What is the term for the percentage of oxygen in the air we breathe?

The term that accurately describes the percentage of oxygen in the air we breathe is known as fractional concentration. This term specifically refers to the ratio of the volume of a particular gas—in this case, oxygen—compared to the total volume of the mixture of gases in the atmosphere. Since the air we breathe is composed primarily of nitrogen, oxygen, and trace amounts of other gases, the fractional concentration helps in understanding the composition of air and is crucial in various fields, particularly in diving, where understanding the levels of oxygen can impact physiological responses and safety.

For instance, the fractional concentration of oxygen in Earth's atmosphere is approximately 21%, reflecting the proportion of oxygen relative to other atmospheric gases. This concept is essential for divers to ensure they are aware of the amount of oxygen they are breathing and to manage their gas mix effectively during dives.