Carbon monoxide is a colourless, odourless gas which has the tendency to bind to haemoglobin, the oxygen carrying molecule in blood and forms a complex called carboxyhaemoglobin. This complex is 300 times more stable than oxyhaemoglobin complex. When the concentration of carboxyhaemoglobin reaches about 3-4 per cent, the oxygen carrying capacity of blood is greatly reduced. This oxygen deficiency, results into headache, weak eyesight, nervousness and cardiovascular disorder and therefore, if once formed can seriously hamper the body and may cause death. That is why CO is said to be more dangerous than CO2.