Document OqNx5LJ2mGrLBX1bXqxz84zQ

FILE NAME: Electrical Insulation (ELEC) DATE: 1991 DOC#: ELEC009 DOCUMENT DESCRIPTION: Article from Scientific Journal AM. IND. HYG. ASSOC. J. 52(11):479-484 (1991) 4350 Exposure to Airborne A sbestos A ssociated with Simulated Cable Installation above a Suspended Ceiling* D.L. Keyes" J. Chessonb + W.M. Ewing0 J.C. Faas R.L. Hatfield11 S.M. Hays' W.E. Longor J.R. Millette6* "Environmental Sciences, Inc., 105 E. Speedway Blvd., Tucson, AZ 85705; bChesson Consulting, Inc., 1717 Massachusetts Ave. N.W., Suite 601, Washington, DC 20036; "Diagnostic Engineering, Inc., 350 Franklin Rd., Suite 300, Marietta, GA 30067; ``Law Associates, 114 Town Park Dr., Kennesaw, GA 30144; "Gobbell Hays Partners, Inc., 217 Fifth Avenue North, Nashville, TN 37219; ` Materials Analytical Service, 3597 Parkway Ln., Suite 250, Norcross, GA 30092; EMcCrone Environmental Services, Inc., 1412 Oakbrook Dr., Suite 100, Norcross, GA 30013 Installing cable above a suspended ceiling in the presence o f asbestos-containing fireproofing is an example o f an activity that may disturb in-place asbestos and associated dust and debris. Two simulations o f cable installation were conducted in a room o f an unoccupied school to test the extent o fsuch disturbance and result ing elevations in airborne asbestos. Average airborne asbestos concentrations in the room increased over 500-fold during the simulations, with severalsamples exceeding 50 structuresper cubic centimeter (slcn?), as measured by transmission electron micros copy (TEM) with an indirectpreparation technique. Elevated con centrations persisted during a subsequent cleaning o f horizontal surfaces in the room and fo r several hours thereafter. Personal samples collected on the cable installers yielded TEM measure ments averaging approximately 68 slcn?fo r the two simulations. T h is work was supported in part by the Plaintiffs Executive Committee of the National Schools' Class Action for Cost Recovery; the authors exercised full control over design and execution of the study and the interpretation and reporting of study results. A paper summarizing the work was presented at the American Industrial Hygiene Conference in Orlando, Florida, May 1990. tD r. Chesson is currently with the Australian Resource Assess ment Commission, Queen Victoria Terrace, Canberra, ACT 2600, Australia. =t=Dr. Millette is currently with Millettc, Vander Wood and Asso ciates, Inc., 5500 Oakbrook Parkway, Suite 200, Norcross, GA 30093. Copyright 1991, American Industrial Hygiene Association S everal studies have indicated that maintenance, repair, and construction activities in buildings with asbestosi _ _ _ ! containing material (ACM) can disturb the ACM and raise levels of airborne asbestos.(1-3>Installing electrical or com puter cable above a suspended ceiling in the presence of asbestos fireproofing is a typical example of such activities. In-place fireproofing may be accidentally contacted and asbestoscontaining dust and debris accumulated on ceiling tile may be disturbed. Resulting release or re-entrainment of asbestos fibers will likely lead to elevated levels of airborne asbestos. Although quantitative evidence of these effects is limited, a few studies have shown airborne fiber concentrations measured by phase contrast microscopy (PCM) to be over 50 fibers per cubic centi meters (f/cm3). Most of these studies are not well characterized with respect to the type and duration of activities and the duration of elevated air levels. Moreover, transmission electron micros copy (TEM), rather than PCM, is now recognized as the method of choice to analyze air samples for asbestos/4 TEM can distinguish between asbestos and nonasbestos fibers and can detect thin fibers that cannot be resolved by PCM. For these reasons, a controlled study of cable installation in the vicinity of asbestos fireproofing was undertaken. The study was designed specifically to examine the effects of normal cable installation procedures without regard to the presence of asbestos. Accordingly, special precautions to mini mize asbestos emissions were not taken. The study thus exam ines situations where workers do not know about the presence of asbestos or where they do not either know or use work practices 479 AM. IND. HYG. ASSOC. J. (52) / November 1991 specially designed for work around asbestos. The authors' expe riences working with owners of hundreds of buildings with ACM has indicated that these situations are not uncommon. The study results provide a rationale for using special work practices and can serve to establish a baseline against which the effectiveness of these practices can be measured. TEST SITE The study was conducted in a 20-yr-old unoccupied school awaiting removal of the asbestos-containing fireproofing (Aspen Middle School in Aspen, Colo.). A room used for science educa tion was selected as the test site (Figure 1). It contained a large STUDY DESIGN Two identical simulations were performed. Each consisted of four phases: cable installation above the suspended ceiling (ap proximately 2.5 hr); an inactive period (approximately 1.5 hr); cleaning of horizontal surfaces in the room