What is the normal percentage of oxygen in the air?

• A. 11%
• B. 14.70%
• C. 19.50%
• D. 21%

Answer: (D)

The normal percentage of oxygen in the air is 21%. This value is well-established and is critical for understanding many aspects of fire behavior, as well as the requirements for human respiration. In the Earth’s atmosphere, oxygen makes up a significant portion of the air we breathe, which is essential for the survival of aerobic organisms and also plays a vital role in combustion processes.

Understanding that air consists of approximately 78% nitrogen and 21% oxygen, along with trace amounts of other gases, helps fire service personnel assess both the environment they are operating in and the potential for fire and combustion. Oxygen levels below 19.5% are considered oxygen-deficient and can lead to hazardous conditions for personnel, while levels higher than 21% can increase fire risk dramatically. Therefore, the 21% figure is a fundamental benchmark for both firefighting and emergency medical operations.