Document E8936wM2Dwr8K7aBm7KX3exR

Applied Occupational and Environmental Hygiene, 18: 786-804, 2003 Copyright Applied Industrial Hygiene ISSN: 1047-322Xprint/ 1521-0898 online DOI: 10.1080/10473220390229240 An Evaluation of the Historical Exposures of Mechanics to Asbestos in Brake Dust Dennis J. Paustenbach, Richard O. Richter, Brent L. Finley, and Patrick J. Sheehan Exponent, Oakland, California This article presents a historical analysis of published data regarding the exposure of brake mechanics to asbestos as a result of doing brake work. Concerns about this possi ble hazard were first raised in the late 1960s. This analysis focuses on 30 years of data collected during the brake re pair event (e.g., a brake job) and 8-hour time-weighted aver age (TWA) personal samples. A brake job TWA represents the average concentration a mechanic experienced during brake servicing, rather than throughout the workday, and an 8-hour TWA represents the average airborne concentration of asbestos for the entire workday (which would involve brake work and other activities). Nearly 200 brake job and 8-hour TWA airborne asbestos samples were analyzed to assess how asbestos concentrations varied by type of vehi cle serviced, country in which mechanics worked, time pe riod, and brake-cleaning method. To facilitate comparisons, brake job TWAs were converted to estimated 8-hour TWAs using the durations and number of brake jobs performed per mechanic each day. Estimated and measured 8-hour TWAs for mechanics servicing automobiles and light trucks ranged from <0.002 to 0.68 f/cc, with a mean of 0.04 f/cc. In contrast, the 8-hour TWAs for mechanics servicing heavy trucks and buses ranged from 0.002 to 1.75 f/cc, with a mean of 0.2 f/cc, suggesting that these mechanics experienced higher daily as bestos exposures than automobile and light truck mechanics. Brake job and 8-hour TWAs for brake mechanics worldwide were found to be similar during the same time periods, and they were consistently below contemporaneous occupational health standards in the United States. The increased use of brake-dust control measures in some garages resulted in at least a 10-fold decrease in the TWA airborne concentrations of asbestos from the 1970s to the late 1980s. Keywords Asbestos, Industrial Hygiene, Auto Mechanics, Brake Dust, Exposure, Occupational Health Asbestos has been used in automobile brake linings world wide since the early 1900s because it has superior performance characteristics. It was not until the 1960s, however, that con cerns arose regarding the potential health hazard associated with exposure to airborne asbestos resulting from automobile brakes. Thomson et al.(2) were the first researchers to address the poten tial impact of emissions from the brake linings of vehicles during braking on ambient air concentrations of asbestos. Specifically, Thomson observed several cases of mesothelioma (a rare form of cancer associated with asbestos exposure) in South Africans who had no history of occupational or other exposure to asbestos and found unexplained asbestos bodies in routine histological sections of lungs collected during autopsies. Based on these find ings, he concluded that these individuals were likely exposed to asbestos products in their homes or to asbestos in ambient air from brake lining wear during vehicle braking, or from disin tegration by natural weathering of asbestos-containing building materials.(3) The question raised by Thomson prompted a number of stud ies on the potential impact of asbestos emissions from brake lin ings on ambient air quality. These studies(4-8) provided data on a number of factors, including (1) the amount of asbestos in the wear debris dust from automobile braking, (2) the size distribu tion of observed fibers, (3) the quantities of asbestos emitted into ambient air from vehicle braking, and (4) the resulting estimated ambient air concentration. An important finding of these studies was that the asbestos content in brake dust ranged from 0.02 to < 1 percent, thereby confirming the hypothesis that most of the asbestos in the original lining material (which was composed of about 35% to 65% asbestos) was thermally degraded during braking. However, none of these studies specifically addressed potential asbestos exposures of mechanics servicing brakes. The first investigation of airborne asbestos in a brake re pair garage was undertaken by Hickish and Knight(9) in the late 1960s. This study was conducted at aFord dealership in London, apparently to evaluate compliance with the 1968 British Occupa tional Hygiene Standard for Chrysotile Asbestos Dust.(10) Soon thereafter, in 1971, similar studies were conducted in the United States, following the promulgation of an occupational standard for asbestos by the Occupational Safety and Health Administra tion (OSHA). From 1972 through 1989, the National Institute 786 EXPOSURE OF MECHANICS TO ASBESTOS 787 for Occupational Safety and Health (NIOSH) conducted numer ous investigations of vehicle brake servicing operations in the United States.(1'11-14) In addition, independent studies of air borne asbestos levels in brake-servicing garages were published in the 1980s and 1990s by researchers in the United States,(16) Germany,(17) Sweden,(18) and Australia.(19) Despite the number of studies conducted over the past sev eral decades, there has been no apparent attempt to use all of the available published air monitoring data to characterize his torical exposures to asbestos from brakes for brake mechanics under various working conditions. In this article, the measured airborne asbestos concentrations from published surveys of ve hicle brake repair facilities were presented and evaluated to char acterize historical exposures for brake mechanics under various working conditions. The concentrations reported for mechanics servicing brakes were characterized and compared based on the period when sampling was conducted, where it was done (United States and overseas), the types of cleaning methods used, and the sizes of the vehicles (cars and light trucks or heavy trucks and buses). The data were then used to estimate the distribu tions of daily airborne concentrations for brake mechanics who performed brake repair work in the 1960s and 1970s, and more recently in the 1980s and 1990s. Finally, estimated 8-hour TWA concentrations experienced by brake mechanics were compared to occupational permissible exposure limits (PELs). METHODS Published studies from the United States and abroad that pro vided long-term personal air sampling data on asbestos con centrations experienced by mechanics during brake servicing operations were evaluated. Generally, only samples with sam pling durations longer than 1 hour were included. Short-term samples collected during blow-out or related "peak exposure events" were not considered informative for assessing the health hazard to mechanics without considering time/motion informa tion; however, the impacts of peak exposures are captured by the longer-term sampling evaluated here. Based on these crite ria, 10 studies were identified/1,9,12-19 In the studies evaluated, personal air samples were collected on filters in the breathing zones of the mechanics, either