Altitude Sickness

Altitude sickness occurs at higher altitudes where it is harder for the blood to get an adequate supply of oxygen.

What Causes Altitude Sickness?

High altitudes have decreased atmospheric pressure, a condition under which humans inhale a decreased amount of oxygen * * that take time to return to normal. The body increases the number of red blood cells to increase oxygen-carrying capacity and alters red blood cells to increase their efficiency for conveying oxygen to body tissues per unit of time. It generally takes a few days for this process to occur at altitudes up to 10,000 feet. The higher the altitude, the longer the acclimatization process.

Did You Know?

During the 1968 Summer Olympics in Mexico City, athletes arrived up to two weeks early to help them become accustomed to the altitude. Mexico City is more than 7,000 feet above sea level, and many of the athletes live at much lower elevations. The athletes who had trained at a high altitude prior to the Olympics may have had an advantage over those who had trained at lower altitudes.

What Are the Risk Factors for Altitude Sickness?

Altitude sickness can affect anyone ascending to a high altitude, especially if the ascent is done too quickly for the body to adapt accordingly. Rate of ascent to the higher altitude is a key factor. The geographical point above sea level at which an individual experiences altitude sickness varies from person to person. Altitude sickness usually starts to affect people at an elevation of 7,000 feet to 9,000 feet above sea level, especially when the ascent occurs within one day. However, some people experience effects at an altitude as low as 5,000 feet. Physical condition and level of fitness are factors that affect the onset of altitude sickness, although being physically fit will not prevent an individual from developing altitude sickness. The amount of physical exertion placed on the body affects how much oxygen the body needs, making greater exertion a risk factor for altitude sickness when individuals are ascending. Altitude sickness is often seen in mountain climbers, trekkers, and skiers who travel to high-altitude areas. Previous episodes of altitude sickness and living at low altitudes predispose individuals to have an episode. Young children are most susceptible to altitude sickness. Maintaining hydration, eating frequent high-carbohydrate meals, avoiding high salt intake, and avoiding alcohol help prevent the occurrence of altitude sickness.

What Are the Symptoms and Treatment of Altitude Sickness?

Altitude sickness may occur in varying degrees of severity. The general set of symptoms includes headache, fatigue, nausea or vomiting, or dizziness. There may also be irritability, muscle aches and pains, and sleep disturbances. Symptoms are exacerbated by physical exertion and dehydration * . Upon reaching a high altitude, individuals usually take 6 to 10 hours to develop symptoms. Given a halt in ascent and rest from physical exertion, symptoms usually either resolve in a day or so or become worse. If altitude sickness increases in severity, the individual may experience confusion, difficulty walking normally, and even development of stupor * or coma * . Difficulty breathing and other respiratory problems may develop.

Risk Categories for Acute Mountain Sickness

Risk Categories for Acute Mountain Sickness
SOURCE: “Altitude Sickness.” Travelers' Health, Centers for Disease Control and Prevention, (accessed July 23, 2015). Table by Cenveo Publisher Services. © 2016 Cengage Learning ?.

Physiological Adaptation to Altitude Sickness

Acute adaptations

When first confronted with an increased altitude, the body automatically changes some of its normal processes to adapt in a process called acclimatization. The goal of adaptation to high altitude is to increase the availability of oxygen to the body tissues. One of the first adaptations is ventilatory acclimatization, which involves a change in breathing patterns. There is an increase in respiratory rate (the number of breaths taken per minute). This type of breathing, called hyperventilation, increases the amount of oxygen available to the tissues and increases the amount of carbon dioxide expired from the body. As the body releases more and more carbon dioxide in greater amounts than with normal breathing, a change occurs in the blood pH. As the blood pH becomes higher (more alkaline) than normal, the brain receives signals that tell it to keep the hyperventilation in check and not let it go on for too long. In the meantime, the kidneys excrete substances that help keep the body pH normal. This process may take up to four days and is improved by the use of acetazolamide.

Chronic Adaptations

In order to maximize the ability of the blood to carry oxygen to the tissues, the amount of hemoglobin present in the blood increases. This process requires weeks to accomplish. Red blood cells are made in the bone marrow and used to transport oxygen to the tissues. The more red blood cells present in the blood, the greater the oxygen-carrying capacity of the blood. In addition to an increase in the number of red blood cells, the change in pH of the blood also triggers an alteration in the red blood cell so that it has a greater than normal capacity to unload oxygen into the tissues.

How Can Altitude Sickness Be Prevented?

The best way to prevent altitude sickness is to ascend to high altitude gradually. For mountain climbers and hikers, a slow ascent allows the body time to acclimatize. When people drive or fly to a high-altitude location, making a slow ascent impossible, they should minimize physical activity for the first few days during the acclimatization process. Maintaining hydration, eating frequent high-carbohydrate meals, avoiding high salt intake, and avoiding alcohol help prevent the occurrence of altitude sickness. Acetazolamide is sometimes used preventively to avoid the occurrence of altitude sickness. The narrowing of blood vessels caused by lack of oxygen to tissue can be caused in part by endothelins (compounds in the body that cause blood vessels to constrict) and may potentially be prevented or greatly improved by endothelin antagonists * .


Books and Articles

Harrington, Rebecca. “Why Do You Feel So Awful at High Altitudes?” Science Line. February 26, 2015. (accessed June 16, 2015).


BBC News. “Altitude Sickness ‘Two Illnesses’ Says Edinburgh University Study.” March 31, 2014. (accessed July 9, 2015).

MedlinePlus. “Acute Mountain Sickness.” U.S. National Library of Medicine, National Institutes of Health. (accessed July 9, 2015).


U.S. National Library of Medicine, National Institutes of Health. 8600 Rockville Pike, Bethesda, MD 20894. Telephone: 301-4964000. Website: (accessed July 9, 2015).

* oxygen (OK-si-jen) is an odorless, colorless gas essential for the human body. It is taken in through the lungs and delivered to the body by the bloodstream.

* pH a logarithmic scale, from 1 to 14, used to describe the acidity or alkalinity of a solution or fluid. A pH level of less than 7 indicates an acidic solution while a pH greater than 7 indicates an alkaline solution.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

* tupor a state of sluggishness or impaired consciousness.

* coma (KO-ma) is an unconscious state, like a very deep sleep. A person in a coma cannot be awakened and cannot move, see, speak, or hear.

* prophylaxis (pro-fih-LAK-sis) taking specific measures, such as using medications or a device, to prevent infection, illness, or pregnancy.

* antagonist (an-TAG-oh-nist) is a chemical that acts within the body to reduce or oppose the effects of another chemical.

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

(MLA 8th Edition)