More Americans are taking to the woods as backpackers and cross-country skiers invade the traditional domain of campers, hunters, and fisherman. And every year an estimated 600 die there -- needlessly.

Authorities say that of the mishaps that could befall anyone outdoors, the greatest killer is also the most preventable. It's hypothermia, the medical term for an excessive loss of body heat, more commonly described as death from exposure or freezing to death.

That last term is misleading, for temperatures needn't be freezing for hypothermia to take its toil. Many victims have "frosen" on sunny days in 50-degree temperatures.

Hikers caught in rain showers and canoeists dunked within a few feet of shore have succumbed to hypothermia. Because wet clothing loses up to 90 percent of its insulating power, a pleasant breeze can suddenly feel like an icy blast as the wind carries off body heat faster than it can be replaced.

The normal response when someone feels chilly is to exercise to get warm. Yet this burns up energy, and as soon as the person stops to rest the cold strikes with redoubled force.

As body temperature drops, defense mechainsms go into action, reducing circuilation in the arms, logs, and head in order to conserve warmth for vital organs in the trunk. This is why people first notice numbing cold in their hands and feet. This restricted blood flow to the brain leads, in turn, to euphoria and disorientation. With slurred speech, the victim may insist that "I'm fine, don't worry about me."

That's exactly when to worry, say park rangers and physicians. Hypothermia can develop so unobtrusively that by the time others notice it, the person may be close to death. A hypothermia victim needs to get into warm, dry clothes and into a warm room or beside a campfire. Warm drinks also help, but they should not contain alcohol. Alcohol, by constricting the blood vessels, only makes the condition worse.

If the sufferer is just barely conscious or even unconscious, he should be undressed and placed in a bed orsleeping bag that has been warmed, possibly by another person or with heated rocks.

Hypothermia strikes the unprepared. A few simple precautions, such as wearing proper clothing on an outing and packing extra garments to copy with a shift in the weather, can prevent it, authorities say.

Public health officials note that rain and immersion are not the only causes of damp chill. Fog, melting snow, or perspiration -- whether from hiking, jogging, or shoveling a snow-covered drive -- can lead to hypothermia if a person isn't warmly dressed. Their advice: Be prepared, be alert for the onset of a chill, and take prompt action to halt it.