Tempting as it is to dip your bottle into a clear alpine stream, risking a bout with giardia is not worth it. Err on the side of caution and treat your drinking water to prevent illness not only from giardia, but also from cryptosporidium and other nasties. You can treat it four ways:
>> Boiling Since this kills all pathogens, the water used for cooking meals and hot drinks is safe. You might have heard it takes five to 10 minutes of boiling, but this wastes fuel; once water reaches a roiling boil (even at high altitude), all microorganisms are destroyed.
>> Water filters These have improved in recent years, making them faster, smaller, lighter, and less finicky. For large groups, gravity-feed filters (versus pump) work great since physics does the work for you, but they take longer. Also, some filters are ineffective against viruses, so don’t rely on them in developing countries, and since freezing can crack them, they’re not suited for winter. We like MSR HyperFlow ($100, msrgear.com) and, for groups, Platypus GravityWorks ($100, cascadedesigns.com)
>> Chemical treatment While once very popular, iodine is no longer recommended because of its inefficacy against crypto, short shelf life once the bottle is open (three months), and a pretty nasty aftertaste. A better method is chlorine dioxide tablets or drops. They impart little aftertaste and kill crypto. (Although the name sounds similar, it is chemically very different from the chlorine in bleach—not recommended.) It takes about 30 minutes (four hours to kill crypto). Downside: It can cost more. We like Katadyn Micropur MP1 ($10, katadyn.com)
>> Ultraviolet (UV-C) Growing fast in popularity, this method takes only two minutes to zap bugs in clear water (longer if it’s cloudy). Drawbacks: UV-C relies on batteries; it doesn’t remove silt; and it only works with a liter at a time, which may be impractical for large groups. We like Editors’ Choice-winning SteriPEN Adventurer Opti ($100, steripen.com); see backpacker.com/steripen.
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