What concentration is considered an oxygen-deficient atmosphere?

An oxygen-deficient atmosphere is defined as one in which the concentration of oxygen is below a certain threshold that is necessary for normal human respiration. The standard atmosphere for healthy oxygen levels in the air is approximately 20.9%. When the oxygen concentration drops below 19.5%, it is considered insufficient for safe breathing and can lead to symptoms of hypoxia. This level signifies that there is a potential risk for individuals entering such environments, making it critical for safety protocols and monitoring in potentially hazardous situations, such as firefighting or confined space entry.

The other provided concentrations do not indicate an oxygen-deficient atmosphere. At 20.5%, the air is still safe for breathing, and greater than 23.5% actually means there is an increased concentration of oxygen, which can pose a fire hazard rather than being deficient. Thus, the threshold of 19.5% is crucial for determining when an environment may become dangerous due to insufficient oxygen.