and 60 days, while the soil pH in the control group showed insignificant changes. It was proved that the increase of soil pH could be the result of the catalytic effect of hydrogen ions in the esterification reaction 45 . However, in our experiment, it may be due to the reaction between hydrogen ions and other substances in the soil that causes an increase in soil pH. Meanwhile, with the prolongation of the application time of TF-Zr, the pH value of the soil increased, which shows that the soil itself has a strong buffering effect. Although the pH value of the soil decreased at the beginning, through our tests, we found that the content of the two elements in the soil was decreasing in the mobile component, and the content of the residual state increased, indicating that at this time, pH is not the key to affect the morphology of F and Pb, but rather the TF-Zr material itself reacts with the fluorine and lead in the soil.

The soil pH with different TF-Zr dosages at different experimental days.

Changes of F forms in soil

It has been identified that different forms of F exist in tea garden soil, including water-soluble state(water-F), exchangeable state(Ex-F), ferromanganese oxidation state(Fe/Mn-F), organic bound state(Or-F) and residue state(Res-F), and their mobility and biological effectiveness in soil vary greatly 46 . It was commonly believed that the main forms of F that could migrate from soil to tea trees were the water-soluble and exchangeable states instead of total F 47 . In this work, the effect of TF-Zr applying in tea garden soil on different forms of F was investigate to evaluate the migration ability of fluorine from soil to tea trees. As shown in Fig.  6 a, the content of different forms of F in all treatment groups changed greatly after 60 day’ simulating experiment. It was found that the content of water-F and Ex-F gradually and significantly decreased and the content of Fe/Mn-F and Or-F decreased slightly, while the content of Res-F gradually increased. It was obviously found that after 60 days, the water-F and Ex-F could gradually and significantly transform to Res-F in all treatment groups.

( a ) The changes of different forms of F in soil with different amount of TF-Zr addition. ( b ) The changes of different forms of Pb in soil with different amount TF-Zr addition.

Most importantly, the changes of different forms of F with the addition of TF-Zr were quite different from those of the control group. With different amount of TF-Zr addition (0.8–2.0 g), the content of various forms of soil F in all the experimental periods changed significantly in comparison with those of the control group, especially the water-F, Ex-F. With the increase of TF-Zr addition from 0.8 to 2.0 g, the contents of Water-F and Ex-F in soil decreased gradually and significantly. Especially when 2.0 g TF-Zr was added, the water-F and Ex-F deceased by 86.07% and 71.65% after 60 day’ simulating experiment, which was 12.54 and 3.80 times higher than those of the control group. Besides, the contents of Res-F increased to 99.03% after 60 day’ simulating experiment, which was higher than that of the control group (98.37%). These results showed that adding TF-Zr could promote the transformation of Water-F and Ex-F in soil to Res-F, and the more TF-Zr was added, the more effective Water-F and Ex-F were immobilized. Thus, these findings confirmed that the addition of TF-Zr could effectively prevent fluoride migrating from tea garden soil to tea plants. It was identified that TF-Zr was a natural polymer composite and an organic material. When applied in adsorption of F in soil, it could facilitate the formation of soil agglomerates and provided more adsorption sites on the surface of the soil, resulting in more fluorine to be adsorbed onto the surface of the material and further be immobilized 48 . The ability of TF-Zr to reduce the bio-availability of soil fluorine was based on the fact that the Zr(IV) pair on the TF-Zr surface had a strong affinity for F, which could make the Zr-F structure easily formed.

