Tobacco chewing has been strongly associated with cancer of the mouth and esophagus. A metastasis describes the spread of a tumor to distant sites in the body. Benign tumors are characterized by being surrounded by a fibrous capsule.
Benign tumors are never life-threatening.
Asbestos exposure is commonly associated with increased cancer in the pleural lining of the chest. A chemical may produce colon cancer in an experimental animal system, but be completely harmless to human beings. In 1775 Dr. Percival Pott observed that x-rays caused skin cancer. Lung cancer incidence has shown a dramatic decline in the US during the past 50 years Tumor metastasis involves the penetration of malignant cells into the blood stream or lymphatics. Chronic exposure to benzene has been associated with a significant decline in the population of predatory birds such as the bald eagle and the brown pelican.