What is the most common cause of lung cancer?

The most common cause of lung cancer in people is smoking, which is responsible for 87 percent of all cases of lung cancer, according to the American Lung Association. Smokers are at an increased risk of developing lung cancer because combustible tobacco products such as cigarettes and pipe tobacco contain many harmful cancer-causing chemicals.

People who do not smoke and who are only exposed to second hand smoke from other tobacco users are also at an increased risk of developing lung cancer. Many people die each year and develop other health issues as a result of repeated second hand smoke exposure, according to the American Lung Association.