Document peeyRznON9B1vKeQ1RyDXwvQ6

FILE NAME: Z< DATE: DOC#: Z< DOCUMENT DESCRIPTION :ZZZ ZZZ,ZZ Alln OcellI' H)R, pp 1-19 The Author 2009 PublJ~htd by Oxford Umver.-lty Pre:-..... on behalf of the Bntl~h Occupdtlonal Hygiene Society dOl 10 1093/annhyg/mep056 Airborne Asbestos Concentrations Associated with Heavy Equipment Brake' Removal A. K. MADLl*, S. H. GAFFNEY\ J. L BALZER2 and D. J. PAUSTENBACH1 1ChemRisk. Inc., 25 Jessie Street, Suite 1800, San Francisco, CA 94105, USA; 2408 Horse Trail Court. Alamo, CA 94507, USA , Received 6 January 2009; in final fonn 25 June 2009 Asbestos-containing brake linings were used in heavy-duty construction equipments such as tractors, backhoes, and bulldozers prior to the 1980s. While several published studies have evaluated exposures to mechanics during brake repair work, most have fpcused on automobiles and light trucks, not on heavy agricultural or construction vehicles. The purpose of this study is to characterize the airborne concentration of asbestos to workers and bystanders from brake wear debris during brake removal from 12 loaderlbackhoes and tractors manufactured between 1960 and 1980. Asbestos content in brake lining (average 20% chrysotile by polarized light microscopy) and brake wear debris [average 0.49% chrysotile by transmission electron = microscopy (TEM)] was also quantified. Breathing zone samples on the lapel of mechanics (n = 44) and area samples at bystander (n = 34), remote (n = 22), and ambient (n 12) lo- cations were collected during 12 brake changes and analyzed using phase contrast microscopy (PCM) [National Institute for Occupational Safety and Health (NIOSH) 7400] and TEM (NIOSH 7402). In addition, the fiber distribution by size and morphology were evaluated according to the International Organization for Standardization method for asbestos. Applying the ratio of asbestos fibers:total fibers (including non-asbestos) as determined by TEM to the PCM results, the average airborne chrysotile concentrations (PCM equivalent) were 0.024 flcc for the mechanic and 0.009 flcc for p~rsons standing 1.2-3.1 m from the activity during the period of exposure (",0.5 to 1 h). Considering the time involved in the activity, and assuming three brake jobs per shift, these results would convert to an average 8-h time-weighted average of 0.009 flcc for a mechanic and 0.006 flcc foroa bystander. The results indicate that (i) the airborne concentrations for worker and bystander samples were significantly less than the current occupational exposure limit of 0.1 flec; (ii) ~2% of respirable fibers were >20 J.lm in length; and (iii) ~95% of chrysotile in the brake linings dElgraded in the friction process. The industrial hygiene data presented here should be useful for conducting retrospective and current exposure assessments of individuals, as well as hazard assessments of work activities that involve repairing and replacing asbestos-containing brakes in heavy construction equipment. Keywords: asbestos; brakes; heavy equipment; industrial hygiene INTRODUCTION Once thought to be a 'miracle' mineral, asbestos gained widespread use beginning in the ,early 1900s, and has been reportedly incorporated in some 3000 different products because of its low cost and desirable qualities, such as heat and fire resistance, wear and friction characteristics, tensile strength, heat, electrical and sound insulation, adsorption capacity, and resistance to chemical and biological at- "Author to whom correspondence should be addressed. Tel: +1-415-896-2400; fax: +1-415-896-2444; e-mail: amadl@chemrisk.com tack [Agency for Toxic Substances and Disease Registry (ATSDR), 2061]. For these reasons, asbestos, specifically chrysotile, was used for many decades by the automobile, heavy equipment, crane, railroad, and airline industries as a component of brakes. Chrysotile's frictional characteristics, such as good tensile strength, durability, flexibility, and heat resistance, provided the auto industry with a friction material that could withstand extreme temperatures, pressure, and stress (Skinner et aI., 1988; Paustenbach et al., 2004; Maines, 2005). These characteristics were particularly necessary for safety, as automobiles throughout the 20th century became larger, heavier, and faster (Harper, 1998). 1 of 19 2 of 19 A. K. Madl et al. Because of regulatory and societal concerns about the health effect~ caused by exposure, asbestos use in the USA has precipitously declined since the mid-1970s (ATSDR, 2001; Maines, 2005; dos Santos Antao et al., 2009; Kelly and Matos, 2009). Over the last 30 years, significant attention has been paid to evaluating asbestos exposures and the potential risk of asbestosrelated diseases among automobile garage mechanics (Paustenbach et al., 2004). Chrysotile asbestos was also used as a friction material in heavy construction equipment, but potential asbestos exposures to mechanics repairing bralces on such equipment has been less well studied and understood (Boelter et aI., 2007). Although the asbestos content in automobile bral(es is generally between 30 and 50% (Lynch, 1968; Anderson et al., 1973; Madl et al., 2008), heat and pressure, such as that exerted during vehicle bral6ng, can cause chrysotile asbestos to degr,ade to non-fibrous amorphous decomposition products, as well as form other non-fibrous minerals, such as forsterite and olivine (lacko et al., 1973; Candela etal., 2007). The dehydration or retention of water in chrysotile decomposition products has been shown to influence the extent to which forsterite is forIfl.ed .under heat and pressure (Candela et al., 2007). It has been suggested, however, that friction during mechanical braking disaggregates mineral bundles in the brake lining, which liberates water and results primarily in amorphous, non-fibrous, magnesium silicate degradation products (Candela et al., 2007). Because the elemental ratios and X-ray diffraction (XRD) patterns of chrysotile and these degradation products are similar, transmission electron microscopy (TEM) is often necessary to identify fibrous structures of chrysotile at low bulk concentrations. Using microscopy, historical studies have shown that bralce wear debris collected from an automobile dynamometer or drum brakes contains on average between 0.02 and 4.5% asbestos, with the majority of wear debris samples containing < 1% chrysotile (Hickish and Knight, 1970; Luxon, 1970; Anderson et al., 1973; lacko et al., 1973; Rohl et aI". 1977; Rowson, 1978; Williams and Muhlbaier, 1982; Cha et al., 1983; Sheehy et al., 1989). While these studies do not directly measure forsterite or other degradation products in brake wear debris, the breakdown of chrysotile is inferred by comparing asbestos content in the lining to that in brake emissions or aotu.mulated dust in the bralce assembly. Although it has been assumed that the forces that convert chrysotile in automotive brakes are at work during the use of heavy equipment (e.g. dozers, backhoes, and graders), little work has been conducted that confirms the degradation of chrysotite to forsterite or an amorphous form. This matter is of particular interest since it is not well understood how the different speeds or weights of heavy construction equipment, compared to passenger automobiles, can influence the frictional mechanisms, and thus, by extension the conversion to forsterite or other nonasbestos amorphous materials. Understanding this matter will inform hygienists whether workers conducting bralce repairs on heavy construction equipment, during the p.eriod between the 1950s and the 1980s or in modern times, were exposed to appreciable concentrations of asbestos. Only one published study has evaluated potential asbestos exposures to mechanics repairing heavy construction equipment brakes (Boelter et a!., 2007). In this paper, personal short-term (30 min) and long-term [8-h time-weighted average (TWA)] samples for airborne asbestos were collected during repair activities involving the replacement of asbestos-containing products (i.e. engine gaskets,' bralce, and clutch linings) in a dozer, grader, and two loaders. Area samples were also analyzed to characterize potential exposures to a bystander nearby these activities. The asbestos-containing products removed from the construction equipment, as well as bralce wear debris were analyzed for asbestos content. White this study filled an important data gap, it did not address directly whether chrysotile asbestos was degraded to a similar extent a, that observed