Abstract
Researchers have developed a model to describe airborne respirable dust (ARD) generation on surface coal mine drills. By measuring a few basic parameters and using a graph, a drill operator or engineer can estimate the relative severity of drill dust emissions as well as how much of a reduction in ARD can be obtained by changing any given parameter. Geometric parameters include: drill deck cross-sectional area, shroud leakage associated with the deck shroud as well as the operational parameters of bailing airflow, and dust collector airflow. The relationships yield predictive ARD values which fall in the range measured on operating drills for collector/bailing air flow ratios greater than 2. Overexposure to airborne respirable crystalline silica dust can cause serious or fatal respiratory disease. Exposures of surface coal mine rock drillers to respirable crystalline silica are of particular concern. In a 1992 alert on silicosis in rock drillers, the National Institute for Occupational Safety and Health (NIOSH) reported on 23 cases of advanced silicosis (acute, accelerated, and chronic) ranging in ages from 25 to 60 with drilling tenures ranging between three and 20 years. Most of the cases involved drill operators in their 30's and 40's, indicating high silica exposure levels are associated with their occupation. A more recent lung x-ray surveillance study of a 664 volunteer population of surface coal miners showed that the prevalence of silicosis-like abnormalities was 9%. The two most significant factors associated with these abnormalities were increasing age and years of drilling experience. The Mine Safety and Health Administration (MSHA) permissible dust exposure for coal mine workers is a shift average of 2 milligrams per cubic meter (mg/m3) of airborne respirable coal mine dust, as defined by the British Medical Research Council (BMRC) Criterion. If the ARD sample contains more than 5% crystalline silica, the dust standard is reduced to the quotient of 10 divided by the percentage of silica in the dust, limiting the respirable crystalline silica exposure to 100 micrograms (microg/m3, BMRC equivalent) for the working shift. Compliance with these respirable dust standards are expected to significantly reduce a worker's risk to occupational lung disease throughout an average life expectancy. A recent analysis of the MSHA data from 2000-2006 shows that the percentage of the DWP drill dust samples exceeding the permissible exposure limit has dropped to 16%, indicating that overexposure to silica dust is an ongoing surface coal mine dust problem for the highwall drill operator.
