How do you make water safe to drink in an emergency?

Short answer. Bring clear water to a rolling boil for 1 minute ²⁵. If you cannot boil, add 2 drops of unscented household bleach per litre (about 5-9% sodium hypochlorite), stir, and wait at least 30 minutes ¹²⁴. Boiling and bleach kill germs only: they do not make water with fuel, toxic chemicals or radioactive material safe ².

What is the safest way to make water drinkable?

Boiling. The World Health Organization states that heating water to a rolling boil is sufficient to inactivate pathogenic bacteria, viruses and protozoa, and that it works even in turbid water and at high altitude ⁶. The CDC calls for a rolling boil of 1 minute ², as does Health Canada ⁵. Health Canada defines a rolling boil as "a vigorous boil, where bubbles appear at the centre and do not disappear when the water is stirred" ⁵.

The WHO summarizes the science behind boiling: bacteria are killed rapidly above 65 °C, viruses are inactivated between 60 °C and 65 °C but more slowly, and Cryptosporidium oocysts are inactivated in under a minute once the water exceeds 70 °C ⁶. A full boil leaves a wide safety margin.

After boiling, let the water cool naturally, without adding ice, and store it in clean, covered containers ¹⁶. Boiling has two limits: it uses fuel, and it gives no residual protection, so boiled water can be recontaminated if handled carelessly ⁶.

How much longer should you boil at high altitude?

Water boils at a lower temperature as altitude rises, and agencies set different thresholds:

If you live in the mountains, use the longest time your own agency gives.

How much household bleach do you add to water?

Use only regular, unscented chlorine bleach suitable for disinfection and sanitization ¹. Check the concentration on the label, because dosages depend on it.

If your bleach is only 1% sodium hypochlorite, the CDC gives 10 drops per litre and 40 drops per gallon ².

Stir, then let the water stand for at least 30 minutes before drinking ¹²⁴. The EPA adds a check: the treated water should have a slight chlorine smell; if it does not, repeat the dose and wait another 15 minutes ¹. If the taste is too strong, pour the water between clean containers and let it stand for a few hours ¹.

Double the dose if the water is cloudy, coloured or very cold ¹².

What do you do with cloudy water?

Clear it first. Let it settle and draw off the clear water, or pour it through a clean cloth, paper towel or coffee filter, then boil or disinfect it ¹². Chemical disinfection works less well in cloudy or coloured water ¹, and particles shield germs from UV light ³. The WHO advises that if cloudy water needs to be clarified, this should be done before boiling ⁶.

Do portable filters make water safe?

Only partly. A filter's protection depends on its absolute pore size ²³:

Because ordinary portable filters do not stop viruses, the CDC says to add a chemical disinfectant after filtering ². The order matters: filter first, then disinfect, because chlorine bleach and iodine do not work well against parasites ³.

Are purification tablets as good as boiling?

They work, with limits. Tablets may contain chlorine, iodine or chlorine dioxide, and products differ in strength, so follow the label exactly ¹².

• Iodine and chlorine tablets do not kill Cryptosporidium ².
• Chlorine dioxide tablets kill it if label instructions are followed correctly ²; another CDC page describes them as effective against Giardia and "somewhat effective" against Cryptosporidium ³.
• For liquid iodine, the EPA gives 5 drops of 2% tincture of iodine per litre (10 drops if cloudy), then a 30-minute wait ¹.
• Avoid iodine-treated water if you are pregnant, have thyroid problems or are sensitive to iodine, and no one should drink it for more than a few weeks at a time ².

Portable UV units can kill parasites, bacteria and viruses, but only in small amounts of clear water ³.

What none of these methods remove

This is the most important limit to know. The CDC states plainly: "You cannot make water containing fuel, toxic chemicals, or radioactive materials safe by boiling or disinfecting it" ². Use bottled water or another source and contact your health department ².

• Chemicals and metals. Boiling and disinfection do not destroy heavy metals, salts or most other chemicals ¹. Boiling can even concentrate non-volatile contaminants such as metals and nitrates as water evaporates ⁵.
• Radioactive material. "Boiling tap water will not get rid of radioactive material" ⁷. Until officials say other sources are safe, the CDC says bottled water is the only water free of contamination, and drinks in sealed containers are safe ⁷. Water already inside your home, such as a toilet tank (not the bowl) or a hot water heater, is free of radioactive material ⁷. Tap water remains safe for washing and decontamination ⁷.

