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Any run or hike longer than a few miles in the heat of summer, I bring with me a running vest with one of these stashed inside. I usually carry with me a liter or two of water, but this is a great backup when you need a rapid refill and the stuff coming out of the nearest water fountain looks a little suspect.

If the water source that's in your nearest proximity isn't inspiring confidence, this will filter it to 99.999% purity, clearing it of bacteria (like E.coli and salmonella), parasites, microplastics, dirt, sand and sediment.