This is the main mechanism by which fluoride exerts its anticaries benefits. It has been established that hydroxyapatite starts to dissolve when pH drops below 5.5, and fluorapatite starts to dissolve when the pH drops below 4.5. If biofilm pH is lower than 5.5 but higher than about 4.5 and fluoride is available in low concentrations, fluorapatite forms on the surface layers of enamel even if hydroxyapatite dissolves in the subsurface enamel. The overall effect is reduced dental demineralization, thanks to the protective outer layer of fluorapatite. When oral pH normalizes after an acid attack and rises again above 5.5, fluoride enhances enamel–dentin remineralization. If fluoride is no longer available, the oral environment begins to favor demineralization if the pH falls below 5.5.1
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