What gas do humans breathe out as a waste product of respiration?

Humans breathe out carbon dioxide as a waste product of cellular respiration.

Cells release energy by breaking down nutrients, especially glucose, through cellular respiration. In aerobic respiration, oxygen helps drive reactions that produce ATP, the cell’s main usable energy currency. Carbon dioxide is generated during these reactions and carried in the blood to the lungs, where it diffuses from pulmonary capillaries into the air sacs and is exhaled.

Exhaled air still contains oxygen, because the body does not use every oxygen molecule inhaled. It also contains much more carbon dioxide than inhaled air—about 4% in a typical estimate—and increased water vapor. The diaphragm relaxes during ordinary exhalation, reducing chest volume and helping push air out of the lungs.

Carbon dioxide should not be confused with carbon monoxide. Carbon monoxide is a poisonous gas produced by incomplete combustion, such as from faulty heaters or vehicle exhaust. Nitrogen makes up most of both inhaled and exhaled air and is not the principal respiratory waste product. The lungs also remove some water vapor, but carbon dioxide is the classic waste gas produced by respiration.