Which altitude is critical for the deployment of crew cigarette oxygen masks in emergencies?

The critical altitude for the deployment of crew oxygen masks during emergencies is 40,000 feet. At this altitude, the atmosphere contains insufficient oxygen to support normal physiological function, which makes the risk of hypoxia significantly high for crew members and passengers.

Deployment of oxygen masks is essential at this altitude to ensure that crew members can maintain cognitive function and physical ability to manage the aircraft and attend to passenger needs. Above this altitude, the partial pressure of oxygen drops to levels that can lead to rapid deterioration of mental and physical performance without supplemental oxygen. Thus, at 40,000 feet, it is crucial to have oxygen readily available to prevent life-threatening consequences.

In the context of the other altitude options, 20,000 feet, 30,000 feet, and 45,000 feet do not represent the same level of critical response requirement for crew safety. Specifically, at 20,000 feet and 30,000 feet, oxygen is not typically needed under normal operating conditions, and while 45,000 feet is technically above the critical threshold, the standard protocol designates 40,000 feet as the specific altitude at which oxygen deployment becomes absolutely necessary to counter the effects of hypoxia.