Document RjEVOwRYkajJp5DZk4oMao687

1 H ct V) PLAINTIFF'S EXHIBIT XCOMPARISON TESTING MONITORING FOR AIRBORNE ASBESTOS FIBERS: SHEET VINYL FLOOR COVERING, WET VERSUS DRY SCRAPING Final Report December 1979 By: Richard Walcott, Industrial Hygienist James Warrick, Industrial Hygienist Prepared for: Resilient Floor Covering Institute 1030 15th St. N.W. Suite 350 Washington, O.C. 20005 SRI Project 7988 SRI International 1611 N. Kent Street Arlington, Virginia 22209 COMPARISON TESTING MONITORING FOR AIRBORNE ASBESTOS FIBERS: SHEET VINYL FLOOR COVERING, WET VERSUS DRY SCRAPING Final Report December 1979 By: Richard Walcott, Industrial Hygienist James Warrick, Industrial Hygienist Prepared for: Resilient Floor Covering Institute 103016th St. N.W. Suite 350 Washington, O.C. 20005 SRI Project 7988 Approved by: Sherry J. Hanen, Associate Director Center for Occupational and Environmental Safety and Health Ernest J. Moore, Vice President SRI INTERNATIONAL SRI International is a problem-solving organization that provides consulting and research under contract for clients in business and government throughout the world. Since SRI was founded in 1946, the problems confronting it's clients have so increased in complexity that few of them can be solved within the confines of a single discipline. The organization is uniquely qualified to deal with such intricate problems. Professionals representing more than 100 disciplines comprise nearly two-thirds of the 3,000 staff members. SRI performs consulting and research for the public sector in such areas as urban management; health, education, and welfare; environmental management, and national security. In addition to broad-ranging activities for the U.S. and other national governments. SRI serves city, state, and county governments in the U.S., and also their international equivalents. One of the most attractive aspects of life at SRI for the staff is the opportunity to share in important work in the public interest. For private commercial and industrial clients, SRI engages in management consulting and corporate strategy determination; technoeconomic, market, and operational studies; new product, process, and equipment development; and the provision of information services. A significant percentage of SRI's professional staff have had a nunber of years of major responsibility in business prior to joining SRI. SRI is an independent, nonprofit corporation, using revenues in excess of operating costs to purchase advanced scientific equipment and to enhance its ability to continue to provide the highest quality consulting and research for its clients. SRI International has no shareholders and no endowment. SRI's annual revenues for consulting and research contracts exceed $125 million. SRI's headquarters are on a 70-acre site in Menlo1 Park, California, a suburban community about 20 miles south of the San Francisco International Airport and a few miles from the campus of Stanford University. Other offices and laboratories are located in major centers in North Anerica, Europe, East Asia, and the Middle East. STATEMENT OF OBJECTIVE The principal objective of this study was to monitor for airborne asbestos fibers, if any, that were generated during the complete removal of sheet vinyl floor covering when different methods of removal (wet versus dry scraping) were used. The effect of using different length handles on the scraping Instrument was also measured. TABLE OF CONTENTS I. II. III. IV. V. VI. VIII. Introduction, Sheet Vinyl Floor Covering ................................... Page 1 Test Site Location ................................................................................... 2 Methodology....................................................................................... .... . 4 Summary of Results..................................................................... 5 Conclusions ................................................................................................ 6 Complete Removal........................................................................................ 7 Attachments: A. NIOSH Analytical Method B. Representative Ambient Air Asbestos Concentrations in Cities Copyright (c) 1979 by Resilient Floor Covering Institute \ I. INTRODUCTION Sheet vinyl floor coverings are widely used in many residential, institutional, and commercial locations. Asbestos is used as a constituent in the felt backing of many sheet vinyl floor coverings, but there is no asbestos in the various types of top or wear layers. Attention has recently been centered on the health effects resulting from certain exposures to asbestos. SRI was retained to measure whether or not airborne asbestos fibers were produced under certain conditions of sheet vinyl floor covering removal. The industry recommends that the felt backing of the sheet vinyl be removed from the subflooring by a method known as "wet scraping." Wet scraping is essentially a wetting of the felt remaining after the wear layer has been removed before scraping Is performed. In this test both wet scraping and dry scraping were used and the asbestos fibers collected and counted. 1 II. TEST SITE LOCATION The site for* these evaluations was a six-year-old home located in a suburban Maryland comnunity. The attic insulation material in this home was, according to the occupant, a pink fiber-glass material. This could not be confirmed since access to the attic was not feasible. The heating system in this house was forced-air ventilation, and the air was filtered by the furnace unit. This operation was performed in the kitchen of the home. The total floor surface area was approximately 350 square feet. The tests were done in the Spring (March 15 and 16) of 1979. A diagram of the test site is included as Figure 1, page 3. 2 Refrigerator and Cabinets d[ To Dining Room Dry Scrape Section To Family Boom -- Electric Range Sink and Dish Washer Wet Scrape Section Figure I SITE PLAN 3 III. METHODOLOGY 1. Complete Removal Recommended procedures were not followed in that dry scraping was also done. 2. Determination of Airborne Asbestos Fibers The NIOSH method for determining asbestos fibers in the air was employed (See Attachment "A," NIOSH Method No. PACAM 239, "Asbestos Fibers in Air"). 4 IV. SUMMARY OF RESULTS The following Table lists the TWA concentrations of airborne asbestos fibers measured at the test site for the specified operations. TWA concentrations are stated assuming a full-day exposure to the conditions that existed during the test period. NOTE: The OSHA Standard for a safe workplace allows a concentration of 2 fibers per cc of air on an 8-hour time-weighted average (TWA). Operation Wear Layer Removal Wet Scrape Dry Scrape Ory Scrape Table No. 1 8 Hr. TWA (Asbestos Flbers/cc) Mechanic 1 Mechanic 0.1637 0.0844 0.4093 NA 2.0248 1.1207 0.8279 1.0583 5 V. CONCLUSIONS Utilization of dry scraping rather than wet scraping resulted in a two to five-fold increase in asbestos exposure. 