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FILE NAME: Phenolic Resins (PHR) DATE: 1976 DOC#: PHR001 DOCUMENT DESCRIPTION: Journal Article - Asbestos Exposure during Brake Lining Maintenance & Repair EN V IR O N M EN TA L RESEA RC H 12, 110~ 128 ( 1976) Reprinted from Environmental Research 12, 110-128 (1976) Copyright 1976 Academic Press, Inc. Printed in U.S.A. Asbestos Exposure during Brake Lining Maintenance and Repair1 Arthur N. Rohl, A rthur M. L anger, Mary S. Wolff, and Irving Weisman Environmental Sciences Laboratory, Mount Sinai School o f Medicine o f the City University o f New York, New York, New York 10029 Received December 10, 1975 Data obtained on asbestos exposure of garage mechanics during brake lining maintenance and repair work show that fiber concentrations frequently in excess of regulated limits are common. The presence of chrysotile, ranging from 2 to 15%, in brake drum dusts, was demonstrated by X-ray diffraction, transmission electron microscopy, selected area electron diffraction, and electron microprobe analyses. Unaltered chrysotile was found, both in fiber and fibril form, in air and brake drum dust samples. The chrysotile asbestos content of personal air samples, taken during automobile brake repair work, was measured both by optical and electron microscopic techniques. While a positive correlation exists between the types of measurements, the present technique of optically counting asbestos fibers may considerably underestimate the levels of total asbestos exposure. INTRODUCTION During the past decade, significant disease risk has been found associated with the inhalation of asbestos fibers in a number of occupational and environmental circumstances other than in asbestos mining, milling and manufacturing, where serious hazard was already known (Wagner et al., 1960; Newhouse and Thompson, 1965; Selikoff et al., 1964, 1965; Harries, 1968). Such exposures were found in the construction industry and in shipbuilding, as well as in other industrial settings where asbestos products were used. More recently, asbestos exposure has been suggested to occur during automotive brake lining repair and installation work, and measurable concentrations of asbestos fiber were observed in the work environment of workmen involved in these opera tions (Hickish and Knight, 1970; Hatch, 1970; Boillat and Lob, 1973). With limited data available, however, uncertainty remained regarding the type and extent of asbestos exposure during this work. Some investigators have questioned whether free asbestos fibers survive the high temperatures produced during braking action (Lynch, 1968; Hickish and Knight, 1970; Hatch, 1970) contending that asbestos decomposes as a result of the high point contact temperatures produced at the interface of the brake drum or disc and brake lining. We have sought to obtain information concerning asbestos exposure of work men engaged in brake lining maintenance and brake shoe installation, by analysis of residual dusts recovered from brake linings and by direct measurement of the 1This research was supported by Center Grant ES 00928 of the National Institute of Environmental Health Sciences of the U.S. Department of Health, Education and Welfare. Assistance was also provided in part by the Health Research Council of the City of New York HRC U 2329 and by the Ford Motor Company. 110 Copyright 1976 by Academic Press, Inc. At! rights o f reproduction in any form reserved. asbestos exposure 111 free asbestos fiber content of workroom air in areas where M c t v M b m take ,,lace In the United States, an estimated work force of at least 900,000 auto mechanics and garage workers is potentially exposed to asbestos in the servicing of both brake and clutch linings. Furthermore, much brake environment during automobile use (Jacko and DuCharme, 1973), to add more to the burden of asbestos air pollution (Selikoff, Nicholson, and Langer, 1972). Asbestos in Friction Materials . In the United States, an estimated 118 million pounds of asbestos is used annu ally for the production of brake friction materials (Jacko and DuCharme, 1973). After processing (cutting, grinding, punching), the asbestos m the approximately 103 million pounds per year. In addition, asbestos ^ntarne automotive clutch