NEET 2020 Chemistry Question 120 Solution

Carbon monoxide ($$\text{CO}$$) is a dangerous gaseous air pollutant produced by incomplete combustion of carbonaceous fuels (coal, firewood, petrol). When inhaled, it crosses the alveolar membrane and binds to the ferrous iron ($$\text{Fe}^{2+}$$) of blood haemoglobin to form carboxyhaemoglobin.

NCERT states verbatim in Environmental Chemistry: "Carboxyhaemoglobin is about 300 times more stable than the oxygen-haemoglobin complex." Because CO binds with 300-fold higher affinity, it displaces oxygen and prevents reversible oxygen binding.

When concentration of carboxyhaemoglobin reaches 3-4%, the oxygen-carrying capacity of blood is drastically reduced, leading to headache, impaired vision, cardiovascular stress, and fatal asphyxiation.

Statement (3) asserts that carboxyhaemoglobin is "less stable than oxyhaemoglobin." This directly contradicts biochemistry, making Statement (3) the correct answer to the question.