1 Introduction

Respirable coal mine dust (RCMD) is a mixture of more than 50 elements and their oxides. It is estimated that 40%–95% of respirable dust in the underground coal mine is pure coal, and the rest contains particles originated from cutting roof and floor rock, diesel exhaust from equipment, and rock dusting (Walton et al. 1977; NIOSH 1995). Cumulative inhalation of RCMD can lead to severe lung diseases, including coal worker’s pneumoconiosis (CWP), silicosis, mixed dust pneumoconiosis, dust-related diffuse fibrosis (DDF), and progressive massive fibrosis (PMF) (Cullinan et al. 2017; Hall et al. 2019). In the United States, an unexpected and severe increase in coal miners’ lung diseases in the late 1990s prompted researchers to investigate the causes of the disease resurgence.

Several potential reasons have been hypothesized for the increased rate of CWP among U.S. coal miners, including geological factors (e.g., Laney et al. 2010; Blackley et al. 2014), thin-seam mining (e.g., Antao et al. 2006; Sarver et al. 2019), quartz exposure (e.g., Sarver et al. 2020), coal rank (e.g., Laney and Attfield 2010; Pollock et al. 2010), and mine size (e.g., Blackley et al. 2014; Gamble et al. 2011). However, there has been no comprehensive systematic review investigating the role of all potential contributing factors.

Systematic review is a type of review that uses explicit and systematic methods to collect reliable data from all available databases to reduce random and systematic errors of bias; thus, providing reliable findings to draw conclusions, and make decisions (Torgerson 2003). Systematic review is a technically robust methodology for summarizing and testing the consistency of the primary studies in a given topic. In conducting a systematic review, all available evidence, whether they support or refute a hypothesis, are included. Therefore, the rejection or confirmation of a hypothesis is with fewer biases compared to other types of review (Shekarian and Mellat-Parast 2020).

A few researchers investigated some aspects of occupational exposure to RCMD. Beer et al. (2017) conducted a systematic review of the coal dust exposure to investigate whether the prevalence of occupational lung diseases is pure coal or coal mixed with silica minerals. In their study, a total of 2945 papers were collected, and nine papers were selected for evaluation of the topic by excluding the nonrelated articles. The study found that the evidence is limited for causal links between exposure to pure coal powder and lung diseases and suggested conducting further analysis of the data related to miners exposed to either pure coal or coal with very low minerals content. Jenkins et al. (2013) conducted a systematic review based on 34 selected articles to examine the links between coal dust exposure and cancer incidence and mortality. The study concluded that the available data are too limited to draw any final conclusions concerning a population cancer risk from coal mining.

Currently, there is no comprehensive systematic review on RCMD health effects in the U.S. This study aims to conduct a systematic review of RCMD exposure and investigate the impacts of mining parameters, including mine size, coal rank, geographical location, and coal seam thickness on the prevalence of CWP. The systematic review was performed based on the defined review protocol and then irrelevant papers were excluded. A total number of 401 publications were included in conducting this systematic review. The preferred reporting items for systematic reviews and meta-analysis (PRISMA) method was used and 18 articles were selected for data evaluation. A detailed discussion of the current state of knowledge on the potential role of mining parameters in the prevalence of CWP was provided. Furthermore, characterization of the selected studies, and the knowledge gaps were identified. The systematic review yields information on the role of various mining factors contributing to the coal mine disease. The information will be used to develop hypotheses during the statistical analysis of RCMD occupational health outcomes.

2 Materials and methods

A systematic review was conducted to investigate the contributing factors in recent resurgence of CWP among US coal miners. The steps for conducting a systematic review are shown in Fig. 1, and were followed in this study. Determination of an appropriate research question in early stages of a systematic review reduces the time and cost in identifying and studying relevant publications (Torgerson 2003). In this study, the following research questions were identified:

Fig. 1
figure 1

The steps for conducting a systematic review. (Adapted from Tsafnat et al. 2014)

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  1. (1)

    What are the potential contributing factors in the resurgence of CWP among the US coal miners at the end of the 1990s?

