An atmosphere is oxygen-enriched if the oxygen concentration exceeds what percentage?

• A. 23.5%
• B. 20.9%
• C. 19.5%
• D. 25.0%

Answer: (A)

An atmosphere is considered oxygen-enriched when the oxygen level is higher than about 23.5 percent. Normal air contains roughly 20.9 percent oxygen, so exceeding 23.5 percent changes how materials burn and ignitions occur more easily. This is the standard boundary used in safety regulations to distinguish enriched atmospheres. The other numbers describe either normal air (around 20.9%) or oxygen-deficient conditions (below 19.5%), while 25.0 percent is also enriched but not the defined threshold asked for in the question.