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  • Key Points:
  • Chlorine, in doses recommended for wilderness and foreign travel, is not effective in eradicating Cryptosporidium cysts from water, and may be poorly effective in eliminating Giardia.

  • Chlorine dioxide, however, is effective against these parasites, and will also eliminate bacteria and viruses.

  • Chlorine dioxide is an oxidizing chemical—it does not use chlorine as the disinfectant.

  • Boiling is unnecessary to purify water—heating water for 2 minutes at 149° F (65° C) or 20 minutes at 113° F (45° C) will make it safe to drink.

  • Hand washing is an effective and underused method of reducing disease transmission-especially infectious diarrhea and respiratory illnesses.

What's New

  • Poultry was the most commonly identified source of food poisoning in the United States in 2006, followed by leafy vegetables and fruits and nuts. (June 2009 report from the CDC-Centers for Disease Control & Prevention.)

  • Most poultry-related illnesses were associated with Clostridium perfringens, a bacterium that commonly causes abdominal cramping and diarrhea usually within 10 to 12 hours after ingestion. The spores from this bacterium often survive cooking, so keeping poultry meat refrigerated during storage can be critical to prevent contamination by bacterial growth.

  • While poultry is the most common source of illness among the 17 different foods tracked by federal officials, the CDC found that two-thirds of all food-related illnesses traced to a lone ingredient were caused by viruses, which are often introduced to food by restaurant workers who fail to wash their hands.

  • These viruses often cause what many people refer to as a "stomach flu," one to two days of of nausea and vomiting that is unrelated to the influenza virus.

  • Salmonella, the bacteria found in nationwide outbreaks of contaminated peanut butter, spinach and tomatoes, was the second-leading cause of sole-source food illnesses, the centers found.

  • While dairy products accounted for just 3 percent of traceable food-related outbreaks, 71 percent of these cases were traced to unpasteurized milk. (Source: NY Times June 12, 2009)

Most Americans take for granted the safety of their food and water. If we do worry, we may focus on sugar, salt, cholesterol, saturated fat, food additives—and now carbohydrates! We forget that modern methods of food preparation, packaging, refrigeration, and the use of preservatives—combined with efficient municipal water purification and sanitation—have given the United States and other developed countries unparalleled safety and freedom from infectious diseases transmitted by contaminated food and water. Probably the main health hazard we face from food is its abundance. We eat too much. Obesity, not food-borne illness, is the greater health threat.

Food and Drink in the United States and Canada

Despite our excellent safety record, hundreds of sporadic outbreaks of food- and water-borne illnesses are officially reported each year in the United States. Undercooked food—contaminated eggs, meat, and chicken transmit most cases of disease. The most common illnesses are enterocolitis (usually caused by Salmonella and Campylobacter bacteria) and hepatitis A. Outbreaks often affect hospital patients, nursing home residents, or school children. Cases of hemolytic-uremic syndrome (HUS), associated with the consumption of undercooked hamburger meat containing Escherichia coli bacteria, have received extensive publicity, but HUS has also been associated with consuming raw cider, person-to-person contact, as well as bathing in the “kiddie pool” at water parks. Gastroenteritis caused by various Vibrio species of bacteria is occasionally reported from the Gulf of Mexico; most cases are related to the consumption of raw shellfish. Botulism, caused by the improper home canning of food, is sporadically reported, and giardiasis, a water-borne parasitic illness, sometimes afflicts hikers and wilderness campers who drink from contaminated ponds, lakes, or streams. Giardiasis and cryptosporidiosis outbreaks have also resulted from breakdowns in municipal water treatment plants. In addition, we have suffered from “travelers' diarrhea” without even traveling. In 1995 to 1997, multistate outbreaks of cyclosporiasis were traced to raspberries and mesclun lettuce imported from Central America!

Food and Drink Overseas

Outside of the United States, Canada, Europe, Australia, and parts of Asia, the situation is far more serious. Most less developed countries don't have our standard of living, our sanitation technology, or our cultural attitudes toward the disposal of human fecal material. Raw sewage may drain into the sources of drinking water, and agricultural fields may be contaminated with various bacteria, viruses, and parasites because human feces (night soil) are often used as fertilizer.

