Suggest a reason as to why `CO` is poisonous.

Explanation

In the lungs, haemoglobin present in red blood cells combines with molecules oxygen loosely and reversibly to form oxyhaemoglobin.
`"Haemoglobin" + O_(2) hArr "Oxyhaemoglobin"`
Oxyhaemoglobin thus formed in the lungs then travels to different parts of the body through blood stream and delivers `O_(2)` to the various tissues of the body. However, `CO` combines with haemoglobin irreversibly to form carboxyhaemoglobin which is about 300 times more stable than the oxyhaemoglobin.
`"Haemoglobin" +CO rarr "Carboxyhaemoglobin"`.
As a result, the oxygen carrying capacity of haemoglobin is destroyed and the man dies of suffocation. Thus, the highly poisonous nature of `CO` arises due to its ability to form a complex with haemoglobin which is about 300 times more stable than the oxygen -haemoglobin complex.