When ascending to high altitude, people often describe the air as being “thin.”  This perception is accurate; at high altitude there are fewer gas molecules per volume than at sea level. Travelers to high altitude are exposed to low atmospheric pressure (hypobaria) and low arterial oxygen levels (hypoxia).

Even though the atmosphere becomes less dense with elevation, the composition of the atmosphere remains stable from sea level to about 25 km. In other words, oxygen comprises 20.9% and nitrogen about 78.8% of air molecules from sea level to 25 km. At altitude, the oxygen molecules are just farther apart.

Most people experience some degree of shortness of breath when ascending to high altitudes, especially with exertion.  To compensate, they automatically breath faster and deeper to maintain normal arterial oxygen levels. This is known as the Hypoxic Ventilatory Response (HVR).  HVR is part of the normal acclimatization process that the body goes through to adjust to decreased atmospheric oxygen. However, some otherwise healthy people do not acclimate readily, and can develop serious or life threatening high altitude sickness.

Oxygen availability (per inspiration) decreases with elevation above sea level:

  • 17% less at 5000 feet
  • 20% less at 6000 feet
  • 26% less at 8000 feet
  • 31% less at 10,000 feet

High Altitude Sickness refers to an array of cerebrovascular and pulmonary vascular syndromes that result from a maladaptive response to hypobaric hypoxia. Statistics vary, but it appears that at least 20-30% of people who ascend above 8000 feet, in less than a day, develop some symptoms of high altitude sickness.  At 14,000 feet, that number rises to about 75%.

There is no single profile of the person who will experience high altitude sickness. It can affect both genders, all ages, and patients with different levels of physical fitness. Children appear to be the most susceptible. Incidents decrease slightly with age over 50 years.

There are 3 primary types of high altitude sickness: 

In addition, there are 2 other types of high altitude sickness that are less common:

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The three primary types of high altitude sickness include: AMS, HACE, HAPE.