**3. Epidemiology**

The main risk factor in developing malignant mesothelioma is asbestos exposure [13]. Asbestos refers collectively to a group of naturally occurring hydrated mineral silicate fibers that include two major forms: serpentine, represented by chrysotile (white asbestos); and the amphiboles, including crocidolite (blue asbestos), amosite (brown asbestos), anthophyllite, actinolite and tremolite[13],[14].Crocidolitefibersareregardedasthemostoncogenictypeofasbestosbecause they are long and thin, and are believed to persist longer in the pleura, but the exact way in which asbestos induces the development of malignant mesothelioma is still not well understood [13], [14]. Inflammation appears to play a critical role as following asbestos exposure in vivo, recruit‐ ment of mononuclear phagocytes (which differentiate into macrophages that in turn phagocy‐ tize asbestos) was observed, resulting in the release of tumour necrosis factor-alpha (TNF-a) by the phagocytes and mesothelial cells [14]. Exposure to asbestos can also lead to the accumula‐ tion of DNA damage in mesothelial cells through interaction with reactive nitrogen and oxygen species,whichcoupledtotheactivationoftheNF-kBpathwaybyTNF-aperpetuatesthesurvival of the DNA-damaged mesothelial cells [14–16].

Crocidolite asbestos is found only in South Africa and Western Australia but has been exported all over the world for various industrial uses [17]. Chrysotile accounts for 97% of worldwide asbestos production and has been mined principally in Russia, Canada (Quebec Province), South Africa, Italy, and Cyprus [17]. Chrysotile itself is not thought to cause malignant pleural mesothelioma but is often contaminated with amphibole fibers, such as tremolite or amosite [18], [19]. However, the issue of chrysotile as a cause of malignant mesothelioma remains still controversial [17–19].

Individuals can be exposed to asbestos in many situations because of its widespread use [17], [20]. However, the areas of the world that have a high incidence of malignant pleural meso‐ thelioma are those with asbestos mines and countries that have shipyards, insulation, con‐ struction, and automobile industries that use large amounts of asbestos [17], [21–23].Regarding tobacco use, there is no evidence that smoking increases the risk of development of malignant mesothelioma [1], [24]. On the other hand, past radiotherapy is considered as a risk factor from case series [1], [25].
