A team of researchers examined Why Climbers Die On Mount Everest:
The overall mortality rate for Everest mountaineers during the entire 86-year period was 1.3 percent; the rate among climbers was 1.6 percent and the rate among sherpas was 1.1 percent. During the past 25 years, a period during which a greater percentage of moutaineers climbed above 8,000 meters, the death rate for non-Himalayan climbers descending via the longer Tibetan northeast ridge was 3.4 percent, while on the shorter Nepal route it was 2.5 percent.Factors most associated with the risk of death were excessive fatigue, a tendency to fall behind other climbers and arriving at the summit later in the day. Many of those who died developed symptoms such as confusion, a loss of physical coordination and unconsciousness, which suggest high-altitude cerebral edema, a swelling of the brain that results from leakage of cerebral blood vessels. Symptoms of high-altitude pulmonary edema, which is involved in most high-altitude-related deaths, were suprisingly rare.
“High-altitude cerebral edema symptoms were common among those that died, but signs of pulmonary edema, or excessive fluid in the lungs, were unusual” Firth says. “We also were surprised at how few people died due to avalanches and ice falls in recent years – those usually happen at lower altitudes, and overwhelmingly people died during summit bids above 8,000 feet – and that during descents, the mortality rate for climbers was six time that of sherpas.”
While the reduced mortality rate among sherpas during descent suggests that taking time to acclimatize to high altitude could improve climber survival, Firth notes that many other factors may be involved. “Most of the sherpas are born and live their lives at high altitudes, and the competitive process for expedition employment probably selects those who are best adapted to and most skilled for the work. So the ability of lowlanders to acclimate to these very high altitudes needs further investigation.”