Water is a treasure trove of myth-making. And while the story about drinking 4 litres daily is clear (we hope!), the issue of boiling water is still a myth-busting one. We’ve tried to get to the bottom of the fears of reboiling.

First, it’s worth understanding what we’re boiling. Tap water can have different compositions, even within the same city. And in general, sanitary requirements for tap water vary from country to country. Therefore, it is only possible to state some average values for impurities in tap water. Chlorine, which is used for disinfection, calcium and magnesium salts – which form a scale on kettle walls – as well as minor amounts of heavy metals can be found in tap water.

Over boiled water becomes ‘heavy’

There is a myth about creating “heavy water” at home. Namely producing water with a high deuterium content. It really is in the water, it really is dangerous to health in large quantities, it is also true that with each repeated boiling and addition of new water the amount of deuterium in the new mixture increases. But! In order to get the really dangerous deuterium content, you have to repeat boiling and adding water to the kettle a huge number of times.

There is no way to get heavy water with the kettle

Academician Igor Petryanov-Sokolov once calculated how much water must evaporate from the kettle for the residue to noticeably increase the deuterium content. It turned out that in order to get 1 litre of water, in which the deuterium concentration equals 0.15%, i.e. only 10 times more than in nature, a total of 2.1⋅1030 tons of water should be added to the kettle, which is 300 million times the mass of the Earth.

Thus, even if you don’t pour the rest of the water out of the kettle, you won’t get deuterium poisoning. Alas, you can’t get “heavy” water with a kettle. But if you do manage to do it, be sure to patent it – this discovery will bring you a lot of money!

Too many minerals

Another common misconception is that evaporation of water in our tea will be too high concentration of minerals. However, there is some maths at work here too.

Canadian scientists from McGill University (one of the top 50 in the world) explain it with the example of fluoride. Suppose we poured 1 litre of water containing 1 ppm (one-millionth) of fluoride into a kettle and boiled it. Then we poured 200 ml to make a cup of coffee. Together with this 200 ml we will get 0.2 mg of fluoride. Now boil the remaining volume of water again, but this time wait until 100ml has boiled off. We drink another 200 ml of water and this time we get 0.22 mg of fluoride. So the difference will only be 0.02 mg, which is insignificant. That is, even assuming that we let 100 ml of water evaporate, which is not a quick and easy process if you have an electric kettle, we will get an increase in the amount of fluoride in the water at the level of error. The same logic works with other substances as well.

As for magnesium and calcium salts, of which there is usually a surplus in tap water, they are simply deposited on the kettle walls in the form of the familiar scale. It is harmless, but its thermal conductivity is much lower than that of metals, so the kettle will boil a little slower. In order not to increase the electricity or gas bills, it is better to clean the kettle periodically.

There is no scientific research that proves that boiling water repeatedly causes any harm to the human body. There is a theory that due to displacement of oxygen from water by the boiling process, the taste of prepared drinks made from re-boiled water may be different from the original. There is no scientific evidence that a person can distinguish the taste of tea made from water boiled once and several times. Therefore – do not be afraid to boil water! Once, twice, even three times.