Until recently, few people other than those who climbed Mt. Everest had to worry about experiencing serious altitude sickness.

These days, however, there are more skiers, hikers and climbers going high in the mountains and many travel agencies are offering "adventure vacations" that include backpacking excursions in the Andes and treks in the Himalayas.

In the U.S., for example, skiers can ride lifts to over 11,000 feet above sea level. Hikers and climbers in the Rockies, the Sierra Nevada mountains and on the volcanic peaks of the Pacific Northwest can ascend above 14,000 feet. And trekkers in Nepal often follow trails that wind beyond 18,000 feet -- or nearly four miles above sea level.

Studies in Nepal and on Alaska's Mount McKinley have found that one in three people who travels to these altitudes will experience some form of mountain sickness; about three people in 100 will develop potentially life-threatening forms of the disorder.

"For the skier in the Colorado mountains, altitude sickness is usually benign. It can make you feel like you have a hangover for a couple of days," says Robert Schoene, a specialist in high-altitude medicine and critical care at the University of Washington School of Medicine. "But the people, for instance, who go to Nepal, who fly to 10,000 feet and who after two days of hiking are suddenly at 14,000 feet -- they can die." The reason they die is not that they go too high, says Schoene, it's that they go too high too fast.

At 10,000 feet, the body has to function with about a third less oxygen than it does at sea level. At 18,000 feet, the amount of oxygen available is half that at sea level. The body reacts to the falling oxygen levels with a complicated cascade of changes, not all of them completely understood. The respiratory and heart rates increase, and there are important shifts in the blood flow to the brain, lungs and other organs.

Given adequate time, the body can acclimate to extreme altitudes. Everest climbers, for example, can reach the mountain's 29,000-foot summit without using oxygen tanks but only after they have taken weeks to adjust. Too rapid an ascent, however, overwhelms the body's ability to adapt. If Everest climbers were flown straight from sea level to the summit, they would die of oxygen starvation.

Altitude sickness rarely develops below 7,000 feet. Between 7,000 and 9,000 feet, the symptoms usually are mild. But a lot depends on how rapidly a person ascends. A vacationer who flies from Washington, D.C., to Denver, then takes a plane to the ski resort in Aspen, has jumped in a few short hours from sea level to about 8,000 feet.

At this altitude, the rapid drop in oxygen causes the cerebral blood vessels to dilate and blood flow to the brain to increase. It is thought that the dilated blood vessels can cause headache and that the increased blood flow causes fluid to leak into the brain tissue, bringing on mild cerebral edema.

At 8,000 feet, a traveler might develop mild altitude sickness with headache, insomnia, loss of appetite and some nausea. These symptoms usually clear in a day or two, and doctors recommend rest, aspirin or acetaminophen for headache, small meals and the avoidance of alcohol or sedatives, which can blunt the normal physiological response to low oxygen.

Above 10,000 feet, the physiological changes are greater and the symptoms of acute mountain sickness are usually more severe. More important, at these altitudes, mountain sickness can develop into two particularly dangerous conditions: high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE).

In HACE, the brain swelling causes headaches and nausea, as it does in mild mountain sickness, but the symptoms may be more serious. The swelling also can cause subtle changes in personality. Victims of HACE at first may seem tired, withdrawn or apathetic, later to become confused and disoriented. They often lose coordination and stagger or stumble as they walk. If they do not descend, they quickly can become comatose.

Fluid collects in the lungs of HAPE victims, for reasons that are not well understood. As fluid accumulates, breathing becomes more difficult. Studies have found that as many as one in four trekkers in Nepal develops some degree of HAPE. They become short of breath even while resting and develop a dry, nagging cough, which is often worse at night. As pulmonary edema progresses, they begin to cough up frothy, bloody sputum, and as their blood oxygen levels fall dangerously low, their lips and fingernail beds turn blue. Both HAPE and HACE can kill within a few hours.Altitude sickness, however, is easy to prevent, according to physician Peter Hackett. He is the director of the Denali Medical Research Project that, in association with the University of Alaska, runs a high altitude physiology lab at 15,000 feet on Mt. McKinley.

Don't rush, Hackett advises. If flying to 9,000-10,000 feet, spend a day or two at that level to adjust before going higher. Above 10,000 feet, limit the ascent to 1,000 feet a day; with every 2,000 to 3,000 foot gain, spend two nights at that altitude.

For those who develop symptoms of mild mountain sickness, don't go any higher. "It's easy to diagnose acute mountain sickness," says Hackett. "It's just like a hangover. If you haven't been drinking and you feel like you're hung over, that's altitude." After a day or two of rest, the symptoms should clear and it's acceptable to continue the ascent, he says.

It's a different story with someone who shows signs of severe mountain sickness -- HACE or HAPE. Sufferers must descend immediately, even at night if necessary, says Hackett. "They don't have to go that far down. Even a descent of one or two thousand feet is often extremely helpful and can save a life. One rule of thumb is to go down to the last previous altitude where the person had no symptoms," he says. "They can often go back up after two or three days at lower altitude, but they have to proceed more slowly and watch for the early signs of illness."

In recent years, two controversial drugs to treat altitude sickness have become popular with climbers and trekkers. Acetazolamide, developed to treat high blood pressure, can prevent mild mountain sickness. "But people shouldn't get a false sense of security when they take it," says Schoene of the University of Washington. "Taking acetazolamide does not guarantee you won't get acute mountain sickness." He describes it as relatively safe, although people who are allergic to sulfa drugs shouldn't take it.

This is not the case with dexamethasone. One of the most powerful corticosteroids, or anti-inflammatory drugs, available, it has dangerous side effects. The drug blunts the immune system, rendering the body susceptible to infection. It often elevates blood sugar to diabetic levels and can cause ulcers, muscle weakness and psychosis. Even so, many climbers and trekkers have taken to using dexamethasone, which in many Third World countries can be bought without prescription.

Dexamethasone should be used only in emergencies, says Schoene. "Where dexamethasone has a real role is in the backpack of someone who knows how to use it. It should be used as a rescue drug. When you get someone on Mt. Ranier or Denali with true cerebral edema, you should give them dexamethasone and get them down."

Anyone who plans to travel to high altitudes should learn the signs and symptoms of mountain sickness, says Schoene. "No one should die of altitude sickness."

Michael McCarthy is a physician and writer in Seattle.

Spend a day or two at a safe level - below 9,000 feet - to adjust before going higher.

Above 10,000 feet, climb only about 1,000 feet per day.

At intervals of 2,000-3,000 feet of ascent, stop for two days to acclimate.

If symptoms appear, stop and rest until they clear up.

With severe mountain sickness, descend at once, even at night if necessary.