Exposure to crystalline silica is known as the lung cancer in human. There are a few studies on estimation of lung cancer risk of crystalline silica in workplaces. So, the aim of this study was to lung cancer risk assessment of occupational exposure to crystalline silica in Iran. For overview, databases including Web of Sciences, Scopus, PubMed, Google Scholar, and SID (Scientific Information Database), were searched. All of the available studies in the field of occupational exposure to crystalline silica were provided. According to the cumulative exposure, risk of silicosis-related mortality in exposed workers was estimated. Exposure to crystalline silica was ranged from a geometric mean of 0.0954 to 0.2784mg/m3, which was higher than that recommended standard exposure limit by ACGIH (0.025mg/m3), NIOSH (0.05mg/m3) and OSHA (0.01mg/m3). The relative risk of death from silicosis was in the range of 1 to 63.63 per thousand people and the risk of lung cancer in workers was ranged from 36 to 104 per thousand. Workers in the mines and cement manufacturing had highest and lowest of the risk of lung cancer, respectively. Result indicated that workers are at considerable cancer risk due to exposure to crystalline silica, the exposure control programs need to be implemented in workplaces to decrease concentration of crystalline silica.
Keywords: Crystalline silica, Exposure, systematic review, Lung cancer risk, Iran
Crystalline silica (SiO2) is the second most common mineral which has covered more than ninety percent on the earth’s crust (Yaroshevsky, 2006). Hence, sand, rock, and soil have the most abundant crystalline silica (Möhner et al., 2017). Workers are exposed to crystalline silica in mining, smelt, sandblast, mason, ceramic, and glass industrials (Azari et al., 2009). Previously published studies showed that masons, plasters, and miners have the highest exposure to crystalline silica (Sanjel et al., 2018; Hoy and Chambers, 2020). Respiratory system is known as the primary pathway of exposure to crystalline silica dust in the human body (Liu et al., 2020).
Exposure to crystalline silica has long been identified to be related to lung diseases (Barnes et al., 2019). Silicosis is a well-known lung disease (Leso et al., 2019). In the exposure to higher concentrations of crystalline silica, “acute silicosis” can happen which has a high mortality (Steenland et al., 2002). “Chronic silicosis” is the most common form of the pulmonary fibrosis among crystalline silica-exposed workers (Möhner et al., 2017). The studies suggest that pulmonary fibrosis elevate the risk of lung cancer (Lacasse et al., 2009). International Agency for Research on Cancer (IARC) has introduced crystalline silica in the form of quartz or cristobalite as a human carcinogen (IARC, 1997). However, workers are exposed to low concentration of crystalline silica in workplace, but they are may be at risk of lung cancer (Rice et al., 2001).
There are an estimated 23 million crystalline silica-exposed workers in China (Chen et al., 2012). Also, over three million in India (Anlar et al., 2017), and over two million employees in the United States (Bang et al., 2015) are exposed to crystalline silica. Yearly, almost 800 workers died from lung cancer as a result of inhaling crystalline silica in Britain (Ahadzi et al., 2020). Published studies reported that the workers in developing countries have exposure to crystalline silica in workplaces (Moyo and Kgalamono, 2017).
Recently, risk assessment has become one of the most important aspects in managment of occupational diseases (Klimova et al., 2018). There is few studies on estimation of relative risk of the lung cancer due to exposure to crystalline silica (Harati et al., 2017; Mohammadi et al., 2017b; Klimova et al., 2018; Rokni et al., 2016). For example, in Iran Azari et al.that the relative risk of death from silicosis was in the range of 1 to 63.6 per thousand people, andthe risk of lung cancer in workers was 124.08 per thousand people (Rezazahehazari et al., 2020). Some studies in other countries have been collected quantitative exposure data for estimating of risk of lung cancer due to exposure to crystalline silica (Lacasse et al., 2009; Poinen-Rughooputh et al., 2016).
