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ASBESTOS and OTHER FIBERS by PCM:METHOD 7400,Issue 2,dated 15 August 1994-Page 7 of 15 <br /> (2) Do not count any fiber which crosses the graticule boundary more than once. <br /> (3) Reject and do not count all other fibers. <br /> c. Count bundles of fibers as one fiber unless individual fibers can be identified by observing both <br /> ends of a fiber. <br /> d. Count enough graticule fields to yield 100 fibers.Count a minimum of 20 fields.Stop at 100 <br /> graticule fields regardless of count. <br /> 19.Start counting from the tip of the filter wedge and progress along a radial line to the outer edge. <br /> Shift up or down on the filter,and continue in the reverse direction.Select graticule fields randomly <br /> by looking away from the eyepiece briefly while advancing the mechanical stage.Ensure that,as a <br /> minimum,each analysis covers one radial line from the filter center to the outer edge of the filter. <br /> When an agglomerate or bubble covers ca. 1/6 or more of the graticule field,reject the graticule <br /> field and select another.Do not report rejected graticule fields in the total number counted. <br /> NOTE 1:When counting a graticule field,continuously scan a range of focal planes by moving the <br /> fine focus knob to detect very fine fibers which have become embedded in the filter.The <br /> small-diameter fibers will be very faint but are an important contribution to the total count. <br /> A minimum counting time of 15 s per field is appropriate for accurate counting. <br /> NOTE 2:This method does not allow for differentiation of fibers based on morphology.Although <br /> some experienced counters are capable of selectively counting only fibers which appear to <br /> be asbestiform,there is presently no accepted method for ensuring uniformity of judgment <br /> between laboratories.It is,therefore,incumbent upon all laboratories using this method <br /> to report total fiber counts.If serious contamination from non-asbestos fibers occurs in <br /> samples,other techniques such as transmission electron microscopy must be used to <br /> identify the asbestos fiber fraction present in the sample(see NIOSH Method 7402).In some <br /> cases(i.e.,for fibers with diameters>1 µm),polarized light microscopy(as in NIOSH Method <br /> 7403)may be used to identify and eliminate interfering non-crystalline fibers[151. <br /> NOTE 3:Do not count at edges where filter was cut.Move in at least 1 mm from the edge. <br /> NOTE 4:Under certain conditions,electrostatic charge may affect the sampling of fibers.These <br /> electrostatic effects are most likely to occur when the relative humidity is low(below 20%), <br /> and when sampling is performed near the source of aerosol.The result is that deposition of <br /> fibers on the filter is reduced,especially near the edge of the filter.If such a pattern is noted <br /> during fiber counting,choose fields as close to the center of the filter as possible[5]. <br /> NOTE 5:Counts are to be recorded on a data sheet that provides,as a minimum,spaces on which to <br /> record the counts for each field,filter identification number,analyst's name,date,total fibers <br /> counted,total fields counted,average count,fiber density,and commentary.Average count <br /> is calculated by dividing the total fiber count by the number of fields observed.Fiber density <br /> (fibers/mm2)is defined as the average count(fibers/field)divided by the field(graticule)area <br /> (mm2/field). <br /> CALCULATIONS AND REPORTING OF RESULTS <br /> 20.Calculate and report fiber density on the filter,E(fibers/mm2),by dividing the average fiber count <br /> per graticule field,F/nf,minus the mean field blank count per graticule field,B/nb,by the graticule <br /> field area,Af(approx.0.00785 mm2): <br /> E_(F/nf—B/nb) fibers/m m2. <br /> Af <br /> NOTE: Fiber counts above 1300 fibers/mm2 and fiber counts from samples with >50%of filter area <br /> covered with particulate should be reported as"uncountable"or"probably biased"Other <br /> fiber counts outside the 100-1300 fiber/mm2 range should be reported as having"greater <br /> than optimal variability"and as being"probably biased" <br /> 21.Calculate and report the concentration,C(fibers/cc),of fibers in the air volume sampled,V(L),using <br /> the effective collection area of the filter,Ac(approx.385 mm2 for a 25-mm filter): <br /> NIOSH Manual of Analytical Methods(NMAM),Fourth Edition <br />