Many countries have only rudimentary water treatment facilities and water distribution systems, and where these facilities do exist there are often breakdowns in the system. Public health regulations and inspections may not be enforced or nonexistent. The hygiene of restaurant personnel is usually below Western standards. The importance of hand washing may not be emphasized to kitchen workers. Refrigeration of food in restaurants may be inadequate, or totally lacking, and countertops and cutting surfaces may not be cleaned as required. Such practices not only promote the transmission of diarrheal diseases caused by bacteria and viruses, but also help spread hepatitis A, typhoid fever, trichinosis, tapeworm, and other bacterial and parasitic diseases rarely found in this country.

Food Safety

Sources of Risk

When you choose foods to eat, evaluate each item in terms of its ability to harbor dangerous organisms or harmful toxins. Eating undercooked, raw, or unpasteurized products is potentially hazardous. Remember that thorough cooking will destroy bacteria, parasites, and viruses. Even simple heating is usually sufficient to destroy harmful microorganisms.

Food contamination can result from any of the following:

  • Contamination at the source: Shellfish, for example, may be harvested from polluted water containing hepatitis A virus, Aeromonas, Salmonella, or cholera bacteria; chicken and beef can be fecally contaminated with E. coli during slaughter; lettuce and other uncooked vegetables may be contaminated in the field from contact with fecally contaminated soil and transmit a variety of bacteria and parasites; unpasteurized dairy products, made from milk produced by sick cattle, can transmit brucellosis, listeriosis, and tuberculosis.

  • Contamination from handling: This includes foods that require a lot of touching during preparation, such as salads and raw vegetables. Salads may have also been rinsed with contaminated water during preparation.

  • Contamination from bacterial growth: Foods that are prepared moist and warm and that are allowed to sit unrefrigerated are risky. Under these conditions, bacteria, such as staphylococci, can rapidly multiply, producing toxins that cause sudden, severe vomiting and diarrhea. The term “food poisoning” describes this type of toxin-related gastrointestinal illness. Reheated foods are particularly dangerous in this regard.

  • Contamination from parasitic larvae: Beef, pork, fish, and shellfish may contain parasitic larvae encysted in their flesh. Aquatic plants (watercress, water chestnuts) may have parasitic cysts attached to their shoots. Examples of illness transmitted by encysted parasites include trichinosis, beef and pork tapeworm disease, anisakiasis, and clonorchiasis, paragonimiasis, and fascioliasis (liver fluke and lung fluke diseases).

Guidelines to Reducing Risk

No matter where you decide to eat, if you follow the guidelines below, you'll improve your chances of staying healthy.

  • Eat only meat and fish that have been thoroughly and recently cooked, not re-warmed. Beef and pork should be well done without any pink areas.

  • CAUTION: Because of uneven heating, microwaving may not completely destroy surface bacteria. Microwave thoroughly.

  • Eat only cooked fruits and vegetables or fruits that you can peel.

  • Wash the surface of melons before slicing. Bacteria can otherwise be carried onto the cut surface.

  • Foods that require little handling are safer.

  • Order hard-boiled eggs served in the shell.

  • Choose dairy products from large, commercial dairies. Boiled milk is safe.

  • Milk and dairy products in Canada, Western Europe, Australia, and Japan are considered safe. Canned milk is safe.

  • NOTE: Commercially prepared mayonnaise is safe. The combination of vinegar, lemon juice, and salt in mayonnaise actually helps kill bacteria, such as Salmonella.

Foods to Avoid

  • Rare or raw meat; raw fish, shellfish, crayfish, and sushi that have not been previously frozen. In the United States, Food and Drug Administration regulations stipulate that fish to be eaten raw—whether as sushi, sashimi, ceviche, or fish tartare—must be frozen first to kill parasites. (Tuna is the only exception.) Other countries have enacted similar laws.

  • Raw vegetables, especially leafy salads served in restaurants

  • Fruits not peeled by you and fruits with punctured skins. Watermelons, for example, are often injected with tap water to increase their market weight.

