Causes
The percentage of oxygen in air, at 21%, remains almost unchanged up to 21,000 metres (69,000 ft). The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen. However, it is the air density itself, the number of molecules (of both oxygen and nitrogen) per given volume, which drops as altitude increases. Consequently, the available amount of oxygen to sustain mental and physical alertness decreases above 3,000 metres (9,800 ft). Although the cabin altitude in modern passenger aircraft is kept to 2,100 metres (6,900 ft) or lower, some passengers on long-haul flights may experience some symptoms of altitude sickness.
Dehydration due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.
The rate of ascent, altitude attained, amount of physical activity at high altitude, as well as individual susceptibility, are contributing factors to the onset and severity of high-altitude illness.
Altitude sickness usually occurs following a rapid ascent and can usually be prevented by ascending slowly. In most of these cases, the symptoms are temporary and usually abate as altitude acclimatisation occurs. However, in extreme cases, altitude sickness can be fatal.
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