dur ing the brake servicing operations or throughout the mechan ics' workdays. All of the samples were analyzed for asbestos fibers greater than 5 micrometers (^m) in length and with at least a 3-to-1 length-to-width ratio, (called an aspect ratio) using phase contrast optical microscopy (PCM). Although, as shown in Table I, slightly different sampling and analysis methods(20-23) were used to quantify airborne asbestos concentrations in these studies, the reported methods are sufficiently similar that the data are comparable and representative of the range of airborne as bestos concentrations that mechanics likely encountered during brake servicing operations in various countries and eras. Measurements reported in the 10 studies were generally ex pressed as time-weighted average (TWA) airborne concentra tions of asbestos for mechanics as they performed one or sev eral brake services (i.e., "brake job TWAs"), or as average daily concentrations of asbestos for individual mechanics as they per formed various activities throughout the course of their day (i.e., "8-hour TWAs"). Unless specifically referred to in a report as 8-hour TWAs, it was assumed that the data collected were for brake job TWAs. This assumption could usually be confirmed by examining the duration of the sampling event. Table I summarizes the 10 published studies that we could identify that provide data on airborne asbestos concentrations for mechanics servicing car, truck, and bus brakes. Together, over the past 30 years, the concentrations of airborne asbestos in the breathing zones of mechanics have been measured in 53 brake repair facilities in the United States, 76 facilities in Germany, 23 facilities in Sweden, 4 facilities in Australia, and 1 facility in the United Kingdom. The reported brake job and 8-hour TWA concentrations for the mechanics in all of these studies are presented in Table II for automobile and light truck mechanics and in Table III for heavy truck and bus mechanics. In the studies by Rodelsperger et al.(17) and Moore,(16) the sampling data were not available; only the mean asbestos con centrations for multiple mechanics doing various types of brake servicing activities were reported, rather than the results for in dividual mechanics. We did, however, attempt to obtain their original data. Although it is not possible to use their summary information in this statistical analysis of TWA concentrations, the mean concentrations that they measured can be compared to exposures among brake mechanics during relevant time peri ods in the various countries where samples were collected. The studies evaluated are summarized in the following sections. United Kingdom Study The first published study ofoccupational exposure to airborne asbestos due to brake dust was carried out in 1968 at a Ford dealership in London.(9) Likely prompted by the then-new 1968 British standards for asbestos in the workplace, Hickish and Knight(9) estimated the exposure of a mechanic engaged in the removal of brake dust from a brake drum using compressed air during an entire work shift, during which 11 cars were serviced. No other study has ever reported that a single mechanic worked on so many vehicles during a single day. (Even 2-3 brakejobs per day is high for a single person in a brake shop.) The measured 8-hour TWA concentration for this mechanic, 0.68 fibers per cubic centimeter (f/cc), is the highest reported value for any mechanic servicing car brakes in any of the subsequent studies that were conducted. These researchers also reported an 8-hour TWA of 1.75 f/cc for a mechanic servicing truck brakes, which, again, is the highest reported value for truck and bus mechanics servicing brakes encountered in a review of the literature. United States NIOSH Studies Between 1976 and 1989, researchers at NIOSH collected personal samples for 75 mechanics in 22 settings, including commercial brake and front-end repair shops, fleet service 788 D. J. PAUSTENBACH ET AL. TABLE I Published industrial hygiene studies with time-weighted average (TWA) asbestos concentrations for mechanics servicing car, bus, and truck brakes Type of facility Number mechanics employed Number mechanics evaluated Number brake jobs/week Personal samples collected Reported TWA values TWA durations (hours) Study Method Service bay Ford dealer Fleet service garage Brake/front-end Brake/front-end Brake/front-end Large brake service (1976) Large brake service (1977) Brake/front-end/muffler Truck brake maintenance (1976) Truck brake maintenance (1977) Commercial brake Brake/front-end Brake/front-end Large brake service Municipal garage Municipal garage Municipal garage Fleet service garage 76 garages Germany NR 3 3 3 3 5 5 3 7 7 5 NR 3 5 60 5 3 4 NR 31 garages United States Government maintenance Postal maintenance Fleet service garage Postal maintenance Private utility garages Outdoors Service station Garages Sweden Garage Australia Garage Australia Garage Australia Bus service depot Australia NR 6 11 2 10 5 1A 1 NR NR NR NR NR 1 11/day 6 1 daily 8.0 (9) BOHS (1968) 1 2-5 3 1 brake job 5.7 (12) OSHA/NIOSH 1 10-14 2 1 brake job 7.9 (12) OSHA/NIOSH 1 4-6 3 1 brake job 5.9 (12) OSHA/NIOSH 1 20-30 7 2 brake jobs 1.5, 3.0 (12) OSHA/NIOSH 4 35-45 16 4 brake jobs 4.7-7.7 (12) OSHA/NIOSH 3 35-45 45 15 brake jobs NR (12) OSHA/NIOSH 2 4-5 2 2 brake jobs 1.6, 3.0 (12) OSHA/NIOSH 3 NR 9 2 brake jobs 4.3, 5.0 (12) OSHA/NIOSH 3 NR 6 3 brake jobs 2.3-6.4 (12) OSHA/NIOSH 2 35-40 5 2 brake jobs 6.2, 6.2 (14) OSHA/NIOSH 1 NR 2 1 brake job 4.3 (13) OSHA/NIOSH 1 4-6 4 4 brake jobs 4.5-5.9 (15) OSHA/NIOSH 3 35-40 2 2 brake jobs 4.7, 5.7 (15) OSHA/NIOSH 2 40 2 2 brake jobs 3.3, 5.0 (15) OSHA/NIOSH 3 1 3 3 brake jobs 2.3-6.2 (15) OSHA/NIOSH 1 NR 1 1 brake job 5.4 (15) OSHA/NIOSH 1 1/shift 1 1 brake job 4.0 (15) OSHA/NIOSH NR NR 67 Mean NR (17) AIA (1979) "combination" car NR NR 35 Mean brake job 0.2-1.0 (16) MF-PCM (400x) NR 6-10 18 9 brake jobs 2.0 (31) OSHA/NIOSH NR 28-36 22 11 brake jobs 2.0 (31) OSHA/NIOSH NR 1 13 7 brake jobs 2.0 (31) OSHA/NIOSH NR 6 20 10 brake jobs 2.0 (31) OSHA/NIOSH NR 7-9 8 4 brake jobs 2.0 (31) OSHA/NIOSH 1A NR 2 1 brake job 2.0 (31) OSHA/NIOSH 1 NR 2 1 brake job 2.0 (31) OSHA/NIOSH NR NR 23 23 8-hour 8.0 (18) NR 1 20 1 1 brake job 1.3 (19) NOHSC (1988) 1 4 1 1 brake job 1.3 (19) NOHSC (1988) 1 4 1 1 brake job 2.2 (19) NOHSC (1988) 1 2 1 1 brake job 2.1 (19) NOHSC (1988) Note: NR--not reported. ADo-it-yourself mechanic. garages, truck brake maintenance garages, municipal garages, private utility maintenance garages, one service station, and one driveway sample for a do-it-yourself mechanic. These evaluations were apparently prompted by the meet ing among NIOSH, researchers at Mt. Sinai Hospital, and in dustry representatives, which was held in July 1975.