In a confined space, an enriched atmosphere is defined as oxygen above which percentage?

In a confined space, an enriched atmosphere is defined as oxygen above which percentage?

• A. 19.5%
• B. 23.5%
• C. 21.0%
• D. 25.0%

Answer: (B)

In confined-space safety, the type of atmosphere is defined by the oxygen level. An oxygen concentration above 23.5 percent is considered enriched. Normal ambient air is about 21 percent, so crossing the 23.5% threshold signals enrichment, which increases fire and explosion risk and triggers stricter safety controls, monitoring, and ventilation requirements. Values around 19.5 percent indicate oxygen deficiency, while 25 percent is still enriched but simply demonstrates it can go higher; the standard definition uses the 23.5% cutoff.

In confined-space safety, the type of atmosphere is defined by the oxygen level. An oxygen concentration above 23.5 percent is considered enriched. Normal ambient air is about 21 percent, so crossing the 23.5% threshold signals enrichment, which increases fire and explosion risk and triggers stricter safety controls, monitoring, and ventilation requirements. Values around 19.5 percent indicate oxygen deficiency, while 25 percent is still enriched but simply demonstrates it can go higher; the standard definition uses the 23.5% cutoff.