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Cold water is a relentless killer. It knifes through clothing, overwhelms the insulating capacity of the body's fat and sucks the warmth out of the body's core. And water does not need to have ice to qualify as "cold water." A person can become hypothermic in 77-degree water. Most North American lakes, rivers and coasts are colder than that year-round.

A person immersed in cold water loses heat about 25 times as fast as someone exposed to cold air. When the Titanic sank, 1,500 people died within 90 minutes, yet there were ample life preservers to go around.A person's cooling depends upon several factors:

1. Body fat. Eskimos know what an excellent insulator blubber is. The fatter a person is, the slower cooling occurs.

2. Body type. Big people cool slower than smaller people. Children cool faster than adults. Women have more fat but are usually smaller, so they cool at the same rate as men.

3. Physical fitness. Cardiovascular fitness can help meet the stress of cold water immersion, but fit people usually have less subcutaneous fat for insulation.

4. Water temperature. The colder the water, the faster a person cools. The U.S. Coast Guard defines "cold water" as any below 70 degrees.

5. Clothing. Clothing can insulate and some types of fabric, such as wool, are better than others.

6. Alcohol. Drunks are more likely to get into dangerous situations. It impairs judgment and coordination. Research studies have found alcohol to be implicated in 10 percent to 50 percent of all drownings.

7. Behavior. Swimming and treading water increases the flow of warm blood from the body's core to the muscles, thus increasing the cooling rate. The swimmers often die first, since they are more likely to try to tread water or swim rather than float. Likewise, "drownproofing," a technique of bobbing in the water, will markedly increase heat loss as water circulates about the head.

A heat escape lessening posture (HELP position) has been devised, in which the victim draws the knees up close to the chest, presses the arms to the sides and remains as quiet as possible. For more than one person, huddling quietly and closely together will decrease the heat loss from the groin and front of the body. Both of these positions require life preservers.

Surviving long periods of submersion has been explained by the diving reflex found in all mam-mals. Researchers say that the diving reflex slows the heart rate, shunts blood to the brain and closes the airway. Recent research, however, suggests that the diving reflex is present in marine mam-mals (i.e., seals) but not in humans. If the diving reflex is discounted, the most likely explanation for prolonged submersion survival is that cold water produces hypothermia. This condition reduces the body's demand for oxygen and protects the brain.

Cold water immersion victims have to be handled very carefully. Move the victim as little as possible. Keep him or her horizontal, do CPR if necessary, and keep the victim from getting any colder by gently removing wet clothing, drying the skin, and keeping him or her out of the wind. Mild hypothermic victims can be rewarmed in the field, but severe hypothermics should, whenever possible, be transported by the EMS system and rewarmed under medical supervision.

Alton Thygerson is a professor of health sciences at Brigham Young University.