(24) The NIOSH investigations provide perhaps the most extensive and well-documented characterizations of asbestos concentrations in brake servicing facilities, providing data for car and truck mechanics using a variety of cleaning techniques during various time periods. During the 1970s and early 1980s, most of the sur veys by NIOSH were conducted in garages that used cleaning methods consistent with those used in previous decades, such as the use of compressed air to blow off brake parts. In contrast, during the late 1980s, some of the sampling was done in garages that used more sophisticated cleaning techniques for removing (Continued on next page) I -O M o a so -C w -a c -9 03 foCcf2cocf2cocfc2ofcc2focc2foccf2cocf2cocfc2focc2focc2fcocf2cocfc2otcL2noct2loHc2toclH2octl2Hoct2lHoc2tlocH2toclH2cot2lHco2tlcHo2tcolH2 O CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN CN OOOOOOOOOOOOOOOOOOOOOOOOO OhOMMMMMNMMMMMMOMMMMMMMMO OOOOOOOOOOOOOOOOOOOOOOOOO <D O M X) O go CQ *~* 3 T3 g - CQ w^ 11^1 tin -a <D -O s z3 cnmmmcnininininininininininininininininininincn < CQ PQ CQ CQ ticnQ ticnQ ticnQ cQ cQ in in in in in ooooo in IN IN M M O NN N N o oo o o <N o oooo Co in in in in Is OOOOOOOOOOOOOOOOOOOOOOOOO - $ .T3 <d h ti 3 ^ o o> iajo-?ohEl . 1 c* cu ts ^ o 1 n. -- OX C* <N <N cn in r-CNOOOO(NTf(NTf'Or^-(N <N O O O O OOOOOOOOOOOOOOOOOOOOOOOOO W) 03 <3 c3 <D 'a -a a> T-HfOIOO-cHn(mDO-H'O(iNO-'HOO-HOOON dddd<Ndddddddddo o dddddddo! 4 'O'OMmiOMOMri oooooooo 4o mo ooo -oh co-- dddddddddddddo dodo \oQ to-oo ii in <m OO Ph ooooo ooooo ooooo oo _C -a 03 n0 zz C2c2c2c2c2c2c2c2c2c2c2c2c2c2c2c2c2c2c2c2 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 "3 ^ ^ ^ <D <D w W ww Ww WOT WW5 <D <D OOTT WW5 <ODT <WD5 OT W <D <D <D <D <D OT W5W OT WWW WWW XOT s--_ 2* b S3 3u g3 S *3 ^ (fUli b w w ^ 3 *3 D(ii w w C 1) > <D <D <D <D <D <D <D <D <D S S o<D <D <U <D <D "3 P "3 ^^ssssssssssssssssssss S O< OhOhOhOhOhOhOhOhOhOhOhOhOhOhOhOhOhOhOhOh T3 Oh -3 Cu Cu ^ ttttOOOOOOOOOOOOOOOOOOOO oo ^ ZZOOOOOOOOOOOOOOOOOOOO UU cQ cQ cQ cQ cQ cQ OC oOO UU! ?D---?D(U(U(U(U---?D--- ^Oh Oh ^ g * Uo Uo Uo Uo Uo g* -a c 03 0^ 0^ 0^ 0^ 0^ 0^ 04 Z ZZZZZ Z -a a> s Oh <D oc -oCo-oCo-oCo-oCo-oCo-oCo-oCo-oCo-oCo-oCo-oCo-oCo-Coo-Coo-Coo-Coo-Coo-Coo-Coo-Coo-Coo>> -C -C 00 00 > -C -C -C -C > 00 00 00 00 -C 00 K'"" h3Jhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh 3hJhh Hi--I o ffi ffi J ffi ffi ffi ffi ffi 789 p S2 O o 3 3 UUQQQW c3 tin tin tin (J_ Hn Hn Hn inio'oiriiriioiohhh'o r^-mcNOCNr^-H'O'OfN OOO-hOOOOOO dddddddddo MMMNMOJNMOJNMO)NMO)MMiNMMmMMiNNMMNMMMMNMOJN oooooooooooooooooooooooooooooooooooo oooooooooooooooooooooooooooooooooooo dddddddddddddddddddddddddddddddddddo 3 3 a oooooooooo _6p a -g* _oo p?3 .Ss io^OOO^^OOO M3M3000M3M3CMCMCM CQ P d' CC CC CC -H -H -H oooooooooooooooooooooooooooooooooooo M3 M3 M3 o o in in in ^ O ^ 1- t B oooooooooo 1010101010101010OOOOOOOOOOOOOOOOOOOOOOOOOOOO -C p -a Z <D C 3 O o c <u C/3 C 40 P is c <u o co o <u $ts ^<D -o on. 1-- O X5 c* o o io in in cc oo 'O'O'O'O'O'O'O'O CMCMCMCMCMCMCMOOOOOOOOOO .MCMMMCMMMCSMMCMMMCSMMCSMMCSMMMMiNCSMMMMMMCMiNMM CtiOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO goooooooooooooooooooooooooooooooooooo oooooooooo dddddddddddddddddddddddddddddddddddo o CM O tu TfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTflO CMTfTfiOOOTfTfTfTfTfTfTfTfTfTflO On m 00 Tf -h -h OONTfccOOOOOOOOOOOOOOOOOOO ooooooooooooooo M '--1 N CM CM CM O oo oooooooooooo ooooooooooooo ooooooooooo o o o o o o o dodo dodo dodo dodo dodo o o o o o dodo dodo dodo odd -HTfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTfTf <u `---'OoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOoOo ? ? ? ^O"OpOpOpOpOpOpOppOOOO^ gp <3 oooooo ddddddddddddddddddo OVVVVVVVVVVVVVVVVVV NO 00 oo oo ovovovovovovovovov o & -a <D <<<<<<<<;p;p;p;p;peQeQeQeQeQeQ da .2 pOoWWopO WopO oooPo oPo ooopwO opwO ooPWO oWPO oWPO ooPwOoPwOo oooooooooo 'o'o'o'o'o'o'o'o'o'o oooooooooooooooooooo -a <u <s g ns 32 Q 32 Q 32i & 33 & 35 & s .t3s 3S 03a0- ^>O W W CO gpc a _o 3 gg O Jr O UQU 2<>82<>82<>82<>82<>82<>82<8-cW=-W!=-W!=-W!=W-!=W-!=W-!=W-!=W-!=W-!= 3<< gss ffi ffi>i W 3 M W W W ffi ffi ffi ffi JgD tskPtskPtskPtskPtskPtskPtskPtskPtgkPt <u c-~- c-~- c-~- c-~- c-~- c-~- c-~- -a OOOOOOC^C^C^C^C^M3M3M3M3M3M3M3M3C-C-C-C-C-C-C-C-C-C-C-M3M3M3M3M3M3M3M3M3M3M3M3M3M3M3M3M3 W& -a ton Oh <d Cd " ^b "b O^ ^O ffi ffi 3 3 %O 3 ct-H Z %o 3 ozi ozi ozi ozi &0&0&0&0&0&0&0&0"}^) "^}) "^}) "^}) ^"})^"S)^"}^) "S^) "S^) "^S) "0S) 0is0is0isi0si0si0si0sis0is0is0is0is0is0is0is0is0is 33333333ffiffiffiffiffiffiffiffiffiffiffi33333333333333333 790 (1) (1) (1) (1) 0.003 0.002 0.003 0.002 0.003 0.002 0.003 0.002 OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO ON ON ON io in in odd -O o 120 120 120 120 40 6 40 6 40 6 40 6 89 89 89 89 0.002 0.002 0.002 0.002 -- 0.004 -- -- 0.006 0.014 0.004-<0.005 0.004 -- 0.005-0.007 <0.004 0.011-0.016 'Oc sozo q S^ZZZZZZZZZZZZZZZZZZZZSS <N<N<N<mc<')inint-'-oocN mTfioior^-CNOOO OOOOOOOOOO'--i '--i '--i '--i '--i '--i '--i M iN iN dddddddddddddddddddo -D o q TC CO OPqh S-H O^QnN 0o z1 ^00 0<I ^ a 'Oo oo V Q JoCp o V *q a) uS |> 3| c<c<c<c<c<c<c<c<c<c<c<c<c<c<c<c<c<c<c<c< ZZZZZZZZZZZZZZZZZZZZ Z < zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC<zC< O %C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<C<%i% O hJhJZZZZZZZZZZZZZZZZZZZZKhJhJ 5^ -qo q -D o & -S oqSo -CCcOo -cC3 dO -7>C3 d Tct s s-h > WPS-H ,<4co---( P .O C0q3 cq 3< <2 CCQQ UZ Qs PsU FMean for 35 mechanics. GEstimated daily TWA assuming zero exposure for unsampled portion of shift. HMechanic in service station. IDo-it-yourself mechanic working outdoors. 791 Aerosol spray Aerosol spray Wet brush Aerosol spray 1986-7 1986-7 1986-7 1986-7 Low Low Low Low 3 3 -a 3 ^ f3a -D -- <D 3) _o 03 3 "* T33 ^ <*) !_ -^ 5S 3 a ooooo C C C On lo> lo> OOOOOO oooooo oooooo ooooo ^ MMM MCM3 MCM3 MCM3 a> O00h 3_o a> a a> QS t2 * -a <d -O z33 CM CM ^ ^ ^ o o o o o oO O Cm IN. Cf- Cm o oo o tos 3 Oh ',< & 00 "3 <r s& gto 3j h3 to W3) c3 <D "E <3 -aa> CoN CoN "r ? o o00 cc o o CM oO oO iOn i-nH ooo ion oooooo to to to zzz VV c3g<D -O^o -a n s t3o GG a _3 3g g to g 5 &5 GQG 3^ -c3a -a tttoO<oooD h ZZZZ tzo tzo tzo tzo tzo tzo 5 o to to to 6 J JZZZ J 792 EXPOSURE OF MECHANICS TO ASBESTOS 793 the brake dust, such as enclosed high-efficiency particulate air (HEPA) vacuum systems. The first NIOSH investigation of a brake repair facility was conducted in 1972;(11) however, the report is unclear as to whether the samples collected were personal samples. Also, be cause the sampling durations were not included, it is unknown whether the results are for peak or long-term exposures; thus, they were not used in this analysis. In 1976 and 1977, NIOSH researchers sampled 16 mechanics in seven different brake service facilities/12,25 Six of the 7 fa cilities were commercial automobile brake service centers that performed from 2 to 45 brake jobs per week, at an average of 65 minutes per vehicle (Table I). The seventh facility, a truck ser vice center, performed an average of 3 brake jobs per week, but servicing took an average of 6 to 9 hours per vehicle. Johnson et al.(12) reported TWA concentrations for 26 automobile (Table II) and five truck (Table III) mechanics. Because none of the results are referred to as 8-hour TWAs, and where given, the sampling durations are all less than 8 hours, it was assumed that the results were for brake job TWAs. The description of brake servicing operations was as follows: "The vehicle is driven into the repair stall or bay for a brake examination. Pending repairs, the wheels are elevated, removed and then inspected. Loose dust is then cleaned from the drums and brake assemblies by vacuuming, wiping, brushing, using compressed air, or a combination of these methods. Parts are then replaced or repaired as needed and the brake system is reassembled and adjusted." Although not provided in the report, the mean and standard deviation calculated from the data for the 26 automobile brake job TWAs are 0.095 and 0.053 f/cc, respectively, with a maxi mum of 0.20 f/cc. For the five brake job TWAs for trucks, the mean and standard deviation are 0.59 and 0.64 f/cc, respectively, with a range of 0.05 to 1.68 f/cc. Two commercial automobile brake service facilities were evaluated by NIOSH in 1979.