The maximum allowed in the US is 4mg/L and the CDC recommends giving children up to 8 years old water with at least 2mg/L [1]. There are plenty of places with more than 2mg/L in the US and Kentucky has places over 4mg/L [2].
I bet everyone down-voting my comment is doing it out of reflex and has never read anything about this.
Wikipedia isn’t correct here. PHS now recommends an optimal fluoride concentration of 0.7 milligrams/liter (mg/L). In this guidance, the optimal concentration of fluoride in drinking water is the concentration that provides the best balance of protection from dental caries while limiting the risk of dental fluorosis. The earlier PHS recommendation for fluoride concentrations was based on outdoor air temperature of geographic areas and ranged from 0.7–1.2 mg/L. This updated guidance is intended to apply to community water systems that currently fluoridate, or that will initiate fluoridation, and is based on considerations that include:
Which goes on to say: For many years, nearly all of these fluoridated systems used fluoride concentrations ranging from 0.8 to 1.2 mg/L; fewer than 1% of these systems used a fluoride concentration at 0.7 mg/L (Unpublished data, Water Fluoridation Reporting System, CDC, 2010). When water systems that add fluoride implement the new PHS recommendation (0.7 mg/L), the fluoride concentration in these systems will be reduced by 0.1–0.5 mg/L, and fluoride intake from water will decline among most people served by these systems.
> the CDC recommends giving children up to 8 years old water with at least 2mg/L [1].
Actually, the Wikipedia article says the opposite (the citation it gives is a dead link, of course)--it suggests to parents that they should be careful with having children use fluoride toothpaste or water with > 2mg/L of water.
> There are plenty of places with more than 2mg/L in the US and Kentucky has places over 4mg/L [2].
If you read that study carefully as well, that's talking about the natural fluoride concentration of well water, not fluoridation within water treatment plants. (Another reason to generally prefer municipal treatment plants as a source for water: they are more likely to follow more stringent guidelines for water contaminants than individual wells).
I bet everyone down-voting my comment is doing it out of reflex and has never read anything about this.
[1] https://en.wikipedia.org/wiki/Water_fluoridation_in_the_Unit...
[2] http://www.uky.edu/KGS/pdf/ic12_01.pdf