The pH of the mouth varies depending on the person. A healthy pH is above 7, and damage to teeth can occur if the pH is below 6.5.
As the pH in the mouth drops, the acidity causes more damage to the teeth. The surfaces of the teeth begin to be affected at a pH of about 6.5, but major enamel decay begins when the pH drops below 5.5.
By contrast, an alkaline environment in the mouth can help remineralize teeth, which repairs tooth decay. Human saliva is naturally alkaline, and provides some defense against tooth decay caused by acidic foods.