When humans exhale, they breathe out mostly carbon dioxide, which is a waste product of cellular respiration during the production of energy. Exhaled air is about 4% carbon dioxide. The air we inhale contains about 21% oxygen, which diffuses into the blood and is taken to the cells of the body, while carbon dioxide diffuses from the blood into the lungs and is expelled as we breathe out. The composition of the air we breathe out is almost the same as the air we inhale, except that the percentage of carbon dioxide and oxygen changes. The amount of inhaled air contains 21% of oxygen and 0.04% of carbon dioxide, while the air we breathe out contains 16.4% of oxygen and 4.4% of carbon dioxide.