Treating water in the backcountry: boiling, filters, chemicals and UV compared

CDC says drinking water straight from a clear mountain stream is generally not safe. NPS agrees: never drink untreated water from a natural source, even if it looks clean. The question is which treatment to use, and the honest answer is that they don't all do the same thing.

What each method does (CDC)

The best options, in order

1. Boil. CDC calls boiling the best way to kill germs: bring clear water to a rolling boil for 1 minute, or 3 minutes above 6,500 feet.
2. Filter, then disinfect. CDC's next best option. NPS puts it plainly: filtration by itself doesn't purify water; follow it with boiling or disinfection.

Cautions

• Iodine: avoid it if you're pregnant, have thyroid problems or are sensitive to iodine, and no one should drink iodine-treated water for more than a few weeks at a time (CDC).
• UV purifiers work only on small amounts of clear water, depend on batteries and need the right contact time (NPS, CDC). Cloudy water shields germs from the light.
• Never use pool-cleaning tablets to disinfect drinking water (NPS).
• No method makes water with fuel, toxic chemicals or radioactive material safe (CDC).

Collecting and protecting water

• Collect from moving water in streams, or the top few inches of a lake (NPS).
• Wash or sanitize your hands before collecting (NPS).
• Bury human waste at least 8 inches deep and at least 200 feet from lakes, rivers and other water (CDC).

For storing and disinfecting tap water at home, including bleach drop counts, see Making water safe in an emergency.

Our suggestions

• Carry a filter and a backup chemical treatment, and know your stove's fuel use for boiling.
• Pre-filter cloudy water through a bandana to make every method work better.
• Keep "dirty" and "clean" bottles clearly marked.