Q.Carbon monoxide gas is more dangerous than carbon dioxide gas. Why?

Both CO and CO2 are present in the troposphere, but they act very differently once inhaled.

Carbon monoxide (CO) is extremely dangerous because of its chemical affinity for haemoglobin, the oxygen-carrying pigment in red blood cells. CO binds to haemoglobin far more strongly than oxygen does, forming a stable complex called carboxyhaemoglobin (COHb):

Carboxyhaemoglobin is roughly 300 times more stable than oxyhaemoglobin (HbO2). Once formed, it does not readily dissociate, so the haemoglobin bound to CO is no longer available to transport oxygen to body tissues. Even a small concentration of CO in inhaled air can therefore tie up a large fraction of the body's haemoglobin, reducing the oxygen-carrying capacity of blood. This causes oxygen starvation (hypoxia), leading to symptoms such as headache, dizziness, poor vision, nervous system disturbance, cardiovascular problems and, at higher concentrations, death by asphyxiation.

Carbon dioxide (CO2), on the other hand, does not bind to haemoglobin in this way. It is a normal product of respiration and is transported and exhaled through the blood's own buffering and gas-exchange mechanisms. CO2 becomes harmful mainly at very high concentrations, where it can cause suffocation simply by displacing oxygen in the air or by disturbing the blood's acid–base balance — a much less potent mechanism than CO's direct, essentially irreversible poisoning of haemoglobin.

This is why CO — a colourless, odourless gas produced by incomplete combustion of fuels (e.g. in vehicle exhaust) — is a much more insidious and dangerous poison than CO2 at comparable concentrations.

Compare the biochemical mechanism: CO forms a very stable complex with haemoglobin (carboxyhaemoglobin) that prevents oxygen transport, while CO2 does not bind haemoglobin at all and is only harmful in very high concentrations by displacing oxygen/disturbing pH balance.

• Saying CO2 is 'harmless' — it is a greenhouse gas and does cause suffocation at very high concentration, just via a different, much weaker mechanism.
• Missing the key biochemical reason (haemoglobin affinity) and giving only a vague answer like 'CO is more poisonous'.
• Forgetting to mention that CO is colourless and odourless, which makes exposure hard to detect.