6 VI. COMPLETE REMOVAL Discussion An existing sheet vinyl floor covering was removed by using both wet and dry scraping. First the wear layer was removed according to standard practice and part of the floor was dry scraped and the rest was wet scraped. A water-detergent solution was used for wetting the felt. Two different length handles were also used in the scraping: Mechanic 1 used a regular handle on a stiff bladed putty knife while Mechanic 2 used a handle of 3-4 feet. Sampling Procedure: The sampling protocol was designed to: 1. Evaluate the background asbestos levels in the home and outside ambient air. This was accomplished using area samples taken several days before the test sampling. 2. Determine any increase above background concentrations of asbestos fibers. Personal samples were taken for each worker during the operation. Separate filters were used for each phase of the operation. Ory scraping was done in two phases so there are two samples for each worker for dry scraping. See Figure 1, page 3, for a diagram of the sampling.area. Sampling Results: The results of the individual-filter analyses are shown below in Table 2. 7 Table 2 Description Background Samples Outside Outside Kitchen Wear Layer Removal Mechanic 1 Mechanic 2 Wet Scrape Mechanic 1 Dry Scrape Mechanic 1 Mechanic 1 Mechanic 2 Mechanic 2 Fil ter No. Air Vol. (liters) Time (min) Fibers/ Total Asbestos** sq. mm* Fibers/cc Fibers/cc 8-Hr. TWA AB-9 C-1089 XX009 760 766 748 380 383 374 00 00 00 u 0 0 AB-12 AB-2 150 140 75 39 0.218 0.1637 0.1637 70 14 0.0844 0.0844 0.0844 AB-14 110 55 96 0.4837 0.4093 0.4093 AB-13 AB-11 AB-10 AB-15 126 80 126 90 63 327 40 96 63 191 45 121 2.168 1.0035 1.2669 1.1259 2.0248 0.8279 1.1207 1.0583 2.0248 0.8279 1.1207 1.0583 Fibers/sq mm of 1total filter surface area. **Analyst estimated that about 90% of the fibers were asbestos. 8 Evaluation The background samples obtained prior to and on the day of the test showed no asbestos fibers. Asbestos fiber concentrations found during the wear-layer removal were very low (0.1637 and 0.0844 fibers/cc). As sometimes happens when the wear layer is removed, a good deal of the felt was separated from the subfloor and removed with the wear layer. The strips of wear layer were rolled and placed in trash bags for disposal. The higher concentration was from the worker who bagged the material. Wet scraping, which took 55 minutes, resulted in a concentration of 0.4093 fibers/cc. Ory scraping samples, however, showed concentrations ranging from 0.8279 to 2.0248 fibers/cc. Dry scraping was done with two different-length handles. The results from the worker using the longer handle (1.1207 and 1.0583) are, on the average, a good deal lower than the results from the worker using the short handle (2.0248 and 0.8279). The longer handle does provide, as would be expected, some protection from the asbestos concentration. 9 ATTACHMENT A ASBESTOS FIBERS IN AIR ASBESTOS FIBERS IN AIR National Institute, for Occupational Safety and Health Analytical Method Analyte: Matrix: Procedure: Date Issued: Date Revised: Asbestos fibers Air Filter collection, microscopic count 3/30/77 Method No.: P&CAM 239 Range: 0.1-60 fibers/cm* Precision (CV*): 0.24 to 0.38 - Classification: D (bperational) 1. 'Principle of the Method 1.1 This method describes the equipment and procedures for collecting, mounting, and counting asbestos libers on cellulose ester membrane filters in the evaluation of personal samples of airborne asbestos fibers. The purpose of the method is to determine an employee's index of exposure to airborne asbestos fibers. The method is primarily a personal monitoring tech nique, but can be used for area monitoring. 1.2 The sample is collected by drawing air through a membrane filter by means of a batterypowered personal sampling pump. The filter is transformed from an opaque solid membrane to a transparent optically homogeneous gel. The fibers are sized and counted using a phase* contrast microscope at 400-450X magnification. 1.3 Definitions. Asbestos fiber, for counting purposes, means a particulate which has a physical dimension longer than S micrometers and with a length to diameter ratio of 3 to 1 or greater. Asbestos includes chrysotile, cummingtonite-grunerite (amosite), crocidolite, fibrous tremolite, fibrous anthophyllite, and fibrous actinolite. 1.4 Any laboratory attempting to use this procedure should have at least one counter attend a training course conducted by an experienced, proficient laboratory. Novice, untutored counters, using only published instructions, can easily obtain counts of half those performed by experi enced, proficient counters. Large differences between laboratories can be caused by: 1) dif ferences in technique and observing ability among counters and 2) small, but significant, dif ferences between microscopes meeting the basic specifications of Section 6.2. -The following procedures are recommended: 1.4.1 All microscopists who perform asbestos counting should meet together for an "asbestos counting workshop" at least quarterly. This is best accomplished with counters from several laboratories using their own microscopes. 1.4.2 Each microscopist should count the same series of slides and with -the results being compared. 1.4.3 Differences between counters should be resolved with side-by-side counting of the fields by the different counters. 1.4.4 Individuals who are found to be persistent outliers over several sessions should be encouraged to seek other tasks in their respective laboratories. 239-1 2. Range and Sensitivity 2.1 The usable range is primarily a function of sample volume, microscope count field area, and background airborne particulates. The influence of these variables is discussed in 8.1.2. For a microscope count field area of 0.003 mm1 (see Figure 1) and a pump flow rate of 1.7 Ipm, ' the optimal fiber densities would be produced over the range of 0.4 fiber/cm1 (8-hour sam ple) to about 60 fibers/cm* (15-minutc sample). For a field area of 0.006 mm* (see Figure 2) and a pump flow rate of 1.7 Ipm, the optimal range is 0.2 fiber/cm* (8-hour sample) to about 30 fibers/cm* (15-minute sample). In each case, the optimal detection limits are in versely proportional to pump flow rate. The upper detection limit can be extended by using sample times less than 15 minutes or using lower flow rates. The lower detection limit can be extended by increasing the flow rate up to about 2.51pm. Filter surface fiber densities less than optimal (less than about 0.5 to 1.0 fiber per count field) are still adequate, but will lead to decreased precision for the method Gncrcased coefficient of variation, see Section 4). The minimum total fiber eount in 100 fields considered adequate for reliable quantitation is 10 fibers. Thus, the lower limit of reliable quantitation is 0.1 fiber/cm* (100,000 fibers/ m*). For this level, a flow rate of about 2.5 1pm is recommended. For a field area of 0.003 mm*, the minimum sample time would be about 2 hours. For a field area of 0.006 mm*, the minimum sample time would be about 1 hour. 2.2 This method considers only fibers with a length to diameter ratio of 3 to 1 or greater and a length greater than 5 micrometers. 3. Interferences In an atmosphere known to contain asbestos, all particulates with a length to diameter ratio of 3 to 1 or greater, and a length greater than 5 micrometers should, in the absence of other information, be considered to be asbestos fibers and counted as such. 4. Precision and Accuracy 4.1 In the past decade, there have appeared a number of articles examining sources of variation in the asbestos sampling and counting procedure. These include: Lynch et al. (11.1), Weidner and Ayer (11.2), Conway and Holland (11.3), Lcidel and Busch (11.4), Beckett and Attfield (11.5), and Rajhans and Bragg (11.6). The sources of variation will be discussed by stages in the membrane filter evaluation procedure. 