Back when fluoridation was removed from Calgary water I dug into some of the studies on cognitive effects from fluoride. For a lot of those studies the "low dose control" group had higher concentrations of fluoride in their water than what was being added to Calgary water. I haven't kept up on the latest studies though. Certainly worth continuing to study and if necessary adjust levels (current fluoridation levels in Canada are lower than they used to be), but it is quite common for substances like fluoride to be beneficial at lower concentrations and have negative effects at higher concentrations. The goal is to optimize the concentration.
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