Measuring respirable dust concentrations in coal mine environments is currently done using approved personal respirable dust sampling equipment operating at a flow rate of 2.0 liters per minute (Lpm). Measurements made with approved coal mine dust sampling equipment are converted to equivalent concentrations that would be obtained with a Mining Research Establishment (MRE) instrument known as the Casella 113A gravimetric sampler, using a conversion factor of 1.38. NIOSH has recently recommended that coal mine dust samplers (CMDS) used to measure respirable dust levels in mine environments be operated at 1.2 Lpm and measured concentrations be multiplied by 0.91 to obtain an equivalent MRE concentration. The purpose of this recommendation is to reduce systematic error caused by the variation in mine dust distributions. This paper presents and discusses data collected in the laboratory and in underground coal mines to evaluate the recommended 1.2 Lpm flow rate and 0.91 conversion factor. Comparative measurements were obtained in the laboratory with the CMDS operating at 2.0 and 1.2 liters per minute and the MRE, using aerosols of coal and limestone dust of varying particle size distribution. Similar comparative measurements were made in a number of underground coal mines at locations with environments having particle size distributions representative of different underground mining operations. It was concluded from this study that there is no significant change in the variability associated with the constant factor used to convert respirable dust measurements, obtained with approved respirable CMDS, to equivalent MRE measurements when the flow rate of the CMDS is reduced from 2.0 to 1.2 Lpm.