The Science of Purifying Backcountry Water
Up-to-date guidelines on the pros and cons of filters, ultraviolet light, chemicals, and other options
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Halfway through a 12-day canoe trip in the Yukon a few years ago, my tripmates and I ran into a problem with our drinking water. Thanks to the frigid temperatures, we’d burned through all of the batteries for our ultraviolet purifier. When we switched to our backup mechanical filter, it immediately clogged with glacial silt—and then, when we tried to force it, it broke. We were above the treeline, so finding enough fuel to boil all our water would have been somewhere between inconvenient and impossible.
What’s amazing to me, in hindsight, is how long it took us to come up with the radical alternative: just drink it. I’m a rule-follower and a risk-minimizer, and I side with Wes Siler on the importance of purifying your water when you’re in the middle of nowhere. So I was interested to read the new Wilderness Medical Society guidelines for water disinfection, which were assembled by a team led by Howard Backer of the California Emergency Medical Services Authority and published this fall in Wilderness and Environmental Medicine.
The guidelines offer a comprehensive look at the evidence for when, why, and how you should disinfect your water. Here are some of the key points.
Beware of Cows
To figure out how risky a water source is, you need to know what’s upstream. A stream that drains from a spring, glacier, or snowpack is likely to be low-risk. If there’s any form of upstream human activity, be worried—especially if it’s agriculture. Several studies have found that cows are the leading source of waterborne disease in North America, since they excrete E. coli and salmonella. It’s also a good idea, the guidelines suggest, to treat water in developing countries and during disaster situations that affect normal water treatment.
Does that mean that you’re in the clear if you can’t hear any mooing? Well, the guidelines recommend treating all water in wilderness settings, but they make a distinction on the strength of the evidence. In areas with “nearby agricultural use, animal grazing, or upstream human activity,” treating water is backed by evidence that gets a grade of 1A: “strong recommendation, high-quality evidence.” Treating water in “wilderness settings without evidence of domestic animal and little to no wildlife or human activity” is also recommended, but the evidence is only 2B: “weak recommendation, moderate-quality evidence.” Your choice.
Clarify and/or Disinfect
If you’ve got a bottle of turbid swamp water, there are several things you can do to make it look, smell, and taste better. The simplest is just letting it sit for an hour or so, allowing sand and silt to settle to the bottom. This “clarifies” the water—i.e. makes it look clearer—but it’s not sufficient to remove the microbes that can make you sick. There are various other options for clarification, including using activated carbon to suck up contaminants, or adding coagulants like lime to make smaller suspended particles clump together for easier removal. None of these approaches actually disinfects water, although for really dirty water it may be necessary to clarify it before you try disinfecting it.
Actually disinfecting water involves getting rid of three key scourges: bacteria (like E. coli), protozoa (like giardia), and viruses (like norovirus). Again, you’ve got various options:
Heat: Boil the water, and you’ll be fine. Actually, all you need is a few seconds above about 140 degrees Fahrenheit (60 degrees Celsius) to kill bacteria, protozoa, and viruses. That means even at 19,000 feet, where water boils at 178 F (81 C), boiling will do the trick. The Centers for Disease Control and Prevention recommend boiling for one minute, mainly to account for differences in what people count as “boiling.” Anything more than that is overkill.
If you’re low on fuel, you may be able to get away with lower heat for a longer time—even, in some cases, hot tap water that’s been sitting in a hot water tank for 30 minutes or more. This gets dicey to judge, but if you can’t keep your finger in it for more than five seconds, it’s probably close 140 F.
Ultraviolet: This is my technique of choice these days (I use a SteriPen), mainly for the convenience. This approach also takes care of all relevant pathogens; the caveat is that water needs to be relatively clear so the UV waves actually hit all the organisms. If it’s too cloudy, you should pre-filter it.
It’s also possible to improvise on UV disinfection by leaving water in a clear plastic bottle in the sun for at least four hours. That sounds nice in theory, but evidence that it actually works in practice is a little more mixed (and fate doesn’t often grant me four consecutive hours of sunshine on my trips anyway), so it’s more of an emergency back-up.
Filtering: My standby before I got a SteriPen was an MSR ceramic filter, which I still carry with me as a backup on trips. The relatively cheap old-school model I have filters out particles as small as 0.2 microns, which takes care of bacteria and protozoa but not all viruses, which tend to be smaller. That’s a trade-off I’m willing to deal with, since the risks I’m mostly trying to guard against on backcountry trips are things like giardia and E. coli, as opposed to, say hepatitis A and poliovirus. That said, even 0.2 micron filters tend to remove between 99 and 99.9 percent of viruses present in water, since the viruses often stick to larger particles and get filtered out.
Here’s a graph from the paper that shows (from the bottom) the typical size of bad stuff in the water and (from the top) the size of particles removed by different types of filter device.
The filter I have, with a pore size of 0.2 microns, is considered microfiltration. But there are other easily portable products on the market that now offer ultrafiltration, like LifeStraw’s gravity purifier or MSR’s Guardian hollow-fiber filter, that also block viruses. If I were buying a new filter, I’d probably opt for something like that. The best option depends in part on what you plan to be doing: the gravity filter makes sense if you’ll be spending a reasonable amount of time in camp. If I’m going to be on the move most of the day, I prefer a quicker and more compact option.
In a pinch, you can improve the quality of water by filtering it through sand, though it won’t catch everything. One emergency suggestion from the paper for the aftermath of a natural disaster or non-backcountry travel in areas without reliable water treatment: fill a 5.3-gallon (20-liter) bucket with about 4 inches (10 cm) of gravel, then a layer of cotton cloth held in place by some wire mesh, then 9 inches (23 cm) of sand. Make a hole in the bottom of the bucket to let the “clean” water out.
Chemicals: Both iodine and chlorine do the job fairly well. The one exception is certain protozoa: iodine and chlorine can handle giardia, but not cryptosporidium or cyclospora. When I first started canoeing almost 30 years ago, I used to use Polar Pure iodine crystals. It works, but iodine has fallen out of favor because no one is really sure that it won’t mess up your thyroid with prolonged use. Chlorine tabs also work, without that hypothetical downside.
Both these options give you water that some people find tastes and smells a little off. That said, I used chlorine tabs on a three-week trekking trip in Nepal because the logistics of filtering would have been more complicated, given that we were often getting water from a tap rather than a river or lake. (That was before I got the UV purifier.)
It’s Not Just the Water
On that trip in the Yukon, after six days of drinking straight from the river, none of us got sick. There’s definitely an argument to be made that if you’re deep enough in the backcountry, and high enough that you’re close to the source of whatever water you plan to drink, you can skip the whole water purification thing. As Ethan Linck argued in Slate last year, the real culprit for backcountry illness is often bad (or nonexistent) hand-washing after crapping in the woods.
Personally, even if I head back to that same uber-remote Yukon river, I’ll plan to bring a couple of water disinfection options—probably UV plus a ceramic filter, since that’s what I’m used to. On trips closer to home, I’m not in the mountains, so it’s impossible to know how far the water I’m drinking has traveled. That makes purifying is a no-brainer. But I’m also a convert to another innovation that a friend introduced me to a decade ago: I keep a bottle of Purell tucked into the center tube of the toilet-paper roll.
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