What percentage of air is oxygen?

The percentage of oxygen in the air is approximately 21%. This composition is a fundamental aspect of Earth's atmosphere, where air is made up of about 78% nitrogen and a mixture of other gases, including argon, carbon dioxide, and trace gases, making up the remainder. It is important for various physiological processes, including respiration in both humans and other aerobic organisms. Understanding the composition of air is crucial for fields such as diving, where changes in partial pressures of gases can significantly impact physiology and safety. This knowledge aids divers in recognizing how the available oxygen affects their body during dives at different depths.