J_Sflouride is added in most water we drink, toothpaste, etc. It is actually quite toxic and a dose of 5-10g is lethal to a full grown adult. Dentists say it prevents cavities and whatnot, but there's still loads of controversy on the topic, and opposers of the idea state that it causes serious health problems. Wikipedia says: "Although helpful for dental health in low dosage, chronic exposure to fluoride in large amounts interferes with bone formation"
on the 'tin foil hat' side of things, i've heard people say that water fluoridation is an attempt to sedate the population. (not sure where i stand on that one.)
NikolausI would guess that the amount we ingest is much too low to cause any damage.
There's no denying that too much fluoride is harmful, but it is very beneficial in small doses. It's also a very cheap way of providing some sort of dental protection to a lot of people at once who could otherwise not afford it. The EPA MCL for fluoride is 4 mg/L and states usually have more restrictive standards. For example my state, Connecticut, sets their fluoride MCL to 2.0 mg/L which is also the EPA secondary standard. For some context, at over 4 mg/L fluoride can cause staining of teeth in young children (dental fluorosis). At 10-11 mg/L, it can cause skeletal fluorosis aka not fun for anyone. Unless you are downing tubes of toothpaste and chugging gallons of water with excessive fluoride in them, you'll be fine.
Yes, it's added to public water supplies, but again in very low amounts. They aren't dumping 1000 mg/L into the system. I believe the level of fluoride in New York City water was tested to be an average of 0.7 mg/L last year.
It would be interesting to see long term studies on its impact though. I can't find any long term research.