Coal remains one of the principal sources of energy for the United States, and the nation has been a world leader in coal production for more than 100 years. According to U.S. Energy Information Administration projections to 2050, coal is expected to be an important energy resource for the United States. Additionally, metallurgical coal used in steel production remains an important national commodity.
Coal production, like all other conventional mining activities, creates dust in the workplace. Respirable coal mine dust (RCMD) comprises the size fraction of airborne particles in underground mines that can be inhaled by miners and deposited in the distal airways and gas-exchange region of the lung.1 Occupational exposure to RCMD has long been associated with lung diseases common to the coal mining industry, including coal workers’ pneumoconiosis (CWP), also known as “black lung disease.” In the 1960s, this disease was found in more than 30 percent of coal miners who had worked at least 25 years in underground coal mines (Figure S-1). Recognition that coal mine dust exposure caused CWP was one of the factors that led Congress to pass the Federal Coal Mine Health and Safety Act of 1969. Title II of the act stated that “it is the purpose of this title to provide, to the greatest extent possible, that the working conditions in each underground coal mine are sufficiently free of respirable dust concentrations in the mine atmosphere to permit each miner the opportunity to work underground during the period of his entire adult working life without incurring any disability from pneumoconiosis or any other occupation-related disease during or at any time at the end of such period.”2
RCMD has many sources, including particles generated by coal extraction (coal and minerals associated with the coal being mined), rock adjacent to the coal seam being mined, rock dust products used to control explosions in mines,3 and other particles associated with mining activities (for example, diesel fuel burning and belt abrasion). Likewise, RCMD has many components, including coal particles, crystalline silica,4 silicate minerals, carbonates (especially associated with rock dusting), and particulate matter from diesel engines.
No specific medical treatment is effective in reversing coal mine dust lung disease or in controlling disease progression. Consequently, efforts to minimize RCMD exposure along with medical surveillance for early disease detection and removal from exposure are the mainstays in protecting a miner’s health. Over the years, U.S. federal regulations have placed increasingly stringent upper limits on the allowable airborne RCMD concentrations in underground mines to
1 For dust sampling and regulatory compliance purposes, respirable dust has been defined as dust collected with a sampling device approved by the Secretary of Labor and the Secretary of Health and Human Services. See Chapters 1 and 4.
2 Title II, Sec.201(b).
3 Coal dust represents an explosion hazard in underground coal mines, which often can be mitigated effectively through a proper rock-dusting program. The practice involves applying an inert rock dust material to the surfaces of an underground coal mine and maintaining sufficient quantities so that the incombustible content of the combined coal dust, rock dust, and other dust is not less than 80 percent (30 Code of Federal Regulations [CFR] 75.403).
4 Crystalline silica is a collective term that refers to quartz, cristobalite, tridymite, and several other rare silica minerals. All of the crystalline silica minerals have the same chemical composition but have different crystal structures and are thus termed polymorphs. Quartz is the most common form of crystalline silica. See Appendix D.
which coal miners can be exposed. A limit of 3.0 mg/m3 was established in 1969. The limit was lowered to 2.0 mg/m3 beginning in 1972. In 2014, the Mine Safety and Health Administration (MSHA)5 issued a dust rule that reduced the limit to 1.5 mg/m3, effective August 1, 2016.6 The concentration limit for respirable crystalline silica (technically, quartz) remained at 0.1 mg/m3 (100 micrograms per cubic meter or µg/m3).
Those regulatory requirements, beginning in 1969, were followed by several decades of decreased prevalence of CWP in underground coal miners, such that by 2000 the recognized prevalence of disease in underground coal miners with more than 25 years of work tenure decreased from more than 30 percent in 1970 to about 5 percent in 2000. However, since around 2000 there has been an unexpected increase in the proportion of CWP in coal miners with 25 or more years of work tenure, and with an increase or plateau of disease prevalence in those with shorter mining tenure. Moreover, recent reports have described rapidly progressive, severe and fatal forms of disease including progressive massive fibrosis occurring mainly in the central geographic region of Appalachia.
