Mount Everest is the world's highest mountain and a particularly desirable peak for mountaineers. Many tried to reach the 8,848 meters altitude, about 200 of them died trying. The cause of death is usually avalanche, injury from fall or ice collapse, exposure, or health problems related to conditions on the mountain.
The last 800 meters are the hardest to climb. At that point, the oxygen level is not sufficient to sustain human life. A human uses more oxygen than he can get by breathing the thin air of less than 356 millibars. An extended stay without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness and, ultimately, death.
Not strangely, this zone is called the death zone.