  • Aquatic plants in Asia (e.g., watercress, water chestnuts)

  • Raw eggs, undercooked eggs, unpasteurized milk and cheese. Some cooking techniques (sunny-side up, “soft” scrambled) won't kill Salmonella bacteria.

  • Street vendor food unless it is hot and well cooked

  • All food that has been left out in the sun, especially dairy products

  • Buffet food that has been re-warmed or recycled (e.g., the same cheeses that are at each meal)

Street_Vendor_Guidelines">Street Vendor Guidelines

  • Choose food that is cooked, boiled, steamed, or grilled directly in front of you. These items are safe if served fresh and hot.

  • Avoid food handled excessively by the vendor after cooking.

  • Avoid juices and other drinks unless they are commercially bottled.

  • Eat only food that is served in a clean container.

Wash Your Hands

Wash your hands with soap or detergent, or use a hand sanitizer gel, before you eat. Proper hand hygiene reduces the incidence of travelers' diarrhea by 30%, AND there's good evidence that it also reduces the risk of viral respiratory illnesses. If you already have travelers' diarrhea, or if you are caring for someone with this problem, be sure to wash your hands after using the toilet, or after having personal contact with the patient. Shigellosis, giardiasis, and viral gastroenteritis are some of the diseases that can be spread from person to person.

Contact with Animals and Eating

Outbreaks of E. coli O157: H7 gastroenteritis have occurred among visitors to a petting zoo and a farm in the United Kingdom. Visitors to zoos and farms should know that various pathogens such as E. coli, Salmonella, and Campylobacter can be transmitted to humans from animals, and that hand-to-mouth contact, eating, drinking, and smoking should be discouraged during these visits. Hand washing should be encouraged.

“Safe” Restaurants

Appearances can be deceiving. It's not always possible to tell if a particular restaurant serves safe food. Although the big, established restaurants and hotels may have better safety records, even their kitchens can have lapses in sanitation. As for eating in local restaurants, ask for a recommendation from business contacts, hotel managers, tour guides, etc. When in doubt, don't hesitate to eat in a deluxe hotel restaurant. Some travelers say that Chinese restaurants are often the safest. These restaurants use fresher ingredients, cooked at high temperatures (not reheated), which are served immediately. Mexican-style restaurants are riskier because many dishes require more handling to prepare and often contain eggs, lettuce, and uncooked vegetables.

The following checklist will also help you decide which restaurants may be safer than others.

  • Are the silverware, tablecloths, glasses, and plates clean?

  • Are the toilets clean? Are soap and water provided for hand washing?

  • Are there many flies inside? (Flies can carry disease germs.)

  • Is there adequate screening to keep out flies and other insects?

  • Is there excess/uncovered garbage outside?

  • Are the waiters well groomed?

  • Is the restaurant recommended by knowledgeable people?

Remember that the enjoyment of eating is partly what travel is all about. Eating well will also help you stay well, provided you use common sense. Getting enough to eat and drink helps avoid fatigue and dehydration. Within reason, you can often eat what the locals eat. For example, if you're traveling in Europe, choose a tasty but well-cooked specialty such as wiener schnitzel and pass up the risky, uncooked, steak tartare. In Asia, enjoy the Peking duck but skip the raw fish.

Water and Beverages

All surface water supplies can be expected to be contaminated at one time or another, no matter how pristine the source, and water from most streams, ponds, wells, and irrigated areas should be considered unsafe to drink. Tap water is often contaminated in less developed countries, but many hotels and resorts maintain safe water systems. You need to judge each facility individually. If you are on a typical tourist itinerary, you are probably going to play it safe and stick to commercially bottled water, soft drinks, fruit juices, beer, and wine, etc. If you plan to obtain drinking water from potentially contaminated sources you need to employ methods that lessen your risk of illness.

Remember that water is only one potential source of illness. Harmful microorganisms also can be transmitted by contaminated food, person-to-person contact, and touching contaminated objects, so there is no absolute protection against the acquisition of an infectious disease. The goal is to reduce your risk by methods that are not too inconvenient or too expensive.

Safe Water and Beverages

  • Boiled water—The traditional “standard” of water purification

  • Chemically treated water—Chlorine dioxide is becoming more widely used as a replacement for iodine and chlorine.