4.2 Sources of Variation in the Sampling Process. These include variations in pump flow rate, proximity of the filter to the employee's body, and filter location Gelt to right) in the em ployee's breathing zone. 4.2.1 Section 9.1 requires that the personal sampling pump be . calibrated with sufficient accuracy such that the 95 % confidence limits on the flow rate are efc'10%. This is equivalent to a coefficient of variation (CV) of about 5%. However, this CV makes a negligible contribution to the total CV for the method due to the relatively large CV of the counting procedure. 4.2.2 Conway and Holland (11.3) concluded that positioning of the filter cassette on the wearer (regarding the angular portions of the filter and their proximity to the wearer) is not a significant factor in determining the fiber distribution on filters. 4.2.3 Weidner and Ayer (11.2) concluded that there is no appreciable difference between samples collected on either the right or left sides of a breathing zone or between samples collected sidc-by-sidc, especially for samples with concentrations less than 2.5 fibers/cm*. 239-2 4.3 Sources of Variation In the Counting Procedure 4.3.1 Random variations exist in the fiber distribution on a filter wedge (intra-wedge vari ability). The industrial hygiene literature has seen considerable debate in the last 20 years concerning whether or not the distribution of mineral dust or asbestos fibers on a filter surface is adequately described by a Poisson distribution probability density function. Leidel and Busch (11.4) found excellent agreement between empirical error variance and theoretical variance calculated from the assumption of Poisson dis tributed true counts. They concluded that there was not excessive variation among count fields for a filter wedge and that clumping of fibers (non-random coalescence) did not occur. 4.3.2 Variations exist in the fiber distribution on the total filter surface (inter-wedge vari ability) due to the random or non-random distribution of fibers across the total sur face of the filter. This type of variation is easily confused with intra-wedge variations. The count procedure does not require counting of multiple sectors of the filter. There may be significant differences between average counts for different wedges, or the fiber distribution variations for the total filter surface may be greater than the variations of the Poisson distribution. If either of these occur experimentally, one must use the experimental variations to estimate the minimum precision of the count procedure. The minimum precision is governed by the variations of the fiber distribution on the totat surface of the filter. Conway and Holland (11.3) concluded the distribution of fibers on filters is not uni form and the distribution of fiber counts is more disperse than Poisson. For their filters which had significant variations in fiber concentrations between sectors (as much as 50-60% of the total filter mean), they described the following relation for the standard deviation of the total number of fibers counted on a .wedge (N) empirical s(N) * 1.6 (N)*'* where N is about 100. The Poisson standard deviation would be: Poisson 9 (N) - (N),s Rajhans and Bragg (11.6) in Series I of their study found significant variation between filter segments and rejected the Poisson distribution for the total filter surface. How ever, in Series II of their study, utilizing various experimental modifications, they found no significant variation between filter segments and no reason to reject the assumption of Poisson distributed fiber counts. 4.3.3 Systematic variations due to differences between microscopes were studied by Leidel and Busch (11.4). In their study using five different brands of microscopes, they found no significant differences among four, but the fifth gave counts approximately 45% higher on the average than the other four. 4.3.4 Variations due to differences between counters should be examined at three levels: experienced counters occasionally counting, experienced counters routinely counting, and inexperienced (newer untutored) counters. Leidel and Busch (11.4) studied five experienced counters, with one counting only occasionally. There were no significant differences among three of the counters, but a fourth was 16% lower than the first three. The fifth, who occasionally counted, averaged 27% higher than the first three. Conway and Holland (11.3) studied three experienced counters and three inexperienced counters. They found statistically significant differences between the means of both the experienced and inexperienced counters that typically were in the range plus or minus 5 to 15%. They concluded that experience as a fiber counter is not a significant parameter affecting intercountor variations. 239-3 Rajhans and Drags 0L6) found no significant differences among mean* of five experi enced counters in Series I of their study. But in their carefully controlled Series II, an analysis of variance showed significant variations between counters that were plus or minus I to 15%. 4.3.5 Variations between laboratories are most likely due to systematic biases and are not a significant additional source of random variations. Any additional variations are most likely due to differences in counting technique. Beckett and Aufield (11.5) ob served that standard counters improved greatly after personal instruction; also new counters, after instruction, tended to overcompensate and get exceedingly high counts. Additionally, they found that counts from an experienced laboratory that had not had contact with other laboratories performing the same analysis were as far from the standard values as were the counts by new counters. 4.4 Sources of variations between samples taken at different times on one employee during one work shift can affect the exposure estimate for that employee. These are primarily due to a) differences in exposure concentrations during the day, b) differences In location of the employee within the plant, and c) differences in work operation performed by the employee during the day. These sources of variation can be controlled by proper choice of sampling strategy. Refer to Leidel and Busch (11.7) and Leidel, Busch, and Lynch (11.8) for an extended discussion of sampling strategies. Interday temporal variations can affect the ex posure estimates obtained on different days. Refer to Leidel, Busch, and Crouse (11.9) for a discussion of this type of variation. 4.5 Until recently, the total coefficient of variation (CVT) for the sampling and counting proce dure was best estimated from the work of Conway and Holland (11.3). The conclusions of their study included: 4.5.1 The precision of their procedure for filters not containing an abundance of fine fibers can be estimated by a coefficient of variation of 16.2%. This value includes variation among counters and observed interaction effects. 