Cigarette smoking is the main cause for developing lung cancer. Air pollution and industrial exposures also may cause problem to the lungs. Persons exposed to harmful substances, such as arsenic, asbestos, radioactive dust, and radon have a greater chance of developing lung cancer if they smoke. Not all smokers develop lung cancer and not all people with lung cancer are smokers.

The most significant cause of lung cancer is exposure to carcinogenic (cancer-causing) agents. Harmful substances, called carcinogens, in tobacco damage the lung cells. Squamous cell carcinoma and small cell carcinoma are the most common types found in smokers. Lung cancer in smokers is related with the age at which smoking began, how long the person has smoked, the number of cigarettes smoked per day, and how deeply the smoker inhales. Smoking at a younger age raises the risk and the death rate from lung cancer. Stopping smoking greatly reduces a person's risk for developing lung cancer.