What percentage of air is comprised of oxygen?

The correct answer is 21%. In the Earth's atmosphere, oxygen constitutes approximately 21% by volume. This percentage reflects the standard composition of air in the atmosphere at sea level, which is essential for supporting life as many organisms, including humans, rely on oxygen for respiration. The remaining components include nitrogen, which makes up about 78%, along with other gases such as carbon dioxide and argon, which are present in much smaller amounts.

Understanding the composition of air is critical in various fields, including environmental science, meteorology, and building performance analysis, as it affects everything from air quality to combustion processes.