page_title: Altitude sickness
section_title: Cause
hierarchical_section_title: Altitude sickness / Cause
caption_attribution_description: Mount Everest as seen from an aircraft from airline company Drukair in Bhutan. The aircraft is south of the mountains, facing north.
context_page_description: Altitude sickness, the mildest form being acute mountain sickness, is the negative health effect of high altitude, caused by rapid exposure to low amounts of oxygen at high elevation. Symptoms may include headaches, vomiting, tiredness, trouble sleeping, and dizziness. Acute mountain sickness can progress to high altitude pulmonary edema with associated shortness of breath or high altitude cerebral edema with associated confusion. Chronic mountain sickness may occur after long term exposure to high altitude.
  Altitude sickness typically occurs only above 2,500 metres, though some are affected at lower altitudes. Risk factors include a prior episode of altitude sickness, a high degree of activity, and a rapid increase in elevation. Diagnosis is based on symptoms and is supported in those who have more than a minor reduction in activities. It is recommended that at high-altitude any symptoms of headache, nausea, shortness of breath, or vomiting be assumed to be altitude sickness.
  Prevention is by gradually increasing elevation by no more than 300 metres per day. Being physically fit does not decrease the risk.
context_section_description: Altitude sickness can first occur at 1,500 metres, with the effects becoming severe at extreme altitudes (greater than 5,500 metres). Only brief trips above 6,000 metres are possible and supplemental oxygen is needed to avert sickness.
  As altitude increases, the available amount of oxygen to sustain mental and physical alertness decreases with the overall air pressure, though the relative percentage of oxygen in air, at about 21%, remains practically unchanged up to 21,000 metres (70,000 ft). The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen until stratospheric heights.
  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 acclimatization occurs. However, in extreme cases, altitude sickness can be fatal.
caption: Climbers on Mount Everest often experience altitude sickness.
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