The lungs are two sponge-like organs found in the chest. The right lung has three sections, or lobes, while the left lung has two lobes. The left lung is smaller because the heart takes up more room on that side of the body. The lungs bring air in and out of the body, taking in oxygen and expelling carbon dioxide gas, a waste product. Lung cancer often takes many years to develop. First, there may be areas of pre-cancerous changes in the lung. These changes are not a mass or tumor. They can’t be seen on an x-ray and they don’t cause symptoms. But these changes can be found by special tests of cells in the lining of the airways of lungs damaged by smoke. As these pre-cancerous areas go on to become true cancer, they may make chemicals that cause new blood vessels to form nearby. These new blood vessels nourish the cancer cells and allow a tumor to form. Finally, the tumor becomes large enough to be seen on an x-ray. Once lung cancer occurs, cancer cells can break away and spread to other parts of the body in a process called metastasis. Lung cancer is a life-threatening disease because it often spreads in this way before it is found. Credits:  CBS/AP/American Cancer Society