Is it dangerous to boil water more than once?

A common myth suggests that repeatedly boiling water makes it dangerous by concentrating harmful substances. But is this true?

In most developed countries, tap water is subject to strict regulations and contains small amounts of dissolved minerals like calcium, magnesium, and fluoride. It may also contain traces of chlorine and low levels of natural contaminants such as nitrates and heavy metals.

Boiling water kills bacteria, viruses, and parasites. Chemically, it causes volatile substances and dissolved gases to evaporate, while metals and heavier compounds remain.

The theory that reboiled water is toxic is flawed. Each time you boil water, some of it evaporates, increasing the concentration of dissolved components. However, this change is negligible. To significantly increase the concentration of dissolved substances, you would need to repeatedly boil and evaporate most of the water.

The idea that reboiled water becomes dangerous is not supported by scientific studies. In 2015, an article in Scientific American noted that while boiling can slightly concentrate minerals, "the differences are minimal and not a cause for concern."

While safe, reboiling can affect the taste of tea or coffee due to the loss of dissolved oxygen. Fresh, cold water contains higher levels of dissolved oxygen, considered essential for better taste.

While continuous boiling is safe in areas with clean tap water, in areas with contaminated water, boiling can concentrate dangerous chemicals. In such cases, use a certified water filter or bottled water.