CO is a poisonous gas while `CO_2` is not-why?

Explanation

Carbon monoxide `( :overset(Θ) -=overset(o+)O: )` combines with the Fe- atom present in haemoglobin of blood through the lone pair of electrons on carbon to form a highly stable complex known as carboxyhaemoglobin (HbCO). Due to formation of this complex, haemoglobin cannot further act as oxygen-carrier. As a result, the body cells get slackened due to the deficiency of oxygen and this ultimately results in death. Thus, CO is poisonous: `Hb + CO to HbCO` (Hb = Haemoglobin)
On the other hand, the structure of carbon dioxide is `:overset(* *)O = C = overset(* *)O:`. Although the oxygen atoms in `CO_2` molecule contain lone pairs of electrons, unlike CO it cannot combine with the Fe -atom of haemoglobin because of its larger size. Therefore, `CO_2` cannot prevent haemoglobin from carrying oxygen and so it is not poisonous.