Methodological studies were conducted as part of our ongoing effort to develop standardized methods for quantifying respirable organic fiber exposure in the workplace. The first study was designed to evaluate the influence of electrostatic potential of respirable organic fibers (also known as RFP) on the quantification of aerosolized RFP samples. Counts derived from highly electrostatic RFP such as p-Aramid are postulated to result in an underestimate of the actual respirable fiber count. Accordingly, studies were performed to compare the RFP counts from the filters directly exposed to p-Aramid or cellulose RFP with other filters directly exposed and supplemented with any RFP that may have deposited on the supporting cowl. The results demonstrated no significant differences between the two sets of samples for either the highly electrostatic p-Aramid RFP or the low electrostatic cellulose RFP samples.

The objective of the second study was to compare the results of aerosolized organic RFP counts from three different laboratories and from four different individual counters, using light microscopy methods. Atmospheres of p-Aramid RFP were generated in an inhalation chamber. Fifteen methylcellulose filters were exposed to a p-Aramid aerosol for 5 minutes at estimated concentrations of 20 - 30 f/cc. Subsequently, filters were prepared for PCOM (phase contrast optical microscopy) counting by standard techniques. The prepared slides containing a portion of the fiber-exposed filters were first counted at DuPont Haskell Lab., and then the same slides were sent to the Denkendorf Institute and finally to the IOM. For quantification of fibers, the NIOSH 7400 method was used at DuPont Haskell Lab., while a WHO/EURO MMF fiber counting method was utilized in the European laboratories.

The results demonstrated that Laboratory A had consistently lower counts when compared to Laboratory B (mean values for the 15 filters = 18.4 + 4.3 f/cc vs. 27.7 + 4.3 f/cc). Laboratory C, with 2 different counters, was frequently intermediate between the counts of Laboratory A and B (24.2 + 1.1 f/cc and 22.1 + 2.2 f/cc). The differences in fiber counts may be related to variation in counters, or to the slight differences in counting rules between the US and European methods. With a few exceptions, the intra-laboratory variability between counts was rather low, while the inter-laboratory variability among counts was higher. Studies are ongoing to better understand the expected variability for organic RFP counts when comparing the results from one laboratory to another.