At what altitude does the requirement for 100% crew oxygen start?

The requirement for 100% crew oxygen begins at 14,000 feet. This regulation is in place due to the physiological effects of altitude on human performance and safety. As altitude increases, the partial pressure of oxygen decreases, which can lead to hypoxia—a condition where the body does not get enough oxygen. At 14,000 feet and above, this risk becomes significant enough to necessitate the use of 100% oxygen for crew members to ensure they maintain optimal cognitive and physical performance. It helps to mitigate any adverse effects that can impair the ability to operate the aircraft safely. Maintaining this oxygen requirement helps ensure the crew is alert and responsive, especially in situations that could require quick decision-making or emergency procedures.