Effects As A Function of Altitude
The human body performs best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O2) in sea-level air is 20.9%, so the partial pressure of O2 (pO2) is 21.136 kPa. In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells.
Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about 100 km, so pO2 decreases exponentially with altitude as well. It is about half of its sea-level value at 5,000 m (16,000 ft), the altitude of the Everest Base Camp, and only a third at 8,848 m (29,029 ft), the summit of Mount Everest. When pO2 drops, the body responds with altitude acclimatization.
Mountain medicine recognizes three altitude regions that reflect the lowered amount of oxygen in the atmosphere:
- High altitude = 1,500–3,500 metres (4,900–11,500 ft)
- Very high altitude = 3,500–5,500 metres (11,500–18,000 ft)
- Extreme altitude = above 5,500 metres (18,000 ft)
Travel to each of these altitude regions can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high altitude pulmonary edema (HAPE) and high altitude cerebral edema (HACE). The higher the altitude, the greater the risk. Research also indicates elevated risk of permanent brain damage in people climbing to extreme altitudes. Expedition doctors commonly stock a supply of dexamethazone, or "dex," to treat these conditions on site.
Humans have survived for two years at 5,950 m (19,520 ft), which appears to be near the limit of the permanently tolerable highest altitude; the highest permanent settlement known is at 5,100 m (16,700 ft). At extreme altitudes, above 7,500 m (24,600 ft), sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.
Read more about this topic: Effects Of High Altitude On Humans
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