Document RpgXwpon4pbwLbDjkerpEG5JV

FILE NAME: Phenolic Resins (PHR) DATE: 1981 Apr 27 DOC#: PHR005 DOCUMENT DESCRIPTION: NIOSH Report - Assessment of Asbestos Exposu to Mechanics Performing Brake Service Operations U.S. DEPARTPv1E!MT OF HEALTH AMD HU r l Public Health Ser. INDUSTRIAL HYGIENE REPORT -A'SDCGTOG ASSESSMENT OF ASBESTOS EXPOSURE TO MECHANICS PERFORMING BRAKE SERVICE OPERATIONS REPORT WRITTEN BY: Dennis R. Roberts Ralph D. Zumwalde DATE OF REPORT: April 27, 1981 REPORT #: 32.4 Industrial Hygiene Section Industrywide Studies Branch Division of Surveillance, Hazard Evaluations and Field Studies National Institute for Occupational Safety and Health Cincinnati, Ohio INTRODUCTION A major objective of the National Institute for Occupational Safety and Health (NIOSH) is to determine environmental exposures of working populations through occupational health research, field surveys, and industrywide epidemiologic studies. Accordingly, NIOSH conducted comprehensive industrial hygiene surveys to characterize dust exposures resulting from vehicle brake servicing oper ations and to make a thorough assessment of work practices utilized, as well as document the types of personal protective equipment used. Of particular interest was the potential for exposure to asbestos fibers which could be generated by these types of operations. Limited studies of workers involved in brake servicing have suggested that their work--related exposures may be asso ciated with asbestos-induced diseases.^- NIOSH estimates that a workforce of 151,000 brake mechanics and garage workers 2 in the U.S. is potentially exposed to asbestos. Potential exposures are a re sult of 128 million pounds of asbestos used annually in the U.S. for the pro- 3 duction of brake friction materials. Besides asbestos, other materials (e.g. binders, friction modifiers, fillers, etc.) are used in the manufacture of brake linings, which can likewise have.a potential for exposure. As noted in one study, thirty materials or compounds that make up the binders, fiber re inforcers, property modifiers, etc., were identified during brake lining manu- fCacturing.2 1 BRAKE MATERIALS, PRODUCTS, AND USAGE . . 4 Historical Development of Friction Products The requirements of early automobile friction materials were relatively mini mal. Passenger cars were light and designed for low speed operations. Brakes were of an external contracting type and utilized a variety of materials; this included leather and impregnated cotton products which were commonly used along with wool and felt. In 1903 woven asbestos friction materials were first marketed in the United States by the Keasbey and Mattison Company of Ambler, Pennsylvania.^ Because of its superior heat resistance and durability they rapidly increased in use and soon dominated the market. The Model T Ford pro vides an example of the changing use of materials. Initially, cotton bands, reinforced with zinc, copper, or brass wire and impregnated with oils and bituminous material, were used for the brake bands. These were soon replaced by mixed cotton--asbestos materials and finally by woven asbestos products. The woven asbestos brakes continued to be the dominant product used in automobiles until about 1930. They typically contained 70% or more wire-cored asbestos yarn, impregnated with drying oils, such as linseed, and bituminous material. Molded brake linings were developed in the .early 1920's and gained increasing use with the introduction of internal shoe brakes in 1927. By 1940 virtually all automobiles were equipped with molded brake linings, although woven pro ducts continue to be used in trucks, heavy equipment and for specialized applications. The molded linings in use were cut to length, usually by the manufacturer, and mounted on brake shoes using rivets. Until the mid 1920's brakes were only mounted on rear wheels. However, with the development of internal shoes, four wheel mountings soon became standard, and by 1930, were generally operated hydraulically. 2 As automobiles were designed for use at even higher speeds, brake linings im proved in both quality and performance. Various new materials were introduced as fillers, binders, and friction modifiers. In 1948 bonded brake linings were developed and soon accounted for approximately 407 of the original equipment brake market. However, they rapidly dominated the replacement market because of the considerable savings in labor during installation. In 1965 the first disc brakes were introduced on American automobiles and rapidly increased m use. In 1975 virtually all original equipment front wheel brakes on cars were of this type. However, because of less stringent braking requirements and the difficulty of adapting mechanical parking brakes to the disc configuration, the rear wheel brakes on 957 of currently sold cars are still of the drum variety. Requirements for Brake Linings A constant or slightly decreasing coefficient of friction (C.F.) with tempera tures up to about 1000F is required for an efficient brake lining; values of from 0.30 to 0.45 C.F. are normally sought. Lower values produce brakes re quiring excess pedal pressure and those with higher values are too sensitive to pressure and develop excess wear. Ideally, the desired frictional qualities should be maintained throughout the life of the lining material. During braking, chemical and physical changes occur in the material at the braking surfaces. These changes may produce an increase (build-up) or a decrease (fade) in friction. Satisfactory linings will fade slightly upon repeated .. 6 applications, but will return to their initial state upon cooling. 