Tobacco is one of the five most common risk factors for chronic diseases worldwide; the others are age and heredity, unhealthy diet, and physical inactivity. Of these five risk factors, only smoking and the last two are modifiable.

When a person smokes, the heat breaks down the tobacco in the cigarette. This process releases at least 400 toxic substances which are then absorbed by the body through the lungs and the respiratory passages.

The most harmful of these toxic substances are tar, nicotine and carbon monoxide. Tar is a carcinogen (i.e., a substance that is known to cause cancer). Nicotine is addictive and it increases blood cholesterol levels while carbon monoxide reduces the amount of oxygen that the red blood cells can carry. The toxins in cigarettes represent an enormous health hazard. They constitute a major risk factor for various diseases. What are these diseases that are associated with cigarette smoking?

First and foremost, of course, is lung cancer. Scientific evidence has long ago established that there is a cause and effect relationship between smoking and lung cancer. The risk of dying from lung cancer for "never smokers" is less than 1 percent while for heavy smokers (i.e., those who smoke more than 15 sticks of cigarettes a day) it is a high 30 percent.

Smoking is also a risk factor for many other forms of cancer including mouth and throat cancers (which are hardly seen in non-smokers), and cancers of the esophagus, stomach, pancreas, kidney, urinary bladder, prostate (in men) and cervix (in women).

Actually, the most common cause of death that is attributable to smoking is not cancer but cardiovascular disease. Smoking contributes to cardiovascular disease by accelerating atherosclerosis — the hardening of the arteries that occurs because of deposition of cholesterol and other fatty substances into the blood vessel walls. Atherosclerosis makes the arteries rigid and narrow making them favorable for the development of blood clots.

The ultimate presentation of cardiovascular disease depends on the organ that is involved. It can be in the form of a heart attack, if the arteries that supply the heart are involved; or it can be in the form of a stroke, kidney failure, or gangrene leading to amputation of the extremities.

Smoking also contributes to the development of hypertension (high blood pressure), another risk factor for cardiovascular disease.

Another major illness that is associated with smoking is chronic obstructive pulmonary disease (COPD). COPD is the collective term for a group of conditions that block airflow into the lungs making breathing more difficult.

Smoking is the cause of as many as 80 percent of all COPD cases. The more common forms of COPD are chronic bronchitis (long-term inflammation of the air passages that presents as chronic cough that is often productive of a lot of mucus) and emphysema (irreversible destruction and weakening of the walls of the air passages). COPD is a progressive disease that ultimately leads to an agonizing death.

Smoking also worsens asthma. It likewise contributes to the development of cataract and other eye diseases, low-back pain, peptic ulcers, fertility problems and impotence (in men). It also stains teeth and causes periodontal diseases. It also affects the skin. Smokers have paler and more wrinkled skin.

Smoking harms not just the smoker, but also those around him/her who are placed at risk for a variety of illnesses by being second-hand or passive smokers.

On the average, a stick of cigarette shortens a person’s life by about 10-11 minutes. Consequently, a heavy smoker, at the least, reduces his/her life expectancy by seven to eight years.

Worldwide, tobacco currently accounts for about 5 million deaths per year. This means tobacco is the underlying cause of one in 10 deaths among adults. World Health Organization (WHO) estimates reveal that if the present smoking pattern of people continues, tobacco will cause 10 million deaths per annum by 2020.