That air you suck into your lungs when you take a breath has a certain chemical composition -- it's nearly 79 percent nitrogen, 21 percent oxygen and an extremely small amount of carbon dioxide. However, that's not the chemical composition of the air you exhale, because when you inhaled the air, your body changed it.

Chemical Composition of Exhaled Air

When you take in a breath of fresh air, that air's chemical composition is changed due to a gas exchange, thanks to your alveoli, which are tiny air spaces in your lungs. While the air you exhale is also made up of nitrogen, oxygen and carbon dioxide, these levels change. Exhaled air is about 78 percent nitrogen, 18 percent oxygen and 4 percent carbon dioxide.