In some areas of the country, water sources contain natural Fluorides. Where the concentrations fall within a certain range, the incidence of dental caries has been found to be below the rate in areas without natural fluorides. It has been established that the presence of about 1 mg/l of fluoride in a water supply will help to prevent tooth decay in children. The effect is the same whether the fluoride occurs naturally or is added to the water during treatment. The optimal fluoride level for a given area depends on air temperature, since that is what primarily influences the amount of water people drink. Optimal concentrations from 0.7-1.2 mg/l are recommended. Excessive fluorides in drinking water supplies may produce fluorosis (mottling) of teeth, which increases as the optimum fluoride level is exceeded. Higher levels adversely affect bone structure.

Excess fluoride can be removed by ion exchange using bone char or activated alumina, a relatively complex process requiring trained operators. Reverse osmosis is a simpler alternative that may be appropriate for smaller systems having access to waters of high fluoride content.