query: When was fluorine first introduced in tap water?
passage: Water fluoridation
Health and dental organizations worldwide have endorsed its safety and effectiveness.[3] Its use began in 1945, following studies of children in a region where higher levels of fluoride occur naturally in the water.[33] Further research showed that moderate fluoridation prevents tooth decay.[34]
passage: Tap water
In many areas a compound of fluoride is added to tap water in an effort to improve dental health among the public. In some communities "fluoridation" remains a controversial issue. (See water fluoridation controversy.)