Smoking is the main risk factor for lung cancer and is responsible for more than 80% of lung cancers. The longer you have smoked and the more you smoke, the more likely you are to get lung cancer. If you stop smoking before cancer cells develop, lung tissue that has been damaged by smoking will start to repair. An ex-smoker's risk will not be as low as that of a person who never smoked, but over time, their risk will go down. Cigar smoking and pipe smoking are almost as likely to cause lung cancer as cigarette smoking.
Even secondhand smoke, the kind inhaled from nearby smokers, can cause lung cancer. Nonsmokers who are married to smokers have a 30% greater risk of developing lung cancer than spouses of nonsmokers.
Living in an environment with high air pollution or working with radioactive minerals or asbestos can also increase the risk of cancer. Research has helped us to understand how these risk factors produce certain changes in the DNA of lung cells. These changes cause the cells to grow abnormally and form cancers.
DNA is the genetic material that carries the instructions for nearly everything our cells do. Some genes (parts of our DNA) contain instructions for controlling when cells grow and divide. The risk factors discussed earlier can trigger changes, also called mutations, in these genes that result in cancer. A risk for some types of cancer (e.g., breast, ovarian, colorectal, and several others) can be inherited from parents. However, inherited gene mutations are not thought to be a cause of very many lung cancers.