Prior to 1950, reported cases of malignant mesothelioma were so rare that even its existence was questioned by some pathologists. However, post World War II, the use of asbestos increased significantly, and a link was established between exposure to the substance and onset of mesothelioma in 1960. The western world had mostly abandoned its use in the 1980s. However, the total number of mortalities from mesothelioma still continues to rise due to the long latency period (15-60 years) between asbestos exposure and development of mesothelioma. Mesothelioma mortality rates in the USA had peaked in 2002, reaching 3060 in a single year, after which they have witnessed a declining trend. However, incidences of mesothelioma in the UK will continue to rise and are expected to reach peak levels around the year 2020. Through 1973-1992, data gathered under the purview of the US Surveillance Epidemiology and End Results programme shows a constant rate of mesothelioma in females, but a consistently higher incidence among males. A mesothelioma register was set up in the 1960s in the UK to systematically record mesothelioma mortality rates. It also aimed at identifying patients who had developed tumors, but did not have any known history of asbestos exposure. Mortalities due to mesothelioma are expected to be reach 90,000 by 2050 and 65,000 of these are predicted to occur from 2002 onwards.

Asbestos occurs naturally as a fibrous silicate. Risks associated with onset of mesothelioma depend on the type of asbestos mineral fiber an individual may have been exposed to. Fibers that make up most of the primary asbestos mineral groups include the serpentine fibers that are curly & long and amphibole fibers that are straight (rod-shaped). It is essential to make a distinction between the two because it is easier to clear the serpentine fibers from the respiratory tract. Epidemiologic data indicates that risks of developing mesothelioma are highest with exposure to amphibole, crocidolite fiber and lowest with chrysotile, a serpentine fiber. Although, occupational asbestos exposure has been identified as the primary cause of mesothelioma in patients, there have been instances where mesothelioma has also developed due to non-occupational “environmental” exposure and para-occupational exposure (industrial workers inadvertently carrying home the asbestos fibers stuck on their clothing which affects their family members). There are possibilities that idiopathic or spontaneous mesothelioma can also develop in both animals and humans without any known exposure to asbestos. The estimated occurrence rate of spontaneous mesothelioma in humans is approximately one per million.

Exposure to certain non-asbestos mineral fibers can also lead to mesothelioma. These include erionite, which is found in a few parts of Turkey, and tremolite, found in north-western Greece. Tremolite is not mined commercially, but it has been identified as a contaminant of chrysotile asbestos and linked with the potential onset of mesothelioma. A relatively more controversial aspect involves the Simian virus (SV-40) and its role in the pathogenesis of mesothelioma. In UK, during the 1950s and 1960s, the virus is said to have contaminated polio vaccines and while it was linked to the development of mesothelioma, recent studies have not found any definitive links.

The Evolving Face of Occupational Medicine

 

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