What is the normal percentage of oxygen in the air?

The normal percentage of oxygen in the air is 20.9%. This value is consistent across different environments and is recognized as the standard for atmospheric air composition at sea level. Oxygen is essential for combustion and respiration, making its concentration critical in both natural and operational contexts, particularly for fire service professionals who deal with hazardous materials and potential respiratory risks.

Options reflecting lower percentages, like 19.5%, indicate an oxygen-deficient atmosphere that could be hazardous to health, particularly in confined spaces or areas prone to gas displacements. Higher percentages, such as 21.5% and 22.0%, do not represent typical atmospheric conditions and can lead to increased fire hazards due to enhanced combustion properties of materials in an oxygen-rich environment. Understanding the standard concentration aids in assessing safety protocols and scenario planning for hazardous environments.