Water fluoridation is widely seen as one of the great public health achievements of the 20th century, credited with substantially reducing tooth decay. But there has been growing controversy among scientists about whether fluoride may be linked to lower IQ scores in children, with the latest research adding to the clamour.
A comprehensive federal analysis of scores of previous studies, which was recently published in JAMA Paediatrics, suggested a significant inverse relationship between exposure levels and cognitive function in children – but not, said the scientists, in the USA.
The researchers from the National Institute of Environmental Health Sciences suggested that while fluoride exposures were indeed linked to lower IQ scores, none of the studies included in the analysis had been conducted in the United States, where recommended fluoridation levels in drinking water are very low. At those amounts, evidence was too limited to draw definitive conclusions.
Observational studies cannot prove a cause-and-effect relationship. Yet in countries with much higher levels of fluoridation, the analysis also found evidence of what scientists call a dose-response relationship, with IQ scores falling in lock step with increasing fluoride exposure, reports The New York Times.
Children are exposed to fluoride through many sources other than drinking water: toothpaste, dental treatments and some mouthwashes, as well as black tea, coffee and certain foods, such as shrimp and raisins. Some drugs and industrial emissions also contain fluoride.
For every one part per million increase in fluoride in urinary samples, which reflect total exposures from water and other sources, IQ points in children decreased by 1.63, the analysis found.
“There is concern that pregnant women and children are getting fluoride from many sources,” said Dr Kyla Taylor, an epidemiologist at the institute and the report’s lead author, “and that their total fluoride exposure is too high and may affect foetal, infant and child neurodevelopment.”
Taylor said that the analysis was meant to contribute to the understanding of the safe and effective use of fluoride. But she said it did not address the benefits and was not intended to assess “the broader public health implications of water fluoridation in the United States”.
Several scientists, including many dentists, criticised the report, pointing to what they said were methodological flaws and emphasising that the research did not have implications for US drinking water.
The subject is so divisive that JAMA Paediatrics commissioned two editorials with opposing viewpoints to publish alongside the report.
In one, Dr Steven Levy, a public health dentist at the University of Iowa, said that many of the studies included in the analysis were of very low quality. He also warned against concluding that any changes should be made in American fluoridation policies.
“A lay reader or policymaker at a water board in a small community somewhere may see the evidence and think that every way you analyse it, it’s a concern,” Levy said. “It isn’t as clear-cut as they’re trying to make it.”
The report’s findings align in some ways with statements by Robert F Kennedy Jr, President-elect Donald Trump’s choice to head the national Department of Health & Human Services. He has questioned the safety of fluoride and said one of the first acts of the Trump administration would be to advise water systems to remove fluoride.
Criticism of fluoridation has popped up frequently since the practice was initiated in many US communities in the 1950s. But opposition was originally dismissed, as it was strongest among those with extremist or fringe views, and right-wing groups like the John Birch Society, which called fluoridation a Communist plot.
That is changing. Last September, US District Judge Edward Chen in San Francisco ordered the Environmental Protection Agency to strengthen regulations for fluoride in drinking water because of research suggesting that high levels might pose a risk to the intellectual development of children.
In a second editorial published alongside the new study, a public health expert, Dr Bruce Lanphear, noted that as far back as 1944, the editor of The Journal of the American Dental Association expressed concern about adding fluoride, which he termed “a highly toxic substance”, to drinking water. He wrote that “the potentialities for harm far outweigh those for good”.
Some studies have suggested that dental health has improved not because fluoride was added to water, but because of fluoridated toothpastes and better dental hygiene practices. (In some countries, fluoride is added to salt.)
According to this argument, topical application of fluoride to teeth is effective enough to prevent tooth decay, and ingestion is not necessary. But other studies have reported increases in cavities after public water fluoridation initiatives ceased in some countries.
Currently, the recommended fluoride levels in the United States are 0.7 parts per million, and the study did not find a statistically significant inverse association between fluoride levels and IQ scores at below 1.5 parts per million based solely on fluoride levels in water.
But nearly 3m Americans still drink water with fluoride levels above 1.5 parts per million from wells and some community water systems.
Linda Birnbaum, former director of the National Institute of Environmental Health Sciences, called for more research into the potential effects of fluoride levels below 1.5 parts per million.
She said the study had concluded that exposure can be damaging to developing brains. “The answer is pretty clear: yes,” she said.
To protect foetuses and babies who are especially vulnerable, she advised parents to avoid drinking fluoridated water during pregnancy and to use fluoride-free bottled water when preparing formula for their infants.
“My recommendation is that pregnant women and infants shouldn’t be exposed to excess fluoride,” said Birnbaum, who is not an author of the new analysis.
Women who are breastfeeding need not be concerned, she added, as very little fluoride is passed on through breast milk.
“The more we study chemicals, especially those that affect IQ, like lead – there’s really no safe level,” Birnbaum said.
Some 74 studies from 10 countries, including China, Mexico, Canada, India and Denmark, were examined. Lanphear noted that the consistent links between fluoride and IQ were found in very different populations.
