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Water is heavy, and so there is always a temptation to carry less so as to reduce packweight. But I have learned to resist this temptation. Too many times, I've come upon a perfect campsite in the mid-afternoon, secluded, soft and level ground, sheltered from the wind, free of trees and branches that might fall in the night, no insects, and perhaps even a fine view, but without enough water on hand, so that I was forced to continue onwards, and ended up with a much less pleasant campsite as a consequence. There is also the possibility of being injured while hiking, and forced to make an emergency bivouac. Having water on hand might mean the difference between full recovery and permanent injury or death. Having water on hand at all times also allows for cleaning out wounds that might occur while hiking, and for cleaning up after defecating.
Finally, having plentiful water available at all times reduces the risk of kidney stones. I once allowed myself to become chronically dehydrated by hiking in hot dry weather without sufficient water-carrying capacity, so that my urine was always deep yellow. My diet then included a large amount of almonds, which are high in calcium and oxalate and known to promote kidney stone formation. And sure enough, I formed a kidney stone (or several stones, I'm not sure). The stone passed without medical treatment, but this is not an experience I ever want to repeat.
Under normal conditions, I load 3 liters water into my bottles/bladder whenever possible, with more water for dry conditions and less where water is everywhere, such as the Sierras. The only time I allow my bottles/bladder to remain empty in the presence of a water source, is when I am absolutely sure I can get all the water I want whenever I want and there is a good reason to not carry water, such as during a steep ascent alongside a stream. In this case, I would wait until I got to the top of the steep ascent and then fill up.
See here for a discussion of how to extend water supplies by about 2 liters by being careful about the type of, quantity and timing of food eaten.
In North America and Europe, the serious dangers from water are: (a) inorganic chemicals (e.g. arsenic in certain water sources); (b) organic chemicals (e.g. herbicides and industrial wastes, and also toxins released by certain algae); (c) helminthic parasites (tapeworms, liver flukes) and their eggs; (d) high concentrations of bacteria in heavily polluted water, such as runoff from a densely packed livestock feeding lot. The much ballyhooed Giardia and Cryptosporidium parasites are a minor problem by comparison. A healthy adult with a strong immune system will shake off Cryptosporidium with ease, and will shake off or learn to live with Giardia after a minor amount of intestinal distress. Same story for the various bacteria and viruses that are commonly present in North America and Europe (other than in the concentrations that occur only in heavily polluted water, which is readily identifiable by smell)—if you haven't been exposed to these recently, you'll get diarrhea, which lasts for a few days, then you'll recover and be immune. These microorganisms have always been present in the water, and that didn't stop our ancestors (or at least adults with a strong immune system) from drinking the water without problems. The big threats in the past were cholera and typhoid fever, but the bacteria for these are not present in the wilderness areas of North America or Europe.
It is difficult to remove chemical pollution in the field (though activated charcoal can help), so if severe chemical pollution is suspected, the best procedure is probably to look for another water source. Some chemical water purification treatments are themselves potentially toxic to some people (iodine) or can create potentially toxic byproducts (sodium hypochlorite aka common bleach, chlorine-dioxide, sodium dichloroisocyanurate), but this is a minor consideration in most cases.
I have been unable to get any clear answer as to the risk of helminthic parasites from drinking wilderness water in North America and Europe. It is known that there are some very dangerous helminthes in the arctic region of North America, and that backcountry explorers have died from such helminthes, as a consequence of eating uncooked fish or drinking untreated water in these parts. In October 2007, there was news about foxes in Munich exposing people to the echinococcosis parasite (see here), also known as the hydatid tapeworm. The echinococcosis helminth is currently present in the arctic and Great Lakes regions of North America, where it is carried by wolves as the primary host and caribou or moose as the intermediate host. Echinococcosis is also known to be common in the Alps of Europe, where is carried by foxes as the primary host and voles as the intermediate host. Echinococcosis is common wherever sheep are raised and dogs are allowed to eat the uncooked offal of slaughtered sheep—here dogs are the primary host and the sheep are the intermediate host. But echinococcosis can be carried by any canine as the primary host, including wolves, wild or domesticated dogs, dingos, foxes and coyotes, and by any other mammal (including humans) as the intermediate or accidental host. Since just about every area of North America, Europe and Australia has one or more of the above canines present, there is potential for this helminth to spread in all of these areas. More info on echinococcosis available at MedScape (search on echinococcosis hydatid).