4.5.2 The accuracy of the procedure for similar filters may be estimated for a 100-fiber count by a coefficient of variation of 21.4%. This assumes that the contribution of the overall variance from the nonuniform fiber distribution is additive. 4.5.3 A high percentage of very fine fibers on the filter can significantly .affect the standard deviation and confidence limits for counts by different counters. After combining variations in fiber concentrations over the entire filter with those for different counters, it was concluded: a. For filters with a low concentration of fine fibers, the coefficient of variation is estimated at 21% and the 95% confidence interval is -- 43%. b. For filters with a high concentration of fine fibers, the coefficient of variation is estimated at 25% and the 95% confidence interval is :fc 50%. Lynch. Kronoveter, and Lcidd (11.1) have also reported on variations of the method. Their intralaboratory study utilized the data from a large number of dust counts mads by different methods by experienced counters over a period of yean in an epidemiologic study of the asbestos products industry. They concluded that the standard deviation of counts of fibers longer than 5 micrometers on membrane filters could be estimated from the relation a * (N)*SM. Thus for counts of about 100 fibers, the coefficient of variation could be estimated at about 15.2% and the 95% confidence limits at de 30.4%. These values are lower than the values reported by Conway and Holland (11.3). Recently, the Johns-Manville Corporation conducted an in-house investigation of the asbestos count method (11.10). The study data contained total fiber counts for over 239-4 i !. \ I ! i.` 1 1 i 1! i'll 1 'lit i 100 filters with each filter counted by (wo to five counters. From the Johns-Manville * data. NIOSH calculated over 100 estimates of the count CV for the method (11.11). The NIOSH CV estimates included random intrafilter variations and intercounter ^ variations, but did not include random pump flow rate variations. It was found that * the count coefficient of variation (all random variations except for pump variations) . was a function of the total fiber count. NIOSH then included a CV of 0.05 for random pump variations (see Section 9.1) in the CV-estimator equation,to obtain a CVj-estimator. The CVj-esiimator line is plotted on Figure 3 for total fiber counts in the range 10 to 100 fibers. Or the following equation can be used: CVT - [antiloglo(--0.215 - 0.203 (log,JFB)) + 0.00251* where FB is total fiber count as discussed in Section 10. Figure 3 demonstrates that for a total fiber count of 100, the best CVT is attainable with the appropriate sampling times given in 8.1.3 and the count rules in 8.3.9. When making decisions regarding compliance with the OSHA asbestos exposure standards in 29 CFR 1910.1001, the statistical procedures given in Leidel et al. (11.11) should be followed. The procedures are based on statistical theory and assumptions given in References 11.12, 11.13. , Because of the possibility of systematic biases due to differences between microscopes, counters, and laboratories as discussed above, it is strongly recommended that any laboratory counting asbestos should participate in an intcrlaboratory quality control program that includes the counting of standard reference filters. These standard filters are available from NIOSH through the Proficiency Analytical Testing (PAT) Pro gram. The PAT Program is used by the American Industrial Hygiene Association (AIHA) as part of its Laboratory Accreditation Program. Each laboratory's quality control program must indude protocols for routinely adjusting and calibrating sampling and counting equipment plus training and evaluation programs for counters.' 5. Advantages and Disadvantages of the Method y) 5.1 The method is intended to give an index of employee exposure to airborne asbestos fibers of specified dimensional characteristics. 5.2 It is not meant to count all asbestos fibers in all size ranges or to differentiate asbestos from other fibrous particulates. 6. Apparatus 6.1 Sampling Equipment The personal sampling equipment train consists of 1) personal sampling pump, 2) tubing, 3) clothing spring clip, 4) tubing-to-field monitor metal adaptor, and 5) field monitor (filter and holder). 6.1.1. Personal Sampling Pump. The pump must be capable of sampling ar 1.0 to 2.5 liters per minute (1pm) against a flow resistance of 7.5 inches of watcr'(i.4 cm Hg) for 8 continuous hours on a fully charged battcry6.1.2 Tubing. Laboratory tubing such as rubber or plastic with 6-mm bore and about 100 cm length. 6.1.3 Clothing Spring Clip. The clip attaches the rubber tubing to the lapel or shirt of the individual being monitored. 6.1.4 Tubing-to-ficld Monitor Adaptor. A short metal adaptor with ridges on one end to grip the inside of the tubing. The other end is designed for a pressure fit into the field monitor. 6.1.5 Field Monitor (Filter and Holder). The only field monitor currently considered acceptable by NIOSH is manufactured by the Mitiiporc Corporation. The unit con- 239-5 sists of 1) three section styrene plastic case designated Millipore Aerosol Monit Case, 2) * 37-nun diameter plain white cellulose ester membrane filter designated Millipore AA (pore size of 0.8 micrometer), 3) a support pad, and 4) two plastic sealing caps. If a large number of samples are to be taken, it may be less expensive to reuse the plastic cases. Great care must be taken in the cleaning and reassembly process. The outside mating surfaces of the field monitors may be covered with a "shrink-fit" band to provide proper sealing and a writing surface for filter identifica tion. 6.2 Optical Equipment and Microscope Features 6.2.1' Microscope body with binocular head. 6.2.2 10X Huygenian eyepieces are recommended. Other eyepieces can be substituted if necessary. Wide field eyepieces can be used; however, wide field eyepieces may yield a count field area less than 0.003 mm* with the Porton reticle. This is not always desirable from the standpoint of obtaining optimum sampling times (see Sec tion 8.1.3). If wide field eyepieces are used, it is preferable to use the Patterson 4 Globe and Circle reticle to obtain a larger count field area. 6.2.3 Koebler illumination (preferably built- in with provisions for adjusting light intensity). 6.2.4 A Porton reticle is recommended. Others such as the Patterson Globe and Circle can be substituted. 6.2.5 Mechanical stage. 6.2.6 Phase-Contrast condenser with a numerical aperture (NA.) equal to or greater than the N.A. of the objective. 6.2.7 40-4SX phase contrast achromatic objective (NA. 0.65 to 0.75). 6.2.8 Phase-ring centering telescope or Bertrand lens. 6.2.9 Green or blue filter, if recommended by microscope manufacturer. 6.2.10 Stage micrometer with 0.01 mm subdivisions. 6.2.11 For general guidance on phase contrast microscopy, consult Needham (11.12), Clark (11.15) and McCrooe (11.14). 6.3 Filter Mounting Equipment Experience has shown that certain equipment is useful for efficient sample mounting. The following items are recommended for extracting and mount ing a portion of the filter for counting. 6.3.1 Microscope slides. 2.5 by 7.5 cm glass slides are most commonly used. Sample number, data, initials, etc, can be conveniently written on a frosted end slide. 