What is the typical cabin altitude range in modern pressurized aircraft, and how does it affect oxygenation?

• A. 2,000–3,000 ft; no effect on PAO2.
• B. 6,000–8,000 ft; lowers PAO2 and can worsen hypoxemia.
• C. 15,000–18,000 ft; improves oxygenation.
• D. 500–1,000 ft; drastically lowers PAO2.

Answer: (B)

The main idea here is how cabin pressure affects oxygen availability. Modern pressurized aircraft keep the cabin at about 6,000–8,000 feet equivalent. That means the ambient pressure inside the cabin is lower than sea level, even though the fraction of inspired oxygen stays about 21%. Because the alveolar oxygen tension depends on the ambient pressure (via the alveolar gas equation), lowering the surrounding pressure reduces PAO2, so the oxygen delivered to the blood falls and can worsen hypoxemia, especially in people with lung or heart disease.

In healthy travelers this drop is usually tolerated, but in those with limited oxygen reserve it can become clinically meaningful. The other suggested ranges don’t reflect how cabins are generally pressurized: a much lower altitude would imply more breathable air than is typical in flight, while a much higher altitude would impose greater hypoxemia risk than passengers usually experience. The 6,000–8,000 ft range best represents the usual cabin environment and its effect on oxygenation.