My understanding is that it is extremely difficult to kill the eggs of some of the helminthic parasites, since these eggs are designed to remain dormant for up to two years while exposed to high-altitude ultraviolent sunlight. Some of these eggs are supposedly far more resistant to chemical treatment than Cryptosporidium cysts, which can withstand up to four hours of exposure to chlorine-dioxide, which is considered the most powerful of the chemical treatments. (Note that it is almost certain that any of iodine, sodium hypochlorite or common bleach, sodium dichloroisocyanurate or chlorine-dioxide would kill the adult forms of any helminthic parasite, since these lack the protective covering of the eggs.) In any case, if the water to be treated contains significant quantities of dissolved iron, manganese, sulfides or certain organic substances (such as those in brown-colored "tea" water), then the chlorine-dioxide will react with these substances first, leaving insufficient residual chloride dioxide to kill all the pathogenic microorganisms, unless extra chlorine-dioxide is added. In order to know when to add extra chlorine-dioxide, and how much extra to add, we would have to test the water to be treated, such as by means of the test strips included as part of the MSR Miox system. I have my doubts as to how many users of chlorine-dioxide or other chemical water treatments actually perform such tests—it seems like a huge nuisance to me. On the other hand, it is very easy to filter helminthic parasites and their eggs, since the eggs are at least 25 microns in size and the adult parasite is even larger. Compare to less than 3 microns for Giardia and less than 1 micron for the smaller bacteria. Thus any filter which will filter Giardia will also filter the helminthic parasites and their eggs.
A particularly dangerous helminthic parasite in North America and Europe is the Baylisascaris procyonis roundworm, which is carried by raccoons as the primary host and rodents, rabbits and birds as the intermediate host, with humans as an accidental intermediate host. The parasite disables or kills the intermediate host, allowing the raccoon to easily capture and eat it. Injury to humans depends on the number of eggs ingested, since each egg gives rise to one parasitic worm. Injury is typically most severe in young children, who may ingest large amounts of raccoon feces and whose nervous system is more susceptible to injury. Like the eggs of other helminthic parasites, those of Baylisascaris procyonis are resistant to chemical/ultraviolet methods of water treatment, but can be easily filtered, since the egg measure 50 microns or more in diameter (www.ncbi.nlm.nih.gov/pmc/articles/PMC1265913/), which is much larger than the specified pore size of the Steripen filter. Boiling water will also destroy eggs in the water, though at high altitutdes it might be necessary to pressurize the water at high altitudes and/or boil for much longer than at sea level.
Another possible parasite to be worried about in North America and Europe is Toxoplasma gondii, which is carried by domestic cats (and perhaps other felines) as the primary host, and rodents and other prey animals as the intermediate host, with humans as an accidental intermediate host. Occysts or eggs excreted in cat feces are consumed by intermediate hosts. When the intermediate host is consumed by a cat, the cycle is completed. There is a theory that animals infected with T. gondii develop an attraction to cats, due to the parasite manipulating the dopamine receptors in the brain so that cat smell is associated with pleasure. In mice and other small animals, this attraction makes the animal easy prey for cats. In humans, this attraction makes the person a cat lover, and may also cause other personality changes, including schizophrenia in severe cases. T. gondii can cause serious birth defects in fetuses, which is why pregnant women are warned to be careful about exposure to cat feces. About 20% of the population of the United States is supposedly infected with T. gondii. Rates of infection are higher in other countries. As with helminth eggs, T. gondii eggs are highly resistant to chemical and ultraviolet water treatment methods, but are easy to filter, since they measure at least 10 microns in diameter (www.ncbi.nlm.nih.gov/pmc/articles/PMC2643564/).
Toxocariasis is name of a group of related and common helminthic parasites whose definitive hosts are domestic dogs (Toxocariasis canis), domestic cats (Toxocariasis catis) and other felines/canines, with humans as aberrant hosts. Infection in humans can cause damage to liver, lungs, central nervous system, eyes. Eggs resistant to chemical/ultraviolet treatment, but easily filtered, since diameter of eggs is 75 microns or greater (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC153144/).