friction materials amounts to 4.5 million pounds annually. Major Constituents o f Brake Linings A number of materials is commonly used in the manufacture of the three major automotive brake lining components (binder, fiber reinforcer, and property m fieri These are listed in Table 1. , . Binder. The binders used in the automotive industry today are primarily phenolic-type resins, which are noted for high binding efficiency and ability to withstand pyrolytic breakdown. Other materials have been used in varying pro portions and in addition to resins, for binder improvement (Table 1). TABLE 1 Common C om ponents of A utom otive B rake L inings" Binder and organic friction modifiers Phenolic-type resin Rubber Tire scrap Pitch Cork Gilsonite Cashew nutshell resin and particles Drying oils Fiber reinforcer Chrysotile asbestos* (grades 4-7) Unaltered Calcined Mixed fiber Property modifier Lead compounds Zinc compounds Antimony oxide Iron oxide Copper metal Brass chips Clay minerals Barite (BaS04) Wollastonite (CaSi03) Quartz (Si02) Cryolite (NagAlFj) Rottenstone (Si02) Coke (C) Coal (C) Gilsonite (C) Graphite (Q Carbon black (C) Molybdenum sulfide (MoS2) Fluorspar (CaF2) " See Carroll, 1962; Anderson, 1969; Anderson, 1973; Jacko and DuCharme, 1973; Bark et at.. 1975. Chrysotile fiber constitutes about 50% by weight of most automotive brakes currently manufac- tured in the United States. 112 ROHL ET AL. Fiber. For fiber reinforcement of the friction product, chrysotile asbestos is used almost exclusively. The mineral typically comprises from 40 to 50% of the brake product. Fiber grades 4 through 7 are used, and occasionally, several sizes are admixed or even calcined to improve performance characteristics. Modifiers. Perhaps the widest range of materials used in friction products are the property modifiers. Nineteen representative compounds are listed in Table 1. Modifiers are used for a number of purposes; they are included to increase brake shoe " density," making the brake surface able to withstand high pressures (e.g., barite); they are included as " lubricants" to reduce the coefficient of friction along the brake surface, and thereby prevent " grabbing" (e.g., lead compounds); they act as " friction agents" increasing the coefficient of friction and enhancing the braking action of the shoe (e.g., brass chips); they act as internal abrasives, which help to " recondition" the braking surface and remove deposited decompos ition products (e.g., rottenstone, quartz); they act as heat sinks, reducing bin der pyrolysis and fiber decomposition thereby extending the useful life of the lining (e.g., brass chips, metals, etc.). It is important to note that one major purpose of the reconditioning agents is to retard the formation of forsterite (a mineral not originally present in the brake material, but created by dehydroxylation and recrystallization of chrysotile asbes tos at high temperatures) which may accumulate on the surface of the brake lining. The hardness of the forsterite (hardness 6.5-7.0) is such that it tends to score and gouge brake drums and discs (hardness 3-3.5), degrading them prematurely. Therefore, recrystallization of chrysotile to forsterite is an unwanted effect, hin dered insofar as possible by the modifiers present in the matrix. Materials o f Biological Interest _ Asbestos, quartz, and heavy metals are constituents of automotive brake lin ings, each warranting special consideration from the viewpoint of biological activ ity. The focus of this report is limited to the problem of chrysotile asbestos exposure. Mechanisms o f Degradation o f Brake Linings during Use Brake wear is dependent upon many factors, such as the temperature generated at th surface of the brake shoe during braking operations. At any one time, only a small percentage of the rubbing area is in contact with the wheel, with " hot spots generated, ranging up to 800 to 1000C (Carroll, 1962; Anderson, 1969). It is not uncommon during moderate braking action, to attain temperatures as high as 500C (Carroll, 1962). Some investigators have suggested that, in addition to bin der pyrolysis, chrysotile completely dehydroxylates under these conditions and " reduces to powder" where it is swept off the brake facing (Carroll, 1962). How ever, this