Carbon and oxygen combine to form either carbon monoxide (CO) (incomplete combustion) or carbon dioxide (CO,) (complete combustion). Whereas carbon dioxide is inert, carbon monoxide is a potent poison owing to its ability to form strong bonds with the blood pigment, haemoglobin. Carbon monoxide is produced naturally, but the major man-made source is the motor vehicle - especially petrol-driven cars. Cigarette smokers subject themselves to further exposure by inhaling CO in the smoke.