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Summary of Wilderness Medical Society Clinical Practice Guidelines on Water Disinfection for Wilderness, International Travel, and Austere Situations


To provide guidance to clinicians and disseminate knowledge about best practices, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines that offer a practical explanation of how to improve drinking water quality in a broad range of wilderness and austere settings situations. The guideline reviews commonly recommended methods based on medical, public health, and sanitation engineering literature; they do not include opinions. While it refers to types of commercial devices, it does not evaluate individual products. The recommendations provide methods of improving water quality that can be applied to individuals, groups, or households and for a wide spectrum of applications, including wilderness, international travel, and for austere situations such as survival, disaster, and refugee conditions. Please refer to the original article for the supporting evidence, as well as for discussion of additional methods, including clarification techniques, other chemical methods, and new methods like electrolytic and nanoparticles.

Without treatment of drinking water, waterborne diseases can spread rapidly, resulting in large scale disease. In industrialized nations, the population is generally protected from waterborne disease by sophisticated water supply systems that disinfect water and provide continuous monitoring. In contrast, travelers to wilderness and recreational areas anywhere in the world and to underdeveloped regions of some countries may be confronted with untreated or contaminated water that poses a risk of acquiring enteric disease. In addition, disaster situations, may result in a breakdown of municipal water systems, exposing victims to contaminated water. These situations necessitate knowledge of how to disinfect water at the point-of-use, prior to drinking.

Risk of Waterborne Infection

The environment and activity upstream from wilderness travelers’ surface water source defines the risk of contamination. Side streams draining springs, snowmelt, and glaciers where there is no human or animal activity provide lower risk. In contrast, upstream usage by humans, farm animals, or wildlife pose a significant risk. Cattle have been found in multiple studies to be the major animal species contributing to waterborne disease in North America. Giardiasis has been linked to numerous host species including farm animals, deer and other wild ungulates, beavers, and even household animals.

Non-alpine wilderness areas in developed countries may have streams and rivers that are contaminated with animal waste, including farm animal runoff, or may be contaminated with incompletely treated sewage from towns and urban areas.

Substantial progress has been made in the past 20 years toward the goal of safe drinking water and sanitation worldwide, particularly in Asia and Latin America; however, 780 million people (11% of world population) still lack a safe water source and 2.5 billion people lack access to improved sanitation. Africa and Oceania are the regions with the greatest need for improvement. Underdeveloped regions around the world show high levels of microbes in the environment and water sources.

In both developed and developing countries, after natural disasters such as hurricanes, tsunamis, and earthquakes, one of the most immediate public health problems is a lack of potable water.

In both developed and developing countries, after natural disasters such as hurricanes, tsunamis, and earthquakes, one of the most immediate public health problems is a lack of potable water.

Wilderness visitors and international travelers have no reliable resources to evaluate local water system or surface water quality. Appearance, smell, and taste are not reliable indicators to estimate water safety.

Infectious agents with the potential for waterborne transmission include bacteria, viruses, protozoa, and nonprotozoan parasites. Different microorganisms have varying susceptibilities to these methods. The risk of waterborne illness depends on the number and type of organisms consumed, host factors, and the efficacy of the treatment system.

The Guideline recommendation is to treat all wilderness surface water, since it is very difficult to exclude animal and human activity in the watershed. Treat all water in developing countries. Treat water in disaster situations that affect municipal or private drinking water sources.

Water Treatment Methods

Methods of field water treatment summarized here include the use of heat, ultraviolet light, filtration and chemical disinfection. Bottled water may be a convenient and popular solution but creates ecological problems.

Potable implies “drinkable” water, but technically means that a water source, on average, over a period of time, contains a ‘minimal microbial hazard,’ so that the likelihood of illness is acceptably low. The term disinfection is used here to indicate the removal or destruction of harmful microorganisms to reduce the risk of illness. This is sometimes used interchangeably with purification, but that term more accurately means the removal of organic or inorganic chemicals and particulate matter to improve color, taste and odor.

Heat is an ancient and highly reliable means of water disinfection. Heat inactivation of microorganisms is a function of time and temperature, so organisms are killed in a shorter time at higher temperatures, while lower temperatures are effective if applied for a longer time. Pasteurization uses this principle to kill food pathogens and spoiling organisms at temperatures well below boiling, generally between 60°C (140° F) and 70°C (158° F). All common enteric pathogens are readily inactivated by heat at pasteurization temperatures.

