Although smoking is not the only cause of lung cancer, 90 percent of all lung cancer cases are caused by smoking, according to WebMD. In nonsmokers, lung cancer can be caused by exposure to radon gas, secondhand smoke, asbestos, air pollution and gene mutations, according to the American Cancer Society.
Notably, the risk of lung cancer goes up with the number of cigarettes that a person smokes over the course of a lifetime. To calculate a smoker's risk of developing lung cancer, doctors multiply the number of packs of cigarettes that the smoker smoked daily by the number of years the person smoked. A two-pack-a-day habit for 10 years yields a 20 pack-year smoking history. The incidence of lung cancer is greatest in those with pack-year histories of 30 or greater.
Not all smokers die from lung cancer. Among smokers who smoke two or more packs daily, only one out of every seven dies from lung cancer. Inversely, as many as 24,000 people in the United States who have never smoked cigarettes die from lung cancer each year, as of 2014. Still, the best way to avoid lung cancer is to avoid the use of tobacco products altogether, according to the American Cancer Association.