We have read with interest the recent work by the University of Wuppertal group (Bach et al., 2013) on dustiness determination using the University of North Carolina (UNC) Dustiness Testing Device (Boundy et al., 2006). We have referred to the UNC device as the "Venturi" device (Evans et al., 2013), as that describes the underlying dispersal mechanism; we continue with this terminology. The Wuppertal paper is presented in two parts. In Part 1, the dustiness of nine industrial powders was measured with the Venturi device, and results compared with their earlier measurements (Bach and Schmidt, 2008) using macroscopic techniques: EN 1505 standardized continuous drop (CEN 2006, 2013) and the commercial Heubach rotating drum and commercial Palas single drop. In Part 2, dustiness values for 11 pharmaceutical powders were determined solely with the Venturi device. We would like to comment on these Wuppertal results, especially in light of our previous and extensive use of the Venturi device for fine and nanoscale powders (Evans et al., 2013). Unfortunately, insufficient detail is provided on the provenance of the Wuppertal powders (Bach and Schmidt, 2008; Bach et al., 2013), to allow an interlaboratory comparison with identical materials. (By contrast, our measurements (Evans et al., 2013) for Holland lactose of Dtot = 5.2 (0.4)% and Dresp = 0.9 (0.1)% are fully consistent with those of the UNC group (Boundy et al., 2006), with Dtot = 5.1 (0.9)% and Dresp = 1.3 (0.5)% for the same material.) In the technique comparison, Part 1, of the Wuppertal study, only three Venturi measurements were made for each powder, and no ranges or statistics were reported. In the pharmaceutical, Part 2, of their study, five Venturi measurements were made for each powder, and standard deviations were reported, permitting some analysis of possible error. Finally, we observed an empirical correlation between respirable and total dustiness, as measured with the Venturi device, to hold for a wide range of powders (Evans et al., 2013). It is informative to test that empirical correlation with these additional Wuppertal results.
Douglas E. Evans, Chemical Exposure and Monitoring Branch, Division of Applied Research & Technology, National Institute for Occupational Safety & Health, Centers for Disease Control & Prevention, Cincinnati, OH 45226, USA