He tried to sleep, but every time he rolled over, he felt the fluid rise, and would often cough up some of the yellowish liquid.

"I felt like I was choking," Boyer, an emergency room physician in Portland, Ore., says of the experience now. "I knew I had to get back down to base camp before I drowned." The sickness was a crushing blow. Boyer, an avid climber who was considered by his peers as the best athlete on the American Medical Research Expedition in 1981, knew he had to get off the mountain. He also knew he was seriously ill and wouldn't reach the summit. To high altitude climbers, Boyer's sufferings are well known.

For at least a century, the world's tallest mountains have beckoned to climbers eager to bag their peaks. But even the very experienced, most of them exceptional athletes, know how quickly anyone can be incapacitated by ailments at high altitude, ranging from minor headaches and dizziness to severe bouts of nausea and vomiting that can sometimes lead to death.

Trekkers Beware

The boom in popularity of high altitude trekking over the last two decades has brought many people to high altitude who are unaware of its potential dangers. Not surprisingly, altitude sickness is common among travelers to the Himalayas, often befalling newbie trekkers despite their brightly-colored fleece jackets and newly buffed hiking boots.

The Merck Manual of Medicine says that 25 percent of those who travel to altitudes of 9,000 feet get some form of altitude sickness, and 50 percent over 14,000 feet.

And, as many skiers know, even as low as 7,000 feet — roughly the altitude of the ski resort city of Park City, Utah — mild cases of Acute Mountain Sickness or AMS can occur, usually as headaches that go away after a few days.

Getting High

It is when climbers go to extreme altitudes above 14,000 feet that severe cases of AMS and other high-altitude sickness can kill. While AMS is the most common, the two most feared problems are a build up of fluid in the lungs or brain, known as high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE), respectively.

"HAPE and HACE are the real killers," says Dr. Peter Hackett, professor of medicine at the University of Washington and an expert on high altitude physiology. Hackett says that while both ailments are dangerous, HACE is the least understood and the more dangerous of the two.

The reason, Hackett says, is that so little is known about the brain. "It's a whole lot easier to look down into someone's lungs than it is to crack somebody's skull open to look at their brain," he says.

While altitude sickness can affect anybody, Hackett's studies suggest that some people are more susceptible than others. "There's definitely a genetic component," says Hackett. But even Nepal's legendary Sherpas, who regularly live at altitudes as high as 14,000 feet, can be afflicted by altitude sickness.

Hackett says scientists are only beginning to understand what happens to the body and the brain at altitude. What they do know is that above about 8,000 feet the decrease in air pressure causes a person's capillaries to expand, and in some cases leak, causing a build up of fluid in a part of the body such as the lungs or the brain.

This can lead to extreme pain, dementia and death. At 10,000 feet, the atmosphere is only 50 percent of that found at sea level. Above 25,000 feet, an altitude ominously known as the "death zone," the atmosphere is just 25 percent of that found at sea level. The summit of Everest stands at 29,035 feet or 5.4 miles above sea level.

Outfitter Awareness

Dr. Brownie Schoene, professor of medicine at the University of Washington who studies altitude sickness, says that one reason altitude sickness is so common at high altitude is that many people aren't educated about how to deal with it.

While many well-established guiding outfits make stops at lower altitudes to allow clients to adjust to altitude, known as acclimatization, Schoene says some of them still try to get people up to the top of a mountain as quickly as possible, which is a recipe for getting ill.

"It's better than it used to be," Schoene says. "But for the most part people don't have a clue."

Gordon Janow, program director for Alpine Adventures International in Seattle, says guiding outfits, particularly those in the United States, are well aware of altitude's dangers, and that their clients' health is always a top concern.

"We're constantly watching our clients, keeping tabs on how they're feeling," he says.

Schoene says the most important thing is to be aware of the symptoms of altitude sickness and, if symptoms occur, to drop to a lower altitude and wait for them to disappear. Persons with HAPE — men get it five time more frequently than women — often experience an irritating cough that produces frothy, often bloody sputum.

People with HACE can experience dizziness, extreme tiredness, nausea and a loss of basic motor skills, especially in the hands. They often exercise poor judgment which can be particularly dangerous because it can lead people to ignore or deny their condition or to lose their footing and fall into a crevasse.

Treating the Symptoms

While the best treatment is to reduce altitude, there are some immediate ways to treat altitude sickness.

A drug called Diamox is often prescribed for HAPE and acts by stimulating breathing, so the sufferer takes faster and deeper breaths, thereby getting more oxygen. Another, called Decadron, reduces tissue swelling and can be effective for HACE. Hackett also says new, unconfirmed studies suggest that Viagra can be an effective means of treating both HACE and HAPE.

It's important to remember that these drugs can reduce symptoms but, Hackett warns, should not be a substitute for reducing altitude.

Also, new devices such as or hyperbaric chambers known as Gamow bags are provided by some guiding companies. Gamow bags are collapsible, portable chambers made of fabric. A person afflicted with AMS can be placed inside of the bag and the pressure can be increased by another climber operating a foot pump.