A compound produced when carbon monoxide links with red blood cells. It is breathed into the lungs and enters the bloodstream. It blocks the sites on the cells that carry oxygen. Oxygen in the blood decreases and, when it decreases is too much, suffocation and death result.

Hemoglobin in which the iron is bound to carbon monoxide (CO) instead of oxygen.()

The compound formed in the blood when inhaled carbon monoxide combines with hemoglobin, thereby restricting the amount of oxygen that the blood can carry; the resulting condition is known as carboxyhemoglobinemia.