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

Altitude	Elevation	Effects
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