What is Altitude Sickness?

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  • Written By: J. Dellaporta
  • Edited By: Niki Foster
  • Last Modified Date: 16 July 2019
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Altitude sickness refers to any of several illnesses whose symptoms appear above a certain elevation. All types of altitude sickness are brought on by the same condition - the reduced level of oxygen at high altitude - but vary in their seriousness.

At high altitude, barometric pressure - the pressure of the air over the earth - decreases. Consequently, every breath taken contains fewer molecules of oxygen, so one must breathe deeper and faster to obtain sufficient oxygen. In addition, the body must adjust its chemistry and fluid balance, including an increase in urination and an increase in red blood cell concentration to improve the blood’s ability to carry oxygen. This process is known as acclimatization, adjustment for the decreased availability of oxygen. Acclimatization is slow, taking place over days to weeks, after which the body can function comfortably at moderately high altitudes.

Altitude sickness occurs when the body fails to acclimatize. High altitude is defined as 5000 to 11,500 feet (1,500 to 3,500 meters), while very high altitude is 11,500 to 18,000 feet (3,500 to 5,500 m), and extreme altitude is anything above 18,000 feet. However, altitude sickness rarely occurs at elevations lower than 8,000 feet (2,500 m).


Altitude sickness with non-life-threatening symptoms is often called acute mountain sickness (AMS). It is the least dangerous form of altitude sickness and occurs in roughly half of all people who climb to high altitude with insufficient time to acclimatize. Symptoms of AMS include a headache, nausea or vomiting, dizziness or light-headedness, difficulty sleeping, lack of appetite, and weakness. Mild symptoms can be relieved by resting before going to higher elevation. More severe symptoms of this type of altitude sickness require descent to a lower altitude.

A more extreme form of altitude sickness develops after symptoms of AMS. High-altitude cerebral edema is characterized by severe headaches and vomiting, confusion, exhaustion, hallucinations, and an impaired thought process resulting in unusual behavior. For this reason, the patient often does not realize his or her symptoms have worsened; instead a traveling companion will notice.

High altitude pulmonary edema may occur with or without other symptoms of altitude sickness. A low oxygen concentration causes blood vessels in the lungs to tighten, resulting in a higher pressure in the lung arteries, causing fluid to leak from the blood vessels to the lungs. Symptoms include chest tightness, fatigue, inability to catch one’s breath, blue or gray lips or fingernails, coughing up pink fluid, and noises when breathing.

Some medications can be prescribed to prevent altitude sickness, but it is still best treated by changing altitude gradually, limiting exertion, and drinking plenty of fluids. At the onset of symptoms of altitude sickness, one should stop ascending until they are gone. More serious symptoms can become fatal if the patient does not descend to a lower altitude immediately.



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