Since micro-organisms that cause diarrhea are killed within seconds by boiling water, and rapidly at temperatures >60°C (140°F), the traditional advice to boil water for 10 min to ensure potable water is excessive. Any water brought to a rapid boil should be adequately disinfected. Boiling for 1 min is recommended by the US Centers for Disease Control and Prevention (CDC) to account for user variability in identifying boiling points and adds a margin of safety. The boiling point decreases with increasing altitude, but this is not significant compared with the time required for thermal death at these temperatures. Although attaining boiling temperature is not necessary to kill microorganisms, boiling is the only easily recognizable endpoint without using a thermometer.

Ultraviolet (UV) light
At sufficient doses, all waterborne intestinal pathogens are inactivated by UV radiation. Both large high-volume units and portable, light-weight battery-operated units for disinfection of small quantities of water are commercially available. The UV waves must strike the organism, so the water must be free of particles that could act as a shield. The UV waves do alter the taste or look of the water.

In austere situations, UV irradiation by sunlight can substantially improve the microbiologic quality of water and reduce diarrheal illness. The optimal procedure for the solar disinfection (SODIS) technique is to use transparent bottles (e.g., clear plastic beverage bottles), preferably lying on a dark surface, exposed to sunlight for a minimum of 4 h with intermittent agitation.

Filters are rated by their ability to retain particles of a certain size. Many different types of media, from sand to vegetable products to fabric, have been used for water filtration throughout history. Filters are simple to use and do not add unpleasant taste, but they do not improve the taste and appearance of water. The primary determinant of a microorganism’s susceptibility to filtration is its size (Figure 1). Portable filters for water treatment can be divided into microfilters with pore sizes down to 0.1 µm; ultrafilters that can remove particles as small as 0.01 µm; nanofilters with pore sizes as small as 0.001 µm or less; and reverse osmosis filters with pore sizes of 0.0001 µm or less.

Figure 1. Levels of filtration and susceptibility of common microbial pathogens and other contaminants. Adapted from Backer H. Water disinfection for international travelers. In: Keystone JS, Kozarsky PE, Connor BA, eds. Travel Medicine. 4th ed. Philadelphia, PA: Elsevier; 2019:31–41. Copyright 2019, reprinted with permission from Elsevier.

Most portable filters are microfilters that can readily remove protozoan cysts and bacteria, but may not remove all viruses, which are much smaller. Ultrafiltration membranes are required for complete microbial removal, including viruses.

Several factors influence the decision of which filter to buy: (1) flow volume sufficient for the number of persons relying on the filter; (2) whether the filter functional claims matches the microbiologic demands that will be put on the filter; (3) the preferred means of operation (e.g., hand pump or gravity); and (4) cost. All filters eventually clog from suspended particulate matter, present even in clear streams, requiring cleaning or replacement of the filter.

Filtration using simple, available products such as rice hull ash filters, crushed charcoal, sponges, and various fabrics and paper have all been used in developing countries and in emergency situations. Their effectiveness for decreasing turbidity (cloudiness) may be used as an indicator that a filter material will reduce microbiologic contamination. Ceramic clay is widely available and very inexpensive to locally manufacture ceramic filters in the shape of a sink or flower pot that is set into a larger container to collect the filtered water.

Sand filters can be highly effective at removing turbidity and improving microbiologic quality. Sand filters are constructed by forming layers of aggregate increasing in size from the top to the bottom. The top layer is very fine sand and the bottom layer consists of large gravel. The container needs an exit port on the bottom. An emergency sand filter can be made in a 20 L (5.3 gal) bucket, composed of a 10 cm (3.9 in) layer of gravel beneath a 23 cm (9.1 in) layer of sand; a layer of cotton cloth, sandwiched between two layers of wire mesh, separates the sand and gravel layers. A sand filter also can be improvised with stacked buckets of successive filter layers with holes in the bottom to allow water passage.

