Boil or filter? How to treat water when camping

See how the common squeeze filters compare in our Sawyer Squeeze vs Katadyn BeFree and Grayl GeoPress vs Sawyer Squeeze pages.

The CDC Yellow Book sorts water by source:

• Wilderness water with little human or animal activity: the main concerns are Giardia and bacteria, and any single method (heat, UV, a microfilter or chemicals) is listed as effective.
• Clear water near human or animal activity, such as cattle grazing upstream or campgrounds: treat for everything, including viruses. Boil, use UV, use an ultrafilter, or filter and then disinfect.
• Cloudy water: settle or pre-filter it first, then boil, ultrafilter or disinfect.

The National Park Service is more conservative for park visitors: it says filtration by itself does not purify water and should be followed by boiling or disinfection, because viruses are too small for most filters.

1. Collect well. The NPS suggests a clean container and clean hands, moving water or the top few inches of a lake, and a spot away from campsites and grazing areas. Avoid stagnant water.
2. Let cloudy water settle and pour or strain off the clear water. Filter it even if you plan to boil.
3. Boil: bring clear water to a rolling boil for 1 minute, or 3 minutes above 6,500 feet, then let it cool before drinking.
4. Or filter, then disinfect: run it through your filter, then add a disinfectant and wait the full contact time on the label. The NPS notes contact times vary by product, from about 30 minutes to 4 hours.
5. Keep treated and untreated water apart, and wash your hands before handling food.

If you run out of tablets on a car-camping trip, the EPA’s emergency guidance for regular unscented household bleach is below. Double the dose for cloudy, colored or very cold water, stir, and wait 30 minutes; the water should smell slightly of chlorine.

• Trusting clear water. Germs that make you sick can be in water that looks clean.
• Treating a filter as virus protection. Unless a filter is specifically rated for viruses, add a disinfectant where viruses are a concern.
• Letting a hollow-fiber filter freeze. Sawyer says its 0.1-micron filters handle 32–140°F, recommends keeping the filter in a pocket by day and in your sleeping bag at night in freezing weather, and says to replace it if you suspect it froze, because there is no reliable way to tell.
• Skipping the contact time. Chemicals need the full time on the label, longer in cold or cloudy water.
• Boiling water that might have chemical contamination. The CDC says boiling and disinfecting cannot make water with chemicals, toxins or radioactive material safe; find another source.
• Using iodine for long stretches. The CDC says people who are pregnant, have thyroid problems or are sensitive to iodine should avoid iodine-treated water, and no one should drink it for more than a few weeks at a time.