National Institute for Occupational Safety and Health

“ It is, therefore, incumbent upon all laboratories using this method to report total fiber counts. If serious contamination from non-asbestos fibers occurs in samples, other techniques such as transmission electron microscopy must be used to identify the asbestos fiber fraction present in the sample (see NIOSH Method 7402) . In some cases (i.e., for fibers with diameters >1 μm) , polarized light microscopy (as in NIOSH Method 7403) may be used to identify and eliminate interfering non-crystalline fibers [15] . NOTE 3:
Do not count at edges where filter was cut. ”

“ Reject and do not count all other fibers. c. Count bundles of fibers as one fiber unless individual fibers can be identified by observing both ends of a fiber. d. Count enough graticule fields to yield 100 fibers. Count a minimum of 20 fields. Stop at 100 graticule fields regardless of count. 19.
Start counting from the tip of the filter wedge and progress along a radial line to the outer edge. Shift up or down on the filter, and continue in the reverse direction. Select graticule fields randomly by looking away from the eyepiece briefly while advancing the mechanical stage. ”

“ Note that the of 0.45 used to derive Figure 1 is used as an estimate for a random group of laboratories. If several laboratories belonging to a quality assurance group can show that their interlaboratory is smaller, then it is more correct to use that smaller . However, the estimated of 0.45 is to be used in the absence of such information. Note also that it has been found that can be higher for certain types of samples, such as asbestos cement [9] . Quite often the estimated airborne concentration from an asbestos analysis is used to compare to a regulatory standard. ”

“ In relatively clean atmospheres, where targeted fiber concentrations are much less than 0.1 fiber/cc, use larger sample volumes (3000 to 10000 L) to achieve quantifiable loadings. Take care, however, not to overload the filter with background dust. If ≥50% of the filter surface is covered with particles, the filter may be too overloaded to count and will bias the measured fiber concentration. NOTE 2:
OSHA regulations specify a minimum sampling volume of 48 L for an excursion measurement, and a maximum sampling rate of 2.5 L/min [3] . 5. ”

“ In this program, one sample out of ten was a quality control sample. For laboratories not engaged in an intensive quality assurance program, the subjective component of variability can be higher. In a study of field sample results in 46 laboratories, the Asbestos Information Association also found that the variability had both a constant component and one that depended on the fiber count [14] . ”

“ The purpose of adjusting sampling times is to obtain optimum fiber loading on the filter. The collection efficiency does not appear to be a function of flow rate in the range of 0.5 to 16 L/min for asbestos fibers [7] . Relatively large diameter fibers (>3 μm) may exhibit significant aspiration loss and inlet deposition. A sampling rate of 1 to 4 L/min for 8 h is appropriate in atmospheres containing ca. 0.1 fiber/cc in the absence of significant amounts of non-asbestos dust. Dusty atmospheres require smaller sample volumes (≤400 L) to obtain countable samples. ”

“ When an agglomerate or bubble covers ca. 1/6 or more of the graticule field, reject the graticule field and select another. Do not report rejected graticule fields in the total number counted. NOTE 1:
When counting a graticule field, continuously scan a range of focal planes by moving the fine focus knob to detect very fine fibers which have become embedded in the filter. The small-diameter fibers will be very faint but are an important contribution to the total count. A minimum counting time of 15 s per field is appropriate for accurate counting. ”

“ 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. 1⁄2 fiber A fiber which crosses into the graticule area one time is counted as 1⁄2 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. ”

“ Care must be taken to provide a nonstressful and comfortable environment for fiber counting. An ergonomically designed chair should be used, with the microscope eyepiece situated at a comfortable height for viewing. External lighting should be set at a level similar to the illumination level in the microscope to reduce eye fatigue. ”

“ Move in at least 1 mm from the edge. NOTE 4:
Under certain conditions, electrostatic charge may affect the sampling of fibers. These electrostatic effects are most likely to occur when the relative humidity is low (below 20%) , and when sampling is performed near the source of aerosol. The result is that deposition of fibers on the filter is reduced, especially near the edge of the filter. If such a pattern is noted during fiber counting, choose fields as close to the center of the filter as possible [5] . ”

“ 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. ”

“ METHOD 7400, Issue 2, dated 15 August 1994 - Page 5 of 15
acetone into the vaporization chamber with a slow, steady pressure on the plunger button while holding pipet firmly in place. After waiting 3 to 5 s for the filter to clear, remove pipet and slide from their ports. CAUTION:
Although the volume of acetone used is small, use safety precautions. Work in a well-ventilated area (e.g., laboratory fume hood) . Take care not to ignite the acetone. Continuous use of this device in an unventilated space may produce explosive acetone vapor concentrations. ”

“ Microscope, positive phase (dark) contrast, with green or blue filter, adjustable field iris, 8 to 10× eyepiece, and 40 to 45× phase objective (total magnification ca. 400×) ; numerical aperture = 0.65 to 0.75. 13.
Graticule, Walton-Beckett type with 100-μm diameter circular field (area = 0.00785 mm2) at the specimen plane (Type G-22) . Available from Optometrics USA, P.O. Box 699, Ayer, MA 01432 [phone (508) -772-1700] , and McCrone Accessories and Components, 850 Pasquinelli Drive, Westmont, IL 60559 [phone (312) 887-7100] . NOTE:
The graticule is custom-made for each microscope. ”

“ The following discussion indicates how this estimation can be carried out based on measurements of the interlaboratory variability, as well as showing how the results of this method relate to the theoretically attainable counting precision and to measured intra- and interlaboratory . ”