In 1945 fluoride was first used. 1962  the  USPHS (United States Public Health Service)) said  that the  0.7-1.2mg/L ideal but this was adjusted in 2015 to 0.7. Currently the
EPA's (enforceable) MCL is 4.0 mainly for Skeletal Fluorosis risks but their
SMCL is 2.0 for dental fluorosis which is usually permanent cosmetic tooth damage.
76 out of 85 human studies showed decreased IQ from elevated exposure.
Many municipalities have stopped flouridation altogether as increasing debate over risks and benefit continue. Bottled water is not the answer! Listen to this one yourself for more info....