Comparison of coal mine dust size distributions and calibration standards for crystalline silica analysis

Since 1982 standard calibration materials recommended for respirable crystalline silica analysis by the Mine Safety and Health Administration (MSHA) P7 Infrared Method and the National Institute for Occupational Safety and Health (NIOSH) X-ray Diffraction (XRD) Analytical Method 7500 have undergone minor changes in size distribution. However, a critical assumption has been made that the crystalline silica in ambient mine atmosphere respirable dust samples has also remained essentially unchanged in particle size distribution. Therefore, this work compared recent particle size distributions of underground coal mine dust and the silica component of these dusts with estimated aerodynamic particle size distributions of calibration standard materials MIN-U-SIL 5, Berkeley 5, and SRM 1878 used by two crystalline silica analysis techniques. Dust impactor sampling data for various locations in 13 underground coal mines were analyzed for the respirable mass median aerodynamic diameters. The data suggest that the MSHA P7 method will underestimate the silica content of the sample by at most 7.4% in the median size range 0.9 to 3.6 microm, and that it is unlikely one would obtain any significant error in the MSHA P7 method analysis when the method uses Berkeley 5, MIN-U-SIL 5, or SRM 1878 as a calibration standard material. The results suggest that the NIOSH Analytical Method 7500 would be more appropriate for a dust sample that is representative of the total (no cyclone classifier) rather than the respirable airborne dust, particularly because the mass fraction in the size range below 4 microm is usually a small percentage of the total airborne dust mass. However, NIOSH Analytical Method 7500 is likely to underestimate the silica content of an airborne respirable dust sample by only 5 to 10%. The results of this study also suggest that any changes that may have occurred in the median respirable size of airborne coal mine dust are not significant enough to cause any appreciable error in the current methods used for respirable crystalline silica analysis.