5 As part of the U.S. Department of Labor, MSHA develops and enforces safety and health rules for all U.S. mines and provides technical, educational, and other types of assistance to mine operators.
6 Lowering Miners’ Exposure to Respirable Coal Mine Dust, Including Continuous Personal Dust Monitors (79 Fed. Reg. 24,814 ).
7 Laney, A. S., and D. N. Weissman. 2014. Respiratory diseases caused by coal mine dust. Journal of Occupational and Environmental Medicine 56 (Suppl 10):S18-S22.
As late as 2014, personal exposures to airborne RCMD were monitored using a device that collected a gravimetric sample of particles onto a filter during a miner’s work shift. The sample was then sent to an analytical laboratory to obtain a single value of RCMD mass concentration that was used to represent a miner’s exposure during the entire sampling period or calculate a time-weighted average exposure for a standard period (such as 8 hours).8 That approach imposed a time lag in getting the monitoring results and initiating appropriate dust controls to reduce elevated exposure concentrations. It was anticipated that the use of a personal monitoring device, being developed with the capability of measuring RCMD concentrations in near real time, could result in lower dust exposures because coal mine operators and miners could respond quickly if an increase in dust concentrations occurred. That capability came to fruition with the availability of the continuous personal dust monitor (CPDM). The 2014 dust rule required monitoring of personal dust concentrations using a CPDM beginning in February 2016. The CPDM must be used to monitor miners in occupations expected to be exposed to the highest RCMD concentrations and miners who have medical findings of CWP and who have opted to transfer to a less dusty job in the mine. CPDM readings obtained for regulatory compliance must be transmitted within 24 hours to MSHA. For the period from August 2016 to May 2017, with the allowable exposure limit of 1.5 mg/m3 in place for airborne RCMD mass concentration, MSHA reported that, for 25,441 valid measurement samples submitted by mine operators, greater than 99 percent of those samples were in compliance with the regulatory limit. It is important to note, however, that most miners incurred much of their exposures when previous regulations were in effect. Given that the latency period of CWP disease onset is typically 10 or more years, sufficient time has not elapsed to assess the effect of the 2014 requirements on disease rates and severity.
There are likely a number of factors that have contributed to an increase in the prevalence and severity of coal mine dust related lung diseases. Determining the causes of that increase and eliminating occupational lung disease in coal miners is a complex scientific, engineering, medical, regulatory, social, political, economic and legal problem. However, a full analysis of that problem is beyond the scope of this report. The primary focus of this report is an examination of monitoring and sampling approaches for the control of RCMD and miners’ exposure.
As dust-generating processes in mining have intensified over the past several decades associated health hazards might increase if there have been unanticipated changes in the characteristics of RCMD exposures that are important to the risk of coal mine dust lung diseases. Also, questions might arise as to whether the monitoring required by the 2014 dust rule is targeting the most important exposures metrics.
Currently, dust control technologies are widely available and have the potential to protect miners’ health from RCMD exposure. As discussed in later chapters, optimal strategies are needed to assure that the recent surge in prevalence and severity of coal mine dust lung diseases does not continue.
THE COMMITTEE’S STUDY
In the Consolidated Appropriations Act of 2016, Congress directed the National Institute for Occupational Safety and Health (NIOSH) to arrange for a study by the National Academies of Sciences, Engineering, and Medicine to assess monitoring and sampling approaches for informing underground coal mine operators’ decision making regarding the control of miner exposure to RCMD.9 The study comprised four aspects:
8 The same general approach is currently required to determine the respirable quartz concentration in the RCMD.
9 NIOSH is a research agency focused on the study of worker safety and health, and empowering employers and workers to create safe and healthy workplaces. NIOSH is part of the U.S. Centers for Disease Control and Prevention in the U.S. Department of Health and Human Services.
