Below approximately which plaque-fluid pH does the oral environment become undersaturated with respect to enamel hydroxyapatite, favouring demineralization?

Explanations

• The critical pH for hydroxyapatite enamel is about 5.5. About 4.5 is the corresponding value for fluorapatite, which is why fluoride exposure makes enamel more acid resistant.

  Source: Fejerskov, Nyvad and Kidd, Dental Caries: The Disease and its Clinical Management — oneguy