Fluoride in water has always been the subject of some controversy. Fluoride was originally added to drinking water because populations drinking naturally fluoridated water suffered fewer dental cavities. But is has been known for a while that very high levels of fluoride can compromise health. The controversy debates where the safe fluoride level lies. Treated water should have far less fluoride than the amount needed to cause issues.
Both New Zealand and Israel have seen this debate resurface recently: Israel actually suspended fluoridisation for about two years before reintroducing it in mid-2016. New Zealand television has been running an anti-fluoridation campaign, strongly criticized for giving misleading information.
According to its critics the fluoride in water can cause kidney strain, child development problems and (ironically) dental issues. It is known that these issues have some connection to fluoride, but only under extremely high levels of exposure. The human kidneys should be more than able to deal with the minute amount than most individuals are exposed to. And the dental issues from fluoride consist of white patches on teeth that are only of aesthetic importance. The effects on child brain development do not seem to be an issue in developed countries with regulated fluoride, only in unregulated, extremely high level supplies; and even then there is suspicion that the problem might be caused by other pollutants. Low levels of fluoride seem to pose no threat.
The cost of water fluoridisation in many developed countries is about $1.oo per person. This is much less than the cost of fixing the dental problems that would exist without fluoridisation. Many studies support the idea that this practice is justified.
The human body needs many trace elements to survive. These same elements can sometimes be toxic in higher doses. Copper is one example of this; deficiencies are a health issue, but toxicity occurs beyond a very small dose. Fluoride is probably similar. It is useful in small doses, but causes problems in human when consumed in extremely high doses.