with a commercial vacuum cleaner, broom, and dry cloths (approximately 1 hr); and a final inactive period (about 3 hr). At the end of each 8-hr simulation, wet cloths and a high-efficiency particulate air (HEPA) vacuum cleaner were used to clean the floor and other horizontal surfaces and two HEPA air filtration units were used to clean the air. To measure the effect of cable installation and subsequent cleaning of the test site, air samples were collected before each simulation began, during the installation activity, and at various times during the 6.5-hr postinstallation period. The experiment was designed to detect 10-fold or greater differences in average airborne asbestos levels among these periods with a probability of at least 95%. Assuming that the coefficient of variation for the TEM asbestos measurements would be approximately 100%, the number of samples was determined to be eight per period. Because the differences were actually much larger than 10-fold, it was not necessary to analyze all samples. The quality assurance program included side-by-side sam ples and field and laboratoiy blanks. In addition, duplicate analyses of selected filters were performed by independent laboratories. central space, a separate area in one comer created by partitions extended to the suspended ceiling, and two office areas and two storage areas separated by masonry walls extending to the deck above. Total floor area was approximately 2000 ftz. Floor cover ings were carpet and vinyl tile. All movable furnishings had been removed before the start of the study. Most of the room contained a suspended ceiling of 61 x 122 cm (2 x 4 ft) lay-in, asbestos-free tile; one office contained 30.5 x 30.5 cm (1 x 1 ft) hidden spline tile. Friable fireproofing with 15-20% chrysotile asbestos, 45-55% veimiculite, and 20-40% binder was sprayed on steel beams with overspray on the deck and utilities above the suspended ceiling. The fireproofing gen erally was in good condition. Some dust and debris similar in appearance to the fireproofing was visible on top of the ceiling tile. The space between the ceiling tile and the deck above was approximately 0.9 m (3 ft) in height and served as a return air plenum during normal building operations. This space had been entered a few times per year for utility system maintenance during the life of the building. The room was prepared for the study by covering the doors and wall penetrations with 6-mil plastic sheets and duct tape. The HVAC system was shut down and was to be demolished and replaced as part of the subsequent asbestos abatement project. A three-chamber decontamination facility for personnel and equipment was con structed at one entry to the room, as shown in Figure 1. EXPERIMENTAL MATERIALS AND METHODS Simulation Procedures Cable was installed by using procedures typical o f those used in the trade. From the northwest comer of the room, 12 separate cables were run to 12 simulated computer work stations, as illustrated in Figure 1. The simulation began by removing ceiling tile above each "work station" and as needed along the routes each cable would trace above the suspended ceiling. Altogether, about 15 out of roughly 200 individual tiles were moved. The removed tiles were then stacked on the floor. Working from ladders, installers attached a string weighted with a 2.54-cm (1-in.) nut to one end of each cable. The nut was then thrown in the direction of the appropriate work station. A second installer located the nut and pulled the string and cable to that location. This "throw and retrieve" operation was used to bring a cable to each work station. Typically, one or two throws were required for each retrieval. In the process, in-place fireproofing was struck about five times during the process of installing 12 cables. Figure 2 illustrates the installation process. All cables were then pulled back to the starting point, recoiled, and set aside, and all ceiling tiles were replaced. The entire operation of installing, removing, and recoiling cable is essentially equivalent to a 2.5-hr period of installation. The cleaning operation consisted of vacuuming all carpeted areas; wiping all lab benches, window sills, and other horizontal surfaces with a dry cloth; and sweeping the tiled portion of the floor. No attempt was made to suppress re-entrainment of dust. AM. IND. HYG. ASSOC. J. (52) / November 1991 480 Equipment and Data Collection Methods Air-sampling equipment used during the study included highflow sampling pumps (Dawson Associates, Tucker, Ga.) and personal sampling pumps (Mine Safety Appliance Company, Pittsburgh, Pa.). Flow measurements were performed on-site with a Mini-BuckTM Model M-30 automated soap bubble cali brator (A.P. Buck, Orlando, Ha.). The sampling media consisted of 0.45-pm pore size mixed cellulose ester filters housed in 25-mm sampling cassettes (Nuclepore Model SN32275, Cam bridge, Mass.). Other equipment used during the study included a HEPA vacuum cleaner (Hako MinutemanTM, Addison, HI.), HEPA air filtration units with nominal flow rates of 1950 ft3/min (Model Mach 2, Critical Systems, Houston, Tex.), a Dial-a- maticTM upright vacuum cleaner (Hoover