Airborne Asbestos Analysis of Low Fiber Density Samples: A Comparison of the A and B Counting Rules of the NIOSH Method 7400

Abstract

This study was conducted to evaluate and compare the NIOSH analytical Method 7400 A and B counting rules for samples with a low fiber density (< 100 fibers/mm²). One hundred forty-eight employee breathing zone exposure samples collected following the NIOSH Method 7400 were chosen for this experiment. Fiber counts were made on each mounted wedge. Each field of each sample was first counted using the A rules and then the B rules before moving to the next graticule field. Each sample was counted independently without knowledge of sample numbers or concentrations, and 10 percent of the samples were randomly chosen and recounted. Ninety-four samples which met the criteria of fiber density greater than or equal to 7 fibers/mm² and less than or equal to 100 fibers/mm² were chosen for statistical analysis. The study demonstrated a significant correlation between the two methods (p < 0.01 level). The results of the B counting rules were significantly different from the A rules (p < 0.05; paired t-test). The correlation between the two methods was anticipated due to the similarity between the two counting methods and provides a basis for comparing low fiber density data counted by the two sets of rules. Differences between the two counting strategies were found to be due to the counting of fibers attached to particles. This study demonstrates that the A rules required by the Occupational Safety and Health Administration (OSHA) yield higher concentrations than the B rules but that the two methods can be predictably compared.