"Acids from plaque cause the loss of minerals from the tooth (a process called demineralization), resulting in tooth decay. The formation of small cavities, or carious lesions, can be reversed by remineralization--that is, the deposition of minerals into previously damaged areas of tooth. Topical fluoride, when applied frequently in low concentrations, increases both the rate of growth and the size of enamel crystals. The accelerated growth of enamel crystals within the demineralized lesion initiates reminerization of the tooth. Also, the larger crystals are less prone to future attack from the acids.
"Systemic fluoride--ingested fluoride that is absorbed mainly through the stomach and intestine into the bloodstream--helps to strengthen teeth while they are growing. The fluoride is carried to developing tooth buds, where the interaction with the developing crystals initiates the replacement of hydroxyapatite (the tooth enamel's normal crystalline composition) with fluorapatite (a related crystal which incorporates fluoride). Fluorapatite is more resistant to decay than is hydroxyapatite."
Mary Hayes of the American Dental Association presents some further information:
"Fluoride is a naturally occurring element that prevents tooth decay when ingested systemically or applied to teeth topically. The fluoride ion comes from the element fluorine. Fluorine, the 13th most abundant element in the earth's crust, is never encountered in its free state in nature. It exists only in combination with other elements as a fluoride compound. It is found in this form as a constituent of minerals in rocks and soil everywhere. Water passes over rock formations containing fluoride and dissolves these compounds, creating fluoride ions. The result is that small amounts of soluble fluoride ions are present in all water sources, including the oceans.
"Fluoride is present to some extent in all foods and beverages, but the concentrations vary widely. All water contains some fluoride naturally. Water fluoridation is the process of adjusting the fluoride content of water to the recommended level for optimal dental health. In the U.S., the optimum concentration for fluoride in the water has been established in the range of 0.7 to 1.2 parts per million (ppm). The specific optimum for a locality is dependent on the average annual temperature for the region.
"Researchers believe there are several mechanisms by which fluoride achieves its anticaries (cavity-preventing) effect. It reduces the solubility of enamel in acid by converting hydroxyapatite into less soluble fluorapatite; it may exert an influence directly on dental plaque, reducing the ability of plaque organisms to produce acid; and it promotes the remineralization of tooth enamel in areas that have been decalcified by acids.
"Most likely, fluoride works by a combination of these effects. But the remineralization effect of fluoride is of prime importance, because it results in a reversal of the early caries process and it gives rise to an enamel surface that is more resistant to decay.