with passenger automobiles, and it only characterized a limited number of equipment representing a wide array of types and brake assembly configurations (e.g. disc, drum, and band). Complete enclosure of a bralce,system, size of asbestos-containing friction lining, location, and access configuration in relation to the mechanic's breathing zone, as well as method ofmaint~nance work are all likely to influence occupational exposures to airborne asbestos during brake repair activities. With these factors in mind and without additional information, it was uncertain how the information presented in the Boelter et al. (2007) might compare to results from other types of heavy construction. The handling and cleaning of contaminated work' clothing worn in some occupational envIronments have been suggested as a possible SOurCf of paraoccupational or 'talce-home' chemical exposure. Studies that have reported exposure through this possible secondary exposure pathway include industries where beryllium, lead, or even asbestos (e.g. insulation workers) exposures in the workplace were excessive. For example, Eisenbud et al. (1949) found mean air concentrations of 500 ).lg beryllium m- 3 when the clothing of beryllium manufacturing workers was shaken out. Piacitelli et al. (1997) found elevated lead concentrations in the vehicles and home~ of leadexposed construction workers. Some per~ons, who live in the homes of workers exposed to free asbestos fibers, developed as'bestos-related disease (Lieben and Pistawka, 1967; Anderson et al., 1976, 1979; Li et al., 1978; Epler et al., '1980; McDonald and McDonald, 1980; loubert et al., 1991; Magnani et al., 1993). Asbestos concentrations during heavy equipment brake removal 3 of 19 Generally, workers in asbestos manufacturing, mining, and shipyard industries are exposed to very high airborne concentrations of asbestos and come in direct contact with large amounts of bulk asbestos and, in the majority of cases, amphibole asbestos. The takehome exposure of other household members (so-called secondary exposure or para-occupational exposure) is thought to occur as a result of bringing very dusty work clothing into the home, which was usually contaminated due to daily contact with bulk or raw asbestos. Although exposures associated with handling work' clofues worn during brake repair work were expected to be extremely low, it was felt that this issue deserved greater characterization as it ha~ implications for both historical and current asbestos exposures of a group of individuals not previously studied. Since a broader range of data would increase the confidence in the preliminary study, we evaluated a number of vehicles for the purpose of understanding the extent of potential conversion of chrysotile asbestos in brake linings to forsterite and amorphous materials. We also characterized worker and bystander exposures to airborne asbestos during brake removal in various types of heavy construction equipment and assessed the potential of take-home exposures from clothing worn during the brake removal activities. In this study, worker and environmental exposures were evaluated during the maintenance of 12 pieces of heavy construction equipment with similar brake assembly configurations. Because of the relatively large number of pieces of equipment tested with similar brake assemblies, the influence of the ex- tent of equipment use (e.g. hours of operation), methods of brake removal used by different mechanics, oily versus dry brake assemblies, and type of equip- mel\t (loader backhoe and tractor) on the variability of the airborne asbestos measurements could be assessed. In addition, short-term samples were collected and 8-h TWA exposures were calculated for comparison to historical and current occupational exposure limits for asbestos. The fiber size and morphology distributions were also measured to characterize the proportion of respirable airborne fibers, free or associated with a matrix, released during brake removal activities. It is anticipated that this information will not only provide useful information regarding potential historical exposures experienced by mechanics conducting brake repair work on heavy construction equipment but will also provide a basis for correlating this information to the exposures and health experience of automobile mechanics. METHODS Description of backhoeltractor brake assemblies Table 1 provides a summary of the type of vehicles evaluated in the study, as well as the years during which the equipment was manufactured, total hours of operation, and the facility in which the brake repair work was performed. Each of these vehicles contained a left and right dry drum and disc brake assembly (Fig. I), each of which possessed an inner and outer drum lining and two band linings comprised friction material. Equipment that potentially had asbestos-containing linings was selected for the study based on the age of the equipment and repair maintenance records provided by the equipment owners. A total of 12 pieces of equipment (two tractors and 10 loader backhoes) manufactured between 1960 and 1980 and operated between 943 and 6741 h were included in this study. It should be noted that the hours of operation for each piece of equipment may not necessarily reflect the total number of hours on the brakes; however, mea&urements of lining thickness for each brake assembly showed significant brake wear. It was not determined until after the testing through bulk sample an'alysis whether the equipment contained asbestos brake linings. In fact, all vehicles tested did have asbestos in the friction materials. Description of test site and study conditions Brake repairs were performed at two heavy-duty equipment service centers on six different days over a period of 17 months (April 2005 to September 2006); 5 days were spent at one center located in Stockton, CA, and the other day at a center in Big Rock, IL (Table I). These service centers were selected because they were (and continue to be) active repair facilities for heavy-duty construction equipment, including tractors and backhoe loaders. The weather conditions in Stockton were generally sunny and clear, with temperatures ranging from 21 to 27C during the 5 days of testing. The Big Rock testing took place during cloudy conditions, with temperatures ~ IS.6C. All work was performed by two currently employed mechanics (one in Stockton and one in Big Rock), who had between IS and 30 years of professional experience repairing heavy-duty construction and agricultural equipment. The mechanics performed the brake removal in the same manner they had reportedly used throughout their careers. The service shop in Stockton, CA, was relatively large, with four service bay doors and approximate dimensions of 30 m wide by 14.9 m deep, with a ceiling height of 6.1 m (Fig. 2). The facility in Big Rock, IL, was less than half the size of the service center in Stockton, CA, with only one service bay door and approximate dimensions of 14.3 m wide by 11.9 m deep with a 6.I-m ceiling (Fig. 3). All service bay and entry doors were closed while the brake removal was being conducted. In addition, both repair shops were not equipped with any active heating, air conditioning, or ventilation systems. Prior to conducting the study, permission from a medical institutional review board (IRB) was .,.. o :"0" Table I. Summary of equipment tested and bulk ~ample asbesto~ concentrations of brake linings and wear debri~ EqUlpmem Date Equipment HoUl; Model Site type type AIr exchange (ACH) Avem.ge thick ne" (mm) by assembly Brake hning ... XRD (% weight chrysotile) PLM (% area chry,otile) Brdke wear debrh TEM (% weight chrysotlle) Decomposition Left RIght n n Average SD Range n n Average SD Range 12 11 Average SD Range % (ND) (NO) (NO) Eql 13 Apnl 2005 580C 4360 Backhoe Stockton 433 4.31 o 32 4 25-37 8 0 40 8 24-54 2 0 0.150 0.071 0.1-0.2 99.5 Eq2 14 September 580C 1232 Backhoe Stockton 0.6 4.46 4.26 o 19 3 15-22 8 0 26 8 20-35 2 0 0.009 0.001 0.008-0.009 100.0 2005 Eq3 16 September 580 1562 Trac"!or Big 0.6 4.76 4.73 8 0 23 2005 Rock Eq4 16 September 430 2005 943 Tractor BIg 0.6 Rock 411 4.42 8 0 29 Eq5' 14 November 580C 6230 ~ackhoe Stpckton 0 66 257 3.15 8 14 2005 Eq6' 14 November 580C 2005 3600 Backhoe Stockton 0.66 409 442 4 8 Eq7 Eq8b 14 November 2005 14 June 2006 580C 580C 6741 Backhoe Stockton 0.66 3.41 4.36 8 0 17 3264 Backhoe Stockton 1.55 4.04 4.13 o 27 Eq9 b 14 June 2006 580CK 2744 Backhoe Stockton 1 55 4.43 4.56 6 2 15 3 19-28 8 0 28 4 24-36 8 0 33 6 1-19 8 15 9 1-25 8 3 14-22 8 0 19 7 18-39 8 0 17 12 1-29 6 0 15 5 20-35 2 0 10 15-40 2 0 8 05-25 2 2 12 0.05-30 2 2 2 15-20 2 6 10-27 2 0 12 0.5-28 2 0 0.075 0.021 0.06-0.09 0.450 0.354 0.2-0.7 0.050 0000 005-0.05 0.050 0000 0.05-0.05 0.825 I 096 0.05-1.6 0.700 0.141 0.6-0.8 3.015 4.221 0.03-6 99.7 98.5 99.6 ?