Higher estimated fluoride levels in public drinking water during pregnancy were associated with lower scores on some measures of childhood cognition in a large U.S. study, including at concentrations near levels commonly used for community water fluoridation.
The observational study, published in the American Journal of Epidemiology, included 2,514 children born between 2006 and 2019. Researchers linked public water-system fluoride measurements to mothers’ residential addresses during pregnancy, then compared those estimates with cognitive testing completed when the children were ages 3 to 17. Because researchers estimated fluoride exposure rather than measuring how much fluoride individual women consumed, and because the study was observational, the findings cannot establish that fluoride caused differences in cognition.
Researchers examined two broad types of cognitive ability. Fluid cognition includes skills such as reasoning, attention, memory and adapting to new information, while crystallized cognition reflects accumulated knowledge, including vocabulary.
Higher estimated fluoride concentrations were associated with lower fluid cognition scores once concentrations reached higher levels, although the point at which the relationship appeared varied depending on the statistical method researchers used.
In one analysis, researchers identified about 675 micrograms of fluoride per liter of water, or 0.675 milligrams per liter, as the point above which fluid cognition scores began declining as fluoride concentrations increased.
Above that estimated level, each additional 500 micrograms per liter of fluoride was associated with an average 3.36-point lower fluid cognition score.
Another statistical approach suggested the inverse relationship became significant only at substantially higher concentrations, around 1,107 micrograms per liter. That difference means the study does not establish 675 micrograms per liter as a clear biological threshold separating safe from harmful exposure.
The researchers did not find the same overall relationship with crystallized cognition.
Associations with fluid cognition also appeared stronger among children tested before age 7 than among older children, although the researchers said more work is needed to understand why age at testing might matter.
“This study addresses an important gap by evaluating prenatal water fluoride and child cognition in a large U.S. sample,” said lead author Katrina Simon of Columbia University Mailman School of Public Health. “Findings from this work offer evidence that can inform ongoing guidance aimed at supporting children’s development.”
Fluoride occurs naturally in some water supplies and is also added to many public water systems to reduce tooth decay. The U.S. Public Health Service recommends a concentration of 0.7 milligrams per liter for community fluoridation.
That recommendation is different from the Environmental Protection Agency’s maximum contaminant level of 4 milligrams per liter. The lower public health recommendation is intended to guide fluoridation for dental benefits, while the EPA limit is a regulatory standard for drinking water.
The current study did not evaluate the overall benefits and risks of community water fluoridation and should not be interpreted as showing that fluoridated water causes cognitive impairment.
Exposure measurement is one of the study’s most important limitations. Researchers did not directly measure fluoride levels in the women’s bodies or track how much tap water they drank. Instead, they matched residential information with fluoride concentrations reported by public water systems.
That approach also could not capture total fluoride exposure. Researchers did not know how much fluoride participants may have received from bottled water, tea, food, toothpaste, supplements or other sources. Nor could they determine whether participants regularly filtered their tap water.
The study population also may not fully represent the broader U.S. population. Participants included in the analysis were disproportionately white, non-Hispanic and college educated, and only a portion of ECHO participants with cognitive data had sufficient information to be included. Analyses intended to account for potential selection bias weakened some of the associations, although the overall direction remained negative.
The researchers adjusted their analyses for factors including parental education and age, prenatal tobacco exposure, child sex, season of conception and neighborhood characteristics. Even with those adjustments, observational studies cannot rule out the possibility that other differences between families contributed to the results.
The findings add to an ongoing scientific debate over fluoride exposure during pregnancy and child development. Previous research has raised concerns primarily at considerably higher fluoride concentrations, leaving uncertainty about whether associations also occur at levels more typical of U.S. drinking water.
The new study moves that question closer to concentrations encountered in some U.S. communities, but it does not settle it. Replication using more direct measures of individual fluoride exposure, along with research that considers both potential developmental risks and established dental benefits, will be important for understanding what the findings mean for public health recommendations.
The research was supported by the National Institutes of Health through the Environmental influences on Child Health Outcomes, or ECHO, Program and other NIH institutes and centers.