  2. (2)

    What is the current state of knowledge in RCMD exposure health risk?

  3. (3)

    What are the knowledge gaps that future research should address?

The inclusion and exclusion criteria are rigorously and transparently reported in a high-quality systematic analysis. Eligible criteria are defined to reduce the reviewer’s selection and inclusion bias in conducting the review. The protocol defines a set of pre-determined written conditions for inclusion and exclusion of studies. For example, the inclusion and exclusion may include a specific type of review, publication period and language, experiments carried on using a real experimental design, or other types of criteria related to the research questions. In this research, the eligibility criteria were defined in Table 1.

Table 1 Inclusion and exclusion criteria for RCMD systematic review
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For this research, the systematic search for studies was initiated from various electronic databases such as PubMed, ScienceDirect, and Google Scholar. The keywords used for collecting research articles included coal, respirable coal mine dust, coal dust characteristics, exposure limits and regulations, respiratory diseases, occupational health regulation, RCMD exposure, miners lung, black lung disease, lung deposition, CWP, silicosis, anthracosis, anthracosilicosis, interstitial lung disease (ILD), lung deposition, respiratory system, systematic review, occupational exposure, mine size, seam height, underground coal mining, accident analysis, occupational lung diseases.

The database was set by PRISMA method. Briefly, in this method, duplicate articles are removed, and at least two independent reviewers screened the search results. Screening is conducted based on titles and abstracts (first screening) and then by complete articles (second screening) to ensure that the articles meet the eligibility criteria. Following the identification of relevant publications, data must be retrieved using a standard data extraction form by at least two independent researchers (double data extraction). The studies are also evaluated in order to establish their quality (quality appraisal). Those publications that met the criteria and have the key features of the application (e.g., explicit rigorous protocol, summarized table, statistical analysis) are considered as high-quality articles. The data from those publications are then extracted and summarized in a synthesis. If the data is not in a format that allows a statistical summary, this can be done as a ‘qualitative’ overview. Statistical analysis can be performed if the data is numerical and homogeneous enough. Finally, the synthesis data will be analyzed in a report that will be subjected to peer review before publishing (Torgerson 2003).

In this research, a total of 401 articles were collected from the computer-based literature search. Upon excluding duplicate publications, the database included 372 articles. The titles were independently screened by two researchers against the inclusion criterion (Table 1). Furthermore, the references in each paper were examined to identify additional publications, which did not appear in the original literature search (snowballing). In the next step, the titles and abstracts were screened and 148 papers were excluded from the database. Afterward, full-text screening was performed on the remaining 224 publications. A total number of 204 papers were eliminated from the database by reviewing the outlines, objectives, conclusions, and health outcomes related to RCMD. Finally, using the inclusion and exclusion criteria, a total of 18 articles (16 from the aforementioned stages and 2 from snowball search) were considered for the systematic review. The inclusion process is presented as a flowchart in Fig. 2. The information extracted from selected papers, including authors, data sources, study period, number of data, geographic location, type of analysis, research category, results, conclusions, and recommendations, were provided in Table 2.

In this research, a comprehensive database was created, and the publications were categorized according to the project outline and the publication year (Fig. 3a and b). The categories include RCMD characteristics, health-diseases, dust control, respiratory deposition, mining methods, monitoring techniques, RCMD sources, standard and regulations, and systematic review. As shown in Fig. 3b, the majority of the selected articles were related to lung deposition (24%), health–diseases (23%), regulation-exposure (19%), and a very few studies were systematic review articles (3%).