  • Filtered water—Filters remove bacteria and parasites, but not viruses (see “Recommendations”).

  • Hot tea and coffee are generally safe. Even if the water was never actually boiled during preparation, heating water over a period of time is similar to pasteurizing it. Be sure the drinking cup is clean.

Avoid—If Possible

  • Untreated tap water by the glass, in mixed drinks, or in the form of ice cubes Commercial ice in blocks should be suspect.

  • Locally bottled water: Be suspicious: These bottles are sometimes refilled with local tap water.

  • Uncapped bottled water: These bottles may also have been refilled locally and are best avoided.

  • Sea water is always unfit to drink because of the salt content, but it can be used for cooking.

  • Pristine-looking water in wilderness lakes and streams may be contaminated with Giardia or Cryptosporidium parasites, or pathogenic bacteria, such as Campylobacter.

Planning Your Water Needs

Review your itinerary to determine what your water needs and sources will be. Will you be vagabond traveling, wilderness trekking, living in tropical countries, or touring under-developed countries? All present different problems and require different strategies. You may be faced with preparing quantities of safe drinking water from polluted sources or simply disinfecting small amounts of tap water in your hotel room. If you will be depending on a filter for much of your water, pump speed, ease of pumping, and ease of cleaning are important factors. Can the filter element be cleaned and re-used, or must it be replaced? When planning your water needs, consider the following:

  • Will you be in an urban, rural, desert, mountain, or jungle environment?

  • For how long will you be there?

  • Will you be hiking or trekking? Need to disinfect water en route?

  • Will you be staying at a fixed location or base camp?

  • Will you be storing drinking water on a boat or vehicle?

  • How many people will be in your group?

  • How much water (maximum amount) will you need to disinfect at one time?

  • How close will you be to rivers, lakes, and streams? Will you be using that water to drink? How safe is that water to drink?

  • What illnesses are common at your destination?

  • What type of disinfection equipment or chemicals are you planning to take on your trip?

  • What type and how many water containers will you carry?

Wilderness hiking and camping in the United States and Canada expose you mainly to Giardia, whereas drinking water in less developed countries is potentially more dangerous—especially near population centers where raw sewage may contaminate the drinking water. In less developed countries, additional protection against bacteria and viruses (especially for certain groups) is essential. See “Recommendations” subsequently. Unless you are in resorts, first-class hotels, and cities that properly filter and chlorinate their water, you should disinfect tap water.

What About Viruses?

You may be advised when traveling in less developed countries, and obtaining water from unsafe sources, not to rely on filters because they do not eliminate viruses. The Wilderness Medical Society states: “Filtration may be used for Giardia and . . . bacteria, but for field use, filtration is not practical for viruses (although many are removed by adhering to larger particles).”

But what viral illnesses are we protecting against? And can they be prevented? Should everyone traveling in less developed countries use a water purification device, apply disinfecting chemicals, or boil their water to avoid this threat? Consider the following points:

  • Polio is the most dangerous water-borne virus to avoid, but this disease can be prevented by immunization and it has been eradicated from most countries in the world.

  • Hepatitis A is spread by contaminated water, but there is an effective vaccine.

  • Aside from the common cold, viral gastroenteritis is the most common infectious disease in the world. How preventable is this illness? Recent epidemics on cruise ships illustrate how widespread the problem can be. It is easy to acquire viral gastroenteritis not just by consuming contaminated food or beverages, but also by touching contaminated objects, having close contact with infected people, and not washing your hands. This is usually a self-limited illness, but can be potentially serious for infants and very young children, the elderly, and people with immune disorders. (These groups are unlikely to be exposed to unsafe water while camping or trekking in a wilderness setting, or traveling off the usual tourist routes.)

  • Hepatitis E is the most common form of hepatitis in many countries, and is usually transmitted by contaminated water. There is no vaccine. This disease is much more serious in pregnant women.


Total protection against gastrointestinal illness is not possible, but some groups of travelers are at higher risk than others. These considerations lead the Health Guide to recommend the following:

  • All travelers should be fully immunized against polio and hepatitis A.