6.3.2 Cover Slips. Cover slips are a necessary part of the slide mount and optical system. The shape should be appropriate for the size of the filter wedge. The appropriate cover slip depends upon the objective to be used. Ordinarily, objectives are optically cor rected for a #1V5 (0.17 millimeter) thickness cover slip. Improper cover glass thick ness will detract from the final image quality. 6.3.3 Scalpel. A scalpel is needed to cut oulja portion of the filter to be examined. A number-ten curved blade scalpel is recommended. 6.3.4 Tweezers. A pair of fine-tipped tweezers is used to remove the membrane filter slice from the field monitor and place it upon the slide. 6.3.5 Lens Tissue. To insure cleanliness, a lint-free tissue is recommended. This tissue should also be used for wiping mounting tools and for cleaning slides and cover slips. 6.3.6 Glass Rod. A fire-polished glass rod may be used to spread the mounting solution on the slide. 239-6 6.3.7 Wheaton Balsam Bottle. This special glass container has a glass top which prevents contamination of the mounting solution. A glass rod is included for dispensing the solution. 7. Reagertts ' Chemicals should be reagent grade, free from particles and color, conforming to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. 7.1 Dimethyl phthalate 7.2 Diethyl oxalate Avoid getting the mounting solution on the skin. Wash skin promptly with soap and water if skin contact occurs. 8. Procedure 8.1 Sampling 8.1.1 General Information Guidelines for the monitoring of employee exposures to industrial atmospheres arc given in Reference 11.8. The Federal requirements for monitoring employee expo sure to airborne asbestos are found in 29 CFR 1910.1001. 8.1.2 Mounting the Sampling Pump on the Worker Fasten the sampling pump to the worker's belt and fasten the field monitor to the' Iape! or shirt front (as close to the breathing zone as is practical). Remove the top cover of the plastic monitor, then invert the monitor making certain the exposed filter is facing downward. Turn the pump on and adjust to the calibrated flow rate (1.0 to 2.5 1pm). Record the following information in a logbook. 1. Filter number 2. Pump stan time and date 3. Flowrate 4. Subject's name and job title 5. Type of operation or process 6. Ventilation controls and is the worker wearing a respirator approved for asbestos? The pump should be checked periodically during the sampling period for proper oper ation and flow rate. 8.1.3 Optimum Sampling Times The requirement for the minimum count of 100 fibers or 20 fields in 8.3.9 was determined to be the best compromise to achieve adequate precision for the airborne fiber estimate and reasonable counting times. An optimum fiber density of about 1 to 5 fibers per microscope count field is recommended. To estimate appropriate sampling times for feasible counting and optimal counting, one must consider the following constraints: 1. microscope count field area (generally 0.003 to 0.006 mm3) 2. pump flow rate (typically 2.5 1pm maximum) 3. average airborne fiber concentrations 4. counting rule range of 20 to 100 fields 5. adequate fiber density to obtain a minimum count of 10 fibers in 100 fields, which is the least total fiber count that yields an acceptable count precision 6. background airborne particulate levels that can reduce the count precision due to an obscuring of fibers on the filter surface 239-7 The preceding constraints were considered in drawing Figures 1 and 2. These figures were developed from the following relationship: sampling time (FB/FL) (ECA/MFA) minutes (FR) (AC) (1000) where: FB/FL * 1 to 5 fibers/field ECA * effective collecting area of filters (855 mm* for 37-mm flitter with effee tive diameter of 33 mm) MFA microscope field area (generally 0.003 to 0.006 mm*) FR =* Pump flow rate (generally 1.0 to 2.5 1pm) AC " Air concentration of fibers in fibers/cm*. Figure 1 (microscope field area * 0.003 mm*) and Figure 2 (microscope field area = 0.006 mm*) show optimum and feasible sampling times for a pump flow rate of 1.7 1pm. Each individual responsible for sampling asbestos should prepare a similar chart for his particular pump flow rate and microscope field area before sampling is per formed to aid in estimating proper sampling times. On Figures 1 and 2, the areas with solid shading lines are generally the optimum conditions for counting. The broken shading lines are for conditions very close to optimal. However, feasible counting conditions may extend down to about 0.1 fiber/field and and above 5 fibers/field. Recommended sampling times are most strongly influenced by background airborne particulate levels, once all the other constraints have been estimated. For heavy particulate levels, it may be necessary to limit each filter to about 60 to 180 minutes sampling duration. Each individual responsible for sampling should work closely with the mieroscopist to attain as high as possible filter surface fiber densities (up to about 5 fibers/field), while avoiding filter surface background particulate levels that create very difficult or. impossible counting conditions. If one . has very little idea of airborne fiber and particulate levels, the best procedure is to take several long samples (as one 8-hour or two consecutive 4-hour samples) in con junction with several short samples (as four consecutive 2-hour or eight consecutive 1-hour samples). If the longer samples prove very difficult to count, the mieroscopist will have the shorter samples to fall back on. From Figures 1 and 2, it can be seen that there are certain sampling times which will yield optimum fiber densities on the filter for almost all airborne fiber concen trations from 1 to 10 fibers/em*. These optimum times have been calculated and are presented in Figure 4. Note that the optimum times given by Figure 4 are approxi mate and can be varied by as mueh as -- 25%. The nomogram is intended as a guide to be used where no prior knowledge of the air concentration is available. 8.1.4 End of Sampling Period Remove the field monitor, replace the plastic top cover and the small end caps, and store the monitor. Always shut of! the pump when changing monitors to avoid contaminating or damaging the pump. Record the pump shutofT time ar.d flow rate in the logbook. 8.1.5 Blanks With each batch (25 to 50 filters) of samples sent for analysis, submit two unopened field monitors which have been subjected to the same treatment as the samples except that they were not exposed to the sampling environment. Label these as blanks. If the blanks yield fiber counts greater than S fibers/100 fields, then the entire sam pling procedure should be examined carefully for the cause of contamination. The 239-K mounting solution of Section 8.2.1 should also be examined for contamination and/or crystal growth. 8.1.6 Shipping The field monitors in which the samples are collected should be shipped in a rigid container with sufficient packing material to prevent crushing. 