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 fiber