Smoking addiction is harmful because it produces polycyclic aromatic hydrocarbons, which cause

Cancer is caused by cells dividing repeatedly out of control. They cease to respond to the normal signals around them and form unspecialised masses of cells called tumours. Lung cancer usually starts in the epithelium of the bronchioles, so-called bronchial carcinoma. It then usually spreads throughout the lungs. It is caused almost exclusively by smoking. Tobacco smoke contains chemicals responsible for lung cancer. The most important of these are polycyclic hydrocarbons which are converted in the body to carcinogens. Besides, carbon monoxide combines with haemoglobin and reduces oxygen transport by about $15 \%$ in smokers and nicotine increases blood pressure, heart rate and constriction of blood vessels.