(13,14) One of these facilities was previously sampled by NIOSH in 1977, while the second was an active shop that operated 6 days per week and performed from 35 to 45 brake maintenance operations per week (Table I). These investigations provided three brake job TWA concentration es timates, ranging from 0.006 to 0.04 f/cc (Table II). During 1979 and 1980, NIOSH collected additional sam ples in 6 brake service facilities, including 2 commercial brake service garages, 3 municipal automobile service garages, and 1 fleet service garage/15,26) One of the commercial facilities, a large brake service garage, was previously sampled by NIOSH in 1976 and 1977.(12) These evaluations, which involved sampling times of about 2 to 6 hours, provided brake job TWA estimates for 13 additional mechanics using six different cleaning meth ods for brake servicing, including compressed air, compressed air and solvents, dry brushing, wet brushing, squirt bottle wash ing, and vacuuming (Table II). Although not reported by the authors, the mean and standard deviation are 0.15 and 0.09 f/cc, respectively, with a range of 0.01 to 0.28 f/cc. The final NIOSH investigations were conducted in 1986, 1987, and 1988 and are summarized in Sheehy et al.(1) Thor ough sampling surveys were conducted at five facilities, each using a different method for controlling exposure to asbestos during brake repair.(27-31) The stated purpose of these surveys was to evaluate dust control technologies (Table I). They eval uated the effectiveness of two commercial enclosure devices with ventilation provided by a HEPA vacuum, a HEPA filterequipped vacuum alone, a brush with recirculating cleaning so lution, and cleaning solvents in aerosol cans. The investigators also evaluated a small service station garage where no dust con trol measures were used, and brake repair conducted outdoors by a do-it-yourself mechanic who used a spray can of solvent to wet the brake surfaces and water from a garden hose to flush surfaces. These investigations provide 44 additional brake job TWA measurements, 42 for autos and 2 for light trucks, all of which were collected for the maximum of either 2 hours or the duration of the brake job (Table II). Using detection limits in the statistical analysis for those samples in which asbestos was not detected, we calculated a mean and standard deviation for the 42 auto brake job TWAs of 0.005 and 0.002 f/cc, respectively, with a range of <0.004 to 0.014 f/cc. Other U.S. Studies The only published study of mechanics servicing brakes in the United States other than those conducted by NIOSH was that of Moore in 1998.(16) Asbestos samples were collected in 31 different brake service facilities (Table I). He describes the objective of his study as the collection of personal samples from the breathing zones of automobile mechanics to estimate their exposures while performing actual brake removal and repair work in different facilities with widely diverse work practices. No specific descriptions of the facilities are supplied, nor are TWA data provided for individual mechanics; however, he does indicate that the brake job durations ranged from 10 to 60 min utes, with an average time of 29 minutes. For the 35 samples collected, the mean and median values were <0.03 and <0.02, respectively, with a range of <0.01 to 0.15 f/cc (Table II). German Study Rodelsperger et al.(17) sampled 31 mechanics servicing pas senger car brakes and 36 mechanics servicing truck and bus brakes in West Germany (Table I). Personal samples were col lected generally during the whole service, but, in some cases, samples were limited to specific dust-generating activities. Based on personal samples of 30 minutes or longer, the authors estimated mean concentrations for mechanics performing brake services blowing out drums with compressed air (0.10 f/cc), dry brushing drums (0.09 f/cc), and for combinations of activities involving cleaning out brakes (0.04 f/cc). Brake job TWA con centration data for individual mechanics were not provided. The authors concluded that the mean asbestos concentration during 794 D. J. PAUSTENBACH ET AL. brake service operations was less than 0.1 f/cc for sampling du rations greater than 30 minutes. Swedish Study Plato et al.(18) provide an evaluation based on twenty-three 8-hour TWA personal samples at different car and bus repair shops for the period 1976 to 1988 from a Swedish National Board of Occupational Health database (Table I). Although a table of the measurements is not provided in their article, 20 of these 23 TWA concentrations were estimated from one of the figures presented in their paper. These values ranged from 0.02 to 0.20 f/cc, with a mean of 0.10 and a standard deviation of 0.07 f/cc. Although the values from Plato et al.(18) are for a combination of vehicles, they were included in the analysis for automobiles and light trucks, even though some of the higher values are more likely for heavy truck and bus brake repair work. Plato et al.(18) also provide independent estimates of asbestos concentrations, rather than direct measurements, experienced by car, truck, and bus mechanics servicing brakes between 1938 and 1986, based on the sampling data described above and ap plication of factors accounting for the types of vehicles serviced (car, bus, truck), sizes and ventilation of repair shop rooms, task activities, frequency of work, and whether work as a vehicle mechanic was performed before or after 1975. Based on their model, the authors estimated an 8-hour TWA for all car mechan ics of 0.09 f/cc during the period from 1938 to 1986. Australian Study The most recent investigation of asbestos concentrations in brake service garages was carried out in Australia in the late 1990s by Yeung et al.(19) Personal samples were collected on mechanics in three garages servicing brakes for passenger and light commercial vehicles and one garage servicing bus brakes (Tables I, II, and III). All four of the samples yielded results below the detection limit of 0.05 f/cc. undertaken during brake servicing. The estimated TWA concen trations were calculated using the measured mean and maximum concentrations and the mean durations of numerous operations (although the duration data and calculations are not provided in the article). It was assumed that brake maintenance of four wheels was carried out during the workday and that other infre quent operations, such as the grinding of brake linings, were one hour in duration. Based on these data and assumptions, Kauppinen and Korhonen(34) estimated that the mean asbestos workday concentration for mechanics was under 0.05 f/cc during the repair of car brakes and under 0.1 to 0.2 f/cc during the repair of truck and bus brakes. Because these TWA concentration esti mates were not derived from long-term sample measurements, they are not included in this analysis of TWA concentration data. However, their estimates of the workplace concentrations of as bestos due to brake repair are virtually identical to the results reported in the United States and elsewhere. DATA ANALYSIS A total of 162 TWA asbestos concentrations were found in the published literature for individual mechanics servicing auto mobile and light truck brakes from the late 1960s to the present. An additional 19 TWA concentrations were reported for indi vidual mechanics servicing heavy truck and bus brakes during the same time period. Most of the reported TWA asbestos air concentrations represent brake job TWAs, whereas only a few studies (e.g., Plato et al.