Changes of Pb forms in soil

The forms of heavy metals in soil can greatly affect the soil environmental quality and uptake of heavy metals by plants. It has been identified that the exchangeable state is considered to be the most readily available for plants uptake in soil, while the residual state is extremely stable, which can be tightly bound to the mineral lattice and within the crystalline oxide and is difficult to release into the soil environment 49 . Therefore, enhancing the transformation of exchangeable state of heavy metals to residual state has been regarded as a feasible and effective way to reduce the pollution and threat of heavy metals in soil. In this work, the effect of TF-Zr applying in tea garden soil on different forms of Pb was investigated to evaluate the immobilization ability and remediation performance of soil Pb. In order to better observe the influence of TF-Zr on different forms of Pb in soil, a certain amount of exogenous lead (about 1250 mg/kg dry soil) was added to the original soil in this study. Therefore, the content of exchangeable state (Ex-Pb) in the soil was relatively high at the initial experimental stage, accounting for 77.36% of the total Pb content. As shown in Fig.  6 b, the content of different forms of Pb in all treatment groups changed greatly after 60 days’ simulating experiment, especially Ex-Pb and residue state (Res-Pb). It was found that the content of Ex-Pb gradually and significantly decreased, while the content of Res-F gradually and significantly increased, indicating the gradual and significant transformation of Ex-Pb to Res-Pb in all treatment groups with 60 days’ experiment.

Besides, it was obviously found that the changes of different forms of Pb with the addition of TF-Zr were quite different from those of the control group. After 20 days’ experiment, the Ex-Pb content decreased by 51.88%, 56.19% and 63.95% with the addition of 0.8, 1.4 and 2.0 g TF-Zr respectively, which was 2.08, 2.26 and 2.57 times higher than that of the control group (without TF-Zr addition). Meanwhile, the Res-Pb content in the total Pb in soil with the addition of 0.8, 1.4 and 2.0 g TF-Zr accounted for 22.01%, 23.30% and 33.11% after 20 days respectively, which increased by 18.39%,19.68% and 29.49% compared with the control group. These results indicated that the addition of TF-Zr could significantly accelerate the transformation of Ex-Pb to Res-Pb in soil and the more TF-Zr was added, the faster Ex-Pb was transformed to Res-Pb. After 60 days, the decrease of Ex-Pb with 2.0 g TF-Zr addition was as high as 751.25 mg/g and the increase of Res-Pb reached to 471.90 mg/g. Moreover, the changes of Fe/Mn-Pb and organic bound state(Or-Pb) contents were similar with that of Ex-Pb, but it was not as obvious as the changes of Ex-Pb content, which was possibly due to the relative lower content of Fe/Mn-Pb and Or-Pb in comparison with Ex-Pb. In conclusion, the application of TF-Zr in tea garden soil could promote and accelerate the transformation of Ex-Pb to Res-Pb, and also promote the transformation of other forms of Pb to Res-Pb, which could effectively prevent the pollution of Pb in soil and control the migration of Pb to tea trees. The ability of TF-Zr to reduce the pollution and threat of Pb in soil was based on the complexation of hydroxyl group on the surface of TF-Zr with the Ex-Pb, in which the highly reactive Ex-Pb could be converted to the most stable form (Res-Pb).

The analysis of Pearson correlation

The immobilization of Pb and F in soil by application of the material TF-Zr could be affected by many factors, such as soil pH, TF-Zr dosage and different forms of Pb and F. The Pearson correlation analysis were carried out to reveal the potential important factors, as shown in Tables 2 and 3 . During the whole 60 days’ experiment, the soil pH value was significantly correlated with TF-Zr dosage (positive) and different forms of Pb and F (positive or negative) (P < 0.01) only with 20 days (Tables S2 - 1 , S2 - 2 and S3 - 1 , S3 - 2 ). It was mainly because the excess addition of TF-Zr could release H + in soil at the initial periods, which would cause the decrease of soil pH and affect the existing forms of Pb and F. However, the influence of TF-Zr addition was insignificant on soil pH thereafter.

