Water, Water Everywhere - What Is Fit To Drink?

Obtaining safe drinking water can be a dilemma for travelers and people in the wilderness. While drinking lots of water is important for staying fit, much of the available water is not fit to drink.

Most people realize the importance of staying well-hydrated; many carry water even for everyday activities. The need for water increases with conditions of travel and recreation- exertion, hot weather, air travel, and altitude, to mention just a few. There is ample evidence that minimal dehydration - which occurs even before you feel thirsty- has subtle adverse effects on your sense of well being and safety; it causes mild fatigue and slightly slows thought processes and reaction times.

But finding safe water can be difficult. Overseas, the fact that tap water is marked "chlorinated" or "safe to drink" does not necessarily make it so. Sophisticated water purification requires proper disposal of sewage, optimum quantities of chlorine or other disinfectants, proper storing, filtration, clean and intact pipes, frequent testing, and continuous electric power. When power fails, for example, purification machinery stops operating. But the water keeps flowing.

(A safe rule of thumb; if there is a candle in your hotel room - which is often the case in some countries - be suspicious of the drinking water. The candle is a sign that power outages are common.)

Tap water is somewhat risky even in areas of good sanitation. "Safe" tap water contains some microorganisms, albeit, generally harmless ones. The US Public Health Service says that water containing up to 10 coliform organisms per liter of tap water is acceptable and unlikely to make anyone ill. But while your stomach is accustomed to the organisms in your own water, new "harmless" organisms may be sufficiently different to cause mild gastrointestinal (GI) upsets the first few days you drink it. Ironically, some of the mild GI upsets that are blamed on the vagaries of travel - fatigue and jetlag, for example - and for which water is prescribed as part of the cure, may in part be due to the drinking of local tap water.

In the wilderness, consider all natural water contaminated. Twenty years ago common wisdom was that clear, cool water upstream from large animals was potable. Not so. Many organisms survive months in cool water, far longer than in warm water (though pH and nutrients play roles).

Shigella, salmonella, and hepatitis A organisms resist freezing. Giardia, the most common diarrhea-producing organism in the wilderness originates in small animals and birds. You can’t get upstream from them. In a study in Colorado, 40% of beavers and all muskrats tested carried giardia. (However ingesting giardia does not necessarily cause illness; many cases appear to be asymptomatic.) Also, water gurgling from a spring may be flowing into the ground from a polluted area on the other side of the hill. And chemicals from agricultural and industrial runoffs are an increasing problem in most parts of the world and have become an important source of GI illnesses.

Canteens and other reusable containers that people carry to store "safe" water can be a source of contamination. These containers often have narrow openings making them difficult to clean, sometimes resulting in the containers becoming sources of infection. Disposable bottles are better.

In developing countries suspect locally bottled water. The water is only as good as the product that goes in the bottle. When there is doubt, choose international brands or carbonated water. Carbonation acidifies water and destroys organisms.

Inadvertently swallowing water while swimming or engaging in water sports is another important cause of GI diseases.

Optimum water purification requires the removal of enteric pathogens (bacteria, viruses, parasites, and other organisms that cause diarrhea). The best technique in a given situation depends on your location, the type of water available, pathogens likely to be in the water, the number of individuals in your group, type and length of the trip, space and weight considerations, and individual preferences.


Boiling sterilizes water. All known enteric pathogens are destroyed by a contact time of 10 minutes at 212 degrees F (100 C). Ideally, cool the water in the container used for boiling.

But boiling for 10 minutes is often impractical, especially in the wilderness where time and fuel are considerations. Boiling one liter of water for a minute requires about two pounds (one kilogram) of wood. Other fuels are difficult to transport in the wilderness. People in the wilderness generally use two liters of water a day.

Most pathogens are killed at lower temperatures. Thermal death is a function of both time and temperature. Lower temperature is effective with longer contact time. Pasteurization - used to ensure safe milk and other foods - occurs at 131 degrees F (55 C) for 30 minutes or at 149 F (65 C) for 1 minute.

Water can be considered pasteurized when it comes out of the tap too hot to touch or when it is merely brought to a momentary boil. Such tap water generally has a temperature of more than 131 degrees F and, often, more than 149 degrees F. The necessary contact time takes place in the water heater and pipes. While "too hot to touch" is subjective - unless you carry a thermometer - it should suffice in most situations. The contact time for momentary boiled water occurs while the water is brought to the boiling point plus the cooling time.

Pasteurized water is not sterile. Neither is pasteurized milk. Both contain some organisms. While there are bacterial spores that can survive brief boiling, no known intestinal pathogen survives pasteurization.

The conventional wisdom is that water must be boiled longer at higher elevations is not correct. True, as altitude increases, atmospheric pressure decreases, and water boils at lower temperatures. But at elevations travelers are likely to reach, the boiling point remains well above temperatures required to kill pathogenic enteric organisms.

When boiling is not practical, chemicals and filters can be used to purify water. First, obtain the best water available. In the wilderness, ideally use water found away from humans and animals. In developing countries use water from piped taps, wells, and springs. These tend to be less polluted than streams and ponds.

Then do one or more of the following before purifying the water:

  • Allow it to stand undisturbed for as long as possible. Then carefully decant off the clear water, leaving any sediment behind. Even if the water is clear, discard the bottom third.
  • Pour it through a crude filter (a coffee filter or clean cloth, for example).
  • Add a small amount of alum. This helps remove organic and solid matter that can interfere with the purification process. Many organisms adhere to matter.

Chemical Purification

The most commonly used chemical disinfectants are halogens - iodine and chlorine. These kill organisms by oxidizing essential cellular structures. Halogens are inexpensive, relatively safe, not bulky, and can disinfect any quantity of water.

