For this latest study, researchers from the National Institutes of Environmental Health Sciences, led by Dr. Kyla W. Taylor, performed an analysis of 74 cross-sectional studies conducted in China, Canada, Denmark, India, Iran, Mexico, New Zealand, Pakistan, Spain and Taiwan. The team examined fluoride in drinking water at levels less than 4 mg/L, 2 mg/L, and less than 1.5 mg/L. They also measured fluoride in urine at the same levels.
None of the analysis was conducted in the U.S., because fluoridation levels in drinking water are much lower than in the other countries studied. Countries with higher levels of fluoride in the water are better able to highlight the evidence linking fluoride and lower IQ scores, according to researchers.
Researchers said they found a “significant inverse association” between fluoride exposure and children’s IQ scores. Among fluoride measurements taken from the participant’s urine, the data showed an IQ score decrease of 1.63 points for every 1 mg/L increase of fluoride in the urine. Average levels of fluoride in the urine range from 0.5 mg/L to 3.5 mg/L in the United States.
In addition to measuring levels of fluoride in urine, they also compared IQ to levels added to drinking water, finding an IQ score decrease of 1.14 points for every 1 mg/L increase of fluoride in the water.
Overall, higher fluoride levels were linked to lower IQ scores in children, researchers determined, concluding that the findings from the analysis will help inform the risk-benefit assessment of using fluoride in children’s tooth care.
“This systematic review and meta-analysis found inverse associations and a dose-response association between fluoride measurements in urine and drinking water and children’s IQ across the large multicountry epidemiological literature,” the researchers concluded. “These findings may inform future comprehensive public health risk-benefit assessments of fluoride exposures.”