NIOSH Mining Safety and Health Research

Health surveillance studies on underground and surface mine workers show that lung disease from exposure to respirable dust continues to be a major health threat in the mining industry. Coal workers' pneumoconiosis (CWP) is a chronic, debilitating lung disease. In severe cases, it is disabling and fatal. The Coal Workers´ X-ray Surveillance Program gives underground coal mine workers the opportunity to periodically have a chest radiograph taken to evaluate their lungs for CWP. X-ray data from 2000-2006 shows that nearly 8% of examined miners with 25 or more years of mining experience were diagnosed with CWP. During 1995-2004, CWP caused the deaths of 10,406 U.S. miners. During the same period, the Federal Black Lung Program paid over $12.2 billion in benefits to coal miners and/or surviving widows and dependents.

During 1996-1997, a special silicosis screening program examined 1,236 surface coal mine workers in Pennsylvania. Silicosis, an irreversible and potentially fatal lung disease caused by exposure to excessive amounts of respirable silica dust, was diagnosed in 6.7% of these miners. During 1990-1999, silicosis caused the deaths of 2,405 workers across all industries. A number of states record the workers' industry on death certificates, and the mining industry accounted for 23% of the silicosis deaths reported during 1990-1999. These screening programs indicate that lung diseases are still contracted by miners in both surface and underground mines.

The Mine Safety and Health Administration (MSHA) conducts dust sampling on a periodic basis to determine if mining operations are maintaining dust concentrations below permissible exposure limits. MSHA compliance dust sampling results during 2003-2007 showed that dust overexposures continue to occur for many high-risk occupations in underground and surface coal and noncoal mines. For example, the percentages of inspector samples exceeding the silica exposure levels for selected high-risk occupations are: dimension stone cutter, 33%; tailgate shearer operators, 32%; jack setters, 20%; and continuous miner operators, 19%. Continued exposure to elevated respirable dust levels and continued development of lung disease indicate that improved dust monitoring and control are needed throughout the mining industry.

Dust Monitoring

The goal of the monitoring program is to develop and evaluate various instruments that can be used to assess worker exposure to respirable particulate matter on a real-time or near real-time basis. Real-time particulate monitoring can be used to effectively locate areas where dust liberation is elevated and determine how well implemented controls are working. Real-time instruments can enable workers and management to assess if exposure levels are acceptable and encourage corrective action before full-shift exposures exceed allowable limits. In addition, these types of instruments could potentially be used by MSHA for compliance purposes to ensure that particulate levels are within acceptable limits.

Personal Dust Monitor

NIOSH entered into a contract with Rupprecht & Patashnick Co., Inc., (now Thermo Scientific) to develop and build a gravimetric-based, person-wearable instrument designed to measure airborne respirable dust in real time. This device has become known as the personal dust monitor (PDM). The PDM provides the wearer with an average dust exposure from the start of sampling, as well as, an exposure level that is projected to the end of the shift. This dust sampling instrument was coupled with the cap lamp worn by the miners into a one-piece unit that is shown below.

NIOSH completed a series of laboratory and in-mine tests to examine instrument accuracy and suitability for in-mine use. Test results have shown that the PDM meets or exceeds the performance of the gravimetric sampler that is currently being used for MSHA compliance dust sampling. In-mine tests indicated that the PDM could be used in the underground environment and provide acceptable performance, while being accepted by mine workers.

Personal Dust Monitor (PDM) with filter unit shown.

MSHA has recently published proposed modifications to 30 CFR § Part 74 that establishes performance-based criteria that must be met for dust samplers to be approved for use in underground coal mines. A public hearing was held on July 8, 2009 to solicit comments on the proposed rule change. It is anticipated that the rule change will be finalized by 2010. The PDM meets these new performance criteria for compliance dust samplers. Thermo has completed a commercialized version of the PDM, received MSHA approval for use in explosive environments, and initiated sales of PDMs to coal mining companies.

As originally developed, the filter within the PDM could not be used to conduct silica analysis on the collected dust. NIOSH is currently evaluating an appropriate filter material and filter containment system that would allow PDM filters to be sent to MSHA for silica analysis in a process similar to the procedures used with the current gravimetric samplers. In addition, NIOSH is working with a software development company to design a software program that can be used by the mining industry to analyze data files generated by the PDM and provide summary information to mine personnel.

Silica Dust Monitoring Technologies

Currently, gravimetric samples collected by MSHA inspectors or mine operators are sent through the mail to MSHA´s processing laboratory in Pittsburgh to be analyzed for silica content. Silica analysis is completed and results are then provided to the mine. As a result, mine operators and sampled workers often wait for over a week to learn if silica overexposures have occurred. In the interim, these overexposures may continue to occur.

NIOSH is initiating research to investigate analytical technologies that may lead to the development of an end-of-shift monitor that can complete an on-filter analysis for silica content in a short time frame. The goal is to have this instrument available to MSHA inspectors so that a silica determination can be made at the end of the working shift. If the sample shows that allowable limits are being exceeded, changes to the dust control measures or operating practices can be made on the next shift to prevent further overexposures.

Dust Control Technologies

For underground coal operations, longwall face workers, continuous miner operators, and roof bolter operators are at high risk for overexposure to respirable coal and/or silica dust. As a result, NIOSH is currently conducting research into control technologies to reduce the dust exposures for these high risk occupations.