Vacuum Cleaners, North Canton, Ohio), and various personal respirators. Number 16/3 electrical cord was used as the computer cable. The flow rate of each air-sampling pump was measured before and after each experiment. High-flow pumps were placed at various locations both inside and outside the room, as shown in Figure 3. Personal pumps were placed on selected workers. FIGURE 2. Cable installation process Airflow rates were approximately 5.4-8.1 L/min for the highflow pumps and 1.9 L/min for the personal pumps. Overloading of filters during the simulations required changes of cassettes on each pump. All cassettes were taped closed and transported by one of the authors to the laboratories for analysis. Asbestos Analysis Subsets of area and personal samples were selected to reflect a wide spatial distribution within the room. Each of the selected samples was analyzed by TEM. Because the filters were heavily loaded, they were prepared for analysis by TEM with an indirect rather than a direct technique. Direct sample preparation, such as required for asbestos abatement projects in schools (Asbestos Hazard Emergency Response Act--40 CFR 763), is appropriate for measuring low concentrations of airborne asbestos in rela tively clean environments. A quantitative measure of the limita tions of direct TEM preparation is provided by the National Institute for Occupational Safety and Health (NIOSH) for Method 7402--the upper boundary of airborne asbestos concentrations that can be reliably measured is 0.5 f/cm3.<6) Where asbestos concentrations are high or the air is heavily contaminated with nonasbestos particles, obscuration of asbestos fibers by other fibers or by nonasbestos particles reduces the reliability of fiber counting.17,85Indirect sample preparation includes filter ashing to remove organic materials, followed by water suspension, sonication to mix the ashed material, and refiltering. Suspension, sonication, and refiltering allows for dilution of the sample as needed and more even distribution of fibers for TEM viewing, both of which improve the reliability of fiber counting/7,85 The indirect method of sample preparation and microscope counting rales as described in the Yamate method, Level H(9> were employed with one exception--sonication time was 10 min 481 AM. IND. HYG. ASSOC. J. (52) / November 1991 rather than 3 min. Either onefourth or one-eighth o f each filter (depending on the degree of dust loading) was ashed in a plasma asher. The ashed material was then suspended in distilled water, sonicated, refiltered, and mounted for TEM examination by using the Jaffe-Wick method. All asbestos structures that could be resolved were reported. TABLE I. Descriptive Statistics of TEM Results for the First Simulation Phase Arithmetic Mean (s/cm3) Arithmetic Std. Dev. (s/cm3) B efore installation D uring installation (area sam ples) D uring installation (personal sam ples) A fter installation 0 .0 5 2 2 8 .9 1 0 .5 8 .4 0 .0 3 0 1 2 .6 1 1 .6 7 .0 Geometric Mean 0 .0 4 6 2 6 .9 7.1 6 .2 Number of Observations 5 5 3 6 Personal samples were also analyzed by PCM. NIOSH Meth od 7400 was used with the "A" counting rules.(10) Statistical A nalysis TABLE II. Descriptive Statistics of TEM Results for the Second Simulation Phase Arithmetic Mean (s/cm3) Arithmetic Std. Dev. (s/cm3) Geometric Mean B efore installation 0 .1 5 8 0 .0 9 4 0 .1 2 9 Number of Observations 5 Where there were two or more D uring in stallatio n consecutive samples at a particular location during a given sampling period, a time-weighted average was calculated so that all measure (area sam ples) D uring installation (personal sam ples) A fter installation ments of airborne asbestos concentrations are based on equivalent time periods. Statistical analyses were applied to the logarithm (Base 10) of the measured concentration. The log transformation tends to equalize variances and permit the use of standard statistical tests that would otherwise be inappropriate. Previous studies of indoor air pollution have demonstrated that air pollution data tend to be lognormally distributed.01' 1 0 0 .2 9 1 .9 6 7 .4 4 1 2 4 .8 8 5 .6 1 0 2 .7 3 1 7 .0 1 3 .5 1 2 .3 4 installation is over four times higher than during the postinstal lation period (Cl = 1.8 to 11, p = 0.01). Table II reveals similar patterns for the second simulation. Average airborne asbestos concentrations during cable installation are 520 times higher (67.4 versus 0.129 s/cm3) than those before the experiment began (Cl = 140 to 1950, p < 0.0001) and those during the postinstallation period are 95 times higher than those before (Cl The effect of "sampling period" as a variable was tested with = 25 to 360, p = 0.0001 ). The ratio of average concentration during a standard one-way analysis of variance and pairwise compari sons between individual sampling periods were performed by using the Scheffe test.02' Results are expressed as p-values. The smaller the p-value, the stronger the evidence for a difference between the sampling periods being compared. P-values less than 0.05 are usually regarded as statistically significant. Ratios of airborne asbestos concentrations and their 95% confidence