What we know is scary - especially given that we actually don't know nearly enough
Fluoridation of municipal water first came into being both/either due to a need to deal with a waste product from the Manhattan A-bomb project, and/or as a way to prove for legal reasons that fluoride was not a noxious poison (the Manhattan Project was facing lawsuits over people having been poisoned)
Fluoride chemicals are currently almost exclusively sourced for inclusion into tap water as a highly toxic waste byproduct of superphosphate fertilizer production.
No significant studies have been done on the human toxicology of this particular chemical. However, in the '90's those products that used these chemicals for pesticide / insecticide / rodenticide purposes had their American licences revoked (and I have not been able to ascertain why, though I have read anecdotal materials indicating 'colateral damage' issues when these products were not used correctly... perhaps indicating a relatively unsafe product?)
Fluoridation does historically seem to have had a positive effect on dental health, insofar as there was (maybe... ) a reduction of anywhere from 10-60% in cavity formation. However, those statistics are mostly based on very dated studies (1940's - 50's). Newer information seems to indicate a significantly lower positive effect to the point of being negligible (more on that on the next page)
There is a very small dosage window between fluoride being possibly useful to our teeth, and it being definitely toxic. There is a benchmark of 0.6 - 1.2 ppm for inclusion in tapwater... but that benchmark was established in the 1950's, and does not in any way reflect the potential dosage potential (which is based on milligrams dosage per kilogram of body weight, rather than ppm in tap water) of a typical individual, as there is no ‘typical’ available at this time for proper dosage analysis
To answer the question, then: we know enough to be certain that this is a dangerous chemical, but not enough to be certain that at the dosage dumped into our tap water it is safe "enough".