Occupational characterization of workers exposed to asbestos: an integrative review

Asbestos is a mineral fiber abundant in nature and classified as a carcinogen since 1987. The present study aimed to identify, in the scientific literature, what are the occupation and activities developed by sick workers and which categories would be affected with asbestos-related diseases. Through a literature review performed in the following databases: PubMed, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Web of Science, and Regional Portal of the Virtual Health Library, 23 studies published from 2015 to 2020 were selected and evaluated. The occupations that showed greater illness due to exposure to asbestos were general asbestos workers (40%), miners (22%), and textile workers (9%), followed by naval, automotive, carpentry, doll-making, construction, and upholstery workers, as well as workers involved in the rescue, recovery, cleaning, and restoration of the World Trade Center (4%). Of the disease associated with exposure to asbestos, the most described is malignant mesothelioma (43%). Evidence found corroborate pre-existing information in the literature showing that exposure to asbestos may be harmful to health. Moreover, the importance of using personal protective equipment was emphasized, in order to prevent the development of asbestos-related diseases.


INTRODUCTION
Asbestos is a collective term used for commercial identification of a heterogeneous group of minerals whose crystals form bundles of easily separable fibers. These minerals derive from eruptive metamorphic rocks that, due to a natural process of recrystallization, undergo changes, forming a fibrous material. They belong to two different groups: a) amphiboles, which includes amosite (brown asbestos), crocidolite (blue asbestos), anthophyllite, actinolite, and tremolite; and b) serpentines, represented by chrysotile (white asbestos). 1,2 The applicability of this mineral goes beyond the construction industry. There are reports of more than 3 thousand products made of asbestos, such as water tanks, asbestos-cement tiles, canvas and brake pads for cars and trucks, fabrics, fireproof blankets, thermal insulation fabrics, vinyl flooring, hydraulic cardboards, automotive gaskets, inks, asphalt, and reinforced plastics. 1,3 Surely, exposure to asbestos contributes to the development of asbestos-related disease (ARD), resulting from biopersistence of fibers in the lung. 4,5 Furthermore, the José de Alencar Gomes da Silva National Cancer Institute (Instituto Nacional de Câncer José de Alencar Gomes da Silva, INCA) and the International Agency for Research on Cancer (IARC) state that the aforementioned fibers are potentially carcinogenic agents. 6 With regard to ARDs, it is worth highlighting malignant mesothelioma (MM), lung neoplasms, and non-malignant conditions, such as asbestosis and development of pleural plaques. 7 Occupational exposure is one of the main factors for the onset of ARDs and other diseases as well, since some workers are routinely exposed to different substances harmful to health. 8,9 Due to the applicability of asbestos in several products, its use was widespread in some countries such as Russia, China, Brazil, Thailand, Kazakhstan, India, and Ukraine. 10 Brazil also stood out as producer and exporter, a fact that deserves attention and monitoring by health services due to the possible occurrence of ARDs. 11,12 Total ban of asbestos is already a reality in at least 66 countries, 13,14 and, in Brazil, only in 2017 the Federal Supreme Court (Supremo Tribunal Federal, (STF) established the prohibition of extraction, manufacturing, and sale of chrysotile asbestos in the entire national territory. 15 ARDs are often identified lately, due to their long period of latency, which lasts from exposure to the first clinical symptoms, thus contributing to disease underregistration. 16,17 In light of the foregoing, knowledge of the possible at-risk groups and of the prevalent diseases resulting from occupational exposure has become indispensable. Based on these considerations, the aim of this study was to identify, in the scientific literature, what are the occupations and activities developed by sick workers and what categories are most affected with ARDs.

METHODS
This was an integrative literature review, a method that allows for gathering and synthesizing evidence available in articles produced about a topic. 18 This research was developed based on Whittemore & Knaff 19 , who proposed the following steps: 1) formulation of the research question; 2) data collection; 3) data assessment according to inclusion and exclusion criteria; 4) data analysis; and 5) presentation of results and knowledge synthesis.
The guiding research question was structured using the non-clinical research strategy characterized by the acronym formed by the words population/patient/ problem, interest, and context (PIC), being P: workers; I: disease caused by asbestos; and C: exposure to asbestos. Therefore, the resulting question was "what characterizes workers with diseases resulting from exposure to asbestos?".
Inclusion criteria consisted of studies published from January 2015 to January 2020 and written in English, Italian, Portuguese, or Spanish. Exclusion criteria consisted of duplicate articles and those not fully available online, as well as theses, dissertations, letters, and editorials from scientific journals. The search for studies was performed from February to March 2020, in the following databases: PubMed, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Web of Science, and the Regional Portal of the Virtual Health Library (VHL), which gathered findings from the MEDLINE and IBECS databases. The following Health Science Descriptors (Descritores de Ciências da Saúde, DeCS) were used: "Workers", "Asbestos", "Amianto", "Occupational exposure," and "Inhalation exposure".
This search identified 320 studies, 175 in BVS, 22 in CINAHL, 82 in PubMed, and 41 in Web of Science, and, after application of exclusion criteria, 237 publications remained in the analysis. These publications were then analyzed after reading of abstracts, and, finally, 23 articles were considered eligible, which constituted the sample of this review.
The study selection process was based on the recommendations of Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA), 20 as shown in Figure 1.

