Peritoneal mesothelioma is recognized only to be caused by exposure to asbestos, a microscopic natural fiber that was used heavily in industry. Asbestos is said to reach the abdominal wall by one of two methods. The first is through ingested asbestos fibers which are processed through digestion and become lodged in the peritoneum. The other method is through the lungs and lymph nodes by inhaled asbestos fibers. Malignant mesothelioma of the pleura is also known to metastasize directly into the abdominal cavity if its spread is not slowed.