Normal breathing air contains what percentage of oxygen?

Normal breathing air contains approximately 21% oxygen. This concentration is essential for the process of respiration in humans and many other organisms, as it provides the necessary oxygen required for cellular metabolism. The composition of the atmosphere, which includes oxygen, nitrogen, carbon dioxide, and other gases, has been relatively stable, with oxygen levels averaging around this percentage. This balance is vital not only for human health but also for maintaining ecological systems.

The percentages of oxygen listed in the other options do not accurately reflect the typical composition of the air we breathe. For instance, 15%, 18%, and 25% would either indicate hypoxic conditions (too low for effective respiration) or an unusually high level of oxygen (such as in controlled environments where supplementation occurs) that is not found in natural atmospheric conditions. Understanding this composition helps firefighters and first responders ensure they are properly equipped and prepared in various environments, especially those with compromised air quality.