Q: Does all wilderness water require treatment before it is consumed?

A: Not likely, say many researchers and medical experts—an assertion that may surprise a lot of backpackers. These experts insist the filter-every-drop approach to backcountry water is flawed thinking and that water in more remote locales at high elevations is much, much cleaner than commonly perceived.

Q: So is every drop of water found in wilderness areas safe to drink without any type of treatment?

A: Also not likely, say other medical authorities and land managers. Humans, pack animals, domestic pets and wildlife all have the potential to contaminate backcountry water sources. The experts in this camp generally agree that water in the wilderness is cleaner than once believed, yet argue that even if the risk of infection is small, it is still a risk—one not worth taking.

Quick Tips for Safe Drinking Water

So how should you handle water you collect in the wilderness? Here's a summary of the guidance based on the scientific/medical opinions expressed in this article:

  • Always carry some treatment method to disinfect any water source you consider even faintly suspect.
  • Be observant at water sources. If you're in a remote, high-elevation, infrequently traveled area and you see no obvious signs of human or animal activity, the water is quite probably clean and instantly drinkable.
  • Be diligent to maintain good personal hygiene when in the wild. Health complications often believed to be caused by ingesting contaminated wilderness water are usually the result of poor sanitation practices, particularly dirty hands.
  • Behave respectfully around backcountry water sources. Sustain their purity by bathing, cooking, urinating and defecating at least 200 feet away.

Know Your Waterborne Threats

It's possible that any water source on Earth could contain waterborne pathogens—microorganisms that live in water and that can cause human illness. This invisible fluvial zoo, spawned when water becomes contaminated with human or animal waste, is measured in microns. A micron equals one-millionth of a meter. (The period at the end of this sentence is roughly 500 microns wide.) The abbreviation for microns (μm) uses the Greek letter Mu.

The main waterborne disease-causing agents, from largest to smallest:


Examples: Cryptosporidium parvum, Giardia lamblia.

Size: Protozoa are single-cell parasites that range between 1 to 20 microns. Cryptosporidium (aka "crypto") typically measures between 4 to 6 μm; Giardia cysts range between 1 and 10 μm.

When symptoms appear: From 2 days to a few weeks. Individual protozoa are capable of causing infection.

Characteristics: Cryptosporidium and Giardia can both survive weeks, even months, in cold water. Crypto is a thick-shelled egg (an oocyst). In still water, the shell's weight usually causes the cyst to sink. The protective shell boosts the resistance of Cryptosporidium to disinfectants such as iodine and chlorine.

Impact on humans: Profuse, often watery diarrhea, vomiting, gas and intestinal discomfort. Most infections last 1 to 6 weeks; rare chronic cases can last up to a year. People with weakened immune systems need to be extra vigilant against contracting cryptosporidiosis.

Effective treatment methods: Boiling, mechanical filtration/purification; UV light. Chemicals and halogens are effective against Giardia, but not always Cryptosporidium.


Examples: Escherichia coli (E. coli), Salmonella, Campylobacter jejuni, Yersinia entercolitica, Leptospira interrogans and many others.

Size: 0.1 to 10 microns.

Time needed for symptoms to appear: From a few days to a few weeks. The number needed to cause infection can vary widely. With Salmonella, for example, it can take ingesting just a handful of organisms or several thousand.

Likely impact: Diarrhea and potentially prolonged intestinal discomfort. A more serious bacteria-caused disorder, cholera, is rare in the United States but fairly common in some countries. It is usually associated with dehydration. It can result in death if not quickly treated.

Effective treatment methods: Boiling, mechanical filtration/purification; UV light; chemicals/halogens.


Examples: Hepatitis A, rotavirus, enterovirus, norovirus.

Size: 0.04 to 0.1 micron.

Time needed for symptoms to appear: From 1 day to several weeks.

Likely impact: Diarrhea, intestinal discomfort. Potentially a variety of other ailments.

Effective treatment methods: Boiling, purification; UV light; chemicals/halogens (though contact time, ranging from 5 minutes to 4 hours, is required).

Pesticides, herbicides and other toxins and pollutants associated with agriculture and urban development do not normally impact water sources found deep in wilderness areas. However a growing threat to backcountry water is the increase in blue-green algae (cyanobacteria) such as Microcystis, which may secrete harmful toxins. Treat any water that shows an algae bloom or harbors significant floating algae. Avoid it if possible.

