The Role of Critical pH

Critical pH is the term given to the highest pH at which there is a net loss of minerals from tooth enamel. This is the pH at which saliva and plaque fluid are no longer saturated with calcium and phosphate, allowing hydroxyapatite to begin dissolving. Critical pH is generally accepted to be 5.5, but it can be a little higher or lower depending on individual factors. During the demineralization process, acid diffuses between the rods and reaches deeper areas into the enamel and dentin, where carbonated hydroxyapatite crystals are more susceptible to dissolution. The calcium and phosphate ions released from the tooth diffuse into dental plaque fluid and saliva. If the acid exposure is frequent and prolonged, increasing amounts of these minerals are lost, leading to a reduction in the size of the crystals and enlargement of enamel pores. Eventually, a carious lesion may develop. The rate at which it forms depends on the extent of fluid undersaturation in the surrounding environment and the rate at which ions diffuse into and out of the enamel.^14,19