3 Low wear of the linings is obviously desirable for economical and practical considerations. However, high wear resistance can be associated with the tendency of the lining to glaze, with a concomitant reduction in the coeffi cient of friction. This can be overcome by allowing a slow alteration of the brake lining material to occur. Pyrolysis of the organic binders and thermal decomposition of the chrysotile fibers under braking provide the necessary continuing renewal of the lining surface. The lining should be non--abrasive to the drum surface. In addition to causing rapid drum wear, abrasive linings score the drums, which, in turn, leads to a rapid wear ing of the lining. Drums made of cast iron and steel are common, with steel being the more susceptible to scoring. Since brake drums have a hardness o f from 3.5 to 4.0 on the MOH scale, virtually all lining materials used have lower hardness values. Other necessary or desirable properties of brake linings include: physical strength, dimensional stability, quiet operation, and safe and non-offensive degradation products. Of the various properties desired in the linings, greatest attention is paid to build-up/fade and recovery characteristics. Wear problems are not as serious and can more readily be overcome with the mate rials available. Compounding Ingredients of Brake Linings To achieve the desired friction properties, a wide variety of ingredients arc commonly used in the manufacturer of automobile brake linings These include: 4 Asbestos Asbestos is used for fiber reinforcement of the friction product. Chrysotile is used almost exclusively and comprises from 40 to 50 percent of the brake lining. Fiber grades 4 through 7 are used, and occasionally, several sizes are mixed or even calcined to improve performance characteristics. Amosite, crocidolite, or other amphibole asbestos varieties are not used because they are too harsh and tend to score the brake drums. Organic binders Organic binders are primarily phenolic type resins selected for high binding strength. Unmodified phenolic resins, when subjected to heat usually become hard and brittle. To prevent this, linseed, cashew nut, or China wood oils or cresols are added. Rubber, which also finds use as a binder, imparts desirable friction qualities and improves the flexibility of the lining material. Friction modifiers Friction modifiers are added to achieve a desirable coefficient of friction over all operating conditions. These modifiers also produce a more homogeneous lining surface. Included among these materials are lead, zinc, brass, cashew nut oil, graphite, and oxides of iron and copper. Fillers . Fillers such as rubber scrap, barites, clays, silica, coke, coal and other minerals are used. These also have utility in achieving desired friction properties, in some cases through action as abrasives to recondition braking surfaces. It is important to note that one major purpose of the reconditioning 5 f agents is to retard the formation of forsterite which may accumulate on the surface of the brake lining. Forsterite is a mineral not originally present in the brake material, but is created by dehydroxylation and recrystallization of chrysotile asbestos at high temperatures. The hardness of forsterite (hardness 6.5-7.0 on the MOH Scale) is such that it tends to score and gouge brake drums and discs (hardness 3.5), degrading them prematurely. Therefore, recrystalli zation of chrysotile to forsterite is an unwanted effect. Curing agents Curing agents and/or accelerators are used to assure that appropriate chemical reactions occur to produce the desired brake quality. Types of Brake Linings and Manufacturing Processes In making the of different types of brake linings various manufacturing processes are utilized to achieve a wide range of potential applications. .These include: Wired Back These are made by a calendering process in which putty-like stock is formed into a ribbon about a wire backing. The wire reinforcing serves to maintain strength during the curing process. Further, as linings of this type are in variably riveted, the wire reinforcing provides long term structural strength and prevents shearing of the lining at the rivets during braking. Linings of this type were extensively produced prior to World War II. They are in little use today. ' 6 Extruded Linings These are manufactured by extruding the soft plastic stock through an appro priately sized rectangular orifice. To minimize structural weakness in this lining, curing agents are added to produce a hard inflexible finished product. Sheet Linings These are laminated structures formed by winding a 0.001-0.002 inch film of stock about a hot roller. Since the fabrication process-is a relatively expen sive one, linings of this type are not commonly produced for general use. Dry Mixed Types This process involves the dry mixing of various ingredients capable of passing a 200 mesh screen, and then molding the lining under pressures of from 1000 to 3000. pounds per square inch. The resulting lining is among the most heat stable of friction materials in use today and obtains wide use in the manu facture of brake blocks for heavy duty service. Millboard Type The manufacture of this type of brake lining material is by a process similar to that utilized in the paper industry. Wet stock is formed and passed over rollers with various drying and baking operations producing sheets of uniform lining material. The equipment for producing such materials is expensive, but. the volume of production leads to an economically produced molded lining. 7 BRAKE LINING REPAIR AND MAINTENANCE PRACTICES To a large extent the changing character of brake lining materials has led to changing work practices and differing asbestos exposures over the years. From 1920 until about 1930, when braking was done through the use of external brake bands made from woven materials, the predominant exposure to asbestos would have come from the cutting and fitting of the woven lining material. It is thought that airborne fiber concentrations were considerably less than those developed in later years when machining of molded materials was common. From 1927, when internal brake shoes were developed using molded linings, until 1948 when bonded brake linings were introduced, all internal brake linings were attached to shoes using rivets. The lining material for use in the replacement market would be precut to appropriate size for various brakes or obtained from rolls of indeterminate length. The precut segments would usually be predrilled at the factory for rapid mounting on shoes. In some cir cumstances, however, drilling for the rivets and bevelling would be done by the mechanic installing them. The use of rolled linings required cutting the friction material to shape, drilling holes for rivets, and bevelling the edges appropriately. In this latter circumstance, asbestos exposure to workers could be considerable. Even when shoes with predrilled and bevelled linings were installed, the processes of punching out the rivets on the old shoes and riveting on of the new shoes would give rise to greater exposures than that accompanying the use of bonded linings. 