Recreational lands close to communities or farmlands may be more susceptible to such contaminants. Carbon components in filters and purifiers will remove some (but not all) organic chemicals (pesticides and herbicides) and inorganic chemicals (such as arsenic and mercury). Carbon often also removes objectionable tastes and odors.

If grazing or farming is permitted on or near the land you are exploring, your best treatment option is boiling—or simply moving on to less-suspicious water sources.

For global travelers, view the companion REI Expert Advice article on choosing a water-treatment method for international travel.

What's the right approach to take against such risks? We sought out the opinions of a number of experts and share them with you here.

Priority One: Good Hygiene

Thomas Welch wants backcountry travelers to come clean.

Dr. Thomas Welch

Dr. Welch is a professor of pediatrics at Upstate Medical University in Syracuse, N.Y. He is also a licensed professional guide, a certifying instructor (and field course instructor) for the Wilderness Education Association and a member of the Wilderness Medical Society. He publishes research on wilderness medical topics and serves as an editorial reviewer for Wilderness and Environmental Medicine, a professional journal for physicians working in the field of wilderness medicine.

As a youthful outdoor explorer decades ago, Welch says water treatment was never a concern. "I drank countless gallons of water from every imaginable source with nothing in the way of a "filter" other than my teeth," he says. "I never got sick. After doing this for most of my then-young life, people started telling me how dangerous this practice was."

A report released in the 1970s, Welch says, did much to alter the public perception of wilderness water. After more than half the people in a group of college students became ill after hiking in Utah's Uinta Mountains, the report concluded their illness was considered water-generated giardiasis. Yet Welch, who reviewed the report years after its release, says no cysts were identified in the suspect water and the report's authors "discounted food or fecal-oral spread."

"This was a classic food/hand epidemic in all respects," Welch says. "The only problem was at the time no one understood that the disease could be spread like that."

While at the University of Cincinnati, Welch conducted a 1995 survey where 48 of 50 U.S. state health departments responded to a questionnaire about giardiasis in their jurisdictions. The agencies studied 80 outbreaks of giardiasis in 1991; 19 outbreaks were linked to contaminated water, and just two were associated with campers or backpackers.

Two departments considered water-associated giardiasis to be a problem for backpackers in their jurisdiction, though the study found neither had any data to reinforce the claim. Welch says this just strengthens his convictions. "There are absolutely no well-documented reports in the peer-reviewed medical literature supporting the concept that North American wilderness waters are inherently unsafe for consumption," he says.

Poor hygiene, Welch believes, is the real issue, as he explained in a 2004 article that appeared in Wilderness and Environmental Medicine:

"If our objective is to protect the backcountry user from enteric [intestinal] infection," he wrote, "then we should emphasize the overwhelming evidence showing that assiduous hand-washing or using alcohol-based hand cleansers is by far the most important strategy. It must be impressed upon backpackers (just as it is impressed upon health care, food industry, and daycare workers) that stopping hand-to-mouth spread is the key to preventing gastrointestinal infection."

Welch stands behind those words today. "Absolutely," Welch told REI.com. "One only needs to review the recent, high-profile episodes of gastrointestinal illness spreading throughout cruise ships to see how such diseases can be spread rapidly from hand-to-mouth contact by people living in close quarters. The drinking water on cruise ships is fine. It's personal hygiene that is the issue."

Water researcher Robert Derlet reports similar findings in his study of backcountry water in California's Sierra Nevada. Read REI's question-and-answer session with Derlet.

Sometimes Water Needs Treatment

Yet Welch and Derlet both acknowledge water sources need to be evaluated individually. Derlet, who has studied high-elevation water sources in the Sierra Nevada for 10-plus years (and has routinely found them to be pathogen-free), recommends caution wherever evidence of grazing, pack-animal activity, pets or messy humans is apparent.

Robert Derlet

Derlet says he would likely treat all water he would gather along the Appalachian Trail, largely due to its high traffic load and its proximity to farmlands and grazing pastures. In the High Sierra, though, Derlet regularly drinks water straight from the source, though he carries iodine in case he encounters suspect water.

Wrote Welch in Wilderness and Environmental Medicine: "Prudence dictates careful attention to the source of drinking water in the same way that it dictates awareness of everything about the wilderness user's environment. Water obtained from areas of high human use, especially if a single source is to be used for an extended time, may merit chemical disinfection, with or without filtration."

"If I were to come up with one rule for most situations," Welch told REI.com, "it would be to treat water at or below established campsites. From what I know about the Appalachian Trail, for example, much of it consists of a series of heavily used established campsites, so I probably would treat water there. Having said that, if I were in a hurry and just wanted a quick drink, I wouldn't get too anxious about it."

