What percentage of oxygen is present in the air?

The correct answer is that the air we breathe contains approximately 21% oxygen. This specific concentration is critical for supporting life, as it is the ideal level for human respiration. Oxygen's presence at this percentage allows for the efficient process of cellular respiration, which is essential for energy production in the body.

While the other options present varying percentages, they do not reflect the actual concentration found in Earth's atmosphere. For example, the figure of 15% is insufficient for sustaining life, as it would lead to oxygen deprivation. The 30% option is above the typical atmospheric level and could pose risks of hyperoxia, which can be detrimental to health in high concentrations. Lastly, 79% represents the approximate percentage of nitrogen in the atmosphere, which, while abundant, is not oxygen and does not support respiration.

Understanding the composition of air, particularly the percentage of oxygen, is vital for fire safety directors. The ability to recognize how oxygen levels influence fire behavior, as well as human health, is crucial in emergencies.