> ?" 9: 99.4 ~ 95.1 ~ 97.4 ',.".... 79.2 EqIO Eqll Eq12 20 September 2006 2rf September 2006 20 September 2006 580C 580B 580C 2743 Backhoe Stockton 1.28 4188 Backhoe Stockton I 28 5320 Backhoe Stockton I 28 462 376 4.46 4.36 2 20 o 19 4.31 2.18 8 5 13 1-36 8 2 25 25 0.5-70 2 0 0.085 0021 007-0.1 99.6 4 12-25 8 0 16 o 1-2 8 4 6 6-25 2 0 0.035 0.021 0.02-O.tr5 99.8 0.5-3 4 0 0.435 0.422 004-0.8 65.2 Eql-EqI2 12 19 3.8 1-39 12 20 6.1 0.05-70 12 0.49 1.20 0.008-6 94.4 ND, non-detectable. aAsbesto~ was not detected in both band and dmm lining on right assembly. "No right dmm lining wm, pre~ent. Non-detectable sample~ entered a~ one-half the limit of detection or ~ensitivity limit. Asbestos concentrations during heavy equipment brake removal Band Uning 5 of 19 Fig. 1. Heavy equipment dry brake systems diagram. i .-...--e.lfl"l:..---...~,~____ ~.~ _____... Z.-w.~~:)7~-+....:l."J:, ...+---):101\---+-'.. 2,~ ...____ 3.1m---..........-l,~ Bay Door 1 Bay Door 2 Bay Door 3 Bay Door 4 1<I.9m Remote Are'd Systander O,9m*o~.sm()9m Bystander . - ~-- t24m ~ B~stander Amtlient D.3ru Fig. 2. Diagram of equipment repair facility (Stockton, CA) and locations of area sampling stations. requested and obtained (Essex Institutional Review Board, Inc., Lebanon, NJ, USA). This IRB complies with the federal regulations of the National Institute of Health, the Office of Human Research Protection (45 CFR 46), and the Food and Drug Administration (21 CFR 50, 56). It is also accredited by the National Committee for Quality Assurance, formerly The. Partnership for Human Research Protection. Description of exposure scenarios Airborne asbestos concentrations were measured during brake removal and disassembly activities related to all 12 pieces of equipment. Clothes handling tasks, such as shaking and folding of coveralls worn during maintenance of 11 pieces of equipment, were also studied. Before any brake work was per- formed, mechanics were fitted with new coveralls. These coveralls were collected after the mechanic completed work on each piece of equipment and were later tested to evaluate the exposure of persons during the handling of these potentially contaminated work clothes. Each coverall (n = 11) was stored in separate plastic-lined bags until the last day of testing, when the clothes handling task was conducted. 6 of 19 A. K. Madl et al. ... ! ~-l... r-~' ~ Byslarnler .1lJ;.b2i=~~lll. Bys.:ander t ~6m al3y$1arlder ' ,..-.- t !!em Bysti;lIl<Jer Q1;;.":n:::i----nn] ~n. ~lslAndef Opeil !hn~stl<lld (Q ad~)Cent ro,m ~2 sm I. Z.St:l) Remo1e. AreA ().!;'H ~Mce8ay DOlls .. Fig_ 3. Diagram of equipment repair facility (Big Rock, ILl and locations of area sampling stallons. The simulated clothes handling task inVOlved repeatedly shaking, folding, and turning clothes inside out for "-' I to 2 min for each pair of overalls by a volunteer to simulate the handling and laundering. Although no fibers or debris were visible on the coveralls, some particles were observed i'h the air during the clothes handling task. TEM analysis of air samples was used to evaluate the proportion of asbestos versus non-asbestos airborne particles released during shaking. The brake removal process was similar for all pieces of equipment, with slight differences only in the work practices exhibited by each meohanic. The mechanics worked on each piece of equipment one at a time. To remove and disassemble the brakes, the external brake housing was first removed f;om the tractor or loader backhoe using a manual or power wrench to loosen bolts that held the housing in place. On four occasions (Eql, Eq9, EqlO, ;nd 'EqI2), a blowtorch had to be applied to facilitate loosening of the external housing bolts. Once the external housing was removed, the entire brake assembly was removed from the vehicle. At this point, the mechanic at the Stockton facility would blowout the assembly and work area with compressed air and then repeat the entire process for the second brake housing. Once both complete assemblies were removed, he performed bench work, which entailed disassembling the drum linings from both brake assemblies. Using a slightly different order' than the Stockton mechanic, the mechanic in Big Rock, IL, completed the entire brake removal process on the first assembly before beginning the process on the second one. More Sipecifically, once the brake assembly was removed from the external housing, the brake assembly wp.s moved to a workbench, and the drum linings were removed from the face plates in preparation of shipment to a specialized shop for refacing. After one complete brake assembly was disassembled and the linings removed, the mechanic from Big Rock would repeat the same process for the second housing and then blowout both brake housings at the end of the brake removal job. While performing the bench work, the Stockton and Big Rock mechanics used different methods to remove the friction linings from the drum facings. Specifically, the Stockton mechanic used a hammer and punch to remove the rivets that attached the lining to the drum face, ~hereas the Big Rock mechanic used a power drill. It is noteworthy that fou r of the 12 pieces of equipment tested contained at least one brake assembly that was saturated in oil that had leaked from an adjacent reservoir. In the~e circumstances, these assemblies were wiped clean before the linings were removed. In general, the 8tockton mechanic took ~30 min to remove and disassemble the linings from two brake housings from one piece of equipment, whereas the Big Rock mechanic took ,,-,45 to 60 min to perform the same job. Sampling and analytical methods All airborne samples for asbestos were collected in accordance with federally established criteria. Airborne asbestos and other particulates were collected onto mixed cellulose ester membranes (25 mm, 0.45 flm pore size; Zefon International, 8t Petersburg, Asbestos concentrations during heavy eqUipment brake removal 7 of 19 FL, USA), with either portable SKC Universal (SKCWest, Inc., Fullerton, CA, USA) or high-volume Dawson 1300 sampling pumps (Ashtead Technology. Rentals, Hayward, CA, USA). The sampling pumps were calibrated with a Bios DryCal DCLite primary flow calibrator (Bios International Corporation, Butler, NJ, USA) before and after each sampling event. The temperature inside the garage was noted (Springfield PreciseTemp, Wind and Weather Instruments, Fort Bragg, CA, USA) during the collection of each air sample. Asbestos sample collection equipment, materials, and procedures were consistent with National Institute for Occupational Safety and Health (NIOSH) Methods 7400 and 7402. Before any brake removal activities began, background samples for airborne asbestos were collected in three different locations in the service centers. Personal samples from workers' lapels, bystander area samples within 1.2-3.1 m of the work activities, remote area samples at more distant locations (9-15 m) from the work activities, and ambient samples for airborne asbestos were collected during tractor brake or baclchoe brake removal and disassembly. Figures 2 and 3 illustrate the location of the bystander, remote area, and ambient airborne asbestos samples collected in relation to the work activities at both the Stockton and the Big Rock facilities. Con- secutive 30-min samples were collected at an airflow rate ranging from I to 10 I min-Ion the right and left lapel of the worker during the brake removal activi- ties. To characterize potential bystander exposures to asbestos, air samples were collected at breathing zone height (1.5 m) at three different locations ~ 1.2 to 3.1 m from the vehicle. Bystander samples were collected at an airflow rate of 5-11 I min-I during brake removal activities on each vehicle (30-60 min). Background samples of ambient air (outside the shop, 90-180 min) and remote area (inside the shop, 30-60 min) were also collected for airborne asbestos at an airflow rate of ~ 10 I min-I during the brake removal activities. At least two field blanks were collected during each day of the a!