Photo illustration by Natalie Matthews-Ramo/Slate. Photos by Thinkstock.

Actually, Backpackers, You Don’t Need to Filter Your Stream Water

The outdoor community (and industry) has made filtration a must. But a look at the scientific evidence shows that this mandate rests on a shaky foundation.

Even in an industry known for tone-deaf excess, Silicon Valley’s “raw water” trend resonated as especially contemptible—selling unfiltered spring water as a luxury good while other American citizens continue to lack access to fresh water following a natural disaster certainly embodies the worst of American capitalism. But what really seemed to set people off was not simply the bad optics. It was the idea that it was also stupid. As Twitter’s hikers and campers were quick to smugly point out, unfiltered water can be dangerous to human health. That’s why treating backcountry water sources for contamination is a fundamental tenet of outdoor recreation education, ignored at the peril of contracting giardiasis, cryptosporidiosis, or worse. In this case, however, popular opinion is wrong: The idea that most wilderness water sources are inherently unsafe is baseless dogma, unsupported by any epidemiological evidence.

Looking at the outdoor industry, you’d be forgiven for thinking otherwise. The online retailer Backcountry.com, for instance, lists 52 purification devices from six different brands, whose rhetoric underscores the urgent necessity of their own products: LifeStraw is “committed to redefining the safe drinking water space,” while Washington state’s MSR provides equipment for “anywhere you need to water away from home.” Hydrating while in nature, it would seem, is a risky proposition best mediated by expensive modern technology. These companies are effective in linking their water filtration devices with the real need to mitigate waterborne diseases in conflict zones, developing countries, and other regions where the burden of the human population on freshwater sources is great. But in claiming the average hiker or camper needs a $99.95 microfilter pump to avoid illness and death, they far exceed the conclusions of a scant medical literature.

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To be clear, there’s no question that Giardia lamblia, Cryptosporidium parvum, and various strains of fecal coliform bacteria—the waterborne pathogens responsible for giardiasis, cryptosporidiosis, and other diseases—can infect humans, with potentially serious health consequences. Yet research to date has failed to demonstrate any significant link between wilderness water consumption and infection with these threats. A 1993 study looking at the incidence of Giardia infection and gastrointestinal illness in backcountry travelers in a high-use area of California’s Sierra Nevada found only 5.7 percent tested positive, none of whom exhibited symptoms. Broader-scale approaches have similarly failed to justify concerns: Both a survey of health departments and a meta-analysis found that while giardiasis was prevalent enough to justify concern, there was no connection between recorded cases and drinking backcountry water.

In fact, it’s unclear that dangerous protozoans and bacteria occur in very many of North America’s wilderness streams and lakes at all—and where they are present, they are usually found far below levels that should concern humans. Though studies have confirmed the presence of fecal coliform bacteria near sites with heavy human or pack animal traffic, they occurred only at a minority of sampled areas, and mostly at concentrations so low they were barely detectable. The data on Giardia and Cryptosporidium are similar: A study in the popular magazine Backpacker again only found pathogens in a minority of sampled sites, with the highest recorded concentration still so dilute that obtaining an infective dose would require consuming 7 liters of water in one sitting.

How, then, did water treatment become the norm? Because the outdoor recreation community is far whiter, wealthier, and better educated than the U.S. population at large, it’s an interesting case study in how misinformation propagates through privileged communities. Asking two friends with different recreation backgrounds about their habits suggests educational programs play a significant role. “I’ve always been taught it’s good practice to keep yourself from getting sick,” says Brooke Warren, a mountain biker, skier, and rock climber based in Colorado, who typically treats her water. Patrick Fink, a medical student and former mountaineering instructor for an industry-leading outdoors school, said concerns about liability may be at play, too. “While we conservatively taught our students to purify all water regardless of source, I found that many fellow guides opted not to purify their water, instead choosing selectively among water sources and drinking from them directly.” (Full disclosure: I’ve also been drinking unfiltered water in the backcountry without incident for a decade, which may have biased my perspective.)

Professional conservatism from outdoor educators can be likely traced back to a 1976 account of a giardiasis outbreak among campers in Utah implicating waterborne transmission. But four decades later, an evolving understanding of waterborne disease pathology implicates a different, more humiliating culprit for even this canonical incident: Lax hygiene standards while out of doors. “The attack rate, temporal clustering, lack of disease in other groups using the same area, and inability to isolate cysts from the implicated water all speak against waterborne disease,” writes Dr. Thomas Welch in a 2004 editorial for the journal Wilderness & Environmental Medicine, concluding that instead, details “point compellingly to hand-to-mouth transmission.” Put more simply: The afflicted campers failed to properly wash their hands after using the bathroom.

This unappetizing anecdote suggests that when it comes to water filtration, foundational knowledge is something closer to foundational myth. Of course, even if the current absence of evidence were widely discussed, it’s likely many outdoors enthusiasts would continue to treat their water out of an abundance of caution. As a personal choice that’s fine, perhaps even commendable. But life is triage, a constant series of negotiations between risks of varying severity. And how we talk about those risks matters. If the real danger comes from eating after a trip to the cathole, then that’s the point that should be emphasized—not an unsubstantiated view of all water in the mountains as suspect. In all likelihood, it’s not the water that’s gross. It’s you.

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