Over a period of a few weeks, the body responds by producing more red blood cells to carry more oxygen to the tissues. The effects of high altitude depend on how high and how fast a person ascends. Noticeable effects are few below 7,000 feet but are common above 9,000 feet after a rapid ascent. They are short of breath, have a rapid heartbeat, and fatigue easily. About 20 percent also have a headache, nausea or vomiting, and sleep disturbances.
Strenuous exertion worsens the symptoms.
Most people improve in a few days. This benign condition, which is seldom more than unpleasant, is more common in younger people and is less common with increasing age. High-altitude pulmonary edema, a more serious condition in which fluid accumulates in the lungs, may follow acute mountain sickness. The risk of developing high-altitude pulmonary edema is somewhat increased in residents of high altitudes, especially children, when they return to a high altitude after spending 7 to 10 days near sea level. People who have had a previous episode are more likely to have another, and even a mild respiratory infection, such as a head or chest cold, seems to increase the risk.