The short answer: to purify water in an emergency, bring clear water to a rolling boil for 1 minute, or 3 minutes at high elevation, then let it cool. If you can’t boil, add 8 drops of plain unscented bleach per gallon, stir and wait 30 minutes. The CDC calls boiling the best way to kill germs. Neither method removes chemicals, fuel or radioactive material.
Bottled water you stored ahead of time is still your first choice. Our home emergency kit list covers how much to keep. This guide is for when that runs out, or when officials say your tap water isn’t safe.
How long do you boil water to make it safe to drink?
Boil water at a rolling boil for 1 minute to make it safe to drink. The CDC says to boil for 3 minutes at elevations above 6,500 feet. The EPA uses a lower line, 5,000 feet, so if you live between the two, boil for 3 minutes to be safe.
- If the water is cloudy, clear it first (see the steps below).
- Bring it to a full rolling boil.
- Keep it boiling for 1 minute, or 3 minutes at high elevation.
- Let it cool on its own, then store it in clean containers with tight lids.
The CDC says boiling kills all types of germs: viruses, bacteria and parasites such as Giardia and Cryptosporidium. Boiled water can taste flat. The CDC suggests pouring it back and forth between two clean containers and letting it stand a few hours, or adding a pinch of salt per quart or liter.
How much bleach do you need to purify water?
You need 8 drops of regular unscented household bleach per gallon of clear water, then 30 minutes of waiting. Use plain chlorine bleach only, the EPA says, never scented, color-safe or bleach with added cleaners. Check the label: the CDC says US bleach is usually 5% to 9% sodium hypochlorite.
| Water | CDC: 5% to 9% bleach | EPA: 6% bleach | EPA: 8.25% bleach |
|---|---|---|---|
| 1 quart or liter | 2 drops (0.1 mL) | 2 drops | 2 drops |
| 1 gallon | 8 drops (1/2 mL, a little less than 1/8 tsp) | 8 drops | 6 drops |
| 2 gallons | 16 drops (1/4 tsp) | 12 drops (1/8 tsp) | |
| 4 gallons | 1/3 tsp | 1/4 tsp | |
| 5 gallons | 40 drops (2 1/2 mL, 1/2 tsp) | ||
| 8 gallons | 2/3 tsp | 1/2 tsp |
Source: CDC, How to Make Water Safe in an Emergency, and EPA, Emergency Disinfection of Drinking Water, as published. Blank cells are amounts that source doesn’t list. Double the dose if the water is cloudy, colored or very cold. If your bleach label has its own drinking water instructions, follow those.
Stir well and let it stand at least 30 minutes. The EPA says the water should then smell slightly of chlorine. If it doesn’t, add the same dose again and wait another 15 minutes. Use bleach that has been stored at room temperature for less than a year, and measure with a clean dropper.
How do you make cloudy water safe to drink?
Clear cloudy water first, then boil or disinfect it. The CDC says to filter it through a clean cloth, paper towel or coffee filter, or let it settle and draw off the clear water on top. This step comes first because the EPA says disinfection doesn’t work as well in cloudy water, and the CDC says small particles can shield germs from UV light and sunlight.
Even after settling, the bleach dose doubles for water that is cloudy, colored or very cold. The EPA also says not to use water with floating material, a dark color or an odd smell, and that flowing water is generally better than still water if you have to collect it outdoors.
Look inside your house before you look outside. The EPA says ice cubes, your water heater tank and the water left in your pipes can all be sources. The CDC warns never to drink water from radiators or boilers that are part of your heating system. In a power cut, our guide to power outage food safety covers the fridge and freezer.
Does a water filter make water safe to drink?
A portable water filter alone doesn’t make water fully safe, because most remove parasites but not viruses or all bacteria, the CDC says. The safest plan with a filter is to filter first, then disinfect. What each method handles:
| Method | Bacteria | Viruses | Parasites (Giardia, Crypto) |
|---|---|---|---|
| Boiling, 1 minute | Kills | Kills | Kills |
| Chlorine bleach | Kills | Kills | Doesn’t work well |
| Iodine or chlorine tablets | Kills | Kills | Not Cryptosporidium |
| Chlorine dioxide tablets | Kills | Kills | Kills Giardia, somewhat effective on Crypto |
| Filter, 1 micron or NSF 53/58 cyst rated | Not all | No | Removes |
| Filter, 0.3 micron or smaller | Removes | No | Removes |
| Reverse osmosis | Removes | Removes | Removes |
| UV light (clear water only) | Kills | Kills | Kills |
Source: CDC, Water Treatment Options When Hiking, Camping, or Traveling, and How to Make Water Safe in an Emergency. Check your filter’s label for its absolute pore size or NSF rating, and follow the maker’s instructions.