Besides, the TF-Zr dosage showed a significantly negative correlation (P < 0.01) with Water-F, Ex-F, Fe/Mn-F, Ex-Pb and Or-Pb and a significantly positive correlation (P < 0.01) with Res-F and Res-Pb, which indicated the addition of TF-Zr could reduce the active forms of F and Pb and increase the stable form of F and Pb (Tables 2 and 3 ). These findings were in line with the changes of different forms of soil F and Pb above mentioned. From the correlation analysis of different forms of F, it was obvious that Water-F, Ex-F and Fe/Mn-F were significantly positive correlation (P < 0.01) with each other, while they were negatively correlated with Res-F suggesting that Water-F, Ex-F and Fe/Mn-F could transform to Res-Pb. The correlation analysis results of different forms of Pb were similar with those of F. The Ex-Pb, Car-Pb and Or-Pb were significantly positive correlation (P < 0.01) with each other, while they were negatively correlated with Res-Pb suggesting that Ex-Pb, Car-Pb and Or-Pb could transform to Res-Pb. However, the Fe/Mn-Pb had little correlation with other forms of Pb. This was probably due to insignificant changes of Fe/Mn-Pb after adding of different of TF-Zr during the whole experimental periods.

In this study, the adsorption properties of TF-Zr on Pb 2+ and F – in water were evaluated. It was found that TF-Zr had a good adsorption capacity for both Pb 2+ and F – , which was because Pb 2+ and F – was effectively combined with hydroxyl group and Zr ion on the surface of TF-Zr. Besides, the presence of F - promoted the adsorption of Pb 2+ by TF-Zr owing to the enhancement of electrostatic effect. Moreover, it was identified that when the TF-Zr material was applied to tea garden soil, the forms of Pb and F in soil changed greatly from highly active state to the most stable residual state, which indicated that the material could effectively and simultaneously prevent the pollution of Pb and F in soil and control their migration to tea trees.

Data availability

All data generated or analysed during this study are included in this article. The data that support the findings of this study are available on request from the corresponding author [email protected]，upon reasonable request.

Havugimana, E., Bhople, B. S., Kumar, A., Byiringiro, E. & Kumar, A. Soil pollution-major sources and types of soil pollutants. Environ. Sci. Eng. 11 , 53–86 (2017).

Google Scholar  

Luo, Y. et al. Multiple factors influence bacterial community diversity and composition in soils with rare earth element and heavy metal co-contamination. Ecotoxicol. Environ. Saf. 225 , 112749 (2021).

Article   CAS   PubMed   Google Scholar  

Peng, J. Y., Chen, Y. A., Xia, Q., Rong, G. Z. & Zhang, J. Ecological risk and early warning of soil compound pollutants (HMs, PAHs, PCBs and OCPs) in an industrial city, Changchun, China. Environ. Pollut. 272 , 116038 (2021).

Qin, F. X. et al. Fluoride morphology and distribution characteristics of soil in tea gardens in high fluoride affected areas. Chin. Environ. Sci. 34 (11), 2859–2865 (2014).

CAS   Google Scholar  

Guo, Y. L. et al. Environmental quality of lead, cadmium, arsenic, chromium, mercury, copper and fluorine in the soil of Tieguanyin Tea Garden in Fujian Province. Chin. J. Eco-Agric. 19 (03), 676–681 (2011).

Article   CAS   Google Scholar  

Long, H., Jiang, Y. M., Li, C. Q., Liao, S. T. & Liao, Y. Effect of urea feeding on transforming and migrating soil fluorine in a tea garden of hilly region. Environ. Geochem. Health 43 , 5087–5098 (2021).

Sun, J. W. et al. Isotope tracers for lead and strontium sources in the Tieguanyin tea garden soils and tea leaves. Chemosphere 246 , 125638 (2020).

Cao, J., Zhao, Y. & Liu, J. Brick tea consumption as the cause of dental fluorosis among children from Mongol, Kazak and Yugu populations in China. Food Chem. Toxicol. 35 (8), 827–833 (1997).

Zhuang, Z. et al. Accumulation of potentially toxic elements in Chinese tea ( Camellia sinensis ): Towards source apportionment and health risk assessment. Sci. Total Environ. 851 , 158018 (2022).

Gao, H. J., Zhang, Z. Z. & Wan, X. C. Influences of charcoal and bamboo charcoal amendment on soil-fluoride fractions and bioaccumulation of fluoride in tea plants. Environ. Geochem. Health 34 (5), 551–562 (2012).

Zhang, J. et al. Distribution, accumulation, and potential risks of heavy metals in soil and tea leaves from geologically different plantations. Ecotoxicol. Environ. Saf. 195 , 110475 (2020).

