Why must the cabin be pressurized during flight at altitude?

• A. To keep oxygen masks off
• B. To provide sufficient oxygen supply
• C. To prevent turbulence
• D. To maintain cabin humidity

Answer: (B)

At cruising altitude the outside air pressure is much lower, so even though the air is about 21% oxygen, the amount of oxygen per breath (the partial pressure of oxygen) is too low for normal body function. Pressurizing the cabin raises the interior pressure to roughly the equivalent of 6,000–8,000 feet, which increases the oxygen partial pressure inside the cabin. This makes breathing easier and ensures blood oxygen levels stay high enough to prevent hypoxia, keeping passengers and crew alert and functional. Humidity, turbulence, or simply having oxygen masks on or off are not the primary reason pressurization is required; the main purpose is to provide a breathable oxygen supply.