When is an airmass considered an "oxygen-deficient atmosphere"?

An airmass is classified as an "oxygen-deficient atmosphere" when the oxygen concentration drops below 19.5% by volume. This threshold is crucial for human safety and health because normal atmospheric air contains approximately 20.9% oxygen. When the oxygen concentration falls to levels below 19.5%, it can lead to impaired physical and mental function and can pose serious health risks, including the risk of loss of consciousness or even death in extreme cases.

Recognizing this specific threshold helps in assessing environments that may be hazardous, particularly in confined spaces or during operations where oxygen levels can be depleted, such as in certain industrial settings. Maintaining awareness of the oxygen levels is critical for ensuring the safety of individuals working in such environments.