Carbon monoxide gas is a poisonous gas. It binds to haemoglobin of the blood to form carboxyhaemogjobin complex which is 300 times more stable than oxyhaemoglobin complex.
When concentration of carboxyhaemoglobin reaches about 3-4%, the oxygen carrying capacity of blood is greatly reduced. This results into headache, nervousness and cardiovascular disorder.
${{CO}_{2}}$ gas does not combine with haemoglobin so it is less harmful as a pollutant. Its increased amount in air is mainly responsible for global warming.