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. Data from the 1992-1996 round of x-ray surveillance show that nearly 8% of examined miners with 25 or more years of mining experience were diagnosed with CWP. During 1990-1999, CWP caused the deaths of 15,036 U.S. miners. During the same period, the Federal Black Lung Program paid over $16.6 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 1987-1996, silicosis, excluding carcinomas potentially caused by silica, prematurely shortened Americans´ lives by 33,000 years. A number of states record the workers´ industry on death certificates, and the mining industry accounted for over 20% of the silicosis deaths reported during 1990-1999. These screening programs indicate that lung diseases are still contracted by experienced 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 1996-2000 showed that dust overexposures continue to occur for many high-risk occupations in underground and surface coal and noncoal mines. For example, the percentage of samples exceeding the silica exposure levels for selected high-risk occupations are: dimension stone cutter, 35%; highwall drill operators, 31%; roof bolter operators, 25%; and continuous miner operators, 23%. Continued overexposure to unacceptable respirable dust levels and continued development of lung disease indicate that improved dust monitoring and control are needed throughout the mining industry.

Dust Monitoring

Real-time particulate monitoring can both effectively locate areas where dust controls are needed and determine how well they are working. In its Coal Mine Respirable Dust Task Group Report, MSHA recommended development of fixed-site and portable continuous respirable dust monitors that could be used to enhance its compliance-monitoring program. MSHA also increased the number of filter samples that must be analyzed for silica content. For this effort, improved monitoring methods are needed.

The purpose of the monitoring program is to study and develop various tools that can be used to assess worker exposure to respirable particulate matter. The resulting tools could be used by regulatory agencies to ensure that particulate levels are within acceptable limits. Real-time measurement tools will enable workers and management alike to immediately assess if controls are working and encourage corrective action before particulate levels affect workers´ health.

Personal Dust Monitor

NIOSH was asked by the Secretary of Labor to improve sampling instrumentation designed for use in the mining industry. In response to this request and in consultation with labor, industry, and government officials, CDC entered into a contract with Rupprecht & Patashnick Co., Inc., Albany, NY, to develop and build a gravimetric-based, portable instrument designed to measure airborne respirable dust. This device has become known as the personal dust monitor (PDM). Initially, lab tests were performed to examine instrument accuracy. Underground evaluations, designed to measure the in-mine performance of PDM prototypes, have also been completed. Both of these efforts have proven successful.

Person-wearable dust monitor.

Based on these accomplishments, the Federal Government has purchased 25 additional precommercial versions of the PDM for final performance verification. These devices will be used to evaluate the long-term accuracy and precision of the PDM in a variety of underground coal mine environments. While in these hazardous surroundings, researchers will examine the mechanical and electrical performance of the instruments. Secondary efforts will attempt to determine the equivalency of the PDM relative to the U.K. Mining Research Establishment dust sampling criteria, upon which the U.S. dust standards are based. An important aspect of this research will involve face-to-face communications between researcher and miner to determine how the monitor may best improve the quality of the underground atmosphere and provide a more effective tool for protecting miner health.  Underground testing of these devices is currently underway.

Dust Dosimeter

New types of coal mine dust sampling devices are in demand that can more quickly and accurately measure airborne respirable dusts. In addition to the PDM, NIOSH is engineering other devices to meet diverse sampling requirements. For example, a low-cost differential pressure instrument has been developed as a surrogate for gravimetric analysis. This device, known as the SKC Dust Detective (SKCDD), uses a disposable sample tube (DST) and a small handheld flow-controlled pump. The pump, manufactured by SKC, Inc., consists of a programmable microprocessor, flow sensor, pressure sensor, air pump, control switches, and a digital display. It is operated at 0.25 liters of air per minute. The pump pulls air into the DST and onto a glass fiber filter. The DST has a 0.5 in diameter and is 4.0 in long. It contains two cylinders of porous foam-one coarse foam that preseparates particles and a second 4-micron cut-point foam that further segregates dust relative to its aerodynamic diameter. The outlet end of the DST is connected to the sampling pump with a section of 0.25-in OD flexible tubing. Dust levels are represented by an increase in differential pressure displayed on the pump. Pressure differentials increase linearly relative to dust particle buildup on the filter.

