When is the atmosphere classified as oxygen deficient?

The atmosphere is classified as oxygen deficient when the oxygen concentration falls below 19.5 percent by volume. Under normal atmospheric conditions, the concentration of oxygen is approximately 20.9 percent. When levels drop below 19.5 percent, individuals may experience symptoms of oxygen deficiency, providing a threshold for safety in various environments, particularly in occupational settings such as firefighting and confined space entry.

This classification helps ensure that safety protocols are in place to prevent health risks associated with low oxygen levels, such as impaired cognitive and physical performance. Recognizing and responding to oxygen deficiency is critical in maintaining a safe working environment for firefighters and other emergency responders.