In an emergency, boiling is the best way to purify water that is unsafe because of the presence of protozoan parasites or bacteria.

If the water is cloudy, it should be filtered before boiling. Filters designed for use when camping, coffee filters, towels (paper or cotton), cheesecloth, or a cotton plug in a funnel are effective ways to filter cloudy water.

Place the water in a clean container and bring it to a full boil and continue boiling for at least 3 minutes (covering the container will help reduce evaporation). If you are more than 5,000 feet above sea level, you must increase the boiling time to at least 5 minutes (plus approx. one minute for every additional 1,000 feet) Boiled water should be kept covered while cooling.


  • Pathogens that might be lurking in your water will be killed if the water is boiled long enough.
  • Boiling will also drive out some of the Volatile Organic Compounds (VOCs) that might also be in the water. This method works well to make water that is contaminated with living organisms safe to drink, however because of the inconvenience, boiling is not routinely used to purify drinking water except in emergencies.


  • Boiling should not be used when toxic metals, chemicals (lead, mercury, asbestos, pesticides, solvents, etc.), or nitrates have contaminated the water.
  • Boiling may concentrate any harmful contaminants that do not vaporize as the relatively
    pure water vapor boils off.
  • Energy is needed to boil the water