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Wilderness Safety and the TRUTH about Wilderness Survival: Treating "wild water."

My opinion on this topic is in accord with the C.D.C. and the E.P.A., so to keep the record straight: we are generally talking about four types of pathogens; Specifically 1) Giardia, 2) Crypto’ (Cryptosporidium), and generally 3) Bacteria, and 4) Viruses. In all cases, they can get into wild water through human and animal waste. Solids like salts or minerals require specialized water purifiers (if the water is treatable at all). This post also deals with three treatment options: 1) Boiling, 2) Filtering, and 3) Chemicals (i.e. Potable Aqua, Aquamira, etc.)

Giardia and Crypto’, in first-world countries like the U.S., are normally the only things we have to be concerned with. Most good 1-micron filters will be sufficient for these. The symptoms for both present in about a week, and while those symptoms are usually mild, they can be severe for some people. If you’re in the backcountry, and you know you’re going home soon, then it’s really not a big deal. If it’s a problem later, just get a prescription and take some pills. However, people usually DON’T know when they’re going to find themselves in a survival ordeal, so if you’re on day 2 of a 7-day outing after drinking untreated water, THEN find yourself in a survival ordeal, your situation just became that much more complicated with probable dehydration. Yes, any decision to opt for diarrhea instead of dehydration is a calculated risk.

Bacteria presents the next level of consideration with wild water. These are smaller than Giardia or Crypto’, so for this you need a better .03-micron filter (read labels!). These are things like Escherichia coli (or “E. coli”), Salmonella, etc. Symptoms from these can present anywhere from a few days to a few weeks, so again, your risk depends on whether you go home on time, but the consequences are much more risky!

Viruses are the most serious consideration with drinking untreated wild water, most often in third-world countries, they are about .02 to 1 micron in width, and 1 to 10 microns in length, so they’re hardest to filter out. These are things like hepatitis A, rotavirus, enterovirus, norovirus, etc. Symptoms from these can present anywhere from 1 day to a few weeks and have the most serious consequences. So again, ultimately the level of risk depends on going home on time.


Boiling is the “Gold standard” of wild water treatment! Boiling is the ONLY highly effective treatment that WILL make wild water safe!! As long as the water is clear, or at least slightly cloudy (Not muddy or contain a lot of silt), BOILING is the only, single, method that WILL guarantee safe water. Otherwise, always try to arrive at the best possible water before filtering/treating by screening-out particulates (i.e. with a head net or t-shirt, etc.) Water from a “waterfall”, a “fast moving” river, or water that is cold, are NOT indications of clean, safe water. Only BOILING will work, clear? Most knowledgeable people will say it’s only necessary to bring water just to the point of boiling, (which is true). Still, I recommend boiling for 1 minute, 3 minutes if above 6500 feet or if the water has slight particulates, etc.

If boiling is not an option, using a 1-micron filter for Giardia and Crypto’ is fine. For bacteria, a .03-micron filter AND chemical treatment (usually iodine or chlorine) will make it just as safe as boiling. For viruses, while using a .03-micron filter helps a little, with chemical treatment (although not as effective as boiling), should make it reasonably safe. However for viruses, I would recommend using a water purifier (if you have one) with a chemical treatment to be sure, but water filters by themselves are NOT effective against viruses at all.

What if you have no fire, AND no filter? With Crypto’, iodine and chlorine are NOT effective at all, and chlorine dioxide will only give low/minimal effectiveness. With Giardia, iodine and chlorine give low/minimal effectiveness while chlorine dioxide gives good protection. With bacteria and viruses, iodine, chlorine and chlorine dioxide all give good/acceptable protection (but they’re much better with filtration!).

With potassium permanganate, just a few crystals can treat a quart/liter of water (about 0.01% solution or 1 gram per liter, 3 - 4 crystals), just enough to make the water a light pink (That’s LIGHT pink, not “Pink”!), with contact time from 2 to 24 hours. But it’s interesting to note that it’s not as effective as chlorine dioxide or as safe.

UV light (or “solarizing”, done by putting the water in a clear glass or plastic P.E.T. bottle) is also effective, but it depends on following the standardized recommendations and good pre-screening practices. Particulates in the water (and temperature and altitude) always inhibit treatment effectiveness, so screening-out silt, debris, etc. before filtering is always recommended.


How does chlorine dioxide differ from sodium hypochlorite? (common household bleach). Any topic involving science/chemistry can get rather scientific, so I’ll try to keep it simple. The difference between chlorine and chlorine dioxide is HUGE! The only thing they have in common is the word "chlorine" in their name and that they're both water soluble, THAT's where the similarity ends! The chemical makeup, the way they react, their strength, etc. are ALL different.

