Which condition requires all occupants to use supplemental oxygen?

The requirement for all occupants to use supplemental oxygen occurs at an altitude above 15,000 feet. This regulation is rooted in physiological considerations, as the partial pressure of oxygen decreases significantly with higher elevations, which can lead to hypoxia—a condition where the body is deprived of adequate oxygen.

At altitudes above 15,000 feet, the risk of hypoxia increases, and supplemental oxygen becomes essential for maintaining normal physiological function for everyone on board. This regulation helps ensure the safety and well-being of all occupants, not just the pilots, as everyone may be affected by the reduced oxygen levels in the atmosphere at those heights.

While provisions exist for supplemental oxygen use at lower altitudes—specifically above 12,500 feet for pilots and above 14,000 feet for passengers—the universal requirement at 15,000 feet emphasizes the importance of oxygen for all individuals in the aircraft to prevent adverse health effects.