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Understanding and Preventing Hypothermia

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The Rule of 3s: A human being can survive more than three weeks without food, a bit more than three days without water, about three minutes without air and, in a cold environment, approximately three hours without clothing.

—Hal Weiss, Secrets of Warmth for Comfort and Survival, p. 109



A human’s greatest protection against the cold is his intellect. . .  Almost everywhere they live, humans are dependent upon intellectually devised clothing and shelter to insulate them from the environment and reduce heat loss to levels for which metabolism and physiology can compensate. Informed, intelligent behavior is even more necessary in the severe cold of high altitude and extreme latitude.

—James A. Wilkerson, M.D., ed., Medicine for Mountaineering, 5th ed., p. 263


Central Issues Addressed in This Article

What is hypothermia and what are the best ways to prevent it? What are the basic principles of heat loss and heat generation in the human body? What are the best ways to treat the early stages of hypothermia?

Definitions and Distinctions

Before getting into the meat of this article, a few definitions and distinctions are in order. First, a working definition:


Hypothermia is a decrease in the core temperature of the body that impairs intellectual, muscular, and cardiac function.

—James A. Wilkerson, M.D., ed., Medicine for Mountaineering, 5th ed., p. 263

The key phrase in this definition is “core temperature.” An important distinction is that between treatment and prevention. The primary subject of this article and of this part of the book is prevention. For recommendations about the treatment of a hypothermic patient, click on: “Wilderness Medicine.” Having made this distinction there is, however, one complication in trying to separate treatment from prevention: treatment of hypothermia in the early pre-hypothermic and mild hypothermic stages is essential to prevent more serious hypothermia. Consequently, suggestions for the treatment of early pre-hypothermic symptoms will be offered later in this article for consideration.

Another important distinction is the difference between being hypothermic and being cold and miserable. Don't confuse these two! Being hypothermic usually comes with a medical definition like the following: mildly hypothermic (core temperature between 95-97°), moderately hypothermic (core temperature between 91-94°), and severely hypothermic (core temperature below 90°). These delineations become quite important in the various treatment modalities. Because this article deals with prevention (of all of these states) and because of expediency, I will stretch the words “hypothermia” and “hypothermic” to cover the full range from being cold and miserable to being severely hypothermic (unless noted otherwise). 

One last distinction. Be careful not to confuse hypothermia with hyperthermia. These are opposites. Hyperthermia is the condition that results in heat exhaustion and heatstroke.

Principles of Bodily Warmth and Hypothermia Prevention

The depth and breadth of information on this subject is extensive and readily available. Instead of repeating a lot of what is available, the most useful thing, in this context of prevention strategies, is to share only the more important principles. These principles have been derived from a variety of sources (reading, discussions, personal experience); no one source has the full picture. Understanding the principles offered for consideration in this section will provide a big picture of hypothermia, if not the big picture. Having this big picture clearly in mind is essential for effective prevention. Some experience in applying the principles also will be useful.

Some of the principles are controversial, but most should be obvious and apparent when the subject is researched. Here are the most important.

Prevention: Prevention is the key. Prevention is relatively easy in most conditions for the well equipped and knowledgeable backcountry traveler. It is much easier to stay warm than to rewarm a body that has become chilled. Treating mild or severe hypothermia is difficult in the backcountry especially if the patient is unconscious.

Numerous Causes: The physiological causes of hypothermia (reduction in body core temperature) are many: getting wet, colder temperatures, wind chill, inadequate or improper layering of clothing, contact with cold surfaces, fatigue, injury or sickness, stress, inactivity, inadequate food, inadequate fluid intake, use of alcohol, lack of body fat, older age, genetics, metabolism. In addition to these physiological factors, the lack of knowledge (especially of heat loss and heat gain mechanisms) plays a central role. Ignoring any one of these factors can create problems. Ignoring several can create emergency situations.


 Quick Escalation of Symptoms: Pre-hypothermic signs and symptoms can come on gradually, but must be dealt with in the early stages because they usually escalate at a fast rate (especially mental deterioration). If exhaustion or good sense forces you to stop, you can slip into hypothermia in a matter of minutes. Stop and deal with the problem while you still have a reserve of energy.

Mechanics of Heat Loss: There are essentially five ways to lose body heat: conduction (contact with cold surfaces including air and wet clothing), convection (movement of air), radiation (infrared emissions), evaporation (perspiration or wet clothing), and respiration (combination of evaporation and convection from breathing). All but radiation are significant when thinking about strategies for preventing hypothermia while traveling in the backcountry. Radiation could be a factor if it is extremely cold outside or if you are lightly clothed, especially in the during a clear, cold night. All of these methods of heat loss come into play, to one degree or another, when your body is warmer than its environment. The colder the environment, the quicker the loss of heat.

Wind and Water Chill: It is important to distinguish between getting chilled by the wind (convection) and getting chilled by the evaporation of moisture from the body.  While evaporative heat loss is a desert hiker’s best friend, it is a cold weather traveler’s worst enemy. Moisture transfers heat away from the body at a much faster rate than air. The combination of wind and wet clothing accelerates evaporative cooling (wind will act to “refrigerate” wet clothing). However, the first line of defense when base layers are getting wet is to don wind-blocking layers to create a wet suit effect (i.e., warm but wet).