SKCDD disposable sampling tube.

Lab tests were conducted in a Marple Chamber. The tests comprised examining, in triplicate, 5 different coal types for a total of 15 coal dust evaluations. The coal types were Keystone Black, Illinois No. 6, Pittsburgh Coal 18 m aerodynamic diameter, Pittsburgh Coal 10 m aerodynamic diameter, and Pittsburgh Coal 5.77 m aerodynamic diameter. Efforts to date have resulted in the submission of a patent application. Development work is being cost-shared with SKC, Inc. A contract to evaluate the concept in the lab has been entered with the University of Minnesota. Researchers at The Pennsylvania State University have contracted to evaluate the device in two operating coal mines. The U.K. Health and Safety Establishment has received 50 new detector tubes from SKC, Inc., for testing. In addition, the Federal Environmental Agency in Bonn, Germany, is interested in using the concept as an environmental monitor.

Other Dust Monitoring Technologies

Another promising idea is the incorporation of a smoke detector and a light-scattering instrument into a single device that can discriminate diesel particulate from respirable dust. Experimental results indicate that a simple theory is adequate to describe the operation of the sensor. It presents correlations and techniques that will allow the sensor to be used for measurement and characterization of aerosols over a broad spectrum of possible applications related to adverse environmental and health consequences.

Dust monitoring research strives to develop near real-time quantitative and qualitative mass and pressure-based sensors for use in mines. Much of this work is being done in partnership with industry and academia. Successful outcomes will lead to new tools and approaches to particle measurement. The common goal of these new tools and approaches is to protect worker health by providing workers with timely feedback of dust exposures.

Dust Control Technologies

Historically, drill operators at surface mines are typically the occupation at highest risk of overexposure to silica dust. Most drilling machines at surface mines are equipped with dry dust collection systems that have the inlet to the collector located at the corner of the drill table. An improved dust collector inlet for drills has been designed and lab tested. This new design places the collector inlet directly around the drill steel, but was designed so that the inlet will not be blocked by rock chips blown out of the drill hole. Lab tests evaluated the new design under several different operating conditions. Results showed that the new inlet design reduced the amount of dust escaping the drill shroud by as much as 60%. Field testing of this new inlet design was recently completed at a surface mine in Kentucky and revealed that the inlet reduced dust levels at the drill. Longer-term use by the mine operator indicated that some plugging of the inlet duct occurred, particularly when drilling wet material. In future applications, access to the duct for periodic cleaning would be incorporated into the original design.

Past research has shown that dust reductions of 90% can be realized when retrofitting air filtration and pressurization units onto enclosed cabs of older mine equipment. One key in maximizing the dust reductions that can be obtained with these systems is to ensure the integrity of the filtration units. NIOSH has a Cooperative Research and Development Agreement to develop a method that can be used in the field to test the integrity of the filtration units in a relatively short time period. To date, a test method has been developed and successfully demonstrated in the lab, and a patent application has been filed. Additional work is planned to evaluate the method on operating equipment in the field.

Evaluating filtration unit for enclosed cab in laboratory test stand.

At many mineral processing facilities, workers´ clothes can become soiled and these workers expose themselves to dust liberated from the soiled clothes as they continue to complete their normal duties. MSHA does not permit the use of compressed air for cleaning soiled clothes and requires that workers vacuum their clothes. This a time-consuming, difficult, and somewhat ineffective method of cleaning clothes. In cooperation with the plant operator of a silica sand processing plant, a clothes-cleaning booth was designed and evaluated. The booth is connected to a baghouse dust collector and operated under negative pressure. Compressed air is supplied through an air nozzle manifold and used to blow off workers´ clothes. A respirator, eye protection, and ear protection must be worn by the worker during the cleaning process. Test results show that this compressed air nozzle system improves dust removal by over 50% while reducing cleaning time to 5% of the time needed for the current practice of vacuuming the clothing. NIOSH is working with the Industrial Minerals Association-North America in its efforts to have MSHA approve this clothes-cleaning method.

