Why is carbon monoxide dangerous?

• A. It is poisonous and combines with haemoglobin in the blood, preventing oxygen from being carried around the body.
• B. It reacts with water to form carbonic acid.
• C. It is an inert gas that does not interact with the body.
• D. It is a non-flammable gas.

Answer: (A)

Carbon monoxide is dangerous because it binds strongly to haemoglobin in red blood cells to form carboxyhaemoglobin. This lock-up of haemoglobin prevents oxygen from being carried around the body, so tissues don’t get the oxygen they need. CO’s affinity for haemoglobin is much higher than that of oxygen—about 200 times greater—so even small amounts can significantly reduce the blood’s oxygen-carrying capacity. Also, CO causes the haemoglobin to hold onto oxygen more tightly (a left shift in the oxygen dissociation curve), making it harder for oxygen to be released to tissues that need it. Its danger is heightened by the fact that it is colourless and odourless, so people may inhale it without realizing.

Other statements don’t fit because carbon monoxide does not react with water to form carbonic acid (that would involve carbon dioxide). It is not inert; it actively interacts with haemoglobin. And it is not a non-flammable gas—the gas can burn in air, so describing it as non-flammable is incorrect.