Oxygen Deficient atmosphere is defined as any atmosphere containing less than what percent oxygen?

• A. 23.5%
• B. 15%
• C. 19.5%
• D. 21%

Answer: (C)

An oxygen-deficient atmosphere means there isn’t enough oxygen for normal breathing. Normal air has about 20.9% oxygen, and safety standards define oxygen-deficient conditions as less than 19.5% oxygen by volume. This cutoff is used because dropping below 19.5% impairs judgment, coordination, and consciousness, increasing the risk in places like confined spaces. So the idea is that any atmosphere with oxygen below 19.5% is considered deficient. The other numbers don’t fit: 23.5% is richer than normal air, 15% is far below the threshold, and 21% is close to normal.