I see the goalposts are moving from "fluoride in drinking water concentrations" (implication: concentrations commonly found in municipal drinking water) to "fluoride in drinking water at certain concentrations" (i.e. any arbitrary number that could support your position).
Anyway, there's a pretty obvious definition of "drinking water concentrations": the recommended amount for US drinking water. Again, the authors of the study bolded this sentence to ensure you wouldn't miss it:
> It is important to note that there were insufficient data to determine if the low fluoride level of 0.7 mg/L currently recommended for U.S. community water supplies has a negative effect on children’s IQ.
Anyway, there's a pretty obvious definition of "drinking water concentrations": the recommended amount for US drinking water. Again, the authors of the study bolded this sentence to ensure you wouldn't miss it:
> It is important to note that there were insufficient data to determine if the low fluoride level of 0.7 mg/L currently recommended for U.S. community water supplies has a negative effect on children’s IQ.