8 With the introduction of bonded linings, the need for drilling, facing, or grinding operations during installation decreased significantly. However, for a short period of time, in the mid-1950's when automobile shoes were first installed with a fixed anchor, some tapering was necessary on uniform thick ness bonded linings to achieve a proper fit. Previously, the end of the shoe opposite to that of the hydraulic cylinder could be mechanically adjusted. Shortly thereafter, tapered bonded linings were available from the factory. Subsequent to 1960, considerably fewer bevelling or grinding operations were performed by an automobile mechanic replacing brake linings. During replacement of internal shoe brakes it was common practice to remove the brake wear dust from the housing by air blowing or brushing. After 1970 increasing awareness of the hazards of asbestos and its presence in brake lining dust led to wet brushing, wet wiping, dry brushing, or vacuuming work practices in some brake servicing facilities. However, even today such im provements in work practices are not universal. In the 1930's and 1940's most automotive shops were relatively small and most mechanics performed all automobile maintenance and repair activities. In recent years, however, there has been an increasing tendency towards speciali zation, with shops existing for brakes and front end work exclusively. Here, while asbestos exposures during brake work on an individual job may be less than those of previous years, some workers are exposed for considerably longer periods of time. 9 SELECTION OF FACILITIES SURVEYED The purpose of the industrial hygiene study was to investigate and char acterize dust exposures resulting from vehicle brake maintenance and repair operations taking into account the work practices utilized. Therefore, it was necessary to locate facilities where a variety of brake servicing techniques were used as well as where there were differences in number of vehicles ser viced. Six of the eleven sites selected for the investigation were automobile brake service facilities which performed from 2 to 45 brake jobs per week at an average of 65 minutes per vehicle. One of the facilities surveyed only serviced trucks, which often required 6-9 hours per vehicle to service brakes, with an average of three brake jobs per week. The remaining four facilities serviced both autos and trucks and performed from 5 to 45 brake jobs per week which varied in service time depending on the type of vehicle. Detailed air borne dust sampling surveys were conducted at each facility. Description of Brake Servicing Operations The servicing procedures found at each facility were basically as follows. The vehicle is driven into a repair stall or bay for a brake system examination. Pending repairs, the wheels are elevated, removed, and then inspected. Loose dust is cleaned from the drums and brake assemblies by vacuuming, wet or dry wiping/brushing, using compressed air, or a combination of these methods. Parts are then replaced or repaired as needed and the brake system is re assembled and adjusted. Test driving the vehicle for proper fitting and ad justment is the final phase of the servicing operation. A brief description of the individual facilities is outlined as follows. Industrial hygiene survey data collected at each facility are shown in Tables 1-4. 10 Facility A Facility A, a private fleet service garage, was responsible for complete automotive maintenance and repair with the exception of internal engine repai and exterior painting. The shop normally operates 8 hours per day, 5 days per week. Of the seven employees working at the facility, only three were re sponsible for brake servicing. Brake servicing operations were performed (two to five jobs per week) in either of two service stalls. Facility B Facility B, a municipal service garage, Xiieo?e^Mete-1hree mechanics that specialized in brake and clutch service and three employees that operated a separate brake repair shop specializing in brake shoe and drum reconditioning The brake mechanics serviced all vehicles which included, waste collection, dump, and light trucks, autos, and some 2- and 3- wheeled vehicles. The shop operated 8 hours per day, 5 days per week. Facility C At Facility C, a municipal garage, there was an average of one complete brake service job per day, taking about 5 hours per job, with most of the vehicles consisting of cars or light trucks. There were five employees responsible for brake servicing and the facility operated 8 hours per day, 5 days per week. 11 Facility D At Facility D, a municipal garage, an average of eight brake jobs per day were performed on cars and trucks. Brake maintenance was performed by any of the 60 auto mechanics. The hours of operation were 8 hours per day, 5 days per week. Facility E Facility E, a private fleet service garage, performed complete automotive and light truck maintenance. The shop normally operated two shifts, 5 days per week, and there were usually four full-time mechanics per shift. About one brake job per week, per shift was performed. Facility F Although Facility F was an automobile brake service shop, front-end alignment and tire sales were also part of the shop mechanics' duties. The three full-time mechanics worked from two service stalls, 12 hours per day, days per week. Brake maintenance operations consisted of 10 to 14 brake jobs per week. Facility G Major services at Facility G, an automobile brake service shop, consisted of front-end alignment, shock absorber servicing, and brake maintenance. The' ' normal work week was made up of five, 9-hour days and one, 6-hour day. Three 12 service stalls were used by the three full-time employees for brake servicing operations during the 4 to 6 brake jobs per week. Facility H Major services provided at Facility H, an auto brake shop, were front-end alignment, shock absorber service, and brake maintenance. The three full-time employees worked from two service stalls, 9-hours per day, 6 days per week. The number of brake jobs per week averaged from 20 to 30. Facility I Facility I was the largest of the automobile brake service shops surveyed. Other services provided by this facility were front-end alignment and shock absorber replacement or repair. The five full-time mechanics worked from foui service stalls, 9 hours per day, 6 days per week. Brake maintenance operations consisted of 35 to 45 jobs per week. Facility J The major services at this facility were front-end alignment, muffler in stallation, and brake maintenance. Automobile brake repair operations were performed by the shop's three employees and consisted on the average of 4 to 5 brake jobs per week. Normal brake servicing at this facility took about 1 hour and 45 minutes per vehicle. 