Lung Cancer is a type of cancer that begins in the lungs. Your lungs are two spongy organs in your chest that take in oxygen when you inhale and release carbon dioxide when you exhale. Lung cancer is the leading cause of cancer deaths worldwide. People who are exposed to cigarette smoke have the greatest risk of lung cancer, though lung cancer can also occur in people who have never smoked. Smoking (both active smoking and passive/second hand smoking) causes more than 80% of all lung cancers.