What percentage of exhaled air is oxygen?

Exhaled air is comprised of approximately 4 to 5 percent less oxygen than was inhaled. The exchange of gases takes place in the alveoli within the lungs in a process called diffusion.

The air we exhale is called expired air and is typically composed of approximately 4 to 5 more percent more carbon dioxide and 4 to 5 percent less oxygen than inhaled air along with 78 percent nitrogen. Inhaled air contains more oxygen and less carbon dioxide, while expired air contains more carbon dioxide — produced as a waste product of energy production — and less oxygen (which has been absorbed through the capillary wall and into the body's tissues.