Altitude Sickness


Altitude sickness is a general term encompassing a continuum of syndromes that occur at high altitudes because of a decrease in the available oxygen in the air.


Altitude sickness has been described by climbers since the 1800s as improvements in technology allowed mountaineers to climb higher and higher peaks. The severity of symptoms of altitude sickness varies with the individual and the altitude involved. Researchers find that mild physiological responses to increased altitude begin at around 5,000 feet. Although definitions vary somewhat, moderate high altitude is defined as height between 5,000 and 8,000 feet (1,524 m–2,438 m). High altitude is defined as between 8,000 and 11,500 feet (2,438 m–3,505 m). Very high altitude ranges from 11,500–18,000 feet (3,505–5,486 m), and extreme altitude is height over 18,000 feet (5,485 m).

Three major clinical syndromes fall under the heading of altitude sickness: acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE), and high-altitude cerebral edema (HACE). These syndromes are not distinct, individual syndromes as much as they are a continuum of severity, all resulting from a decrease in oxygen in the air. AMS is the mildest. HAPE and HACE represent severe, life-threatening forms of altitude sickness that require immediate treatment.

Altitude sickness occurs because the partial pressure of oxygen decreases with altitude. (Partial pressure is a term applied to gases in a way that is similar to how the term concentration is applied to liquid solutions.) For example, at 18,000 feet (5,486 m) the partial pressure of oxygen drops to one-half its value at sea level (0 ft). That means that air at 18,000 feet contains only half as much oxygen as air at sea level. The condition of lowered available oxygen is called as hypoxia. When there is less oxygen in the air, less oxygen reaches the bloodstream. This condition is known as hypoxemia. Hypoxia and hypoxemia form the basis for the medical problems associated with altitude sickness.

As a person becomes hypoxemic, the automatic response is to breathe more rapidly (hyperventilate) in an attempt to take in more oxygen. This attempt at alleviating the effects of the hypoxia at higher altitudes occurs during the first few days at increased altitude. About 20% of people ascending above 9,000 feet (2,743 m) in one day will develop altitude sickness. The individual may feel short of breath, notice that the heart beats faster, and after a day or two may urinate more. This is part of a process called acclimatization. At moderately high altitudes, most people acclimate, or get used to, the decreased level of oxygen in the air in a few days and these symptoms ease. People who live permanently at very high altitudes produce a hormone called erythropoietin that causes the body to make more red blood cells to carry oxygen around the body. This compensates for the decrease in oxygen in the air. However, producing more red blood cells takes weeks.

Risk factors

It is impossible to determine in advance who will develop altitude sickness. Physical condition appears to matter very little; all healthy people may be at risk, and risk increases altitude and the rapidity of the change in altitude. People with respiratory infections, chronic obstructive pulmonary disease (COPD), emphysema, anemia, and sickle cell disease are all more sensitive to hypoxia than healthy individuals. These people should be alert to symptoms of altitude sickness and avoid rapid ascents to higher altitudes, as they are more likely to develop altitude sickness.

At one time it was thought that young children were more likely to develop altitude sickness, but subsequent studies showed that they develop the syndrome at about the same rate as adults.


There are few good worldwide statistics about the number of people who develop altitude sickness syndromes, in part because the rate of ascent influences the development of symptoms. In addition, many people who develop headache and other early symptoms of AMS simply descend to a lower altitude and do not report the symptoms. A slow ascent with time to acclimate is less likely to cause symptoms than a rapid ascent. Altitude sickness develops independent of age and race in healthy individuals. Some studies suggest women are more susceptible to altitude sickness than men, but these studies were not definitive.

A condition in which there are too few red blood cells, too many abnormal red blood cells or too little iron-containing hemoglobin for normal oxygen transport in the body.
Pertaining to the brain.
Chronic obstructive pulmonary disease (COPD)—
Lung diseases, such as emphysema and chronic bronchitis, in which airflow is obstructed, causing labored breathing and impairing gas exchange.
Accumulation of excess fluid in the tissues of the body.
An abnormally low amount of oxygen in the blood, one of the major consequences of respiratory failure.
A deficiency in the amount of oxygen required for effective ventilation.
Pertaining to the lungs.
Sickle cell disease—
An inherited disorder characterized by a genetic flaw in hemoglobin production. (Hemoglobin is the substance within red blood cells that enables them to transport oxygen.) The hemoglobin that is produced has a kink in its structure that forces the red blood cells to take on a sickle shape, inhibiting their circulation and causing pain. This disorder primarily affects people of African descent.

Causes and symptoms

Acute mountain sickness (AMS) is a mild form of altitude sickness that results from ascent to altitudes higher than about 8,000 feet (2,438 m)—even 6,500 feet (1,981 m) in some susceptible individuals. Although hypoxia is associated with the development of AMS, the exact mechanism by which this condition develops is not clear. Researchers believe that hypoxic conditions cause the blood vessels in the brain to dilate and the brain to swell. This seems to happen to all people, but only some develop symptoms of AMS such as headache.

AMS often is reported to feel like a hangover. Symptoms include headache, dizziness, shortness of breath, nausea, vomiting, loss of appetite, and difficulty sleeping. Symptoms can develop within 2 to 24 hours of reaching a high altitude. People usually acclimate after several days if they remain at the same altitude. However, AMS can recur if individuals travel to an even higher altitude.