>ovedescribed testing. All airborne asbestos samples, including personal, area, background, and ambient samples, were sent to an accredited laboratory (EMS Laboratories, Pasadena, CA, USA) for asbestos analysis by phase contrast microscopy (PCM, NIOSH Method 7400) and TEM (NIOSH Method 7402) (NIOSH, 1994a,c). EMS Laboratories is an asbestos analysis laboratory acicactrieodnitaenddbythteheNAamtioenriaclanVoInludnutsatrryialLHabyogriaetnoeryAsAsoc-- creditation Program (US Department of Commerce, National Institute for Standards and Technology,. Gaithersburg, MD, USA) and utilizes analysts trained according to NIOSH 582 who adhere to the quality assurance and quality control requirements set forth by OSHA (OSHA, 1994) and the most cur- rent version of the NIOSH 7400 Method. For the analysis of air samples by TEM, selected area electron diffraction and energy-dispersive X-ray were used to assess the fiber type via the diffraction pattern and elemental profile of the asbestos fibers, respectively (NIOSH, 1994c). Fibers were counted according to the NIOSH Methods 7400 and 7402, which define fibers as being >5 Ilm in length and 0.25 Ilm in diameter and having at least a 3: 1 aspect ratio (NIOSH, 1994a,c). Air samples were also analyzed according to the International Organization for Standardization (ISO) method for characterization of fiber type, size, and morphology of fibers >5 ~lm in length (ISO, 1995). Fiber size and morphology analYSIS Because OSHA specifies PCM analysis (with or without TEM analysis) for evaluating occupational exposures to airborne asbestos, most studies utilize NIOSH Method 7400. However, PCM analysis under NIOSH Method 7400 does not differentiate asbestos fibers from other structurally similar non-asbestos fibers, so OSHA has indicated that TEM analysis (NIOSH Method 7402) can be used to quantify the ratio of asbestos fibers to total fibers (OSHA, 1994). While NIOSH Methods 7400 and 7402 are still used today to deterlI}ine workplace compliance with the OSHA PEL for asbestos, these methods are limited in their ability to account for fiber morphology (e.g. presence of a resin) that might influence the respirability or the health hazard of airborne fibers. While not widely used, the ISO Standard method allows for characterization of both fiber size and type, as well as determination of the fiber size distribution of airborne asbestos and differentiation of free fibers from fibers associated with a non-respirable matrix (ISO, 1995). This method (in addition to the NIOSH methods) was, used in this study because the data can then be employed in future doseresponse and risk assessment models (while the NIOSH methods are most appropriate for comparing to the OSHA PEL) (OS HA, 1994). Using the ISO methodology, asbestos fibers were classified according to fiber size and morphology. Asbestos fiber morphology was quantified by categorizing asbestos fibers that were >5 Ilm in length as free fibers, free fiber bundles, fiber clusters, or matrix fibers (including matrix fibers, bundles, and dispersed arrangements). In those instances where asbestos fibers were associated wit~ a cluster or matrix, the dimensions of the cluster or matrix structure, as well as those of the individual fibers comprised within the cluster or matrix, were recorded. Asbestos fibers were characterized by their morphology and size to evaluate the proportion of airborne fibers that were potentially respirable. While fibers up to 3.5 Ilm in diameter have been detected in the lungs of workers, 8 of 19 A. K. Madl et at. and while fibers of this dimension may represent the very upper bound limit of respirability (Gross et al., 1971; Morgan and Holmes, 1980; Timbrell, 1980, 1982), a number studies have shown that most fibers that are deposited in the pulmonary region of the lung are thinner than 0.7 11lIl, and almost all are thinner than 1 !lm (Harris and Timbrell, 1975; Sussman et al., 1991a,b; Strom and Vu, 1994; Yu et al., 1995). Respirable fibers (free and bundles) were therefore designated as those with diameters of S;0.7 !lm. The deposition of fibers contained within clusters or matrices was assumed to be based on the dimensions of the overall cluster or matrix structure. Depending on the size and shape of these structures, the fiber cluster or matrix may act aerodynamically more like a particle than as a fiber. Nonetheless, respirability of fiber clusters or matrices was evaluated in two ways, as a respirable fiber of diameter SO.7 '!lm or a respirable particle with diameter S; 10 !lm. In addition, fibers >20 !lm in length were considered in the size distribution analysis. There is a basis of data to suggest that asbestos fibers of this length or greater pose the greatest risk, whereas those <5 !lm do not. Stanton et al. conducted a series of animal exposure experiments with asbestos and non-asbestos fibers and ultimately concluded that fibers S;8 !lm in length have little or no mesotheliogenic potential (Stanton, 1973; Stanton et al., 1977, 1981). Berman et al. (1995) evaluated data from 13 rat inhalation bioassays in which the animals were exposed to nine different types of asbestos dusts and concluded in this analysis that structures contributing to lung tumor risk appeared to be long (~5 !lm) and thin (0.4 !lm) fibers (Berman et al., 1995). Berman further noted that potency appeared to increase with increasing length, with structures longer than 40 !lm. being "",500 times more potent than those between 5 and 40 !lm in length. These researchers suggested that structures <5 !lm in length did not contribute to lfIng tumor risk. Modeling results reported by Miller et at. (1999) indicated that the concentration of fibers longer than 20 !lm and thinner than 1 !lm in diameter is 'most influential in determining the tumorigenic potential of fibers (Miller et al., 1999). In addition, the US Environmental Protection Agency (US EPA) contracted the preparation of a technical support document for a protocol to assess asbestosrelated risk (Berman and Crump, 2003). Based on the modeling results presented in the technical support document, the authors concluded that the best (estimate of risk for both lung cancer and mesothelioma for fibers between 5 and 10 !lm in length is one-three-hundredth of the risk assigned to fibers longer than 10 !lm. Further, the best estimate of the potency of fibers shorter than 5 !lm was zero for mesothelioma and lung cancer. The authors also explained that 'results from [their] review of the supporting literature suggest that the optimum cutoff for increased potency occurs at a length that is closer to 20 !lm than 10 ~m'. Collection and analysis of bu'lk asbestos samples For each piece of equipment, filings of the brake material from each assembly (two drum linings and two band linings) and samples of brake wear debris (one from each assembly) were collected for bulk sample asbestos analysis. Both types of bulk material. were analyzed by EMS Laboratories, using polarized light microscopy (PLM) according to NIOSH Method 9002 (NIOSH, 1994b) and XRD according to NIOSH Method 9000 (NIOSH, 1994d). Because the asbestos concentration in brake wear debris was anticipated to be below the detection limit for PLM or XRD ("'" 1%), a modified approach based on the US EPA methods for detecting asbestos in bulk samples and drinking water was utilized (Chatfield and Dillon, 1983; Perkins and Harvey, 1993). More specifically, this approach involved ashing the sample to remove organic material using muffle furnace, suspending the ashed sample in water, filtering an aliquot of the water suspension, transferring the filtered sample onto a TEM grid, and characterizing dimensions of fibers according to those measured under NIOSH 740017402. Air exchange measurements using tracer gas Sulfur hexafiuoride (SF6) was used as a tracer gas to estimate the air exchange rate within the service centers. Briefly, measurements of the gas were taken according to American Society for Testing and Materials (ASTM) Method E741-00 (ASTM, 2(01), and. air exchange measurements using this method were collected during each day of testing. A steady state concentration of I p.p.m. for SF6 (Sigma-Aldrich, St Louis, MO) was targeted as the initial room concentration for the tracer gas analysis. Tedlar bags (Fisher Scientific, Hampton, NH, USA) tilled with SF6 were then released in the garage with all doors closed. Fans on either end of the garage were used to facilitate the gas dispersion. After steady state was reached, fans were turned off, and SF(, measurements were taken in 30-s intervals with a MIRAN SapphIRe-XL Analyzer (Thermo-Electron