Concept
In mountaineering, the death zone refers to altitudes above a certain point where the pressure of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as , less than of atmospheric pressure). The concept was conceived in 1953 by Edouard Wyss-Dunant, a Swiss doctor, who called it the lethal zone. All 14 peaks above 8000 m in the death zone are located in the Himalaya and Karakoram of Asia. Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly by loss of vital functions or indirectly by wrong decisions made under stress, or physical weakening leading to accidents. An extended stay above without supplementary oxygen will result in deterioration of bodily functions and death. The human body has optimal endurance below elevation. The concentration of oxygen (O2) in air is 20.9% so the partial pressure of O2 (PO2) at sea level is about . In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells. Atmospheric pressure decreases with altitude while the O2 fraction remains constant to about , so PO2 decreases with altitude as well. It is about half of its sea level value at , the altitude of the Mount Everest base camp, and less than a third at , the summit of Mount Everest. When PO2 drops, the body responds with altitude acclimatization. Additional red blood cells are manufactured; the heart beats faster; non-essential body functions are suppressed, food digestion efficiency declines (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves); and one breathes more deeply and more frequently. But acclimatization requires days or even weeks. Failure to acclimatize may result in altitude sickness, including high-altitude pulmonary edema (HAPE) or cerebral edema (HACE). Humans have survived for 2 years at [ of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude.