The alternatives of filter overloading and electrostatic effects as explanations for results of inaccurate asbestos (1332214) fiber measurements using NIOSH Method 7400 were discussed, with reference to a report by Seixas and colleagues on electrostatic effects. In the original report using Method 7400, all the samples were indicated to be significantly overloaded with dust due to high background levels, so that one could expect the filters to become overloaded in a short time period. This would obscure fibers. A second effect when samples were overloaded was that the particles collected on top of each other rather than directly on the filter, making them prone to redistribution on the cowl surface during sample handling and shipment. Fiber counting could also be biased positively with lightly loaded filters, which might have occurred for filters used to collect fibers found on the cowl surface. The author's research on electrostatic effects involved the question of cowl deposition. The electrostatic field was highest at the inlet of the conductive cowl, lower at its outer surfaces, and least on its interior. Any deposit due to oppositely charged particles would be primarily on the inlet edge and outer surfaces of the cowl. Identically charged particles would be completely repelled or propelled toward the center of the filter. Thus, deposits measured in the Seixas paper, which were collected from the cowl interior, were not likely to have come from electrostatic sampling effects but rather from redeposition of losses from the overloaded filter surfaces. The author concludes that electrostatic effects can bias measurements under certain conditions, but that other recognized sources of bias, such as those discussed above, need to be eliminated as well.
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