Boiling Water to Make It Safe
Boiling is the most reliable way to kill germs in water and the most misunderstood. It does nothing for lead, nitrate, or chemicals, and can even make them worse. Here is the right way to use it.
Boiling water is the oldest and most dependable way to make it microbiologically safe, and in an emergency it is often the right move. It is also widely misapplied, because many people assume boiling "purifies" water in general. It does one thing extremely well, kills living organisms, and several things not at all.
What boiling does
Heat kills or inactivates bacteria, viruses, and parasites, including the chlorine-resistant ones. That is why a boil-water advisory is the standard response when a utility suspects microbial contamination. For the pathogens covered in bacteria and waterborne pathogens, boiling is the gold standard, requiring no equipment or chemicals.
What boiling cannot do
Boiling does not remove chemical contaminants, metals, or salts. It does nothing for lead, arsenic, PFAS, or pesticides. Worse, because boiling evaporates some water while leaving these contaminants behind, it actually concentrates them. The clearest example is nitrate: boiling nitrate-contaminated water makes it more dangerous, not less. So boiling is the right tool for germs and the wrong tool for chemistry.
How to boil correctly
Bring water to a rolling boil and keep it there for one full minute. At elevations above about 6,500 feet, boil for three minutes, since water boils at a lower temperature in thinner air. Let it cool on its own rather than adding ice, and store it covered in a clean container. If the water is cloudy, let it settle and filter it through a clean cloth before boiling.
When to boil
Boil when your utility issues a boil-water notice, when you are using an untreated source while camping or after a disaster, or any time you suspect microbial contamination but not chemical contamination. If the concern is a chemical or metal, boiling is not the answer; use the appropriate filter or an alternative water source instead.
After a boil notice
When a boil-water advisory is lifted, flush your plumbing by running cold taps for several minutes, replace any filter cartridges that were in use during the advisory, and discard ice made during that time. For the complete checklist, including what is safe for brushing teeth and washing dishes, see what to do during a boil-water advisory.