Exercise in the cold presents a smaller risk for injury than exercise in extreme heat due to brief exposure, nearby shelter, protective clothing and the heat generated during exercise. There is more risk to exercising in the heat because we operate much closer to our boiling point.
Spectators at cold-weather events have more of a problem than the athletes. Cross-county skiing races have been contested at temperatures approaching minus-30 degrees F with no trouble for the athletes--not so for race officials and spectators.
In the cold, the body tries to preserve heat. Blood vessels in the skin narrow, diverting blood from the cool skin to the warmer depths of the body. Shivering--alternate coordinated contractions of opposing muscles to generate heat--is a protective mechanism. Hormones that elevate the metabolic rate--and thereby body heat--may be released. The most obvious response, goosebumps (piloerection), produces insignificant amounts of heat.
Layering clothing can help control heat when it comes to exercise. The closest layer to the skin should wick sweat away from the skin. Each additional layer traps a little air that is warmed by the body and helps keep us comfortable.
The layers nearest the skin should be as dry as possible. Any trapped moisture exposed to the cold will make us colder. As for the spectators, I am always amazed that news programs tell spectators to "layer up'" before going to watch a game in the cold. Any hunter knows that when you sit in the cold, wear a wicking garment next to the skin, then bulk up. A spectator who layers clothing like the athletes will get cold.
Athletes rarely need more than four layers on their torso and two on their legs, then gloves and a hat. If possible, the outer garment should have a zipper or button front. Adding or removing hats, gloves and opening or closing the outer shirt can control body temperature quite well. As a rule of thumb, dress for exercise as though the outside temperature was 10 degrees warmer than it really is.
Some common questions about cold-weather exercise:
Can the lungs ever freeze when exercising in the cold? No. The air warms very quickly on its way to the lungs.
How much heat is lost through the head? The lower the temperature, the greater the heat loss through the scalp. At rest, about 30 percent of body heat is lost through the head. During exercise, about 19 percent of heat loss is through the scalp. When out training or jogging, use a hat to keep heat in, and carry it as you get warm.