Mountain Sickness Underlying Cause
The cause of high altitude illness remains unclear, but it is probably caused by exposure to lower concentrations of oxygen in the atmosphere.
There are a number of physical effects on the human body when exposed to high altitudes. These are mainly due to a lower percentage of oxygen and lower air pressure. For example, in Denver, Colorado (1610 m), the air pressure is 17 percent less than at sea level, and the air contains 17 percent less oxygen than at sea level.
Ascending to a higher altitude decreases the amount of oxygen available to the body. When the ascent is too rapid, the body cannot compensate quickly enough. This results in high altitude illness.
Humans can adapt to lower oxygen levels at high altitude. This process of acclimatization generally takes days to weeks. Changes in breathing, red blood cell count, fluid balance, and cardiovascular function make this adaptation possible.
Physiologic Divisions of High Altitude
|High||4,900-11,500 ft||decreased exercise performance, increased ventilation|
|Very high||11,500-18,000 ft||extreme low oxygen during sleep & activity|
|Extremely high||>18,000 ft||physiologic decompensation in humans|
Continue to Mountain Sickness Types
- Basnyat B, Murdoch DR. High-altitude illness. Lancet. 2003 Jun 7;361(9373):1967-74. 
- Foster PP, Feiveson AH, Boriek AM. Predicting time to decompression illness during exercise at altitude, based on formation and growth of bubbles. Am J Physiol Regul Integr Comp Physiol. 2000 Dec;279(6):R2317-28. 
- Gallagher SA, Hackett PH. High-altitude illness. Emerg Med Clin North Am. 2004 May;22(2):329-55, viii. Review. 
- Gertsch JH, Basnyat B, Johnson EW, Onopa J, Holck PS. Randomised, double blind, placebo controlled comparison of ginkgo biloba and acetazolamide for prevention of acute mountain sickness among Himalayan trekkers: the prevention of high altitude illness trial (PHAIT). BMJ. 2004 Apr 3;328(7443):797.