An oxygen-deficient atmosphere is characterized by an oxygen concentration of:

• A. 19.5% or greater
• B. 20.9% or greater
• C. 19.5% or less
• D. 21.0% or less

Answer: (C)

An oxygen-deficient atmosphere is defined by having an oxygen concentration of 19.5% or less. Under typical atmospheric conditions, air contains about 21% oxygen. When the oxygen concentration falls below 19.5%, it can pose significant health risks, leading to symptoms such as impaired cognitive function, shortness of breath, and in severe cases, loss of consciousness or death.

This threshold is important for safety personnel, such as those in the HAZMAT field, to recognize when an environment may become unsafe for work without proper respiratory protection. An oxygen concentration lower than 19.5% indicates that the atmosphere could be hazardous, necessitating specific precautions and measures to ensure safety during operations in such environments.