Making Water Safe to Drink

Boiling, chemicals and filters, with the right times and doses

6 minBeginnerUpdated 2026-10-23 Making Water Safe to Drink

You will learn

Boil water for the correct time at any altitude
Dose household bleach and iodine safely per WHO and CDC guidance
Know what filters remove and what they miss

⚠️ Never use scented, splashless or colour-safe bleach. Only plain sodium hypochlorite bleach is safe for water treatment.

⚠️ Iodine is not suitable for pregnant women, people with thyroid conditions, or long-term use.

⚠️ No field method removes chemical pollution or salt. If a source may be chemically contaminated, do not drink it.

What you are removing

Water can carry three kinds of living contamination: bacteria such as E. coli, protozoa such as Giardia and Cryptosporidium, and viruses such as norovirus and hepatitis A. It can also carry chemicals and heavy metals, which no simple field method removes. Every purification method works against some of these and not others, so it helps to know the target. Boiling kills all three living groups. Chemicals kill bacteria and viruses well but are weak against Cryptosporidium. Most portable filters remove bacteria and protozoa but let viruses through. That is why the safest field approach is often two steps: filter, then boil or treat chemically.

Step one: settle and pre-filter

Cloudy water hides germs inside particles and uses up chemicals before they can work. Start by letting the water stand in a container for 30 to 60 minutes so sediment sinks, then pour the clearer top layer off gently. Pass it through a clean cloth, a coffee filter or a bandana folded several times to catch grit, leaves and insects. This does not make water safe on its own, but it makes every next step more effective and makes the water taste far better. Keep your dirty and clean containers separate and never let the treated water touch the untreated one.

💡 Mark one bottle DIRTY and one CLEAN so you never mix them up.

Boiling: the gold standard

Bring the water to a full rolling boil, with large bubbles breaking the surface, and keep it there for 1 minute. Above 2,000 metres of altitude, boil for 3 minutes, because water boils at a lower temperature there and needs longer to do the same job. This is the WHO and CDC guidance and it kills bacteria, protozoa and viruses. Let the water cool with the lid on to keep it clean. Boiled water tastes flat because the air has gone; pour it between two clean containers a few times to bring the flavour back. Boiling does not remove chemical pollution, salt or heavy metals.

💡 No timer? Once you see a rolling boil, count slowly to 60.

Chemical treatment: bleach and iodine

Use plain, unscented household bleach with 5 to 9 percent sodium hypochlorite and no added cleaners. Per CDC guidance, add 2 drops per litre of clear water, stir, and wait 30 minutes before drinking. If the water is cloudy or very cold, double the dose to 4 drops per litre and wait 60 minutes. A faint chlorine smell afterwards means the dose was enough. For 2 percent tincture of iodine, add 5 drops per litre of clear water, 10 drops if cloudy, and wait 30 minutes. Iodine should not be used by pregnant women, people with thyroid problems, or for more than a few weeks. Both methods are weak against Cryptosporidium, so filter or boil when you can.

💡 Purification tablets are pre-dosed. Follow the packet exactly.

Filters and UV

A good portable filter with a pore size of 0.1 to 0.2 microns removes bacteria and protozoa, including Cryptosporidium, but it does not remove viruses. In regions where viral contamination is likely, or where water is near sewage, pair the filter with chemical treatment or boiling. Some filters labelled purifiers include an extra stage that does remove viruses. UV pen devices work on clear water only and need working batteries. Replace filter cartridges on schedule and never let a wet filter freeze, because ice cracks the fibres. Wash your hands or use sanitiser before handling the clean side of any device.

Check yourself

How long should water be kept at a rolling boil at sea level?

WHO and CDC guidance is 1 minute at a full rolling boil, rising to 3 minutes above 2,000 metres.

How many drops of unscented 5 to 9 percent bleach treat 1 litre of clear water?

The CDC dose is 2 drops per litre of clear water with a 30 minute wait, doubled to 4 drops if the water is cloudy.

Which contaminant do most portable filters fail to remove?

Viruses are far smaller than the 0.1 to 0.2 micron pores of most filters, so add boiling or chemicals where viruses are a risk.

Why boil longer at high altitude?

At altitude water boils below 100 degrees Celsius, so it needs 3 minutes to reach the same level of safety.

Try it for real: Practise the boil and bleach methods at home

Fill a pot with tap water, bring it to a rolling boil and time exactly 1 minute
Read the label on your household bleach and confirm it is unscented, 5 to 9 percent sodium hypochlorite
Measure 2 drops into 1 litre of tap water with a clean dropper, wait 30 minutes and note the faint chlorine smell
Write the doses on a label and stick it on the bleach bottle for emergencies