What is the upper limit of the physiological zone where supplemental oxygen is not required?

The upper limit of the physiological zone, where supplemental oxygen is not required, is generally considered to be 10,000 feet. Above this altitude, the partial pressure of oxygen decreases, which can lead to a reduction in the availability of oxygen for the body to utilize, potentially resulting in hypoxia. While some individuals may tolerate altitudes up to around 12,000 feet without supplemental oxygen for a limited time, 10,000 feet is recognized as the standard threshold where supplemental oxygen becomes advisable, especially for prolonged exposure.

Understanding this limit is crucial for pilots and aircrew members to ensure they maintain optimal cognitive and physical performance while flying. Maintaining awareness of this physiological boundary helps in planning flight operations and ensuring safety at higher altitudes.