ASBESTOS and OTHER FIBERS by PCM: METHOD 7400, Issue 2, dated 15 August 1994 - Page 6 of 15
slide per workday. Change the labels on the reference slides periodically so that the counter does not become familiar with the samples. b. From blind repeat counts on reference slides, estimate the laboratory intra- and intercounter precision. Obtain separate values of relative standard deviation ( ) for each sample matrix analyzed in each of the following ranges: 5 to 20 fibers in 100 graticule fields, >20 to 50 fibers in 100 graticule fields, and >50 to 100 fibers in 100 graticule fields. Maintain control charts for each of these data files. NOTE: Certain sample matrices (e.g., asbestos cement) have been shown to give poor precision [9]. 12. Prepare and count field blanks along with the field samples. Report counts on each field blank. NOTE 1: The identity of blank filters should be unknown to the counter until all counts have been completed. NOTE 2: If a field blank yields greater than 7 fibers per 100 graticule fields, report possible contamination of the samples. 13. Perform blind recounts by the same counter on 10% of filters counted (slides relabeled by a person other than the counter). Use the following test to determine whether a pair of counts by the same counter on the same filter should be rejected because of possible bias: Discard the sample if the absolute value of the difference between the square roots of the two counts (in fiber/mm²) exceeds where = average of the square roots of the two fiber counts (in fiber/mm²) and where is the intracounter relative standard deviation for the appropriate count range (in fibers) determined in step 11. For more complete discussions see reference [13]. NOTE 1: Since fiber counting is the measurement of randomly placed fibers which may be described by a Poisson distribution, a square root transformation of the fiber count data will result in approximately normally distributed data [13]. NOTE 2: If a pair of counts is rejected by this test, recount the remaining samples in the set and test the new counts against the first counts. Discard all rejected paired counts. It is not necessary to use this statistic on blank counts. 14. The analyst is a critical part of this analytical procedure. 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. In addition, counters should take 10- to 20-minute breaks from the microscope every one or two hours to limit fatigue [14]. During these breaks, both eye and upper back/neck exercises should be performed to relieve strain. 15. All laboratories engaged in asbestos counting should participate in a proficiency testing program such as the AIHA-NIOSH Proficiency Analytical Testing (PAT) Program for asbestos and routinely exchange field samples with other laboratories to compare performance of counters. MEASUREMENT: 16. Center the slide on the stage of the calibrated microscope under the objective lens. Focus the microscope on the plane of the filter. 17. Adjust the microscope (Step 10). NOTE: Calibration with the HSE/NPL test slide determines the minimum detectable fiber diameter (ca. 0.25 µm) [4]. 18. Counting rules: (same as P&CAM 239 rules [1,10,11]: see examples in APPENDIX B). a. Count any fiber longer than 5 µm which lies entirely within the graticule area. (1) Count only fibers longer than 5 µm. Measure length of curved fibers along the curve. (2) Count only fibers with a length-to-width ratio equal to or greater than 3:1. b. For fibers which cross the boundary of the graticule field: (1) Count as ½ fiber any fiber with only one end lying within the graticule area, provided that the fiber meets the criteria of rule a above. NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
