Abstract
Intralaboratory and interlaboratory investigations were used to evaluate the precision and accuracy of point counting versus visual estimation of asbestos in friable bulk materials. Interlaboratory analyses revealed that the commonly used “equivalent” visual-estimation methods were significantly less accurate than the Environmental Protection Agency (EPA) 400-point-count method, especially at low asbestos concentrations. In an effort to produce a method that is statistically “equivalent” yet less time consuming than the EPA method, several strategies were assessed to reduce preparation and analysis time. False negatives were produced by schemes that unconditionally reduced the number of slides/points analyzed. The best modification was a stratified scheme in which effort was inversely proportional to the Percentage of asbestos in a sample, Although precision was sacrificed at higher concentrations, that precision was deemed superfluous because such materials were categorically asbestos-containing materials (ACMs). A nonpoint scan Option was also developed for non-ACMs. Thus, the time required for preparation and analysis of ACMs by the stratified method can be less than 20% of the time required for the full EPA point count, with only ACMs in the 1% range requiring the full 400 points counted. A series of formulated- Weight bulk samples were analyzed by outside laboratories using the EPA method, the stratified method, and visual estimation. The stratified method produced results that were not significantly different than results by the EPA method. Visual-estimation results were less accurate when compared to formulation values and were usually significantly different than results from either point-count method.