Can you boil water twice?

The persistent claim is that reboiling concentrates nitrates, arsenic and fluoride to dangerous levels. Boiling does evaporate some water and does slightly concentrate dissolved minerals, but the amounts involved are tiny: a kettle loses a few per cent of its volume per boil, so concentrations rise by a few per cent, from levels that are already far below any safety threshold in treated drinking water. You would need to boil the same water down to a fraction of its original volume, many times over, for the change to matter at all. What does change is taste — reboiled water has less dissolved oxygen and tastes flatter, which is the real reason tea drinkers prefer fresh water.

What goes wrong

• No meaningful health risk from ordinary reboiling of treated water.
• The taste flattens noticeably as dissolved gases are driven off.