Zhang, W. H., Yan, Y., Yu, R. L. & Hu, G. R. The sources-specific health risk assessment combined with APCS/MLR model for heavy metals in tea garden soils from south Fujian Province, China. Catena 203 , 105306 (2021).

Jin, C. W., Zheng, S. J., He, Y. F., Zhou, G. D. & Zhou, Z. X. Lead contamination in tea garden soils and factors affecting its bioavailability. Chemosphere 59 (8), 1151–1159 (2005).

Article   ADS   CAS   PubMed   Google Scholar  

Xu, J. C., Wang, S. Q., Yao, T. S., She, X. S. & Gan, Z. T. Vertical distributions and bioavailabilities of heavy metals in soil in an-tea plantations in Qimen county, China. Processes 10 (4), 664 (2022).

Yang, X. D. et al. Effects of long-term nitrogen application on soil acidification and solution chemistry of a tea plantation in China. Agric. Ecosyst. Environ. 252 , 74–82 (2018).

Han, W. Y., Zhao, F. J., Shi, Y. Z., Ma, L. F. & Ruan, J. Y. Scale and causes of lead contamination in Chinese tea. Environ. Pollut. 139 (1), 125–132 (2006).

Ruan, J. Y., Ma, L. F., Shi, Y. Z. & Han, W. Y. Uptake of fluoride by tea plant ( Camellia sinensis L) and the impact of aluminium. J. Sci. Food Agric. 83 (13), 1342–1348 (2003).

Yurak, V. et al. Testing of natural sorbents for the assessment of heavy metal ions’ adsorption. Appl. Sci. Basel 11 (8), 3723 (2021).

Chang, H. Q. et al. The effects of calcium combined with chitosan amendment on the bioavailability of exogenous Pb in calcareous soil. J. Integr. Agric. 19 (5), 1375–1386 (2020).

Souza, L. R. R., Nakadi, F. V., Zanatta, M. B. T. & Da Veiga, M. A. M. S. Reduction of bioaccessibility and leachability of Pb and Cd in soils using sludge from water treatment plant. Int. J. Environ. Sci. Technol. 16 (10), 5397–5408 (2019).

Huang, C. Y. et al. Enhanced fluoride adsorption of aluminum humate and its resistance on fluoride accumulation in tea leaves. Environ. Technol. 41 (3), 329–338 (2020).

Fan, C. F. et al. Stabilization of fluorine-contaminated soil in aluminum smelting site with biochar loaded iron-lanthanide and aluminum-lanthanide bimetallic materials. J. Hazard. Mater. 426 , 128072 (2022).

Xia, Y., Li, Y., Sun, Y. T., Miao, W. & Liu, Z. G. Co-pyrolysis of corn stover with industrial coal ash for in situ efficient remediation of heavy metals in multi-polluted soil. Environ. Pollut. 289 , 117840 (2021).

Zhang, H. Y. et al. Evaluating the remediation potential of MgFe2O4-montmorillonite and its co-application with biochar on heavy metal-contaminated soils. Chemosphere 299 , 134217 (2022).

Qi, X. et al. MgO-loaded nitrogen and phosphorus self-doped biochar: High-efficient adsorption of aquatic Cu2+, Cd2+, and Pb2+ and its remediation efficiency on heavy metal contaminated soil. Chemosphere 294 , 133733 (2022).

Xu, Q. H., Wang, Y. L., Jin, L. Q., Wang, Y. & Qin, M. H. Adsorption of Cu (II), Pb (II) and Cr (VI) from aqueous solutions using black wattle tannin-immobilized nanocellulose. J. Hazard. Mater. 339 , 91–99 (2017).

Wang, J. & Chen, C. Chitosan-based biosorbents: Modification and application for biosorption of heavy metals and radionuclides. Bioresour. Technol. Biomass Bioenergy Biowastes Convers. Technol. Biotransf. Prod. Technol. 160 , 129–141 (2014).

Liu, B. J., Wang, D. F., Yu, G. L. & Meng, X. H. Removal of F− from aqueous solution using Zr(IV) impregnated dithiocarbamate modified chitosan beads. Chem. Eng. J. 228 , 224–231 (2013).

Singh, S., German, M., Chaudhari, S. & Sengupta, A. K. Fluoride removal from groundwater using zirconium impregnated anion exchange resin. J. Environ. Manag. 263 , 110415 (2020).

Cho, D. W. et al. Synthesis of hydrous zirconium oxide-impregnated chitosan beads and their application for removal of fluoride and lead. Appl. Surf. Sci. 372 , 13–19 (2016).

Article   ADS   CAS   Google Scholar  

Wang, J., Lin, X. Y., Luo, X. G. & Long, Y. F. A sorbent of carboxymethyl cellulose loaded with zirconium for the removal of fluoride from aqueous solution. Chem. Eng. J. 252 , 415–422 (2014).

Barathi, M., Kumar, A. S. K. & Rajesh, N. Efficacy of novel Al–Zr impregnated cellulose adsorbent prepared using microwave irradiation for the facile defluoridation of water. J. Environ. Chem. Eng. 1 (4), 1325–1335 (2013).

Gabarrón, M. et al. Effect of land use and soil properties in the feasibility of two sequential extraction procedures for metals fractionation. Chemosphere 218 , 266–272 (2019).

Article   ADS   PubMed   Google Scholar  

Huang, X., Wang, Y., Liao, X. & Shi, B. Adsorptive recovery of Au3+ from aqueous solutions using bayberry tannin-immobilized mesoporous silica. J. Hazard. Mater. 183 (1–3), 793–798 (2010).

Jiang, Y. M. et al. Zirconium loaded on tannin foam for efficient adsorptive removal of fluoride at low concentration in a wide pH range. Desalin. Water Treat. 250 , 159–171 (2022).

Arshadi, M., Abdolmaleki, M. K., Mousavinia, F., Foroughifard, S. & Karimzadeh, A. Nano modification of NZVI with an aquatic plant Azolla filiculoides to remove Pb(II) and Hg(II) from water: Aging time and mechanism study. J. Colloid Interface Sci. 486 , 296–308 (2017).

Li, H. H. et al. Mechanisms for potential Pb immobilization by hydroxyapatite in a soil-rice system. Sci. Total Environ. 783 , 147037 (2021).

Swain, S. K., Patel, S. B., Panda, A. P., Patnaik, T. & Dey, R. K. Pea ( Pisum sativum L.) peel waste carbon loaded with zirconium: Study of kinetics, thermodynamics and mechanism of fluoride adsorption. Sep. Sci. Technol. 54 (14), 2194–2211 (2019).

Liu, H. L., Sun, X. F., Yin, C. Q. & Hu, C. Removal of phosphate by mesoporous ZrO2. J. Hazard. Mater. 151 (2–3), 616–622 (2008).

Yurtsever, M. & Şengil, İA. Biosorption of Pb(II) ions by modified quebracho tannin resin. J. Hazard. Mater. 163 (1), 58–64 (2009).

Prabhu, S. M. & Meenakshi, S. Defluoridation of water using synthesized Zr(IV) encapsulated silica gel/chitosan biocomposite: Adsorption isotherms and kinetic studies. Desalin. Water Treat. 53 (13), 3592–3603 (2015).

Zou, L. Z. et al. Tannic acid-based adsorbent with superior selectivity for lead(II) capture: Adsorption site and selective mechanism. Chem. Eng. J. 364 , 160–166 (2019).

Dou, X. M., Mohan, D., Pittman, C. U. & Yang, S. Remediating fluoride from water using hydrous zirconium oxide. Chem. Eng. J. 198–199 , 236–245 (2012).

Article   Google Scholar  

Wolter, S. D., Piascik, J. R. & Stoner, B. R. Characterization of plasma fluorinated zirconia for dental applications by X-ray photoelectron spectroscopy. Appl. Surf. Sci. 257 (23), 10177–10182 (2011).