(18)) report data that represent 8-hour TWAs. All of the TWA sampling data were compiled from the avail able published studies and entered into a database for analy sis. Additional information was also summarized on factors that might affect airborne asbestos concentrations, such as the year in which samples were collected, the type of cleaning method employed during brake servicing, the type of facility, and the estimated number of brake jobs performed weekly in the garage (level of service) (see Tables I and II). Other Studies Providing TWA Estimates Two other documents provided estimates of 8-hour TWA con centrations for mechanics servicing brakes; however, these es timates are based on data from studies described previously. Nicholson et al.(32) estimated an 8-hour TWA concentration of 0.1 f/cc for mechanics in municipal garages and "an upper limit of the average exposure" of 0.2 f/cc for commercial garage mechanics. These estimates appear to be based largely on the NIOSH data reported by Roberts and Zumwalde.(15) The Fed eral Register(33) reported a mean 8-hour TWA concentration of 0.03 f/cc for mechanics servicing vehicle brakes from 1979 to 1984 and a mean 8-hour TWA concentration of 0.012 f/cc from 1986 to 1989, presumably based on NIOSH data. An additional study, which was conducted in Finland by Kauppinen and Korhonen,(34) provided 8-hour TWA concentra tion estimates for various vehicle mechanics based on short-term samples (generally less than 30 minutes) for specific operations Conversion of Brake Job to 8-Hour TWAs To characterize the historical exposures of brake mechanics in the United States so that one can compare the information with occupational exposure limits, such as OSHA permissible exposure limits (PELs) and American Conference of Govern mental Industrial Hygienists (ACGIH) threshold limit values (TLVs), it was necessary to convert the brake job TWA values to 8-hour TWA concentrations. This approach is a standard and customary practice in industrial hygiene, and in the field of risk assessment, when attempting to characterize the risk of lifetime exposure to chronic toxicants. The 8-hour TWAs were estimated by assuming that mechan ics were exposed at the brake job TWA levels during the period of the day they serviced brakes and at the background levels of the garages during the rest of their 8-hour days. The fraction of the day during which a mechanic was actually doing brake EXPOSURE OF MECHANICS TO ASBESTOS 795 work at each facility was estimated by using the number of brake mechanics at the facility, the number of hours mechanics work per week, the number of brake jobs per week, and the average duration for a brake job at each facility. Daily Fraction jobs per week x duration mechanics x hours per week [1] The brake job TWAs were then converted to 8-hour TWAs using the fractions of day, brake job TWAs, and mean background concentrations for the facilities. 8-Hour TWA = (Brake job TWA x Daily Fraction) + [Background x (1 - Daily Fraction)] [2] The values used were either taken directly from the applicable report or estimated based on available information. The parame ters used and the results for the conversions are shown in Tables II and III. Concentration Distributions and Statistical Comparisons The TWA concentration data were analyzed statistically to characterize and compare asbestos exposure levels for mechan ics performing brake services under various exposure condi tions. The data were characterized based on when the sampling was conducted, where it was done (United States and other coun tries), the types of cleaning methods used, and the types of vehi cles serviced (cars and light trucks or heavy trucks and buses). Statistical comparisons were made to determine whether differ ences existed between exposure levels as a result of (1) the time of sampling (1970s vs. 1980s), (2) the location (United States vs. Europe), and (3) the methods of cleaning brakes. Statistical comparisons were conducted where possible, given the limitations of the data to assess how locations and cleaning methods may have affected asbestos concentrations. To eliminate the impact of the amount of brake work done at a site, the comparisons were made only for those facilities with the same level of service. Low was assigned to shops with fewer than 10 brake jobs per week, medium for 11 to 20, and high for more than 20 brake jobs per week. Nonparametric methods (Kruskal-Wallis and Wilcoxon ranksum tests) were used for comparisons, because the concentra tion data could not satisfy the homogeneity of variance and normality assumptions underlying analysis of variance meth ods. Significance was determined after correcting individual test p-values for the multiple comparisons that were made; specif ically, individual test p-value was multiplied by the number of tests. Differences among locations were assessed for early U.S. (1970s), late U.S. (1980s), and Swedish data, only because these data sets all have sufficient measurements for individual me chanics to support such an analysis (42, 42, and 20, respec tively). Assessment of cleaning methods was conducted within a location and service level to avoid confounding the interpreta tion of cleaning method influence with other factors. Cleaning methods that were very similar, such as compressed air only and compressed air with a dry brush, were combined to cre ate cleaning method categories for evaluating differences. Early U.S. and late U.S. studies both reported multiple concentrations for multiple cleaning method categories. Early U.S. cleaning method categories included compressed air with and without dry brush, wet brush, and vacuum-HEPA for low-service-level shops, and compressed air with and without dry brush, com pressed air-solvent, and dry brush for high-service-level shops. Late U.S. low-service-level shops were assumed to use the fol lowing cleaning method categories: aerosol spray, vacuumHEPA, wet brush-recycle, vacuum enclosure, wet brush, wa ter hose-squirt bottle, and no cleaning. RESULTS The TWA concentrations measured for automobile and light truck mechanics, and heavy truck and bus mechanics, are de scribed separately. Differences in asbestos concentrations re lated to different brake cleaning methods are evaluated. Com parison of the asbestos concentrations experienced by U.S. and European mechanics repairing