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Layer clothing, drink fluids to reduce chance of hypothermia

Anywhere in the world where there are cool or cold temperatures, people get cold. Most people know that it doesn't require subfreezing temperatures to get cold -- it can also happen during the summer.

Your body produces heat, much as a stove does. The "fuel" is the food you eat. It is changed by digestion into a variety of things, like sugars that are "burned" (metabolized) by your body to produce the energy you need to live. Much of that energy is heat, and most of that heat is produced in the liver and in the large muscles.If your core body temperature cools slightly -- to about 95 degrees -- you will shiver, get clumsy, be unable to speak clearly, and your ability to reason will deteriorate. At this point mild hypothermia begins, but you may not recognize it because you're cooled brain is not working well.

Our bodies lose heat in four main ways: radiation, conduction, convection and evaporation. You probably can remember from your own experience the feeling of each.

Radiation

This happens when heat is given off by a warm object. For example, it is the kind of heat you feel from the sun or when you hold your hand beside a fire or near a lightbulb.

You may have heard that up to 70 percent of your body heat can be lost through an uncovered head. While this depends on lots of variables, the point is worth stressing.

The scalp is well supplied with lots of heat-shedding blood vessels that don't constrict like the body's other peripheral blood vessels. The head lacks insulating layers of fat to keep the heat inside. Wear a hat when you start to feel chilled.

Conduction

This occurs when your body contacts something that is colder than you are. For example, the cold you feel when you hold a snowball in your bare hand is the rapid movement of heat by conduction from your hand into the snowball.

Since water conducts heat much more efficiently than air does, wet clothes bleed your body's warmth up to 25 times faster.

When you know you are going to be active in cold weather, it is important to anticipate sweat production. The key is layering: a hydrophobic inner layer, an insulation layer or two, plus a wind-proof outer layer. Maintain dry feet by applying antiperspirant. Avoid cotton -- it is a water-retaining heat-drainer. Stick with synthetics or wool for cold weather.

Convection

Convection is another name for air movement. Your skin warms a very thin layer of the air that lies against it. Moving air or water can snatch the heat from the skin faster than radiation or conduction. Wind quickly removes the layers of the warm air in clothes and whisks it away -- along with body heat.

Fleece jackets and pants won't keep you warm without a windproof layer over them. Wearing a windproof layer alone will keep you warm when a breeze kicks up.

Evaporation

Evaporation is the changing of water into vapor. When sweat evaporates or dries on your skin, it cools you. The risk is especially high in winter, though, because people are not as aware of their fluid intake -- or lack thereof -- during cold weather.

When you don't drink enough fluid, your blood thickens, and the sluggish blood can't get through your body fast enough to keep you warm.. Consequently, dehydrated people are more susceptible to cold injuries.

So drink plenty of liquids. Avoid coffee and tea because they are diuretics and will cause fluid loss.

Whether you are skiing in the winter or working outside in the summer, you can get cold and lose body heat which can make you susceptible to hypothermia.

Alton Thygerson, a professor of health sciences at Brigham Young University, is the National Safety Council's first aid and CPR author and technical consultant.