If you suspect chemical or radiological contamination, do not try to treat the water yourself: rely on stored or sealed water and follow official instructions.

What official agencies recommend

• United States (EPA, CDC): Use bottled water first; otherwise boil, and use bleach if you cannot boil ¹². The EPA and CDC agree on 2 drops per litre and 8 drops per gallon for typical bleach, but set different altitude thresholds (5,000 ft vs 6,500 ft) ¹².
• Canada (Health Canada): Follow local authorities and use bottled water if tap water is not safe; boil water for drinking, preparing food and drinks, brushing teeth and making ice ⁴. If boiling is not practical, 2 drops (0.2 mL) of 5.25% bleach per litre, with a 30-minute wait ⁴. Boil 1 minute, or at least 2 minutes above 2,000 m ⁵.
• World Health Organization: A rolling boil is sufficient against bacteria, viruses and protozoa, including in turbid water and at altitude; cool without ice and protect from recontamination ⁶.
• Radiation emergencies (CDC): Boiling does nothing against radioactive material; drink bottled or sealed water until officials clear other sources ⁷.

Where agencies differ, follow the agency of the country you are in, and always follow a local boil-water advisory over general guidance ⁴.

Frequently asked questions

Can I use scented bleach?

No. The EPA specifies regular, unscented chlorine bleach suitable for disinfection and sanitization ¹. Health Canada also specifies unscented bleach ⁴.

Does boiling for longer than a minute make water safer?

Not against germs, beyond the altitude rules: one minute at a rolling boil inactivates bacteria, protozoa and viruses ⁵⁶. Longer boiling does not remove chemicals and can concentrate metals and nitrates ⁵.

Will a camping filter protect me after a nuclear accident?

The agencies cited here give no filter as a way to remove radioactive material. The CDC says boiling and disinfecting cannot make radioactive water safe and that bottled or sealed water is the only water free of contamination until officials say otherwise ²⁷.

How long should bleach-treated water sit before I drink it?

At least 30 minutes ¹²⁴. If it has no slight chlorine smell after that, the EPA says to repeat the dose and wait another 15 minutes ¹.

Next step: build your kit

Purification is a backup; stored water is the plan. Keep unscented bleach, a clean dropper, coffee filters and the treatment method your own agency recommends with your water supply. Your Phaenyx plan counts the drops of bleach for your stored water. Build your Phaenyx plan.

Sources

1. U.S. Environmental Protection Agency — Emergency Disinfection of Drinking Water — https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water (accessed 2026-10-20)
2. Centers for Disease Control and Prevention — How to Make Water Safe in an Emergency — https://www.cdc.gov/water-emergency/about/index.html (accessed 2026-10-20)
3. Centers for Disease Control and Prevention — About Water Treatment Options When Hiking, Camping, or Traveling — https://www.cdc.gov/drinking-water/prevention/water-treatment-hiking-camping-traveling.html (accessed 2026-10-20)
4. Health Canada — Food and drinking water safety in an emergency — https://www.canada.ca/en/health-canada/services/food-drinking-water-safe-emergency.html (accessed 2026-10-20)
5. Health Canada — Guidance for Issuing and Rescinding Boil Water Advisories in Canadian Drinking Water Supplies — https://www.canada.ca/en/health-canada/services/publications/healthy-living/guidance-issuing-rescinding-boil-water-advisories-canadian-drinking-water-supplies.html (accessed 2026-10-20)
6. World Health Organization — Boil water (technical brief WHO/FWC/WSH/15.02) — https://www.who.int/publications/i/item/WHO-FWC-WSH-15.02 (accessed 2026-10-20)
7. Centers for Disease Control and Prevention — Food, Drinking Water, and Medicine Safety in a Radiation Emergency — https://www.cdc.gov/radiation-emergencies/safety/food-drinking-water-medicine-safety.html (accessed 2026-10-20)