Fluoride, chemical compound of the element fluorine, and an important trace constituent of the human body. The bones and teeth contain most of the body's fluoride. Sea fish and tea are rich sources, but intake is mainly from drinking water. Fluoridation of water that contains the ideal level of 1 part per million (ppm) significantly reduces the incidence of dental decay (caries). Excess accumulation of fluoride (called fluorosis) occurs in teeth and bone in proportion to the level and duration of intake, and communities in which the level in drinking water exceeds 10 ppm are commonly affected.