Initially mesothelioma was considered a disease that is entirely related to asbestos exposure. Recent reports suggest that a history of asbestos exposure can be obtained only in about 80% cases of mesothelioma. It is also to be noted that less than 10% of people who had exposure to asbestos develop mesothelioma. A comprehensive survey of adult mesothelioma cases in Canada and the U.S. has found that asbestos exposure had been unlikely in about 25% to 33% of cases (McDonald & McDonald, 1980).
A number of other fibrous and non-fibrous materials have been associated with mesothelioma induction. Durable, long and thin fibers have cancer inducing properties. A number of natural and man-made fibers with these characteristics have been established as the causative agents for mesothelioma in laboratory animals. These include glass fibers, aluminum oxide, attapulgite, dawsonite, silicon carbide and potassium titanate (Stanton et al., 1977).
A reported outbreak of mesothelioma in rural Turkey has been associated with exposure to fibrous zeolite found in these regions. In his report, Baris had identified 185 cases of erionite/zeolite-related mesothelioma in two areas of Turkey with no local asbestos deposits or industry.
In addition to fibrous agents, several non-fibrous agents, both organic and inorganic, have also been shown to induce malignant mesothelioma. For example, a causal link between mesothelioma and radiation has been established based on numerous case reports of mesotheliomas developing at the exact sites of radiation therapy. Patients with history of Hodgkin's disease have a higher risk of development of mesothelioma probably because of radiation therapy associated with the treatment of this disease. In these patients the mesothelioma develops usually within the area of radiation therapy field with an average delay of 15 years from the time of radiation therapy. Other suspected causes include biogenic silica fibres, chronic irritation stemming from tuberculosis and other factors, and heavy metals such as beryllium.
It is clear from these observations and studies that malignant mesothelioma is not uniquely related to asbestos exposure. Mesothelioma is malignancy that arises from the serosal covering of the tissues. Various materials and physical agents can induce this malignancy. Asbestos is the commonest of these agents that can cause mesothelioma.