Lung cancer is caused by a mutation in your DNA. When cells reproduce, they divide and replicate, forming identical cells, so that your body is constantly renewing itself. Inhaling harmful, cancer-causing substances (carcinogens) such as cigarette smoke, asbestos, and radon, damages the cells that line your lungs. At first your body may be able to repair itself, but with repeated exposure, your cells become increasingly damaged. Over time, they begin to act abnormally, growing uncontrollably. Eventually, cancer may develop.
Several precancerous changes have to occur before cancer actually manifests. The buildup of extra cells causes tumors, which are either benign or malignant. Benign lung tumors are not cancerous. They usually don't need to be removed and don't spread. Cancerous lung tumors can be life threatening, can spread, and can come back once removed. Nearly 90% of all lung cancers can be attributed to smoking.