Carboxyhemoglobin (COHb) is the term used to describe hemoglobin (the protein in red blood cells that normally carries oxygen from the lungs to the rest of the body) that has bonded with carbon monoxide instead of oxygen.

Carbon monoxide's bond with hemoglobin is much stronger than oxygen's bond with hemoglobin. When hemoglobin binds with carbon monoxide instead of oxygen, the blood can no longer carry enough oxygen.