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 concentration s of synthetic vitreous fibers (SVFs), including the National Institute for Occupational Safety and Health (NIOSH) Method 7400 A and B rules and the W orld Health Organization European Reference Method for Man-Made Mineral Fib ers. Differences in counting rules have led to a lack of comparability of s ampling results across studies. The purpose of this article is to compare N IOSH 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 recen t investigations that used B rules. A total of 792 airborne fiber samples w ere collected according to NIOSH Method 7400 and separately evaluated using both NIOSH A and B counting rules. The results of this investigation indic ate that A-rule fiber counts give statistically higher fiber density estima tes 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 approx imately 33% greater than corresponding B-rule estimates. In contrast, prima rily due to their smaller diameter distribution, fiber glass blowing wool w ithout binder produced B-rule fiber counts only 8% greater than the corresp onding A-rule fiber counts.