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Inmountaineering, thedeath zone refers to altitudes above which the pressure ofoxygen isinsufficient to sustain human life for an extended time span. This point is generally agreed as 8,000 m (26,000 ft), where atmospheric pressure is less than 356 millibars (10.5 inHg; 5.16 psi).[1] The concept was conceived in 1953 byEdouard Wyss-Dunant, a Swiss doctor, who called it thelethal zone.[2] All14 peaks above 8000 m (the "eight-thousanders") in the death zone are located in theHimalaya andKarakoram regions of Asia.
Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly by the loss of vital functions or indirectly by poor decisions made under stress (e.g., not turning back in deteriorating conditions, or misreading theclimbing route), or physical weakening leading to accidents (e.g., falls). An extended stay above 8,000 m (26,000 ft) withoutsupplementary oxygen will result in deterioration of bodily functions and death.[3][4][5]
Thehuman body has optimal endurance below 150 m (490 ft) elevation.[6] Theconcentration of oxygen (O2) in air is 20.9% so thepartial pressure of O2 (PO2) at sea level is about 21.2 kPa (6.3 inHg; 3.07 psi). In healthyindividuals, this saturateshemoglobin, the oxygen-binding red pigment inred blood cells.[7]
Atmospheric pressure decreases with altitude while the O2 fraction remains constant to about 85 km (53 mi), so PO2 decreases with altitude as well. It is about half of its sea level value at 5,500 m (18,000 ft), the altitude of theMount Everest base camp, and less than a third at 8,849 m (29,032 ft), the summit of Mount Everest.[8] When PO2 drops, the body responds withaltitude acclimatization.[9] Additional red blood cells are manufactured; the heart beats faster; non-essential body functions are suppressed, food digestion efficiency declines (as the body suppresses thedigestive system in favor of increasing its cardiopulmonary reserves);[10] and onebreathes more deeply and more frequently. But acclimatization requires days or even weeks. Failure to acclimatize may result inaltitude sickness, includinghigh-altitude pulmonary edema (HAPE) orcerebral edema (HACE).[11][12]
Humans have survived for 2 years at 5,950 m (19,520 ft) [475 millibars (14.0 inHg; 6.89 psi) of atmospheric pressure], which appears to be near the limit of the permanently tolerable highest altitude.[13] At extreme altitudes, above 7,500 m (24,600 ft) [383 millibars (11.3 inHg; 5.55 psi) of atmospheric pressure], sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.[11][14][15]
In the death zone and higher, no human body can acclimatize. The body uses up its store of oxygen faster than it can be replenished. An extended stay in the zone withoutsupplementary oxygen will result in deterioration of body functions, loss of consciousness, and ultimately, death.[1][4][5] Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance tohypoxia in patients withchronic mountain sickness and normal fetuses in-utero, both of which present pO2 levels similar to those at the summit of Mount Everest.[16]
Mountaineers use supplemental oxygen in the death zone to reduce deleterious effects. An open-circuit oxygen apparatus was first tested on the1922 and1924 British Mount Everest expeditions; the bottled oxygen taken in1921 was not used (seeGeorge Finch andNoel Odell). In1953 the first assault party ofTom Bourdillon andCharles Evans used closed-circuit oxygen apparatus. The second (successful) party ofEdmund Hillary andTenzing Norgay used open-circuit oxygen apparatus; after ten minutes taking photographs on the summit without his oxygen set on, Hillary said he "was becoming rather clumsy-fingered and slow-moving".[17]
PhysiologistGriffith Pugh was on the1952 and 1953 expeditions to study the effects of cold and altitude; he recommended acclimatising above 4,600 m (15,000 ft) for at least 36 days and the use of closed-circuit equipment. He further studied the ability to acclimatise over several months on the1960–61 Silver Hut expedition to the Himalayas.
In1978,Reinhold Messner andPeter Habeler made the first ascent of Mount Everest without supplemental oxygen.
Several expeditions have encountered disaster in the death zone that led to multiple fatalities, including:
Consequences ofexternal causes | |||||
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| Temperature |
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| Radiation |
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| Oxygen | |||||
| Pressure | |||||
| Food | |||||
| Maltreatment | |||||
| Travel | |||||
| Adverse effect | |||||
| Other | |||||
| Ungrouped skin conditions resulting from physical factors |
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