If you’re like many Americans, you drink fluoridated water without questioning its safety or efficacy. For decades municipalities have been adding fluoride to water supplies to reduce the incidence of tooth decay at the advice of numerous highly credentialed entities, including the World Health Organization, the U.S. Public Health Service, the American Dental Association and the American Academy of Pediatrics.

But in late 2016, a coalition of activist groups — Food and Water Watch, The American Academy of Environmental Medicine, Fluoride Action Network, the International Academy of Oral Medicine and Toxicology, the Organic Consumers Association, and Moms Against Fluoridation — together with private citizens from around the United States petitioned the U.S. Environmental Protection Agency (EPA) to ban the addition of fluoride to water supplies, citing research pointing to health risks linked to exposure to fluoride.

What exactly are the benefits of fluoridating municipal water supplies? What are the risks? And how does a conscientious society balance the two? These questions are finding new urgency as a growing body of research calls into question its safety.

Fluoride’s Benefits

The practice of adding fluoride to drinking water began in the 1940s after dentists observed that children living in communities with naturally high levels of fluoride had fewer cavities than those who didn’t. Several communities began adding fluoride to municipal water supplies as a public health measure to lessen the prevalence of dental decay. Studies revealed that cavity rates in those communities were lower than in comparison communities that did not fluoridate, and increasing numbers of municipalities decided to fluoridate in ensuing decades. Rates of dental decay continued to drop. Today, more than 200 million Americans live in communities with fluoridated water. (Most, but not all, major U.S. cities fluoridate.)

However, some argue that other factors during this period, such as advances in dental care and widespread use of fluoridated toothpaste, are likely behind these declines. In fact, European countries that never fluoridated have seen very similar drops in cavity rates.

U.S. communities started fluoridating water in the 1940s to reduce risk of tooth decay. However, some argue that other advances such as use of fluoridated toothpaste deserve the credit for the decline in cavities that followed. Image © iStockphoto.com/FatCamera

Howard Pollick, professor at the University of California, San Francisco School of Dentistry and spokesperson on fluoridation for the American Dental Association, says it has become “harder and harder to separate the benefit of water fluoridation in light of the use of fluoride products” such as toothpaste and fluoride varnishes. Designing a study that can tell us with certainty that fluoridated water confers benefits above and beyond what we derive from other sources of fluoride and dental care like sealants, he says, is “very difficult.”

In an effort to quantify the benefits of fluoridating water, the Iowa Fluoride Study has followed 570 participants from infancy to young adulthood. Researchers have found that children living in communities that fluoridate their water don’t have statistically significantly fewer cavities than those in unfluoridated communities.

Steven Levy, who directs the study, says that the demographics of study participants, who report brushing regularly and receiving consistent dental care, likely affect results. For those who lack access to fluoridated dental products and preventive dental care, Levy speculates that fluoridated water’s impact on dental health may be considerably greater. In fact, the U.S. Centers for Disease Control and Prevention estimates that fluoridated water reduces cavities by 25 percent which includes low-income populations who Levy says generally have much higher rates of tooth decay. Critics point out that this reduction amounts to perhaps one cavity per person and argue that these benefits to teeth — minimal for those who brush regularly with fluoride toothpaste and see a dentist regularly — must be weighed against the risks to other parts of the body.

Potential for Harm?

The EPA in February 2017 denied the 2016 petition on the grounds that it lacked “scientifically defensible” proof of neurotoxic harm caused by fluoridated water. But Kathleen Thiessen, senior scientist at Oak Ridge Center for Risk Analysis who has three decades experience assessing fluoride’s risk, maintains that the agency dismissed many high-quality studies. Further, Thiessen says the sheer number of studies, despite their limitations, requires taking the possibility of harm from fluoride more seriously. And Harvard adjunct professor of environmental health Philippe Grandjean, who has spent his career investigating the effects of environmental pollutants on children’s development, says evidence suggesting fluoride’s potential for harm warrants reconsidering its addition to drinking water.

Indeed, over the past two decades, numerous studies have begun to challenge assumptions about the safety of ingesting fluoride.

2014 paper on developmental neurotoxins Grandjean and colleague Philip Landrigan of the Icahn School of Medicine at Mount Sinai in New York published in The Lancet included fluoride in a list of chemicals with developmental neurotoxic effects. In 2012 Grandjean worked on a systematic review of studies from China showing lower IQ in areas with naturally high levels of fluoride. Though these studies had some design issues, Grandjean contends that when more than 20 discrete studies point to a similar effect, they should receive serious consideration rather than be dismissed for their limitations. If anything, he says, the imprecision in these studies likely “led to an underestimation of the effects” of fluoride on neurodevelopment.

study published in Environmental Health Perspectives in September 2017 sought to address the shortcomings of previous studies. Looking at nearly 300 mother-child pairs, researchers  analyzed samples of pregnant mothers’ urine archived in the Mexican Early Life Exposures in Mexico to Environmental Toxicants (ELEMENT) project. The study measured maternal urinary fluoride and cognitive performance in children and found significantly lower cognitive performance in children whose mothers had higher levels of fluoride. Howard Hu, founding dean of the Dalla Lana School of Public Health at the University of Toronto and co-author of the study, says that results may underpredict the relationship between fluoride and IQ.

