An atmosphere is considered oxygen deficient if it contains less than what percentage of oxygen by volume?

An atmosphere is classified as oxygen deficient when the oxygen concentration falls below a certain threshold that could pose health risks. The standard level of oxygen in the air at sea level is around 20.9% by volume. As the oxygen level decreases, the risk of physical effects due to oxygen deficiency increases.

Specifically, an atmosphere containing less than 19.5% oxygen is recognized as an oxygen-deficient atmosphere. This is the point at which individuals may experience symptoms of hypoxia, which could include impaired cognitive and motor functions. Therefore, the correct percentage defining an oxygen-deficient atmosphere is indeed 19.5%, which is crucial for maintaining safety standards in environments where adequate oxygen supply is necessary for health and safety.