Higher levels of urinary fluoride during pregnancy are associated with more ADHD-like symptoms in school-age children, according to University of Toronto and York University researchers.

"Our findings are consistent with a growing body of evidence suggesting that the growing fetal nervous system may be negatively affected by higher levels of fluoride exposure," said Dr. Morteza Bashash, the study's lead author and researcher at the Dalla Lana School of Public Health.

The study, "Prenatal Fluoride Exposure and Attention Deficit Hyperactivity Disorder (ADHD) Symptoms in Children at 6-12 Years of Age in Mexico City," published today in Environment International, analyzed data from 213 mother-child pairs in Mexico City that were part of the Early Life Exposures in Mexico to Environmental Toxicants (ELEMENT) project, which recruited pregnant women from 1994 to 2005 and has continued to follow the women and their children ever since.

Tap water and dental products have been fluoridated in communities in Canada and the United States (as well as milk and table salt in some other countries) by varying amounts for more than 60 years to prevent cavities. In recent years, fierce debate over the safety of water fluoridation -- particularly for children's developing brains -- has fueled researchers to explore the issue and provide evidence to inform national drinking water standards.

The research team -- including experts from the University of Toronto, York University, the National Institute of Public Health of Mexico, University of Michigan, Indiana University, the University of Washington and Harvard School of Public Health -- analyzed urine samples that had been obtained from mothers during pregnancy and from their children between six and 12 years of age to reconstruct personal measures of fluoride exposure for both mother and child.

The researchers then analyzed how levels of fluoride in urine related to the child's performance on a variety of tests and questionnaires that measure inattention and hyperactivity, and provide overall scores related to ADHD. Analyses were adjusted for other factors known to impact neurodevelopment, such as gestational age at birth, birthweight, birth order, sex, maternal marital status, smoking history, age at delivery, education, socioeconomic status and lead exposure.

"Our findings show that children with elevated prenatal exposure to fluoride were more likely to show symptoms of ADHD as reported by parents. Prenatal fluoride exposure was more strongly associated with inattentive behaviours and cognitive problems, but not with hyperactivity," said Bashash.

This work builds off of previous research the team published on this population demonstrating that higher levels of urine fluoride during pregnancy are associated with lower scores on tests of IQ and cognition in the school-age children.

ADHD is the most common psychiatric disorder diagnosed in childhood, affecting between five and nine per cent of all school-aged children.

"The symptoms of ADHD often persist into adulthood and can be impairing in daily life," said Christine Till, Associate Professor of Psychology at York University and co-author on the study.

"If we can understand the reasons behind this association, we can then begin to develop preventive strategies to mitigate the risk," said Till, who is also the principal investigator of another National Institutes of Health-funded grant examining fluoride exposure in a large Canadian sample of pregnant women.

The National Institute of Environmental Health Sciences, part of the National Institutes of Health (NIH), funded this study.