Moreover, disinfection can be accomplished overnight or while you carry the water. Most experts prefer iodine over chlorine. Iodine is less affected by pH or organic wastes and the taste is less offensive. After extensive testing, the U.S. Army now uses iodine in the field.

The disinfecting ability of halogens is a function of concentration (mg/liter or parts per million) and contact time with water. Increasing one allows a reduction in the other. Other relevant factors are the temperature of the water and the presence of organic contaminants. Low temperature slows reaction time requiring longer contact, especially to kill giardia. Organic contaminants require increased concentration. Such contaminants are most commonly due to decomposing vegetation and are recognizable by a cloudy appearance in the water.

Optimal concentrations of halogens readily kill bacteria, viruses, and most parasites. But one parasite, cryptosporidium, discovered in the mid-1980s, can withstand halogenation. Cryptosporidium causes diarrhea, cramps, and vomiting. It is found in water in developing countries and in the wilderness, and was responsible for the outbreak of gastrointestinal illness that affected more than 400,000 people in Milwaukee several years ago. In one survey in the western U.S., cryptosporidium was found in 77% of rivers, 75% of lakes, and 28% of samples of treated drinking water. As few as ten organisms may be sufficient to cause illness.

The significance of cryptosporidium as a cause of diarrhea is unknown; it is responsible for only a few percent of such illnesses among travelers and people in the wilderness. Ingesting water containing the organism does not necessarily produce illness, perhaps because of some immunity from previous cryptosporidium infection. Also, stomach acidity and other body defenses may reduce the number or decrease the virulence of organisms. Moreover, some individuals appear to harbor the organism without ill effects. Therefore, halogen-treated water presents only a small risk of carrying cryptosporidium-related disease, a risk that may be acceptable for healthy individuals.

Cryptosporidium presents problems for individuals with immunodeficiency diseases and with decreased stomach acidity, due to disease, medication, or surgery. Such individuals are more likely to become infected, and the disease may be far more serious.

Chlorine and iodine is available in liquid and tablet form and iodine is available in crystals. In the U.S., iodine tablets (Potable Aqua) are popular. Use one tablet in a liter of water. Wait 20 minutes. Use two tablets and wait 30 minutes if the water is cold or cloudy.

A popular and inexpensive water purifier is laundry bleach. Most products contain 4-6% chlorine. Add a drop or two (one drop = 0.05ml) of the bleach to each quart or liter of water. Let it stand for 20 minutes before use. Add 4 drops if the water is cloudy or very cold and let stand for several hours, if possible.

Tincture of iodine (found in some first aid kits) contains 2% iodine. Add 4 drops to one quart or liter of water; 8 drops if very cold or cloudy. Let it stand as with bleach.

Many failures in using halogens are due to improper use. Read instructions. Check expiration dates. The potency of some products (tablets and solutions) decrease with time and are affected by prolonged exposure to moisture or heat (tablets) and air (iodine crystals). Liquids are corrosive and stain.

Theoretically, iodine-treated water may be a problem for individuals with thyroid diseases and also during pregnancy, causing goiters in newborns. Likely, drinking such water for a week or two causes no problems.

Halogens have an unpleasant taste, especially at higher concentrations. Taste can be improved by: using charcoal to remove halogens; reducing the concentration and increasing the contact time; or adding flavoring or ascorbic acid (vitamin C) or sodium thiosulfate. These substances should be added after the necessary contact time. Sodium thiosulfate reduces iodine or chlorine to iodide or chloride which have no taste or flavor. Sodium thiosulfate is non-toxic and available in chemical supply stores.


A large variety of sophisticated, user-friendly filters are available, ranging from small, light, simple-to-use, and relatively inexpensive models designed to deliver one glass of water at a time (in a hotel room, for example) to large units that deliver hundreds of liters for expeditions. Filtration imparts no taste and requires no contact time. Filters remove organisms by a variety of methods: small pore size: membranes: mazes: adsorption: exchange resins: and osmosis, for example. Filters also remove sand, clay, and other matter, improving water taste and appearance.

Filters effectively remove bacteria and parasites, including cryptosporidium and giardia. Filters also remove most viruses, even though viruses are far smaller than the openings in the pores. The filters causes the viruses to clump together and adhere to other particles, forming masses to large to enter the pores. But some viruses may pass through. As few as ten viruses may cause illness. GI disease-producing viruses are frequently present in water in developing countries but less often in the U.S. wilderness. Therefore, good filters are fairly effective in purifying water in the U.S. wilderness but less so in developing countries.

Removal of all enteric pathogens requires both filtration and halogenation. Several available filters do this by having the water pass through silver-impregnated elements or by using iodine exchange resins to remove viruses. The iodine resin creates an electrically charged structure. When negatively charged contaminants contact the resin, iodine is instantly released killing the microorganism. By this process, organisms are killed, without large amounts of iodine actually being in solution.

In addition, as a final step, many filters are granulated activated carbon to remove organic pollutants, radioactive particles, pesticides and other chemicals, including halogens used in the filter. This greatly decreases objectionable color, taste, and odors.

While filters are quite useful, problems remain:

  • All claims made by manufacturers have not been tested by impartial laboratories.
  • Many filters require periodic cleaning to remove trapped matter and organisms.
  • Clogged pores require increased water pressure which can damage filters.
  • The cleaning process can cause damage.
  • It is difficult to recognize when filters are damaged and are no longer functioning effectively.
  • The relationship between pore size vs. organism is misleading. Organisms of the same size vary (e.g. all amoeba are not the same size), and are somewhat elastic, especially under pressure, allowing ones larger than filter pores to squeeze through pores.