hypothesis is oversimplified, in that other important processes, besides thermal wear, contribute to shoe breakdown, and brake shoe degradation. (Bur well, 1957). For example, the effects of abrasive wear and macroshear have been investigated. When monitored by X-ray diffraction, chrysotile in brake materials displays structural strain and substructure fragmentation, caused by shear during braking processes (Mizutani et al., 1973). This shear strain produces material fatigue which, with binder pyrolysis, can cause brake lining disintegration at 113 asbestos exposure brake lining disintegration may liberate partially altered, or fibers. Thermal Decomposition f Chryso*Ue chrvSotile undergoes dehydroxyla- Differential thermal analysis mdieates th hiT | to forsterite) tio n at650to680Candrecrystallizes(anhydrousn^gnesmms ^ , 971; (Mg,SlO.) al bou. 8W to 821TC ranges ^ sntgect to great variations, sample packing, etc. s ' s rs temperatures (Bates and Comer, prolonged static heating, at and Dresher, 1966). 1957; Martinez, 1966; Bond ey an ' reqUired for dehydroxylation and In general, temperatnres tn excess of 5W C m e w m a i ^ ^ ^ S TM rtfT h ^ " nt,:nake Uning composition rTM. design indicates that chrysotile fiber may survive in the decomposed lining dust. methods A m ly m o f Brake Drum Duet Ten samples of antonH .bde^edrnrndns ,, f c a e d and exanrined by dectro,, microScopy and *^ 2 ^ ssa trs & " ,th ir = -- ^ f " E * S S S m- r ^ 'z - r J r ro^ ; e p a r a . i o n s , readiiy technique. Identification o f Chrysotile by Electron Microscopy- i __a iffro rtin n an d elec- Teoncmkginn electron microscopy, selected area i We acknowledge he cooperation of the United the time. sz* TM **" * * " under way at 114 ROHL ET AL. iron microprobe analysis of the brake dusts were carried out on each of the ten samples after preparation by a technique which disperses the dust particles in a nitrocellulose film without altering particle size distribution. Free chrysotile fiber bundles and fibrils were observed in all ten samples (Fig. 1). Selected area elec tron diffraction analysis of representative fibers demonstrated the preservation of the chrysotile structure (Figs. 2A, B). Some patterns displayed arcuate reflections suggestive of interfibril rotation and intrafibril displacement (Figs. 2A, B). Occa sionally, fibers were observed without characteristic chrysotile morphology, with mottled surfaces and obliterated fibrils, indicating partial or complete recrystalli zation. Electron diffraction patterns obtained from these particles displayed Fig. 1. Electronphotomicrograph of large chrysotile bundle in brake drum dust (38,000 x magnifica tion). Other particles include phenol resin binder and road dust debris. ASBESTOS EXPOSURE 115 Fig 2. Selected area electron diffraction patterns obtained on fibers of chrysolite obtained dunng iur sampling at brake repair shops. In A, the reciprocal a axis is marked a* as are the layer lines in the (Okl) series. Indexing of upper right quadrant yielded 16 reflections corresponding to sin^e crystal X-ray diffraction amdysis of Whittaker and Zussman, 1956. Pattern in(B) displays " smearing ofreflections m a " clockwise" manner suggesting interplanar rotation. polycrystalline characteristics of multiple random reflections or Debye-Scherrer rings rather than the distinctive single fiber chrysotile pattern (Fig. 2B). Mtcrochemical analysis with a probe technique on the unaltered fibers showed them to possess the usual Mg:Si ratio of chrysotile. In addition to free chrysotile fiber bundles and fibrils, chrysotile was also frequently observed projecting from the margins of binder fragments (Fig. 3). Free asbestos fibers present in the decomposed lining dusts were sized at 42,000x magnification. The results, seen in Table 2, show that most fibers are too small to be seen by optical microscopy; almost all of them are shorter than 0.4 /tm in length; virtually all are of respirable size (-5 fim). Hatch (1970) in reporting on optical fiber counts obtained from brake cleaning operations with compressed air jet found that 94% of the fibers fell in the 2-5 *tm length category, while only 6% were longer than 5 fim. Jacko and DuChanne (1973) made size distribution meas urements of asbestos fibers in brake dusts