The human body can do little to adapt to a cold environment. This is in contrast to the body’s ability to adapt and become more efficient when exercising in a hot, humid environment. Cold, wetness, and wind challenge the body to maintain core temperatures above 35oC (94oF). Heat loss or inadequate heat production elevates the risk of physical discomfort, hypothermia, or surface injury such as frostbite. Blood flow bears principal responsibility for maintaining peripheral temperature in cold weather and is the metabolic vehicle for the transportation of oxygen and generation of heat.
The one adaptable characteristic that can contribute to better tolerance of recreational activity in a cold environment is aerobic capacity (physical work). When working muscles and the thermoregulation system must compete for the same limited blood supply, reduced demand for the same level of work in persons with higher aerobic capacity can mean an increased margin of safety when temperature regulation becomes critical. A second advantage is that at the same workload, aerobically fit individuals derive a greater percentage of energy from stored fat. This is in plentiful supply even in the slimmest of individuals. Therefore, a lesser percent is required from the limited supply of carbohydrate foods, which need to be conserved in any survival situation in cold weather.
It can therefore be concluded that the ability to exist safely in cold, wet, or windy environmental conditions does not depend on a robust, adaptable body, but on mastery and use of information that enables self-preservation. Two major areas of information are critical: (1) knowledge of physiologic phenomena relative to exercise and temperature regulation and (2) knowledge of the insulation, ventilation, and protective properties of outdoor clothing and how to employ such clothing to gain the greatest advantage in a cold weather survival situation.
Management of Thermoregulation:
As metabolic machines, humans produce heat profusely during recreational activity. Heat energy increases as the rate of physical activity increases. Roughly between 80% and 90% of the energy produced is in the form of heat. Just sitting on the couch produces 60 to 70 kcal/hr, or a body temperature rise of 2oF if none of the heat is dissipated. A moderate hiking pace with a day pack could raise core temperature 8oF in an hour if the heat were not dissipated. Thus generation of metabolic heat can be a threat to proper thermoregulation. It is remarkably easy to overdress for activity in cold weather, to sweat needlessly, and to thereby lose heat rapidly. The adverse effects of sweating in a winter survival situation can be compounded by wearing clothing that sweat can permeate. This reduces garment temperature to that of a refrigerator. Clothing manufacturers have done a marvelous job of producing materials that preserve a warm microclimate for the body to maintain warmth at rest or at low levels of physical activity. However, most have not designed garments that can efficiently ventilate metabolic heat during more vigorous activity. To minimize the risk of this situation, a person surviving in a harsh environment must know what clothing is appropriate and how to use their garments correctly.
At rest body heat is lost primarily by radiation of body surface area. Radiant heat forms a barrier of warm air around a person, unless there is a breeze. In the presence of moving air or when a person is moving, significant amounts of heat are lost by convection. Loose fitting clothing pulled by body movement creates a bellows-like convection action of air between the skin and clothing, purging body heated air out, like smoke up a chimney. The neck, waistband, sleeves, pockets, and pant legs are the usual orifices. Using garments that have the ability to selectively loosen or close these “chimneys” to intentionally lose or conserve heat from the microclimate within the garment is always prudent.
Heat loss by conduction is the least frequent mode of transfer in a wilderness environment, although conductive heat loss occurs across the skin whenever it is in physical contact with matter that is 2 degrees C cooler or warmer. Some examples of heat conduction that occur in the outdoors include sitting on rocks, lying on the ground, or being in contact with clothing that has been cooled by evaporation of sweat or environmental moisture. Unquestionably, the most important mode of heat loss is through evaporation. A body engaged in physical activity of sufficient vigor to produce sweating will lose 70% of body heat loss through evaporative cooling. Because cooling occurs at the site of evaporation and, of most consequence, when evaporation takes place on the surface of the skin, the value of garments that can transfer, or “wick,” moisture away from the skin to be evaporated on outer layers of clothing is readily understood.
It helps to know the mechanisms of heat loss to critically evaluate the design and type of clothing material selected to be worn in a hostile environment. Being able to selectively control the amount of heat loss by evaporation and convection is the key to outfitting. Most important is the ability to regulate skin temperature in the trunk, where most sweat glands are located, the head and neck, and the areas of natural folds in the body such as the axillary (arm pits), crotch, and backs of knees. Using buttons, zippers, and Velcro fasteners and simply adding or shedding layers of clothing are methods by which to regulate heat loss. Despite manufacturer’s claims about product ventilatory capability, any activity of greater activity than walking requires conscious temperature regulation. The challenge is to maintain near normal core body temperature, to conserve body energy stores, and to lose body heat to the extent that sweating is minimal. This requires balancing clothing to be worn against expected climatic conditions and properly assessing the amount of physical activity that one will endure. All of these factors influence thermoregulatory balance.
