You have probably heard from friends or even your parents that you shouldn't boil the same water in a kettle multiple times. They say you should always pour out the water, refill it with fresh water, and only then bring it to a boil and drink it. But what is the reasoning behind this? Is it a tradition, an urban legend, or simply superstition? When someone explains why you shouldn't boil water twice, they often mention the formation of heavy water. But is it really true that repeatedly boiling the same water leads to the creation of so-called heavy water? Does this happen if you boil the water ten times or even a hundred times?
First, we should clarify what exactly heavy water is, as many people fear it. Heavy water is water that contains deuterium—a heavy isotope of hydrogen. It consists of one oxygen atom and two deuterium atoms, which is where the name "heavy water" comes from. Heavy water occurs naturally and has a concentration of about 0.015%. This means that it is present in both drinking water and our bodies since we are made up of about 80% water. Therefore, the natural content of heavy water in our bodies does not harm us.
So how is heavy water formed? The most well-known method is through repeated electrolysis—a physical-chemical process for separating dissolved substances at electrodes. This shows that it is not repeated boiling that causes the formation of heavy water but rather electrolysis—which is definitely not the same thing. However, the boiling process does lead to an increase in the concentration of heavy water in regular water! How much does the concentration increase with each boiling?
Scientists have found that to achieve a tenfold increase in concentration to 0.15%, you would need to put a total of 2.1 x 10^30 tons of water into the kettle. First: How long would it take to bring that much water to a boil? A lifetime certainly wouldn't be enough for that. Second: There simply isn't that much water on our planet.
And should we really be afraid of this heavy water? In fact, it is not as toxic as regular table salt. A person can drink up to three glasses of heavy water without any health issues; it will be excreted within a few days.
Thus, the myth is debunked! Repeatedly boiling water does not lead to the formation of heavy water, and even heavy water is by no means dangerous for the human body. So if you're worried that constant high temperatures in a water dispenser create heavy water, you can set your concerns aside and safely drink from kettles and dispensers!