  • Pregnant women in regions endemic for the hepatitis E virus should drink only commercially bottled or boiled water, or purified water. Using iodine tablets for longer than 3 weeks is not recommended because of the possibility of thyroid suppression.

  • Infants, young children, the elderly, and those with immune disorders should take precautions against exposure to viruses in drinking water.

  • All other travelers should be concerned primarily with eliminating bacteria and parasites from their drinking water.

In reality, many travelers may use several methods of water acquisition and treatment as availability, sanitary conditions, itinerary, length of stay, convenience, and personal preference dictate. One method or technology doesn't necessarily exclude the other. Each method has advantages and disadvantages. You may not have your filter or purifier with you at all times. It may clog or break, or you may not have a replacement cartridge. If you are at a base camp you may need large quantities of water for many people and a gravity drip filter plus granular chlorine might be the most convenient and cost-effective method. If on a solo hike, a small water filter or purifier, or disinfecting tablets, may be the most convenient. Whatever the scenario, you often need a backup method of treatment.

Regardless of what system is used to disinfect water, travelers' diarrhea is still a threat because it is also caused by the consumption of contaminated food and spread by person-to-person contact. Treating water—even sterilizing it—reduces, but does not eliminate, the risk of illness.

Water Treatment and Disinfection

Obtaining Clear Water

If you are drawing water from a polluted source, it may be grossly contaminated with organic material. For esthetic reasons alone, you wouldn't want to drink cloudy, scummy water. Furthermore, cloudy water requires more time and bigger doses of chemicals to disinfect, especially if it is cold. Chlorine, in particular, reacts with, and is neutralized by, organic material such as rotting vegetation. Unless you are literally dying of thirst, you should take enough time to clarify your drinking water before it is treated. Here are some techniques:

Sedimentation Let the turbid water stand undisturbed for several hours, then pour off the upper, clear portion. This works best if the cloudiness is due to sand, silt, or other inorganic material.

Flocculation Organic impurities may not settle out with gravity alone. Add a pinch of alum (available over the counter in drugstores) to each quart. Flocculation (clumping) of suspended organic impurities will occur, and these clumped particles will settle to the bottom of the container. Pour off the clarified water. To save time, pour the water through a coffee filter, commercial filter paper, fine cloth, or a canvas filter bag to remove the flocculated sediment more rapidly.

Filtering Ceramic and glass fiber filters that filter bacteria and parasites also filter out turbidity, but clogging will occur. Ceramic filters can be cleaned many more times than glass fiber filters. Use a pre-filter on the intake hose to eliminate large particles.

Methods of Disinfecting Water

Heating Water Cryptosporidium and Giardia cysts (oocysts) are very susceptible to heat. Two minutes at 149° F (65° C) or 20 minutes at 113° F (45° C) (similar to pasteurization) will inactivate the cysts. Bacteria, such as cholera germs, are killed at 144°F (62°C) for 10 minutes.

Boiling Water Water that is brought just to a boil and then allowed to cool is safe to consume. Boiling water for 10 to 20 minutes, even at high altitudes, is unnecessary and wastes time and fuel. Some people even question the need to boil water at all—they just “pasteurize” it by heating it for a period of time at a sub-boiling temperature (as mentioned earlier). NOTE: Boiling water at 10,000 feet raises its temperature to 194° F (90°C)—adequate for killing all microorganisms.

Advantages of Boiling Boiling water completely eliminates bacteria, cysts of parasites (amoeba, Giardia, Cryptosporidium), worm larvae that cause schistosomiasis, and viruses (the causes of hepatitis, polio, and viral gastroenteritis). NOTE: Briefly boiling water won't eliminate the spores of certain bacteria; hence, the water can't be considered absolutely sterile. However, bacteria do not cause intestinal illness and can be consumed without harm.

Disadvantages of Boiling It is easier said than done. Heating the water is time-consuming, often inconvenient, and may require you to carry a source of fuel with you. Boiling is usually most easily done at a base camp or other fixed location, not on the trail. Other technologies of water disinfection now make the tedious process of boiling water often unnecessary.

