As difficult as it is to scale Mount Everest, coming back down from the world’s tallest peak is far more deadly, a new study shows.

Some 192 of the 212 deaths on the Himalayan mountain that occurred between 1921 and 2006 were above base camp, according to research in this week’s online edition of the British Medical Journal. Among climbers who died after scaling higher than 8,000 meters (26, 246 feet) above sea level, 56 percent succumbed on their descent from Everest’s 8,850-meter (29,000-foot) summit, and another 17 percent died after turning back. Just 15 percent died on the way up or before leaving their final camp.

Most of the deaths were among climbers – not sherpas who are native to the high altitude and have previously been found to have broader arteries and capillaries to deliver oxygen to their blood. While the sherpas who died did so more often at low altitudes — from falls or avalanches — for the climbers, the thin mountain air near the top proved fatal. They died mostly because the lack of oxygen caused blood vessels in their brains to leak fluid into the surrounding tissue, causing a fatal swelling called cerebral edema.

Study co-author Paul Firth, an anesthesiologist at Massachusetts General Hospital, said in a press release that he expected to find that most of the deaths would have been caused by avalanches and falling ice at low altitudes, and for higher-altitude deaths to be caused by pulmonary edema, or fluid in the lungs.

But R. Douglas Fields, chief of nervous system development and plasticity at the National Institutes of Health (NIH), tells that the findings aren’t surprising. "That’s actually a common rule in climbing that more people die coming down than going up," says Fields, who was not involved in the study. "You're spent getting to the top. You get tired, you're exhausted."

Cerebral edema, a feature of altitude sickness, is a known cause of death among oxygen-starved climbers and is curable only by descending, Fields adds. The fatal cases may have occurred higher on the mountain than lethal pulmonary edema, he notes, "because it is impossible to keep climbing if you can’t breathe. People with mental impairment might be able to keep climbing longer."

The overall death rate among the more than 28,000 people who tried to scale the mountain was 1.3 percent, or 212 people. The scientists – from Harvard, the University of Toronto and three British hospitals – couldn’t determine where on the mountain 13 percent of the deaths occurred because there weren't witnesses.

Image of Mount Everest by topgold via Flickr