Rangers and land managers recommend that wilderness visitors treat all water they drink. Beyond the obvious (usually unspoken) liability issues, they do so for other reasons as well.

When dealing with the general public, they recognize not every wilderness traveler has the same level of health. Some people have compromised immune systems and thus a lower tolerance for waterborne pathogens. The same level of contamination will affect people differently. Some can simply shrug it off. Others may become quite ill.

A More Cautious Point of View

Paul Auerbach, editor of the respected textbook Wilderness Medicine and author of Medicine for the Outdoors, advocates a safe-not-sorry approach for backcountry visitors.

Dr. Paul Auerbach

Giardia still exists in the backcountry," says Dr. Auerbach, a clinical professor of surgery (division of emergency medicine) at the Stanford (Calif.) School of Medicine. "It's probably not as prevalent as once believed, as researchers like Derlet have shown, but it's still present.

"I don't want to use the word "random" when describing its presence, and I don't want to cause any unfounded fears about it, but in my view drinking water without any disinfection is a type of dietary Russian roulette. Is there, for example, a cow bathing upstream from where you're gathering water? I've seen that happen."

Auerbach recalls caring for one backpacker who drank straight from a stream while on a trip in Montana. "He hiked another 400 yards and there was a cow in the water," he says. "He was rewarded with a classic case of giardiasis.

"With more people using the backcountry, it's my view that the natural water supply is likely not becoming more pure," Auerbach says. "It's either staying the same or becoming more contaminated. Undoubtedly many water supplies at reasonably high altitudes are uncontaminated. The risks are lower the higher you go. But how do you know which ones are uncontaminated?

"There is no easy field test, no dip stick that can detect pathogens. The only way to know is to do a microbiological study, which is not practical at every water source. Or do a human test and offer yourself up as the culture medium."

Auerbach concedes that treating water is a hassle. "None of us prefers the inconvenience of disinfecting water as opposed to drinking it straight from the source," he says. "It's work to chemically treat water, or to irradiate it, or to pump it through a filter. It's a nuisance. There's nothing fun about it, and some chemical methods mean the water won't taste as good as untreated water. But you do it to avoid becoming ill."

To learn about technologies available for disinfecting wilderness water, read our article on How to Choose a Water Treatment System.

Seeing Both Sides of the Issue

Dr. Howard Backer

In the textbook Wilderness Medicine, Howard Backer, a California-based public-health physician, is author of the 49-page chapter "Field Water Disinfection." Dr. Backer has studied both the pathogens and dozens of portable water-treatment methods created to protect humans. Would he ever drink untreated water straight from the source?

"I tend to fall somewhere in the middle of the debate," Dr. Backer told REI.com. "It's a risk issue. You assume a certain degree of risk when you drink surface water, and everyone has to assume their own degree of risk.

"Even our urban water standards allow some bacteria," he says. "It's not sterile water that we drink from the tap. There's a calculated risk with urban water. It's too expensive to reduce the number of organisms beyond a certain level, and there have been a number of illnesses attributable to tap water. So you don't need to achieve zero risk in the wilderness.

"I've talked to people who say, 'Ah; I've been hiking for years and never got sick drinking the water without treating it.' But that's not informed risk. That's like people who don't wear seat belts, ride a motorcycle without a helmet or engage in unprotected sex. They need to assess, 'What's my real risk?'"

Backer points to cases where people have become ill even after ingesting treated water, such as in swimming pools or the infamous 1993 Cryptosporidium outbreak in Milwaukee where more than 400,000 consumers of a malfunctioning municipal water system became ill. More than 100 deaths were linked to the event, mostly individuals with weakened immune systems.

"It's extremely rare for people to die from the diseases caused by these infections," Backer says. "But the illness is not pleasant. Once people are burned, they're pretty cautious about their water."

A few factors that can influence your odds of getting sick from tainted water:

  • Dosage. Dozens, sometimes hundreds or thousands, of organisms are needed to cause infection, and water quality in a stream can vary over time, depending on activity and weather in the watershed.
  • Personal health; strength of individual immune system. Medically speaking, there are "host characteristics" that also determine the risk of disease from organisms. One person can drink untreated water without experiencing any problems, while another person may become ill. "If, for example, you are HIV-positive or immuno-compromised for some other reason, you would be foolish not to carefully treat all your water," Backer says.

Ultimately, Backer recommends exercising good judgment. "If you know the source of the water, you can estimate your risk," he says. "But you can't tell microbiologic quality of water simply by its appearance.

