Divisive review reignites debate on fluoridation and children’s IQ

Jeremy Booth, Dental Tribune International

Wed. 26 February 2025

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DURHAM, N.C., US: The practice of fluoridating US community water supplies to prevent dental caries is hailed as one of ten great public health achievements of the twentieth century by the US Centers for Disease Control and Prevention and endorsed by the American Dental Association (ADA). A contentious topic and increasingly a political issue, fluoridation was once again cast into the spotlight in August when a US National Toxicology Program (NTP) report linked higher levels of fluoride exposure to lower IQ scores in children. In January, the findings of the report were detailed in a review and meta-analysis that has proved divisive and will likely add weight to the arguments of those who want fluoridation consigned to the past.

Conducted by researchers from the US National Institutes of Health and consulting firm ICF, the review analysed 74 epidemiological studies on the relationship between children’s IQ and fluoride exposure. A total of 59 studies reported on mean IQ scores in relation to fluoride exposure in water at the group level, and 19 reported on IQ scores in relation to fluoride intake and concentrations in urine in individuals. Most of the studies were conducted in China (45) and India (12), and the remainder focused on populations in Canada, Iran, Mexico, Pakistan, Denmark, New Zealand, Spain and Taiwan.  

The researchers identified significant inverse associations between IQ scores and fluoride exposure. Where studies measured fluoride in water, inverse associations were found when fluoride concentration was restricted to less than 4 mg/l and less than 2 mg/l but not lower than 1.5 mg/l. In studies where fluoride was measured in both water and urine, inverse associations persisted when exposure was restricted to less than 4 mg/l, 2 mg/l and 1.5 mg/l. 

The World Health Organization recommends that fluoride concentrations in artificially fluoridated drinking water do not exceed 0.5–1.0 mg/l and that health regulators consider various factors when setting fluoride levels, including local climate conditions and population exposure to fluoride from other sources. Since 2015, US health officials have recommended a fluoridation level of level of 0.7 mg/l to protect against caries while limiting the risk of fluorosis.  

The researchers did not exclude studies based on quality of evidence (risk of bias). They identified 52 studies as having a high risk of bias and analysed study subgroups with and without these studies included. Data from the 22 studies with a low risk of bias confirmed the inverse association at all three dosage restriction levels. Concerning the actual decrease in IQ scores, a group of 13 studies (11 with a low risk of bias) based on individual-level data showed that a 1 mg/l increase in urinary fluoride was associated with a decrease of 1.63 IQ points. This decrease fell to 1.14 IQ points when studies with a high risk of bias were excluded. 

According to the authors, the results are consistent with previous meta-analyses in finding a statistically significant inverse association between children’s IQ and fluoride exposure. Co-author Dr Kyla Taylor, a National Institute of Environmental Health Sciences health scientist, commented to Fox News Digital that the research showed that “the more fluoride a child is exposed to, the more likely that child’s IQ will be lower than if they were not exposed”. The NTP cautioned that an association does not prove cause and effect and advised that further research is needed to determine whether low fluoride exposures pose any health risk 

Accompanying editorials hint at complexity

The Taylor et al. review was accompanied by two editorials praising and criticising the analysis. Dr Steven M. Levy, professor of preventive and community dentistry and of epidemiology at the University of Iowa in Iowa City in the US and a member of the ADA national fluoride committee, warned readers to exercise caution when interpreting the results. According to news agency AFP, Dr Levy and other scientists have criticised the review’s methodology and questioned whether its findings apply to local fluoridation levels in the US. One point of critique, for example, has been the study’s lack of data on any risk posed by fluoridation at the US guideline level. The authors told AFP that the data was insufficient “to determine if 0.7 mg/l of fluoride exposure in drinking water affected children’s IQ”. In addition to raising issues related to the quality of the studies selected, Dr Levy commented, “Almost all of the studies have been done in other settings where there are other contaminants, other things we call confounding factors”, and he cited coal pollution in China as one example. 

The second editorial framed the lowering of US fluoridation levels in 2015 as the beginning of a public health reckoning that is only now coming to fruition, urging critical debate. Its authors were Dr Bruce Lanphear, professor of health sciences at Simon Fraser University in Burnaby in British Columbia, and Dr Christine Till, a clinical neuropsychologist and professor of psychology at York University in Toronto in Ontario—both in Canada—and Dr Pamela Den Besten, professor of orofacial sciences at the University of California San Francisco School of Dentistry in the US. 

The ADA said in January that the Taylor et al. review “does not provide any new evidence to warrant changing current community water fluoridation practices”.  

Having been followed by a federal court ruling in September that current water fluoridation levels in the US pose “an unreasonable risk of injury” to public health, including cognitive development in children, the NTP report appears to have been a watershed moment. Given that Secretary of the US Department of Health and Human Services Robert F. Kennedy vocally opposes fluoridation and that President Donald Trump has said that Kennedy’s suggestion of banning it “sounds OK to me”, the fresh debate surrounding the practice may end to its detriment. 

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