Which fraction of oxygen in air is commonly used in safety calculations as the normal level?

• A. 25%
• B. 21%
• C. 28%
• D. 18%

Answer: (B)

The key idea is the baseline oxygen content of normal air. In safety calculations, the standard reference is about 0.21 by volume, meaning roughly one-fifth of the air is oxygen. This 21% baseline reflects the typical composition of dry air at sea level and provides a consistent standard for assessing oxygen-deficient or oxygen-enriched environments, sizing ventilation, selecting respirators, and evaluating fire and respiratory hazards. Real air can vary with humidity and altitude, but 21% is the conventional value used in most calculations and codes.