Carbon monoxide, CO, is a toxic gas made by the incomplete combustion of fuels in faulty boilers, gas fires, cookers and car engines. It is colourless and has no smell, so people cannot tell when it is in the air they are breathing.
Oxygen is carried around the body by haemoglobin in the red blood cells. When carbon monoxide is breathed in, it combines with haemoglobin. It binds more strongly than oxygen does, so the haemoglobin can no longer carry oxygen. Less oxygen reaches the body cells.
Without enough oxygen the cells cannot release energy by respiration. A small dose causes headache, dizziness and tiredness. A large dose can lead to unconsciousness and death. Carbon monoxide alarms are fitted in homes to give a warning.