What is 1 fifth of the air?

One fifth of the air refers to the proportion of gases that make up the Earth's atmosphere. The atmosphere is composed of approximately 78% nitrogen, 21% oxygen, and small amounts of other gases like argon and carbon dioxide. Therefore, one fifth of the air would primarily encompass around 20% of the total atmospheric composition, which is roughly equivalent to the percentage of oxygen present. This indicates that oxygen is a critical component of the air we breathe.