Chlorine Dioxide is a more powerful oxidant. It undergoes 5 changes in its oxidative nature, Chlorine is only capable of 1, so Chlorine Dioxide is 2.6 times more powerful. Chlorine Dioxide acts only by oxidation, Chlorine will combine to produce harmful Chlorinated byproducts (e.g. chlorinated phenols, THM's, Dioxins etc.), many of these byproducts are recognized carcinogens. The properties of Chlorine are very pH dependent; Chlorine Dioxide is effective at all pH levels below 12. Chlorine has been found to be ineffective against complex organisms (e.g. cysts and protozoa, both giardia and crypto' are protozoa), but Chlorine Dioxide has been found to be effective against a wide range of organisms. Chlorine cannot be used at high temperatures (104F/40C) because it tends to dissociate evolving Chlorine Gas. Chlorine Dioxide does not dissociate as readily at elevated temperatures. Chlorine does not remove Biofilm. Chlorine Dioxide Does. Basically, Chlorine Dioxide is better and safer.


There are two common methods for water purification tablets: Iodine and chlorine. Iodine is the arguably more effective method, but it leaves an iodine taste. It is suggested the taste may determine how much or how little one drinks. So, how do you deal with it? Some products like Potable Aqua include a second bottle with tablets you can add to remove the taste, OR, simply adding vitamin C (i.e. Tang), or adding a pinch of salt per quart, adding flavorings (e.g., lemonade mix, etc.) should do it or you can just pour it back and forth between containers (again, AFTER it has been adequately treated).

The other option is chlorine. Chlorine should be used for persons with iodine allergies or restrictions like persons with thyroid problems, are on lithium, are allergic to shellfish (they may also be allergic to iodine), women over fifty and pregnant women should consult their physician prior to using iodine for purification. Be aware that iodine is light sensitive and must always be stored in a dark bottle, once opened, it may need to be used within a certain time.


With more medical uses of iodine (especially as a radio contrast media), more instances of reactions have been reported in recent years. It is a commonly held belief among radiologists and other medical professionals that there's a specific cross-reactivity between iodinated radiographic contrast material and other iodine-rich substances.

Iodine is a common element found in, among other places, the human body so reactions to iodine are extremely rare, but they do happen. "Iodine allergy" is used as a collective term for adverse reactions, and though iodine is not generally considered an allergen, some people who are hypersensitive may be considered to have an iodine "allergy." In a few cases, iodine has even been responsible for patient deaths.

Evidence exists of a nonspecific cross-reactivity between contrast material sensitivity and allergy to seafood (as well as other foods). Patients reporting iodine or seafood allergy should be questioned as to the exact nature and severity of the reaction. If possible, seafood allergy should be distinguished from other causes of seafood intolerance. The presence of a seafood allergy places the patient at a threefold risk of an adverse reaction to iodine contrast material. As with any other allergy, the nature and severity of the reaction should be considered when choosing the type of contrast material and when determining the need for a premedication regimen.

Finally, iodine and chlorine will NOT work on toxic water (water that has dissolved chemicals or solids like salt). In that case, it MAY be safe/safer to add the use of absorbent filters (i.e. with charcoal), but the BEST advice is to simply NOT use toxic water!

Potable Aqua, for example (both the iodine and chlorine versions), are effective against viruses, bacteria, and cysts (such as giardia and cryptosporidium). But while the iodine version can work in as little as 30 minutes, the chlorine version may take a 4-hour treatment. ALL chemical treatment is effective, HOWEVER, you need to use clear or virtually clear water, AND it can't be too cold or you may need to increase the treatment time.

The effectiveness of all chemical treatment of water is related to the temperature, pH level, and clarity of the water. Cloudy water often requires higher concentrations of chemical to disinfect. If the water is cloudy, strain it, add the tablets and swish it around. Splash some water with the chemical onto the lid and the threads of the water bottle so all water areas are treated. The water should sit for at least 30 minutes. The colder the water, the less effective the chemical, it works best if the water is over 68F/21C, if the water temperature is below 40F/4C, double the treatment time before drinking.

I always suggest you have at least TWO options for water, fire and shelter. Here, tablets are one, the other is a filter like Sawyer (it has the distinction of being able to be used almost perpetually by simply backwashing it!). But the best method is still boiling, some say you don't actually have to "boil" water to make it safe, that by the time water actually comes to a boil it is already safe, but I like to see bubbles for my own peace of mind!

