At what oral pH does enamel demineralization begin to occur?

The critical pH for enamel demineralization is approximately 5.5. Below this pH threshold, the oral environment becomes undersaturated with respect to hydroxyapatite, and enamel mineral begins to dissolve. This demineralization occurs when oral bacteria metabolize fermentable carbohydrates and produce organic acids (primarily lactic acid), dropping the local pH below 5.5 — a phenomenon described by Stephan's curve. Fluoride shifts the critical pH lower (to approximately 4.5) by forming fluorapatite, which is more acid-resistant. Saliva provides buffering capacity to neutralize acids and promote remineralization when the pH rises above the critical threshold.