Lung cancer is a malignant disease that begins in the cells of the lungs, most commonly in the lining of the air passages. It is broadly classified into non-small cell lung cancer and small cell lung cancer, based on how the cancer cells appear under a microscope and how aggressively they grow. The leading risk factor is cigarette smoking, though exposure to secondhand smoke, radon gas, asbestos, air pollution, and certain occupational hazards also increases risk. Early stages may cause no noticeable symptoms, but as the disease progresses, individuals may experience persistent cough, chest pain, shortness of breath, coughing up blood, unexplained weight loss, or fatigue. Because lung cancer often develops silently in its early stages, it is frequently diagnosed after it has advanced beyond the lungs.