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updated 1/16/2009 8:18:29 AM ET 2009-01-16T13:18:29

A deep freeze has much of the U.S. — from the Midwest to the Northeast — in an icy grip for at least the next several days. It's the coldest weather in years , according to the National Weather Service.

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When the temperature drops below 20 degrees Fahrenheit and the wind is 20 miles per hour or more, it takes only minutes for exposed skin to become frostbitten, explains orthopedic surgeon Dr. Edward V. Craig of the Hospital for Special Surgery in New York.

If you have to go outside, here's some advice on protecting you and your family from the extreme cold.

Hypothermia sets in when the core body temperature drops below 95 degrees. At that point, the brain can be affected, meaning the victim isn’t able to think clearly and loses coordination. Extremities such as the tip of nose, hands, fingers, feet and toes are the most susceptible to the cold. When frostbite sets in, the body tissues actually freeze, causing ice crystals to form and damage the cells.

Young children and the elderly are particularly susceptible to hypothermia and frostbite, as well as people with circulation problems, such as diabetes, and those who take drugs, such as alcohol, nicotine and beta blockers, which decrease blood flow to skin. Slideshow: The big chill

Children affected by hypothermia may shiver and become lethargic and clumsy. Speech may be slurred. If you believe a child has hypothermia or frostbite, don't rub the skin. Give him or her a warm beverage and cover with blankets or clothing. If numbness continues for more than a few minutes, call the doctor.

Hypothermia and frostbite are “easier to prevent than treat,” says Craig, noting that it’s important to dress appropriately and protect your head, hands and feet. “If you get wet, get inside,” says Craig.

Frostbitten areas may feel numb or hard (like a block of wood) and frozen. The skin appears waxy, white or grayish.

Dress in layers
The key to protecting against subzero temperatures is layering.

If you’re going outside, wool, silk or fleece layers are preferable. Avoid cotton because it doesn’t hold body heat well and doesn’t dry quickly if it gets wet. Also, be sure the outer layer of your clothing is tightly woven, or wind resistant.

Adults and children should wear:

  • a hat (a lot of body heat is lost through the scalp)
  • a scarf or knit mask to cover face and mouth
  • sleeves that are snug at the wrist
  • mittens (they are warmer than gloves)
  • water-resistant coat and boots
  • two pairs of socks (wool over cotton)
  • several layers of loose-fitting clothing

Shivering is an important early sign that the body is losing heat. If you can’t stop shivering, it’s time to head indoors.

Exercising in the cold
You don't have to stop exercising outside in the winter, but extreme cold puts an extra strain on the heart. If you have heart disease or high blood pressure, contact your doctor before shoveling snow or doing other hard work or rigorous exercise in the cold. Your body is already working harder to stay warm, so don’t push it.

If you’re jogging or doing other vigorous exercising outdoors, be careful about perspiration. Excess sweat can increase heat loss, so remove layers as you start to feel warmer, but don’t leave areas of skin exposed to the cold. Cold air can also trigger a bronchitis or asthma attack, so take extra precautions.

Sources: U.S. Centers for Disease Control and Prevention; American Academy of Pediatrics; National Institutes of Health

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