generated during dynomometer tests, using both optical and electron microscopy. They found, at magnifications ot 22,000x that 30% of the fibers were from 0.25 to 0.50 /m in length and that 60% were longer than 0.5 p,m. Some discrepancies between our data and those fJack and DuCharme may be attributed to their use of the lower magnification (22,000x vs 42,000x), at which fibers shorter than 0.20 pm may not be easily seen or identified on the electron microscopic screen. Thus, both the optical fiber count data in other studies and the electron microscopic fiber size distribution data 116 ROHL ETAL. Fig. 3. Electron photomicrographs of brake drum dusts. Chrysotile is present in both free fiber and fibril form. Opaque granular material is road dust or phenolic binder, a. x 10,800; b. X9300; c. x 30,000; d. x 30,000. indicate that the chrysotile fiber population generated by brake wear is a strongly skewed one, with almost all fibers concentrated in the smaller than 5 /urn region. No attempt was made to size the asbestos-binder particulates. Personal Air Sampling during Brake Repair Work Personal air sampling for asbestos exposure during brake lining maintenance and repair was carried out at franchised auto dealer garages, taxi fleet repair shops, and a municipal truck repair shop, all located in New York City.3Personal 'Assistance in providing opportunity for sampling was given by the Department of Air Resources, New York City. ASBESTOS EXPOSURE 2 e F ibers in B rake D rum D ust0 Sample 1 2 3 4 5 6 7 8 9 10 750- 1500 (%) 40 32 20 26 57 23 50 29 6 11 1500-2250 (%) 34 23 25 37 17 9 26 30 41 6 2250- 3000A (%) 11 32 25 26 4 12 21 21 18 31 3000- 3750A <%) 11 -- ___ 7 ___ 12 2 17 10 31 Fibers counted and sized at 42,000x; all fibers have diameters from 250 to 500A. 117 Total <%) % 87 70 % 78 56 99 97 75 79 air samples were taken during and after brake repair work and at varying distances to n . the work sites in other areas of the garages and shops. The latter samples were intended to provide information concerning levels of asbestos exposure which garage employees other than those doing brake work might experience. Asbestos Exposure during Automobile Brake Repair Work . Air samples were first taken in the breathing zone of mechanics doing brake repair work. These peak exposure measurements were taken over periods of3-8 minutes during which the workers were blowing dust from brake drums. The a samples, taken on membrane filters, were processed, and fiber counts made m accordance with the procedures which have been adopted by the Occupation^ Safety and Health Administration (OSHA) of the U.S. Department of Labor (Bayer, Brown, and Zumwalde, 1975). Essentially, the analysis consists of count ing fibers 5 to 100 pm , in a fixed area of a Porton graticule, using phase contrast microscopy at a magnification of 400x. This microscopic method enhances image contrast and allows' large asbestos fibers to be readily seen and counted. When a vehicle is brought into a repair shop for brake lining inspection or replacement, the wheel is removed and loose dust is removed from the drums and back plates, generally by means of a compressed air jet. A recent survey of brake repair establishments in Baltimore and Washington revealed that fins is the stand ard method in those cities (Castleman et al., 1975). A similar- situation exists m New York City. The cloud of dust that is produced is visible for several minutes afterwards (Fig. 4). Table 3 shows that fiber concentrations are high in the operator's area under these conditions (an average concentration of 16 fibers/ml), and that there are significant concentrations at least 20 ft away. Background or area sampling during the same operation shows that, at least 14 minutes after jet air blowing and up to 75 ft away, asbestos concentrations are still measurable even by optical microscopy. The data in Table 3 indicate that an asbestos concentration gradient, dependent on distance and time, is associated with this operation. It is evident that any person 65-75 ft away can be exposed. Current (interim) regula tions of OSHA prohibit concentrations of 5 fibers/ml or more, longer than 5 pm , as 118 rohl et al. a urne illegal aftef^976.aR e ^ a tio n r^ ^ a r ^ fc o n c nCt ntr^t,0nS ab Ve 2 fibers/ml wiu