Q. 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 due to the higher hemoglobin affinity for carbon monoxide thus for example, in intoxication from car exhausts there is no oxygen transport and the individual loses conscience, undergoes hypoxia, inhales more carbon monoxide and may even die.

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