Normally, lung cells divide to produce more cells only when the body needs them. Lung cancer occurs when cells divide and form more cells uncontrollably, creating a mass of tissue called a tumor. Tumors can be benign and rarely a threat to life, or they can be malignant. Malignant tumors are cancers that can invade and damage nearby tissues and organs. Cancer cells can also break away from a malignant tumor and enter the bloodstream, spreading to other parts of the body where they can form new tumors.
Most lung cancer is caused by cigarette smoking.
Tobacco smoke contains many carcinogens harmful substances that damage cells. Over time, these cells can become cancerous. The more a person smokes, the higher the risk of getting cancer not just lung cancer, but also cancers of the mouth, throat, esophagus, larynx, bladder, kidney, cervix and pancreas. Although smoking is by far the major cause of lung cancer, it is not the only cause. Scientists have found that nonsmokers who live or work with smokers have a higher lung cancer risk than nonsmokers who do not face this type of exposure to environmental tobacco smoke.