"If you know there has been substantial human or animal activity in the area you better treat it. If you're pretty confident this is untainted alpine water, you're probably safe to drink it without treatment," he says.

A Backcountry Strategy

So what's a wilderness explorer to do? When should you treat water, and when could you simply dip and sip?

1. Always be equipped to treat drinking water. Unless, based on your familiarity with a backcountry area, you are exceedingly confident that a water source is isolated from potential contamination, you should use some method of water treatment. Here is guidance offered to wilderness travelers by the Water Resources Division (Fort Collins, Colo.) of the National Park Service:

The policy adopted by parks that manage backcountry use is to purify the drinking water you collect in the backcountry by using one of the following methods:

  • Boil water for 1 minute plus an additional minute for each 1,000 feet above sea level.
  • Filter water with a portable water filter system that eliminates Giardia.
  • Use water-purifying tablets or drops that kill Giardia.

Most medical experts we interviewed, however, argue that simply bringing water to a boil is sufficient. Boiling water for several minutes is considered excessive. For details, see the "Boiling" section of our Water Treatment for International Travel article.

2. Learn to discern backcountry water. Before deciding to drink untreated water, wilderness travelers should look for danger signs around a water source. Consider treating water if you spot any of the following cautionary tipoffs:

  • Water (particularly at lower elevations) near meadows or pastures where animals have grazed or near popular, established campsites.
  • Evidence of pack animal traffic or other domesticated animal activity.
  • Signs of sloppy human behavior or a prolonged human visit.

If no such signs are apparent, a water source (particularly any at elevations exceeding 7,000 feet) may offer some of the purest, most delectable water on the planet. Derlet says he drinks water freely in the Sierra because he knows the region so well.

Even so, adventurers always want to carry some option for treating water available for any situation when a water source raises uncertainty or suspicion. If at any time you feel unsure about the purity of a water source, definitely treat it.

Shop REI's selection of water treatment options.

Water Treatment Tips

  • Seek calm, clear water. Calm water (a lake, a pool, a slow-moving stream) will likely contain less sediment or silt. Such water passes through a filter more swiftly and reduces the chance of clogging.
  • Calm water offers another advantage: Hard-shelled Giardia and Cryptosporidium cysts tend to sink in still water, reducing the chance you will encounter such micropests.
  • Yet another advantage of calm water: Water near the surface of a still body of water undergoes prolonged exposure to UV radiation—an enemy of waterborne pathogens.
  • Avoid drinking untreated water shortly after a heavy rain, when streams are "on the rise." Significant rain transports surface material from within the watershed into streams and other water bodies. This increases bacterial loads by several orders of magnitude.
  • Seek out water sources that show little or no evidence of animal or human activity. Leave none behind yourself.
  • Clear water unavailable? Use the pot in your cookset and scoop water from near the surface of the cleanest-looking section of water you can locate. Let the pot sit so sediment sinks to the bottom. Float your filter's intake hose along on the top of the water and draw in the cleanest water you can see.
  • Use a prefilter. Attached to the ends of some filter intake hoses, they help keep larger debris from reaching the filter media.
  • No prefilter? Strain especially murky water into a pot through a bandanna or a paper coffee filter before filtering. Or tie the bandanna around the tip of the intake hose.
  • Follow the rules: Camp at least 200 feet away from a water source and properly dispose of human waste at least as far, if not farther.
  • Snow or ice? Choose clean snow. Bacteria can live for months in ice.
  • Carry a backup treatment system. A filter could be lost; batteries could fail; device could be broken. Chemicals such as Potable Aqua or chlorine dioxide tablets offer extra security with negligible weight.
  • Diligently practice good sanitation when in the wilderness. Clean hands and fingernails with soap and water or sanitizers. Carry water you intend to use for cleaning away (preferably 200 feet or more) from the source in a portable, fold-up container. Never use soap in or near a water source. See the companion REI Expert Advice article, Hygiene and Sanitation in the Backcountry.
  • How best to dispose of soapy water? Pour it (or better, disperse with a fling) on soil rather than rocks, advises Derlet, the water researcher. "Soap may interfere with some lichen species growth," Derlet says. "It's better to toss the water on the ground, where trillions of microorganisms will 'fix,' or metabolize, the pollutants, mostly soil fungi." Never toss soapy water into backcountry water sources. It could spawn algae, which can later establish a population of microscopic pathogens in the water. Soap residue, Derlet says, is becoming a significant threat to backcountry water.

Shop REI's selection of soaps and accessories.

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