13 Facility K Facility K, a truck brake maintenance facility, involved a somewhat different operation and exposure. Servicing operations were more complex and, therefore, involved more employees with fewer vehicles serviced than auto maintenance facilities. The four service bays at the facility were used by seven mechanics. Other service operations included pad grinding, riveting, and punching (pad removed and/or replaced on shoe), sand blasting of old shoes, and milling of wheels. SAMPLE COLLECTION AND ANALYSIS Airborne Samples Personal and general air samples were collected at each facility on different occasions during a 3-year period. Brake servicing operations and areas not in the immediate vicinity of brake work within each facility, were monitored <-o provide asbestos exposure data. Personal air samples were collected in the breathing zone of the brake mechanics using Millipore Type AA, 37 millimeter (mm) diameter, 0.8 micrometer (pm) pore size, membrane filters at a calibrated sampling flow rate of 2.0 liters per minute (1pm). The filters were changed periodically during the work shift to prevent particulate overloading on the filter. Time-weighted average (TWA) fiber concentrations were determined for the time spent performing brake service at all facilities and peak concentra tions determined for time spent cleaning brake dust from drums and assemblies. Samples for peak exposures were collected using Gast. pumps calibrated at ll.v. or 10.6 1pm using identical media as above. At facilities B, C, and D, a 2.0 14 lpm sampling train was used for peak samples. Analysis of the membrane filters for asbestos fibers was conducted in accordance with the procedures outlined by the Occupational Safety and Health Administration11 and the NIOSH Manual of Analytical Methods P&CAM #239.12 These procedures require the counting of fibers greater than 5 micrometers (pm) in length and with at least a 3 to 1 length to width aspect ratio utilizing phase contrast optical microscopy at a magnification of 400-450X. Concentrations are expressed as fibers greater than 5 ym in length per cubic centimeter of air (fibers/cc). Random samples from each facility surveyed, as well as those samples having high fiber concentrations, as determined by the optical counting method, were analyzed on a transmission electron microscope (TEM) utilizing selected area electron diffraction (SAED) and ^ e n e r g y dispersive X-ray analysis (EDXRA). S.amples were observed at 17,000X magnification with fibers (> 3:1 aspect ratio) sized by length and diameter. SAED was attempted on all observed fibers for possible identification. In addition, EDXRA was performed on individual fibers to determine their elemental composition. SAED patterns and EDXRA elemental spectrum ratios were compared with reference minerals (UICC chryso.tile, and forsterite obtained from the U.S. Smithsonian Institution). Sample preparation and analysis were performed using the NIOSH method described in the Technical Report "Review and Evaluation of Analytical Methods for Environ13 mental Studies of Fibrous Particulate Exposure . Genetal area samples for trace metals (lead, zinc, copper, iron, and manganese) were collected at most facilities using Staplex Type TF-1A high-volume samplers at a flow rare oi 10 cubic feet per minute, and also 15 a sampling train and Low rate like that used for asbestos fiber collection. Samples were analyzed for metals by atomic absorption spectrophotometry in accordance with the NIOSH methods P&CAM Number 222, S186, S341, and S366.1 4 '15 Bulk Samples Samples of brake wear dust were collected from the brake drums of several vehicles that were being serviced during the surveys. These samples were analyzed by TEM for characterization and identification of fibrous particulates and to determine fiber size distributions. Work Practice Characterization Considerable emphasis was placed during the surveys on detailing work pro cedures during brake servicing in order to document the types of cleaning practices (i.e. vacuum, compressed air, brushing, etc.) used in replacing brakes. As previously described in the section "Description of Brake Servicing Operations" the brake servicing work-practices utilized were similar for all facilities surveyed; the major difference observed was the methodology utilized for clean-off of brake wear dust from the brake-shoe/backing-plate assemblies. There were six different types of clean-off methods observed during the study. Those six methods are described as follows: 1. Compressed Air Blow-Off. A compressed air stream was used to blow away brake wear dust from the brake assemblies and drums. 16 2 . Compressed Air-Stoddard Solvent Mist: Blow-Off. The same as #1 except a spray gun containing Stoddard solvent was used to produce a solvent mist for blow-off. 3. Dry Brushing. Brake wear dust was brushed away with a small utility brush (usually 1" diameter). 4. Wet Brushing. Basically the same as dry brushing except the brush was kept saturated with a liquid such as gasoline, water, or Stoddard solvent. 5. Squirt-Off. A liquid squirt-bottle containing water was used to wash away brake dust. This was followed by drying off assemblies with a cloth. 6. Vacuum Cleaning. A shop type vacuum cleaner, equipped with a HEPA* filter, was used in combination with compressed air. This system included a brake encapsulation cylinder that completely enclosed the brake-shoe/backing plate assembly. The mechanic operated a compressed air nozzle fixed inside the enclosure to blow off the brake dust, which was immediately drawn into the vacuum system. A separate vacuum hose was used to vacuum the dust from the brake drums. *HEPA: High Efficiency Particulate Air - 99.7% Al? efficiency for 0.3 ' ym diameter aerosols. 