He urged the US Public Health Service to set up a committee, perhaps one that does not include researchers who have studied the subject in the past, which can take a fresh look at the topic to examine two questions seriously: whether fluoride is neurotoxic, and whether it is as beneficial for oral health as it is believed to be.
“If that doesn’t happen urgently, my concern is there will be growing distrust of public health agencies amid the public, and they will have deserved it,” he said.
Study details
Fluoride Exposure and Children’s IQ Scores: A Systematic Review and Meta-Analysis
Kyla Taylor, Sorina Eftim, Christopher Sibrizzi, et al.
Published in JAMA Paediatrics on 6 January 2025
Key Points
Question
Is fluoride exposure associated with children’s IQ scores?
Findings
Despite differences in exposure and outcome measures and risk of bias across studies, and when using group-level and individual-level exposure estimates, this systematic review and meta-analysis of 74 cross-sectional and prospective cohort studies found significant inverse associations between fluoride exposure and children’s IQ scores. For fluoride measured in water, associations remained inverse when exposed groups were restricted to less than 4 mg/L or less than 2 mg/L but not when restricted to less than 1.5 mg/L; for fluoride measured in urine, associations remained inverse at less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L; and among the subset of low risk-of-bias studies, there were inverse associations when exposed groups were restricted to less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L for analyses of fluoride measured both in water and in urine.
Meaning
This comprehensive meta-analysis may inform future risk-benefit assessments of the use of fluoride in children’s oral health.
Abstract
Importance
Previous meta-analyses suggest that fluoride exposure is adversely associated with children’s IQ scores. An individual’s total fluoride exposure comes primarily from fluoride in drinking water, food, and beverages.
Objective
To perform a systematic review and meta-analysis of epidemiological studies investigating children’s IQ scores and prenatal or postnatal fluoride exposure.
Data Sources
BIOSIS, Embase, PsycInfo, PubMed, Scopus, Web of Science, CNKI, and Wanfang, searched through October 2023.
Study Selection
Studies reporting children’s IQ scores, fluoride exposure, and effect sizes.
Data Extraction and Synthesis
Data were extracted into the Health Assessment Workplace Collaborative system. Study quality was evaluated using the OHAT risk-of-bias tool. Pooled standardised mean differences (SMDs) and regression coefficients were estimated with random-effects models.
Main Outcomes and Measures
Children’s IQ scores.
Results
Of 74 studies included (64 cross-sectional and 10 cohort studies), most were conducted in China (n = 45); other locations included Canada (n = 3), Denmark (n = 1), India (n = 12), Iran (n = 4), Mexico (n = 4), New Zealand (n = 1), Pakistan (n = 2), Spain (n = 1), and Taiwan (n = 1). Fifty-two studies were rated high risk of bias and 22 were rated low risk of bias. Sixty-four studies reported inverse associations between fluoride exposure measures and children’s IQ. Analysis of 59 studies with group-level measures of fluoride in drinking water, dental fluorosis, or other measures of fluoride exposure (47 high risk of bias, 12 low risk of bias; n = 20 932 children) showed an inverse association between fluoride exposure and IQ (pooled SMD, −0.45; 95% CI, −0.57 to −0.33; P < .001). In 31 studies reporting fluoride measured in drinking water, a dose-response association was found between exposed and reference groups (SMD, −0.15; 95% CI, −0.20 to −0.11; P < .001), and associations remained inverse when exposed groups were restricted to less than 4 mg/L and less than 2 mg/L; however, the association was null at less than 1.5 mg/L. In analyses restricted to low risk-of-bias studies, the association remained inverse when exposure was restricted to less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L fluoride in drinking water. In 20 studies reporting fluoride measured in urine, there was an inverse dose-response association (SMD, −0.15; 95% CI, −0.23 to −0.07; P < .001). Associations remained inverse when exposed groups were restricted to less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L fluoride in urine; the associations held in analyses restricted to the low risk-of-bias studies. Analysis of 13 studies with individual-level measures found an IQ score decrease of 1.63 points (95% CI, −2.33 to −0.93; P < .001) per 1-mg/L increase in urinary fluoride. Among low risk-of-bias studies, there was an IQ score decrease of 1.14 points (95% CI, –1.68 to –0.61; P < .001). Associations remained inverse when stratified by risk of bias, sex, age, outcome assessment type, country, exposure timing, and exposure matrix.
Conclusions and Relevance
This systematic review and meta-analysis found inverse associations and a dose-response association between fluoride measurements in urine and drinking water and children’s IQ across the large multicountry epidemiological literature. There were limited data and uncertainty in the dose-response association between fluoride exposure and children’s IQ when fluoride exposure was estimated by drinking water alone at concentrations less than 1.5 mg/L. These findings may inform future comprehensive public health risk-benefit assessments of fluoride exposures.
JAMA linked editorial – Time to Reassess Systemic Fluoride Exposure, Again (Open access)
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