Qiao, D. W., Yao, J., Song, L. J. & Yang, J. Y. Migration of leather tannins and chromium in soils under the effect of simulated rain. Chemosphere 284 , 131413 (2021).

Gan, C. D., Jia, Y. B. & Yang, J. Y. Remediation of fluoride contaminated soil with nano-hydroxyapatite amendment: Response of soil fluoride bioavailability and microbial communities. J. Hazard. Mater. 405 , 124694 (2021).

Zhao, S. L. et al. Influence of fertilizers on fluoride accumulation in tea leaves & its remediation using polyphenol-Ce adsorbents. RSC Adv. 5 (8), 6085–6091 (2015).

Erktan, A. et al. Immediate and long-term effect of tannins on the stabilization of soil aggregates. Soil Biol. Biochem. 105 , 197–205 (2017).

Sut-Lohmann, M. et al. Using modified Tessier sequential extraction to specify potentially toxic metals at a former sewage farm. J. Environ. Manag. 304 , 114229 (2022).

Download references

This work was supported by the National Natural Science Foundation of China (41641010 and 21406147), the Foundation of Sichuan Science & Technology Committee (2020YFH0162 and 2022YFS0476).

Author information

Authors and affiliations.

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, 610066, People’s Republic of China

Xiaolu Huang, Mei Zhang, Minghui Wang, Zhuoyu Wen, Yamei Jiang, Yunhao Sui, Jun Ma, Yang Liao & Xiaoting Li

Key Laboratory of Land Resources Evaluation and Monitoring in Southwest, Ministry of Education of China, Chengdu, 610068, People’s Republic of China

Xiaolu Huang, Mei Zhang, Minghui Wang, Zhuoyu Wen, Yamei Jiang, Yunhao Sui, Yang Liao & Xiaoting Li

Engineering Research Center for the Development of Farmland Ecosystem Service Functions, Chengdu, 610068, Sichuan, People’s Republic of China

Jun Ma, Yang Liao & Xiaoting Li

You can also search for this author in PubMed   Google Scholar

Contributions

This manuscript is a joint effort by all the authors. Xiaolu Huang mainly did some experiments and wrote this paper, Mei Zhang, Minghui Wang, and Zhuoyu Wen participated in the drawing of the paper, and Yamei Jiang, Yunhao Sui did the experiments, Xiaoting Li, Jun Ma and Yang Liao proposed the experimental ideas and methods, guided the writing and revision of this paper. All the authors have given approval for the publication of this paper.

Corresponding authors

Correspondence to Yang Liao or Xiaoting Li .

Ethics declarations

Competing interests.

The authors declare no competing interests.

Consent to participant

All the authors have given approval for the final version of manuscript.

Additional information

Publisher's note.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary Information

Supplementary information., rights and permissions.

Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/ .

Reprints and permissions

About this article

Cite this article.

Huang, X., Zhang, M., Wang, M. et al. Efficient and simultaneous immobilization of fluoride and lead in water and tea garden soil by bayberry tannin foam loaded zirconium. Sci Rep 14 , 20901 (2024). https://doi.org/10.1038/s41598-024-71767-8

Download citation

Received : 20 May 2024

Accepted : 30 August 2024

Published : 08 September 2024

DOI : https://doi.org/10.1038/s41598-024-71767-8

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

• Combined pollution
• Simultaneous immobilization
• Bayberry tannin
• Fluoride and lead
• Adsorption mechanisms
• Tea garden soil

By submitting a comment you agree to abide by our Terms and Community Guidelines . If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings

Preview improvements coming to the PMC website in October 2024. Learn More or Try it out now .

• Advanced Search
• Journal List
• J Dent (Shiraz)
• v.16(3 Suppl); 2015 Sep

The Effectiveness of Home Water Purification Systems on the Amount of Fluoride in Drinking Water

Behrooz eftekhar.

a Dept. of Endodontic, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Masoume Skini

b Postgraduate Student, Dept. of Endodontic, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Milad Shamohammadi

Jaber ghaffaripour.

c DDS, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Firoozeh Nilchian

d Dental Students Research Center, Dept. of Dental Public Health, School of Dentistry, Isfahan University of Medical Sciences, Isfahan, Iran.