Iodine and Chlorine Under proper conditions, both iodine and chlorine are excellent water disinfectants for eliminating bacteria and viruses; they are less effective, or even ineffective, against parasites, especially when contact time is brief and/or the water is cloudy and cold.

Iodine has been used to disinfect water since the turn of the 20th century. U.S. Army studies have demonstrated that under field conditions with dirty, cold water, a 10-minute contact time with iodine kills bacteria, Giardia, and viruses; other field studies have shown that at least a 50-minute contact time, and perhaps longer, is necessary to kill Giardia under “worst case water” conditions. Cryptosporidium cysts are not eliminated by either iodine or chlorine. Boiling, filtering, or using chlorine dioxide are the only ways to eliminate this parasite.

Iodine Tablets Potable Aqua comes in a small glass bottle that holds 50 tablets and can treat 25 quarts of water. The directions state that it is not for continuous use but for occasional use or emergencies. Potable Aqua is a lightweight, convenient item to carry as a backup purifier. It is not effective against Cryptosporidium. Disadvantages are that the need for adequate contact time prevents immediate use. There is a disagreeable taste and odor, but Potable Aqua Plus provides a neutralizer that improves taste and smell. Travelers should avoid prolonged use (more than 3 weeks) of iodine-treated water (as the sole source of drinking water) to avoid potential suppression of thyroid function. This limitation of iodine intake is most important during pregnancy because of the potential adverse effects on fetal thyroid gland development. However, iodine should be used by pregnant women if there is no other short-term alternative to purifying water, especially in areas endemic for hepatitis E.*

Liquid Chlorine Bleach (4% to 6% Clorox) Household bleach is easily available and cheap, but it doesn't kill Cryptosporidium and may not kill Giardia. The Health Guide does not recommend chlorine as a first-line treatment for water disinfection, but it may have a role when other methods are not available or when the water can also be filtered to remove parasites, if these are a threat. Chlorine is cost effective for treating large quantities of drinking water. The percentage of available chlorine in bleach is usually written on the label. Be sure to use clear water. Mixing directions are as follows: If the strength of the chlorine is unknown, use 10 drops per quart. Double the amount of chlorine if the water is cloudy or cold. Allow the water to stand for 30 minutes. The water should have a slight chlorine odor; if not, repeat the dosage and let stand for an additional 15 minutes.

Granular Calcium Hypochlorite This method is useful for disinfecting large quantities of water. It uses the granular chlorine often used in swimming pools. Add and dissolve 1 heaping teaspoon of high-test granular calcium hypochlorite for every 2 gallons of water. This will prepare a stock chlorine solution that can be used to disinfect larger quantities of water. Add the chlorine stock solution 1 part to every 100 parts of water. For example, add 1 pint of stock solution to 12.5 gallons of water to be treated.

Chlorine Tablets Chlorine tablets were once a popular product in the United States, sold under the brand name Halazone. Chlorine tablets are no longer sold in the United States, but are still available in the United Kingdom and elsewhere. Other disinfecting tablets, using iodine or chlorine dioxide, have replaced chlorine tablets in availability and popularity.

Chlorine Dioxide (ClO2) Chlorine dioxide is an extremely effective disinfectant, which rapidly kills bacteria, viruses, and Giardia, and is also effective against Cryptosporidium. ClO2 also improves taste and odor, destroys sulfides, cyanides, and phenols, controls algae, and neutralizes iron and manganese ions. It is an effective biocide at concentrations as low as 0.1 ppm (parts per million) and over a wide pH range. It is ten times more soluble in water than chlorine, even in cold water. Unlike iodine, chlorine dioxide has no brdverse effects on thyroid function. Chlorine dioxide is widely used by municipal water treatment facilities.

The term “chlorine dioxide” is misleading because chlorine is not the active element. Chlorine dioxide is an oxidizing, not a chlorinating agent. ClO2 penetrates the cell wall and reacts with amino acids in the cytoplasm within the cell, killing the microorganism. The by-product of this reaction is chlorite, which is harmless to humans. Chlorine dioxide is available in both tablet (Micropur MP 1) and liquid (Pristine) preparations.

Micropur MP 1 These tablets are a stabilized form of chlorine dioxide and have a shelf life of 3 years. Each tablet will treat 1 quart of water and they are sold in a 30-tablet blister pack. Effective against Giardia, Cryptosporidium, bacteria, and viruses, but a 4-hour wait time is required to eliminate Cryptosporidium from cloudy, cold water.

Pristine This is an economical liquid chlorine dioxide product that will purify 30 gallons (120 quarts) of water for about the same price as 30 tablets of Micropur MP 1 (which will treat 30 quarts). This product is slightly more complex to use than Micropur MP 1 because mixing of two chemicals is required and there is the chance of spillage.

Micropur MP 1 is the only single-tablet chemical treatment approved by the Environmental Protection Agency (EPA) to kill bacteria, viruses, and parasites, including Giardia and Cryptosporidium.

Water Filters and Purifiers

A filter's basic task sounds simple and straightforward: remove microorganisms and other particles larger than a specified size from water. This mission isn't so easy, given the small size and variety of particles, pathogens, and chemical contaminants that may be encountered. Therefore, a variety of devices have evolved: ceramic filters, depth filters, surface filters, etc. These filters all come with a rating of their pore size, which determines what size particles can be physically removed. Pore sizes are measured in microns, and the period at the end of this sentence is about 600 microns across. In practical terms, the most important number is the “absolute” pore size rating, which means the filter element will pass no particle above a certain size.

To strain out common parasites (protozoa), such as Giardia and Cryptosporidium, a pore size of no larger than 4.0 microns absolute is necessary (protozoa range in size from 5 to 15 microns). For bacteria that range in size from 0.2 microns to 10 microns an absolute pore size of 0.2 microns is desirable. A filter this fine is subject to more rapid clogging and will require frequent cleaning. A pre-filter helps reduce clogging.

Water Purifiers Because viruses can be as small as 0.0004 microns, no field device that relies entirely on filtration will remove them. Water purifiers have both a filter and a demand-release iodine-resin element.* A popular model is the Katadyn Exstream. The VirusStat cartridge kills bacteria and viruses and the filter removes parasites. The cartridge needs to be replaced after 100 liters of water have been treated.

Note: Choosing a filter/purifier—Aside from removal of microorganisms, consider the following factors when selecting one of these devices:

  • Rated output in liters/minute

  • Life of filter/purifier element before replacement

  • The size and weight of unit with accessories

  • Cost, not only of the device, but also of the replacement filter

  • Whether a carbon cartridge is attached to reduce residual iodine and other chemicals in the filtered water

  • *A Peace Corps study published in 1998 in the medical journal Lancet described a group of Peace Corps volunteers in Niger who used two-stage iodine-resin filters for more than 24 months as the sole source of potable water. Forty-six percent developed enlarged thyroid glands (goiter) and 34% had abnormalities of thyroid function. The authors recommended not using an iodine-resin filter (or iodine tablets) as the sole source of potable water for more than 3 weeks in any 6-month period.

    *How the demand-release iodine system works: iodide ions are bound to an anion exchange resin, creating an electrically charged structure. When negatively charged microorganisms contact the resin, iodine is instantly released, penetrating the microorganism. By this process, bacteria and viruses, and some parasites, are killed. NOTE: the residual iodine in the water may have a concentration of 10 mg/liter, which is significant considering that the RDA (recommended daily allowance) of iodine is only 0.15 mg/day. Some purifiers have a charcoal filter that reduces the iodine content of the treated

Facts about activated carbon filters—What they can and cannot do:

  • Carbon filters won't “soften” water.

  • Carbon filters will not remove iron, lead, manganese, and copper or chlorides, nitrates, and fluorides.

  • Carbon filtration can remove more than 90% of cadmium, chromium, manganese, mercury, silver, and tin.

  • Carbon filters are not effective against bacteria, cysts, or viruses; in fact, they may promote bacterial growth when not changed at proper intervals.

  • They remove many objectionable tastes and odors.

  • Carbon filters are effective for removal of chlorine and iodine taste and smell as well as potentially dangerous and carcinogenic organic compounds.

  • If turbidity of the water is the main problem, fiber pre-filters are preferred because they are more cost effective.