Characterization of Asbestos Fiber Distribution on Membrane Filters from 25- and 37-mm Sampling Cassettes

Abstract

In the occupational assessment of airborne asbestos fibers, non-uniform fiber distributions on membrane filters could be a significant factor in the overall precision of the analytical method because counting is allowed in any region of the filter and because only a small portion of the membrane filter is selected for counting. A laboratory study was conducted to characterize fiber distribution properly both within and among filter sectors (wedges) for two 25-mm cassettes with and without air flow trenches in the cassette bases, a 37-mm cassette with air flow trenches in the base, and a 37-mm circumferential orifice cassette. Chrysotile asbestos fibers were sampled at a 70 percent relative humidity to minimize electrostatic effects. Sampling variables were adjusted to obtain a fiber density which was statistically valid for counting purposes and approximately equivalent between filters of differently sized sampling cassettes. Fiber identification and counting conditions conformed to OSHA and NIOSH specifications. Along each of 6 (60°) radial sectors, 9 counting fields in the 25-mm filters and 20 counting fields in the 37-mm filters were selected in the middle of rings of equivalent area so that no region of the filter, on a per unit area basis, was under or over sampled relative to another region. In the 25-mm cassettes, no differences in fiber density among sectors attributable to cassette design were found. In the 37-mm cassettes with air flow trenches, fiber densities in on-trench sectors were greater than some, but not all, off-trench sectors. Fiber density in the region directly above the air flow channel in the base of the circumferential orifice cassette was greater than that observed in adjacent regions.

In all cassette types examined, a trend of decreasing fiber density was observed from the middle to the outer regions of filters. This trend was especially prominent in the outer one-fourth to one-third, by distance, of the 25-mm filters. When filters were counted along a randomly selected radial line according to NIOSH counting rules, average fiber density did not differ from the average fiber density obtained in the detailed characterization. The results of the study suggest that the selection of radial or nearly radial counting lines, which can be made in 25-mm filters without regard for location relative to air flow trenches in the cassette base, are probably necessary to minimize the variability of the method and to make comparisons among different samples more meaningful.