17 In Methods 1 to 5, and especially 1 to 3 > the brake dust " enter the ambient air of the facility, Conversely, in M-rho contained by the HEPA filter which may be removed from the va> system and disposed of. SUMMARY OF SURVEY RESULTS " " P - a l microscopy flber count anaiysis for eh6 TWA and ^ ^ backgrou,,d area samples collected durlng t,,. , tudy summati2ed ^ Tpbie ; ^ TM and peak fiber concentrations f,, raechanicg tended eo fiuctuate ' depending on the brake clean-off method used a,,d the ^ ^ eXa"Ple' reSatdUSS f U i , . d , peak exposures gh (up to 15.0C flber/cc) when compered to their respective TWA ex posures (0.0, to 0 , 3 fibers/cc,. The differences between the TWA exposures =nd the peek exposures ere perhaps best expieined by variations in work pnactrces utilised, the inconsistencies in performing brake work that existed t noughout the various repair shops and because clean-off is done inter mittently and therefore^ rreepprreesseenntt*s aa semalnl percent r , f , percent of the work performed u - n g the shift, for example, the amount of time spent servicing brakes -- among the mechanics in addition to the number of brake Jobs that were performed per shift; likewise, there were differences in c rrerences m cleaning methods e.g., compressed air, brush, vacuum, etc,, and procedural t e c h n i c s (e.g. b" ke - Edition, there were some - antes who dropped the brake drums on the floor, causing airborne disperse, EnVir~ l ~ - - a k e service facilitv, such as: shop size, ventilation control* nnH ,, . . ' "d Pen wlr,dows and doors, would also atiect individual worker exnn*nrQ j . , P SUte ^ A g r o u n d concentrations of airborne 18 asbestos fibers. Regardless of the cleaning method utilized, TWA exposures for mechanics at all facilities were relatively consistent, and did not differ significantly from their respective background (ambient levels in facilities) TWA exposures. The similarities between mechanic TWA and background exposures suggest that all individuals in the immediate work environment are potentially exposed to the same fiber concentrations during a normal work shift. This observation is further supported by the fact that the mechanics spend much of the work shift away from the work site and in other areas of the facility. TEM Fiber Characterization Ajfbo.rne Samples^ Samples were randomly selected for transmission electron microscopy (TEM) with fibers sized by length and diameter. In addition, fiber concentrations (fibers/cc) were determined for total fibers and fibers >5 pm in length. These concentrations were compared to those found by the optical microscopy method and are reported in Table 2. In all but 3 samples, the concentrations of fibers >5 ym in length determined by optical microscopy were somewhat higher than those determined by TEM. This difference could have been caused by particulate loss during sample preparation for TEM, or because of the small number of fibers actually counted on each sample^ at low fiber counts, small differences in fibers counted by both microscopy methods would have a significant effect in the calculation of fiber concentrations. Besides 19 determining the concentrations for fibers >5 ym in length, total fibers observed were counted and concentrations calculated. As would be expected, the greatest proportion of fibers observed was shorter than 5 ym in length (80 to 90%). Fibers observed by TEM were identified utilizing SAED and EDXRA. Approx imately 50% of the fibers analyzed by SAED could not be identified due to ambiguous diffraction patterns. The remaining fibers which were identi fied indicated the presence of chrysotile (V30%) and forsterite (^20%). The presence of fibrous forsterite was probably due to the dehydroxylation and recrystallization of chrysotile as a result of high temperatures (>650C) encountered during braking. ' Some of the fibers which revealed ambiguous diffraction patterns appeared to have crystalline structures similar to both chrysotile and forsterite (probably a transition inter mediate2 ) while others were too small for diffraction analysis. When EDXRA was performed on the fibers, confirmation of the SAED analysis was made for the chrysotile and forsterite fibers. Some of those fibers which gave ambiguous SAED patterns indicated magnesium, silicon, and iron in various elemental ratios. As shown in Table 3, when a fiber size distribution was performed for all fibers observed by TEM, a geometric mean length of 1.66 ym, and a geometric mean diameter of 0.14 ym were determined. Likewise, for those fibers identified as asbestos (chrysotile) a geometric mean length and diameter of 1.70 ym and 0.15 ym were determined, respectively. 20 Brake Dust Samples of brake wear dust were analyzed by TEM in the same manner as the air borne samples. Identification was attempted on all fibers using SAED and EDXRA. Approximately 45% of the fibers analyzed by SAED could not be identi fied due to ambiguous or the absence of diffraction patterns. EDXRA was per formed on all fibers observed with elemental analysis being successful on about 707 of the fibers. When utilizing SAED and/or EDXRA many of the fibers observed were positively identified as chrysotile (40%) while the remaining were either forsterite (20%) or unknown (40%). Many of the unknowns were thought to be intermediate recrystallized forms between chrysotile and forsterite. In addition, a fiber size distribution was performed which indicated somewhat shorter lengths (0.24-5.88 ym vs. 0.24-10.0 ym) and smaller diameters (0.06-0.29 ym vs. 0.06-1.0 ym) than those observed in the airborne samples (Table 3). Trace Metal Analyses Trace metal analyses were performed on airborne samples collected at Facil ities B, C, D, I, and K with the results reported in Table 4. Samples were analyzed for the following metals: lead, iron, zinc, copper, and manganese. As noted in Table 4 the metals were often non--detectable (n.d.) or found in trace amounts. The range of concentrations for all facilities were: Pb, n.d. - 63.3 3 3 3 3 yg/m ; Fe, n.d. - 1.5 mg/m , Zn, n.d. - 352 yg/m ; Cu, n.d. - 8.7 yg/m ; and 3 Mn, n.d. - 3.5 yg/m . 21 DISCUSSION The human toxicological significance for the inhalation of chrysotile asbestos fibers is well documented; and instances of mesothelioma in auto repair workers have been identified. 