Outdoor survivalists often teach the rough rule of three’s: humans can only survive three minutes without oxygen, three hours without proper shelter (given extreme conditions), and three days without water. Though oxygen is likely a given on land, and warm, water-resistant clothing can suffice as shelter in a short-term survival scenario, natural water is the most suspect of the bunch. Regardless of how clean or clear it appears, unseen contaminants are likely lurking beneath the surface. Which is why survival expert Ted Smith, a senior instructor at the REI Outdoor School with decades of knowledge and experience in outdoor education, recommends arming yourself first with some basic knowledge: how and where to collect water, and the difference between the two main types of water treatment, filtration and purification. We worked with Smith to gather the insight and know-how for each method, and discussed the proper gear to help you stay hydrated when it’s needed most.

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via Guide to Water Purification and Filtration – Gear Patrol.