Research is ongoing with an iron ore plant to explore methods to reduce dust generation during iron ore processing. Recently, dust sampling was conducted to quantify dust levels throughout the plant while making changes to the plant-wide ventilation system. A new plant ventilation layout was developed that reduced dust levels in the dustiest part of the plant by 31%.

Surveys have been conducted at an underground limestone mine to evaluate the potential of using diesel-powered propeller fans to improve the local and/or mine-wide ventilation systems. Results show that the propeller fans can be successfully used to improve mine-wide ventilation and significantly reduce the dust retention time in the mine after face shots. Additional research is ongoing to evaluate local ventilation changes for reducing dust levels around an underground crusher.

Water sprays are used throughout the mining industry as a primary means of controlling dust liberation. Water sprays possess several properties (wetting, capture, air movement) that can be used to improve dust control. Past research has shown that water-powered scrubbers can be effective in moving air and removing dust, but have to operate at pressures of about 1,000 psi. Lab tests have shown that a new design of the water-powered scrubber can move about 500 cfm of air and remove over 90% of the dust from this induced airflow while operating at only 250 psi. Arrangements have begun for installing this water-powered scrubber at a dimension stone shop in an effort to capture dust being generated by a chop saw.

For underground coal operations, research has focused on improving dust control for longwall and continuous mining operations. Longwall face workers are typically at highest risk for overexposure to coal dust, while continuous miner operators and roof bolter operators are at high risk for overexposure to silica dust. To date, dust surveys have been conducted on six longwall faces to quantify dust being generated by major sources and benchmark the control technologies being used. These surveys will identify effective controls and areas of additional research need. One such area is the application of directional spray systems on shearers, particularly at higher seam heights. An upgrade of the external spray system on the shearer at NIOSH´s full-scale longwall gallery was recently completed in preparation for testing to optimize these spray systems.

As longwall production has increased, airflow on the face has also been increased. Operators are concerned about increased dust entrainment at these high air velocities. The greatest potential for dust entrainment may result from shield advance, where relatively dry dust from the shield canopies is dropped directly into the ventilating air as the shields are advanced. Lab testing has been conducted in a wind tunnel to simulate dust liberation and entrainment from shields at air velocities up to 2,000 fpm. Results suggest that respirable dust particles in the feed material that adhere to oversize particles or to each other can be separated as the air velocity increases, thereby increasing airborne respirable dust concentrations. Additional testing to evaluate the impact of added moisture is being planned.

Wind tunnel used to evaluate dust entrainment in high-velocity airstreams.

Roof bolter operators are subjected to dust generated by the bolter and, depending on the ventilation used, dust generated by the continuous miner. Technology to filter a portion of the mine air and supply cleaned air to the breathing zone of the bolter operator has been successfully demonstrated in the full-scale continuous miner gallery. A mine site evaluation of this canopy air curtain is being planned at an Indiana mine. Fletcher Manufacturing has retrofitted the air curtain onto a bolter at the mine in preparation for evaluation by NIOSH.

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 has been used to evaluate gas levels in the continuous miner gallery while monitoring dust at the same time. To date, a spray fan system and a flooded-bed scrubber system have been evaluated in a series of tests in the box cut, and testing is ongoing to complete evaluations with the miner repositioned into the slab cut. The ultimate goal of this research is to optimize water spray systems so that both dust and gas can be effectively controlled.

Testing in NIOSHs full-scale continuous miner gallery.

NIOSH researchers are also working on a DART project 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 and 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. Recently, two surveys were completed to evaluate the impact of changing water flow to these spray manifolds.

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