What percentage of air is oxygen by volume?

Oxygen makes up approximately 21% of the Earth's atmosphere by volume. This percentage is consistent and widely accepted in scientific literature, highlighting the crucial role oxygen plays in supporting life and various chemical processes.

The value of 21% is significant because it represents a balance within the atmosphere, allowing for sufficient oxygen for respiration in living organisms while also leaving ample space for other gases that are essential in our environment, such as nitrogen, which constitutes about 78% of the atmosphere. Understanding the composition of air is fundamental for various fields, including biology, environmental science, and even engineering, where air properties might affect system performance and design.

Other percentages provided in the options do not accurately reflect the volume of oxygen in the atmosphere. For example, while 19%, 23%, and 75% may represent different contexts or conditions, they do not represent the standard atmospheric composition found at sea level under normal conditions.