- Compare the monitoring technologies and sampling protocols (including sampling frequency) currently used or required in the United States, and in similarly industrialized countries for the control of RCMD exposure in underground coal mines.
- Assess the effects of rock dust mixtures and their application, as required by current U.S. regulations, on RCMD measurements.
- Assess the efficacy of current monitoring technologies and sampling approaches.
- Develop science-based conclusions regarding optimal monitoring and sampling strategies to aid mine operators’ decision making related to reducing RCMD exposure to miners in underground coal mines.
Those four aspects are specified in the committee’s formal statement of task.10 In addition, the committee was asked to identify important research gaps regarding monitoring and sampling protocols for controlling miners’ exposure to RCMD. It was asked not to recommend changes to the requirements of MSHA’s 2014 final rule for lowering miners’ exposure to RCMD, as the development of those requirements involves considerations beyond the scientific and technical focus of this study.
TRENDS IN DISEASE EPIDEMIOLOGY AND MINING PRACTICES
The regulatory focus on controlling the RCMD mass concentration and the mass concentration of respirable crystalline silica in the RCMD has not changed over the past several decades. As described above, this approach was associated with a substantial decline in CWP prevalence from 1970 to 2000 across all coal mining regions in the United States. However, since around the year 2000, an increase in prevalence and severity of CWP has been observed in various hot-spot geographic areas. The reasons for this increase are not obvious but could be related to changes in mining practices and conditions (for example, increases in equipment size and horsepower and mining increasingly thinner coal seams) leading to the increased extraction of rock containing crystalline silica and other RCMD components.
The 2014 dust rule lowered the allowable airborne RCMD mass concentration in underground mines and improved other protections for miners. However, as mentioned previously, given that the latency period of CWP disease onset is typically 10 or more years, sufficient time has not elapsed to assess the effect of the 2014 requirements on disease rates and severity. It is important to note, that compliance with regulatory requirements by itself is not an adequate indicator of the rule’s effectiveness in protecting miners’ health.
EFFICACY OF CURRENT MONITORING AND SAMPLING IN UNDERGROUND MINES IN THE UNITED STATES
Effective exposure control is a key means of addressing the occurrence of CWP in coal miners, which continues to be an important and complex problem. Effective monitoring technologies and sampling approaches would provide information on not only the RCMD mass concentrations for meeting regulatory requirements, but also the RCMD particle characteristics (such as composition) of greatest relevance to disease risk in miners. That information would inform a continual assessment of the RCMD standard and, ultimately, approaches for optimal protection of the health of the miners.
The CPDM is an important technological advancement compared to monitoring methods used previously, as it provides near-real-time readings of airborne RCMD concentrations in the workplace. If a measurement collected over a full shift exceeds allowable limits, mine operators
must take corrective actions immediately. In addition, miners wearing CPDMs receive information about their personal exposures and sometimes can modify their activities or locations within a mine in response to elevated readings.
However, only a small fraction of coal miners are required to use a CPDM during any given shift, and it is possible that those coal miners using the CPDMs are not representative of the dust exposure to other miners who are not using the CPDMs. When miners wearing CPDMs react to high monitor readings to limit their personal dust exposure (for example, by altering their locations while carrying out their job duties), the required RCMD sampling might no longer be representative of the miners with the highest exposures. Whether the airborne RCMD concentration is being maintained at or below the permissible limit for only those miners wearing the CPDM, or all personnel in the work area, is unknown.
In addition, changes in mining technologies over the past several decades might have led to changes in typical particle size distributions of RCMD. If so, there might have been a change in the relationship between CPDM measurements of RCMD mass concentrations and the health effects associated with particle type, size, concentration, and deposition in the lung.
