Air normally contains what percentage of oxygen?

Air normally contains approximately 21% oxygen. This percentage is significant because it is the optimal concentration for supporting combustion and sustaining life. Oxygen is critical for various chemical reactions, including those involved in fire behavior. When firefighters understand the concentration of oxygen in the environment, they can better assess the potential for combustion and the safety of an area during firefighting operations. The presence of sufficient oxygen is essential for effective ventilation tactics, influencing fire behavior and smoke development. Recognizing that standard atmospheric air holds around 21% oxygen helps firefighters make informed decisions about their approach to a fire scene and personal safety measures, including the use of breathing apparatus.