Selection of Clothing:
Material properties important to outdoor activities: (1) THICKNESS. The thicker the material, the greater the insulative value, so long as it stays dry. (2) FIBER REACTION TO MOISTURE. Four qualities are important: (a) The ease of “wicking” action. Transferring moisture from body surface to material. (b) Evaporative ability. The rate of drying. (c) Moisture regain. The amount of moisture the material can absorb before it feels cold. (d) The amount of insulative value a material loses when wet. (3) THERMAL CONDUCTANCE. The less the conductance, the better the insulation. (4) RESISTANCE TO WIND.
The most commonly used clothing materials for outdoor activities are wool, cotton, nylon, polyester, and polypropylene. The four material properties are different for each of the fibers cited.
WOOL is a poor conductor of heat and therefore a good insulator. It has a moderate affinity to absorb moisture, but it can absorb a great deal, about 35% to 55% of saturation, before it feels wet. Its evaporative ability is poor, but its fiber suspends water vapor without decreasing its insulative value.
COTTON feels great in summer time, however has meager value in a harsh environment, where conservation of heat may be needed. Cotton loses up to 90% of its insulative value when wet. It readily regains moisture therefore its moisture regain is poor.
NYLON evaporates moisture quickly, is a good insulator, and has good quality of moisture regain. Because of its durability it is often the material preferred for outerwear. However unless nylon is tightly knit, it doesn’t screen wind and water well.
POLYESTER is justifiably the most widely employed material in outdoor clothing today. Polyester is a poor conductor (good insulator), high in moisture regain, and in some forms good in wicking.
POLYPROPYLENE, like cotton, wicks moisture well, but unlike cotton it has a very low conductive index and high evaporative qualities. These properties are what make it so popular as an under layer material for active outdoorsmen.
DOWN and SYNTHETIC LOFT material are not often appropriate for clothing to be worn by the physically active. They certainly have value when insulation is needed for quiet situations such as fishing, sitting around a camp fire, using a sleeping bag, or other relatively inert functions. The greater amount of “loft” possible in the material, the better the insulative value. There are other synthetic hollow-core fibers such as QUALLOFIL, THINSULATE (THERMALOFT), or POLARGARD that approach the insulative value of down, and are much less bulky, lose less insulative value when wet, and, being predominantly hydrophobic, dry more rapidly when wet.
Layering Clothing:
UNDERLAYER: Warmth and wicking ability are the principal requisites for layers next to the skin. Polyesters designed for moisture transfer and polypropylene best satisfy the needs of this layer. Some manufacturers have added a small percentage of Lycra to the polyester to achieve a consistent snugness to the skin. This enables the garment to be somewhat more effective in both insulation and moisture transfer. On days when the temperature is above freezing , the under layer may not be needed.
INSULATION LAYERS: Adequate insulation and ability to selectively ventilate are by far the most important characteristics of the insulative layers. When protection from wind and moisture is not necessary, an insulation layer may also be the outermost layer. Finding garments that are well designed for selective adjustment can be a challenge. Zippers or Velcro fasteners that vent areas around the trunk (core) are extremely important. Also ability to adjust tightness around waist, sleeves, and collar can augment the bellows action of clothing movement by providing a chimney for air circulation.
PROTECTIVE LAYER: wind and moisture can be serious challenges to thermoregulation, so protection against the elements and selective ventilation are the most important functions of the outer layer. Tightly knit, tough shells of nylon or webbed layers of nylon polyesters are the most popular materials for this layer. Gore-Tex laminate remains the gold standard for qualities of both water resistance and breathability. In vigorous activity performed in rain or wet snow, however, no garment will satisfy the weather because body heat production overwhelms the breathability of any material. Special finishes can be sprayed or laminated to polyester weave or microfiber garments to be used as outer layers which may be somewhat less expensive and less moisture repellent but the tradeoff would be for more breathability.
The wide variety of gloves made from polyester fleece, synthetic down, and wool, with a nylon outer cover are appropriate. Glove liners should be used when more insulation is needed. As with all cold weather clothing, gloves should not fit so tightly that peripheral blood flow is restricted.
Appropriate footwear remains a problem in cold environments. Boots are vulnerable to moisture and cold wherever they are stitched, although sealing compounds and waterproof tape can help. Instead of trying to keep moisture and cold out of the boot at the expense of sweaty feet, an alternative strategy may be to use breathable and less waterproof boots such as Gore-Tex or comparable sock liners with the intent of keeping the inner sock dry.
All for one and one for all.