automobile brakes during the same time period are made, and changes in exposure concentra tions over time from the 1970s to 1990s are evaluated. Automobiles and Light Truck Mechanic Exposures Automobiles and light trucks were evaluated together, be cause the tasks involved in servicing the brakes and the dura tions of the brake servicing were similar. The means and ranges for the nine studies that provided data on the airborne concen trations during the actual brake job (rather than 8-hour TWA samples) are presented in Figure 1. The mean brake job TWA for the 141 automobile mechanics evaluated between 1968 and 1996 is 0.05 f/cc, with a range of 0.004 to 0.28 f/cc. It is notable that brake job TWA concentrations for the more recent surveys in the late 1980s and 1990s are generally much lower than those reported for mechanics working in the 1970s and early 1980s. These lower concentrations are not surprising, given the greater awareness and concern about exposure to airborne dusts of any type by most workers. The estimated 8-hour TWA concentrations for the above nine studies, and the measured 8-hour TWA concentrations from Hickish and Knight(9) and Plato et al.(18) for automobile and light truck mechanics, are presented in Figure 2. The mean 8-hour TWA concentration based on 162 values is 0.04 f/cc. The maximum reported 8-hour TWA was 0.68 f/cc from Hickish and Knight,(9) with the next highest reported value being 0.20 f/cc. The relatively high value from Hickish and Knight is for a me chanic who cleaned the brakes of 11 cars in one shift, which was an unusually high number of vehicles serviced in a day (two to three was more typical). 796 D. J. PAUSTENBACH ET AL. FIGURE 1 Brake job time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on automobiles and light trucks. Heavy Truck and Bus Mechanic Exposures Fewer TWA concentration measurements are available for mechanics servicing heavy truck and bus brakes than for those servicing automobile and light truck brakes. The means and ranges for the four studies providing brake job TWAs for heavy trucks and buses are presented in Figure 3. The mean brake job TWA concentration for the 18 mechanics evaluated is 0.21 f/cc, with a maximum of 1.68 f/cc. The estimated 8-hour TWA concentrations for the above four studies, and the measured 8-hour TWA from Hickish and Knight(9) for heavy truck and bus mechanics, are presented in Figure 4. The mean 8-hour TWA concentration, based on 19 data points, is 0.25 f/cc. The maximum reported TWA was 1.75 f/cc from Hickish and Knight,(9) with the next highest value being 0.87 f/cc. As for the automobile and light truck mechanics, the brake job and 8-hour TWA concentrations for truck and bus mechanics were generally lower in surveys from the late 1980s and 1990s than from surveys for earlier periods. As shown in Figure 5, the 8-hour TWA concentrations for mechanics servicing heavy truck and bus brakes are significantly greater than those for mechanics servicing automobile and light truck brakes. This difference is likely due to several factors. The brakes on trucks and buses are larger and require more time to service. In addition, truck and bus mechanics generally replace just the brake linings, which often requires riveting and machining. Because auto mechanics normally replace the entire brake shoe--not just the lining-- these dust-generating operations are not usually performed by auto mechanics. Eight-Hour TWA Concentration Distributions It is apparent that the data from the NIOSH surveys provide the most complete characterization for estimating the 8-hour TWA concentrations experienced by brake mechanics in the United States in the 1970s and late 1980s. The cumulative distribution plots of the NIOSH survey data (Figure 5) show that 8-hour TWA concentrations for automobile mechanics in the 1970s were generally within a narrow range, with a 50th percentile value of 0.07 f/cc and a 90th percentile of 0.10 f/cc. EXPOSURE OF MECHANICS TO ASBESTOS 797 FIGURE 2 Eight-hour time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on automobiles and light trucks. (That is, it can be expected that at least 90% of the mechan ics were exposed to concentrations below 0.1 f/cc during that 20-year period.) The cumulative distribution plot for the later NIOSH survey in the 1980s shows a substantially lower and narrower 8-hour TWA concentration distribution, with both the 50th and 90th percentiles at 0.002 f/cc. The plot for heavy truck and bus brake mechanics indicates that the 50th percentile value for this group (0.08 f/cc) was similar to that for automobile brake mechanics from the 1970s NIOSH studies but that the 90th percentile values for truck and bus mechanics (1.65 f/cc) was substantially higher than that for automobile mechanics. centrations, respectively). The distributions for the three data sets are shown in Figure 5. A statistical comparison of 8-hour TWA concentrations derived from the NIOSH studies of United States mechanics from 1976 to 1982 and the 8-hour TWAs re ported for Swedish mechanics from 1976 to 1988 found no sig nificant difference in mean concentrations (see Table IV). U.S. mechanics evaluated in the late 1980s by NIOSH were exposed to significantly lower airborne concentrations of asbestos than either the early U.S. or Swedish mechanics. This difference is likely due to the improved cleaning methods discussed below. Comparison of U.S. and European Brake Mechanic Exposures Differences among locations were assessed using the early U.S. (1970s), late U.S. (1980s), and Swedish data sets, because they were the only ones that had sufficient numbers of individ ual samples for statistical analysis (42, 42, and 20 TWA con TWA Concentrations Relative to Cleaning Method As early as 1970, studies or surveys of the concentration of airborne asbestos for vehicle mechanics servicing brakes noted that the cleaning process, particularly compressed air blow-out ofdrum brakes, was the major contributor to airborne fiber levels. The mean and ranges for brake job TWAs using the various cleaning methods are presented in Figure 6. The distributions of 798 D. J. PAUSTENBACH ET AL. FIGURE 3 Brake job time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on heavy trucks and buses. FIGURE 4 Eight-hour brake job time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on heavy trucks and buses. EXPOSURE OF MECHANICS TO ASBESTOS 799 8-HOUR TWA ASBESTOS CONCENTRATION (f/cc) Note: Grapn presents data from studies tor which there are measerenacts repo-tec 'o ndi victual mechanics. Data Iron Moore (1988) arid Rodeisperger et at. (1988; are excluded because only group averages were presented. Data `mm Hiekish and Knight {1970} ar?u Yeung el ai. (1999) did no! provide sulioeni data <oi i\ sepal ate analysis'. '" FIGURE 5 Cumulative distributions of 8-hour time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on automobiles, trucks, and buses. the most frequently used methods are shown on Figure 7. With the exception of a few TWA concentrations, most of the data are from NIOSH studies. It is evident from the plots and the statistical test results summarized in Table IV that the introduction of dust control technologies significantly reduced both the magnitude and