Carbon monoxide is dangerous to human beings mainly because it:

The correct answer is A, combines with haemoglobin to form carboxyhaemoglobin. Carbon monoxide binds to the haemoglobin of the blood roughly two hundred times more readily than oxygen does, and the carboxyhaemoglobin so formed cannot carry oxygen. Tissues are therefore starved of oxygen, which causes headache, giddiness and, in a closed room with a burning coal fire or a running engine, unconsciousness and death. Option B is wrong because it is sulphur dioxide and the oxides of nitrogen that dissolve and oxidise to give sulphuric and nitric acid in rain. Option C is wrong because chlorofluorocarbons and halons deplete stratospheric ozone. Option D is wrong because settling in the alveoli describes fine particulate matter such as PM2.5, not a gas. Remember that carbon monoxide is colourless and odourless, which is why it gives no warning of its presence, and that a well-ventilated room is the simplest protection against it.