OPTIMAL MONITORING AND SAMPLING STRATEGIES
Historically, the primary focus of RCMD monitoring and sampling efforts had been based on compliance with federal regulations. Additional monitoring efforts were undertaken by coal mine operators to support improvements in mine ventilation and other dust controls, for instance, to resolve noncompliance conditions. Over three decades, the compliance-driven approach led to a significant reduction in the incidence of lung diseases associated with occupational exposure to RCMD among U.S. coal mine workers. However, it has not resulted in attainment of the ultimate goal of the Coal Mine Health and Safety Act of 1969, which is to eliminate such diseases. Given current uncertainties about the cause of increase in disease prevalence and severity, the committee noted the possibility that high rates of operator compliance with the 2014 dust rule requirements may not guarantee that RCMD exposures will be controlled adequately or that future disease rates will decline. To continue progress toward reaching this goal, a fundamental shift is needed in the way that coal mine operators approach RCMD exposure control and, thus, sampling and monitoring.
Optimal sampling and monitoring strategies are created for the protection of miner health through the control of RCMD exposure. Those strategies would embrace additional voluntary monitoring and sampling that go beyond regulatory compliance to gain information on potentially important factors affecting miners’ health as well as the temporal and spatial variation of RCMD within a mine. Optimal strategies are implemented in the context of practical constraints (such as cost, availability of technology, existing regulatory requirements, and program acceptance by various stakeholders). The committee concludes that optimal monitoring and sampling strategies manifest as programs that, in principle, exhibit these attributes:
MONITORING AND SAMPLING PRACTICES USED IN DIFFERENT INDUSTRIALIZED COUNTRIES
In its statement of task, the committee was asked to compare the monitoring technologies and sampling protocols currently used or required in the United States, and in similarly industrialized countries for the control of RCMD exposure in underground coal mines. In addition to the United States, the committee selected the countries of Australia, Germany, India, the People’s Republic of China, Poland, and the Republic of South Africa to compare monitoring technologies and sampling practices for the control of RCMD exposure in underground coal mines. Required medical surveillance programs for detection of diseases in coal miners were also included. Those countries are among the leading coal producers and have regulatory programs in place for RCMD monitoring and miner health surveillance. Differences in exposure monitoring and sampling approaches among major coal-producing countries make it difficult to compare exposure measurements among different countries directly.
Despite those differences, there are important commonalities, such as using gravimetric sampling devices to monitor mass concentrations of RCMD and respirable crystalline silica. Those commonalities point to potential opportunities for harmonizing monitoring data collected in different countries, including RCMD and silica content. Additionally, a more complete understanding of international approaches to medical surveillance for coal mine dust diseases, including strengths and limitations, would lead to opportunities for improved understanding of the relationships between RCMD exposure and disease prevalence and ensuring that monitoring approaches are targeting the most important aspects of RCMD exposure.
The various approaches to medical surveillance among coal-producing countries warrant an in-depth and appropriately critical analysis, which was beyond the scope of this report. Such an analysis would provide insight into the country-specific prevalence of coal mine dust lung disease over time and would inform an understanding of the success of various strategies for monitoring and controlling exposures.
EFFECTS OF ROCK DUSTING ON RCMD MEASUREMENTS
The committee was asked to assess the effects of rock dusting on RCMD measurements in underground coal mines. Rock dusting (that is, the application of rock dust products) is a proven practice that has been utilized in some way since the early 1900s. Most of the commercially available rock dust is composed of pulverized limestone or marble, although regulations allow for other materials. Rock dust particles must be small enough to pass through a sieve having 20 meshes per linear inch and 70 percent or more of the particles must be able to pass through a sieve with 200 meshes per linear inch (30 CFR 75.2).11 Smaller rock dust particles (approaching the size of respirable particles) are more effective than larger ones at mitigating explosion risks. Smaller particle sizes have a greater surface area per unit mass than larger particles and thus greater ability to absorb heat. Because rock dust products that meet the regulatory size requirements also contain particles in the respirable size range, rock dusting can contribute substantially to the RCMD mass concentration measured by CPDMs.
