1. Tobacco: Tobacco includes the smoking of cigarettes, pipes, cigars, marijuana and crack cocaine. Tobacco smoke is the most common cause of lung cancer. There are many harmful chemicals (carcinogens) in the tobacco, which damage cells within the lung, airways, mouth and throat. These cells can then turn cancerous. The amount of tobacco smoked each day, multiplied by the number of years smoked, determines the level of risk level. Quitting smoking can significantly reduce the likelihood of developing lung cancer, however, never to the level of a lifetime nonsmoker.
   
2. Second-Hand Smoke: Exposure to this type of smoke can also lead to lung cancer.  It is estimated that 3,000 people each year die from lung cancer caused by second-hand smoke exposure. It may be worse than smoking directly because no filter exists when one inhales the smoke in the air.  Nonsmokers living directly with smokers sustain one-quarter of the smoker's increased risk.
   
3. Radon:  Radon is a radioactive gas from the breakdown of uranium in soil and rocks. Radon is found in some basements with little or no ventilation, as well as in mines. Because it is invisible, tasteless and odorless, special kits are required to determine its existence. Kits used to test for radon are found in hardware stores, online, and sometimes your local health department can supply a kit. If high levels are found the problem can be corrected by proper ventilation. Exposure to radon and tobacco smoke can significantly increase the risk for developing lung cancer.
   
4. Asbestos:  Asbestos is a mineral used in many industries including steel plants, shipbuilding, brake repair, insulation, plumbing, and construction. It consists of tiny inhalable fibers that cause cell damage and increase cancer risk. A smoker exposed to asbestos has an extremely high risk for developing lung cancer. Asbestos also causes malignant mesothelioma, a type of lung cancer.
   
5. Chemicals:  Certain industrial chemicals cause lung cancer including benzene, chromium, nickel, arsenic, vinyl chloride, polycyclic aromatic hydrocarbons, and Bis-chloromethyl ether (BCME).
   
6. Genetics:  A genetic predisposition to developing lung cancer may exist if many smokers in the same family develop the disease. Researchers are studying genetic defects that can be linked to lung cancer. A common one is the p53 gene, which is a tumor suppressor gene. If this gene is altered or absent, tumor cells may develop.
   
7. Lung Disease:  Problems such as emphysema, chronic bronchitis, or tuberculosis are associated with greater lung cancer risk.