Can you purify water with sunlight or iodine?
Yes, both work when better options aren’t available. For sunlight, the CDC says to fill clean, clear plastic bottles with clear water and lay them on their sides in the sun for 6 hours, or 2 days if it’s cloudy. A dark surface underneath helps. The CDC says this reduces germs, so treat it as a last resort.
For iodine, the EPA says to add 5 drops of 2% tincture of iodine per quart or liter, or 10 drops if the water is cloudy, then wait 30 minutes. The CDC says to avoid iodine-treated water if you are pregnant, have thyroid problems or are sensitive to iodine, and that no one should drink it for more than a few weeks.
Does boiling water remove chemicals?
No. Boiling and disinfecting kill germs, but they don’t remove chemicals. The CDC says you can’t make water containing fuel, toxic chemicals or radioactive materials safe by boiling or disinfecting it. The EPA adds heavy metals and salts to that list.
If you know or suspect your water has fuel or chemicals in it, the CDC says to use bottled water or another source and to call your health department. The same goes for radioactive material, which is why our guide on what to do in a nuclear explosion starts with sealed bottled water.
What does a boil water advisory mean?
A boil water advisory means your water supplier or health officials think germs may be in your tap water, so you should use bottled water or boil tap water before you drink it or cook with it. The CDC lists three kinds of advisory:
- Boil water: boil tap water for 1 minute (3 above 6,500 feet), even if it went through a home filter or filter pitcher. Use boiled or bottled water to brush your teeth.
- Do not drink: the problem may be chemicals or toxins. Boiling won’t help, so use bottled water for drinking and cooking.
- Do not use: rare, and any contact with the water could be dangerous. Use bottled water for everything.
During any advisory, the CDC says not to use water or ice from your fridge dispenser. You can usually shower, but don’t swallow any water, and give babies a sponge bath. Wash dishes in a dishwasher with a sanitizing cycle, or soak them for at least a minute in 1 teaspoon of unscented bleach per gallon of warm water.
If tap water goes off during a bigger emergency, the Nuclear Emergency Plan gives your household a printed plan for water, food and shelter.
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Quick questions
How long do you boil water to purify it?
Boil water at a rolling boil for 1 minute to kill germs, the CDC says. Boil it for 3 minutes at elevations above 6,500 feet. The EPA sets that line at 5,000 feet, so use 3 minutes if you live in between. Let the water cool naturally before you drink it.
How much bleach do you put in a gallon of water to drink?
Add 8 drops of regular unscented bleach to 1 gallon of clear water, according to the CDC for bleach with 5% to 9% sodium hypochlorite. The EPA says 6 drops is enough for 8.25% bleach. Double the amount for cloudy, colored or very cold water, stir, and wait 30 minutes before drinking.
Does boiling water purify it from chemicals?
No. Boiling kills germs, but it doesn’t remove chemicals. The CDC says you can’t make water containing fuel, toxic chemicals or radioactive materials safe by boiling or disinfecting it. The EPA adds heavy metals and salts. Use bottled water or another safe source, and ask your health department for advice.
Does a water filter remove E. coli?
Only some filters remove bacteria like E. coli. The CDC says a filter needs an absolute pore size of 0.3 micron or smaller to remove bacteria, and most portable filters remove parasites but not viruses or all bacteria. After filtering, add a disinfectant such as chlorine bleach to kill any viruses and bacteria that got through.
Can you shower during a boil water advisory?
Yes, you can usually shower during a boil water advisory, but be careful not to swallow any water, the CDC says. Give babies and young children a sponge bath to cut the chance they swallow water. Brush your teeth with boiled or bottled water, and follow any extra advice from your local officials.
Can you purify water with sunlight?
Sunlight can reduce germs in water when you have no better option, the CDC says. Fill clean, clear plastic bottles with clear water and lay them on their sides in direct sun for 6 hours, or 2 days if it’s cloudy. It doesn’t work well in cloudy water, so filter or settle it first.
This article is general information, not medical advice. Always follow instructions from your local water utility and health officials. If you are pregnant, have a thyroid condition or are caring for a baby, ask your doctor which water treatment suits you.