RESULTS
The final sample consisted of 23 articles that addressed the occupational relationships of workers with diseases resulting from exposure to asbestos. Of these articles, 17 (74%) were cohort studies, 5 (22%) were case studies, and 1 (4%) was a pilot study. Concerning the country where the study was conducted, Italy was the leading country, with seven (30%) articles, followed by the United States (USA), with five (22%); South Korea, China, and France, with two (9%) articles each; and then Germany, Belgium, Great Britain, Japan, and Spain, with one (4%) article each.
As for workers' occupations reported in the studies, nine (40%) articles included general asbestos workers; five (22%), miners; two (9%), textile workers, followed by workers in the naval, automotive, carpentry, doll making, construction, upholstery industries, and workers who participated in the rescue, recovery, cleaning, and restoration of the World Trade Center, with one (4%) publication each.
Corning the study population, it was observed that, in 16 (70%) articles, sick workers were men; whereas four (17%) studies involved both men and women and three (13%) studies included only female participants.
Among the ARDs that affected workers, articles reported cases of MM in ten (43%) publications; lung cancer and other neoplasms, such as those affecting mouth, pharynx, esophagus, liver, bile duct, ovary, and colon, were found in eight (35%) publications; and respiratory diseases, asbestoses and the development of pleural plaques were observed in five (22%) studies.
Chart 1 presents the main characteristics of the articles selected for this review.

DISCUSSION
The selected studies revealed the hazards of asbestos and characterized workers with diseases resulting from exposure to this substance. Measures to restrict its use have already been implemented by several countries; however, its deleterious effects will last for decades in the health of many workers. 44,45 ARDs are increasingly more common and are especially observed in regions where this mineral was widely employed, being also considered an occupational disease. 43,44,46 Of the ARDs that affect the workers included in this research, it is worth noting the prevalence of MM, a rapidly evolving disease whose period of latency depends on time of exposure and may reach up to 50 years. 21,27,47,48 A study conducted in Belgium in 2015 showed that mortality for MM was higher among bluecollar asbestos workers. 22 It bears highlighting that, in other studies, workers of the following industries also presented with diagnosis of MM: naval, textile, automotive, chemical, metal, construction, and furniture. [21][22][23]25 Furthermore, MM occurred among employees of a doll-making factory, whose exposure to asbestos has not been known so far. 32 The incidence rates of MM were higher in the male population. 23,29,37 The prevalence in men is justified by the occupations they perform, which often involve a direct handling of asbestos. 34,36 Women diagnosed with MM were exposed to asbestos in the workplace, especially in the textile industry, or had a history of paraoccupacional exposure, probably derived from the household environment or from contact with a family member who worked directly with the mineral. 34,36,49 Lung cancer was also identified, with incidence for workers of the naval industry 21,28,39 and for those who handle asbestos directly. 22 Trachea and bronchus cancer was also observed in this population. 21 A relevant issue for the development of these diseases is the possible association between smoking and exposure to asbestos: the frequency of selfacknowledged smokers in a study was 79.5%, 31 corroborating another research that found 74.4% of smokers with lung cancer. 41 Therefore, smoking cessation is also necessary in the population exposed to asbestos. 31,50 The synergy between smoking and exposure to asbestos potentiates the toxic effects of fibers and increase the risk for lung cancer.
Other neoplasms attributable to exposure to asbestos are those affecting mouth, pharynx, esophagus, liver, and bile duct, and mortality for these causes was found in blue-collar asbestos workers, followed by automotive, naval, and construction workers. 21,22 This circumstance confirms the carcinogenic potential of the fibers. 51,52 Contrary to the already reported types of cancer and in line with a study conducted in Italy, the occurrence of ovary cancer stands out in the female population, especially among women working in the textile industry. 41,42,53 For the male population, in turn, findings point out to development of colorectal cancer. 30,54 With regard to non-cancerous asbestos-related diseases, they include asbestosis and pleural plaques. 24,28 Asbestosis is caused by the deposition of asbestos fibers in the lungs, leading to fibrosing interstitial pneumonitis. 55,56 Pleural plaques represent a predisposing factor of ARDs. 38,40,55 Miners are found to be the workers with the most extensive pleural plaques and asbestosis. 24 In others studies, there are reports of the prevalence of these conditions in construction workers, followed by metal, mining, and furniture workers. 33,31,40 This differs from findings reported in studies conducted in Marysville (United States) and in Libby (United States), which did not evidence an association between single pleural plaques and reduced lung function. 26,35 However, the identification of these data, which are related to risk factors, help predict malignant comorbidities. 38,40 Studies emphasize that workers exposed, directly or not, to asbestos should be informed about the hazards it causes, so as to provide better work conditions, as well as better health surveillance, for this population. 25,39,57 It is important to reinforce the importance of characterizing workers, in order to promote health care measures aimed at those whose occupation is directly related to ARDs.

CONCLUSIONS
Evidence found in this study makes it possible to characterize workers with diseases resulting from exposure to asbestos. Findings reveal that workers exposed, directly or not, to asbestos, are prone to the development of any type of ARD.
The main occupations of these workers consists of general asbestos workers, miners, carpenters, as well as those working in the textile, naval, and automotive industry. Associated diseases were MM, lung, ovary and colorectal neoplasms, and non-neoplastic conditions such as asbestosis and pleural plaques. In most studies, workers with diseases resulting from any type of ARD were men.
These data corroborate information showing that exposure to asbestos may be detrimental to health. The hazards of occupation exposure are highlighted; however, the risk is still imminent, since restriction of asbestos is not still a reality in many countries. Therefore, it is proposed to develop scientific productions and to implement effective health surveillance measures aimed at the most vulnerable population.
One limitation of this study was the non-inclusion of grey literature. However, the articles selected for this integrative review contributed to the outcome of the research, since they were recent articles published in international journal and addressing a relevant topic.