Amoebas are uncommon in North America and Europe, though not unheard of. Supposedly, Deep Creek Hot Springs along the southern California Pacific Crest trail is contaminated with Naegleria fowleri, the so-called "brain-eating amoeba", which causes primary amoebic meningoencephalitis, which has a fatality rate of 98%. There are reports of deaths from Naegleria fowleri every so often from people drinking or swimming in bodies of water in the warmer parts of the United States, including swimming pools not properly treated with chlorine, and especially from swimming in natural hot springs. I have read no reports of Naegleria fowleri being found in cold mountain streams. Like all cysts, those of amoebas are highly resistant to chemical and ultraviolet treatment methods. Amoebas and their cysts are typically 10 microns or larger in diameter, so easy to filter.
I normally drink unpurified water straight from natural sources in the mountains (streams, ponds) or else filter with a mesh screen of some sort, 10 micron or finer pore size, to eliminate any helminthic eggs. I never attempt to filter or treat water for viruses, bacteria or Giardia, but I do try to avoid water that is heavily contaminated with bacteria (runoff from a pig farm, for example) to avoid overloading my immune system. Examples of contaminated water sources I have drunk without treatment and also without any intestinal problems: (a) streams in the Pyrenees mountains of Europe, which were contaminated by feces from the sheep, horses and cows which graze in large numbers in the pastures surrounding these streams; (b) ponds in Maine (northeast United States), which are contaminated by feces from moose, beavers, ducks and other animals. I have probably been harboring Giardia for many years without problems. The only instance where I suffered diarrhea in the United States or Europe in the past 20 years occurred when I was not hiking and had not been hiking for at least a year prior to the diarrhea. (That one episode of diarrhea in the United States was most likely due to contaminated food rather than water. I have suffered diarrhea twice while traveling in Central America, where I stayed in hotels rather than hiking in the wilderness. Here also, I suspect food rather than water was the source of the bacteria or virus which caused the diarrhea.) In the past, I have had pinworms, a very common helminthic parasite in North America and Europe, more annoying than dangerous, and I am certain this had nothing to do with drinking untreated water. If I am currently harboring any helminthic parasites, I'm not aware of it. Then again, helminthic parasites can work in a hidden manner, until one day the victim has all sorts of severe and sometimes incurable health problems.
DudaDiesel sells biodiesel filter bags with 10 micron or finer pore size, which are perfect for screening amoebas and helminth eggs. Polyester 1 micro additionally screens Giardia cysts.
For those who are more concerned about water purification, my recommendation would be the Katadyn Hiker Pro water filter, which uses a pleated fiberglass filter with 0.3 micron screen size. The Katadyn Hiker filter is about $15 cheaper than the Hiker Pro, but this is a very foolish economy, since the only difference is a field-cleanable prefilter to reduce clogging of the main filter by silt, which is an extremely useful feature. I have experimented with this filter myself and it is lightweight, has reasonably fast throughput, has various mechanisms to prevent clogging and allow for field cleaning if clogging does occur, is not damaged by freezing, and is otherwise reliable. Ceramic filters (Katadyn Pocket, MSR Miniworks) and filters which use hollow glass fibers as the filter element (MSR Hyperflow, Sawyer filters) can crack if allowed to freeze when wet (and they will stay wet for a long time after being used) or if dropped, which I consider to be unacceptable limitations for 3-season backpacking in North America or Europe.
The whole topic of water purification tends to be confusing and to raise strong emotions. There is a large amount of money to be made manufacturing and selling water treatment products, and more money to be made accepting advertisements from the manufacturers and vendors of such products, and there is a widespread perception that persons in positions of responsibility who fail to warn others of the possible dangers of drinking untreated wilderness water are likely to be sued for damages (I've been unable to find evidence that such a lawsuit has ever been filed, much less that damages have ever been awarded for such a suit), and finally there are complex sociological reasons why even those with no monetary interest in the issue feel compelled to conjure up visions of doom for those who fail to conform with the accepted wisdom that all wilderness water must be treated. Also, similar to the situation with stoves, tents, multi-compartmented backpacks, elaborate first-aid kits, and other backpacking contraptions, there are always plenty of hobbyists with discretionary income, who relish the opportunity to have yet another outlet for their consumerist and tinkering urges, and plenty of manufacturers, retailers and publishers of outdoor magazines enthusiastic about making money satisfying these urges. Also, plunking down a large sum of money for water purification gear and then carrying this heavy gear on the back tends to provide a sort of psychological comfort to many people: "According to the laws of universal justice, everyone should suffer equally. Since I have taken my dose of suffering up front, by buying and carrying this heavy and expensive gear, I shouldn't have to suffer additionally from getting sick. Only the people who didn't buy and carry water purification gear should get sick." Those who accept the preceding logic (I don't) will indeed probably get sick if they try to emulate me in not purifying their water other than to screen for the larger protozoa and helminths. Those who want to get sick or think they deserve to get sick generally do get sick, and contrarywise.
