The relationships between low levels of urine fluoride on children’s intelligence, dental fluorosis in endemic fluorosis areas in Hulunbuir, Inner Mongolia, China.

Many animal experiments document that exposure to high levels of fluoride in drinking water may have certain structural and functional damages to nervous system [1], [2], [3] as well as cumulative toxic symptoms like dental fluorosis and skeletal fluorosis [4]. Epidemiological studies have consistently confirmed that excessive exposure to fluoride in drinking water may lead to the decrease of children’s intellectual ability [5], [6] and the rise in the prevalence of dental fluorosis [7], [8], [9]. Meta-analyses focusing on the effect of fluoride exposure in drinking water to children’s intelligence also suggest a strong negative relation of fluoride exposure on IQ performance [10], [11]. These findings provided evidences and references for the health administrative departments in some countries to revise their screening guidelines of fluoride concentration in drinking water to safe levels. For instance, the fluoride concentration in drinking water has been set to no more than 1 mg/L in China (GB 5749-2006) [12]. However, most studies examining the effects on the damages to children’s intellectual ability and other aspects were conducted in regions where residents were exposed to relatively high fluoride levels (more than 3 mg/L) [5], [6], [13], [14] while the majority of residents living in endemic fluorosis areas have been exposed to lower fluoride levels in their daily life. Although some studies [15], [16] in which the fluoride exposure levels were relatively low had reported associations between low fluoride exposure and the negative effects among children, the analysis focused specifically on children’s dental fluorosis condition rather than intellectual performance.

Fluoride concentration in drinking water is long-term stable in years in a certain region [17]; kidney as a site of active metabolism excretes 50–80% of fluoride intake from drinking water and other sources [18]. Therefore, urine fluoride concentration as an internal exposure index can systematically reflect the burden of fluoride exposure in drinking water. In the present ecologic study, we explored the dose–response relationships between urine fluoride levels and children’s intelligence quotient (IQ) as well as dental fluorosis condition using multiple linear regression model and Cochran–Armitage test for trend respectively to estimate the possible negative effects of long-term low fluoride exposure in drinking water to young children.