vari ability of asbestos exposures for brake mechanics. As shown in Figure 7, the 50th and 90th percentile brake job TWA concentra tions for mechanics using compressed air for cleaning during the 1970s and early 1980s were approximately 0.10 and 0.18 f/cc, respectively. The use of solvents with compressed air in the early 1980s reduced the 50th and 90th percentiles to about 0.03 and 0.08 f/cc, respectively. The use of wet brush/recycle, HEPA vac uum, and vacuum enclosure reduced most asbestos exposures to less than 0.01 f/cc--levels at or below the detection limit for NIOSH sampling. The use of these dust control technologies served several purposes. They dramatically reduced the peak exposures during cleaning, they prevented the distribution of airborne dust into other areas of the work environment, and they kept brake dust from being deposited on the floor of the work area, where it could subsequently be resuspended by foot traffic or sweeping. The 50th and 90th percentile brake job TWA concentrations for mechanics using compressed air for cleaning are similar to those for U.S. mechanics as a group in the 1970s, likely because compressed air was the method most frequently used for brake cleaning during that period. Changes in Exposures Over Time The decrease in 8-hour and brake job TWA asbestos concen trations in the late 1980s is evidenced by the distributions of early and late NIOSH values for auto brake mechanics shown in Figure 5. This statistical comparison is summarized in Table IV. Based on the available data, it is clear that much of the decrease is due to dust control measures that were first introduced in the late 1970s. However, the introduction of disc brakes during this period would also have led to decreased exposures for mechan ics. During the period of nearly all these studies, disc brakes were placed predominantly only on the front wheels because of problems in using disc brakes in conjunction with the rear parking brake system. Because disc brakes have many fewer places where dust can collect than do drum brakes, there would have been much less brake dust available for suspension into the 800 D. J. PAUSTENBACH ET AL. Results of statistical comparisons of time-weighted averages (TWAs) by time and location and cleaning method and level of service Type Cleaning method Service Number of Standard level results Mean deviation Test conclusions Time and location (8-hour TWAs) Early United StatesA NA Late United StatesB NA NA NA SwedenC NA NA Cleaning methods and level of service (brake job TWAs) Early United StatesA Com air w/ & w/o dry Low brush Wet brush Vacuum & HEPA Early United StatesA Compresed air w/ & w/o Dry brush High Compressed air w/solvent Dry brush Late United StatesB Aerosol spray Low Vacuum & HEPA Wet brush/rec Vacuum enclosure No cleaning Wet brush Water hose & squirt bottle 42 0.063 0.032 42 0.0021 0.00036 Significantly different from early U.S., Sweden (w/multiple test correction) 20 0.10 0.067 4 0.028 0.0050 Significantly different from wet brush (w/o multiple test correction) 3 0.25 0.026 Significantly different from com air w/ & w/o dry brush (w/o multiple test correction) 1 0.010 -- 22 0.10 0.053 Significantly different from com air w/solvent, dry brush (w/o multiple test correction) 4 0.049 0.033 Significantly different from com air w/ & w/o dry brush (w/o multiple test correction) 2 0.20 0.0071 Significantly different from com air w/ & w/o dry brush (w/o multiple test correction) 3 0.0080 0.0053 Significantly different from wet brush/rec, vac encl (w/o multiple test correction) 7 0.0069 0.0035 Significantly different from wet brush/rec, vac encl (w/o multiple test correction) 10 0.0041 0.000316 Significantly different from aerosol spray, vac & HEPA (w/o multiple test correction) 8 0.0040 0 Significantly different from aerosol spray, vac & HEPA (w/o multiple test correction) 1 0.0080 -- 1 0.0040 -- 1 0.0070 -- Note: Significant differences determined by overall type-1 error rate of 0.05. AEarly United States are samples from the 1970s by NIOSH. BLate United States are samples from 1980s by NiOsH. CSweden are samples from 1980s by Plato et al. air during brake servicing operations. In addition, non-asbestos brake pad materials began to be used in disc brakes in the 1980s. Although there are no published data on the exposure of ve hicle mechanics to asbestos during brake repair activities prior to 1968, it is possible to speculate on earlier exposure lev els for automobile brake mechanics. With the introduction of bonded brakes in 1948, the need for drilling, facing, and grinding was significantly reduced, and with factory-supplied, tapered, bonded brake replacements available in the 1950s, the need for beveling was also greatly reduced.(1) By the early 1960s, the EXPOSURE OF MECHANICS TO ASBESTOS 801 FIGURE 6 Brake job time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on automobiles and light trucks using different cleaning methods. methods for performing brake jobs on autos were similar to those used in the 1970s--cleaning brake assemblies with com pressed air or a wire brush and replacing the entire brake shoes with factory supplied, tapered, bonded shoes. Thus, it is quite likely that the exposure levels experienced in the late 1950s and the 1960s were similar to those in the 1970s. DISCUSSION This evaluation of TWA airborne concentrations of asbestos due to brake dust experienced by mechanics presents a compre hensive picture of exposure levels for brake repair work over the past 30 years. The mean concentration identified (e.g., about 0.04 f/cc) appears to be consistent with those reported in other evaluations of mechanic exposures and are substantially lower than levels reported for workers in other occupations handling asbestos-containing products. Similarities to Other Studies This analysis found that the 8-hour TWA concentrations for brake mechanics obtained or derived from the early NIOSH sur vey data(15) were similar to those estimated by Plato et al.(18) for Swedish automobile mechanics in the 1970s (the mean for both was about 0.04 to 0.09 f/cc). Likewise, data from the United States (e.g., NIOSH) were similar to those estimated for Finnish mechanics by Kauppinen and Korhonen(34) using short-term concentration data (mean of about 0.05 f/cc). The consistency among these independent 8-hour TWA airborne concentrations for asbestos due to brake dust indicates substantial similarities in brake repair operations and the asbestos content in brake linings in various countries during the 1970s and 1980s. The similarity in results by so many different researchers, in such different geographic locations, gives significant confidence in the conclusions. Comparison to Regulatory Standards The U.S. regulatory standard for occupational exposure to asbestos has changed numerous times over the last 30 years.