be fibers/ml of air. Newly proposed (max,mum excursion) of 10 fibers/ml (500,000 f i b e ' l / m ^ h a^ m l L T d" * ied t0 Set a ,imit * 0 J It was generally found that th Xln?um excursion o f 5 fibers/ml. most garages. Woricmen do not u s i r C i r a t o r ^ t e c T ' l C ntT l dust in ness of the potential hazard of brake dust ^ 'On. There was little aware- r 35 do" a * At Z * Z sZ T ' ns Tn1 B,ake* " * *<**. Sanitation truck repair sfaTp^whS; kind's o n , ^ * Y * Depar,ment of work are performed. Used truck brake linim f brake aPPlcation and repair to remove grease and dirt. edges of new linings are beveled on a g r i S ^ S * * * * * the " * * l hreak' in- The <Fig-5>-" * " -- < 2 2 b* S ? S ^ 5 S ASBESTOS EXPOSURE 119 TABLE 3 A sb esto s C o n c e n t r a t io n s d u r in g A u t o m o b il e B rake S er v ic e 0,0 Operation Blowing dust out of brake drums with compressed air jet Background samples taken at varying distance and lapsed times, after brake drum blowing Distance (ft) Number of samples 3-5 4 5-10 3 10-20 2 Distance from operation (ft) Time lapse (min) 10 0 20 0 12 5 50 5 65 7 75 14 Fiber concentration (fibers/ml) Mean Range 16.0 6.6-29.8 3.3 2.0--4.2 2.6 0.4--4.8 Concentration (fibers/ml) 0.3 0.8 0.2 0.1 0.1 0.1 Distance Number of Fiber concentration (fibers/ml) (ft) samples Mean Range Cleaning brake drums with dry brush 1-3 Background samples taken 3 minutes after cleaning brake drums with dry brush 12 2 2.5 1.3--3.6 3 0.1 0 1 NJ * Fibers 5--100 fim in length, counted by optical microscopy. The new proposed Asbestos Standard of the U. S. Department of Labor records asbestos exposure in fibers/m3, noting that a workman might respire approximately 8 m3 of air per working day, retain ing an unstudied proportion of inhaled fibers. The above table omits reference to air content of fibers < 5 pm in length. is then riveted onto a steel plate. Some of these operations are similar to those done during the manufacture of brake shoes. Table 4 summarizes the results of personal air sampling in the course of this work. During light grinding of truck brake shoes (Fig. 6), an average peak concentration of about 4 fibers/ml was found in the breathing zone of the operator. The data show that measurable fiber con centrations are found 25 ft or more away. At a distance of 25 ft, for example, a concentration of 1fiber/ml (1,000,000 fibers/ml3) was found. Much larger numbers of shorter fibers would simultaneously be inhaled. During the beveling of truck brake shoes on a grinding machine, very high concentrations of fibers were found in the vicinity of the operator. The average of five air samples was about 37 fibers/ml. Area samples, taken up to 30 ft away from this operation, demonstrated the presence of airborne fibers. It was of interest to note that, at the time of this sampling, from eight to 15 other garage mechanics were working within this J 120 ROHL BTAL. Fig. 5. Beveling of truck brake linings at municipal garage. Arrow indicates accumulation of asbes tos dust. perimeter and were exposed to asbestos. Fiber levels for other kinds of operations at the truck garage are given in Table 4. Boillat and Lob (1973) have reported fiber concentrations measured during drilling holes for rivets and grinding. They found values ranging from 0.3 to 29.2 fibers/ml; four o f the nine values exceeded 5 fibers/ml. A Comparison o f Fiber Levels Visible by Light Microscopy and Electron Microscopy In the ten brake drum dust samples examined, it was found that asbestos fibers shorter than 0.4 /tm predominated (Table 2). The OSHA Asbestos Standard does not require that short fibers (< 5 /xm in length) be counted or controlled. This oversight may have considerable biological significance in that small chrysotile fibers readily produce asbestos disease (Holt, Mills, and Young, 1964, 1965; Davis, 1965; Pott, Huth, and Friedrichs, 1972; Wagner, Berry and Timbrell, 1973; 122 ROHL ET AL. TABLE 4 A sbestos C oncentration D uring T ruck Brake Serv ice" Distance Number of Fiber concentration (fibers/mi) Operation (> Mean Range Renewing used linings by grinding 3-5 10 Background to grinding 10 2 used linings 25 2 60 1 Beveling new linings Background to beveling new linings 3-5 5 8 1 12 2 30 1 Punching rivets into brake linings 3-5 2 Chipping rust off used brake linings 3-5 1 Sweeping floor around grinder 3-5 1 Background to sweeping floor around grinder 15 1 " Fibers 5-100 pun in length, counted by optical microscoDy. 