Which gas from the combustion of hydrocarbons is poisonous?

• A. Carbon monoxide
• B. Carbon dioxide
• C. Water
• D. Oxygen

Answer: (A)

Incomplete combustion of hydrocarbons creates carbon monoxide. This gas is poisonous because it binds to haemoglobin in red blood cells much more readily than oxygen does, forming carboxyhaemoglobin and preventing oxygen from being carried to tissues. This leads to oxygen deprivation in organs, which is why carbon monoxide is dangerous even at relatively low concentrations.

Carbon dioxide, water, and oxygen are not acutely poisonous in normal conditions. Carbon dioxide can be hazardous at very high levels by displacing oxygen, but it doesn’t bind to haemoglobin like carbon monoxide. Water is harmless in its gaseous and liquid forms at everyday exposures, and oxygen is essential for life.