People worried about the fluoridation of municipal water supplies seem to occupy the same fringe space in American life as flat-earthers. But a new study published by JAMA Pediatrics appears to lend credence to the idea that fluoridated water has an impact on the IQ of children. The Washington Post reports that the study was subjected to additional scrutiny but ultimately found an impact on the IQ of boys:

Pregnant women reported their consumption of tap water and black tea, which is high in fluoride, in questionnaires. The authors of the new study also calculated the amount of fluoride in municipal water, based on the levels at wastewater treatment plants linked to the women’s postal codes. The researchers estimated the women’s fluoride intake based on a combination of those measures.

The researchers compared the fluoride intake of 400 women, some who lived in fluoridated cities and some who did not. They controlled for factors such as household income and the women’s education. A 1 milligram daily increase in fluoride intake was associated with a 3.7-point drop in children’s IQ, they found…

The scientists observed that a 1 milligram-per-liter increase in urine fluoride predicted a drop in IQ of 4.5 points in young boys. When the researchers examined the urine of mothers who had daughters, however, fluoride had no association with IQ…

Several researchers unaffiliated with the report applauded this work’s publication in the face of intense review. “I believe that, in general, the dental community will discount these findings, minimize their importance and continue to recommend the use of fluoridated water during pregnancy,” said Pamela Den Besten, a pediatric dentist who studies tooth enamel at the University of California at San Francisco. She added: “This study has been carefully conducted and analyzed.”

Indeed, the Post article has a statement from the American Dental Association in its second paragraph. A spokesperson for the group says it will continue to support fluoridation as a significant way to prevent tooth decay for large populations. The American Academy of Pediatrics also supports fluoridation.

But Philippe Grandjean at the Harvard School of Public Health called the study “excellent” and said, “CDC has to come out and look at the risk-benefit ratio again, because they can’t continue relying on studies that were carried out decades ago.”

NPR spoke to Christine Till, one of the authors of the study, who said that if the results are accurate the impact on the population as a whole would be significant even if the impact on a particular child was small:

The difference was typically a couple of IQ points, though the spread was wider when comparing those with highest exposure and those with the least. In general, there was a small difference for any individual child.

“We would feel an impact of this magnitude at a population level,” Till says, “because you would have millions of more children falling in the range of intellectual disability, or an IQ of under 70, and that many fewer kids in the gifted range.”

The study was funded by the Canadian government and the U.S. National Institute of Environmental Health Science.

“It’s actually very similar to the effect size that’s seen with childhood exposure to lead,” says David Bellinger, a professor of neurology at Harvard Medical School and Boston Children’s Hospital. He reviewed the paper before it was published and wrote a commentary about it.

No one thinks that one study of a few hundred women in Canada is going to change US policy any time soon. But this study will encourage other researchers to try and replicate the findings. If the findings do hold up then maybe we’ll see some changes in a few years.