Which atmospheric condition defines an oxygen deficient atmosphere?

• A. Any atmosphere that has an oxygen level below 19.5%.
• B. An atmosphere with oxygen at 21%.
• C. Any atmosphere that has an oxygen level above 23.5%.
• D. An atmosphere with oxygen below 18%.

Answer: (A)

Oxygen deficiency means there isn’t enough oxygen in the air to support normal breathing. In safety terms, this is defined by an oxygen concentration below 19.5%. Normal outdoor air is about 21% oxygen, so anything below 19.5% poses a risk to people in that space.

The statement that matches this standard threshold is the one describing an atmosphere with oxygen below 19.5%. A value of 21% is normal air, not deficient; an atmosphere with oxygen above 23.5% is oxygen-rich, not deficient; while an oxygen level below 18% is deficient, the established safety definition uses 19.5% as the cut-off.