intervals were estimated by calculating the ratio of the corresponding sample geometric means. Geometric as op posed to arithmetic means are the appropriate quantities to use to estimate the ratio when the distributions are lognormal and the coefficients of variation are equal. RESULTS Table I shows the results for the area samples for the first simulation. The average concentration of airborne asbestos (as measured by TEM) during the cable installation operation is estimated to be 590 times higher than the concentration be fore the experiment began (geometric means of 26.9 and 0.046 s/cm3). The 95% percent confidence interval (Cl) for this ratio is 230 to 1500. Likewise, the average concentration during the postinstallation period is 140 times higher than before the experi cable installation to average concentration during the postinstalla tion period is over 5 (Cl = 1.4 to 22, p = 0.12), but the difference is not statistically significant at the 0.05 level. Tables I and n also show the TEM results for the personal samples during cable installation for each o f the two simulations. The relationship between area and personal samples differs between simulations. In the first simulation, the average concen tration measured by area samples during installation is higher than the concentration measured by personal samples (p = 0.04). In the second simulation, the average concentration measured by area samples during installation is lower, but the difference is not statistically significant at the 0.05 level (p = 0.6). Nevertheless, both the area and personal samples are significantly elevated compared with the levels prior to cable installation. Figure 4 summarizes the results for the two simulations. Note that the vertical axis is on a logarithmic scale. The average levels of airborne asbestos (geometric means, or means on the log scale) are represented by the height of each bar for each of the three phases of the experiments. The variation about the mean (standard deviation on the log scale) is indicated by the vertical lines. Also shown are the means and standard deviations of the personal samples during the installation and postinstallation periods. ment began (Cl = 50 to 330). Both these differences have a A few personal samples collected during cable installation p-value (p) of less than 0.0001. The concentration during cable were also analyzed by PCM for comparison purposes. The AM. IND. HYG. ASSOC. J. (52) ! November 1991 482 arithmetic means are 0.13 f/cm3(SD = 0.03) for the first simula tion and 0.34 f/cm3 (SD = 0.11) for the second simulation. The quality assurance results indicated that field and laboratory contamination was minimal with one exception. Of the 13 field and laboratory blanks analyzed, 1 field blank showed a higher than expected structure loading (441 s/mm2). The laboratory blanks averaged 35 s/mm2and the field blanks averaged 151 s/mm2. Initial comparison of side-by-side samples analyzed by two independent laboratories showed substantial differences in re sults. Investigation of these results suggests that differences in sample preparation and analysis--time of sonication, size of the filter used for refiltration o f the ashed sample, and failure of one laboratory to count small asbestos structures (less than 0.5 |xm in length)--accounts for many of these differences, in addition to any inherent difference in the air sampled between side-byside samples. To further test this conclusion, the quality assur ance laboratory reanalyzed a different portion of the same filter (to avoid differences between side-by-side samples) for six stratified random samples, holding constant the sonication time, filter size, and stmcture counting variables. Although the second laboratory produced results that were approximately half as large as those o f the first laboratory, there was a high and statistically significant correlation (0.88, p =0.02) between the two sets ofresults. DISCUSSION The results show that airborne asbestos concentrations as mea sured by TEM are significantly elevated during cable installa tion. Concentrations generated during the study were high both relative to concentrations before the experiment and in absolute terms. Concentrations peaked during installation. Whether cleaning activities resuspended asbestos structures and thereby contributed to the elevated levels after cable installation cannot be determined directly from the data. However, elevated concentrations persisted for several hours after the installation was finished. For the personal samples, asbestos concentrations measured by TEM are substantially higher than might be expected from the fiber levels recorded by PCM. This suggests that most o f the measured asbestos structures are thinner than the resolution limit of PCM or shorter than those fibers counted according to NIOSH Method 7400. An examination of fiber size distributions reported for the TEM analyses verifies this conclusion--over 95% of measured structures are thinner than 0.25 pm and shorter than 5 pm. Health scientists continue to debate the relationship between fiber size and potency; the arguments have recently been sum marized by Chesson et al.