8.1.7 Numbers of Samples When sampling for the Federal ceiling standard of 10 fibers (>S/im)/cro*t (29 CFR 1910.1001(b) (3), effective July 7, 1972), only one sample (15 minutes maximum duration) is necessary, theoretically. However, several samples should be taken dur* ing expected periods of peak air concentrations to allow for detection of gross sam pling or counting errors. When sampling for determination of noncompliance with the Federal 8-hour TWA standard of 2 fibers (>5pm)/cm*, {29 CFR 1910.1001(b) (2)), one should contin uously sample as large a portion of the work day as is feasible for airborne concen trations of about 2 to 10 fibers/cm*. However, for a lower airborne concentration such as 0.5 fiber/cm*, one sample might require 4 to 8 hours sampling time in order to get the proper filter fiber density (Section 8.1.3). For tijis situation, the 8-hour TWA exposure would be determined from one 8-hour or two 4-hour samples as ap propriate. 8.2 Sample Preparation 8.2.1 Preparation of Mounting Solution A very important part of the sample evaluation is the mounting process. This proc ess involves a special mounting medium of prescribed viscosity. The proper viscosity is important in order to expedite filter dissolving and still minimize pvticle migration. After the sample has been mounted, an elapsed time of approximately sixty minutes is needed before the sample is ready for evaluation. Combine the dimethyl phthalate and diethyl oxalate in a one to one ratio by volume and pour into a Wheaton balsam bottle. Add approximately 0.05 ( 0.005) grams of new membrane filter per milliliter of solution to reach the necessary viscosity. The mixture must be stirred periodically until the filters have dissolved and a homogeneous mixture is formed. The normal shelf life of the mounting solution is about three months. Twenty milliliters of mounting solution will prepare approximately 300 samples. 8.2.2 Sample Mounting Cleanliness is important! A dirty working area may result in sample contamination and erroneous counts. The following steps should be followed when mounting a sample. 1. Clean the slides and cover slips with lens tissue. Lay each slide down on a clean surface with the frosted end up. It is a good practice to rest one edge of the cover slip on the slide and the other edge on the working surface. By doing this, you keep the bottom surface (the one which contacts the filter) from becoming contaminated. 2. Wipe all the mounting tools clean with lens tisshe and place them on a clean surface (such as lens tissue). AU tools should be wiped clean prior to mounting each sample. 3. Using the glass rod supplied with the Wheaton balsam bottle, apply a drop of mounting solution onto the center of the slide. It may be necessary to adjust the quantity of solution so that after the cover slip has been placed on top, the solu tion extends only slightly beyond the filter boundary. If the quantity is greater than this, particle migration may occur. 239-9 I 4. Using another glass rod, spread the mounting media into a triangular shape. The size of this triangle should coincide with the dimension of the filter wedge. 5. Separate the middle and bottom sections of the field monitor case to expose the filter. Cut a triangular wedge from the center to the edge of the filter using the scalpel.* The size of the wedge should approximate one-eighth of the filler surface. The filter can be very carefully removed from the cassette for cutting, but this should only be done with great care.. 6. Grasp the fitter wedge with the tweezers on the perimeter of the filter which was clamped between the monitor case sections. Do not touch the filter with your fingers. Place the wedge, sample side up, upon the mounting medium. 7. Pick up a clean cover slip with tweezers and carefully place it on the filter wedge. Once this contact has been made, do not reposition the cover slip. 8. Label the slide with the sample number and current date before proceeding to the next filter. On the bottom (backside) of the slide, trace the perimeter of the filter wedge with a felt tip marking pen. This will enable the counter, after the filter has become transparent, to stay within the filter perimeter when counting. 9. The sample should become transparent within fifteen minutes. If the filter appears cloudy, it may be necessary to press very lightly on the cover slip. This Is rarely necessary; however, counting should not be started until an hour after the mount ing. This allows the microscopic texture of the filter to become Invisible to micro scope viewing. 10. Discard the sample mount after two days if it has not been counted. Crystals appearing similar to asbestos fibers may begin to grow at the mounting media/air interfaces. They seldom present any problems if the slide s examined before two days. In any case, stay away from the filter's edges when counting and sizing. Counting of Fibers 8.3.1 Place the slide on the mechanical stage of the microscope and position the center of the wedge under the objective lens and focus upon the sample. Start counting from one end of the wedge and progress along a radial line to the other end (count in either direction from perimeter to wedge tip). Random fields are selected, without looking into the eyepieces, by slightly advancing the slide in one direction with the mechanical stage control. 8.3.2 It is essential to continually scan over a range of focal planes (generally the upper 10 to 15 micrometers of the filter surface) with the fine focus control during each field count. This is especially necessary for asbestos fibers due to their impaction into the filter matrix. 5.3.3 On most airborne samples, asbestos fibers will generally have fiber diameters less than one micrometer. Therefore, it is necessary to look carefully for faint fiber images. 8.3.4 Regularly check phase ring alignment. 8.3.5 When an agglomerate (mass of material) covers a significant portion of the field of view (approx 1/6 or greater) reject the field and select another. (Do not include it in the number of fields counted.) However, report the fact as it may have meaning on other data collection. 8.3.6 Bundles of fibers are counted as one fiber unless both ends of the fiber can be clearly resolved. 8.3.7 Count only fibers with a length to width ratio greater than or equal to 3:1. 8.3.8 Count only fibers greater than 5 micrometers in length. (Be as accurate as possible in accepting fibers near this length.) Measure curved fibers along the curve to esti mate the totat length. 239-10 g.3.9 Count as many fields as necessary to yield a total count of at least 100 fibers. Ex. ceptions: a) count at least 20 fields even if you count more than 100 fibers, and b) stop at 100 fields even if you haven't reached 100 fibers. 8.3.10 For fibers that cross either one or two sides of the counting field, the following pro* cedure is used to obtain a representative count. COUNT any fiber greater than 5 micrometers in length, that lies entirely within the counting area. COUNT as "Vi fiber" any fiber with only one end lying within the counting area. DO NOT COUNT any fiber crossing any two sides. Reject and do not count all other fibers. Refer to Figures 5 through 10. Note that the fibers in Figures 5 through 10 are not representative of the appearance of most as bestos fibers. Most fibers have a very faint image. 