Smoking produces carbon monoxide which is inhaled. Carbon monoxide is an odourless, colourless gas. Inhaled carbon monoxide passes easily into the bloodstream where it enters the red blood cells and binds to the haemoglobin, thus preventing the carriage of oxygen from the lungs to the tissues, so that the tissues become hypoxic. Unfortunately, carbon monoxide binds to haemoglobin much more strongly than oxygen and, the resulting compound, carboxyhaemoglobin, is more stable than the equivalent compound with oxygen. In numerical terms, carbon monoxide’s affinity for haemoglobin is over 200 times as great as that of oxygen. This means that even a very low concentration of carbon monoxide in the inspired air will result in a progressive build-up of carboxyhaemoglobin to harmful levels.