there are places where fluoride occurs naturally in drinking water at higher concentrations than it is added artificially and there don’t seem to be significant health problems.
I’m simply replying that there is places where fluoride occurs naturally in drinking water at higher concentration that it is added artificially and there is significant health problems in these places.
Does it means that fluoride in low concentration like in the US water system is dangerous ? No, it just means that very high concentration can be dangerous.