Fluoride: A compound of fluorine with another element or radical. Fluoride containing compounds are used topical and systemic in the prevention of tooth decay. Fluoride ions replace hydroxyl ions in hydroxyapatite in teeth, forming fluorapatite, which leads to fewer cavities. Fluoride compounds are therefore added to some toothpastes and dentists may also give a yearly fluoride treatment. Fluoridation of water is considered by the U.S. Centers for Disease Control and Prevention as “one of the 10 treat public health achievements of the 20th century. The addition of fluoride to water systems is one of the major achievements in public health in the 20th century. It is generally recommended that fluorides be added to water supplies in which the natural level is less than 0.7 ppm in water to increase the concentration to 0.7-1.2 ppm. Dental fluorosis is considered the only accepted adverse effect of fluoridation. Fluoridation of water has its critics. See also: Water fluoridation.