Comparison of NIOSH method 7400 A and B counting rules for assessing synthetic vitreous fiber exposures

A number of counting rules are available to quantify airborne concentrations of synthetic vitreous fibers (SVFs), including the National Institute for Occupational Safety and Health (NIOSH) Method 7400 A and B rules and the World Health Organization European Reference Method for Man- Made Mineral Fibers. Differences in counting rules have led to a lack of comparability of sampling results across studies. The purpose of this article is to compare NIOSH Method 7400 A and B fiber counts using samples collected as a part of a large assessment of end-user exposures to SVFs. This comparison provides a link between previous investigations that utilized A rules and more recent investigations that used B rules. A total of 792 airborne fiber samples were collected according to NIOSH Method 7400 and separately evaluated using both NIOSH A and B counting rules. The results of this investigation indicate that A-rule fiber counts give statistically higher fiber density estimates than corresponding B-rule fiber counts, with fiber glass (except fiber glass blowing wool without binder) and mineral wool products A-rule counts producing counts approximately 70% greater than corresponding B-rule counts. A-rule fiber counts for refractory ceramic fiber products averaged approximately 33% greater than corresponding B-rule estimates. In contrast, primarily due to their smaller diameter distribution, fiber glass blowing wool without binder produced B-rule fiber counts only 8% greater than the corresponding A-rule fiber counts.