Let’s take a closer look at what happens when water boils. At first glance, you see it bubble and steam, but there is also something happening at the chemical level. By cooking the same water multiple times, the oxygen content changes and potentially dangerous substances accumulate. You can really taste this with tea. Tea brewed with fresh water tastes very different to water that’s been re-used.

Boiling water is a great way to kill bacteria, that’s true. But when you boil water over and over again, dangerous gasses and poisonous substances like arsenic, nitrates, and fluoride can form as a result of the changed chemical structure. Various studies have shown a connection between this fluoride and the damaging effects it has on the nervous system and brain.
Keep in mind that the minerals found in water don’t evaporate. If you were to boil water over and over again until it had all evaporated, you’d find a crust of minerals is still there. So if you’re regularly drinking concentrated amounts of these minerals — calcium, for instance — it could lead to problems such as kidney stones.

So this is the reason why you really shouldn’t ever boil the same water twice. Obviously, it depends on how clean your tap water may be to begin with. Smaller amounts for your tea isn’t really a huge problem, but the accumulated effect over a long period of time is really what might be better to avoid. If you’ve boiled too much and would rather start fresh, let it cool and water your houseplants with it instead — it isn’t dangerous for them!

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