Asbestos has been discovered and mined by mankind for thousands of years . The modern asbestos industry started in the 1800s and the production and use have raised ever since . The link between occupational exposure to asbestos and mesothelioma was first proposed in 1935 . The popularity comes from the unusual and practical properties of these fibers such as heat resistance, flexibility, and high tensile strength, thermal, electrical, and sound insulation [4, 23].
Followed by the extensive use of asbestos in the industry, the annular mortality due to MM increased dramatically in the 1900s . That was a wake-up call for many countries to start banning and limiting the use of these hazardous fibers in industry. Laws on asbestos ban started with Denmark in 1972 with the ban of thermal and noise insulation and waterproofing use of asbestos and soon after, other countries came up with new legislation limiting the use of asbestosis too; Iceland was the first country to completely ban asbestos use in 1983 . The effect of the new legislation can be seen from 1980 when worldwide asbestosis usage started to decline. By 2010, 52 countries banned all forms of asbestos use; but Iran was still one of the 8 countries (Brazil, China, India, Iran, Kazakhstan, Russia, Thailand, and Ukraine) with the highest usage in 2003 [21, 26]. With the estimated median latent period of 32 years after the first occupational exposure, it is obvious that MM is still going to be of a concern in the following decades and it can be expected that the peak has not yet reached in Iran like other countries such as Italy which banned asbestos in 1992 [24, 27, 28].
Numerous studies have been conducted worldwide to raise awareness around asbestos carcinogenicity which led to this strict legislation we have today. Regarding our study, the mean age of the patients was 61 years, and male to female ratio was 3 to 1 which is corresponding with many articles such as Haining Yang et al. that demonstrated most of the patients are between 50 and 70 years of age and that men are more at risk due to occupational exposure than women . Furthermore, their study, similar to ours, stated that there is no evidence showing an association between MM and smoking. Contrary to our study, Abdel-Hamid MA et al. regarding the environmental and occupational risk factors among malignant pleural mesothelioma, reported that smoking was considered as a risk factor for MM of pleura .
In our study oil company workers were the second group, after housekeepers, who were diagnosed with MM. a study on occupational exposure in 2002 on 1445 confirmed cases of mesothelioma in the USA showed that shipbuilding, US navy, and construction industry were the top three industries with the most cases of MM while oil and chemical industry was in the 5th place . Taking into account that the oil and gas industry is the main industry in the south of Iran, it is safe to say that our results are comparable to the results of the mentioned study. Another study on employees of refinery and petrochemical in Lambton County, Ontario also confirmed that employment as a maintenance worker in the refinery and petrochemical sector was associated with increased risk of asbestos-related MM .
Also, other forms of occupational exposures , such as non-occupational and environmental exposures should also be taken into account; although lower compared to occupational exposure, many people are at risk by various means during their lifetime especially women [31, 32]. In 2011 Alessandro Marinaccio and his colleagues studied on Pleural malignant mesothelioma epidemic which demonstrates occupational asbestos exposure was in 69.3% of pleural MM cases (N = 4,577 cases), while 4.4% was due to cohabitation with someone (generally, the husband) who was occupationally exposed, 4.7% by environmental exposure from living near a contamination source and 1.6% during leisure activity . Excluding the 6 oil company employees, 7 of our patients lived near the oil company which could be a potentially contaminated source which our study showed that increases developing MM by threefold. In a similar study on 272 cases of MM in Iran from 2006 to 2010, which was reported by the ministry of health of Iran, high-risk occupations included construction workers with 20% of cases followed by oil company workers with 13.3% . In our study no significant difference was seen between our case and control group regarding the construction workers; however, this could be because in this study some of the cases were excluded because they could not be contacted and the sample size was small, but oil company employees are proven to be a high-risk occupation in Iran regarding MM.
Almost all articles reviewed concurred that due to the tumor's aggressive nature and limited efficacy of current therapies, the median survival of patients with MM from the time of diagnosis is about 12 months. This result is consistent with our findings demonstrating an average of 11 months [4, 31].