“Smoking tobacco is by far the leading cause of lung cancer. About 80% of lung cancer deaths are caused by smoking, and many others are caused by exposure to secondhand smoke.

Smoking is clearly the strongest risk factor for lung cancer, but it often interacts with other factors. People who smoke and are exposed to other known risk factors such as radon and asbestos are at an even higher risk. Not everyone who smokes gets lung cancer, so other factors like genetics probably play a role as well (see below).”

American Cancer Society (https://www.cancer.org/…/causes-risks…/what-causes.html)