Boiling water correctly, and what boiling cannot fix

What boiling does

Heat inactivates bacteria, viruses and protozoa, including the organisms that cause diarrhoeal disease. The World Health Organization advises bringing water to a rolling boil as a household treatment: the water must visibly bubble vigorously throughout, not merely steam.

How to do it

1. Let cloudy water settle first, then pour off the clear water or strain it through a clean cloth. Heat works on the water, not on the sediment carrying the organisms.
2. Bring the water to a rolling boil.
3. Let it cool on its own. Do not add ice or cold tap water to speed it up, since that can reintroduce the contamination you just removed.
4. Store the cooled water in a clean, covered container with a narrow opening or a tap.

Boiled water tastes flat because the dissolved air is driven off. Pouring it between two clean containers restores the taste. That is a taste issue, not a safety issue.

What boiling does not do

Boiling does not remove chemical contamination. Lead, arsenic, nitrates, pesticides and industrial solvents are not destroyed by heat. Because boiling evaporates water while the chemical stays behind, prolonged boiling can slightly concentrate these contaminants rather than reduce them.

Boiling also does nothing about salt. Boiled seawater or brackish water is still not drinkable.

If you have reason to suspect chemical contamination, for example a fuel smell, a chemical spill upstream, or an advisory from your water utility, do not rely on boiling. Use a different source and report the problem.

When to boil

Boil when a water utility issues a boil water advisory, when supply has been interrupted and pressure has dropped, after flooding, or whenever the microbiological safety of a source is unknown and no safer source is available.