It’s the first time that scientists have systematically studied the relationship between the behavioral disorder and fluoridation, the process wherein fluoride is added to water to prevent cavities.
The study, published in the journal Environmental Health, found that states with a higher portion of artificially fluoridated water had a higher prevalence of ADHD. This relationship held up across six different years examined. The authors, psychologists Christine Till and Ashley Malin at Toronto’s York University, looked at the prevalence of fluoridation by state in 1992 and rates of ADHD diagnoses in subsequent years.
“States in which a greater proportion of people received artificially-fluoridated water in 1992 tended to have a greater proportion of children and adolescents who received ADHD diagnoses [in later years], after controlling for socioeconomic status,” Malin says. Wealth is important to take into account because the poor are more likely to be diagnosed with ADHD, she says. After income was adjusted for, though, the link held up.
Take Delaware and Iowa, for instance. Both states have relatively low poverty rates but are heavily fluoridated; they also have high levels of ADHD, with more than one in eight kids (or 14 percent) between the ages of four and 17 diagnosed.
In the study, the scientists produced a predictive model which calculated that every one percent increase in the portion of the U.S. population drinking fluoridated water in 1992 was associated with 67,000 additional cases of ADHD 11 years later, and an additional 131,000 cases by 2011, after controlling for socioeconomic status.
“The results are plausible, and indeed meaningful,” says Dr. Philippe Grandjean, a physician and epidemiologist at Harvard University. This and other recent studies suggest that we should “reconsider the need to add fluoride to drinking water at current levels,” he adds.
Thomas Zoeller, a scientist at UMass-Amherst who studies endocrine disruptors—chemicals that interfere with the activity of the body’s hormones, something fluoride has been shown to do—says that this is “an important observation in part because it is a first-of-a-kind. Given the number of children in the U.S. exposed to fluoridation, it is important to follow this up.” Since 1992, the percentage of the U.S. population that drinks fluoridated water has increased from 56 percent to 67 percent, during which time the percentage of children with an ADHD diagnosis has increased from around seven percent to more than 11 percent, according to the Centers for Disease Control and Prevention (CDC).
Others felt more strongly. “The numbers of extra cases associated with a one percent increase in the 1992 artificial fluoridation [figures] are huge,” saysWilliam Hirzy, an American University researcher and former risk assessment scientist at the Environmental Protection Agency, who is also a vocal opponent of fluoridation. “In short, it clearly shows that as artificial water fluoridation increases, so does the incidence of ADHD.”
But scientists were quick to point out that this is just one study, and doesn’t prove that there is necessarily a causal link between fluoridation and ADHD. They also noted a number of important limitations: Individual fluoride exposures weren’t measured, ADHD diagnoses weren’t independently verified and there may be other unknown confounding factors that explain the link.
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