At high altitude, the air outside an aircraft still contains about 21% oxygen, just like air at sea level. The danger is that total air pressure is lower, so each breath contains fewer oxygen molecules. When too little oxygen reaches the brain and body tissues, a person can develop hypoxia.
This matters in aviation because hypoxia can reduce judgment, coordination, and consciousness before a pilot fully realizes what is happening.
Aircraft protect people from high-altitude hypoxia by using cabin pressurization, supplemental oxygen, or both. Time of useful consciousness is the short period after oxygen becomes inadequate during which a person can still make effective decisions and take corrective action. As altitude increases, this time becomes much shorter because oxygen pressure in the lungs drops quickly.
Understanding pressure, breathing, and oxygen delivery helps explain why altitude training, oxygen masks, and pressurization systems are essential for safe flight.
Key Facts
- Dry air is about 21% oxygen at sea level and at altitude, but total pressure decreases with altitude.
- Partial pressure of oxygen is P_O2 = 0.21 P_air for dry air.
- Fewer oxygen molecules per breath at altitude means less oxygen diffuses from the lungs into the blood.
- Hypoxia can cause headache, confusion, poor coordination, blue lips or fingers, and loss of consciousness.
- Time of useful consciousness decreases as altitude increases, from many minutes near 18,000 ft to seconds near 40,000 ft.
- Cabin pressurization raises cabin air pressure, while supplemental oxygen raises the oxygen available to the lungs.
Vocabulary
- Hypoxia
- Hypoxia is a condition in which body tissues do not receive enough oxygen to function normally.
- Partial pressure
- Partial pressure is the portion of the total gas pressure caused by one gas in a mixture.
- Time of useful consciousness
- Time of useful consciousness is the time a person can still perform helpful actions after oxygen supply becomes insufficient.
- Cabin pressurization
- Cabin pressurization is the process of keeping aircraft cabin pressure higher than the outside pressure at altitude.
- Oxygen saturation
- Oxygen saturation is the percentage of hemoglobin in the blood that is carrying oxygen.
Common Mistakes to Avoid
- Saying high-altitude air has much less than 21% oxygen is wrong because the oxygen fraction stays about the same, while total pressure and oxygen partial pressure decrease.
- Ignoring time of useful consciousness is dangerous because a pilot may have only seconds to minutes to recognize hypoxia and use oxygen at high altitude.
- Assuming deeper breathing fully solves hypoxia is wrong because low oxygen partial pressure can still limit oxygen diffusion into the blood.
- Confusing pressurization with adding pure oxygen is wrong because pressurization mainly increases total cabin pressure, while oxygen systems increase the oxygen supplied to the mask or breathing air.
Practice Questions
- 1 At sea level, air pressure is about 101 kPa. If dry air is 21% oxygen, what is the partial pressure of oxygen?
- 2 At an altitude where air pressure is 50 kPa, estimate the partial pressure of oxygen in dry air. Compare it with the sea-level value of 21.2 kPa.
- 3 Explain why a pilot at high altitude can become hypoxic even though the air still contains about 21% oxygen.