Have you ever accidentally let boiling water return to room temperature before you have a chance to use it? Perhaps you start the water boiling in the kettle and get distracted after it has reached a boil. If the water has completely cooled, you might be tempted to simply turn the stove back on and reboil the water. Most people don’t give reboiling water a second thought, because how could it possibly be dangerous? It turns out there are numerous negatives associated with reboiling water. This may sound counterintuitive since so many people are aware that boiling water once will make it safer to drink by removing or killing bacteria and other contaminants.
The truth of the matter is much more subtle. There are actually several chemicals in tap water that can become dangerously over-concentrated if water is boiled for too long or multiple times. Arsenic is perhaps the most dangerous of these compounds, and high levels can lead to heart disease, cancer, diabetes, and other debilitating conditions. Nitrates can also reach unsafe levels in water that has been boiled more than once. They can also be converted into dangerous nitrosamines, and strong evidence suggests that nitrosamines cause cancer.
Fluoride is another chemical additive in tap water that can reach unhealthy levels when water is boiled too much. Children consuming too much fluoride are at risk for developing cognitive problems and chronic neurological conditions. When combined, each of the elements within water that could cause negative health effects are even more dangerous to the human body.