Humans predominantly exhale carbon dioxide (CO2). This gas is a byproduct of the metabolic processes in our bodies, particularly cellular respiration, where glucose is broken down for energy. When oxygen (O2) is inhaled, it is used by cells to produce energy, and carbon dioxide is produced as a waste product. The body regulates the levels of carbon dioxide in the blood, and the respiratory system manages its exchange with oxygen. While we breathe in about 21% oxygen and exhale about 16% oxygen, the concentration of carbon dioxide in exhaled air is significantly higher, reflecting our metabolic activity.