CO binds to haemoglobin for which it has 200 times more affinity than oxygen and forms carboxyhemoglobin. In blood, when the concentration of carboxyhemoglobin reaches 3-4, the oxygen carrying capacity of the blood is greatly reduced. In other words, the body becomes oxygen starved This results into headache, nervousness, cardiovascular disorder, weak eyesight, etc. On the other hand, `CO_2` does not combine with hemoglobin and hence is less harmful as pollutant `CO_2` is the main contributor towards greenhouse effect and global warming
