THURSDAY, Aug. 24 (HealthDay News) -- One in 10 mountain climbers die while trying to scale Mount Everest, at 29,035 feet (8,850 meters) the world's tallest peak.

Injuries and exhaustion are the leading causes of mortality. But altitude sickness is a significant threat -- and a factor that becomes increasingly deadly above 27,000 feet (8,300 meters) because too many climbers take too long to reach the summit.

That's the contention of a British doctor who was part of an Everest expedition this summer.

Fifteen climbers have died so far this year, a rate that's remained fairly constant over the years. But even for those who make it to the top, there's a one-in-20 chance they won't survive the return to the bottom.

Dr. Andrew Sutherland, a Wellcome Research Training fellow in the Nuffield Department of Surgery, in Oxford, said it seems that many climbers who suffer severe altitude sickness are in denial and keep pushing to the top of Everest. But, they move slower and slower as they go. And once reaching the summit, their determination dwindles and they fall prey to altitude illness and die on the way down, he said.

Sutherland's views were printed in an article in the Aug. 26 issue of the British Medical Journal.

Sutherland said it's a popular misconception that the deaths are limited to inexperienced climbers who pay $60,000 or more to be guided up the mountain. "Of all the people I knew who died, most had 8,000-meter climbing experience before. It has more to do with people exceeding their altitude ability," he said.

He stressed that the main reasons for deaths on Everest are injuries and exhaustion. But, a significant proportion of climbers die from altitude-related illness, specifically from high altitude cerebral edema (HACE) and high altitude pulmonary edema (HAPE). Both conditions are deadly if not treated by getting to a lower altitude, he said.

"The only way you can tell that someone is suffering from HACE or HAPE is that they are taking too long to get to the top of the mountain," Sutherland said. "When you look at a number of deaths this year and how long it took them to get to the summit, it was far too long."

Sutherland thinks that anyone moving slower than 1 to 1.5 hours per 100 vertical meters is going too slowly and should turn back.

Most of the deaths occur on the descent after having reached the summit. "Essentially, we are all slowly dying when we are going up to the summit, and we have a limited amount of time we can spend in the death zone," Sutherland said. "If we are not out of the death zone by a certain time, we will die up there."

"On our summit attempt we were able to help with HAPE at 7,000 metres, but higher up the mountain we passed four bodies of climbers who had been less fortunate," he wrote. "The last body we encountered was of a Frenchman who had reached the summit four days earlier but was too exhausted to descend. His best friend had tried in vain to get him down the mountain, but they had descended only 50 metres in six hours and he had to abandon him."

Sutherland said many deaths would not have occurred if the climbers had set a time limit and realized they were going too slowly. "Unfortunately, once they see the summit, they believe they still have a chance to get there," he said. "People push to get to the top and only then realize they haven't got enough left to get themselves down."

Even though many climbers have experience on peaks above 8,000 meters, "things start getting significantly harder over 8,300 meters and there aren't many peaks at that height that are as easy to climb as Everest," Sutherland said.

One expert agrees that climbing Everest is very different from other mountains.

"Although we have a good handle on how to prevent altitude illness in trekkers and climbers at altitudes up to 6,000 meters, the situation becomes less clear at higher altitudes, especially above 8,000 meters," said Dr. Ken Zafren, the associate medical director of the Himalayan Rescue Association and a clinical assistant professor of emergency medicine at Stanford University Medical Center.

Probably the majority of those who have died on Everest in recent years were quite experienced, but it's difficult to get experience above 8,300 meters without climbing Everest, Zafren said.

In addition, Zafren believes that many climbers are using the corticosteroid dexamethasone. "The potential of this medication to cause an altered sense of reality is well known, even at sea level," he said. "One climber who took dexamethasone on summit day felt that he could fly down, but talked himself out of it. We will never know how many climbers have fallen on the upper reaches of Everest because of similar, but more compelling, delusions," he said.