ASBESTOS and OTHER FIBERS by PCM: METHOD 7400, Issue 2, dated 15 August 1994 - Page 8 of 15
. NOTE: Periodically check and adjust the value of , if necessary. 22. Report intralaboratory and interlaboratory relative standard deviations (from Step 11) with each set of results. NOTE: Precision depends on the total number of fibers counted [1,16]. Relative standard deviation is documented in references [1,15–17] for fiber counts up to 100 fibers in 100 graticule fields. Comparability of interlaboratory results is discussed below. As a first approximation, use 213% above and 49% below the count as the upper and lower confidence limits for fiber counts greater than 20 (Figure 1). EVALUATION OF METHOD: Method Revisions: This method is a revision of P&CAM 239 [10]. A summary of the revisions is as follows: 1. Sampling: The change from a 37-mm to a 25-mm filter improves sensitivity for similar air volumes. The change in flow rates allows for 2-m³ full-shift samples to be taken, providing that the filter is not overloaded with non-fibrous particulates. The collection efficiency of the sampler is not a function of flow rate in the range 0.5 to 16 L/min [10]. 2. Sample preparation technique: The acetone vapor-triacetin preparation technique is a faster, more permanent mounting technique than the dimethyl phthalate/diethyl oxalate method of P&CAM 239 [2,4,10]. The aluminum “hot block” technique minimizes the amount of acetone needed to prepare each sample. 3. Measurement: a. The Walton-Beckett graticule standardizes the area observed [14,18,19]. b. The HSE/NPL test slide standardizes microscope optics for sensitivity to fiber diameter [4,14]. c. Because of past inaccuracies associated with low fiber counts, the minimum recommended loading has been increased to 100 fibers/mm² filter area (a total of 78.5 fibers counted in 100 fields, each with field area = 0.00785 mm².) Lower levels generally result in an overestimate of the fiber count when compared to results in the recommended analytical range [20]. The recommended loadings should yield intracounter in the range of 0.10 to 0.17 [21–23]. Interlaboratory Comparability: An international collaborative study involved 16 laboratories using prepared slides from the asbestos cement, milling, mining, textile, and friction material industries [9]. The relative standard deviations ( ) varied with sample type and laboratory. The ranges were: Rules
Intralaboratory
AIA (NIOSH A Rules)* Modified CRS (NIOSH B Rules)†
0.12 to 0.40 0.11 to 0.29
Interlaboratory 0.27 to 0.85 0.20 to 0.35
Overall 0.46 0.25
- Under AIA rules, only fibers having a diameter less than 3 µm are counted and fibers attached to particles
larger than 3 µm are not counted. NIOSH A Rules are otherwise similar to the AIA rules. † See Appendix C.
A NIOSH study conducted using field samples of asbestos gave intralaboratory in the range 0.17 to 0.25 and an interlaboratory of 0.45 [21]. This agrees well with other recent studies [9,14,16].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
