What percentage of oxygen is normally present in the air?

The correct answer, which indicates that 21% of oxygen is normally present in the air, reflects the composition of Earth's atmosphere. The atmosphere is primarily made up of nitrogen (approximately 78%), with oxygen making up about 21%. This percentage remains relatively constant under normal conditions and is essential for the survival of most life forms on the planet.

Understanding this composition is critical for firefighters, as the availability of oxygen significantly impacts fire behavior and human survivability in smoke-filled environments. A higher or lower oxygen concentration can alter combustion processes, making knowledge of the normal atmospheric percentages vital for effective firefighting and safety measures.