There is not comprehensive study on status of exposure to the crystalline silica and its health risk in Iranian work places. So, the aim of this study was to provide a systematic review of exposure to crystalline silica in workplaces and estimate of its health risk
Material and Methods
All of available studies in the field of occupational exposure to crystalline silica including case reports, case series, editorials, case-control, cohort studies were provided. The literature search strategy was conducted by the main keywords such as “Silica”,” Crystalline silica “, “Exposure, “Occupational exposure “, “Industrial”, “Workplace”, “Factory” and “Iran”. All of the articles that the concentration of crystalline silica in air samples have been reported, were selected for this study. All of articles published in English and Persian languages were chosen. The search for articles was limited between 2000 to 2021 and included citation or lack of citation. The Cochrane review method was used as a method guideline for systematic review (Chandler and Hopewell, 2013; Nasirzadeh et al., 2020).
Web of Sciences (WOS), Scopus, PubMed, Google Scholar and SID (Scientific Information Database) were selected to implement the search strategy. The search strategy was performed by the following databases:
- PubMed: (((((((((Silica[Title/Abstract]) OR (Crystalline silica[Title/Abstract]) AND Exposure[Title/Abstract]) OR Occupational exposure[Title/Abstract]) OR workplace[Title/Abstract]) OR factory[Title/Abstract]) OR Industrial[Title/Abstract]) AND Iran[Title/Abstract])).
- Scopus and WOS: ((keyword (Silica) OR keyword (Crystalline silica) AND keyword (Exposer) OR keyword (Occupational exposure) OR keyword (workplace) OR keyword (Factory) OR keyword (Industrial)) AND ((keyword (Iran)).
Screening of Articles
The screening of articles was performed by title, abstract, and full-text of the articles, separately. The inclusion criteria were all of articles performed in occupational exposure to crystalline silica in Iran. The excluded criteria were the articles worked on biomonitoring of individuals. Moreover, abstracts (without their full-text available online), review and mini-review articles, conference papers, meta-analyses and modeling studies, books, and unpublished studies were excluded.
We used EndNote X9® (Thomson Reuters, Toronto, Canada) software (Hupe, 2019) to prepare the list of the articles and finally downloaded the full text of the screened articles. In order to reduce the error, two researchers conducted the search strategies, separately. When there were disagreements, a third researcher was involvement.
As a set out table 1; data extraction was performed based on year, city, monitoring station number, mean and standard deviation concentration of crystalline silica and method of detection.
Risk assessment of death due to exposure to crystalline silica was performed by using of Mannetje’s method (Steenland et al., 2001). In this method, exposure history and crystalline silica concentration are two main factors. Also, exposures of all industrial workers in different studies were classified according to Mannitejie category for cumulative exposure categories (Steenland et al., 2001).
Relative risk of silicosis related mortality was calculated according to the model of Rice et al (Rice et al., 2001) by formula 1. This model is based on the geometric mean of exposure to crystalline silica and 45 years of exposure. In this formula, (A) is the risk of death from lung cancer in workers and (GM), geometric mean of exposure to crystalline silica.
A= 0.77 + 373.69 × GM (1)
Results & Discussion
The research reports of the databases showed that all of the articles were 72 from September 2000 to September 2020. Of these, 28 were indexed in PubMed, 8 in Scopus, 5 in WOS, 24 in SID and 7 in other databases. Due to duplication, 15 articles were put aside. Finally, 36 papers were selected to the study, which was analyzed by Preferred Reporting Items for Overviews of Reviews (PRIOR) method. The total of 24 articles were published in Persian language. The number of 1421 measuring stations in various industries were studied in 36 studies conducted in the field of worker’s exposure in Iran.
