What is the standard composition of the earth's atmosphere in terms of oxygen concentration?

The earth's atmosphere is composed of approximately 21% oxygen by volume. This concentration is a result of various natural processes, including photosynthesis performed by plants, which release oxygen as a byproduct. The stable presence of this level of oxygen is vital for the survival of aerobic organisms, including humans, as it supports cellular respiration. The measurement of atmospheric oxygen concentration is a critical aspect of environmental science and atmospheric studies, reflecting its importance to life on Earth.

Understanding why 21% is the accepted figure is essential; reaching this number balances the need for oxygen in the atmosphere with other gases present, such as nitrogen (which makes up about 78%) and trace gases that encompass the rest. Variations from this standard can occur due to local environmental conditions, but the global average aligns closely with this figure.