Boiling water

Microorganisms are made of proteins and other delicate structures that are permanently destroyed at high temperatures. A rolling boil reaches a temperature at which bacteria, viruses and protozoa – including Cryptosporidium, which is very resistant to chlorine – are inactivated within a short time.

The boiling point depends on air pressure. The higher you are, the lower the air pressure and the lower the temperature at which water boils. The World Health Organization (WHO) regards reaching a rolling boil as sufficient. As a safety margin, the CDC recommends one minute at a rolling boil and three minutes above about 2,000 metres.

Boiling also works with moderately cloudy water, because the heat penetrates all of the water and is not shielded by dirt particles the way light or chlorine is. You should still clarify very cloudy water first.

You recognise a rolling boil by the water bubbling vigorously and continuously, with the bubbles not disappearing even when you stir briefly. A few bubbles on the bottom of the pot or a quiet hum are not enough. From this moment the boiling time counts.

Afterwards, let the water cool in the covered pot. Do not add ice or raw water to cool it – that would contaminate it again immediately. Pour the cooled water into a clean, closable container and scoop with clean vessels, not with your hands.

Boiled water often tastes flat because dissolved air has escaped. If you pour it back and forth several times between two clean vessels, or let it stand covered for a few hours, it tastes fresher.

Never operate stoves or fires inside a tent, vehicle, shelter or indoors without adequate ventilation. Carbon monoxide can form – a gas you can neither see nor smell and which can kill. Headache, nausea or dizziness while cooking are alarm signs: get into fresh air immediately and, if symptoms persist, call the local emergency number (112 in the EU). With open fires, check local rules and fire bans.

Scalds are among the most common accidents when boiling water. Set the pot down stably, keep children away, hold handles with a cloth and do not pour hot water into plastic containers that are not made for it.

Boiling removes neither chemicals nor salt. Because water evaporates during boiling, dissolved substances such as nitrate or heavy metals can even become more concentrated. If fuel, industrial or flood contamination is suspected, only another source helps.

Boiling costs fuel and time, especially for large volumes over several days. A lid on the pot and a windscreen save a noticeable amount of fuel. Plan consumption for the whole period, not just for one meal.

If you are cooking anyway, you can combine boiling and cooking: water for tea or soup is automatically disinfected after a rolling boil. In winter, boiling can be combined with melting snow.

At home, during an official boil-water advisory: follow the authorities' instructions exactly, including for brushing teeth, ice cubes and washing fruit and vegetables if they say so. Boiling longer than recommended adds no extra safety against pathogens but costs fuel.

Boiling is regarded as one of the most effective disinfection methods, because it works against all three groups of pathogens – bacteria, viruses and protozoa – and because the result is easy to recognise: either it is at a rolling boil or it is not.

Its big weakness is that it leaves no residual protection. Boiled water that comes into contact with dirty hands, vessels or raw water is immediately contaminated again. Clean, covered containers are therefore an integral part of the method.

Boiling is one building block, not a cure-all: excellent against pathogens, useless against chemicals and salt. Seawater needs distillation or a desalination device.