Usually when you have some water left over after boiling, you tend to leave it and then don’t hesitate to reuse it later.
What’s the harm right? It’s already been boiled once, seems there shouldn’t be an issue with reboiling. As it turns out the exact opposite is true. It’s actually a serious health hazard to reboil. When you boil water, you change the chemical makeup of it. And this is good, as you get rid of germs. But, when reboiling occurs, the chemically changed dissolved gasses and minerals actually now become toxic! This can lead to arsenic, nitrates and fluoride.
Reboiling can cause even the good minerals like calcium salts to turn harmful. Kidney stones and gallstones can develop. Nitrates will turn into nitrosamines when you reboil and arsenic becomes more profound in reboiled water, turning them carcinogenic leading to cancer and heart disease. Children may suffer from developmental disorders and neurological problems.