(35) In 1971, OSHA set a PEL for asbestos in workplace air of 12 f/cc as an 8-hour TWA, which was based on the TLV established by ACGIH in 1968. Since then, the PEL has become more strict over time. Specifically, in December 1971, OSHA issued an emergency temporary standard that lowered the asbestos PEL to 5 f/cc, and in June 1972, OSHA officially lowered the PEL standard to 5 f/cc for 1972 and 2 f/cc commencing in July 1976. In 1986, the PEL was lowered to 0.2 f/cc, and in 1994, it was again lowered to its present value of 0.1 f/cc. Comparisons of the measured asbestos airborne concentrations in garages to the OSHA PEL at the corresponding times indicate that both the 802 D. J. PAUSTENBACH ET AL. BRAKE-JOB TWA ASBESTOS CONCENTRATION (fee) Note: QfcUjf; presents data from studies for which there a<s rneasmemeMs repot tea `or ind-v aoa' mechanics and a reported cleaning method Clear mg motrods with fowo' lira" five data po Ms are not shown FIGURE 7 Cumulative distributions of brake job time-weighted average (TWA) asbestos concentrations for mechanics servicing brakes on automobiles and light trucks using different cleaning methods. brake job and 8-hour TWA concentrations of asbestos for brake mechanics were consistently below the applicable federal stan dard. This observation is further confirmed by the lack of OSHA citations of garages for exceeding the PEL for asbestos from 1972-2002. Comparison to Other Occupational Exposures To better understand the relative magnitude of brake me chanic exposures, one can evaluate other occupations that han dled raw asbestos and used it in the manufacture of other goods. From 1964 to 1985, for example, the U.S. Public Health Ser vice and NIOSH conducted seven studies at brake manufactur ing facilities.(36-41) During the 1960s, the mean asbestos con centration for 381 samples collected during three studies was 6.0 f/cc, with a range of 0 to 130 f/cc. In the 1970s, the mean value for 269 samples from two facilities was 1.5 f/cc, with a range of 0 to 17.5 f/cc. In the 1980s, the mean value for 49 samples from two facilities was 0.38 f/cc, with a range of 0.01 to 1.1 f/cc. By comparison, the mean brake job TWA for au tomobile brake mechanics was 0.11 f/cc during the 1970s and 0.005 f/cc during the 1980s. Thus, at comparable times, auto mobile brake mechanics had asbestos exposures that were 10 to 100 times less than for those persons who manufactured the brake linings. Characterizing Variability As with any analysis of multiple studies, there were some limitations in the available data for estimating the mean 8-hour TWA concentrations to which brake mechanics were exposed over time. For example, nearly all of the sampling data collected in the field on brake mechanics involved 1- to 2-hour samples; thus, the data needed to be corrected to predict the typical 8-hour TWA airborne concentration to which mechanics were exposed. Few 8-hour TWA samples were collected on brake mechanics, because the industrial hygienists apparently concluded that if the results of a shorter sampling period (e.g., during a brake job) did not exceed the PEL, then surely the 8-hour TWA would be less than the PEL, given that the remainder of a mechanic's day would have been spent away from brake repair activities. Also, because exposures were sporadic and of short duration, they could be estimated by conducting a time/motion analysis, which accounted for the actual time during which exposures occurred. This analysis was limited by the lack of certain information in the reports regarding the various tasks performed by mechanics EXPOSURE OF MECHANICS TO ASBESTOS 803 during the sampling period (as well as the entire workday). Ex cept for several of the NIOSH surveys, little information was provided on the number of brake jobs performed per day by mechanics, or the amount of time spent each day actually per forming brake repair work. None of the long-term studies pro vided specific mechanic time/motion data for activities, which would have enabled better comparisons of the data. Several of the original authors of these studies were contacted in an attempt to gain a better understanding of what was actually done and for how long. They provided an adequate verbal account of what they did, but either the original notes describing their work had been discarded, or much of the desired information was never recorded. Though there are some limitations associated with these data, compared to the majority of occupations that were surveyed for asbestos concentrations prior to 1980, the accumulated data are quite good. For example, almost 200 measurements were iden tified and included in this analysis. The data were sufficient to compare the ranges of exposure for brake mechanics working on different types of vehicles, in different countries, during dif ferent time periods, and using different cleaning methods. The brake job TWA data set establishes the upper bounds of the as bestos concentrations to which mechanics could be chronically exposed if they performed brake jobs 40 hours per week for many years. Because the process of changing drum brakes and the asbestos content of the linings did not change appreciably from about 1940 to 1975, these data should be applicable for the entire 35 years during which drums were the primary braking mechanism used on automobiles in the U.S. Airborne concen trations of asbestos for auto mechanics would have decreased substantially only over the past 20 years due to the fewer as bestos brake linings on cars (conversion to nonasbestos linings), the conversion to disc brakes, and better techniques for control ling exposure to brake dust in repair shops. CONCLUSIONS Based on this review and analysis of the available data on the concentrations of airborne asbestos for vehicle mechanics servicing brakes, several conclusions were reached: TWA concentration data from the 10 published studies are sufficient to characterize exposures of vehicle brake mechanics from the 1970s to the present with a high degree of confidence. The 8-hour TWA concentrations for auto brake me chanics in other industrialized countries studied were similar during the same time periods for the years 1970 to 1990. As dust control methods were implemented, the 8-hour TWA concentrations dropped by at least an order of magnitude from 1970 to 1990. Airborne concentrations of asbestos during the process of changing brake linings (1- to 2-hour samples), as well as the 8-hour TWA concentrations, were consis- tently below the applicable OSHA occupational stan dard for the 1970s, 1980s, and 1990s. Because both the cleaning and brake replacement meth ods were similar after the introduction of tapered bonded brakes, it is quite likely that asbestos exposure levels in the late 1950s and the 1960s were similar to those in the 1970s for auto brake mechanics. For comparable time periods, asbestos TWA concen trations for vehicle brake mechanics were 10 to 100 times lower than those experienced by friction product manufacturing workers. ACKNOWLEDGMENTS This work was supported, in part, by funding from Daimler Chrysler, Ford Motor Company, and General Motors Corpora tion. They have been and are involved in litigation related to the exposure of mechanics to brake dust. REFERENCES 1. Sheehy, J.W.; Cooper, T.C.; O'Brien, D.M.; et al.: Control of As bestos Exposure During Brake Drum Service. National Institute for Occupational Safety and Health, Cincinnati, OH (1989). 2. Thomson, J.G.; Kaschula, R.O.C.; McDonald, R.R.: Asbestos as a Modern Urban Hazard. S Afr Med J 37:77 (1963). 3. 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