The idea of twice boiled being dangerous sounds ludicrous. We are told that boiling dirty water will kill off all the ‘bugs’, making it safer to drink. So why, then, would boiling it twice be such a misdemeanour? The bacteria are already dead! But as silly as it sounds, there is some sense behind the saying. (But only a little.)
The first thing to know is that the water that you put in your kettle isn’t really pure. Dissolved within tap water are lots of things, including dissolved gases (like oxygen and carbon dioxide) and minerals (like calcium and magnesium). Boiling the water makes the gases bubble off – and the longer you boil it, the more that will escape. But unless you’re going to put the water in a fish tank, this isn’t going to have any meaningful difference (fish use the oxygen in the water to breathe).
Everything that is dissolved that doesn’t easily evaporate when heated (called non-volatile substances) will stay in the water no matter how long you boil it for. These substances will get more concentrated the longer you boil the water – this is exactly what happens when you boil a gravy to ‘reduce it’ – the water evaporates, leaving a saltier, stronger-tasting liquid behind.
Most of these dissolved minerals are not harmful (calcium, for example) so it doesn’t matter if they are a little more concentrated. Occasionally though, there may be some harmful chemicals in the water, like barium or uranium. You need to check your local water authority for more details, but it is theoretically possible that boiling water for a long time – i.e. until the kettle is nearly dry – could raise the concentration significantly.
But that said, the levels of contaminants in drinking water is generally so low, and the amount of time we boil water for is so short (electric kettles turn themselves off pretty quickly), that boiling water twice isn’t going to cause any health problems. One thing that is worth noting is that if you do use an electric kettle then it is probably worthwhile using one that has an enclosed heating element at the bottom. Using an old-style kettle with an exposed nickel heating has been linked to nickel allergies.
So yes, boiling water is important for killing any bacteria – something you should do before giving tap water to a baby. Boiling it twice is overkill but you needn’t fret if you do.
Oh, and don’t worry about the limescale scum at the bottom of the kettle – it’s not harmful. Make sure you fish it out of your tea though, because it tastes awful.
Answer by Dr Stu
Question from Zoe via website
Image source: Vélocia, on Flickr