16 '17 '18 In a detailed examination of 90 union vehicular maintenance workers in New York City,^ with 10 or more years of shop work, 29% had decreased vital capacity; the percentage increased with age and most markedly after 20 years from the outset of auto work. Many of the workers examined showed signs consistent with asbestosis, with observed changes noted in chest x--rays and indication of restrictive pulmonary function. The prevalence of these changes was significantly higher after 20 years exposure, , . 19 a result expected after occupational exposure to asbestos. Unlike chrysotile, the health effects of exposure to forsterite, or transiti on series fibers (chrysotile/forsterite) with altered crystalline structures are not well documented. In studies by Davis and Coniam,^ and Koshi in which fibers of chrysotile, chrysotile/forsterite, and forsterite were injected into the pleural and peritoneal cavities of mice, the results suggested varying degrees of toxic effects. Fiber implantation animal studies conducted by Pott, et. al. ' and Davis, et. al. suggest that the morphology and size of a fiber, regardless of fiber type, are responsible for its carcinogenicity. Likewise, Stanton, et. al. suggests that fibers <1.5 ym in diameter and >8 ym in length pose the greatest risk in producing pleural sarcomas. These studies tend to suggest that the physical morphology (size dimensions), and to a lesser degree chemical and surface characteristics of a fiber are the determining factor for inducing a biological effect. The precise fiber dimensional characteristics required for these observed pathologic responses 22 have been difficult to determine experimentally because of the difficulties encountered in producing fibers of specific size dimensions. SUMMARY The TWA airborne asbestos sample results for all facilities were within the 26 current OSHA asbestos standard. This standard states: "The 8-hour time-weighted average (TWA) airborne concen tration of asbestos fibers to which any employee may be exposed shall not exceed 2 fibers, longer than 5 micro meters in length, per cubic centimeter of air (fibers >5 ym/cc). The ceiling airborne concentration to which no employee may be exposed shall not exceed 10 fibers >5 ym/cc." . However, two of four peak sample results for samples collected at Facility I (Table 1) during the compressed air cleaning of brake assemblies exceeded the OSHA ceiling standard. The compressed air cleaning method also indicated the highest overall peak exposures (up to 15 f/cc) for all cleaning methods examined. When the overall TWA and peak sample results are compared to the NIOSH recommended standard for asbestos, 8 of 13 of the TWA exposures for mechanics indicated concentrations exceeding the recommendation. This standard recommends a 8-hour TWA exposure of 0.1 fibers ;5 ym/cc (fibeis/cc) with a ceiling exposure of 0.5 fibers/cc for any 15-minute sampling period. 23 Many of the samples collected yielded such low fiber counts that their co efficient of variation (CV ) was above what is considered reliable (i.e. greater than 0.38). 27 Consequently, the fiber concentration data are best utilized for comparing exposure variations among the different cleaning methods/work practices. CONCLUSION 1 2 The results of this and other studies ' indicate varying concentrations of asbestos fiber exposure to brake mechanics. The exposure concentrations are apparently affected by the work practices utilized, and the existing environ mental conditions and controls at each facility. The results of this study show that 8 of 13 of the mechanics engaged in brake service had TWA fiber exposures above the NIOSH recommendation, however, all TWA exposures were below the OSHA standard. Furthermore, when samples were analyzed by TEM, only 30% of the fibers observed were identified as asbestos with the remaining fibers being categorized as forsterite (20%) or unknowns (507o). As a result of the TEM analysis the interpretation of the asbestos concentrations, as determined by optical microscopy, is questionable. The analysis indicates that actual asbestos fiber concentrations (>5 ym length) are often less than those reported (see Table 2). However, this does not preclude the possibility of high airborne asbestos fiber concentrations when all fiber size ranges are considered. As determined 28 29 by the TEM analysis in this study and from other reported studies ' a significant number (up to 100%) of short fibers (<5 ym in length) are always 24 present (see Table 2). Likewise, some epidemiologic studies29 have indicated that a potential health risk exists for asbestos fibers <5 ym in length. Additional supportive evidence has been shown from the results of animal 22 23 24 25,30 studies in-which various size fibers were implanted in animals. These studies concluded that the physical morphology (size dimensions), and to a lesser degree chemical and surface characteristics of a fiber, are the determining factor for inducing a biological effect. The mechanics surveyed during this study were either full-time brake mechanics, who performed only brake maintenance service and may have serviced up to five cars per day/six days a week, or mechanics who undertook only one brake job per week. Regardless of the number of brake jobs performed (per work shift), the TWA fiber exposures were similar for all the mechanics surveyed. The exposures to mechanics who performed full-time brake work were not significantly higher than the exposures for those who did much less brake servicing. Conversely, the short term (<3 minutes) peak exposures encountered in this 'study during the dust clean-off of braking assemblies were often higher when compressed air was used. All of the cleaning methods surveyed, except for vacuum cleaning, had peak fiber concentrations that were near or above the NIOSH-recommended ceiling exposure level of 0.5 fibers/cc. However, the com pressed air cleaning method was the only type which approached, and in two cases even exceeded, the OSHA ceiling exposure limit of 10 fibers/cc. These findings strongly suggest that brake mechanics are at a higher risk of air borne exposure to asbestos fibers during the cleaning of brake assemblies, 25 except when the vacuum cleaning methods are properly utilized. The peak fiber exposures found for mechanics