Operate with asbestos is the major risk cause for mesothelioma. In the USA, asbestos is the leading cause of malignant mesothelioma and has been considered "indisputably" connected with the development of mesothelioma. so, the relationship between asbestos and mesothelioma is so strong that many consider mesothelioma a “signal” or “sentinel” tumor. A past of asbestos exposure exists in most cases. However, mesothelioma has been reported in several individuals without any notable exposure to asbestos. In rare cases, mesothelioma has also been associated irradiation, intrapleural thorium dioxide (Thorotrast), and inhalation of other fibrous silicates, such as erionite. Some researches suggest that simian virus 40 (SV40) may act as a cofactor in the progress of mesothelioma.