ASBESTOS and OTHER FIBERS by PCM: METHOD 7400, Issue 2, dated 15 August 1994 - Page 14 of 15
These rules are sometimes referred to as the “A” rules: Object
Count
1
1 fiber
2
3
4
5 6
7 8 9
Discussion
Optically observable asbestos fibers are actually bundles of fine fibrils. If the fibrils seem to be from the same bundle, the object is counted as a single fiber. Note, however, that all objects meeting length and aspect ratio criteria are counted whether or not they appear to be asbestos. 2 fibers If fibers meeting the length and aspect ratio criteria (length >5 µm and length-to-width ratio > 3 to 1) overlap, but do not seem to be part of the same bundle, they are counted as separate fibers. 1 fiber Although the object has a relatively large diameter (>3 µm), it is counted as fiber under the rules. There is no upper limit on the fiber diameter in the counting rules. Note that fiber width is measured at the widest compact section of the object. 1 fiber Although long fine fibrils may extend from the body of a fiber, these fibrils are considered part of the fiber if they seem to have originally been part of the bundle. Do not count If the object is ≤ 5 µm long, it is not counted. 1 fiber A fiber partially obscured by a particle is counted as one fiber. If the fiber ends emanating from a particle do not seem to be from the same fiber and each end meets the length and aspect ratio criteria, they are counted as separate fibers. ½ fiber A fiber which crosses into the graticule area one time is counted as ½ fiber. Do not count Ignore fibers that cross the graticulate boundary more than once. Do not count Ignore fibers that lie outside the graticule boundary.
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. “B” Counting Rules 1. Count only ends of fibers. Each fiber must be longer than 5 µm and less than 3 µm diameter. 2. Count only ends of fibers with a length-to-width ratio equal to or greater than 5:1. 3. Count each fiber end which falls within the graticule area as one end, provided that the fiber meets rules 1 and 2 above. Add split ends to the count as appropriate if the split fiber segment also meets the criteria of rules 1 and 2 above. 4. Count visibly free ends which meet rules 1 and 2 above when the fiber appears to be attached to another particle, regardless of the size of the other particle. Count the end of a fiber obscured by another particle if the particle covering the fiber end is less than 3 µm in diameter.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
