One of the halogen series of elements. In the form of fluoride, it is a constituent of bone and teeth. Supplementing the daily intake of fluorine lessens the incidence of dental caries (see TEETH, DISORDERS OF). In America and in Britain, evidence indicates that people who, throughout their lives, have drunk water with a natural fluorine content of 1 part per million have fewer dental caries than those whose drinking water is fluorine-free. Provided the concentration of fluorine in water does not exceed 1 part per million, there are no toxic effects. Several water companies in the UK have added fluoride to the public water supply, but opponents of this policy, who claim that fluoride has serious side-effects, have prevented fluoridation being introduced nationwide.