MODEL RELEASED. Fluorosis. Brown staining of an 11-year-old boy's tooth enamel due to excess ingestion of fluoride. Fluoride is a mineral compound that is useful in preventing dental caries. It is thought to work by strengthening the mineral composition of tooth enamel making it more resistant to acid attacks. It may also reduce the acid-producing ability of microorganisms in dental plaque. The water supply may naturally provide fluoride, or it may be added. Mild fluorosis causes white mottling of the teeth; severe cases show brown staining and usually occur in areas where the level of fluoride in the water is many times greater than the recommended level of 0.7 to 1.2 parts per million.