Tobacco smoking is the cause of lung cancer in as many as 90% of cases.
A person who smokes is 13.3 times as likely to develop lung cancer as is a person who has never smoked. The risk also varies with the number of cigarettes smoked per day; people who smoke more than 20 cigarettes per day have a much greater risk of developing lung cancer than do those who smoke fewer than 20 cigarettes per day.
Once a person quits smoking, the risk of lung cancer increases for the first 2 years and then gradually decreases, but the risk never returns to the same level as that of a person who has never smoked.
Not all people who smoke develop lung cancer, and not all people with lung cancer smoke. Clearly, other factors, including genetic predisposition, also play a role.
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