9. Calibration and Standards 9.1 Sampling Train Calibration The accurate calibration of the sampling pump is essential to the correct calculation of the air volume sampled. The frequency of calibration is dependent on the use, eare, and hand ling to which the pump is subjected. Pumps must be recalibrated if they have just been repaired, misused, or received from the manufacturer. If the pump receives hard usage, more frequent calibration may be necessary. Ordinarily, pumps should be calibrated in the labora tory both before they are used in the field and after they have been used to collect a large number of field samples. The accuracy of calibration is dependent upon the type of instrument used as a reference. The choice of a calibration instrument will depend largely on where the calibration is per formed. For laboratory testing, a 1-liter buret used as a soap bubble flow meter or wet-test meter is recommended. Other standard calibrating instruments, such as a spirometer, Mar riott's bottle, or dry gas meter can be used. The calibration should be of sufficient precision -that the 95% confidence lunits on the flow rate are . 10% (95% of the flow rates wilt fall within 3: 10% of the calibrated value). Instructions for calibration with the soap bubble flow meter follow. The sampling train used (pump, hose, filter cassette) in the pump calibration should be the same as the one used in the field. 9.1.1 Check the voltage of the pump battery with a voltmeter both with the pump off and while it is operating to assure adequate voltage for calibration. If necessary, charge the battery to manufacturer's specifications. 9.1.2 Fill a beaker with 10 ml of soap solution. 9.1.3 Connect the filter cassette inlet to the top of the buret with a length of hose. 9.1.4 Turn (he pump on and moisten the inside of the soap bubble meter by immersing the . open end of the buret into the soap solution and drawing bubbles up the inside of the buret. Perform this task until the bubbles are able to travel the entire length of the buret without breaking. 9.1.5 Adjust the pump rotameter to provide a flow between 1.5 to 2.5 1pm. 9.1.6 With a water manometer, check that the pressure drop across the filter is less than 13 inches of water (about 1 inch of mercury). 9.1.7 Start a soap bubble up the buret and measure the time it takes for the bubble to travel a minimum volume of 1 liter. 9.1.8 Repeat the procedure In 9.1.7 at least three times, average the results, and calculate the calibrated flow rate by dividing the volume traveled by the soap bubble by the elapsed time. If the range between the highest and lowest of the three flow rates is greater than about 0.33 Ipm, then the calibration should be repeated since it is likely that the precision is not adequate. 239-11 9.1.9 Data required for the calibration include the volume measured, elapsed time, pressure drop, air temperature, atmospheric pressure (or elevation), pump serial number, date, and name of person performing the calibration. 9.1.10 Corrections to the flow rate for pumps with rotameters may be necessary if the pres sure (elevation) or temperature where the samples are collected (actual flow rate) differs significantly from that where the calibration was performed (indicated flow rate). Actual flow rates at time of sampling may be calculated for a linear scale rotameter by using the following correction formula: n sA / Pt Wmctua' ui<*** y p^x, * Ticlul where both pressure (?) and temperature (T) are in absolute units such as: psia = psig + 14.7 deg Rankin = deg Fahrenheit + 460 deg Kelvin = deg Celsius + 273 * 2 Microscope Setup 9.2.1 Porton Reticle and the Counting Field The asbestos fiber count procedure consists of comparing fiber length to the diam eters of calibrated circles of a Porton reticle, and counting all fibers greater than 5 micrometers in length lying within a given counting field area. The Porton reticle is a glass plate inscribed with a senes of circles and rectangles. The left half of the reticle Is divided into six rectangles constituting the counting field. The counting field is illustrated in Figures 5 through 10. 9.2.2 Placement in Eyepiece The Porton reticle is placed inside the Huygenian eyepiece where it rests on the fieldlimiting diaphragm. If other types of eyepieces are used, it may be necessary to insert a counting collar for retaining the reticle. The reticle should always be kept clean, since dirt on the reticle is in focus and could complicate the counting and sizing process. 9.2.3 Stage Micrometer The Porton reticle cannot be used for counting until it has been properly calibrated with a stage micrometer. Most stage micrometer scales are approximately two millimeters long and are divided into units of one-hundredth of a millimeter (ten micrometers). 9.2.4 Microscope Adjustment When adjusting the microscope, follow the manufacturer's instructions while,observing the following guidelines. 1. The light source image must be in focus and centered on the condenser iris or annular diaphragm. 2. The particulate material to be examined must be in focus. 3. The illuminator field iris must be in focus, centered on the sample, and opened only to the point where the field of view is illuminated. 4. The phase rings (annular diaphragm aind phase-shifting dements) must be con centric. 9.2.5 Porton Reticle Calibration Procedure Each eycpiecc-objectivc-rctidc combination on the microscope must be calibrated. Should any of the three be changed (disassembly, replacement, zoom adjustment, etc.), the combination must be recalibrated. Calibration may change if intcrpupillary dis- 239-12 Unce is changed. For proper calibration, the following procedure should be followed closely. With a 10X objective in place, place the stage micrometer on the mechanical stage, focus the millimeter scale, and center the image. Change to the 40-45X objec tive and adjust the first millimeter scale division to coincide with the left boundary of - the Porton rectangle. Measure the distance between the left and extreme right bound aries of the Porton rectangle, estimating any portion of the final division. This meas urement represents 200 L units. The rectangle is 100 L units on the short vertical dimension. The calculated **L" is inserted into the formula D * L(25f),,s where "N" is the circle number (indicated on the reticle) and "D" is the circle diameter. Since the circle diameters vary logarithmically, ever)' other circle doubles in diameter. For example, circle number three is twice the diameter of number one; number four is twice the diameter of number two. When the circle sizes have been determined, the count ing field area which consists of the left six smaller rectangles can be calculated from the relation 10,000 L*. This completes the reticle calibration for this specific objective-eyepiece-reticle combination. Example for Porton Retide The following calibration was obtained for a pair of 10X Huygenian eyepieces and a 43X objective: 200 L * 0.148 mm * 148 micrometers 100 L = 0.074 mm * 74 micrometers One L-unit " 0.74 micrometers Thus Circle #1 has a diameter D L(2:c),/* - 0.74<2`)` - 0:74 (1.414) - 1.05 micrometers. Then our circle diameter calibration table looks like: Diameter of Circle #1 1.05 micrometers #2 - 1.48 m * 2.09 #4 - 2.96 #5 - 4.19 #6 - 5.92 Field area (10,000) (L*) (100 L) (100 L) - (0.074) (0.074) 0.0055 mm3 Thus fibers with a length greater than a distance halfway between the diameters of the #5 and #6 circles would be counted. If a Patterson Globe and Circle reticle is used, a different calculation procedure is required. The circle diameters arc rdated as follows. The #25 circle diameter is (O.l) (retide length). The circle diameters are proportional to the ratio of tlieir numbers. Thus the #20 circle diameter is (20/25) or 0.8 times the #25 circle diameter. 