Oxygen availability and altitude

The human body can perform 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 kilopascals (158.53 mmHg). In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells.

Atmospheric pressure decreases following the Barometric formula with altitude while the O₂ fraction remains constant to about 100 km (62 mi), so pO₂ decreases 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 which 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! The blood vessels in the lungs actually constrict when oxygen levels drop. This was only known recently however this was portrayed in the Quran 1400 years before it was discovered.