Because and CO are small molecules with weak van der Waals forces, they remain gases at ordinary temperatures, like and . Although one can smother in because of the absence of oxygen, carbon dioxide is not intrinsically poisonous.

Carbon monoxide is a different story. Oxygen is picked up at the lungs and carried to where it is needed in the tissues by hemoglobin, a protein molecule in the bloodstream. A hemoglobin molecule normally binds , but can be fooled by carbon monoxide, which binds to hemoglobin even more strongly than oxygen does.

Unfortunately for the person who breathes CO, once this happens, that particular hemoglobin molecule is permanently out of action and useless thereafter in carrying its proper cargo of .

Carbon monoxide thus is poisonous in a sense that carbon dioxide is not.