Oxygen mask requirements begin at what altitude in feet MSL?

• A. 10,000 ft
• B. 15,000 ft
• C. 25,000 ft
• D. 35,000 ft

Answer: (C)

Oxygen is essential because as you climb, the air gets thinner and the amount of oxygen per breath drops, increasing the risk of hypoxia. To keep pilots capable during depressurization or high-altitude flight, oxygen masks are provided and a specific altitude is set as the point at which wearing them becomes mandatory. That threshold is 25,000 ft MSL, where the potential for rapid impairment is significant enough that donning the mask is required to maintain safety and performance.

Lower altitudes listed relate to other oxygen rules or system behaviors, but the rule emphasizing actual mask use comes into play at 25,000 ft, making that the best choice.