Smoking is the leading cause of lung cancer. Even if you donât smoke, being around someone who smokes on a regular basis and inhaling their secondhand smoke can also cause cancer. When you inhale cigarette smoke, the carcinogens cause changes in the tissues and cells in the lungs. Over time, these changes damage the cells in the lungs and cause cancer to develop. A healthy lung and one harmed by smoking look very different. A lung damaged by smoking is blackened over time, and its shape becomes irregular.