Methods development studies for quantification of organic fiber exposure in the workplace: results of a round robin study

Methods development studies were conducted to develop standardized techniques for quantifying respirable organic fiber aerosol exposures in the workplace. The aim of this study was to compare the light microscopy results of aerosolized organic respirable-sized, fiber-shaped particulate (RFP) counts from three different laboratories and from four different individual counters (two in one laboratory). p -Aramid was utilized in these studies as a representative organic fiber-type. Atmospheres of aerosolized p -Aramid RFPs were generated in an inhalation chamber. Fifteen randomly distributed methylcellulose filters were exposed to a p -Aramid aerosol for 5 min at estimated concentrations of 20–30 f/ml. Following completion of exposures, filters were prepared for phase-contrast optical microscopy counting using standard preparation methods. The prepared slides containing a portion of the fiber-exposed filters were first counted at DuPont Haskell Laboratory in the USA, and then the same slides were sent to the Denkendorf Institute in Stuttgart, Germany and finally to the Institute of Occupational Medicine, Edinburgh, Scotland, UK. For quantification of fiber counts, the NIOSH 7400 fiber counting method was used at DuPont Haskell, while a WHO/EURO MMF fiber counting method was utilized in the European laboratories. The results demonstrated that the fiber counts from one laboratory (designated here as Lab 1) had consistently lower counts when compared to another laboratory (i.e. Lab 2) (mean values for the 15 filters = 18.4 ± 4.3 versus 27.7 ± 4.3 f/ml). A third laboratory (Lab 3), with two different counters, was frequently intermediate between the counts of Labs 1 and 2 (24.2 ± 1.1 and 22.1 ± 2.2 f/ml). The differences in fiber counts may be related either to the minor differences in counting rules between the US and European methods and/or to variability among counters. With few exceptions, the intralaboratory variability between counts was lower than the inter-laboratory variability among counts. Studies are ongoing to better assess the expected variability for aerosolized organic RFP counts when comparing the results from one laboratory to another and one method to another (NIOSH versus WHO/EURO MMF).