Rock dust used in U.S. coal mines may contain no more than a small percentage of respirable crystalline silica (4 to 5 percent by mass, per 30 CFR 75.2). Rock dusting’s contribution to the respirable silica exposure of miners relative to the contribution from dust created by mining operations has not been documented. The percentage that rock dusting contributes to a miner’s silica exposure would vary according to the percentage of silica in the coal and the surrounding
11 Mesh refers to the number of openings across one linear inch of screen. As the mesh number increases, the size of the openings decreases. The openings of a 200 mesh sieve are approximately 74 µm.
rock being extracted, as well as the proportion that rock dust contributes to the total mass of airborne RCMD in the mine.
Sustained high exposures to rock dust and other so-called nuisance dusts, for which constituent-specific exposure limits are not specified, may trigger respiratory symptoms of irritation and cough and could contribute to a higher risk of chronic obstructive pulmonary disease (COPD). However, in general, the committee found few case reports or studies implicating rock dust exposure in risk for clinically significant coal mine dust lung disease.
Although rock dust contributes substantially to the total mass of RCMD, it is not possible to determine the percent contribution of rock dust using the currently required monitoring technology. As a result, two mines using different rock-dusting approaches (such as dusting at different times and rates during mining operations) might have very different RCMD compositions and yet exhibit similar measured mass concentrations. Therefore, heavy contributions of rock dust could distort health assessments of relationships between RCMD exposure and diseases caused by dust components insignificantly contributed by rock dust.
Measurements of airborne RCMD concentrations include respirable rock dust particles, by definition. However, it appears that complying with the rock-dusting requirements (30 CFR 75.2 and 75.402-403) has not been an obstacle to demonstrating compliance with the 2014 dust rule. It is critical that efforts to comply with both the rock-dusting requirements and RCMD requirements not compromise the effectiveness of either explosion mitigation or RCMD exposure reduction.
The committee identified important information gaps regarding monitoring and sampling protocols for controlling miners’ RCMD exposures. Research and development efforts are needed for better understanding of relationships between miners’ exposures and disease, including studying effects of changes in mining practices, improving monitoring approaches, and increasing participation in medical surveillance programs. Likewise, enhanced worker education and mine operators’ monitoring and sampling efforts would help ensure that all coal miners’ exposures are adequately controlled, in addition to those whose individual exposures are being measured for regulatory compliance purposes.
The recommendations provided below include research and development activities to address the gaps. The recommendations are amplified and expanded in the chapters. The sequence in which they are presented is not intended to imply a sense of priority.
Challenges in Implementing Optimal Monitoring and Sampling Practices
NIOSH and other organizations, such as the National Mining Association and the unions representing miners, should conduct a comprehensive investigation to identify key challenges that coal mine operators face in implementing an optimal, beyond-compliance approach to RCMD exposure monitoring and sampling for informing exposure control efforts. The organizations conducting the investigation also should recommend practical solutions for overcoming those challenges. (Recommendation 1)
Considerations of All Miners’ Exposures
- Conduct studies to evaluate the exposures of miners not wearing CPDMs to ensure that the approach of detecting and mitigating high exposures for designated occupations reliably results in mitigating high exposures of all miners. (Recommendation 2)
- NIOSH and MSHA should carry out a systematic examination of the content and implementation of training and education programs with respect to RCMD exposure. The examination should focus not simply on curricula but also on the way adults learn. It should
seek ways of implementing education and training programs in an effective and consistent manner across the coal mining industry. As a part of being effective, the programs should be relevant to all miners, not just the ones who wear CPDMs, as well as to operators and regulators. Programs should be assessed after they have been implemented for a few years to determine their overall effectiveness. (Recommendation 3)
- NIOSH, in collaboration with MSHA, should evaluate whether the current relationship between the particle-size distributions of RCMD samples and particles deposited in the lung that are associated with or implicated in the development of coal mine dust lung diseases (CMDLD) is similar to the relationship established decades ago, when the monitoring devices used for sampling were first adopted. In studying the particle-size distribution in modern-mining RCMD samples and their relationship to the particles deposited in the lung, it is important to consider associations with or implications in the development of CMDLD. (Recommendation 4)