during the cleaning of brake assemblies were higher than most of the TWA exposure concentrations. However, there is some question as to the accuracy of the fiber counts from the analysis of the peak samples since the number of fibers counted were small, 27 resulting in a large coefficient of variation. RECOMMENDATIONS The data from this study suggests that a potential for airborne fiber exposure exists during brake servicing operations, principally during brake assembly cleaning. While the fiber exposures reported do not represent 100% asbestos fiber, a potential health hazard still exists since at least 30% of the fibers are asbestos. Also, animal studies suggest that fiber shape and size may be more important than chemical nature in terms of biological activity. Further more, the possibility exists for exposure to a much higher asbestos fiber con centrations for fibers <5 pm length. Since there is no known safe asbestos fiber exposure level 31 , and as clinical evidence suggests from a study of union vehicular maintenance workers i.n which over 25% had evidence of x-ray abnormalities consistent with asbestosis, it would seem prudent to conclude that a potential health hazard exists during the performance of brake maintenance operations. Therefore, it is recommended that appropriate control measures for reducing exposures, especially during brake assembly cleaning, be instituted at brake maintenance facilities. This would best be accomplished by 26 using an appropriate vacuum cleaning system to remove all dust from brake assemblies and drums. Above all, any blow-off of brake dust by compressed air must be eliminated. To further protect the health of the brake mechanics, it would be advisable to initiate a personal respiratory protection program. This would include the wearing of NIOSH approved respirators for asbestos, a program for proper fitting, and a routine maintenance program for the cleaning and replacing of respirator filters. As the exposure data indicates, it would only be necessary for mechanics to wear respirators wh.ile performing brake service. In order to minimize asbestos dust exposures to vehicular mechanics performing brake and clutch maintenance NIOSH has prepared guidelines "Recommended Procedures for Asbestos Brake and Clutch Servicing" (see Attachment 1) to be utilized during these types of work tasks. 27 REFERENCES 1. Lorimer, W.V. , Rohl, A.N., Miller, A., Nicholson, W.J., and Selikoff, I.J. '.'Asbestos Exposure J-n Br^ke^ Rgpai.rJJflrker| 5o United States"., Mt. Sinai Journal of Medicine, 43:20/-Id, ^'Crfay-Ju^e^ f9 76 )~ ___ V 2. National Occupational Hazard Survey. U.S. Dept, of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health; estimate, April 1981. 3. Stanford Research Institute (1976). Chemical Economics Handbood, 712.1000C, October 1977. 4. Much of the historical information on the development of fricition products was obtained from interviews by Dr. William J. Nicholson, Mount Sinai School of Medicine, with Edward W. Drisane, Friction Materials Standards Institute, Paramus, N.J., Harry, H. Wagner, Jr., Molded Materials Co., Ridgway, Pa. 5. Keasbey and Mattison products catalog, 1926. Ambler, Pa. 6. Friction materials on automotive brakes. Fleet Owner, (August 1963). 7. Carroll, W.G. The manufacture of brake linings. British Plastics 414-417 (August 1962). 8. Anderson, A.E., and R. Gealer. Ford Motor Co., Detroit, MI. Unpublished notes. 9. White, Andrew J., Brake Dynamics: an introduction to brakes at the inspection station level. Motor Veh. Resh. of N.H., Lee, N.H., Chapter 11, 463-472. (1968). 10. Rohl, A.N., et al. Asbestos exposure during brake lining. 11. U.S. Code of Federal Regulations, Title 29, part 1910.1001. U.S. Department of Labor, Occupational Safety and Health Administration, Occupational Safety and Health Standards. 12. Taylor, D.G., (1977). NIOSH Manual of Analytical Methods. 2nd Edition, Volume 1, P&CAM No. 239, Publications No. 77-157-A. 13. Zumwalde, R.D., and Dement, J.M., (1977). Review and Evaluation of Analytical Methods for Environmental Studies of Fibrous Particulate Exposures. DHEW (NIOSH) Publication No. 77-204. 14. Taylor, D.G., (1977). NIOSH Manual of Analytical Methods. 2nd Edition, Volume 1. P&CAM No. 222, Publication No. 77-157-A. ' . 15. Taylor, D.G., (1977). NIOSH Manual of Analytical Methods. 2nd Edition, Volume 111, S186, 341, 366 Publication No. 77-157-C. 28 16. Newhouse, M.L. and Thompson, H. Mesothelioma of pleura and peritoneum following exposure to asbestos in the London area. Brit. J. Ind. Med. 22:261-269, 1965. 17. McDonald, A.D. et al. Epidemiology of primary malignant mesothelial tumors in Canada. Cancer 26:914-- 19, 1970. 18. Greenberg, M. and Lloyd Davies, T.A. Mesothelioma Register 1967-1968 Brit. J. Ind. Med. 31:91-104, 1974. ' 19. Bader, M.E., Bader, R.A., Teirstein, A.S., Miller, A. and _ Selikoff, I.J. Pulmonary function and radiographic changes in 598 workers with varying duration of exposure to asbestos. Mount Sinai J. of Med. 38:492-500, 1970. 20. Davis, J.M.S., and Coniam, S.W. Experimental Studies on the Effects of Heated Chrysotile Asbestos and Automobile Brake Lining Dust Injected into the Body Cavities of Mice, Experimental and Molecular Pathology, Volume 19, pp. 339-353 (1973). 21. Koshi, K . , Hayashi, H., and Sakabe, H. Biological and Mineralogical Studies on Serpentine Minerals in Heat Treated State, Ind. Health, Volume 7, pp. 66-85 (1969). 22. Pott, F., Huth, F., and Friedricks, K.H. Tumorigenic Effect of Fibrous Dust in Experimental Animals. Environmental Health Perspectives, Volume 9, pp. 313-315 (1974). 23. Pott, F.. , Dolgner, R., Friedricks, K.H., and Huth, F. Animal Experiments Concerning the Carcinogenic Effect of Fibrous Dusts. Interpretation of Results Considering the Carcinogenesis in Humans. Annales d'Anatomie Pathologique, Paris (1976), Volume 21, pp. 237-246. 24. Davis, J.M.G. The Fibrogenic Effects of Mineral Dusts Injected into the Pleural Cavity of Mice, British Journal Exp. Pathology, Volume 53, pp. 190-201, (1972). 25. Stanton, M.F., Layard, M., Tegeris, A., Miller, E., May, M . , and Kent, E. The Carcinogenicity of Fibrous Glass: Pleural Response in the Rat in Relation to Fiber Dimension. J. Natl. Cancer Institute 58:587-603, (March 1977). 26. U.S. Code of Federal Regulations, Title 29, Part 1910, Section 1910.1001. 