The influence of fluoride on the solubility of bone salt.

The decrease in the incidence of dental caries which follows the addition of fluoride to municipal water supplies has been attributed to the inhibition of enzyme reactions and of bacterial growth (2-4), and to a decrease in the solubility of bone salt. Evidence that fluoride decreases the solubility of bone salt consists of many reports which show, principally, that (a) a smaller amout of enamel, or of hydroxyapatite, is dissolved in fluoride-containing solutions than in fluoride-free solutions (5, 6) ; (b) the pretreatment of bone with relatively high concentrations of fluoride decreases the amount of the bone which may be dissolved in acid over a subsequent short time interval (8, 9) ; (c) inclusion of fluoride in acid drinks decreases erosion of the teeth (10); and (d) a smaller portion of the enamel of the teeth is dissolved in acid if the teeth are from individuals who have lived in areas in which the water was treated with fluoride than if the water was not treated with fluoride (11). These studies show that the presence of fluoride decreases the amount of bone salt dissolved over a short time interval but they do not indicate whether this decrease is due to a change in the solubility equilibrium state or to the rate at which equilibrium is approached. With the possible exceptions of two (7, 11)) none of the studies has reported the effect of an amount of fluoride as small as the 1 p.p.m. added to municipal water supplies. In experiments designed to study the effect of a small amount of fluoride on the solubility equilibrium of bone salt formed from inorganic salt solutions, it was observed that, depending upon experimental conditions, fluoride appears to either increase or decrease the solubility. The increased solubility was found to be attributable to the stabilization at a relatively high solubility level of an initial precipitate formed in the presence of fluoride. When, on the other hand, fluoride was added to preformed bone salt, suspended in salt solution, an amount of fluoride as small as 0.2 p.p.m. decreased the solubility. This paper is a report of these experiments.