Fluoride in U.S. drinking water does not reduce IQ, a new study finds

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 results, based on standardized intelligence testing of more than 10,000 people in Wisconsin followed since their senior year of high school in 1957, challenge the idea that typical fluoridation levels in public drinking water pose a neurodevelopmental risk, a central point of contention in ongoing policy debates.

“It’s very strong data,” says Steven Levy, a dentist and public health researcher at the University of Iowa in Iowa City who was not involved in the research. “There’s no strong signal at all coming through that should give us concern.”

However, given the politically charged nature of water fluoridation and continued differences in how researchers interpret the available evidence, the findings are unlikely to be the last word on the issue.

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.

But beginning around the turn of the century, concerns about possible neurological effects began to surface, fueled largely by studies of children exposed to unusually high levels of naturally occurring fluoride in groundwater in parts of China, India and elsewhere.

Those concerns came to a head 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.

That 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.

But many scientists were less convinced. As critics were quick to point out, much of the underlying evidence drew 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 such critic 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. Warren and his colleagues reported in November in Science Advances that they found no evidence that fluoride at levels typical of community water systems harms cognitive performance.

But that study relied on academic achievement measures rather than direct IQ tests, making it more difficult to directly compare its findings with those of the National Toxicology Program analysis. What’s more, it could only approximate childhood fluoride intake based on school location. What Warren needed was 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 the earlier analysis 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, though. For instance, 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 such, the findings “should be interpreted cautiously,” Till says.

As governments weigh whether to keep fluoride in the tap, researchers agree on at least one point: The fight over its effects on the brain is far from over.