LUNG CANCER is a malignant cancer of the bronchial covering. The tumour grows through surrounding tissues invading and often obstructing the air passages. The earliest symptom is often a persistent cough; a physical exam may attribute the symptoms of chronic bronchitis. Chest x-rays sometimes show shadows that indicate tumours and enlarged lymph nodes. However, the definitive diagnosis of lung cancer is based upon microscopic examination of lung tissue. The time between exposure to asbestos and the occurrence of lung cancer is 20-30 years. There is no threshold or limit of exposure below which the risk of lung cancer is not increased.