The Major Cause Of Malignant Mesothelioma

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

Asbestos was known in antiquity, but it was not mined and widely used commercially until the late 19th century. Its use greatly increased during World War II. Since the early 1940s, millions of American workers have been exposed to asbestos dust. Initially, the risks associated with asbestos exposure were not publicly known. However, an increased risk of developing mesothelioma was later found among shipyard workers, people who work in asbestos mines and mills, producers of asbestos products, workers in the heating and construction industries, and other tradespeople. Today, the official position of the U.S. Occupational Safety and Health Administration (OSHA) and the U.S. EPA is that protections and "permissible exposure limits" required by U.S. regulations, while adequate to prevent most asbestos-related non-malignant disease, they are not adequate to prevent or protect against asbestos-related cancers such as mesothelioma. Likewise, the British Government's Health and Safety Executive (HSE) states formally that any threshold for mesothelioma must be at a very low level and it is widely agreed that if any such threshold does exist at all, then it cannot currently be quantified. For practical purposes, therefore, HSE assumes that no such "safe" threshold exists. Others have noted as well that there is no evidence of a threshold level below which there is no risk of mesothelioma. There appears to be a linear, dose-response relationship, with increasing dose producing increasing disease. Nevertheless, mesothelioma may be related to brief, low level or indirect exposures to asbestos. The dose necessary for effect appears to be lower for asbestos-induced mesothelioma than for pulmonary asbestosis or lung cancer. Again, there is no known safe level of exposure to asbestos as it relates to increased risk of mesothelioma.

The duration of exposure to asbestos causing mesothelioma can be short. For example, cases of mesothelioma have been documented with only 1–3 months of exposure. People who work with asbestos wear personal protective equipment to lower their risk of exposure.

Latency, the time from first exposure to manifestation of disease, is prolonged in the case of mesothelioma. It is virtually never less than fifteen years and peaks at 30–40 years. In a review of occupationally related mesothelioma cases, the median latency was 32 years. Based upon the data from Peto et al, the risk of mesothelioma appears to increase to the third or fourth power from first exposure.

Environmental exposures

Incidence of mesothelioma had been found to be higher in populations living near naturally occurring asbestos. For example, in central Cappadocia, Turkey, mesothelioma was causing 50% of all deaths in three small villages — Tuzköy, Karain and Sarıhıdır. Initially, this was attributed to erionite, a zeolite mineral with similar properties to asbestos, however, recently, detailed epidemiological investigation showed that erionite causes mesothelioma mostly in families with a genetic predisposition. The documented presence of asbestos fibers in water supplies and food products has fostered concerns about the possible impact of long-term and, as yet, unknown exposure of the general population to these fibers.

Occupational

Exposure to asbestos fibers has been recognized as an occupational health hazard since the early 20th century. Numerous epidemiological studies have associated occupational exposure to asbestos with the development of pleural plaques, diffuse pleural thickening, asbestosis, carcinoma of the lung and larynx, gastrointestinal tumors, and diffuse malignant mesothelioma of the pleura and peritoneum. Asbestos has been widely used in many industrial products, including cement, brake linings, gaskets, roof shingles, flooring products, textiles, and insulation.

Commercial asbestos mining at Wittenoom, Western Australia, occurred between 1945 and 1966. A cohort study of miners employed at the mine reported that while no deaths occurred within the first 10 years after crocidolite exposure, 85 deaths attributable to mesothelioma had occurred by 1985. By 1994, 539 reported deaths due to mesothelioma had been reported in Western Australia.

Paraoccupational secondary exposure

Family members and others living with asbestos workers have an increased risk of developing mesothelioma, and possibly other asbestos related diseases. This risk may be the result of exposure to asbestos dust brought home on the clothing and hair of asbestos workers. To reduce the chance of exposing family members to asbestos fibres, asbestos workers are usually required to shower and change their clothing before leaving the workplace.

Asbestos in buildings

Many building materials used in both public and domestic premises prior to the banning of asbestos may contain asbestos. Those performing renovation works or DIY activities may expose themselves to asbestos dust. In the UK use of Chrysotile asbestos was banned at the end of 1999. Brown and blue asbestos was banned in the UK around 1985. Buildings built or renovated prior to these dates may contain asbestos materials.