Stored water is the first choice in an emergency, and Ready.gov recommends keeping at least one gallon per person per day for several days. If it runs out, or you're told your tap water isn't safe, you can treat water to make it safer to drink. Which method works depends on what's in the water, so start with the source. Everything here follows guidance from the CDC and the EPA. If local officials have given instructions for your area, follow those first.
Step 1: Decide whether this water can be treated at all
Boiling and disinfecting deal with germs. They don't deal with everything. The CDC says you cannot make water containing fuel, toxic chemicals or radioactive materials safe by boiling or disinfecting it. The EPA adds that treatment won't destroy heavy metals, salts and most other chemicals.
Avoid water that smells of fuel or chemicals, or that has floating material, a dark color or a questionable odor. Look for bottled water or another source, and contact your health department for advice.
Step 2: Clear cloudy water
Disinfection works less well in cloudy water. If it's murky, let it settle, then pour the clearer water through a clean cloth, paper towel or coffee filter. This only removes the visible particles, so the water still needs treating.
Step 3: Boil it if you can
Boiling is the method to use when you have the option. The CDC says it kills all types of germs, including viruses, bacteria and parasites.
- Bring clear water to a rolling boil for 1 minute.
- At higher elevations, boil for 3 minutes. The CDC sets that at above 6,500 feet and the EPA at above 5,000 feet, so if you live in between, use 3 minutes.
- Let it cool, then store it in clean, sanitized containers with tight covers.
- If it tastes flat, the EPA suggests adding a pinch of salt per quart or liter, or pouring it between clean containers a few times.
Step 4: If you can't boil, use bleach
Use only regular, unscented chlorine bleach. Avoid scented or color-safe products and bleach with added cleaners. Check the label for the concentration, then use the EPA's amounts. If yours is a different strength, follow the CDC's guidance for it.
| Amount of water | 6% bleach | 8.25% bleach |
|---|---|---|
| 1 quart or liter | 2 drops | 2 drops |
| 1 gallon | 8 drops | 6 drops |
| 2 gallons | 16 drops (1/4 tsp) | 12 drops (1/8 tsp) |
| 4 gallons | 1/3 tsp | 1/4 tsp |
| 8 gallons | 2/3 tsp | 1/2 tsp |
- Double the amount if the water is cloudy, colored or very cold.
- Stir and let it stand for 30 minutes.
- It should have a slight chlorine smell. If it doesn't, repeat the dose and wait another 15 minutes.
- If the chlorine taste is too strong, pour the water between clean containers and let it stand for a few hours.
Bleach has a limit too. The CDC notes that chlorine bleach kills bacteria and viruses but doesn't work well on some parasites, which is why it recommends filtering before you disinfect. Ready.gov also says not to use iodine or camping water treatment products for this purpose unless sodium hypochlorite is the only active ingredient, at the strength it lists.
Where water filters fit
A portable filter is useful when you have no heat source or need to deal with parasites, but it isn't a stand-alone answer. Here's what the CDC says about filters:
- Most portable water filters remove parasites, but not viruses or bacteria.
- An absolute pore size of 1 micron or smaller removes parasites, but not viruses or all bacteria.
- An absolute pore size of 0.3 micron or smaller removes bacteria and parasites, but not viruses.
- Reverse osmosis filters remove parasites, bacteria and viruses.
The CDC's advice is to follow the maker's instructions, then disinfect the filtered water to deal with viruses and any remaining bacteria. Filters also don't make fuel or chemical contamination safe.
We carry a few filters for this role. The gravity fed filter holds 1 gallon at a time, and its maker rates the membrane at 0.1 micron. The water filter straw is sip-through, while the personal filter also threads onto a 28 mm bottle. The maker rates both at 0.01 micron. Those are the makers' ratings, not independent lab results, so treat these filters as one step in the process. If you're unsure about the source, boil or disinfect the water as well. A sip-through straw leaves no chance to disinfect afterward, so it's best kept as a backup for when you have no other option.
The methods side by side
| Method | Good for | Limits |
|---|---|---|
| Boiling | Viruses, bacteria and parasites | Needs heat and fuel. Doesn't remove chemicals or metals. |
| Unscented bleach | Bacteria and viruses | Doesn't work well on some parasites, and less well in cloudy water. Doesn't remove chemicals. |
| Portable filter | Parasites, and bacteria if the pore size is 0.3 micron or smaller | Most don't remove viruses. Doesn't make fuel or chemical contamination safe. |
Under a boil water advisory
If local officials issue an advisory, the CDC says to use commercially bottled water or boil your tap water for drinking, cooking and brushing your teeth. If you feed a child formula, use ready-to-use formula if possible. It's safe to wash clothes as usual, and you can often wash your hands with tap water and soap, but follow what your local officials say. Keep going until they tell you the water is safe again.
Store treated water safely
Use clean, sanitized containers with tight covers. If you're filling your own for storage, Ready.gov recommends food-grade containers and replacing the water every six months. Plan ahead for how you'd treat more, and add a pot, bleach and a filter to your emergency supplies list. Water may also be a concern after a storm or a long power outage.
Common questions
How long do you boil water to purify it?
A rolling boil for 1 minute, or 3 minutes at high elevation.
Is filtered water the same as purified water?
Not necessarily. What a filter removes depends on its type and pore size, and according to the CDC, most portable filters don't remove viruses or bacteria.
You can see our filters and other supplies in the Emergency Supplies and Survival Gear collections.
Activearo Outdoor sells some of the gear linked in this article. You don't need it to follow this guidance, which comes from the public sources listed below. Check those and your local officials for anything specific to your area.