Altitude Illness

Altitude illness occurs because of a lack of oxygen at high altitudes.

As altitude increases, the atmospheric pressure decreases, thinning the air so that less oxygen is available. For example, compared with the air at sea level, the air in Denver (about 5,300 feet above sea level) contains 20% less available oxygen; the air in Aspen, Colorado (about 8,000 feet above sea level), about 25% less oxygen; and the air near the top of Mount Everest (more than 29,000 feet above sea level), about 66% less oxygen.

Altitude illness occurs when an oxygen deficiency at high altitudes causes fluid in the blood to leak from the smallest blood vessels (capillaries) into the surrounding tissues, resulting in swelling (edema). The forms of altitude illness differ mainly in their degree of severity and where in the body the fluid accumulates. Mild fluid accumulation in the brain causes acute mountain sickness, whereas more severe fluid accumulation in the brain causes high-altitude cerebral edema. Fluid accumulation in the lungs causes high-altitude pulmonary edema. Fluid accumulation in the hands, feet, and face causes high-altitude edema.

Altitude illness is common in people visiting high altitudes. The severity of the illness depends on how high and how quickly a person ascends. For example, most people who, within a day or two, ascend to above 6,000 feet develop high-altitude edema. Acute mountain sickness develops in 10% of people who ascend too rapidly above 8,000 feet, in 25% of people above 9,000 feet, and in almost 50% of people above 14,000 feet. High-altitude pulmonary edema and high-altitude cerebral edema rarely develop at altitudes below 10,000 feet.

People who normally live at sea level or at very low altitudes are more likely to be affected by altitude illness, as are those who engage in strenuous exertion soon after ascent. People with certain lung diseases (such as chronic obstructive pulmonary disease), heart and blood vessel disorders (such as angina, heart failure, or peripheral vascular disease), or blood disorders (such as sickle cell anemia or hemoglobin S-C disease) may have particular difficulties at high altitudes. Asthma, however, does not seem to be worse at high altitudes. Spending less than a few weeks at altitudes below 10,000 feet does not appear to be dangerous for a pregnant woman or the fetus. Physical fitness has no effect on a person's risk of developing altitude illness. Fewer older people than young people develop altitude illness. People who have had high-altitude pulmonary edema and high-altitude cerebral edema previously are particularly likely to develop those conditions again after ascent.

The body eventually adjusts (acclimatizes) to higher altitudes by increasing respiration and heart activity and by producing more red blood cells to carry oxygen to the tissues. Most people can adjust to altitudes of up to 10,000 feet in a few days. Adjusting to much higher altitudes takes many days or weeks, but some people can eventually carry out normal activities at altitudes above 17,500 feet.