Fluorine is a highly toxic gas, for which the recommended maximum allowable concentration for a daily 8 hour exposure is 0.1 ppm. It can, in fact, be detected by its smell in a concentration of 3ppm, while an atmosphere containing 50 ppm is intolerable. The inhalation of high concentration of fluorine causes asphyxia and subsequently would results in severe lung congestion. High fluorine concentrations on the skin produce thermal burns.
All inorganic flourides that have an appreciable solubility in water are toxic when ingested in quite small amounts. Less than 1 g of sodium fluoride is a fatal dose.
It is now well established that drinking waters must contain about 1 ppm to reduce dental caries, but in quantities superior to 2 ppm can be prejudicial to oral health. Usually, the fluorine is supplied by the public water distribution so its composition must be tightly observed.