10. Calculations 10.1 The average airborne asbestos fiber concentration estimated by the filter sample may be calculated from the following formula: _ KFB/FL) - (BFB/BFL)) (ECA) (1000) (FR) (T) (MFA) .239-13 where: AC *= Airborne fiber concentration in (fibers > 5 *m)/cm\ BFB =* Total number of fibers counted in the BFL fields of the blank or control filters in fibers > 5 pm. BFL = Tot21 number of fields counted on the blank or control filters. ECA * Effective collecting area of filter (855 mm* for a 37-mm filter with effective di ameter of 33 mm). FR = Pump flow Tate in liters/min (1pm). FB = Total number of fibers counted in the FL fields in fibers > 5 pm. FL = Total number of fields counted on the filter. MFA =* Microscope count field area in mm* (generally 0.003 to 0.006). T Sample collection time in minutes. 10.2 Recount criteria. It is very desirable for a counter to conduct a "blind recount" for about 1 in ever}' 10 filter wedges (slides) counted. Alternatively, a second counter could perform the blind recount. In training sessions for novice counters, the trainee should conduct a blind . recount for filter wedges counted by an experienced, proficient counter. In all cases, we will observe differences between the first and second counts of the same filter wedge. Most of these differences will be due to chance alone, that is, due to the random variability (precision) of the count method. Statistical recount criteria enable us to decide whether observed dif ferences can reasonably be explained due to chance alone or are probably due to systematic differences between counters or microscopes or due to some other biasing factor. The following recount criterion is for a pair of counts that estimate some airborne fiber con centration (AC) in fibers/cm*. The criterion is given at the type-! error level. That is, there is a 5% maximum risk that wc will reject a pair of counts for the reason that one might be biased, when the large observed difference is really due to chance. Reject a pair of counts because one might be biased if: where: (AC- - AC,) exceeds 2.77(ACXCV,,) AC, * lower estimated airborne fiber concentration AC- * higher estimated airborne fiber concentration AC = average of the two airborne concentration estimates CV, B =* average CV for the two concentration estimates which are a function of the total fiber count (FB) in each case. Use the relation in Section 4 or Figure 3. For a pair of counts on the same filter, reject the pair because one might be biased if: (FB, - FB,) exceeds 2.77(FB)(CVTM) where: - FB, * lower fiber count on the filter (total fibers) FB, ** higher fiber count on the filter (total fibers) FB -- average of the two total fiber counts CVrH * CVT for the value FB. Use the relation in Section 4 or Figure 3. References 11.1 Lynch, J. R., K. J. Kronovctcr, and N. A. Lcidct, "Validity of the Poisson Distribution in Dust Counting", unpublished. 11.2 Wcidner, R. B. and H. E. Ayer, "Dust Exposure in Asbestos Processing", Transactions of the 239-14 I !4I *l i( !> [ j !j ' i - si. American Conference of Governmental Industrial Hygienists, May 1972, pp. 103-121, San Francisco, California. 11.3 Conway, R. E. and W. D. Holland, ^Statistical Evaluation of the Procedure for Counting Asbestos Fibers on Membrane Filters", LFE Corporation, Richmond, California. Prepared for the Asbestos on Membrane Assoc/North America, New York, 1973. 11.4 Leidel, N. A. and K. A. Busch, "An Evaluation of Phase Contrast Microscopes for Asbestos Counting", presented at the 1974 American Industrial Hygiene Conference, Miami Beach, Florida, May 18, 1974, unpublished. 11.5 Beckett, S. T. and M. D. Attfield, "Inter-Laboratory Comparison of the Counting of Asbestos Fibers Sampled on Membrane Filters", Ann Occup Hyg 17:85-96,' 1974. 11.6 Rajhans, G. S. and G. M. Bragg, "A Statistical Analysis of Asbestos Fiber Counting in the Laboratory and Industrial Environment", Am Ind Hyg Assoc J 36(12):909-9I5, 1975. 11.7 Leidel, N. A. and K. A. Busch, "Statistical Methods for the Determination of Noncompliance with Occupational Health Standards", NIOSH Technical Publication 75-159, 1975. 11.8 Leidel, N. A., K. A. Busch, and J. R. Lynch, "Occupational Exposure Sampling Strategy Manual", NIOSH Technical Publication 77-173, 1977. 11.9 Leidel, N. A., K. A. Busch, and W. E. Crouse, "Exposure Measurement Action Level and Occupational Environmental Variability", NIOSH Technical Publication 76-131, 1975. 11.10 Comments of the Johns-Manville Corporation with Respect to the Notice of Proposed Rulemaking: Occupational Exposure to Asbestos, Federal Register, October 9, 1975. Submitted to the public record at the U. S. Department of Labor, Occupational Safety and Health Ad ministration, April 1976. 11.11 Leidel, N. A., S. G. Bayer, R. D. Zumwalde, and K. A. Busch, USPHS/NIOSH Membrane Filter Method for Evaluating Airborne Asbestos Fibers, to be published by NIOSH in 1977. 11.12 Needham, G. H., The Practical Use of the Microscope, Charles C, Thomas Publishing Corporation, Springfield, Illinois, 1958. 11.13 Clark, G. L., The Encyclopedia of Microscopy, Rheinhold Publishing Corporation, New York, 1961. 11.14 McCrone, W. C. and J. G. Delly, I. The Particle Atlas, Edition Two, Ann Arbor Science Publishers, Inc., Ann Arbor, Michigan, 1973. 239-15 ! f.; K* : t .* 1 i i- l `l .5. r.. i i< !' f- i '* * :f -> ti :[ If !I ri! URE 1. Optimum Sampling Times for airborne asbestos where microscopic field area -- 0.003 nun* 239-16 Ji ff .. ri i i! l.l i; ; Jl STo^G &% Tvja **s?i60 Hri,m s*in. <TE$ i5.700Ip1m000. 'its to. V'boi e **b**/Ojf 239. /7 *V, P/c %<# ***4 ss y- 0.40 I P O e ws Cl a 2 *ucs E P W E JL oto 8 o oo 6 AD `NOIIVIBVA JO 1N3I0IJJ30D IVlOdL 239-18 iTo oo 6 i i f : i I i; FIG U R E 4. Nomogram of optimum sampling times to r airborne asbestos fibers In concentrations of 1 to 10 fibcrs/cm* zmm V3MV 01313 3dOOSOMOIW - V CO 10 Csl --* OOP O ^ cJ i-t t t t Il....lmilnHLuj-l l I I . . t. . I I . I I . t I I I ll tit I E/ a Ul E a E E z H Ua_J.l Z < X cm O rooo o z 3 z 32 in IQ. CM O + * a < ui Ul tc t--i--i--}--i--tt--i i~'f"r~T~r~| Og n? tttttttj--r ~t 1` o<ovi ta;nuiui *3W!1 SNndWVS/WnWUdO udT `31VM M013 dWfld-M3 239-19 . FIGURE S 239-20 l !. .11 'I j :i j ; ] t. t i J I ( l I (i Ir : i. i ri ii. 1j ii 7i ! f. \\ l- ! % LIST OF FIGURES - (S through 10) FIGURE 5. DO NOT COUNT. Fiber crosses top and bottom sides. FIGURE 6. COUNT. One fiber. FIGURE 7. .COUNT. One-half fiber. Fiber crosses left side and one end lies within count area. FIGURE 8. COUNT. One-half fiber. Fiber crosses bottom side and one end lies within count area. FIGURE 9. DO NOT COUNT. Fiber crosses two sides. FIGURE 10. DO NOT COUNT. Fiber crosses two sides (bottom left comer). COUNT. One-half fiber. Fiber crosses bottom side and one end lies within count area. COUNT. One fiber (top right comer). 239-21