A concentration threshold used before entry into oxygen-enriched environments is 23.5%. Which option expresses this threshold?

• A. 23.5 percent
• B. 21 percent
• C. 19.5 percent
• D. 25 percent

Answer: (A)

Oxygen-enriched environments increase fire risk, and a specific concentration marks when space is considered enriched. That threshold is 23.5 percent, so the expression 23.5 percent matches this limit. Normal atmospheric oxygen is about 21 percent, so 23.5 percent represents a noticeable enrichment that changes how we treat the space and what precautions we take. The other values aren’t used for defining an oxygen-enriched environment: 21 percent is typical air, 19.5 percent is a cutoff used for oxygen deficiency concerns, and 25 percent is simply higher than the threshold but does not define the official limit.