A risk factor  is anything that increases a person's chance of developing cancer. Although risk factors often influence the development of cancer, most do not directly cause cancer. Some people with several risk factors never develop cancer, while others with no known risk factors do. However, knowing your risk factors and talking about them with your doctor may help you make more informed lifestyle and health care choices. Most lung cancer occurs in people who smoke or in those who have smoked in the past. However, people who don't smoke can also develop lung cancer, so it is important for all people to learn about other risk factors and the signs and symptoms  of lung cancer.
The following factors may raise a person's risk of developing lung cancer:
Tobacco . Tobacco smoke damages cells in the lungs, causing the cells to grow abnormally. The risk that smoking will lead to cancer is higher for people who smoke heavily and/or for a long time. Regular exposure to smoke from someone else's cigarettes, cigars, or pipes (called environmental or "secondhand" tobacco smoke ) can increase a person's risk of lung cancer, even if that person does not smoke.
Asbestos. These are hair-like crystals found in many types of rock and are often used as fireproof insulation in buildings. When asbestos fibers are inhaled, they can irritate the lung. Many studies show that the combination of smoking and asbestos exposure is particularly dangerous. People who work with asbestos in a job (such as shipbuilding, asbestos mining, insulation, or automotive brake repair) and smoke have a higher risk of developing lung cancer. Using protective breathing equipment reduces this risk.
Radon. This is an invisible, odorless gas naturally released by some soil and rocks. Exposure to radon has been associated with an increased risk of some types of cancer, including lung cancer. Most hardware stores have kits that test home radon levels, and basements can be ventilated to reduce radon exposure.
The most important way to prevent lung cancer is to avoid tobacco smoke. People who never smoke have the lowest risk of lung cancer. People who smoke can reduce their risk of lung cancer by stopping smoking , but their risk of lung cancer will still be higher than people who never smoked. Attempts to prevent lung cancer with vitamins or other treatments have not worked. For instance, beta-carotene, a drug related to vitamin A, has been tested for the prevention of lung cancer. It did not reduce the risk of cancer. In people who continued to smoke, beta-carotene actually increased the risk of lung cancer.
Recently, a large study called the National Lung Screening Trial  showed that, in patients age 55 to 75 who currently smoke or who used to smoke heavily, the use of a screening test called a low-dose helical (or spiral) computed tomography (CT or CAT) scan can find lung cancers early and decrease the risk of death from lung cancer by 20%. A CT scan creates a three-dimensional picture of the inside of the body with an x-ray machine. A computer then combines these images into a detailed, cross-sectional view that shows any abnormalities or tumors.
CT scanning is not recommended for every person who smokes. Doctors still need to prove that screening everyone at risk for lung cancer reduces rates of death from lung cancer in the general population. Learn more about ASCO's recommendations for lung cancer screening in current or former smokers.