At higher altitudes, the pressure of the air around you (barometric pressure) decreases so there is less oxygen in surrounding air. People can live comfortably at moderately high altitudes, but the body must make some adjustments, and this takes time. If you ascend to altitudes above 8,000 feet, you will be in danger of developing uncomfortable or dangerous symptoms from the change in altitude.
Symptoms of altitude sickness that are not life threatening are called acute mountain sickness. Mountain climbers on any high mountain and skiers in high-altitude locations such as Colorado are at risk of developing acute mountain sickness. Symptoms from acute mountain sickness improve if you descend to lower altitude quickly. For very mild symptoms, a delay before further climbing may be enough to allow symptoms to go away.
Acute mountain sickness is the least dangerous of several kinds of altitude illnesses that can occur. This sickness affects close to half of all people who begin near to sea level and climb to 14,000 feet of elevation without scheduling enough rest time.
Symptoms that develop at high altitude should be taken very seriously, since some altitude problems can develop into fatal illnesses. One dangerous reaction to high altitude is a condition called high-altitude cerebral edema (HACE), in which the brain accumulates extra fluid, swells and stops working properly. A related illness, high-altitude pulmonary edema (HAPE), can occur with or without warning symptoms that signal altitude sickness. HAPE causes fluid to enter the lungs. A type of altitude sickness called high-altitude retinal hemorrhage (HARH) can cause eye damage. Coma and death are the most serious consequences of altitude sickness.
Altitude sickness is more likely to occur in people who have a previous history of altitude sickness. It is more likely if you climb quickly, if you exercise vigorously during your first few days of altitude exposure, and if you have been living at low elevation prior to your climb. Obesity appears to increase the risk for altitude sickness. Genetics may also put some people at increased risk, particularly for high altitude pulmonary edema (HAPE).