We breathe in a mixture of the first two actually. The air is composed of 78% nitrogen, 21% oxygen, 0.03% carbon dioxide and 0.07% of other gases. Technically, you can also breathe in carbon monoxide if it is present within your environment. However, it has harmful effects. It can enter your bloodstream and mix with hemoglobin to form carboxyhemoglobin. The blood cannot carry oxygen anymore, and this causes the body's cells and tissue to die.