The Cold, Hard Facts: Preventing Hypothermia
The ARS Sojourner
It's cool, but comfortable. You put up an antenna and settle back against a granite boulder to operate for a few hours. Or perhaps you're at the beach, or building a snow cave for a winter operating adventure in sub zero weather.
What do these situations have in common? They may put you at substantial risk for heat loss injury, also called hypothermia!
WHAT IS HYPOTHERMIA?
As warm-blooded creatures, we must maintain our body heat much higher than the ambient temperature. Even on a warm day, you can suffer serious injury due to loss of body heat.
By definition, hypothermia is the loss of two degrees Centigrade (about 3.5 degrees Fahrenheit) of core body temperature. Signs of mild hypothermia are first seen when the body core temperature drops to 96.8 F.
Muscle tone increases, blood pressure becomes elevated and the metabolic rate climbs. At 95 F shivering reaches its maximum. When the core temperature falls to 93 F a person will become confused or irritable, and will stagger and have slurred speech. His memory and judgement will be poor. He might undress himself or fail to perform simple essential tasks of survival.
More extensive heat loss injury can be fatal. "Moderate" hypothermia begins with loss of consciousness (90 F) and hypothermia is "severe" when reflexes and response to pain are completely absent (80 F) and the victim appears to be dead. A few lucky individuals have survived core temperatures as low as 60 F, but such events are rare.
WHO IS AT RISK?
The elderly and the very young, and those with poor health are at highest risk for heat loss injury. Diabetics and those with neurologic illness are more severely affected. Fatigue, sleep deprivation and dehydration increase the risk of heat loss injury, as do alcohol and sedative medications.
Acclimatization to the cold does NOT help protect against heat loss injury. It's good to be in excellent physical condition, but knowledge and preparation is far more important.
MECHANISMS OF HEAT LOSS
Understanding the four ways heat is lost from the body will help you prevent hypothermia:
Radiation: This is the simple "giving up" of heat to a cooler environment. Decreasing the amount of skin you allow to be exposed to the environment will help limit this loss. Pay special attention to your neck and head where blood vessels do not contract in response to cold. Wearing a hat and a scarf is essential!
Conduction: Heat will be conducted away from your body when you expose it to cooler objects, such as rocks, pavement, metal, water and snow. Minimize this loss by putting an insulator such as a chair, a sleeping pad, a wet suit, gloves and insulated boots between you and the things you touch.
Wearing wet clothing will increase your conductive heat loss by five times or more. Direct exposure to water increases your heat loss twenty-five times or more!
Convection: This is heat loss due to wind or movement. Your body radiates heat that remains nearby and surrounds you like a blanket. Wind and movement disperse this "heat shield". Putting up a shelter against the wind and wearing layers of loose clothing can help conserve your radiated heat.
Evaporation: This is by far the most important heat loss mechanism. It takes a tremendous amount of body heat to dry you by converting the sweat, water, or snow on your skin to water vapor. "Get dry!" and "stay dry!" is the best way to preserve your body temperature. Incidentally, you're much better off drinking water from an icy stream than eating snow because of the amount of body heat required to convert snow to water inside you.
CLOTHING
Dressing wisely is important to keep you safe and comfortable in the wilderness. You should think about heat regulation whenever you do something more strenuous than walk in the park. The hard work of climbing, snowshoeing, carrying a pack, or digging a snow cave will cause your body to generate an excess of heat that must be disbursed. Sweating is the result, but the moisture on your skin will cause rapid heat loss when you stop moving.
The Eskimos put it simply: in very cold weather "if you sweat, you die." Your clothing should be quickly adaptable to get rid of heat when exercising and to conserve it when you rest.
It is essential to stay dry! Some military units have soldiers strip off clothing to the skin while they dig snow caves, and then immediately put on dry clothing when the work is done.
It is helpful to think of your clothing in three functional layers:
Undergarments: These should provide insulation and be able to "wick" sweat away your skin. Snug fitting underwear that maximize contact with the skin are best.
A material such as polypropylene that "wicks" well but conducts heat poorly is ideal. A bit of Lyca adds some stretch for a snug fit.
Outer garments: Wear multiple layers of loose fitting clothing. Buttons or Velcro closures around sleeves, cuffs, waist and other openings will allow air movement to get rid of extra heat while you exercise. Quickly close up your sleeves etc. to maintain your body heat when you stop moving.
Cotton is a poor choice of material for outer garments. It absorbs moisture quickly, but loses up to 90% of it's value as an insulator when wet. Wool is a good insulator and can absorb a great deal of water without losing it's properties, but it dries very slowly. Goose down is a superb insulator, but it compresses and works very poorly when wet.
Thinsulate is a new, lightweight synthetic material that has excellent insulation properties and works well even when it's wet.
Protective outer garments: These should be made of tough, non-porous material such as nylon or one of the new polymers to keep out wind, rain or snow.
USE YOUR HEAD!
Heat loss injuries have been devastating to armies and civilians throughout history.
In modern times, knowledgeable outdoorsmen with average physical conditioning can survive and be comfortable over long periods of exposure to subzero weather.
Plan your hikes and adventures and give careful thought to heat regulation. Watch your companions for signs of heat loss injury and provide warmth and shelter for them immediately if they begin to shiver or show other symptoms of heat loss.
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Dr. Bob Armstrong, N7XJ, an avid outdoorsman, QRPer and practicing physician living in Manti, UT, is health and fitness editor of The ARS Sojourner.
barmstrong@mail.manti.com