Statement of the Problem

Water purification systems for domestic use have drawn significant attention over the past few years. This can be related to the improvement of public health and concern for water contamination. 

The aim of this study was to evaluate whether home water purification systems eliminate the essential materials such as fluoride besides filtrating the heavy ions and other unwanted particles out of water.

Materials and Method

In this experimental study, six most frequently used commercial brands of water purifiers were evaluated and compared. Specimens were collected right before and after setting up the device, and 6 months later. Then, spectrophotometry (the Harrison device) was performed to compare fluoride clearance by each home water cleaner device.

Based on the data collected from all water purification devices in different locations, the amount of fluoride was significantly different before and right after using home water purifier and six months later ( p = 0.001 and p = 0.00, respectively).

The filtration of water significantly decreased its fluoride concentration. The fluoride content of purified water was approximately as much as zero in some cases.

Introduction

Fluoride is a natural element branched from Fluorine. This element can be found in all sorts of water and soil. Out of every kilogram of outer layer of earth, 0.3 gram is fluoride. Mineral waters have more amount of this element compared to other sources.( 1 )

About 60 years ago, Grand Rapids in Michigan State was the first city in which fluoride supplement was synthetically added to tap water. In US, adding fluoride to community water supplies of many cities has improved the oral health of millions of American citizens.( 2 )

Fluoridation of community water supplies is adding a specific amount of fluoride (0.7-1.2 ppm) to water in order to reduce the risk of dental caries. By 2002, almost 170 million Americans were provided with this privilege.( 3 )

Since most of the systemic fluoride is provided through tap water to population, many policies have been established to add fluoride to community water regarding its benefits for teeth and bones.( 4 )

In regions and countries that do not have water-fluoridation technology, there are natural supplements as previously mentioned. For example, Iran has many mineral water supplies that contain considerable amounts of fluoride. Amount of fluoride in natural mineral waters depends on weather conditions; the warmer the weather is, the higher the amount of fluoride can be detected. Mineral waters in southern regions that have warmer weather contain more fluoride. In Iran, the highest amount of fluoride has been found in southeast and northeast areas.

Water purification systems for domestic use have drawn much of attention over the past few years. This can be related to improvement of public health and concerns for water contamination. There are several types of home water purification systems that can be categorized into 3 different groups( 5 ) as filtered systems, systems using UV irradiation, and ion-exchange systems.

The aim of this study was to find out whether domestic water purification systems could eliminate the essential materials such as fluoride besides filtrating the heavy ions and other unwanted particles out of water.

In this study, 6 frequently used commercial brands of water purifiers in Ahwaz were compared. The commercial brands evaluated in the current study were CCK (Ceramic and Ceramic/Carbon Cartridges ; RTX-TS DLM filters, Korea), Soft Water (Ceramic Candles; Alpine TJ Series filters, W9332420, USA), Alkusar (Special media cartridges filters; PRB50-IN, USA), Puricom (Special media cartridges filters; Watts 4.5" x 10" Dual Housing, Korea), Water Safe (Granular Carbon Cartridges filters; LCV (Lead, Cysts, VOC's) (Carbon Block Filter Cartridges, Australia), and Aquafresh (Sediment String-Wound; Poly Spun and Pleated Washable Cartridges filters, K5520, USA). The main drinking water supply for Ahwaz is provided by governmental companies. After making arrangement with certain companies that supported these brands, the devices were setup in 6 different regions of Ahwaz. Samples were collected before and right after setting up the device. To reduce the errors and elevate the accuracy of the module, 5 samples were taken from each device. Another sample was collected from each single device 6 months later. A total of 64 samples were collected including 32 unfiltered (control) and 32 filtered samples of tap water (experimental) from 6 regions in Ahwaz. Fluoride sampling kits (Spands; EW-99574-08Hach ® Test Kits, USA) were used to test the amount of fluoride in sample waters. Samples were all collected in polyethylene sampling containers and were then coded. Spectrophotometry (AvaSpec-ULS2048L- USB2 UARS spectrometer, USA) was performed. In order to measure the characteristics of individual molecules, a mass spectrometer converted them to ions so that they could be moved about and manipulated by external electric and magnetic fields.

