Oxygen at High Altitude

At Everest’s altitude, oxygen still makes up about 21% of the air by volume; its fraction does not fall to 7%. But total atmospheric pressure is roughly one-third of sea-level pressure, so the partial pressure of inhaled oxygen is also about one-third of the sea-level value. At 8,000 m, reduced pressure—not a change in oxygen’s share of the mixture—limits oxygen availability. [1–3]