How long do you have to boil water to make it safe?

CDC guidance is to bring water to a rolling boil and hold it for one minute, extended to three minutes at elevations above about 6,500 feet where water boils at a lower temperature. That is sufficient to inactivate bacteria, viruses, and protozoan parasites including Giardia and Cryptosporidium, which are the pathogens that matter in most contaminated water. Pathogens are killed well below boiling, so reaching a full boil is a visible margin of safety rather than a precise requirement. Let the water cool naturally and store it covered in clean containers.

The critical limitation is that boiling addresses biological contamination only. It does nothing for chemical contamination including lead, nitrates, pesticides, and industrial solvents, and because boiling evaporates water it slightly concentrates whatever chemicals are present. That distinction matters during boil-water advisories, which are typically issued for microbial risk after main breaks or treatment failures, and during chemical contamination events, where the guidance is do not drink rather than boil.

Alternatives when boiling is impractical. Unscented household bleach can disinfect water at the ratios published by the EPA and CDC, requiring a 30-minute contact time, though it is less effective against Cryptosporidium. Commercial filters rated to remove protozoa and, if needed, bacteria and viruses work well, with the specification to look for being an absolute pore size of one micron or smaller for protozoa and NSF certification for the claims made. Ultraviolet devices are effective against microbes in clear water. During an advisory, remember that ice, beverages made with tap water, and washing produce all use the same supply, and that brushing teeth with unboiled water is a commonly overlooked exposure.