(13>The current Occupational Safety and Health Administration (OSHA) exposure standards are based on measurements that use NIOSH Method 7400 (or equivalent) and thus on fibers longer than 5 pm and with diameters greater than about 0.25 pm. However, these fiber dimensions do not reflect an official OSHA view of the health question; rather, they derive from historical considerations of fiber counting reliability.(14) OSHA recognizes that PCM is neither specific for asbestos nor sensitive to small (both short and thin) fibers; in this light, PCM measurements are best viewed as an indicator of exposure to airborne asbestos. How adequate an indicator PCM measure ments are depends in part on the health significance of the small, unmeasured fibers. Research continues on the question. The analytical method used in the present study measures asbestos structures of all sizes. The term "structures" is used to identify free asbestos fibers, overlapping fibers identified as "bundles" and "clusters," and fibers combined with matrix ma terials called "matrices." Moreover, sonication of ashed samples may disassociate bundles and clusters and disassemble matrices, thereby increasing the total number of structures counted.055 (Some have suggested that sonication may actually break indi vidual asbestos fibers but this hypothesis does not accord with available evidence.055) The significance of the elevated concentrations of airborne asbestos recorded in the present study will depend on the physi ological importance of the asbestos structures identified and counted. One could argue that the disassociation and disassem bly of complex structures during sonication mimics natural physiological processes in the human lung.065 Thus, indirect preparation of TEM samples not only improves fiber counting reliability, it may provide a better measure of asbestos fibers presented to the lung. However, research is needed to substanti ate this possibility. Despite the remaining uncertainty about the significance of small asbestos fibers and the disassembly of complex structures during indirect sample preparation, cable installation workers are well advised to wear respirators and to use work practices designed to reduce fiber release and resuspension. The recorded concentrations of airborne asbestos are high even if only 5% of the structures are ultimately shown to have health consequences. The persistence of high asbestos levels during the simulation suggests that work areas should be isolated from the rest of the building and the air cleaned with air filtration units before the areas are reoccupied, unless use of wet methods and good work practices can greatly reduce concentrations of airborne asbestos. The extensive quality assurance aspects of this study identi fied two potential problems. The first involves possible contami nation of samples. The one blank sample with an elevated reading (441 s/mm2) suggests that contamination may have contributed to the observed levels of airborne asbestos. However, only one of three field blanks showed elevated loadings and none of the background samples showed unexpectedly high levels. Moreover, even if contamination was widespread, a structure 483 AM. IND. HYG. ASSOC. J. (52) ! November 1991 loading of 441 s/mm2is low compared to the loadings on all but the background samples (the average loading was approximately 38 000 s/mm2for samples collected during the simulation. Thus, contamination is not believed to be a problem. The second potential problem concerns interlaboratory dif ferences in analytical results. Although the investigation identi fied the major causes of the discrepancies, the quality assurance laboratory's results were still approximately one-half as large as the primary laboratory's results. However, the high correlation between the two sets of results means that the study's conclusions would be unchanged regardless of which laboratory analyzed the samples. Systematic differences between laboratories is prob ably not unusual for TEM analysis of asbestos in air; the authors' observations suggest that studies involving airborne asbestos measurements should utilize data from more than one laboratoiy. Other investigators are encouraged to conduct similar quality assurance examinations and to probe the causes of any observed interlaboratory differences. ACKNOWLEDGMENT The authors wish to thank the Aspen School District and its employ ees, particularly Mr. Griff Smith, for their cooperation. Brand Site Services and Kaselaan & D'Angelo Associates also assisted in providing materials, equipment, and logistical support. REFERENCES 1. Sawyer, R.N.: A sbestos E xposure in a Yale B uilding. Environ. Res. 73:146-169 (1977). 2. Paik, N.W., RJ . Waicoti, and P.B. Pairicia: W orker E xposure to Asbestos during Removal o f Sprayed M aterial and Renovation A ctivity in B uildings C ontaining Sprayed M aterial. Am. Ind. Hyg. Assoc. J. 44(6):428-432 (1983). 3. Pielart, J.H. and R.G. Mathey: Guidelines fo r Assessment and Abatement o f Asbestos-Containing Materials in Buildings. W ash ington, D.C.: General Services Adm inistration, 1983. 4. 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