19 Replies

A very fine discussion.  My preference is boiling, simple, effective, and not all that time consuming.  James Wilkerson points out in Medicine for Mountaineering, 6th edition, that milk is pasteurized at160 degrees and coolinggradually.  His opinion is that bringing water to a boil is all that is necessary.

A hot topic for sure....


Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.

Thanks, but I think I should have given "Solarizing" (or S.O.D.I.S) more time...


This method of water treatment is widely used in third-world countries and is perfectly suited for wilderness survival purposes in the right conditions. Solar water disinfection (SODIS) is a simple, environmentally sustainable, low-cost solution for drinking water treatment at the house-hold level for people consuming microbiologically contaminated raw water. SODIS uses solar energy to destroy pathogenic microorganisms causing water borne diseases and improves the quality of drinking water. Pathogenic microorganisms are vulnerable to two effects of sunlight: radiation in the spectrum of UV-A light (wavelength 320-400nm) and heat (increased water temperature). A synergy of these two effects occurs, as their combined effect is much greater than the sum of the single effects. This means the mortality of the microorganisms increases when they are exposed to both temperature and UV-A light at the same time.

SODIS is ideal to disinfect small quantities of water of low turbidity. Contaminated water is filled into transparent plastic bottles and exposed to full sunlight for at least six hours. During exposure to the sun, the pathogens are destroyed. If cloudyness is greater than 50%, the plastic bottles need to be exposed for 2 consecutive days in order to produce water safe for consumption. However, if water temperatures exceed 122°F/50°C, one hour of exposure is sufficient to obtain safe drinking water. The treatment efficiency can be improved if the plastic bottles are exposed on sunlight reflecting surfaces such as aluminum (or corrugated iron sheets in third-world countries). Or, white sand/rock as long as it's NOT cool to the touch.

SODIS is probably the most inefficient method of treating water because it can take 6 to 12 hours of bright, direct sunlight (or more), AND ideally requires the water to be as clear as possible, but if you have a bottle that meets the recommended criteria, it's a great no-cost method.

It works best if: The bottle is protected from wind and shade (Anything that would lower the water's temperature), placed on a reflective surface (Yet another use for aluminum foil!), the container MUST be as clear as possible, transparent and colorless. Labels should be removed and heavily scratched bottles should be avoided. The bottle should be clear, glass, or clear Poly-Ethylene Terephthalate, aka PET), not P.V.C. because PVC can be harmful to your health. PET bottles are recommended because of their durability and ease of availability (For solarizing, the bottle should be replaced every year), but plastic bags can also be used (i.e. freezer bags, etc.). PET and PVC bottles are normally marked, the labels can vary from country to country, but if the bottle is not marked the only way to tell is by burning them. PET burns quickly and easily when held in a flame, when taken out of the flame, the fire goes out slowly or it may keep burning and the smoke smells sweet. PVC does not burn easily, the material does not burn at all when not in the flame and the smoke smells acrid.

NOTE: The treated water should NOT be poured into dirty containers, the bottles MUST be as clear and clean as possible and should be placed horizontal to the sun (To increase exposure and decrease depth) and the bottle should not hold more than 3 liters. The SODIS method does not kill ALL waterborne pathogens, just those which cause diarrhea (i.e. crypto and giardia). However, studies have shown many other bacteria like viruses and protozoa are also neutralized.

Personally, I don't like water bladders for my reservoir, instead I use four 750ml Fiji water bottles with the flip-top sport caps (my favorite bottled water, which comes in P.E.T. plastic bottles) in my pack as my reservoir which I replace nearly every time I go out. I leave the labels on to protect the plastic (until the unlikely event I should need to solarize).


In some circumstances,untreated water may be better than no water at all.

Dehydration has serious consequences, some even fatal since lack of water increases blood density, enhancing the formation of blood clots which can travel to the heart, lungs, or brain with very bad outcomes, including death.

Of course, it is much better to always have the means to treat water, but sometimes that doesn't happen. An episode of the trots, while unpleasant and disabling, is much preferable to a heart attack or lung embolism.  There are other pernicious results from dehydration, including reduced mental capability (never a good idea when out in the woods).

When younger, during the 1950s and 60s, I quite regularly drank untreated water from various sources.  I never experienced adverse effects, but that was a time when trail use was significantly less than today.  I am much more cautious now, but there were times when I drank copiously from raw water sources.  Once you get back to town, medical help is available.

You can reclaim really bad water.  Once, doing a project in Canyon del Muerto (charming name!), we lived for a month on local stream water, which we were sure was polluted - people without sanitary facilities and herds of sheep upstream - by letting the water settle for 24 hours, decanting the clear upper layers, and then boiling.  Results were good.

