APPENDIX C. ALTERNATE COUNTING RULES FOR NON-ASBESTOS FIBERS
Other counting rules may be more appropriate for measurement of specific non-asbestos fiber types,
such as fibrous glass. These include the “B” rules given below (from NIOSH Method 7400, Revision #2,
dated 8/15/87), the World Health Organization reference method for man-made mineral fiber [24], and
the NIOSH fibrous glass criteria document method [25]. The upper diameter limit in these methods
prevents measurements of non-thoracic fibers. It is important to note that the aspect ratio limits
included in these methods vary. NIOSH recommends the use of the 3:1 aspect ratio in counting fibers.
It is emphasized that hybridization of different sets of counting rules is not permitted. Report
specifically which set of counting rules are used with the analytical results.