Corporation, Waltham, MA, USA; Ashtead Technology Rentals) for "'" 1 h. The air exchange in the garage was calculated using the concentration decay (optional regression) test method by plotting the natural logarithm of SF6 concentration over time (ASTM,2001). ' Data and statistical analysis For the purposes of statistical analyses, results below the analytical sensitivity limit were imputed using a value equal to one-half the sensitIvity limit. Analytical sensitivity limits were estimated based. on the presumption that one fiber could be counted Asbestos concentrations dliring heavy equipment brake removal 9 of 19 within 100 microscopic fields and divided by the. volume of air sampled. PCM measurements were adjusted for asbestos fiber content according to the method outlined in NIOSH Method 7402, which. specifies multiplying the ratio of asbestos fibers to total fibers observed in the TEM analysis by the PCM fiber concentration (NIOSH, 1994c). The ratios of asbestos to total fibers (asbestos and non-asbestos fibers) were based on TEM fiber counts for the same filters, from which the PCM fiber counts were obtained. The PCM measurements adjusted by the ratio of asbestos versus total fibers are referred to as 'PCM-equivalent' (PCME) airborne asbestos concentrations. In circumstances where PCM measurements were above the sensitivity limit, but asbestos fibers were not detectable by TEM, a PCME asb'estos concentration was not calculated. Descriptive statistics were performed on PCM, TEM, and PCME measurements of airborne fiber concentrations collected during the removal of asbestos-containing brake assemblies from tractors and loader backhoes, and also during the clothes handling activities. Results were analyzed by sample location (worker, bystander, remote area, and background), by testing location (Stockton and Big Rock), and by assembly oiliness, when applicable. Eight-hour' TWA asbestos exposures during brake removal were faolsrobcoatlhcutlhaetedwobrakseerd aonnd ththeePbCyMstEandmeer,asbuarseemdenotns the "assumption that three brake jobs could be conducted in a single workday and the remaining time the worker would be exposed to background c()ncentrations. For example, 90 min of brake removal activities (representing work on three pieces of equipment) at a concentration of 0.024 flcc and a 390-min exposure to background at a concentration of 0.005 flcc would result in an 8-h TWA of 0.009 f/cc. Air concentration data were determined to be lognormally distributed based on probability plots. 'Oneway analysis of variance and pairwise comparisons based on the Tukey's test for each group were conducted for natural log-transformed air concentration data of worker, bystander, and remote area samples. Two-sample t-test of the log-transformed worker data was conducted with respect to dry versus oily brake assemblies based on unequal variances. Power calculations of the above comparisons were estimated to be 100%. A Pearson and Spearman (nonparametric) analysis was performed to evaluate' whether airborne chrysotile concentrations for the worker (untransformed and natural log transformed) were influenced or correlated with the asbestos con- tenr in the brake lining or brake wear debris and whether the asbestos content in the brake wear debris was influenced by the extent of wear (e.g. lining thickness) or the original asbestos content in the brake lining. RE~ULTS All equipment contained at least one brake assembly with asbestos-containing linings. The thickness of the brake linings ranged from completely worn to the metal support to 5.94 mm, and their ashestos content is presented in Table 1. In summary, the asbestos content of the brake lining averaged 19% chrysotile by weight (range: 1-39%) as measured by XRD and 20% chrysotile by area (range: 0.570%) as measured by ?LM (Table 2). Table 1 also presents the asbestos content measured in the break wear debris. The average asbestos content found in these samples was 0:49% of chrysotile asbestos (range: 0.008-6%). All heavy equipment showed brake wear debris with < 1% chrysotile asbestos, with the exception of Equipment 9, which had brake debris in one assembly containing 6% asbestos. It should be noted that this assembly was also missing both drum linings. These results indicate that nearly all (average of 94.4%, range 58-100%) the chrysotile in the brake linings degraded or was converted to an amorphous material. Specifically, for 10 of the 12 pieces of equipment, over 95% of the chrysotile in the brake linings was degraded to non-fibrous particles in the brake wear debris. Interestingly, there were no correlations found between the asbestos content in the brake lining and brake wear debri~, suggesting that the asbestos content in the bral<e lining does not influence the amount of asbestos remaining in the brake wear debris after degradation processes. Air sampling results are reported in Tables 2 and 3. Table 2 presents the short-term (30 min) and brake removal airborne asbestos concentrations measured by PCM, TEM, and PCME for the worker, bystander, remote, and background locations by facility. The average airborne chrysotile concentrations as measured by PCM, TEM, and PCME were 0.053, 0.087, and 0.024 flcc, respectively, for the Stockton mechanic and 0.558, 0.012, and 0.010 flcc measured by PCM, TEM, and PCME, respectively, for the Big Rock mechanic (Table 2). The overall workel average airborne asbestos concentrations by analytical method are presented and compared to the OSHA 30-min asbestos excursion limit in Fig. 4. No correlations were found between concentrations of airborne asbestos for the worker (PCME, 30 min) and the asbestos content in the brake lining or wear debris; this lack of correlation is likely attributed to the low asbestos concentrations in the air samples and in the bral<e wear debris. The results of the clothes handling activity are also presented in Table 2. The average airborne asbestos concentrations measured on the volunteer (handling the clothes) were 0.231, 0.011, and 0.036 flee when measured by PCM, TEM, and PCME' respectively. Likewise, at the bystander location, average asbestos concentrations of 0.093,0.012, and 0.010 flcc were  . - Table 2. Summary of air sampling results: brake removal and clothe~ handling by PCM, TEM, and PCME (30 min) by worker, bystander, remote, and background, a, well as location o -o . Sample type/activity/location Equipment Asbestos fiber concentrations (f/Cc) ----------------~~------------------------------------------------------------------- :::0 PCM TEM PCMEd ------ n n n Avg GM SO Range n n n Avg GM SO Range n Avg SO Range (NO) (NR) (NO) (NR) Worker Brake and bench work Stockton Eql Eq2 3 0 o 0.056 0.038 0.059 0.013-{l.123 3 o O.O(l9 O'(l(l9 O.(Xl4 0.(X)5-0.013 2 0.038 0.055 O.013-{1.l01 3 0 o 0.156 0.120 0.103 0.037-{l.220 3 0 1 0.055 0.053 0.021 (l.(l40-0.070 2 0.036 0.018 0.023-{l.(l4S Big Rock Eq5 4 0 1 0.038 0.027 0.027 0.007-D.059 4 0 2 0.300 (l.3<Xl (l.(XlO 0.300-0.300 2 0.(l44 (l.O05 O.(l40-0'(l48 Eq6 4 0 o 0.024 0.024 O.OCl4 0.021-{l.O29 4 I 1 O'(l48 O'(l40 0.029 0.0l5-{)Jl70 2 O.()(l9 0.008 O.012-D.016 Eq7 4 0 o 0.016 0.015 0.007 0.0l1-{l.O26 4 o 0.015 O.Oll 0.011 (l.(Xl3-{l.030 3 0.003 0.003 0.OOI-D.()(l8 Eq8 4 0 o 0'{)75 (Ul71 (l.O28 0.042-{l.l03 4 0 o 0.225 O.2(Xl 0.126 O.lOO-{lAOO 4 0'(l42 0.034 0.035-(l.O90 Eq9 4 0 o 0.058 0.057 0.011 (l.O46-{l.()71 4 0 o O'(l48 0'(l39 0'{l36 0.020-{l.100 4 0.033 0.012 0.0 18-D.043 EqIO 4 0 o (l.O29 0.027 0.015 0.021-{l.O52 4 o 0.031 0.022 0'(l23 0'(Xl4-D.060 3 0.0(l9 0.008 0.004-{l.018 Eqll 4 0 o O,l28 0.02~ 0.013 0.017-{l.O46 4 0 o 0.125 0.089 0.120 0'(l~0-D.300 4 0.018 0.014 0.006-{l.(l38 ?> e, Eql2 4 0 o 0.050 0.(l47 0.023 (l.036-D.084 4 1 o 0.011 O'(l09 0.<Xl7 0.CXl3-D.020 3 o.om 0.002 0.(X)3-0.(106 ~ Eq1-Eq2, 10 0.053 0.045 0.029 0.007-0.220 10 0.087 0.077 0.038 0.003-0.400 10 0.024 0.016 0.001-0.090 ~ Eq5-EqJ2 Eq3 4 0 o 0.256 0.230 0.130 0.11O-DA25 4 4 o O'(l03 0.003 O'{lOl 0'(Xl2-D.004 ~ ,\.:.).. Eq4 4 0 o OA19 0.354 0.236 O.131-{l.622 4 0 3 0.020 (l.(l20 0.020 0.010 O.O]() Clothe, handling Eq3-Eq4 2 4 0 0.338 0.292 0.183 0.110-0.622 2 o 0.231 0.199 0.125 0.OSO-{).360 2 2 0.012 0.011 0.001 0.002-0.004 o 0.011 0.007 0.010 0.0024>.020 1 0.010 2 0.036 0.0 0.010 0.0324>.039 Bystander Brake and bench work Stockton Eql Eq2 o o 3 0 o 0.029 0.029 0.006 O.024-{l.035 3 o 0'(l07 0.005 0.<Xl5 0.(Xl2-0.010 2 0.006 0001 0.0()S-0.009 Eq5 Eq6 Eq7 Eq8 Eq9 EqlO Eqll 3 0 1 0.007 0.007 0.002 O.006-{l.008 3 0 3 3 0 o 0.014 0.012 0.010 O'(l07-{l.025 3 3 o 0'(l02 0'(l02 O,{Xll 0.