Chinese children drinking well water with very high levels of fluoride scored poorly on intelligence testing compared to those with lower exposures.

This is one of the first studies in humans to find that too much fluoride is associated with low performance on intelligence tests. More information is needed to ascertain if the sum total amount of ingested fluoride from tap water, consumer products and other sources are enough to inhibit brain development in children living in the US and other countries where fluoridation is common.

What did they do? Wang et al. examined 524 children (exposed and controls) aged 8 to 12 years old in China's Shanxi province for fluoride exposures in relation to intellectual functioning and growth. The families were exposed to naturally occurring high concentrations of fluoride through well water. The children were compared to control children who were recruited from three nearby villages with uniformly low concentrations of fluoride in well water. All children lived in areas with similar geography, cultural conditions and socioeconomic development.

The authors obtained urine samples for fluoride analyses and conducted physical exams. Medically trained professionals administered standardized neurodevelopmental tests.

What did they find? Very high fluoride concentrations -- as high as 8.3 milligrams per liter (mg/L) -- were measured in the well water of the high fluoride group while the control group's well water had just 0.5 mg/L of fluoride.

The percentage of children with IQ scores less than 70 increased from 0 in the control group to 4% in the high fluoride group. The percentage of children with IQ scores greater than 109 was 41% in the control group and decreased to 30% in the high fluoride group. Overall, children in the high fluoride group had a four-point reduction in IQ score as compared to the control children. Measured fluoride in the urine was almost five times higher in the high fluoride exposed children than in the controls (5.1 mg/L versus 1.5 mg/L in controls). All of these associations were statistically significant.

What does it mean? Fluoride may affect brain development in growing children exposed to high levels of the ion through drinking water. Lower overall IQ scores, a greater number of lower scores and fewer higher scores were found in the group of children drinking high fluoride well water when compared with children who drank well water with lower amounts of fluoride. The observed differences in testing could affect a child's ability to perform in school.

The results have significant public health implications given that tap water and many other products contain fluoride. It may be that many children are receiving high of doses of fluoride in the US. The drinking water concentrations of fluoride observed in this study are likely well above concentrations in US public drinking water supplies, but children may be receiving excess fluoride through multiple sources (drinking water, soft drinks/bottled water, toothpaste, mouth rinses).

At this time, there are no studies quantitating the sum of all fluoride exposures for an average child in the US. The American Dental Association has recommended that infants not be given formula reconstituted with fluoridated water to prevent excess exposures. Further studies should measure all fluoride exposures to children to prevent fluorosis and potential neurodevelopmental toxicities.