In the most of the studies, workers worked six days a week and their working hours are more than 8 hours. Meta-analysis suggested that geometric mean concentration for worker’s exposure to crystalline silica was ranged of 0.0954 to 0.2784mg/m3 (Table 2). These estimations show that occupational exposure limit for crystalline silica was higher than recommended by American Conference of Governmental Industrial Hygienists (ACGIH) (0.025mg/m3) (ACGIH, 2010), National Institute for Occupational Safety and Health (NIOSH) (0.05mg/m3) (NIOSH, 2002) and also Occupational Safety and Health Administration (OSHA) (0.01mg/m3) (OSHA, 2019), in all
Workers in various industrial sectors were exposed to different grades of crystalline silica. In this study, we observed that iron-ore miners are exposed to the highest amount of crystalline silica and workers working in the flour mill are the least exposed (Safinejad et al., 2019; Semnani et al., 2007a).
Between 2004 to 2008, Scarselli et al. in a study in Italy, reported that workers in the manufactures and construction industries are the high risk groups of occupational exposure to crystalline silica (Scarselli et al., 2008).
In 2015, according to OSHA compliance data from1979 to 2015, Doney et al. reported that workers in the poured concrete foundation had the highest exposure to crystalline silica. Also, out of 99 700 workers, 100 000 workers had potentially exposed to crystalline silica at higher than the occupational exposure limit recommended by NIOSH in 2014 (Doney et al., 2020).
In 2016, a meta-analysis study, based on worldwide studies up to April 2016, the highest pooled concentration mortality ratio of exposure to crystalline silica was estimated 6.03 (95 % CI 5.29–6.77) in mixed industries of Japan. Also, Italy had the highest observed lung cancer deaths (798 cases) before 2006. Moreover, miners had the highest risk of lung cancer with the pooled standardized mortality ratio of 1.48 (95 % CI 1.18–1.86) (Poinen-Rughooputh et al., 2016). Interestingly, in this study, although estimated health risk was high in Asia countries after Canada, the research studies in Iran have been neglected.
In 2018, Kim et al. suggested that geometric mean concentration of crystalline silica for occupational exposure ranged of 0.006 to 0.399 mg/m3 in Korea. The highest concentration of crystalline silica was estimated in construction (concrete grinding) industries (Kim et al., 2018). However, mean concentration in all of Korean industries was lower than Iran.
Table 3 shows the relative risk (RR) of silicosis- related mortality was in the range of 1 to 63.63 per thousand people based on cumulative exposure category. According to Mannetje’s study, relative risk for exposure to 0.05mg/m3 crystalline silica is six per thousand people (Steenland et al., 2002). In all of industrials, mean concentration is higher than 0.05mg/m3 and relative risk (RR) of silicosis is high.
As a set out in table 4, the risk of lung cancer due to exposure to crystalline silica in workers in all industries in Iran was in the range of 36 to 104 per thousand. According to geometric mean cumulative exposure, in the mines, estimated excess lifetime risks of mortality from lung cancer was at highest level. This estimation is similar to Poinen-Rughooputh et al’s study (Poinen-Rughooputh et al., 2016).
In Liu et al’s study, when the mean cumulative concentration (using a 25-year lag) was 0.01 to 1.12mg/m3-y, estimated excess lifetime risks of mortality from lung cancer was 128 per thousand with rate ratio 1.26 (Liu et al., 2013).
Chen et al. suggested that mortality was 992.6 per 100,000 person-years among dust-exposed workers and risk ratio for lung cancer was 0.90 (0.84–0.97) from 1970 to 2003 in China (Chen et al., 2012). Comparison of studies show that risk of crystalline silica-induced mortality in Iran is approximately 16% higher than that China.
The authors provide a lung cancer risk assessment of occupational exposure to crystalline silica in Iran industrials based on the collected quantitative exposure data. At present study, occupational exposure to crystalline silica was higher than that occupational exposure limits. Also, the relative risk of death from silicosis was in the range of 1 to 63.63 per thousand people and the risk of lung cancer was ranged from 36 to 104 per thousand. It seems that the prevalent occupational health engineering strategies is not sufficient to protect workers. So, workers’ exposure to crystalline silica dust should be controlled in Iranian workplaces.
We would like to express our special appreciation for people who helped to collecting data in this study. Their willingness to give their time so kindly is very much appreciated.
Conflicts of interest
The authors declare that they have no conflict of interest.
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