A. Smoking is by far the leading risk factor for lung cancer. About 80% of lung cancer deaths are thought to result from smoking. The longer you smoke and the more packs a day you smoke, the greater your risk. If you stop smoking before cancer develops, your damaged lung tissue gradually starts to repair itself. No matter what your age or how long you’ve smoked, quitting may lower your risk of lung cancer and help you live longer. If you don’t smoke, breathing in the smoke of others can increase your risk of developing lung cancer. Secondhand smoke is thought to cause more than 3,000 deaths from lung cancer each year. Other risk factors include exposure to radon gas or asbestos, and a family history of lung cancer.