Reanalysis of 1973 activity-weighted particle size distribution measurements in active U.S. uranium mines

Activity-weighted particle size distribution data taken with a lowpressure impactor in two working uranium mines have been reanalyzed using modern unfolding techniques. Both mines used diesel engines and were located in the southwestern United States at an elevation of 2120 m; 14 measurements were made in the winter of 1973 at several mine locations and during typical mining activities. The activity median diameter for the principal mode of the distribution determined using these better methods ranged between a minimum of 72 nm and a maximum of 303 nm; the average value was 169 nm (average geometric standard deviation: 2.42), which is significantly smaller than the estimate of 250 nm (geometric standard deviation: 2.5) obtained originally using graphical methods and currently used in some dosimetric calculations. Four samples had secondary modes with more than 20% of the total activity at diameters less than 60 nm; this was possibly a consequence of bimodal diesel emission and/or aerosols generated by propane combustion.