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 at 19,000 feet (5,800 meters) contains only half the amount of oxygen. In Denver, which is located about 5,300 feet (1,615 meters) above sea level, the air contains 20% less oxygen.
Most people can ascend to 5,000 to 6,500 feet (1,500 to 2,000 meters) in one day without problems, but about 20% who ascend to 8,000 feet (2,500 meters) and 40% who ascend to 10,000 feet (3,000 meters) develop some form of altitude illness.
The organs most commonly affected by altitude illness are the
In the lungs, there is elevated pressure in the smallest blood vessels (capillaries). The capillaries also may leak fluid.
Effects of high altitude vary greatly among individuals. But generally, risk is increased by
Risk is greater in people who previously had altitude illness and in those who normally live at sea level or at very low altitude (below 3,000 feet [900 meters]). Young children and young adults also are probably more susceptible.
People who have disorders such as diabetes, coronary artery disease, and mild chronic obstructive pulmonary disease are not at increased risk for altitude illness. However, such people may have particular difficulties at high altitude because of the low oxygen levels (hypoxia). Physical fitness is not protective. Asthma does not generally seem to be worse at high altitudes. Also, spending less than a few weeks at higher altitudes (but below 10,000 feet) does not appear to be dangerous for a pregnant woman or the fetus.
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 nearly normal activities at altitudes above 17,500 feet (about 5300 meters). However, no one can fully acclimatize to long-term residence above that altitude.
Acute Mountain Sickness:
Acute mountain sickness is a mild form of altitude illness and is the most common form. It may develop at altitudes as low as 6,500 feet (2,000 meters). Symptoms usually develop within 6 to 10 hours of ascent and include headache and one or more other symptoms, such as light-headedness, loss of appetite, nausea and vomiting, fatigue, weakness, irritability, or trouble sleeping. Some people describe the symptoms as similar to those of a hangover. Symptoms usually last 24 to 48 hours. Occasionally, acute mountain sickness progresses to more severe forms of altitude illness.
High-Altitude Pulmonary Edema (HAPE):
HAPE usually develops 24 to 96 hours after a rapid ascent to over 8,000 feet (2,500 meters). HAPE is responsible for most deaths due to altitude illness. Respiratory infections, even minor ones, appear to increase the risk. Symptoms are worse at night and can quickly become more severe. Mild symptoms usually include a dry cough and shortness of breath after only mild exertion. Moderate symptoms include shortness of breath at rest; confusion; pink or bloody sputum; low-grade fever; and a bluish tinge to the skin, lips, and nails (cyanosis). Severe symptoms include gasping for breath and making gurgling sounds while breathing.
High-Altitude Cerebral Edema (HACE):
HACE is a rare but potentially fatal condition. People with HACE have headache, confusion, walking that is unsteady and uncoordinated (ataxia), and coma. These symptoms may progress rapidly from mild to life-threatening within a few hours.
Swelling of the hands, the feet, and, on awakening, the face is common. The swelling causes little discomfort and usually goes away in a few days.
Retinal hemorrhages (small areas of bleeding in the retina at the back of the eye) may develop after ascent to altitudes of 9,000 feet (2,700 meters). These hemorrhages are common above 16,000 feet (5,000 meters). People usually have no symptoms unless the hemorrhage occurs in the part of the eye that is responsible for central vision (the macula). In such cases, people may notice a small blind spot. Retinal hemorrhages resolve rapidly without causing long-term problems.
Doctors diagnose altitude illness based mainly on the symptoms. In people with HAPE, doctors can usually hear fluid in the lungs through a stethoscope. An x-ray of the chest and measurement of the amount of oxygen in the blood can help confirm the diagnosis.
Rate of Ascent:
The best way to prevent altitude illness is to ascend slowly. The altitude at which a person sleeps is more important than the maximum height reached during the day. On the first night, people should not sleep any higher than 8,000 to 10,000 feet (2,500 to 3,000 meters). Mountain climbers should sleep at that altitude for 2 to 3 nights before they sleep at any higher altitudes. Each day thereafter, sleeping altitude can be increased by about 1,000 feet (300 meters), although higher day hikes are acceptable as long as people return to the lower level for sleep.
People vary in their ability to ascend without developing symptoms. Thus, a climbing party should be paced for its slowest member. The pace of ascent should be slowed if symptoms of altitude illness develop.
Acclimatization reverses quickly. If acclimatized people have descended to low levels for more than a few days, they must once more follow a graded ascent.
Acetazolamide taken at the start of the ascent can reduce the likelihood of altitude illness. If taken after the illness has begun, acetazolamide may help lessen symptoms. Acetazolamide should be continued for a few days after ascent. Some doctors believe that dexamethasone can also reduce the likelihood of altitude illness and lessen its symptoms.
People who have had previous episodes of HAPE should be alert for any symptoms of a recurrence and descend immediately if this occurs. Some doctors also recommend such people take the drug nifedipine by mouth or inhaled bronchodilators for prevention of HAPE.
Avoiding strenuous exertion for a day or two after arrival may help prevent altitude illness, as may eating frequent, small meals that are high in easily digested carbohydrates (such as fruits, jams, and starches) instead of fewer large meals. People should drink plenty of noncaffeinated fluids. Alcohol and sedatives, which can cause symptoms similar to acute mountain sickness, should be avoided.
Although physical fitness enables greater exertion at altitude, it does not protect against any form of altitude illness.
People with acute mountain sickness must stop their ascent and rest. They should not ascend to higher altitudes until symptoms disappear. Most people with acute mountain sickness improve within a day or two. Acetazolamide may help relieve symptoms. Acetaminophen or nonsteroidal anti-inflammatory drugs (NSAIDs—see Pain: Nonsteroidal Anti-Inflammatory Drugs) help relieve headache.
If symptoms are more severe, supplemental oxygen should be provided through a face mask. If supplemental oxygen is unavailable, or if symptoms persist or worsen despite treatment, the person should descend to a lower altitude, preferably at least 2,500 feet (760 meters) lower.
People with HAPE should descend to a low altitude as soon as possible. Oxygen should be given if it is available. The drug nifedipine may temporarily help by decreasing blood pressure in the arteries to the lungs.
If HACE develops, the person should descend as far down and as soon as possible. Oxygen and dexamethasone should be taken.
When prompt descent to a lower altitude is not possible and people are seriously ill, a hyperbaric bag can be used to buy time. This device consists of a lightweight, portable fabric bag large enough to completely contain a person and a manually operated pump. The person is sealed tightly in the bag, and the bag's internal pressure is then increased using the pump. The increased air pressure simulates a decrease in altitude. The person remains in the bag for 2 or 3 hours. The hyperbaric bag is as beneficial as supplemental oxygen, which often is not available when mountain climbing, but is not a substitute for descent.
Last full review/revision February 2009 by John B. West, MD, PhD, DSc