Fig. 2
figure 2

Results of systematic review search (adapted from Moher et al. 2009)

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Fig. 3
figure 3

a Distribution of publications by decades b Categories of the RCMD publication database (total number of 372 papers)

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3 Results and discussion

The summary of the papers selected for the systematic review data selection is listed in Table 2. The systematic review conducted in this research included a total of 401 publications. The majority of the selected articles were related to lung deposition, health-disease, and regulation-exposure. Irrelevant publications were excluded from the database based on the inclusion and exclusion criteria (Table 1). Upon screening, a total number of 18 papers were selected to extract reliable quantitative and qualitative information for the systematic review. Individual factors, changes in mining practices, technology advancement, thin seam mining, mine size, coal rank, geographic location, mining method, rock dusting, and new cutting technology were among the parameters that were considered to potentially contribute to the resurgence of CWP.

In the majority of the studies, exposure measurements were taken by gravimetric dust sampling or personal sampling instrument, and expressed as average mean exposure (mg/m3). In two comprehensive studies for the U.S. underground and surface coal mines, 5.5% and 1.5% of RCMD samples in underground and surface mines, respectively, exceeded the permissible exposure limit (PEL) of 2.0 mg/m3 (Doney et al. 2019a, b). In those studies, 15.3% of surface silica samples and 2.0% of underground silica samples exceeded the PEL. In comparison to the rest of the United States, Central Appalachia exhibited statistically greater RCMD, respirable quartz, and precent quartz in the samples. Over 32% of quartz samples exceeded the PEL in the drilling occupations. The arithmetic mean of respirable quartz for drilling occupations was 0.15 mg/m3, approximately three times higher than the overall average (0.04 mg/m3). Duration of RCMD exposure was in the range of 5–60 years of employment. In several studies, the duration of the dust exposure was not stated (Morgan et al. 1973; Laney et al. 2010; Vallyathan et al. 2011; Blackley et al. 2014; Hall et al. 2019, 2020; Sarver et al. 2020). Studies population was in the range of 4491 to 90,973 miners. Most studies’ research populations were entirely male. As a result, the assessment contains no information concerning the specific risks of these exposures for females. There is only one study that includes females in the risk associated with coal mining (Jenkins et al. 2013). Due to the small sample size, those female miners were excluded from the study (Graber et al. 2017). There was no study focusing on the effect of pure respirable coal or silica on health (Beer et al. 2017). The focus of many studies was on Appalachian underground coal mines in Pennsylvania, Kentucky, Virginia, and West Virginia. Characterization of RCMD samples from the various geographic locations in the U.S. was performed in two different studies (Morgan et al. 1973; Sarver et al. 2020). Due to the geographical clustering of coal mines in the U.S., regional variations in dust characteristics exist, which is associated with various lung diseases. In Central Appalachia, for instance, mines may have more rock strata sourced dust than in other mines in the United States (Sarver et al. 2020). A previous study showed that a considerable portion of the coal mine dust collected in different mine sites is in the size of submicron (Sarver et al. 2020).

The characteristics of selected publications in the systematic review are as below:

  1. (1)

    There are consistent data related to the coal mines employee/production, RCMD disease, and exposure in the U.S. provided by MSHA, NIOSH, EIA, etc.

  2. (2)

    All of the studies in the U.S. agreed more investigations are needed to find the contributing factors related to the recent resurgence of lung diseases.

  3. (3)

    There has been significant progress in recent characterization studies, including characterization of submicron RCMD particles.

  4. (4)

    Most studies focused on the prevalence of CWP cases in the Appalachia region (Kentucky, West Virginia, Pennsylvania, and Virginia) since most of the cases were observed in this region.

  5. (5)

    It has been confirmed that RCMD characterization is different between mines.

  6. (6)

    Recent studies have focused on the investigation of contributing mining factors in the prevalence of mine diseases (e.g., the effect of mine size in the prevalence of CWP among Appalachia coal mines).

  7. (7)

    More than 40% of articles in the database were related to RCMD health-disease, and exposure-regulation. Therefore, there has been a tremendous effort to address this issue and study the the efficacy of the current RCMD regulations and monitoring approaches.

  8. (8)

    The total number of publications related to RCMD published recently has been increased; therefore, the topic of RCMD is of concern for many researchers in the U.S.

