What type of oxygen is supplied when cabin altitude reaches lower safe levels?

When cabin altitude reaches lower safe levels, the system typically operates by supplying cabin air rather than pure oxygen or other alternatives. At higher altitudes, the partial pressure of oxygen decreases, requiring the use of supplemental oxygen to ensure passenger and crew safety. However, when cabin altitude is stabilized and returns to lower safe levels, the air in the cabin returns to normal atmospheric conditions, meaning that the air composition will consist of approximately 21% oxygen at sea level.

Cabin air only refers to the breathable air that mixes with the necessary levels of oxygen and nitrogen adjusted for pressure. Thus, at lower cabin altitudes within safe limits, the aircraft environment is maintained with cabin air that is safe for breathing by the passengers and crew.

Other options, such as pure oxygen, are typically reserved for emergencies or high-altitude conditions, where the oxygen levels would be insufficient for normal respiration. Similarly, compressed gas is also used in specific contexts and situations, especially where supplemental oxygen is required, such as high-altitude flights. Reserved oxygen is not a recognized term in standard aviation practices and does not describe any specific type of oxygen supply.