Research currently underway aims to clarify the relationship between fluoridated water and urinary concentrations of fluoride. ADA’s Pollick has reviewed studies on fluoride’s health effects and says that the dental community has taken them into consideration. He reports that at present “the evidence points to its being safe” and notes that we lack information about urinary levels in Americans.

Research currently underway aims to clarify the relationship between fluoridated water and urinary concentrations of fluoride. The Canadian “Maternal-Infant Research on Environmental Chemicals” (MIREC) study is examining urinary fluoride levels of women during pregnancy and subsequent behavior of their children. The MIREC study took samples from 1,960 pregnant women in 10 Canadian cities, half of which fluoridate drinking water. Preliminary findings suggest urine levels in women from fluoridated communities in the Canadian study are comparable to those found in the Mexico research, according to study director Christine Till, associate professor of clinical neuropsychology at Toronto’s York University.

Linda Birnbaum, director of the U.S. National Toxicology Program, oversaw a 2016 review of animal studies on fluoride that she says determined the available evidence inconclusive. The NTP is currently reviewing human epidemiological data, including the recent ELEMENT study. Its findings, says Birnbaum, help confirm conclusions drawn from the Chinese studies on fluoride and IQ and show that smaller increases in fluoride concentrations are associated with impaired cognitive performance. While she calls it “an important study,” she also notes that it requires replication.

Researchers face numerous difficulties, Birnbaum says, including quantifying how much fluoride people ingest, since “it’s everywhere now,” including in prepared foods and beverages, such as soup, soda and coffee  made with water sourced from fluoridated community water supplies that reach people living in communities without fluoridated water. Since everyone is likely consuming some fluoride, finding a comparison group that doesn’t is nearly impossible. What additional research will hopefully help us do, says Birnbaum, is “understand differential susceptibility — are there some people because of genetics or past history who are more sensitive [to fluoride] than others?”

Attention deficit hyperactivity disorder (ADHD) is another area of scrutiny. Attention deficit hyperactivity disorder (ADHD) is another area of scrutiny. A study published in Environmental Health in 2015 found that every 1 percent increase in the prevalence of water fluoridation in 1992 was associated with 67,000–131,000 additional cases of ADHD between 2003 and 2011. Till, who co-authored the study, says that such observational studies cannot prove causation. She and other researchers are using the Mexican ELEMENT and Canadian MIREC study data to better understand possible relationships between fluoride exposure and ADHD.

Because fluoride can interfere with absorption of iodine and affect thyroid function, some research has looked into the possible links between drinking fluoridated water and the prevalence of thyroid disorders. A 2015 study in the Journal of Epidemiology and Community Health looking at the association between drinking water fluoride and hypothyroidism in two regions of the United Kingdom found the prevalence of hypothyroidism prevalence was almost two times higher in the fluoridated area as in the unfluoridated one. Critics have challenged the study design and argued that other confounding factors should have been accounted for. A 2017 Canadian report using data from the Canadian Health Measures Survey (CHMS) found no association between fluoride exposure and incidence of reported thyroid disorders in fluoridated communities. Other research has found some correlation between fluoride exposure and elevated or lowered levels of different thyroid hormones.

Burden of Proof

The coalition that petitioned the EPA in 2016 has since filed a complaint with the U.S. district court in northern California calling for a new review of the evidence. If the court decides that water fluoridation indeed poses an “unreasonable risk” of harm, it can require the EPA to ban the addition of fluoride to drinking water supplies. The EPA has filed a motion to dismiss the case. Plaintiffs have requested a trial they expect to take place in 2018.

Challenging the status quo on chemical safety has become increasingly difficult, says Bruce Lanphear, a public health researcher at Simon Fraser University whose work focuses on disease and disability prevention. The evidence required to alter policy, he says, has risen dramatically in the last several decades, thanks in part to powerful industry lobbies.

Lanphear urges public health officials to reconsider long-held assumptions as science yields results that call them into question. Lanphear urges public health officials to reconsider long-held assumptions as science yields results that call them into question. Scientists used to believe there was a safe threshold of exposure for virtually all chemicals. In the case of compounds such as lead and benzene, Lanphear says, “the evidence suggests that assumption is no longer valid,” with “proportionately greater harms at lower levels” of exposure. Further, he explains, our bodies are “bombarded” by a huge range of chemicals that may collectively predispose us to greater risk of disease. While each individual toxicant may have a limited impact on health, when combined they could greatly increase the risk of negative health effects.

At present, many focused on preventing tooth decay are not persuaded of fluoride’s health risks, while fluoridation opponents argue sufficient evidence exists of fluoride’s negative effects on the body that far outweigh its marginal benefit to teeth. As research on fluoride continues to clarify its biological effects and litigation pushes government to reconsider policy, governing bodies, communities, and individuals may find themselves grappling anew with how to weigh the potential benefits of fluoride with its possible harms to human health. View Ensia homepage

 Editor’s note: Susannah Shmurak produced this feature as a participant in the Ensia Mentor Program.