At what altitude must all passengers be supplied oxygen in an unpressurized airplane?

The requirement for supplying oxygen to passengers in an unpressurized aircraft is established for safety reasons related to the physiological effects of reduced atmospheric pressure at high altitudes. Specifically, at altitudes above 15,000 feet Mean Sea Level (MSL), it becomes necessary to provide supplemental oxygen to all passengers due to the risk of hypoxia, a condition that occurs when the body doesn't receive enough oxygen.

Operating at this altitude can lead to oxygen deprivation for individuals who are not acclimated or who may be sensitive to altitude changes. Supplemental oxygen ensures that passengers maintain adequate oxygen levels in their bloodstream and helps to prevent negative health effects such as loss of consciousness, impaired judgment, or other complications associated with low oxygen levels.

While there are regulations that mention different altitudes for crew members' oxygen requirements, the threshold for all passengers specifically is set at 15,000 feet MSL. Below this altitude, it is generally considered safe for passengers to be without supplemental oxygen, provided that they do not stay at high altitudes for extended periods without acclimatization.