Cold facts about exercise.

The sun was out, the temperature was a comfortable 20 [degrees]F, and there was a slight 5 mph wind blowing. Gary considered it a perfect day for cross-country skiing. After a couple hours of the strenuous activity he started sweating a bit, so he decided to take off his ski parka and ski only in his T-shirt.

As the day progressed, the temperature dropped down to 0 [degree]F, the wind picked up to 15 mph, and it began snowing. Gary put his ski parka back on, but he was still cold and shivering. His feet, hands, and face were starting to feel numb. Thinking it would help him warm up, he drank frequently from his brandy flask. Later in the day his friends persuaded him to continue skiing despite the fact that he was exhausted and his extremities were numb. Gary was fortunate to have survived that day.

Millions of exercise diehards fail to take special precautions when exercising in cold weather, and some, like Gary, get into serious trouble. Whether your sport is skiing, skating, cycling, walking, or jogging, exercising in the cold can be a safe and enjoyable experience if you take the time to plan ahead.

Temperature regulation.

Why are humans able to exercise in extremely cold weather without freezing to death? Two reasons: first, because we are intelligent enough to dress warmly and get out of the cold when we need to, and second, because our body is able to adjust its temperature.

Body temperature is regulated by means of receptors in the skin and muscles that are sensitive to changes in the ambient temperature. And we have a master regulating center in the brain that senses changes in the temperature of the blood. During exposure to cold weather these mechanisms work together to increase heat production in an effort to maintain body temperature.

Keeping body heat in cold weather.

* Shivering - Maximal shivering can increase the body's heat production up to five times.

* Exercise - Exercise that uses large muscle groups develops more body heat than shivering; however, exercise also takes considerable energy, so care must be taken to maintain a reserve of energy for use in emergencies.

* Protective vasoconstriction - During prolonged exposure to cold the body reduces blood flows to the extremities in an effort to maintain heat balance and to protect vital organs. Essentially, the body is willing to lose a few toes and fingers to frostbite in order to maintain internal body temperature.

* Body fat - Body fat serves as a layer of insulation beneath the surface of the skin. Those with more body fat are better insulated from the cold.

* Additional clothing - Adding more layers of clothing increases the body's insulation, thus helping to maintain body heat.

The most immediate threat from exposure to the cold is wind on the body's surface. Wind increases the degree of cold air molecules coming into contact with the skin so that heat loss is accelerated. When Gary began skiing in the late afternoon the temperature dropped from 20 [degrees]F to 0 [degree]F and the wind speed picked up from 5 to 15 mph. Because of the windchill factor, Gary was exercising in - 36 [degrees]F weather. Since he was wearing only a T-shirt and ski parka, he was not adequately prepared for such severe weather.

The windchill index chart outlines the interaction between temperature and wind speed. Use the chart as a guide to plan winter exercise sessions. If possible, begin exercising against the wind, and return with the wind to your back. Use extreme caution when deciding to exercise when the windchill index is in the Increasing Danger zone, and try to avoid exercise altogether if the index is in the Great Danger zone.

Cold-weather injuries.

Exposure to the cold and wind increases the risk of cold-weather injuries. The two most common are frostbite and hypothermia. Frostbite refers to skin that has been damaged from exposure to extreme cold. It usually occurs when skin temperature falls below - 20 [degrees]F. Gary was at risk for developing frostbite in the afternoon since the actual temperature was -36 [degrees]F because of the windchill factor. Exercising when the ambient temperature is above 20 [degrees]F poses little danger of frostbite.

Body parts most likely to be affected by frostbite include the ears, cheeks, nose, fingers, and toes. First the affected area turns white and/or pale and may have a tingling sensation, then the skin turns red, and the affected area becomes numb.

The immediate treatment for frostbite is immersing the affected area in warm (103 [degrees] to 105 [degrees]F water. Seek medical attention if sensation does not return or if the skin turns purple or black after it has been rewarmed.

Hypothermia occurs when your body begins to lose heat faster than it can produce it. In Gary's case, when the temperature with the windchill dropped down to -36 [degrees]F, the combination of inadequate clothing, fatique, and alcohol significantly increased his risk for developing hypothermia. The symptoms of hypothermia include intense shivering, followed by disorientation, apathy, hallucinations, and even euphoria. If someone exhibits the symptoms of hypothermia, get medical help immediately. If that is not possible, do the following:

* Get the victim out of the cold.

* Remove all clothing.

* Give the victim warm drinks, dry clothing, and cover him or her with blankets or a sleeping bag.

* Try to keep the person awake and alert.

* If additional dry clothing is not available, get the person into a sleeping bag with another person to provide warmth.

* Get medical attention as soon as possible.

Breathing cold air during exercise.

A myth regarding exercising in the cold is that lungs can freeze if cold air is breathed for a long period of time. The fact is that the human pulmonary system is extremely efficient at adapting to changing environmental conditions.

The nose, mouth, and trachea add water vapor to inhaled air, warm and/or cool it to body temperature, and trap any foreign matter (e.g., dust, yeast, and bacteria), as well as chemical pollutants in the air.

Because inhaled air is significantly warmed before it reaches the delicate smaller bronchi and other lung tissue, the danger of lung tissue freezing is very remote. To lessen the irritation of breathing in cold air, try wearing a scarf over your mouth or one of the more sophisticated cold-weather face masks.

The right clothing.

Clothing gives protection from the elements by increasing the insulating capacity of the body, and the right clothes will allow you to enjoy outdoor exercise despite very cold temperatures. It's best to dress in layers, so that pieces can be removed as the body heats up. When Gary took off his parka and skied in his T-shirt, he significantly reduced his body's insulation. Don't go from one extreme to another.

Layering should start with a good set of synthetic long underwear, such as those made out of polypropylene. The material actually draws perspiration away from the skin so that evaporative cooling does not increase body heat loss.

The next layer should be one or two long shirts or a sweater, followed by a waterproof windbreaker or jacket. It's important to remember that the insulating quality of clothing is dramatically reduced once it gets wet. Better-quality winter clothing provides insulation and still allows loss of water vapor.

Alcohol and cold weather.

Drinking alcohol when participating even in winter sports can lead to serious problems. Even moderate levels of alcohol in the blood cause dilation of the small arteries, allowing blood to flow more readily to peripheral vessels, especially in the skin, fingers, and toes. Since alcohol causes a greater supply of blood to be delivered to the surface of the body, heat is lost more rapidly from the skin.

In addition, alcohol can impair motor coordination skills, increasing the risk of potential injuries. Alcohol impairs judgment and critical thinking skills, which can cause a false sense of security about being cold.

Summary guidelines:

* Take time to plan ahead before exercising in cold weather.

* Make use of the many technological advances in winter clothing. Although "high-tech" clothing may cost more, it will allow you to have a safer and more enjoyable experience. Also, remember to dress in layers.

* Wear a scarf or winter mask over your mouth to lessen the sensation of the cold air.

* Don't drink alcohol; instead, drink lots of cold fluids to prevent dehydration.

* Be careful of your fatigue level. Fatigue is the fastest way to develop a case of hypothermia.

* Try to avoid facing the wind when exercising in the cold. If there is no alternative, protect as much of your exposed flesh as possible.

* Watch for changes in the weather. Always think ahead. [Tabular Data Omitted]

Scott Roberts, M.S., writes from Rocklin, California.