Above Freezing Temperatures:  Surprisingly, most cases of hypothermia develop in air temperatures between 30-50 degrees. In these temperatures, death can occur in a few hours. There are usually several causes of becoming hypothermic, but one is simply not being on guard for this condition. It can occur in midsummer!

Use the Buddy System: Recognizing the signs and symptoms of hypothermia is problematic; it is best to use a buddy system to look out for each other. Better yet, make the best-protected member of the party responsible for calling a halt before the least-protected member gets into trouble.

Core Temperatures: Core temperature is the key to prevention (maintaining normal core temperature), treatment (restoring to normal) and survival (keeping it above 90°). It is important to know the specific core temperature when effecting treatments for more severe hypothermia. External skin temperature is not a good indicator of hypothermia (i.e., the core can be below normal with the skin feeling warm or the reverse).

Calories to Raise Core Temperatures: Warmth from external sources (e.g., other bodies, the sun, fire) can’t be depended upon to create internal core warmth. Exercise, fluids and food are the key elements for generating body heat. However, the best source for maintaining and restoring normal core temperatures is calorie rich foods. The best calorie source for creating warmth quickly is simple carbohydrates (sugars). Hot food and drink can add some internal warmth, but that source of heat is not nearly as effective as the heat generated from the calories. As I look back on the few times I have been “pre-hypothermic” in the backcountry, it is when I have gotten behind schedule (e.g., to get out to the trailhead before dark) and decided not to stop to eat and drink.



Like most things of this nature, some of these currently accepted principles can and will change over time. For additional information on this topic, directly and indirectly related to the prevention of hypothermia, review the two articles on this website entitled “The Fine Art of Layering” and “Wet and Cold Weather Hiking.For an in depth and more technical exposition of this subject, read the following article at BackpackingLight.com: “Hypothermia: Avoiding and treating hypothermia in the backcountry with ultralight equipment,” by Darin Banner. (To view the full article requires an online subscription to BPL.) For a classic treatment of this subject consult Hal Weiss’ Secrets of Warmth For Comfort and Survival, The Mountaineers, 2nd ed., 1992.

Even though one purpose of this article is to synthesize available knowledge and expertise together in one place, the overriding purpose of sharing the “big picture” is to prevent pre- and actual hypothermic conditions in the field. To repeat the claim made at the beginning of this article: Having the big picture clearly in mind (i.e., understanding the basic principles) is essential for effective prevention.

Early Stage Treatment Strategies

In this article, I have focused mostly on prevention of hypothermia, not on diagnosis or treatment. However, treatment of the early stages (pre-hypothermic and mild hypothermia) is essential to prevent more serious hypothermia. Early stage treatment strategies should be obvious to the experienced hiker. But they are important enough to repeat: stop hiking, get sheltered from the elements, remove wet clothing, add dry insulation layers (including getting into a pre-warmed sleeping bag if you have one), do stationary exercises (e.g., sit-ups, push-ups, jumping jacks), eat calorie-rich foods, drink liquids and if possible prepare hot drinks and hot food. With these strategies in mind, I now carry the following on most fall and spring day hikes: emergency bivy, chemical body heat wrap, fuel tablet stove and pot set, set of long underwear. My current total of these lightweight hypothermia prevention gear items is 27.5 ounces (less than two pounds). During the winter, I sometimes add a 12-ounce lightweight quilt or a 9-ounce vapor barrier shirt to this kit. All of the above information is pertinent to early stage treatment. Expert medical knowledge and equipment is necessary for effective treatment of more advanced stages of hypothermia.

Additional Issues for Reflection

1.    Is hypothermia is the greatest single threat to health and well being when hiking and trekking in wetter and colder mountainous regions? Is hypothermia really more common in summer than in the other seasons?

2.    Assuming there are many causes for hypothermia, does lack of knowledge trump all the rest? If so, what area of ignorance is most prevalent regarding this subject?

3.    Assuming the core is warm, how important is it to keep the extremities warm as a hypothermia prevention strategy? Is keeping the extremities warm more important, equally important or less important than maintaining core temperatures? Is raising the core temperature the best way to warm the extremities?

4.    Is the most important clothing change for a wet patient in hypothermic conditions a set of dry undergarments (base layers)? In colder climates, should one always carry a change of long underwear?

5.    What about the claim by some manufacturers that their fabric (especially base layers) is temperature sensitive (“Provides cooling properties in warm conditions, warming properties in cooler conditions . . ..”)?

6.    How significant is the claim by some manufactures that their base layer fabrics have a “Push-Pull” mechanism whereby the inner fabric “pushes” moisture from the skin while the outer layer pulls it outside to dissipate? Stated technically, how important are untreated hydrophobic fibers (water repelling) on the inside next to skin with treated hydrophilic fibers (water attracting) on the outside?

7.    The claim is sometimes made that synthetic fabrics are preferred because they absorb less water, readily transport moisture from your body and dry quickly. Can Merino wool fabrics be manufactured to perform in a similar manner?

8.   Is Merino wool warmer than synthetic base layers when wet? Or does it just feel less clammy? Are wool/synthetic hybrids the best compromise?

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