Put in the most basic terms, smoking is bad for you. How bad? Smoking is the leading preventable cause of death. It is estimated that it can reduce your life expectancy by 15 to 25 years. Smoking adversely affects the following areas of the body:Heart: The heart is one the main organs damaged by smoking. Smoking lowers the HDL cholesterol, what is commonly referred to as "good" cholesterol. Smoking places more stress on the system that regulates the heart and the blood vessels because it increases the activity of the sympathetic nervous system. Because smoking reduces the elastic properties of the aorta, the body's largest blood vessel, the risk of developing a blood clot is increased. The nicotine in cigarettes acts as a blood coagulant and can cause such blood clots.Smoking can accelerate the process known as atherosclerosis. This is a narrowing the arteries that can cause strokes or heart attacks. The nicotine in cigarettes acts as a vasoconstricter that constricts blood vessels and restricts blood flow. If you are a smoker in the 30 or 40 years age bracket then you are more likely to die from smoking-related heart disease than from the cancer smoking produces. In fact, you are about five times more likely to die of a heart attack than a non-smoker. If you are a male aged between 35 and 64 then you are nearly four times more likely than a non-smoker you age to suffer a stroke.Lungs: As well as contributing to lung cancer, smoking also weakens your immune system making you more susceptible to bouts of colds and flu. It also irritates the lining of your lungs and impairs your respiratory system. Smoking is the major cause of obstructive diseases like emphysema and 'smoker's cough'. A smoker is 10 times more likely to experience chronic obstructive pulmonary disease (COPD) than a non-smoker.Cancer: The carcinogens in tobacco smoke can lead to cancer. Smoking has a huge role to play in the incidence of lung cancer. It is believed that smoking causes about 90 to 95% of male deaths from lung disease and some 70 to 75% of female deaths. Smoking is believed to cause around 30% of all cancer deaths in mature tobacco markets like the US.As well as lung cancer, the carcinogens in cigarettes can cause other types of cancer as well. Smokers have higher rates of cancers of the stomach, bladder, kidney, pancreas and leukaemia. In the case of cancer of the bladder, pancreas, and stomach, smoking approximately doubles your chances of developing them. The tar from cigarettes makes it hard for the lungs to repair the damage done to them by smoking because it damages the lung's DNA. Smoking and chewing (smokeless) tobacco are responsible for over 60% of cancers of the throat, mouth, and esophagus.Children's health: Children who breathe in second-hand smoke are more likely to develop asthma or, if they already have asthma, they risk the condition worsening. It also increases their chances of suffering from pneumonia, colds, and bronchitis.Other consequences: Smoking can play a role in disorders as varied as osteoporosis, peptic ulcers, cataracts, thyroid problems, impotence, and periodontal disease. A person who smokes increases the risk of developing osteoporosis. This is because smoking reduces the blood supply to your bones and slows the production of bone-forming cells. This can affect joints and the spine and may slightly increase the chances of smokers developing rheumatoid arthritis. Smoking can cause peptic ulcers. This is because it increases acid secretion, decreases mucosal blood flow and reduces the body's production of prostaglandin and bicarbonate. This will also delay the healing of gastric and duodenal ulcers. Heavy smokers risk developing cataracts of the eye. Smokers also have twice the chance of developing macular degeneration as non-smokers. Macular degeneration is an eye disorder usually associated with old age. Thyroid disease can be caused by cyanedim, a chemical found in cigarette smoke, which interferes with the production of thyroid hormone. Heavy smoking may contribute to impotence in men by decreasing blood flow to the penis. Male smokers also run a higher risk of infertility because smoking reduces the mobility and density of sperm. This is all rather ironic for those who think smoking makes them sexier. Because tobacco results in tartar build-up, it increases the incidence of periodontal (gum) disease, staining of the teeth, and potential tooth loss.Smoking can be detrimental to your external appearance as well. Tobacco smoke has a drying effect on the surface of the skin. Carbon monoxide enters the bloodstream when you smoke and this decreases the amount of oxygen carried to the tissue in your body. This depletes the skin of essential nutrients and makes your skin look grey, especially around the mouth. It also increases the number of wrinkles visible on your skin. You will appear to age prematurely as these deep wrinkles make you look older than you really are. No only that, but smoking actually damages the ability of the skin to heal. This is why you should not smoke before you undergo any surgical procedure. Smokers also often develop discolouration of the fingers and fingernails on the hand they use to hold the cigarette. Smoking also stains the teeth and can cause halitosis.