Dust surveys have been conducted on ten longwall faces to quantify dust being generated by major sources and benchmark the control technologies being used. These surveys have identified effective controls, as well as, areas of additional research need. Results of these surveys are being compiled into a paper to summarize effective dust controls for longwalls. One area of ongoing research is the application of directional spray systems on shearers. Tests are being conducted in the full-scale longwall dust gallery at PRL to evaluate the application of directional sprays on the tailgate end of the shearer to reduce the dust exposure of the tailgate shearer operator and jacksetter during tail-to-head cutting passes.

Adjusting water sprays during testing in longwall dust gallery.

As longwall production has increased, the number of shields that are advanced and the speed at which they advance has also increased. Consequently, the respirable dust liberated during shield advance is becoming more problematic. NIOSH has initiated tests in the longwall dust gallery to evaluate sprays systems designed to control dust liberation from shield advance.

Water spray systems on continuous miners have emphasized the confinement and control of the dust cloud in an effort to minimize worker dust exposure. In contrast, water spray systems on miners to control methane gas have generated turbulence and promoted mixing at the face. As a result, water sprays developed mainly for controlling dust are often ineffective for controlling gas and vice versa. A tracer gas injection and monitoring system is being used to evaluate gas levels in the full-scale continuous miner gallery at PRL while monitoring dust at the same time. Tests are being conducted to evaluate the performance of different water spray systems when operated with flooded bed scrubbers during the extraction of extended cuts with exhaust face ventilation. The ultimate goal of this research is to optimize the water spray system so that both dust and gas can be effectively controlled.

Testing in full scale continouos miner dust gallery.

NIOSH is conducting baseline dust surveys in underground coal mines to evaluate dust levels generated throughout extended cut faces. Real time sampling is being conducted and time studies being performed to quantify dust levels as the continuous miner and roof bolter progress through their extended cuts. Operational performance of the flooded bed scrubbers on miners and the dust collection system on roof bolters are also being monitored during these cuts. Surveys will be conducted at a total of six mines, with the goal of locating mines with different ventilation methods and operating conditions.

Roof bolter operators are subjected to dust generated by the bolter and, depending on the ventilation used, dust generated by the continuous miner. For bolter-generated dust, NIOSH is evaluating the performance of pre-cleaner cyclones that are used on some bolters to reduce the loading of the dust collector box. In-mine testing is being planned to quantify the amount of respirable dust that may become airborne when the precleaner discharges dust onto the mine floor. In addition, laboratory tests are being conducted to evaluate the potential dust reduction that may result from the use of water boxes that can be installed on the discharge of the dust collection system.

To improve the protection for bolter operators when the bolter must work downwind of the miner, a system is being developed to filter a portion of the mine air and supply cleaned air to the breathing zone of the bolter operator. An air plenum is mounted on the underside of the roof bolter canopy and discharges filtered air down over the operator. Preliminary laboratory and in-mine tests have shown that this canopy air curtain can reduce dust levels in the breathing zone of the bolter operator. Additional research is being conducted in an effort to optimize the performance of the air curtain.

Canopy air curtain being tested at an underground mine.

Surface drill operators are typically the highest exposed occupation at surface mining operations. NIOSH is evaluating modifications to the drill shroud in an effort to improve the capture of drill-generated dust. A physical barrier is mounted on the inside of the drill shroud to help confine dust within the shroud and allow for greater capture by the drill dust collector. Laboratory and initial mine site tests have shown that this system can reduce dust escape from the drill shroud. Additional surveys are planned in order to improve system performance.

As previously indicated, x-ray surveillance data has shown an increase in CWP rates. Examination of the x-ray database shows that the greatest increases have been observed in the southern Appalachian region (SAR) of the coal fields. NIOSH is conducting a research effort to document the control technologies, operating practices, and mining conditions found in mines operating in the SAR. Eight baseline dust surveys have been completed in this region to date. Initial results show that a common factor impacting dust levels in these mines is the large quantity of rock that is being extracted. Silica analysis shows that this rock is typically high in silica content. Survey results also indicate that increased maintenance is required when mining in these difficult conditions. Four additional surveys are planned to further evaluate mining operations in this region.

NIOSH has initiated an effort to document available dust control technologies for both coal and metal/nonmetal mining operations, which will be summarized into two "Best Practices" handbooks. This information will also form the basis of dust control workshops that will be presented at various locations around the country in the coming year. This information will provide mine operators with a convenient source of information that can be used to select appropriate controls for their particular mining operation.

NIOSH is also working in cooperation with the Industrial Minerals Association - North America (IMA-NA) to complete a handbook designed for use by industrial minerals producers. The handbook will contain detailed information on control technologies to address all stages of the minerals handling process, including dumping, crushing, screening, and bagging operations. NIOSH and industry representatives are currently writing various chapters for this handbook.

NIOSH researchers are working on a project, in cooperation with NIOSH personnel from the Division of Applied Research and Technology, to evaluate dust controls on road milling machines. These machines are similar to continuous miners in that they are equipped with a rotating cutter head with associated spray manifolds wetting the head. In addition, the milling machines are equipped with conveyors that are used to transport the cut material to haul trucks. These conveyor systems are also equipped with water sprays. Testing has been completed to evaluate a wide variety of spray system designs on various manufacturers´ machines. Promising design characteristics have been identified and these designs are being incorporated by multiple manufacturers for further evaluation of dust control effectiveness.

Results of this research program will provide improved monitoring of dust exposures, improved understanding of dust sources and generation, and reduced exposures through development of new and improved control technologies.