Question

Why does tooth decay start when the pH of mouth is lower than 5.5 ?

Answer 1

When pH of mouth is lower than 5.5 tooth decay begins. The enamel of our teeth is the hardest substance in our body but gets corroded when pH of mouth falls below 5.5.Bacteria present in our mouth produce acid due to degradation of sugar and other food particles remaining in the mouth after eating thus accelerating the rate of erosion of our tooth enamel.

Enamel doesn't have any living cells, which means there is no way it can repair itself. Any damage done to the enamel on your teeth isa permanent one.