(a) : Lung cancer is a disease where tissue in the lung grows out of control. This may lead to metastasis, invasion of adjacent tissue and infiltration beyond the lungs. The vast majority of primary lung cancers are carcinomas of the lung, resulting from epithelial cells. One of the causes of lung cancer is exposure to coal dust. Exposure to coal dust can cause some coal mine workers to develop pneumoconiosis, or "black lung." This occurs when inhaled coal dust becomes embedded in the lungs, causing them to harden and making breathing difficult.