Safe drinking water is the first problem to solve on any trip that goes longer than planned. Lakes and streams can look clean and still carry viruses, bacteria and parasites. Public health guidance gives three tools: boiling, chemical disinfection and filtering. Each handles different threats, and knowing the difference is the core of backcountry water purification.
Boiling: the surest method
The CDC calls boiling the surest way to kill disease-causing organisms, including viruses, bacteria and parasites. If your water is cloudy, let it settle or strain it through cloth first so you start with clear water.
Elevation matters because water boils at a lower temperature as you climb. Above 6,500 feet, the CDC says to boil for 3 minutes instead of 1. Let it cool before drinking.
Boiling has one important limit: it does not remove chemicals. Water near mines, farms or industrial sites may need a different source entirely.
- Start with the clearest water available.
- Bring it to a rolling boil for 1 minute (3 minutes above 6,500 feet).
- Let it cool, then store it in a clean container.
Disinfecting with household bleach
When you cannot boil, the CDC's emergency guidance allows plain, unscented household bleach with 5 to 9 percent sodium hypochlorite. The CDC says this method kills most viruses and bacteria, while boiling also kills parasites.
- Add 8 drops (a little less than 1/8 teaspoon) of bleach per gallon of clear water.
- Use 16 drops (about 1/4 teaspoon) per gallon if the water is cloudy.
- Mix well and wait at least 30 minutes before using it.
What portable water filters really do
Filters are popular with hikers and paddlers, but the CDC is direct: most portable water filters do not remove bacteria or viruses. A filter labeled to remove parasites handles that threat, and filtered water may still need disinfection to be fully safe.
Read the label before you buy. The best practice for a remote trip is to carry a filter and a backup method, such as a stove for boiling or a chemical treatment.
Carry more water than you think
Hydration is one of the National Park Service's Ten Essentials: bring extra water and a way to treat more. At home or in a vehicle kit, Ready.gov recommends one gallon per person per day for several days, for drinking and sanitation.
Quick answers
How long do you boil water to purify it?
Bring clear water to a rolling boil for 1 minute, or 3 minutes above 6,500 feet, per the CDC.
Does a water filter kill viruses?
Most portable filters do not remove bacteria or viruses, according to the CDC. Check the label and pair a filter with disinfection.
How much bleach purifies a gallon of water?
8 drops of 5 to 9 percent unscented bleach per gallon of clear water, 16 for cloudy water, then wait 30 minutes.
General education, not medical advice or a substitute for training. In an emergency, call 911 or your local emergency number. Updated 2026-10-26.