Among which, a meta-analysis (Choi et al.,) - a statistical analysis that combines the results of multiple studies - found that in 26 out of 27 studies, children in a high-fluoridated community had lower IQ scores, on average, compared to children in a low-fluoridated community. Conversely, another group of researchers found no differences in IQ between children living in communities with artificially-fluoridated and those in a non-fluoridated community. Regarding the latter research, however, substantial methodological limitations have been identified; one key limitation of this study is that the difference in fluoride intake between fluoridated and non-fluoridated communities was small, thereby reducing the power of the study to detect an effect of fluoride on IQ.
Based on research that found a relationship between exposure to fluoride and lowered IQ, which accounted for various confounding variables, a daily dose should not exceed 0.005 mg of fluoride per day, or 0.0010 mg of fluoride per kilogram of bodyweight per day, for children aged 8-13 years.
Several groups of researchers indicate that fluoride is a developmental neurotoxicant in humans. In addition, a separate group of researchers observed a significant association between the prevalence of artificial water fluoridation and and the prevalence of attention deficit-hyperactivity disorder (ADHD) in the United States.
Another group of researchers observed a significant inverse relationship between both urinary and serum fluoride levels and IQ among children. The researchers observed a statistically significant IQ difference of 4.3 IQ points between the high (n=106) and control (n=110) urinary fluoride groups. Similarly, another group of researchers observed that both serum fluoride, and urine fluoride were significantly related to water fluoride levels, and both were also significantly related to lowered IQ. The high urinary fluoride level group, the IQ point difference, compared to the control group, was 2.42 points per milligram of fluoride per liter.
Regardless of the mechanism(s), the researchers observed that children who lived in areas with high fluoride exposure had lower IQ scores, compared to those who lived in low-exposure areas.
The researchers compared the estimated exposed and control doses received in the recent water fluoridation study (Choi et al.,), and compared the estimated differences in those exposures to their findings regarding an adverse effect level. They then used two methods, both with uncertainty factors, to estimate a protective fluoride dose:
The evidence indicates that the effect of fluoride on IQ is quite large, with a predicted mean 5 IQ point loss when going from a dose of 0.5 mg F/day to 2.0 mg F/day, which is an exposure range one might expect when comparing individuals in the United States with a low total intake to those with a higher total intake. However, when comparing a fluoridated area of the United States to an unfluoridated area it would be hard to discern a mean IQ difference, because of the multiple sources of fluoride intake besides drinking water.
While this research does not touch on the question of whether such a level in drinking water offers dental health benefits, it indicates that an intake rate greater than 0.047 mg F/day poses a significant risk of lowering IQ of exposed children.
The current average mean fluoride exposures for US children range from about 0.80 mg F/day to about 1.65 mg F/day. These doses are 17 to 35 times higher than the higher estimated reference dose of 0.047 mg F/day. These results imply that at present the risk of IQ loss among children in the US is high.
Fluoride may be similar to lead and mercury in having no threshold below which exposures may be considered safe.
In a population of 320 million, the population level impact of an average 5 IQ point loss, beyond purely dollars of income loss, is a reduction of about 4 million people with IQ>130 and an increase of almost as many people with IQ<70.
Broadbent, J., Thomson, W., Ramrakha, S., Moffitt, T., Zeng, J., Foster Page, L., & Poulton, R. (2015). Community Water Fluoridation and Intelligence: Prospective Study in New Zealand. American Journal Of Public Health, 105(1), 72-76. http://dx.doi.org/10.2105/ajph.2013.301857
Choi, A., Sun, G., Zhang, Y., & Grandjean, P. (2012). Developmental Fluoride Neurotoxicity: A Systematic Review and Meta-Analysis. Environmental Health Perspectives, 120(10), 1362-1368. http://dx.doi.org/10.1289/ehp.1104912
Malin, A., & Till, C. (2015). Exposure to fluoridated water and attention deficit hyperactivity disorder prevalence among children and adolescents in the United States: an ecological association. Environmental Health, 14(1). http://dx.doi.org/10.1186/s12940-015-0003-1
Wang, S., Wang, Z., Cheng, X., Li, J., Sang, Z., & Zhang, X. et al. (2007). Arsenic and Fluoride Exposure in Drinking Water: Children’s IQ and Growth in Shanyin County, Shanxi Province, China. Environmental Health Perspectives, 115(4), 643-647. http://dx.doi.org/10.1289/ehp.9270
Zhang, S., Zhang, X., Liu, H., Qu, W., Guan, Z., & Zeng, Q. et al. (2015). Modifying Effect of COMT Gene Polymorphism and a Predictive Role for Proteomics Analysis in Children’s Intelligence in Endemic Fluorosis Area in Tianjin, China. Toxicological Sciences, 144(2), 238-245. http://dx.doi.org/10.1093/toxsci/kfu311
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