What causes lung cancer?

Smoking :The incidence of lung cancer is strongly correlated with cigarette smoking, with about 90% of lung cancers arising as a result of tobacco use. The risk of cancer increases with the number of cigarettes smoked over time; doctors refer to this risk in terms of pack-years of smokinghistory (the number of packs of cigarettes smoked per day multiplied by the number of years smoked). For example, a person who has smoked two packs of cigarettes per day for 10 years has a 20 pack-year smoking history. While the risk of lung cancer is increased with even a 10 pack-year smoking history, those with 30 pack-year histories or more are considered to have the greatest risk for the development of cancer. Among those who smoke two or more packs of cigarettes per day, one in seven will die of lung cancer. But even though the risk is higher the more you smoke, there is no safe level of exposure to tobacco smoke.

Pipe and cigar smoking can also cause lung cancer, although the risk is not as high as with cigarettes. While someone who smokes one pack of cigarettes per day has a risk for the development of cancer that is 25 times higher than a nonsmoker, pipe and cigar smokers have a risk of  cancer that is about five times that of a nonsmoker.Tobacco smoke contains over 4,000 chemical compounds, many of which have been shown to be cancer-causing, or carcinogenic. The two primary carcinogens in tobacco smoke are chemicals known as nitrosamines and polycyclic aromatic hydrocarbons. The risk of developing lung cancer decreases each year following smoking cessation as normal cells grow and replace damaged cells in the lung. In former smokers, the risk of developing cancer begins to approach that of a nonsmoker about 15 years after cessation of smoking.

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