3.8 1.5 0.8 0.2 37.3 0.6 0.4 0.3 1.5 2.4 3.6 3.1 1.7-7.0 1.2-1.7 0.6-1.0 -- 23.7-72.0 -- 0.3-0.5 -- 1.9-2.0 -- -- -- oxygen to remove organic materials. The ashed resid -as d . ^ r e e d i n a d r o p o r nitrocellulose solution. The dispersal was accomphshed by a ruhout techmq using the edge of a watch glass (Nicholson, Rohl and Ferrand, 1971). y method large asbestos fiber bundles are broken into their consUtuent smaller fibrils and large agglomerates of inorganic materials, which normally obscure the presence of asbestos fibers, are broken into particles small enough to allow virtu ally all asbestos to be seen. By placing a second slide over the ^ound nitrocellulose solution and then gliding the two slides apart, a thin film is pro duced. The dried film is cut into segments which are then floated off'n ^ t e r . Th film is mounted onto Formvar-coated electron microscopic grids. Typically, four grids are prepared from each sample and one square on each grid is scanned in the electron microscope at 42,000x magnification to determine the quantity of chrysotile present. By estimating the length and diameter of each fiber, and as suming a cylindrical fiber geometry, the mass of chrysotile per grid square is determined. Representative electron photomicrographs of chrysotile fibers and fibrils are shown in Figs. 7 and 8. RESULTS . A comparison of the optical microscopic fiber counts and the e*ectronJ TM r? ' scopic total asbestos mass calculations obtained from the eight samples is shown Table 5. Figure 9, showing the same data, is plotted on logarithmic paper, and visual inspection indicates that a positive correlation exists ^ twe" and electron microscopic results, although the data are limited and the amount ASBESTOS EXPOSURE 123 Fig. 7. Electron photomicrograph of air sample taken during brake drum blowing (see sample No. 4, Table 5). Large numbers (70-100) of chrysolite, some of which are masked by granular particu lates, presumably road dust (65,000 total magnification). scattering precludes a regression analysis. For example, from these data it may be possible to predict that, during the grinding of new brake linings (Sample No. 8, Table 5), a worker could be exposed to about 0.5 mg of asbestos daily in cir cumstances in which the time-weighted TLV of 5 fibers/ml would not have been exceeded. Similarly, Fig. 9 shows that, since a microgram of asbestos represents on the order of 1 million fibers per cubic meter of air (of greatly varying diameters and lengths), extremely high concentrations of submicroscopic fibers are present up to 65 ft away from brake repair work (e.g., Sample No. 5, Table 5), even though fiber levels in such a case are barely detected, if at all, by the standard optical counting technique. These limited data indicate that the standard (OSHA) optical fiber counting method may be only a fractional indicator of total asbestos expo sure, at least in the case of automobile repair work. They also indicate that the total exposure is much higher than the OSHA technique records, in terms of ROHL ETAL. 124 F,o Electroflnii<rograpb of air sample of cluster of chrysodlc fibrils in background sample (see sample No. 3, Table 5) (83,000x magnification). asbestos fiber number, mass, and surinee area. and other kinds of asbestos exposure are needed to confirm and extend tn findings It is important to note that particles of asbestos-containing p b rie lin in g were not included in this mass determination. Their importance, m terms of biologic potential, is presently unknown. SUMMARY AND CONCLUSIONS (1) Chrysotile asbestos fiber is a major component of brake jn>ng ^ at^ s. o i ^ J d S n of the lining is brought about by a combmatton of factors, which 125 asbestos exposure TABLE 5 C omparison of O pt .cal ano E lectron M icroscopio F iber Counts Operation 1 Blowing dust off drum with air je t (10 ft away) 2 Background to blowing out brake drum (10 ft away) 3. Blowing dust off drum with air je t (20 ft away) 4 Background to blowing out brake drum (20 ft away) Optical microscopy (fibers/ml) 2.0 0.3 0.4 0.8 Electron microscopy Otg/m3) 1.27 