CO combines with haemoglobin to forms a complex entity, carboxyhaemoglobin which is about 300 times more stable than oxyhaemoglobin. In blood, when the concentration of carboxyhaemoglobin reaches 3 -4%, the oxygen-carrying capacity of the blood is significantly reduced. In other words, the body becomes oxygen starved. This results into headache, nervousness, cardiovascular disorder, weak eyesight etc.