A closer look at the previous studies reveals several deficiencies. First, the effects of all the contributing mining parameters (i.e., mining operation type, mine size, coal rank, coal seam thickness, geographic location, etc.) on the prevalence of CWP have not been investigated in a multivariable model (Shekarian et al. 2021a). Second, there has been no research to compare the prevalence of lung diseases in surface and underground mines. Third, the confidence levels and p-values of analysis in a few of the selected studies are missing, and it is not clear whether the identified relationships in those studies were statistically significant. Therefore, with the limited data available in previous studies, it is difficult to conclude to what extent those studied factors may have contributed to the recent resurgence of CWP. Fourth, the worker’s health history and any previous illnesses/disabilities have not been systematically collected. Fifth, a major flow in the previous studies was the assumption that miners were exposed to a constant level of dust exposure at the same job during the entire employment tenure (Vallyathan et al. 2011; Graber et al. 2014; Shekarian et al. 2021b). Finally, the primary focus of the selected publications was focused on underground coal mines in the hot spot areas (i.e., West Virginia, Virginia, Pennsylvania, and Kentucky), and the data from other regions and mining methods were excluded.

In addition to the aforementioned drawbacks in previous studies, information on key parameters and their relationships with the prevalence of CWP are missing. Information related to several factors such as mining technology changes, advancement in cutting machineries, and disease latency time is insufficient. There is also limited data on the lung deposition of submicron RCMD particles since most of the data collected in previous studies were in supramicron range of 3–10 microns (NAS 2018; Sarver et al. 2019). There is also a gap in knowledge to investigate which and how the compositions of RCMD may cause high rates of CWP and silicosis among coal miners.

Table 2 Summary of selected papers for systematic review analysis
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There have been tremendous efforts to identify the contributing factors in developing lung diseases among coal miners. The analysis of possible explanations for the recent increase in CWP seems very complicated. Changes in mining practices, technology advancement, thin coal seam mining, rock dusting, the efficacy of water spay and mitigation techniques, engineering dust control, hours of working, and new cutting machinery may each contribute to the occurrence of new CWP cases in the US (See Fig. 4) (Volkwein et al. 2006; Colinet et al. 2010; Brown et al. 2013; Scaggs 2016; NAS 2018; Thakur 2019; Shekarian et al. 2021a). Further investigations are required to identify the root causes of the lung diseases resurgence and the contributing factors (Brown et al. 2013; NAS 2018; Johann-Essex et al. 2017; Sarver et al. 2019).

Fig. 4
figure 4

Key risk factors contributing to the respiratory health of the underground and surface coal miners (Go et al. 2016; Perret et al. 2017)

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4 Conclusions

In the United States, the manifestation of black lung disease was in the 1960s when 30% of workers who had more than 25 years of tenure in underground coal mines reported developing CWP disease. There has been a significant effort since 1969 to address this issue by reducing the PEL of respirable coal and silica dust. This study aimed to investigate various contributing factors in the recent resurgence of CWP among coal miners in U.S coal mines by conducting a systematic review. Hence, available statistics and data on coal mine dust lung disease were collected from the literature to identify the critical medical, engineering, and mining factors contributing to the resurgence of CWP among coal miners in the United States. Of the 401 publications in the database, 148 studies were excluded in the title and abstract screening, 208 studies in the full-text assessment. Finally, a total number of 18 papers were selected by snowball research and full-text assessment of publications for the data extraction. The systematic review results showed that there are various contributing factors, including mine type, geographic location, technological development, level of automation, thin coal seam mining, application of rock dusting, coal rank, and changes in mining practices that can contribute to the increase of lung diseases. However, there has been no comprehensive systematic review to determine all the contributing factors associated with the recent resurgence of lung disease among U.S. coal miners. Furthermore, there are various limitations to those studies. For instance, the gender, race, mean, and median of miners’ age were not provided in many of those studies. Also, the vast majority of research studies focus on RCMD occupational exposure in underground mines. However, it is worth mentioning that respiratory lung disease is also a prevalent health issue among surface coal miners.