Why boiling disinfects water

Waterborne pathogens fall into three big groups: bacteria, viruses and protozoa (single-celled organisms such as Giardia). Different as they are, they all consist of proteins, membranes made of fat-like substances, and genetic material. These building blocks only work within a certain temperature range.

When it gets too hot, the proteins unfold – experts call this denaturation. A protein is like a carefully folded string; its shape determines what it does. If it loses its shape, it no longer works. At the same time, membranes become leaky. The pathogen is then “inactivated”: it can no longer multiply or cause disease.

What counts is two things together: temperature and time. The hotter it is, the faster it happens. Pasteurising milk uses lower temperatures and a set time instead. A rolling boil is so far above what most waterborne pathogens can survive that it is regarded as very reliable.

“Boiling” means a rolling boil: big bubbles rise continuously from the whole body of water. The first small bubbles at the bottom of the pot or a gentle humming are not yet boiling. How long the water should boil is set by health authorities – the practical instructions are in the entry “Boiling water”.

At sea level water boils at 100 °C. As altitude increases, air pressure falls and water boils at a lower temperature. It bubbles just the same but is less hot. Authorities such as the CDC therefore recommend a longer boiling time at high altitude.

After boiling, cleanliness counts: boiled water can be recontaminated immediately if it is poured into a dirty container or scooped with unwashed hands. Letting cloudy water settle first or pouring it through a cloth makes it more pleasant but never replaces boiling.

Boiling is the right choice when germs are the concern: water from streams and lakes, tap water during an official boil-water advisory, or water after flooding, as long as no chemicals are involved. It works against all three groups of pathogens and does not depend on how cloudy the water is, as long as it boils properly.

Boiling does not help against chemicals, heavy metals, fuel or salt. Some substances even become more concentrated because water evaporates and the substance stays behind in the pot. Seawater does not become drinkable by boiling. If chemical contamination is suspected – an oily film, a chemical smell, water downstream of industry or mining – another water source is needed.

If an authority issues a boil-water advisory, its instructions apply – they know what is in the water and how long the advisory lasts. Never run stoves or fires in tents, vehicles or enclosed spaces without good ventilation: burning can produce carbon monoxide, a gas you can neither smell nor see.

Boiling water causes severe scalds. Place the pot securely and stably, keep children away and let the water cool in peace. Anyone who develops severe diarrhoea, vomiting or fever after drinking water, especially children or older people, should get medical help or call the local emergency number (112 in the EU).

Boiling is not the same as sterilising. Some bacteria can form tough resting forms called spores that survive boiling temperature. For drinking water, a rolling boil following official guidance is nevertheless regarded as sufficient. Preserving food by canning, on the other hand, has its own, stricter rules.

A common misconception is that very hot but not boiling water is “almost as good”. Without a thermometer and a clock, however, you do not know which temperature was reached for how long. Bubbling, by contrast, is a sign anyone can recognise without a measuring device – which is why authorities recommend exactly that sign.