(Xl2-{l'(Xl3 3 0 o 0.009 0.009 0.002 O.007-{l.OIO 3 3 o 0.002 O.OOl O'(Xll 0 (Xll-{l.002 3 0 o (l.021 0.019 0.012 O.008-{l.O32 3 1 o 0.010 0.008 0,{Xl9 O.(Xl4-{l.(l20 2 O.Ol7 0.008 0.OOS-0'(l323 3 0 o 0.013 0.013 O.O(l4 O'(l09-{l.016 3 2 o 0.003 0.003 O,{Xll 0.(X)2-{l.(Xl4 0.008 O.O(Xl O.OOS 3 0 o 0.012 0.010 0.008 O'(l07-{l.021 3 3 o O'(l03 0.002 (l.OO! 0.002-{l.(Xl4 3 0 o 0.019 (U1l5 0.015 O.007-{l.036 3 1 o 0.011 O.OOS 0'(Xl9 0.OO3-{l.020 2 (l.O05 0.005 O.(Xl6-{l.OlO Table 2. CQntinued Sample type/activitylloeation Equipment Asbestos flber concentrations (flce) ~PC=M~------------------------------=T=E--M-----------------------------------P-C--ME~d------------------ Eq12 n n n Avg GM SO (NO) (NR) Range n n n Avg GM SD (ND) (NR) Range n Avg SD 2 0 o 0.()()4 O'(X)3 0.005 0.001-H.008 2 2 o O.()()4 0.003 (UXll o.002--D.003 Range Eq1-Eq2, 9 0.014 0.013 0.007 0.001-0.036 9 0.005 0.004 0.003 0.001-0.020 4 0.009 0.004 0.006-0.023 Eq5-Eq12 Big Rock Eq3 5 0 o 0.015 0.014 0'()07 0.007....().()26 5 5 o O'()03 0.002 O'(Xll o.002-<l.O()4 Eq4 3 0 o 0.055 0.054 0.005 0Jl49-<U)59 3 2 o 0.002 O.OOl O'(XlI 0.001-<l.O02 1 0.0024 0.CXl2 ~ Eq3-Eq4 2 0.035 0.034 0.006 0.007-0.059 2 0.002 0.002 0.001 0.001-0.004 1 0.002 0.002 eT Cl> o o ~ Clothe~ handling 2 0 Remote area Brake and bench work o Stockton Eq I o o Eq2 2 0 0.0!)3 0.080 0.066 0.046-0.140 2 0 0.012 0.008 0.012 0.003-().020 2 0.010 0.011 0.()()~.018 '"o Pl :C::ls> ~ 5 O.O()4 Q.004 0.001 0.()()4-<U)05 2 2 o 0.002 0.002 O,(X)O 0.002-<l.0)2 ~ Eq5 ~ 0 o 0.005 O,(X)4 0.002 0.003-<U)06 2 I o O'()02 0.002 O,(X)O O.002-<l.O02 0.00 I (l.(X)O 0.001 0- . Big Rock o ~ Eq6 2 0 0.004 (l.O04 0.000 (W04""().()()4 2 2 .0 0.@()2 0.002 O.CXlO 0.002 ~ o Eq7 2 0 o 0.005 0.cX)5 0.002 0.003-<l.007 2 2 O.O()1 0.001 O.CXlO O'()Ol g~ o Eq8 2 0 (W09 O.(X)9 0.003 0.007-<l.C1Il 2 2 o 0.002 0.002 O.CXlO 0.002 '< o Eq9 2 0 o 0.006 0.(X)6 0.000 0.006-<l.C)06 2 1 O'()02 0.002 0.001 0.002-<l.003 0.0016 O.(X)O 0.CX)2 .gCl> -6. o EqIO 2 0 0.005 O.(X)4 0.004 O.002-<l.O07 2 2 o 0.002 0.002 O.cX)O 0.002 o ~ Eqll 2 0 o O.OOS O.OOS 0.002 0.007-<l.(1I0 2 2 0.002 0.002 O.CXlO 0.002 eT o Eql2 2 0 O.OOS O.(X)S 0.000 O.OOS 2 2 o 0.002 0.002 O.CXlO 0.002 ~ Eq1-Eq2, Eq5-Eq12 Eq3 10 2 0 0.006 0.006 0.002 0.002-0.011 10 o 0.010 O.OlO 0.001 0.010-<l.(1I1 2 2 0.002 0.002 0.000 0.001-0.003 o 0.001 (l.001 0.<Xl0 0.001 2 0.001 0.000 0.001-0.002 o o I Eq4 2 0 0.035 0.035 0.002 O.033-<l.O36 2 2 0.001 0.001 O,(X)O 0.001 Clothe~ handling Eq3-Eq4 2 0.022 0.022 0.001 0.010-0.036 2 0.001 0.001 0.000 0.001-0.001 2 0 o 0.040 0.037 0.021 0.025"'{).055 2 2 o 0.001 0.001 0.000 0.001~.002 o Background Brake and bench work Stockton Eql o o E42 2 () " " 0.023 o.(m 0.006 0.019 0027 2 o () (jfn 0 002 0 002 0 001-0.004 0.005 O.cX)O 0.005 o ~. ::0 -t-.) 0 Table 2. Continued :'":0" Sample type/activityllocation Equipment Asbestos fiber concentrations (f/Cc) PCM TEM PCME" n n n Avg GM SO (NO) (NR) Range n n n Avg GM SO (NO) (NR) Range n Avg SO Range EqS 3 0 0 OJl!O 0.007 O.OOS O.003 ...{j.OI9 3 2 0.001 0.001 (J.(X)O 0.001 Eq6 3 0 0 0.007 (J.(X)6 0.002 O.OO4"'{).OOS 3 3 0 0.002 (>.002 (J.(Xll O'(X)2-0'(Xl3 Eq7 3 0 0 O.OOS OJX)S O.OOS OJ)02...{).()l7 3 3 0 O'()02 (>.002 (J.(Xll (J.(X)2...{J.(X)3 EqS 3 0 0 0.006 O.OOS 0.002 O.003",{J.(X)7 3 3 0 (l.()02 0.002 0.000 0.002 Eq9 3 0 0 O.OOS (J.(X)S 0.001 O.007...{J.(X)9 3 3 0 0.001 O.OOl (J.(X)O 0,(X)[...{J.(X)2 EqlO 3 0 0 0.007 (J.(X)6 (l.OOS 0.002...{J.(l!2 3 3 0 (l.()02 0.002 (J.(X)O O.OIS Eqll Etj12 . 3 0 0 0.007 (J.(X)6 0.001 OJ)06...{J.(X)7 3 3 0 0.002 0.002 OJX)O (J.(X)2 3 0 0 O.OOS O.OOS 0.001 O.006...{J.(X)9 3 3 0 0.002 0.002 (J.(X)O 0.002 Big Rock Ambient Eq1-Eq2, 10 Eq5-EqJ2 0.009 0.008 0.004 0.002-0.027 10 0.002 0.002 0.000 0.001-0.004 1 0.005 0.005 t> Eq3 2 0 0 0.008 (J.(X)S O.O()() O.OOS 2 2 0 0.002 0.002 0.000 (J.(X)2 ~ "' Eq4 2 () 0 0.021 (J.(l21 (l.OI9 OJ)07...{l.()3S 2 2 0 0.002 0.002 0.000 (J.(X)2 ~ e: Eq3-Eq4 2 0.015 0.015 0.010 0.007-0.035 2 0.002 0.002 0.000 0.002 ~ '",.... Brake and bench work Stockton Eql 0 0 Eq2 0 0 0.001 (UXH 0.001 0 O.O(X)4 (U)(X)4 (J.(X)04 EqS-Eq7 3 0 0 0.002 0.002 0.002 O.OO1...{J.(X)S 3 3 0 0.0004 (l.0(X)4 0.0001 O'(X)04-0.000S EqS-Eq9 3 5 0 (l.O02 OJX)2 0.00.2 0.001...{J.()04 3 0 0.001 o.ooi (J.(X)O1 (J.(X)OS"'{l.OO07 2 .O.OO()4 0.0000 (J.(XX)4 Eq lO-Eq 12 3 0 0 0.003 O.(X)3 0.001 OJ)03...{J.(X)4 3 0 0.001 (l.0(l! (J.(X)O1 (J.(X)OS-O.OO10 2 0'()(X)2 O.O(X)O (J.(XX)[...{J.()()()2 Eq1, 4 Eq5-EqJ2 0.002 0.002 0.001 0.001-0.005 4 0.001 0.001 0.000 0.0004-0.0010 2 0.0003 0.0001-0.0004 Big Rock Eq3 0 0 (l.O(H (J.(XH 0.001 0 O.O(Xl3 0.0(X)3 (J.(X)03 Eq4 0 0 (l.001 (J.(X>! (l.O(H 1 0 O.O(X)S O.OOOS O.OOOS Eq3-Eq4 1 0.001 0.001 0.001 1 0.0004 0.0004 0.0003-0.0005 PC~1adj, PCM concentration multiplied by the ratio of asbestos fibers to total fibers measured hy TEM: n numhcr of ,ample,: NR. number of sample, not readable; NO, number of ,ample, in which asbe,tos was not detected; Avg, average; SD, ,tandard deviation; -, not applicable. "Based on samples in which asbestos fibers were detected by TEM. Asbestos concentrations dw-in~ heavy equipment brake removal 13 of 19 Tabre 3. Summary of fiber size and morphology of airborne asbestos fibers collected on the worker dw-ing brake removal Fiber structure n Total Percent fibers (%) classified as fiber or particle with dimensions of classification fibers (%) Respirable fiber Respirable particle* <0.7 Ilm width >51lm length (%) > 2O llm length (%) <31lm width >51lm length (%) >20 J.1ill length (%) <10 Ilm width >51lm length (%) > 2O llm length (%) Total fibers 261 Free fiberlbundle 95 36 Fiber clusters 8 3 Matrix disperse 158 61 18 2 2 0 0 0 35 2 2 7 0 3 1 36 9 1.2 11: 10.5 uS 0_4 o Q.Z I . I 1 ' - Maxltnum Mlnll11um Average D.O ----------L---------------t----------------------------------1--------------------1---------------------, PCM rEM PCME< Fig. 4. Comparison of worker asbestos concentrations (flee, 30 min) by analytical methods (PCM, TEM, and PCME). measured using PCM, TEM, and PCME, respectively (sampling times were 30 min in duratiofl and collected during the anticipated peak times of exposure). Figure 5 presents the average asbestos concentrations as measured by PCME for the worker, bystander, remote, and background locations, as compared to the current OSHA 30-min excursion limit for asbestos. Asbestos was not detected in more than half of the samples collected at the bystander lo- cations (even though < 1.2 m from the work activity) as determined by TEM, and average airborne asbes- tos concentrations at the bystander locations were generally less than half of those measured for the mechanic. Airborne concentrations were 0.014 flcc (PCM), 0.005 flcc (TEM), and 0.009 flcc (PCME) at bystander locations at the Stockton facility and 0.035 flcc (PCM), 0.002 flcc (TEM), and 0.002 fI~c (PCME) at bystander locations at the Big Rock facility (Table 2). Airborne asbestos concentrations found at the remote and background locations samples were even lower than those found at the bystander locations. It is interesting to note, however, that actinolite (one fiber) was detected in two ambient air samples and one worker sample, although this finding is not surprising since actinolite is commonly found in ambient air (Lee and Van Orden, 2008). Because four pieces of equipment (Eq3, Eq4, Eq6, and Eq7) had at least one assembly saturated in oil, airborne asbestos concentrations found on the worker removing those brakes were compared to the concentrations found while removing dry brake assemblies. The resulting average asbestos concentration during oily brake removal (0.009 f/cc) was less than onefourth the concentration of dry brake removal (0.043 f/cc), as measured by PCME. These findings were statistically signific~nt (P "" 0.001) by a twosample t-test of the log-transformed worker data. Estimated 8-h TWA asbestos exposures were .:alculated for the worker and the bystander. Considering the time involved in the brake removal activity, and assuming three brake removal jobs are conducted per shift, the resulting average 8-h TWA was estimated to be 0.009 flcc for a mechanic and 0.006 flcc for a bystander. Therefore, 8-h TWA asbestos exposures for mechanics performing brake removal on heavy equipment and those standing nearby this work are not likely to exceed the current OSHA PEL of 0.1 f/cc. Table 3 presents a su'mmary of fiber size and morphology of the airborne asbestos fibers collected on 14 of 19 A. K. Madl et al. 12 OS~'1. 