Atmospheric pressure was around 760 torr (mm of mercury). The pressure under which ions may be handled is roughly 10 -5 to 10 -8 torr (less than a billionth of an atmosphere). By varying the strength of the magnetic field, ions of different mass can be focused progressively on a detector fixed at the end of a curved tube and also under a high vacuum.

Latin alphabetic words were used to code each commercial device. Numbers were used for samples obtained before and after setting the device.( 6 )

The results were analyzed by using paired sample t-test, with alpha (ɑ) set at 0.05.

The amount of fluoride in water before and after using six brands of water purifier device is summarized in Table 1 .

The amount of fluoride before and after installing water purifier devices

Based on the data gathered from all water purification devices set in different regions, the level of fluoride was significantly different before and after using home water purifier ( p = 0.001). It was found that home water purifiers nearly eliminated fluoride from tap water. Table 2 represents the results of t-test.

Comparison of different study groups with t-test

* p< 0.05 is statistically significant.

Another round of sampling was done 6 months later from the same filters of home water purifier. Details are illustrated in Table 3 and 4.

The amount of fluoride in tap water after 6 months of using a water purification filter

Comparison of the study groups after six mounts with t-test

Fluoride absorption is mostly systemic or local; systemic absorption occurs through eating the element with food, water or fluoride pills, and local absorption by toothpastes and other fluoride-containing hygienic products. In many countries, the highest supply for fluoride absorption is systemic absorption through water consumption.( 6 ) In early 20 th century, the first attempts were made to fluoridate public water supplies, which eventually led to 40% decrease of dental caries in the target population.( 7 )Introduction of water fluoridation in the 1950-1960 and fluoride-containing dental products in the 1970 changed the situation. The main sources of fluoride in established market economies (EME) are drinking water, fluoridated salt, foods and beverages, baby cereals and formulas, fluoride supplements, toothpastes, mouth-rinses, and topical fluorides. Additionally, fluoride in water has a diffusion or halo effect; which means that the drinks and foods manufactured in fluoridated areas are also available to whole population including the residents of non-fluoridated areas.

Although adding fluoride to almost all oral hygienic products has restricted the effect of fluoride water (Halo effect), it is still common to fluoridate the city water supply.( 6 ) In many areas of the world, there is no systematic plan for fluoridation of community water and only the natural sources supply it. Therefore, sometimes the hardness of water and aggregation of different and sometimes poisonous elements drive the population to use bottled water or use home purification devices.

The findings of the present study revealed that all the 6 devices reduced the fluoride in tap water and most of them nearly eliminated it. Different home purification devices have been marketed each of which is claimed to eliminate certain kinds of elements from water.( 9 ) JK Mwabi et al. (2011) used 4 different filters to reduce the hardness and chemical contamination of water in poor villages in Africa, and reported that all of the four filters reduced the fluoride significantly. Bucket filter had the most significant effect and reduced fluoride element 99.9%. These results also indicated that fluoride was the most reduced element of all. Likewise, silver-impregnated porous pot (SIPP) filter reduced 90%-100% of elements.

Clasen et al. ( 5 ) in their study reported that 3 different home purification systems ,the ceramic candle gravity filter, iodine resin gravity filter, and iodine resin faucet filter, reduced bacterial contamination by four logs and decreased ions such as fluoride and arsenic, as well.

Moreover, there are certain methods to reduce the excessive amount of fluoride in the water. One of the best-known methods is absorption technique.( 7 ) Evaluation of 6 different commercial water purifiers has not been done in any other study; therefore, there is no similar study to compare the results exactly. More evaluations are suggested to be performed on home water purification systems, and more strategies should be devised to preserve the essential elements of tap water.

The current study found considerable differences between the amount of fluoride before and after filtration with home purification device; that is filtration significantly decreased the fluoride concentration even as much as 100% in some cases.

Conflict of Interest: None declared