Aeromedical factorsQuestion 717 of 934

What is the percentage of oxygen in the atmosphere from the ground to high altitude?

  1. About 15 percent
  2. About 31 percent
  3. It falls by half at 18,000 feet
  4. About 21 percent
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Correct answer: About 21 percent

Oxygen stays at about 21 percent. Hypoxia at altitude is due to the reduced barometric pressure.

Source: Aeronautical Information Manual (FAA, Basic with Change 3, 9 July 2026), AIM 8-1-2 Effects of Altitude, page 641

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Questions last reviewed: 18 October 2026