According to NIOSH, an atmosphere is oxygen deficient when the oxygen content is less than:

• A. 17%
• B. 18.5%
• C. 19.5%
• D. 20%

Answer: (C)

Oxygen-deficient atmospheres are defined by a specific threshold of oxygen in the air. Normal ambient air contains about 21% oxygen, so when the oxygen content drops below 19.5% by volume, the environment is considered oxygen-deficient and poses breathing and safety hazards. This 19.5% mark is the boundary NIOSH uses to trigger protective controls and response.

So the value that represents the boundary is 19.5%. Readings below that level indicate deficiency, while a reading at or above it is not yet classified as deficient. The other options either fall below the threshold (more clearly deficient) or are above it (not deficient).