To raise the temperature of 1 gram of ice or snow takes about 1 physics calorie per °C (or .001 kilocalorie, kilocalorie is also written as Calorie, with uppercase "C", nutritional information almost always refers to kilocalories). To melt 1 gram of ice or snow at 0°C (normal melting point) takes about 80 calories. To raise the temperature of 1 gram of water (about 1 ml) takes about 1 calorie per °C. To evaporate 1 gram of water at 100°C (normal boiling point) takes about 539 calories. It is also possible to evaporate water without first raising the temperature to the boiling point. To evaporate 1 gram of 0°C water takes about 600 calories. To evaporate 1 gram of 37.6°C (body temperature) water takes about 580 calories. Thus every gram of moisture that leaves the body via respiration or evaporating perspiration causes a leakage of 580 calories from the body.
To evaporate 100 ml (a little over 3 ounces or less than half a cup in the English measurement system) of water from the skin takes about 58,000 calories or 58 Calories. If metabolism is at the rate of 3000 Calories/day, then 58 Calories represents about 30 minutes of metabolic heat. This shows how being forced to evaporate even small amounts of water in cold weather, when the only source of heat is the body (as opposed to the sun or a fire) can easily bring on hypothermia. It is thus critical to avoid getting wet in cold weather. If clothes do get wet in cold weather, it is sometimes better to take all the clothes off and go naked, rather than trying to dry the clothes using body heat.
Drinking 1 liter of water heated to 60°C adds about 23 Calories of heat to the body (since normal body temperature is 37°C), or about 11 minutes of metabolic heat, assuming metabolism of 3000 Calories/day. Drinking moderate amounts of warm liquids won't hurt (excessive drinking of warm or cold liquids can hurt, research hyponatremia) but it also may not offset the loss of metabolic heat due to evaporating water and thus may not cure hypothermia. It is thus critical to avoid hypothermia in the first place.
Rapidly eating large amounts of snow or ice is inadvisable, partly due to the risk of frostbite of the mouth and partly because this could bring on hypothermia. However, provided the snow or ice is eaten gradually, there is little danger in eating snow or ice, contrary to folk wisdom. Eating 2 liters (a day's requirements) of frozen water (in the form of either ice or snow) at -20°C subtracts about 280 Calories from the body, or under 10% of daily production, assuming 3000 Calories/day metabolism. 280 Calories is also approximately the energy content of a small candy bar—for example, a small Snickers® candy bar (59g) contains 280 Calories according to the label. Alternatively stated, a person could eat the equivalent of 2 liters of water in the form of snow or ice at -20°C for 10 days straight, and at the end of these 10 days would have lost less than 1 pound of body fat beyond what they would have lost had they been drinking liquid water at room temperature. Thus eating snow or ice, provided this is done gradually, is unlikely to bring on hypothermia, even in very cold conditions. It is normally better to lose a small amount of body heat by eating snow and ice than to risk dehydration, since dehydration can slow down metabolism. Reduction of body heat due to slowed metabolism is a much greater danger than the loss of body heat due to gradually eating snow or ice. If you are planning to eat snow in lieu of melting snow with a stove, then remember to bring along the equivalent of an extra candy bar of food for each day you will be doing this.
I have personally experimented with eating snow in cold conditions, when I was once forced to make camp during a blizzard and remain where I was for several days, during which time my liquid water ran out. I was also low on food and thus burned mostly body fat. In addition to eating snow, I filled up my water bottle with snow and melted this snow while sleeping using body heat, so as to have some liquid water for brushing my teeth and washing up after defecating. Neither eating snow nor melting snow with body heat caused me to feel chilled. Nor is this surprising, since this is exactly what wolves and other animals do in the Arctic winter, and our bodies are not that different from theirs. On the other hand, washing my hands and then drying them by evaporation (I don't carry a towel and my clothes resist absorbing moisture and so don't work well as a towel substitute) quickly caused them to become chilled. Evaporating water requires much more heat energy than melting ice or snow.