- Develop a real-time crystalline silica monitor. As an interim measure, NIOSH should continue its efforts to develop an end-of-shift silica monitor. (Recommendation 5)
- NIOSH should continue to facilitate the development of a less costly and less ergonomically stressful real-time RCMD monitoring device that would facilitate the use of the personal monitors for engineering studies and other purposes in addition to compliance monitoring. As part of that effort, NIOSH should incorporate appropriate filter media that is compatible with an end-of-shift analyzer for respirable crystalline. (Recommendation 6)
- Explore the broader use of area (fixed-site) monitoring devices for gathering trends information on RCMD concentrations and particle characteristics in underground mines. (Recommendation 7)
RCMD Exposure and Disease Rates
- Conduct a systematic evaluation of changes in mining technology and activities to determine the extent to which those changes have caused increased extraction of rock and the extent to which past rock extraction had been co-located with disease hot spots. The evaluation should identify important focus areas for optimal sampling and monitoring strategies in the future. (Recommendation 8)
- NIOSH should conduct or facilitate a comprehensive assessment of RCMD particle characteristics, including their variability, to help target future exposure studies, because different particle characteristics (for example, composition and surface area) can pose different health risks. In addition, the assessment should characterize and quantify important source contributions to airborne RCMD, including rock dusting and extraction of rock strata adjacent to the mined coal seam. To the extent possible, NIOSH should assess how RCMD characteristics have changed over time and consider making provisions for tracking temporal trends in the future. Further research and development are needed to improve analytic methods for evaluating source contributions of RCMD. (Recommendation 9)
- Link medical surveillance programs directly with exposure monitoring programs and integrate health-related data on active and retired miners. (Recommendation 10)
- Elucidate factors that act as disincentives for participation in the voluntary portions of the NIOSH medical surveillance programs and in the MSHA Part 90 Program,12 with the goal of addressing those disincentives. (Recommendation 11)
- Conduct a comprehensive assessment of the requirements for exposure monitoring, including RCMD and silica mass content, and medical surveillance as well as the implementation of those requirements in major coal-producing countries. The assessment should identify opportunities for data harmonization and the use of those data for improving exposure monitoring approaches and conducting epidemiologic research. (Recommendation 12)
Research Capacity and Resources
NIOSH, MSHA, and other organizations should set priorities for addressing the committee’s recommendations and develop a strategy for addressing them. Federal agencies should provide the capability for research to be conducted in an experimental underground mine. Federal, academic, and coal mine industry researchers should seek opportunities for conducting collaborative research and development activities. (Recommendation 13)
Coal mine dust lung disease continues to be an important and complex problem affecting coal miners in the United States. Reliable information on RCMD exposures in underground coal mines is crucial for predicting, reducing, and preventing mine workers’ disease risks. Additionally, medical surveillance of miners, combined with comprehensive exposure assessment, are important tools for understanding disease trends and risk factors and assessing the efficacy of exposure reduction efforts.
The committee has provided recommendations concerning the efficacy of current monitoring technologies and sampling approaches and developing optimal strategies to aid mine operators’ decision making related to voluntary measures for reducing RCMD exposure to mine workers.
Those recommendations include studies to ensure that the approach of detecting and mitigating exposures for designated occupations reliably results in controlling exposures of all workers, including those not using a CPDM. In addition, training and education programs should be evaluated and enhanced. Improvements in monitoring technology should include reduction in cost and weight of CPDMs. Such improvements also should include development of a real-time silica monitor and, as an interim measure, the continued development of a commercially available end-of-shift monitor, because silica dust is known to pose a serious health risk. Better understanding of health risks from RCMD exposure could result from expanded worker participation in periodic medical surveillance, and from studies that evaluate changes in dust characteristics related to changes in mining practices over the past five decades.
12 The Part 90 Program is for coal miners who have medical findings of pneumoconiosis and who opt to transfer to a less dusty job in the mine (30 CFR 90).