Effects of high altitude on humans

The effects of high altitude on humans are considerable. The percentage oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the 'death zone', where it is generally believed that no human body can acclimatize. The effects of high altitude on humans are considerable. The percentage oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above 8,000 metres (26,000 ft) as the 'death zone', where it is generally believed that no human body can acclimatize. The human body can perform best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O2) in sea-level air is 20.9%, so the partial pressure of O2 (pO2) is 21.136 kPa. In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells. Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about 100 km, so pO2 decreases exponentially with altitude as well. It is about half of its sea-level value at 5,000 m (16,000 ft), the altitude of the Everest Base Camp, and only a third at 8,848 m (29,029 ft), the summit of Mount Everest. When pO2 drops, the body responds with altitude acclimatization. Mountain medicine recognizes three altitude regions that reflect the lowered amount of oxygen in the atmosphere: Travel to each of these altitude regions can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE). The higher the altitude, the greater the risk. Research also indicates elevated risk of permanent brain damage in people climbing to extreme altitudes.Expedition doctors commonly stock a supply of dexamethasone, or 'dex,' to treat these conditions on site. Humans have survived for two years at 5,950 m (19,520 ft, 475 millibars of atmospheric pressure), which is the highest recorded permanently tolerable altitude; the highest permanent settlement known, La Rinconada, is at 5,100 m (16,700 ft). At extreme altitudes, above 7,500 m (24,600 ft, 383 millibars of atmospheric pressure), sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly. The death zone, in mountaineering, refers to altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as 8,000 m (26,000 ft, less than 356 millibars of atmospheric pressure). All 14 summits in the death zone above 8000 m, called eight-thousanders, are located in the Himalaya and Karakoram mountain ranges. The concept of the death zone (originally the lethal zone) was first conceived in 1953 by Edouard Wyss-Dunant, a Swiss doctor, in an article about acclimatization published in the journal of the Swiss Foundation for Alpine Research. Many deaths in high-altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents).In the death zone, the human body cannot acclimatize. An extended stay in the death zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness, and, ultimately, death. Open-circuit oxygen apparatus was tested on the 1922 and 1924 British Mount Everest expeditions; the bottled oxygen taken in 1921 was not used (see George Finch and Noel Odell). In 1953 the first assault party of Tom Bourdillon and Charles Evans used closed-circuit oxygen apparatus. The second (successful) party of Ed Hillary and Tenzing 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'. Physiologist Griffith Pugh was on the 1952 and 1953 expeditions to study the effects of cold and altitude; he recommended acclimatising above 15,000 feet (4,600 m) for at least 36 days and the use of closed-circuit equipment. In 1978 Reinhold Messner and Peter Habeler made the first ascent of Mount Everest without supplemental oxygen.