Chemical Disinfection: Halogens (Iodine and Chlorine)
Worldwide, disinfection with chemicals, chiefly chlorine, is the most commonly used method for improving and maintaining the microbiologic quality of drinking water and can be used by individuals and groups in the field. Disinfection effectiveness is determined by the microorganism, the disinfectant, contact time, and environmental factors. Both chlorine and iodine are widely available worldwide in multiple formulations. Hypochlorite, the major chlorine disinfectant, is currently the preferred means of municipal water disinfection worldwide. The most commonly available form of chlorine is hypochlorite (household bleach 5-8%, or concentrated “swimming pool” granules or tablets 70%). Chlorine is still advocated by the WHO and the CDC as a mainstay of large-scale community, individual household, and emergency use. Another advantage of hypochlorite is the ease of adjusting the dose for large volumes of water.

Iodine is also effective in low concentrations for killing bacteria, viruses, and some protozoan cysts. Because of its effect on the thyroid, which uses iodine, the World Health Organization (WHO) recommends iodine only for short-term emergency use.

Common bacteria are very sensitive to halogens. Viruses, including hepatitis A, have intermediate sensitivity, requiring higher concentrations or longer contact times. Protozoan cysts are more resistant than enteric bacteria and enteric viruses but Giardia and amebic cysts can be inactivated by field doses of halogens. Cryptosporidium oocysts, however, are much more resistant to halogens and inactivation is not practical with common doses of iodine and chlorine used in field water disinfection.

Understanding factors that influence chemical disinfection, chiefly the inverse relation between concentration and time allows flexibility to minimize chemical dose and improve taste or, conversely, to minimize the required contact time. Cold water slows chemical reactions and can be adjusted by longer contact times or higher concentration of disinfectant chemical. Another important factor in chemical disinfection is the presence of organic and inorganic contaminants often associated with turbidity (cloudiness). Typical recommendations for field treatment double the amount of chlorine or iodine in cloudy water; however, it is preferable to use clarification techniques prior to chemical disinfection in cloudy water to improve efficacy and taste. Refer to tables in the Guideline for recommended doses and time for disinfection.

Taste of chlorine or iodine in water can be improved by several means. One method is to use the minimum necessary dose with a longer contact time. Another method is to use higher doses and remove the taste through a chemical reaction by adding a small pinch ascorbic acid (vitamin C), available in crystalline or powder form.

Preferred Technique

The optimal water treatment technique for an individual or group will depend on the number of persons to be served, space and weight accommodations, quality of source water, personal taste preferences, and fuel availability. Since halogens are not effective for killing Cryptosporidium at drinking water concentrations and common microfilters are not reliable for virus removal, optimal protection for all situations may require a two-step process of (1) filtration or coagulation–flocculation (clarification technique), followed by (2) chlorine. Heat (boiling) is effective as a one-step process in all situations but will not improve the look and taste of the water. Cloudy water should first be clarified before using hypochlorite or filtration.

On long-distance ocean-going boats where water must be desalinated as well as disinfected during the voyage, only reverse osmosis membrane filters are adequate. Water storage also requires consideration. Iodine will work for short periods only (i.e., weeks) because it is a poor algaecide. For prolonged storage, water should be chlorinated and kept in a tightly sealed container to reduce the risk of contamination. For daily use, narrow-mouthed jars or containers with water spigots prevent contamination from repeated contact with hands or utensils.


Sanitation and water treatment are inextricably linked. Personal hygiene, particularly handwashing, prevents spread of infection from food contamination during preparation of meals. Disinfection of dishes and utensils is accomplished by rinsing in water containing enough household bleach to achieve a distinct chlorine odor. Use of halogen solutions or potassium permanganate solutions to soak vegetables and fruits can reduce microbial contamination, especially if the surface is scrubbed to remove dirt or other particulates, but neither method reaches organisms that are embedded in surface crevices or protected by other particulate matter. Travelers to remote villages, wilderness areas, and disaster situations should assure proper waste disposal to prevent additional contamination of water supplies. Human waste should be buried 8–12 in deep, at least 100 ft from any water, and at a location from which water run-off is not likely to wash organisms into nearby water sources. Groups of 3 persons or more should dig a common latrine to avoid numerous individual potholes and inadequate disposal.

Backer HD, Derlet RW, Hill VR. Wilderness Medical Society Clinical Practice Guidelines for water disinfection for wilderness, international travel, and austere situations. Wilderness Environ Med. 2019;30(4S):S100–20.

Published March 4, 2020

Volume 37, Issue 1

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