The main issue is hypothermia, which is excessive loss of body heat, explained Dr. Cedric X. Bryant, chief science officer at the American Council on Exercise.
Layers of clothing keep you warm and give you the ability to control your body heat while exercising. A hat or helmet is crucial, because you can lose about 50 percent of your body heat through your head when the temperature is at the freezing mark.
Wear gloves and warm footwear. It can be difficult to keep hands and feet warm when exercising in the cold. Lower air temperatures cause the body to shift blood away from the extremities to the center of the body to warm and protect the internal organs, Bryant said in a council news release.
While superficial warming of the hands will restore normal blood flow, this does not occur in the feet unless the temperature of the torso is normal or slightly higher than normal. This means that, to keep your hands and feet warm, you also have to keep the rest of your body warm at all times, Bryant explained.
Before you go outside to exercise, check the air temperature and wind chill factor. If your skin is properly covered, there is little danger when the temperature is 20 degrees Farenheit, even if there is a 30 miles-per-hour wind, according to National Safety Council data cited in the release.
However, exposed skin is in danger when the wind chill factor (a combination of air temperature and wind) falls below minus 20 degrees.
If you're exercising in the danger zone for skin exposure, you should wear a scarf or mask over your nose and mouth, to warm the air before you inhale it, Bryant advised.