Fluorine (F) is the first element of the halogen family and the most reactive of all chemical elements. The term "fluoride" refers to its ionic form (F-) and "fluorides" to fluoride-containing compounds, both organic and inorganic. Fluorine is never found by itself in nature but fluorides are found everywhere: in soil, air, and water, as well as in plant and animal life. Fluoride is commonly added to tap water, particularly in North America, and used in dental products to help prevent tooth decay. Fluorides are important industrial chemicals with a number of uses but the largest uses are for the production of aluminium and specialty chemicals used for refrigeration and air conditioning through fluorocarbons, for drinking-water fluoridation and for the manufacture of fluoridated dental preparations. In excessive amounts, fluoride can lead to fluorosis. More: Inorganic fluorides are the most important for environmental exposure through water and food. Based on quantities released and concentrations present naturally in the environment as well as the effects on living organisms, the most relevant inorganic fluorides are hydrogen fluoride (HF), calcium fluoride (CaF2), sodium fluoride (NaF), sulfur hexafluoride (SF6) and silicofluorides.