Superusers do not speak on behalf of REI and may have received
one or more gifts or other benefits from the co-op.

> In some circumstances,untreated water may be better than no water at all... Dehydration has serious consequences....

Well of course, all of this goes without saying, but it's situation dependent. If you're in 1st-world countries, giardia and crypto'  are rather mild possible consequences, even if you can't go home on time. In 3rd-world countries, when you're facing the possibility of some rather nasty bugs, you need to think a little harder.  Both before you go and when you get there.

But in the U.S., our backcountry water is fairly clean. Even the possibility of giardia and crypto is minimal. Still, even if it's present, that doesn't mean you will necessarily get it. If you consume 50 cysts,  you probably will not get giardia. If you consume 100 cysts, you MAY get giardia. If you consume  150 cysts, you will probably get giardia. But giardia and crypto' NEED access to your digestive tract, so you can't get them by splashing wild water on your skin, eyes, ears, etc.

In any case, I routinely screen my water collection bag with a head-net before I filter (helps keep the filter clean, promote flow rate, and minimizes the need to backflush). I also lightly backflush my filter after every use. If I'm cooking, I always boil before I add the [dehydrated] food. If I'm drinking, and I have ANY qualms about purity, I have tablets in my survival kit or I'll boil. That's just routine behavior for me.

If I'm going to a riskier area, (and presuming we're talking about backcountry areas), I always know where the possible water sources are before I go out (but then I know how to find water sources wherever I am) and the likelihood of contamiation. I also top-off my water bottles every chance I get, even if I'm not empty. I always look for information on area water often by asking the Ranger Office, if there's one nearby, for ANY updates before I head-out. Etc., etc., etc.

Regardless, you always have to balance immediate need with later consequences. Unfortunately, human behavior dictates we are NEVER so concerned with later consequences (that's why we have a climate change issue!). This is what makes a PROPER wilderness survival education so valuable, it teaches you best practices you can rely on.


Excellent topic and discussion.  Although I do not hike in wilderness areas, my travels bring me to primitive and under-developed locations.  This discussion focuses on avoiding contaminated drinking water.  There are other ways of becoming ill from water borne pathogens.  Last October I camped out on the Sahel in Chad and followed that up with camping in  the Omo Valley of Ethiopia.  Three days into my Ethiopia trip, I became ill.  I now believe that I contacted my sickness while taking a bucket shower in my hotel room that had no running water - just a garbage bucket of water. I had washed my hair and I believe some water had inadvertently gotten into my mouth. I waited two days for it to clear before I started taking Cipro that I was carrying.  It was no helping.  I finally got to a lodge.  I was laying on the bed when I realized that I most likely had dysentery.  Cipro will not get rid of dysentery.  I needed Flagyl. I checked and there was a clinic in the nearby village.  I was able to get some Flagyl there and after the first three doses I was well on my way to recovery after five days of misery.  Once back in civilization with Internet access, I realized that I had suffered from Giardiasis rather than Dysentery.  On my upcoming trip this summer, I will be exercising more caution when I wash my hair (if I do) and I will carry Flagyl as well as Cipro with me.


AH, right. Although giardia/giardiasis (and crypto'/cryptospiridium) is all we have to worry about in North America, it absolutely DOES occur everywhere else. In Africa there are about 200 million cases of giardiasis every year (in addition to what ever else is wrong with the water!).

But yes, Metronidazole (Flagyl) is the most common antibiotic for giardia, but Nitazoxanide (Alinia) and Tinidazole (Tindamax) can also be used. Cipro/ciprofloxacin can be used for Crypto, but you can ALSO use Nitazoxanide (Alinia) for crypto' AND giardia! For good measure, Azithromycin (Zithromax) may be given with one of these medications to people with compromised immune systems.

I wouldn't recommend carrying those medication in 1st-world countries because we have easy access to medical help, but do your research BEFORE you go, that should tell you all you need to know about how careful you should be.



CORRECTION - There are about 200 million cases of giardiasis every year in Africa AND ASIA AND SOUTH AMERICA.


A quick look at the UV transmission spectra for PET(s) shows a cutoff at ~300nm.  This means that PET containers are unsuitable for this purpose since they strongly absorb (remove) the kind of UV light necessary for sterilization.  The insertable type of UV sources will still be effective since the UV light is not filtered out by first passing thru the container. 



The main/common concern of Polyethylene terephthalate (PET) containers for water/SODIS use is transmission/leaching of carcinogens or other harmful elements. The study you point to appears to be focused on milk. While research is, of course, always ongoing, you may want to look for another more recent (and on point) article.