Two U.S. states and more than a dozen cities and counties have moved in the past year to stop adding fluoride to community drinking water, citing research suggesting the mineral could harm children’s brain development.
But a new analysis of cognitive outcomes tracked over decades finds no evidence that water fluoridation is associated with lower adolescent IQ or diminished mental abilities later in life, researchers report April 13 in the Proceedings of the National Academy of Sciences.
The debate is one of those public-health arguments that never really stays in the lab. It spills into policy rooms, court filings, and plenty of kitchen-table conversations. In the real world, the question often gets boiled down to a simple premise: if something is meant to protect teeth, could it also be quietly nudging children’s brains in the wrong direction?
Fluoride has been added to public water supplies in North America since the 1940s, after studies of communities in the western United States showed that naturally occurring fluoride in groundwater strengthened tooth enamel and reduced cavities. The practice became one of the most widespread public health interventions of the 20th century and is widely credited with sharply lowering rates of tooth decay in children.
Then, beginning around the turn of the century, concerns about possible neurological effects began to surface. Those worries were fueled largely by studies of children exposed to unusually high levels of naturally occurring fluoride in groundwater in parts of China, India and elsewhere. The most recent flare-up came last year, when researchers affiliated with the U.S. government’s National Toxicology Program synthesized epidemiological evidence and reported a link between elevated fluoride exposure and lower IQ scores
in children — with the strongest associations observed at fluoride concentrations above the World Health Organization’s guideline of 1.5 milligrams per liter, and mixed results below that threshold. Misryoum newsroom reported that this study drew widespread attention, including from a U.S. federal district court, which cited the finding in ordering the Environmental Protection Agency to review fluoride’s potential neurotoxic effects. Federal health officials also referenced the research when announcing plans to reassess the safety and
benefit of ingestible fluoride supplements and to reevaluate public water fluoridation policies.
Still, many scientists were less convinced. As Misryoum analysis indicates, much of the underlying evidence came from populations exposed to substantially higher fluoride concentrations than those commonly found in North American drinking water. None of the studies were conducted in the United States, and only a handful included data from countries with fluoridation practices similar to U.S. programs, such as Canada and New Zealand.
One critic of the “stop now” conclusion was Rob Warren, a demographer and public health researcher at the University of Minnesota in Minneapolis. After hearing U.S. Health and Human Services Secretary Robert F. Kennedy Jr. tout the research while arguing that fluoride exposure could harm children’s brains, Warren set out to evaluate whether the claim was backed by U.S. data. First, he looked at cognitive outcomes in a nationally representative group of nearly 58,000 high schoolers first surveyed in 1980 and followed for decades, reporting in November in Science Advances that they found no evidence that fluoride at levels typical of community water systems harms cognitive performance.
That earlier work, though, depended on academic achievement measures rather than direct IQ tests, making it harder to line up with the National Toxicology Program analysis. What’s more, it could only approximate childhood fluoride intake based on school location. Warren needed something better: a dataset with both standardized IQ scores and detailed residential histories — and he found it in the Badger State.
The new longitudinal study of Wisconsinites extends that earlier effort with more precise measures of both cognitive ability and duration of exposure to fluoridated water. Ultimately, it arrives at the same conclusion as earlier studies: across multiple statistical models and sensitivity analyses, community water fluoridation at the current guideline level of 0.7 milligrams per liter was not associated with cognitive outcomes across the course of a life. “The claim about IQ just doesn’t hold up,” Warren says.
Not everyone is persuaded, of course. Because the participants were born before widespread water fluoridation, the analysis does not capture exposure during sensitive early life periods such as gestation and infancy, when the brain is developing most rapidly, says Christine Till, a neuropsychologist at York University in Toronto. It also lacks direct measures of fluoride intake, instead inferring exposure from place of residence and overlooking other sources such as supplements. As Till puts it, the findings “should be interpreted cautiously.” And maybe, even with strong data, policy debates like this are more about timing and assumptions than about one study finally ending the story.
Misryoum newsroom reported that Steven Levy, a dentist and public health researcher at the University of Iowa in Iowa City who was not involved in the research, called the dataset “very strong data” and said, “There’s no strong signal at all coming through that should give us concern.”
For now, governments weighing whether to keep fluoride in the tap have at least one shared takeaway: the fight over its effects on the brain is far from over.
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