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Altitude acclimatization is the process of adjusting to decreasing oxygen levels at higher elevations, in order to avoid altitude sickness. Once above approximately 3,000 metres (10,000 feet = 70 kPa), most climbers and high-altitude trekkers take the "climb-high, sleep-low" approach. For high-altitude climbers, a typical acclimatization regimen might be to stay a few days at a base camp, climb up to a higher camp (slowly), and then return to base camp. A subsequent climb to the higher camp then includes an overnight stay. This process is then repeated a few times, each time extending the time spent at higher altitudes to let the body adjust to the oxygen level there, a process that involves the production of additional red blood cells. Once the climber has acclimatised to a given altitude, the process is repeated with camps placed at progressively higher elevations. The general rule of thumb is to not ascend more than 300 metres (1,000 ft) per day to sleep. That is, one can climb from 3,000 (10,000 feet = 70 kPa) to 4,500 metres (15,000 feet = 58 kPa) in one day, but one should then descend back to 3,300 metres (11,000 feet = 67.5 kPa) to sleep. This process cannot safely be rushed, and this is why climbers need to spend days (or even weeks at times) acclimatising before attempting to climb a high peak. Simulated altitude equipment that produces hypoxic (reduced oxygen) air can be used to acclimate to high altitude, reducing the total time required on the mountain itself.
Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to intermediate altitudes (e.g., by aircraft and ground transportation over a few hours), such as from sea level to Template:Convert/ftTemplate:Convert/test/A as in many Colorado, USA mountain resorts. Stopping at an intermediate altitude overnight can alleviate or eliminate occurrences of AMS.