Acute Mountain Sickness (AMS) The percentage of oxygen in the atmosphere at sea level is about 21%. As altitude increases, the percentage remains the same but the number of oxygen molecules per breath is reduced. At 12,000 feet (3,600 m) there are roughly 40% fewer oxygen molecules per breath so the body must adjust to having less oxygen. Altitude sickness, known as AMS, is caused by the failure of the body to adapt quickly enough to the reduced oxygen at increased altitudes. Altitude sickness can occur in some people as low as 8,000 feet, but serious symptoms do not usually occur until over 12,000 feet.
Mountain medicine recognizes three altitude categories:
- High altitude: 4,900 to 11,500 ft (1,500 to 3,500 m)
- Very high altitude: 11,500 to 18,000 ft (3,500 to 5,500 m)
- Extreme altitude: 18,000 ft and above (5,500 m and above)
In the first category, high altitude, AMS and decreased performance is common. In the second category, very high altitude, AMS and decreased performance are expected. And in extreme altitude, humans can function only for short periods of time, with acclimatization. Mount Kilimanjaro's summit stands at 19,340 feet - in extreme altitude.