Today, about two-thirds of people in the United States receive fluoridated tap water, as do many people in Australia, Brazil, Canada, New Zealand, Spain, and the United Kingdom. Now, a controversial new study links fluoridation to lower IQ in young children, especially boys whose mothers drank fluoridated water while pregnant.
Long-time fluoridation critics are lauding the study, but other researchers say it suffers from numerous flaws that undercut its credibility. Either way, “It’s a potential bombshell,” says Philippe Grandjean, an environmental health researcher at Harvard University who wasn’t involved in the work.
Fluoride is well-known for protecting teeth against cavities by strengthening tooth enamel. It’s found naturally in low concentrations in both freshwater and seawater, as well as in plant material, especially tea leaves. Throughout the 1940s and ’50s, public health researchers and government officials in cities around the world experimentally added fluoride to public drinking water; they found it reduced the prevalence of cavities by about 60%. Today, fluoridated water flows through the taps of about 5% of the world’s population, including 66% of Americans and 38% of Canadians.
Yet scepticism has dogged the practice for as long as it has existed. Some have blamed fluoridated water for a wide range of illnesses including cancer, but most criticism has been dismissed as pseudoscience. Over the years, though, a small number of scientists have published meta-analyses casting doubt on the efficacy of water fluoridation in preventing cavities. More recently, scientists have published small-scale studies that appear to link prenatal fluoride exposure to lower IQ, although dental research groups were quick to challenge them.
A study in JAMA Pediatrics offers perhaps the highest profile critique to date. Psychologists and public health researchers looked at data from Canada’s federally funded Maternal-Infant Research on Environmental Chemicals program, a long-term study of pregnant women and their children in six Canadian cities that started to collect data in 2008 on everything from diet to education levels to traces of lead and arsenic in the urine.
About 40% of the nearly 600 women lived in cities with fluoridated drinking water; they had an average urinary fluoride level of 0.69 milligrams per litre, compared with 0.4 milligrams for women living in cities without fluoridated water. Three to 4 years after the women gave birth, researchers gave their children an age-appropriate IQ test. After controlling for variables such as parental education level, birth weight, prenatal alcohol consumption, and household income, as well as exposure to environmental toxicants such as lead, mercury, and arsenic, they found that if a mother’s urinary fluoride levels increased by 1 milligram per litre, her son’s (but not her daughter’s) IQ score dropped by about 4.5 points. That effect is on par with the other recent studies looking at childhood IQ and low-level lead exposure.
Using a secondary method for measuring fluoride intake—mothers’ self-reports of how much tap water and fluoride-rich tea they drank throughout pregnancy—they found a 1-milligram-per-litre increase in fluoride was associated with a 3.7-point IQ score drop in both boys and girls. Self-reporting is a less widely accepted method because it’s considered less reliable and prone to inaccurate recall. The researchers admit they aren’t sure why there’s a sex discrepancy between the two methods, though they say it could arise from the different ways in which boys and girls absorb environmental toxins in utero. For both findings, the authors declined to speculate on the exact mechanism at work.
If the work holds up—a big “if,” as the paper’s findings are already coming under heavy scrutiny—it could hold serious implications for public policy. According to recommendations from the U.S. Department of Health and Human Services, drinking a litre of fluoridated water should provide about 0.7 milligrams of fluoride. “If you just drink 1 litre [of tap water] and then in addition have a couple mugs of tea, then the fluoride concentration in the tea is enough to get you over the limit proposed,” Grandjean notes.
The authors are fully aware of the controversial nature of their work, and one of them—Rivka Green, a neuropsychology doctoral candidate at York University in Toronto, Canada—says she hopes the study will jump-start further research. “We tried to be as cautious and careful as possible,” she says. “We’re not coming in saying that fluoride is poison or anything like that. We’re just … letting the data tell the story.”
Aware that the study’s findings were likely to make waves, JAMA Pediatrics took the unusual step of publishing an editor’s note accompanying the paper. “This decision to publish this article was not easy,” writes the journal’s editor, pediatrician and epidemiologist Dimitri Christakis of Seattle Children’s Hospital in Washington. He adds that the paper was “subjected [to] additional scrutiny for its methods and the presentation of its findings.”
Despite that, several researchers argue that the paper’s methodological shortcomings undercut its importance. In a statement to the Science Media Centre in London, an independent organization that sources expert opinions on science in the news, psychologist Thom Baguley of Nottingham Trent University in the United Kingdom noted the data “are very noisy,” meaning they contain a lot of other factors that could easily lead to false positives. Psychologist Stuart Ritchie at King’s College London added that the findings are just barely statistically significant, calling them “pretty weak and borderline.” By itself, the study “shouldn’t move the needle much at all on the question of the safety of fluoride,” he wrote.
Lindsay McLaren, a public health researcher at the University of Calgary in Canada, disagrees. She tells Science that the study appears both credible and methodologically sound—but she agrees it’s too early to change fluoridation practices. “Public policy is ideally informed not by any one study, but by the best available evidence as a whole,” she says. “It will be important to continue to review and appraise new research on fluoride and fluoridation.”