30 Mml1lll 8ccursion LIfl1l1 1~ --~-----------------~-------------------- - Maxunum MHllmurTl -AIJf.-fage 0.0 +---..:z...---+--~a--'-+----4.t--_.-fl-----;.......- - - ! Worker Bystander Remote Background Fig. 5. Comparison of worker, bystander, remote area, and background airborne asbesto~ concentrations (flee, PCME, 30 min). the worker. Within the worker samples, there were 261 total asbestos fibers counted using the ISO methodology. Of these, only 36% were free fibers or bundles, 18% were free fibers or bundles with diameter <0.7 Ilm (length> 5 Ilm) and only 2% were free fibers or bundles with diameter <0.7 Ilm and length >20 Ilm. The remaining fibers were either in clusters (3%) or attached to a matrix (61 %). Only 4% of the fibers, however, were part of a cluster that may be re- spirable 10 Ilm in width), and 44% of fibers Were part of a matrix that may be respirable. Figure 6 is an image of fiber clusters collected in worker samples and exemplify fibers that are part of a much lafger .matrix. DISCUSSION This study was conducted to assess possible exposures to airborne asbestos during removal and disassembly of asbestos-containing brakes from heavy construction equipment manufactured during the 1950-1980 time frame. The data collected in this simulation study are believed to capture the plausible range of variables that might influence t;xposures during brake removal and disassembly from heavy construction equipment, as well as the potential exposure associated with handling work clothes. The work activities were conducted under low ventilation conditions (e.g. no active local or general ventilation and low building air exchange), by mechanics with varying years of experience and techniques at differ- ent maintenance service centers, and on different heavy construction equipment (tractors and backhoes) with similar brake assembly configurati@ns, but representing a range of equipment use (e.g. hours and brake lining wear). The results collected from Fig. 6. TEM Image of a fiber cluster collected dunng a shortterm worker sample performing brake removal on Equipment 6, courtesy of EMS Laborat01;es. this study provide information not only on airborne asbestos exposures experienced by mechanics re-' moving asbestos-containing brakes from heavy construction equipment and by persons nearby these activities but also on the extent to which chrysotile asbestos degrades into non-fibrous particles during braking of heavy 'construction equipment, the influence that dry versus oily brake assemblies has on airborne asbestos exposures, and the size and Asbestos concentrations duting heavy equipment brake removal 15 of 19 morphological distribution and potential respirability of airborne chrysotile fibers generated during the brake removal activities. Although most of the brake removal work was conducted at the Stockton, CA, service center (10 backhoes) and not at the Big Rock, IL, facility (2 tractors), worker exposures resulting from brake removal and disassembly at these two facilities, mechanics, and types of equipment appeared to be similar. The most striking effect on airborne asbestos concentrations measured on the workers' lapel was the internal dryness of the brake housing (i.e. whether it was saturated with oil), with dry assemblies resulting in worker exposures of 0.043 f!cc (range: 0.01--0.13 f/cc) and oily assemblies resulting in worker exposures of 0.009 flcc (range: 0.0030.016 f/cc). No correlation was apparent using regression analyses for the amount of asbestos present in the brake wear debris when compared to the asbes- tos content in the drum and band linings or to the extent to which the linings were worn (e.g. lining. thickness). It is possible that this lack of correlation refl&:ts the already low chrysotile asbestos concentrations present in the brake wear debris (0.49%), inability of these fibers to become airborne during manipUlation and compressed air blowout of the brake assembly, andlor similar surface area dynamics during the mechanical action of the braking process that is independent of the asbestos content. The precision of airborne fiber concentrations is dependent on the fiber density and proportion of filter surface area (e.g. microscope fields) examined, with statistical uncertainties generally being inversely proportional to the fiber density (lohnston et aI., 1982; Ogden, 1982; Cherrie and Johnston, 1986; Lange et at., 1996). It has been reported that the ac- curacy is not greatly improved for counts beyond 50 fibers and thus has been recommended that at least 50 fibers be counted and the number of fields be only limited where the airborne fiber concentrations are so low that the accuracy is no longer important (Ogden, 1982). These concepts have been incorporated into the current NIOSH method for asbestos' (NIOSH, 1994a), where 100 fibers or 100 microscope fields, whichever criterion is met first, are counted. For the majority of the worker samples col- lected in this simulation study, >50 fibers were counted within the prescribed 100 microscope fields, whereas far fewer fibers 10-20 fibers) were ob- served in samples collected in bystander or remote area locations. The confidence limits would, as a result, be expected to be narrower for worker compared to those for area airborne asbestos concentrations. Based on the data collected in this study, however, it was determined that the data represented a power of 100% at a 95% confidence level to detect a difference between worker, bystander, and remote' area measurements, as well as worker exposures handling dry versus oily brake assemblies. Based on standard t-tests comparisons, worker exposures were found to be significantly higher than those measured at bystander or remote area'locations (P < 0.000 I), and worker exposures while removing brakes from dry assemblies were statist~cally greater than those associated with oily assemblies (P = .0.001). Although Boelter et al. (2007) evaluated airborne asbestos levels during repair of heavy construction machinery; they did not restrict their study to just brake work (Boelter et aI., 2007). Boelter et al. (2007) evaluated asbestos air concentrations during 'in-frame maintenance and repair activities, which included aggressive techniques that resulted in visible dust from work involving friction products and gaskets'. Further, the work performed during this study included dismantling, cleaning, and reassembling engines and clutch,,:s. Because a narrow range of work tasks were involved with brake removal, only a subset of asbestos measurements from the Boelter et al. (2007) study can be directly compared to our study. It was observed that airborne ashestos concentrations observed during brake removal and disassembly were equal to or less than those of comparable work activities reported in the Boelter et al. (2007). Work involving band brake removal, rivets and friction lining removal from brake band or brake shoe, and disc brake assembly removal resulted in average 30-min airborne asbestos concentrations ranging from 0.044 tl:) 0.045 flcc (PCME) in the Boelter et al. (2007) study. The average airborne asbestos concentration for similar activities in our study was 0.016 flcc (range: 0.001--0.090 f/cc) (PCME). Boelter et al. collected and analyzed debris from the brake assembly of each piece of equipment and reported non-detectable or < 1% asbestos levels for every sample. Because PLM was utilized as the method for bulk sample analysis of brake wear debris, and because concentrations below 1% are not detectable, this approach did not allow for precisely quantifying the extent to which chrysotile is degraded during braking. While we did not specifically measure forsterite conce~trations in bral(e wear debris, indirectly we can determine how much chrysotile is degraded by measuring the chrysotile content in the friction lining and in the brake wear debris residing in the brake housing. With TEM analysis, we were able to quantify the amount of chrysotile in the brake wear debris (average 0.49%, range 0.008-6%) and determine that 95% of chrysotile in the hrake lining is degraded to non-fibrous, non-asbestos particles in the friction process. These findings are comparable to those reported for passenger automobiles, with reported averages being between 0.02 and 4.5% asbestos, with the maj~rity of w~ar debris samples containing < 1% chrysotile (Hickish and Knight, 1970; Luxon, 1970; Anderson et aI., 1973; lacko et aI., 1973; Rohl et al., 1977; Rowson, 1978; 16 of 19 A. K. Madl et al. Williams and Muhlbaier, 