0.2 1.1 0.1 5. Background to blowing out brake drum (65 ft away 7 minutes after blowing stopped) 6. Cleaning brake drum with hand brush 7. Light grindings of new linings before installation 8 Light grinding new linings before installation .1 0.2 3.6 6.5 4.7 53.0 2.7 66.0 .etude thermal stress, m a te fatigue * ZZSZXSZJZ * hydrra- Citv and analyzed. Optical microscopy was of limited usefiilnes . y tion 'analysis, using both continuous ence of chrysotile in all dust samples^T P P c" ^ anged 0m ^ free flbers and binder as particulate, Forsterite, the OO o 0.10--1 -- 10 too 100.0 MiCfOqroms pr cubic meter - electron microscopy F.c. 9. Comparison of optical and electron microscopic fiber counts ROHL et a l . thermal transform ationprodi^of chrysotile could w>1 unequivocally identified verified by transmission electron m i c r o c o p y , ^ ^ in fiber and fibril and electron microprobe analyses. C r y composition. Its frequency of occur- * form, with unaltered structure Quantitative determination made by rence was consistent with, but lower t h ^ h q^ ^ ^ x _ray diflraction X-ray diffraction analyse. H o w e v e ^ ^ in clumps; the latter would analysis is based on ^ . ^ ^ ^ c t r o n microscopic study. In addition and completely * - -- -- -- *" S" S) Size distribution analysis a l 0. 4 pm in cate that about four-fifths of allctuy ^ optical microscopic techniques, length. These fibers are too smf ` during brake repair work in automobile (5) Personal air sampling conducted d ic procedures for fiber garages in New York C*ty- ^ ^ S t y o/repairmen blowing dust counting were used. In samples tak ^ an average concentration of 16 from automobile brake drunwwi time-lapse samples indicate that various brake fabrication and aPPHca^ ^ truck brake shoes resulted m an averag about 4 fibers/ml r f 37 fibers/ml was meas- (4,000,000/m3). During beveling, TM rivets, and cleanup were also ured. Exposure levels durmg dnll gt PU that fiber concentration gradients are measured. Background,me ^ u r e m e i "ation work. During Ught grinding of produced during truck brake p concentrations were found 25 ft or more truck brake shoes, measurable fiber 1 operations. The background rS 'm c m ^ T o .b c rlc ta n ic s work were analyzed by( c T r se iS tr uf ^ y exist between optical f f c o m s ( ^ : magnification. These data indicate that s, fiber counts thS^hem t^free " exposure!^6 terrmTof fiber number mass, and surface area is much i greater asbestos, other biologically (8) Attention is caHed to thefact m comp0unds, have been identified active substances, including free s,llC!\ k k environments are not in brake lining dusts. Their concentrations in brake w known, and warrant investigation. ASBESTOS EXPOSURE 127 (9) Potentially hazardous asbestos exposure exists during automotive brake servicing. It has been reported that approximately 900,000 persons are employed in such work in the United States. It is recommended that stringent industrial hygiene measures to control exposure be implemented as rapidly as possible. ACKNOWLEDGMENT We thank Drs. A. E. Anderson, R. L. Gealer, and E. Eichen of The Ford Motor Company for their valuable comments in reviewing the manuscript. REFERENCES Anderson, A. E. (1969). Wear in brake materials. In Proc. Amer. Soc. Metals Wear Conference. Anderson, A. E ,, Gealer, R. L,, McCune, R. C., and Sprys, J. W. (1973). Asbestos emissions from brake dynamometer tests. Soc. Auto Engm. Meeting, Detroit, Michigan, 14-18 May 1973 Paoer No. 730549. ` Bark, L. S., Moran, D., and Percival, S. J. (1975). Chemical changes in asbestos-based friction materials during performance--a review. Wear 34, 131-139. Bates, T. F., and Comer, J. J. (1957). In Proc. 6th Nat'l Conf. Clays and Clay Mineralogy, Int. Monog Sec. 6, 237-248. Bayer, S. G., Brown, T. A., and Zumwalde, R. D. (1975). Document TR-84, U.S. Department of Health, Education and Welfare, Public Health Service, National Insitute for Occupational Safety and Health, Cincinnati, Ohio. Berry, E. E. (1971). Thermal analysis o f various chrysolites using evolved water analysis techniques. In Proc. 2nd Inti Conf. Physical-Chemical Asbestos Minerals, Louvain Univ 6-9 September 1971, paper 2:7 15 pp. ' Boillat, M.A., and Lob, M. (1973). 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