context: 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 lung cancer increases with the number of cigarettes smoked over time; doctors refer to this risk in terms of pack-years of smoking history (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 lung 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 lung cancer that is 25 times higher than a nonsmoker, pipe and cigar smokers have a risk of lung cancer that is about five times that of a nonsmoker. Tobacco smoke contains over 7,000 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 lung cancer begins to approach that of a nonsmoker about 15 years after cessation of smoking. Passive smoking Passive smoking, or the inhalation of secondhand tobacco smoke from other smokers sharing living or working quarters, is also an established risk factor for the development of lung cancer. Research has shown that nonsmokers who reside with a smoker have a 24% increase in risk for developing lung cancer when compared with other nonsmokers. An estimated 7,300 lung cancer deaths occur each year in the U. S. that are attributable to passive smoking. Asbestos fibers Asbestos fibers are silicate fibers that can persist for a lifetime in lung tissue following exposure to asbestos. The workplace is a common source of exposure to asbestos fibers, as asbestos was widely used in the past for both thermal and acoustic insulation materials. Today, asbestos use is limited or banned in many countries including the Unites States. Both lung cancer and mesothelioma (a type of cancer of the pleura or of the lining of the abdominal cavity called the peritoneum) are associated with exposure to asbestos. Cigarette smoking drastically increases the chance of developing an asbestos-related lung cancer in exposed workers. Asbestos workers who do not smoke have a fivefold greater risk of developing lung cancer than non-smokers, and those asbestos workers who smoke have a risk that is 50 to 90 times greater than non-smokers. Radon gas Radon gas is a natural, chemically inert gas that is a natural decay product of uranium. It decays to form products that emit a type of ionizing radiation. Radon gas is a known cause of lung cancer, with an estimated 12% of lung cancer deaths attributable to radon gas, or 15,000 to 22,000 lung cancer-related deaths annually in the U. S. As with asbestos exposure, concomitant smoking greatly increases the risk of lung cancer with radon exposure. Radon gas can travel up through soil and enter homes through gaps in the foundation, pipes, drains, or other openings. The U. S. Environmental Protection Agency estimates that one out of every 15 homes in the U. S. contains dangerous levels of radon gas. Radon gas is invisible and odorless, but can be detected with simple test kits. Familial predisposition While the majority of lung cancers are associated with tobacco smoking, the fact that not all smokers eventually develop lung cancer suggests that other factors, such as individual genetic susceptibility, may play a role in the causation of lung cancer. Numerous studies have shown that lung cancer is more likely to occur in both smoking and nonsmoking relatives of those who have had lung cancer than in the general population. Lung disease The presence of certain diseases of the lung, notably chronic obstructive pulmonary disease ( COPD), is associated with a slightly increased risk (four to six times the risk of a nonsmoker) for the development of lung cancer even after the effects of concomitant cigarette smoking are excluded. Prior history of lung cancer Survivors of lung cancer have a greater risk than the general population of developing a second lung cancer. Survivors of non-small cell lung cancers have an added risk of 1%-2% per year for developing a second lung cancer. In survivors of small cell lung cancers, the risk for development of second cancers approaches 6% per year. Air pollution Air pollution, from vehicles, industry, and power plants, can raise the likelihood of developing lung cancer. Up to 1% of lung cancer deaths are attributable to breathing polluted air, and experts believe that prolonged exposure to highly polluted air can carry a risk similar to that of passive smoking for the development of lung cancer.