Tooth decay starts when the pH of the mouth is lower than ______.

5.5

The correct answer is option (1) 5.5.

Tooth decay starts when the pH of the mouth is lower than 5.5.

When the pH of the mouth drops below 5.5, it becomes acidic enough to demineralize the enamel of the teeth, leading to the beginning of tooth decay. This acidic environment encourages the growth of bacteria and the breakdown of tooth enamel, eventually resulting in cavities and other dental problems. Therefore, maintaining a pH level above 5.5 is crucial for preventing tooth decay and maintaining oral health.