To boil water which is already boiled and cooled down is very harmful for the health. This time and agin boiling of water which is already boiled initiate in change og its molecular structure and according to the specialist it is not beneficial for health. Boiling water time to time makes the amount of arsenic, nitrate and fluorides more in it which harm the body. According to the specialists when water is boiled, there is reaction occur in it and the change in the structure if salts present in water take place. After the boil water get cool down the salt get in to its own position as before. But again when the cooled water is boiled again it brings imbalance in the structure of salts due to the chemical reaction.
When water is boiled the nitrate compound in it gets separated. So when water is boiled time and again it gets change into toxic substance. The high temperature convert nitrate in to nitrosamine carcinogenic which cause cancer. Boil water again and again the arsenic get settle down and reach to the very harmful level. It generates the disease related to heart, cancers and brain. It also affects the reproductive system of person. Also more boiling of the water gives rise to the fluorides. It generated the risk of diseases related to brain.
This is why it is very harmful to boil water many times. Instead keep the boiled water in clean pot and cover it properly. You can make the boiled water luke warm but do not boil is again. And as far as possible do not boil more water at a time but little at time.