Harvard study finding fluoride in water does negatively impact childhood development: http://www.hsph.harvard.edu/news/features/fluoride-childrens...
The current target concentration in the US is 0.7mg/L and is routinely monitored and lowered if the background level is too high where some of the 'hot spots' in China that saw childhood development issues had water containing above 6mg/L and one area had a range from 118mg/kg to 1,361mg/kg (thank you coal mining).
Another confounding factor is that many of the areas with high fluoride levels also had high aluminum, mercury, and arsenic levels as well.
I'd like to see a study comparing IQs or development with actual usage levels and without the heavy metal pollutants from industry.
Full HSPH study:
http://ehp.niehs.nih.gov/wp-content/uploads/2012/09/ehp.1104...