Why is carbon monoxide toxic for humans?

Hemoglobin "likes" carbon monoxide (CO) much more than it likes oxygen. When there is carbon monoxide in the inhaled air it binds to hemoglobin forming carboxyhemoglobin by occupying the binding site where oxygen would bind. Because of the higher hemoglobin affinity for carbon monoxide therefore (e.g., in intoxication from car exhausts) there is no oxygen transport and the individual undergoes hypoxia, loses conscience, inhales more carbon monoxide and may even die.

Intoxication by carbon monoxide is an significant cause of death in fires and in closed garages.