How does carbon monoxide, a poisonous gas emitted by automobiles, prevent transport of oxygen into the body tissues?

Carbon monoxide combines with hemoglobin to form carboxyhemoglobin at any or all of the oxygen-binding sites of hemoglobin, and also acts to increase the stability of the bond between hemoglobin and oxygen, reducing the ability of the hemoglobin molecule to release oxygen bound to other oxygen-binding sites.