At what pH value tooth starts to decay?

5.5

The correct answer is option (3) 5.5.

Tooth decay begins to occur when the pH value of the mouth drops below a certain threshold, causing demineralization of the tooth enamel. The critical pH value at which tooth decay begins to occur is approximately 5.5.

At a pH below 5.5, the acidic environment in the mouth starts to dissolve the minerals in the tooth enamel, leading to the formation of cavities and decay. This acidic environment is typically created by the fermentation of sugars by bacteria in the mouth, which produce acids as byproducts. Therefore, maintaining good oral hygiene, reducing sugar intake, and regularly visiting the dentist are essential for preventing tooth decay.