Explain giving reasons: 'There presence of CO reduces the amount of haemoglobin available in the blood for carrying oxygen to the body cells.'

Carbon monoxide is produced as a result of incomplete combustion of carbon. The largest portion of carbon monoxide comes from the exhaust of motor vehicles.
Carbon monoxide is poisonous because it binds to haemoglobin much more strongly (about 200 times) than oxygen. The presence of carbon monoxide, therefore, reduces the amount of haemoglobin available in the blood for the transport of oxygen to the body cells and, thus with less O₂ levels, the normal metabolism is impaired.