High-altitude pulmonary edema (HAPE) is a life-threatening condition that afflicts a small percentage of those who experience AMS. In this condition, fluid leaks from within the pulmonary blood vessels into the lung tissue. As this fluid begins to accumulate within the lung tissue (pulmonary edema), the individual becomes increasingly short of breath. HAPE is known to afflict individuals regardless of their level of physical fitness.

Typically, an individual who develops HAPE ascends quickly to a high altitude and almost immediately develops shortness of breath, a rapid heart rate, a cough productive of a large amount of sometimes bloody sputum, and a rapid rate of breathing. If no medical assistance is provided by this point, the individual goes into a coma and dies within a few hours.

High-altitude cerebral edema (HACE), the rarest and most severe form of altitude sickness, involves cerebral edema (severe swelling of the brain). The mechanism of HACE development is poorly understood. The symptoms often begin with those of AMS, but neurologic symptoms also develop, such as an altered level of consciousness, speech abnormalities, severe headache, loss of coordination, hallucinations, and even seizures. If no intervention is implemented, death follows.


Researchers use a scale called the Lake Louise Score to assess AMS. The scale rates five categories from zero (no symptoms) to three (most severe). The categories are headache, gastrointestinal symptoms (e.g., nausea, vomiting), fatigue and/or weakness, dizziness, and difficulty sleeping. A score of 3–5 indicates mild AMS. A score over 6 indicates more severe AMS.

In reality, the diagnosis of most altitude sickness is made from observation and the individual's self-reported symptoms during travel to higher altitudes.


Mild AMS requires no treatment other than an aspirin or ibuprofen for headache and avoidance of further ascent. Narcotics should be avoided because they may blunt the respiratory response, making it even more difficult for the person to breathe deeply and rapidly enough to compensate for the lower levels of oxygen in the environment. Oxygen may also be used to alleviate symptoms of mild AMS.

As for HAPE and HACE, the essential course of action is descent to a lower altitude as soon as possible along with oxygen therapy. Various drugs may be given to supplement descent, oxygen therapy, and continued monitoring of the individual are critical.

Public health role and response

Altitude illnesses are not considered a major public health problem. The U.S. Centers for Disease Control and Prevention (CDC) considers AMS a self-treatable condition. The CDC publishes guidelines for responding to AMS in the Yellow Book, which is published every two years as a health resource for international travelers.


The prognosis for mild AMS is good. If the individual descends to a lower altitude, symptoms usually disappear rapidly. Many people with mild AMS acclimate within a few days and do not need to descend to lower altitudes. The prognosis for HAPE and HACE depends upon the rapidity and distance of descent and the availability of medical intervention. Descent often leads to improvement of symptoms; however, recovery times vary among individuals. Without treatment, both HAPE and HACE can rapidly be fatal.


When individuals ascend from sea level, it is recommended that they spend at least one night at an intermediate altitude prior to ascending to higher elevations. In general, climbers should take at least two days to go from sea level to 8,000 feet (2,438m). After reaching that point, healthy climbers should generally allow one day for each additional 2,000 feet (610m), and one day of rest should be taken every two or three days. Should mild symptoms begin to occur, further ascent should be avoided. If the symptoms are severe, the individual should return to a lower altitude. Some reports indicate that acetazolamide (a diuretic) may be taken before ascent as a preventative measure for AMS.


Paying attention to diet can also help prevent altitude sickness. Water loss is a problem at higher altitudes, so climbers should drink ample water (enough to produce copious amounts of relatively light-colored or clear urine). Alcohol and large amounts of salt should be avoided. Eating frequent small, high-carbohydrate snacks (for example, fruits, jams, and starchy foods) can help, especially in the first few days of climbing.

The Institute for Altitude Medicine suggests that individuals can take acetazolamide (Diamox) 24 hours before arrival to altitude and for the first two two days at altitude to speed up the acclimatization process. The institute also reports some studies have found Gingko biloba taken five days before ascending to altitude helps prevent AMS. It is, however, less effective than acetazolamide. Individuals interested in medications to prevent AMS should consult their doctor or other knowledgeable health care professional.

See also Chronic obstructive pulmonary disease .



Hackett, David H., and Peter R. Shlim. “Altitude Sickness.” In 2012 Yellow Book. Atlanta, GA: Centers for Disease Control and Prevention, 2012.

West, John B., Robert B. Schoene, and James S. Milledge. High Altitude Medicine and Physiology. 4th ed. Oxford, UK: Hodder Arnold, 2007.


Harris, N. Stuart. “Altitude Sickness—Cerebral Syndromes.” . (accessed May 18, 2018).

Harris, N. Stuart. “Altitude Sickness—Pulmonary Syndromes.” . (accessed May 18, 2018).

Kale, Rahul. “Altitude-related Disorders.” . .(accessed May 31, 2018).


Institute for Altitude Medicine, PO Box 1229, Telluride, CO, 81435, (970) 728-6767,, .

U.S. Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA, 30333, (800) 232-4636,, .

World Health Organization, Avenue Appia 20, 1211 Geneva 27, Switzerland, 2241 791 21 11, Fax: 2241 791 31 11,, .

Kapil Gupta, MD
Revised by Tish Davidson, AM

  This information is not a tool for self-diagnosis or a substitute for professional care.