Monitoring and Sampling Approaches to Assess Underground Coal Mine Dust Exposures

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 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. (Figure presented) 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.