"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 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. These assumptions were later dismissed by hundreds of scientific publications which showed that internal consumption of fluoridated water had no effect on tooth decay. 

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 Centers for Disease Control and Prevention says dental fluorosis is highest among adolescents between the ages of 12 an 15. One reason for the increase in fluorosis: Americans are now exposed to fluoride from a variety of sources, including toothpaste, mouth rinses and prescription supplements, the Department of Health and Human Services says.

Fluoride was found to be an equivocal carcinogen by the National Cancer Institute Toxicological Program."In point of fact, fluoride causes more human cancer death, and causes it faster than any other chemical," stated Dr. Dean Burk PhD who spent over three decades with the national cancer institute. 

A study published in Neurologia showed "the prolonged ingestion of fluoride may cause significant damage to health and particularly to the nervous system." That was the conclusion by a review of studies by researchers Valdez-Jimenez, et al.

“Fluoridation is no longer effective,” contends Hardy Limeback, head of the preventive dentistry program at the University of Toronto, who says adding the chemical to water is “more harmful than beneficial.” 

Researchers suspect a connection to cancer because half of ingested fluoride is deposited in bones, and fluoride stimulates growth in the end of bones, where osteosarcoma occurs.

Most developed nations do not fluoridate their water. In western Europe, for example, only 3% of the population consumes fluoridated water. Only 11 countries in the world have more than 50% of their population drinking fluoridated water: Australia (80%), Brunei (95%); Chile (70%), Guyana (62%), Hong Kong (100%), the Irish Republic (73%), Israel (70%), Malaysia (75%), New Zealand (62%), Singapore (100%), and the United States (64%). In total, 377,655,000 million people worldwide drink artificially fluoridated water. This represents 5% of the world’s population.

The research team for the EI study include 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.