1982; Cha et al., 1983; Sheehy et al., 1989). We acknowledge that the degraded chrysotile may not, in fact, be chemically equivalent to forsterite and that it may be some other non-asbestos amorphous material (Langer, 2003; Candela et aI., 2007). It is also acknowledged that the material that is called chrysotile in this analysis may not possess the biologic activity of chrysotile asbestos due to dehydroxylation and other stretses 'as has been suggested by Langer (2003). Few studies have characterized the size distribution and morphological characteristics of asbestos fibers associated with handling asbestos-containing friction materials (Atkinson et aI., 2004; Jiang et al., 2008; Madl et al., 2008), and other studies have evaluated the size and type of asbestos fibers retained within the lungs of mechanics (Churg and Wiggs, 1986; Dodson et aI., 1991; Roggli et al., 2002). In the former set of studies, however, fiber characteristics were associated with diretly handling replacement asbestos-containing automobile brakes, and in the latter group of studies, the source of the fibers retained within the lungs can only be qualitatively associated with employment history. To the best of our knowledge, no studies have evaluated the size distribution and morphological characteristics of asbestos fibers in brake wear .debris released during the disassembly of brakes, and in particular in heavy equipment brakes. We found that 61 % of the airborne asbestos fibers were associ:hed with a matrix or resin that can significantly influence the potential respirability of these fibers; of the fibers associated with a matrix, only 9% were potenti~lly .respirable (using cutoff of particle diameter of 10 ~m) and had fiber lengths >20 ~m. Of the free fibers or bundles (not associated with a matrix), only 2% of airborne fibers were respirable (cutoff of fiber diameter of 3 ~m). Thus, even with the low concentrations of airborne asbestos fibers released during heavy construction equipment brake removal and disassembly, only a small percentage of these fibers were likely to be respirable. The exposure and epidemiologic literature for automobile mechanics can provide a useful banchmark for exposures measured in this study. In a recent assessment of all the published and unpublished industrial hygiene data collected during asbestos brake repair by vehicle mechanics, nearly 200 brake job and 8-h TWA airborne asbestos samples were analyzed (Paustenbach et aI., 2003). In this assessment, which encompassed measurements collect~d in seven different countries over the last 30 years, average 8-h TWA concentrations of 0.04 flcc for airborne asbestos were found with individual measurem~nts ranging from <0.002 to 0.68 flcc reported for brake mechanics servicing light trucks and passenger vehicles (Paustenbach et aI., 2003). This value (1.04 .flcc) is identical to that identified by US EPA in the survey that they conducted in 1984 (Weil et aI., 1985). The values are also not dissimilar from the analysis of >200 short-term samples recently reported by Richter et al. (2008). In addition, since 1975, six epidemiologic casecontrol studies and two meta-analyses have evaluated the risk of asbestos-related disease among mechanics (McDonald and McDonald, 1980; Teta et aI., 1983; Spirtas et al., 1985, 1994; Woitowitz and Rode1sperger, 1994; Teschke et al., 1997; Agudo et aI., 2000; Wong, 2001; Hessel et aI., 20(4). These studies have consistently found no increased risk of mesothelioma in brake mechanics. Studies that specifically evaluated mechanics involved in brake lining installation ,and repair also showed a relative risk consistently <1.0 (Spirtas et '11., 1985; Woitowitz and Rodelsperger, 1994; Teschke et al., 1997; Hessel et aI., 2004). It has been noted that the risk of mesothelioma in brake mechanics is similar to that of other occupations that do n\)t involve occupational exposure to asbestos, such as teachers, librarians, and accountants (Teschke et al., 1997). Based on these findings, the available epidemiological data show that employment as a motnr vehicle mechanic or, more specifically, a brake repair worker does not result in an increased risk of developing mesothelioma (Paustenbach et al., 2004). Taking the epidemiologic and industrial hygiene findings together, we can conclude that auto mechanics who repair asbestos-containing brakes as a career are exposed on average to 0.04 flcc (range <0.002--0.68 flcc) of asbestos and are therefore not at an increased risk of asbestos-related disease, including mesothelioma (Paustenbach et aI., 2003). The range of lifetime cumulative doses of chrysotile have been characterized by Fin1ey et al. (2007) and were reported to range from 0.16 to 0.41 flcc year-I for facilities with no dust-control procedures (1970s) and from 0.010 to 0.012 flcc year-I for those employing engineering controls (1980s). Upper bound (95%) estimates for the 1970s and 1980s were 1.96-2.79 and 0.07--0.10 flcc year-I, respectively (Finley et aI., 20(7). These data also suggest that mechanics conducting brake work on heavy construction equipment similar to that described in this simulation study are comparable to exposures of automobile mechanics (Fig. 7) and, as a result, would also not be expected to be at an increased risk of asbestos-related disease. In summary, the short-term airborne asbestos concentrations measured for both a worker removing asbestos-containing brakes from heavy construction equipment as well as for a bystander working in the vicinity of such activity were below both the current OS HA excursion limit for asbestos and all the previous US occupational asbestos standards. Based on a collection of 44 samples, this study found that short-term exposures (30 min) of a mechanic to airborne asbestos during the removal and disassembly Asbestos concentrations during heavy equipment brake removal 17 of 19 Utl9\'H ~'"l ~!f.fnJ L!$!."!~ --- -- - -- -_ .. --- .......... -- "" -- .. ---"f, , , I "-< OSfSA.~'Pfl tiJXt)~)~} j I .. .. .. .... __ _____ __ __ __ ~ SZl-lm.~ J~ ~ ~ ~) c 1).11a*,,~ ~. ~._K.~ ~ ~ ~._~ ~ M1 ... T _ ""'* JoI><ool ... <Il !<"~ "970) 11y(9)!H7t<- I~ '<In) ~'t~ lJ~3l>' !!~t~ -('llWo) {1&1$1 ... . _ " . . _ _ _~~Wo>.(1l1eS1 - " . , . . Z" . . . . . .1<Jo IH32) I'l'~ j19l!e) i1mH,a~ ~v*l'J {=, S1.k~~'''. ~S-NuJrt""o\'6 .W.ooK)!6ft<:J".t'flt(f.'\'.I>JoyM..QXo~x.Wxo:.,~lbc-"'~t.O 9KlU1-tIt-l<:w.w.-h atlr..w: *~Y",.'t(<<IN:~)to~ Y)ft'foX>.",,@:00Q14~".ot"O!"Xcod,.iXJI/I6~o:\~~~ T T CIloSli} "",,_I r~s~ .ay--> (11'>"""") ~i!l(-el' C",'Hl !;'<"i_" T~ ~!IM-~~' Fig. 7. Comparison of airborne asbestos exposures for automobile mechanics to those measured by mechanics handling asbestoscontaining brakes on heavy construction equipment. of asbestos-containing brakes from heavy construction equipment average 0.024 flcc (range: 0.001-0.1 f/cc), whereas 8-h TWA exposures average 0.009 f/cc (range: 0.005-0.23 f/cc), based on the assumption that three brake assemblies could be removed within a workday. The industrial hygiene data presented here should therefore prove useful f6r retrospective and current exposure assessments of individuals and hazard assessments of work activities that involve repairing and replacing asbestos-containing brakes contained in heavy construction equipment (Hickish and Knight, 1970; lohnson et al., 1979; Roberts, 1980a,b; Roberts and Zumwalde, 1982; Rodelsperger et al., 1986; Moore, 1988; Sheehy et al., 1989; Blake et al., 2003; Boelter et al., 2007). FUNDING Case-New Holland. Acknmv!edgements-This research was supported by Case- New Holland, a manufacturer of heavy construction equipment. which has been involved in litigation related to the possible exposure of brake mechanics to asbestos. Some of the authors have served as expert witnesses in litigation regardiag the potential asbestos health hazards to mechanics historically involved in automobile-related and heavy equipment repair work. The authors would like to thank and acknowledge Pearl Moy, Ellen Donovan, and the mechanics for their participation in the simulation study, as well as Paul Scott and James Keenan for their contributions to the analysis and graphical presentation of these data. REFERENCES Agudo A, Gonzalez CA, Bleda MJ et al. (2000) Occupation and risk of malignant pleural mesothelioma: a case-control study in Spain. Am J rnd Med; 37: 157-68. Anderson AE, Gealer RL, McCune RC et al. (1973) A,bestos emissions from brake dynamometer tests. 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