27. Leidei, N.A., Bayer, S.G., Zumwalde, R.D., and Busch, K.A. NI0SH Technical Report "USPHS/NI0SH Membrane Filter Method for Evaluating Airborne Asbestos Fibers". DHEW (NIOSll) Publication No. 79-127, Febtuary 1976. 29 28. Bayer, S.G., Zumwalde, R.D., and Brown, T.A. Equipment and Procedures for Mounting Millipore Filters and Counting Asbestos Fibers by Phase Contrast Microscopy. Available from U.S. Department of Health and Human Services, National Institute for Occupational Safety and Health. 29. Gillam, J.D., Dement, J.M., Lemen, R.A., Wagoner, J.K., Archer,' V.E., and Blejer, H.P. Mortality Patterns Among Hard Rock Gold Miners Exposed to an Asbestiform Mineral. Annals of the New York Academy of Sciences. 271:336-344, 1976. 30. Stanton, M.F., Blackwell, R. , and Miller, E. Experimental PulmonaryCarcinogenesis with Asbestos. Am. Ind. Hyg. Assoc. J. 30:236-244, 1969. 31. U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health. Asbestos: An Information Resource. DHHS Publication Number (NIH) 79-161. May 1978. 30 Table 1 Fiber Air Sample Results for Brake Assembly Clean-Off Methods Cleaning Method Compressed Air Compressed Air Comp essed Ait Cu".pi essed Air Compressed Air CofTipt essed Air Compressed Air C .'Hipl essed Air Comp: eised Ai r Cinipi essed Air Sc 1vent Mist Comp essed Air Solvent Mist Compressed Air Solvent Mist Compessed Air Solvent Mist Dry Brush Dry Brush Wot Brush Wot Brush Wet Blush Wet Brush Ltquid Scpjirt Bottle V a c u u m Cleaning V.h `min C 1ion ing Var turn Cleaning Fa c i1ity Surveyed G G G G G I I I 1 Employee Sampled Mechanic #1 Mechanic #1 Mechanic #1 Mechanic # \ Mechanic # \ Mechanic #1 Mechanic #2 Mechanic #2 Mechanic #3 Fibers/ cc* Peak Exposures Sample Sample T ime Volume (Sec. ) (Liters) 1.82 180 6.0 0.14 38 4.7 0.77 34 4.2 0.33 26 3.2 2.69 20 2.5 2.84 60 10.7 0.91 45 8.0 14.54 30 5.3 15.00 30 5.3 I Mechanic #1 0.25 60 4.0 1 Mechanic #1 0.45 45 8.0 I Mechanic #2 0.68 30 5.3 I Mechanic #2 0.37 180 12 D Mechanic #1 0.81 720 24 D Mechanic #2 0.61 1260 42 C Mechanic 2.62 180 6.0 C Mechanic # 2 2.22 180 6.0 C Mechanic # 2 0.87 540 18 C Mechanic #3 0.67 540 18 B Mechanic 0.54 600 20 E Mechanic H l 0.00 120 16 E Mechanic //l 0.07 120 16 E Mechanic H I 0.03 120 16 I' M ;.s 1 Kir h '.h imoKT of 5,ampled a n TWA Exposures** Sample Samp le Fibers/ T ime Vo 1urne cc* (Min. ) (Liters) 0.03 352 604 0.12 283 566 0.10 298 596 0.19 2 70 540 0.08 343 686 0.07 0.20 0.19 0.23 0.28 0.24 0.21 0.01 283 566 197 394 301 . 602 346 692 369 . 738 135 270 326 652 240 480 Background TWA Exposures** Sample Sample Fibers/ Time Vo 1urne cc* (Min.) (Liters) 0.013 380 760 0.10 133 266 0.08 227 454 0.13 94 188 0.04 231 462 0.03 222 444 0.07 414 828 0.03 382 764 0.07 360 720 0.07 360 720 0.07 360 720 0.06 395 790 0.01 378 75b Table 2 Air Sample Results For Fibers Comparison Between TEM and Optical Microscopy Analysis Optical Microscopy Transmission Electron Microscopy 1 I >5 ym in length >5 ym in length | Total Fibers 7 Fibers | | f ibers/cc f ibers/cc j fibers/cc >5 ym in length | 1| 0.54 | 6.0 | 0.58 | 1.18 | 0.13 | 6.84 1 0.25 | 0.50 50 | 5.97 | 11.33 53 | 0. 17 | 1.01 17 | 0.67 | 2.35 29 | 0.10 | 0.74 14 | 0.07 I 0.43 . 17 | | 5.59 0.33 | 0.39 83 | | 0.82 0.02 | 0.02 100 | | 0.01 0.0 | 0.11 0 | | 0.01 0.0 | 0.0 o 1 | 0.02 0.19 | 2.72 14 i | 0.38 0.16 | 0.48 33 | | 1.44 0.0 | 0.08 0 j | 0.01 0.0 | 1.43 0 | | 0.26 0.09 | 0.09 100 j | 0.24 | 0.06 0.04 0.0 | 0.16 | 0.0 25 | o 1 | 0.12 I 0.01 0.01 | 0.01 0.0 | 0.03 100 | 0 j I 0.03 0.0 I 0.0 (Blank) 0.0 | 0.0 | 0.0 o 1 o I | 0.12 | 0.17 0.42 | 0.86 0.10 | 0.20 48 | 50 | | 0.18 | 0.06 0.14 0.05 j 0.43 | 0.15 33 | 33 | 1 0.12 1 0.50 | 0.73 1 68 j 1 * Note: Fibers counted by TEM represent asbestos fibers only. 32 Table 3 Brake Dust Fiber Size Data irborne Samples Asbestos Fibers Only) irborne Samples All Fibers) ulk Brake Dust Asbestos Fibers Only) ulk Brake Dust All Fibers) Number nf Fi far firs 151 Fiber Diameter um Geometric Range Mean 0.06- 1.0 0.15 Geometric Std. Dev. 2.36 Number of FiJKjiers 151 Fiber Ltngth pm Geometr ic Range Mean 0.24-10.0 1.70 Geometric Std. Dev. 2.27 523 0.06- 1.0 0.14 2.17 523 0.24-10.0 1.66 2.49 0.06-0.18 0.10 1.53 8 0.24-1.76 109 Note: All | | 0.06-0.29 | 0.08 1.53 | 1 I 1 1 1 1 ______________ 1 l fiber size data determined by Transmission 109 | 0.24-5.88 ..... Electron Microscopy. 0.40 | 2.03 0.49 | 1.95 I J___________________ Table 4 Trace Metal Analyses Facility | R>--anfi0e o--f--Tr^ace Metal Concentrations ufi/m Surveyed Pb . Fe Zn Cu Mn B 1.2 - 8.7 1.7 - 2.3 0.2 - 0.3 0.2 - 0.3 0.04 - 0.06 C 19.5 - 24.9 1.2 - 1.4 0.3 - 0.3 0.2 - 0.2 0.04 - 0.05 D 0.4 - 0.8 1.5 - 1.8 0.3 - 0.5 0.1 - 0.2 0.03 - 0.04 I N.D. - 63.3 N.D. - 349.3 N.D. - 351.9 N.D. N.D. K N.D. - 24.0 N.D. - 1,452.8 N.D. - 26 , N.D. - 8.70 N.D. - 3.50 OSHA Exposure Standard us/m OJ 50 10,000* 5,000** ,1,000*** 1 5,000**** P- NIOSH Recommended Standard uR/m 100 1 - . 5,000 - 1_______ I_______ N.D. * ** *** **** - Not Detected by Analysis Iron Oxide Fume Zinc Oxide Fume Copper Dusts and Mists Ceiling ATTACHMENT I RECOMMENDED PROCEDURES FOR ASBESTOS BRAKE AND CLUTCH SERVICING The National Institute for Occupational Safety and Health (NIOSH) has con ducted research on dust exposures which are generated during brake and clutch servicing. Based on data demonstrating the potential for significant asbestos exposures during brake and clutch servicing, NIOSH has investigated various work practices which are utilized in reducing asbestos exposures. These inves tigations have indicated that vacuum cleaning systems to be the most effective method for minimizing asbestos dust exposures during brake and clutch servicing.^ Vacuum cleaner testing have demonstrated that these units operate 2 reliably within design specifications. Therefore, NIOSH recommends vacuum cleaning as the primary method to be used for 'cleaning of asbestos dust during brake and clutch servicing operations. The following are additional procedures recommended by NIOSH to minimize asbestos dust exposures. 1. Where possible, an area shall be designated for brake and clutch repairs and servicing. Entrances into this area shall be posted with the following asbestos exposure warning sign printed in letters of sufficient size and contrast to be readily visible and legible: Asbestos Dust Hazard Avoid Breathing Dust Wear Assigned Protective Equipment Do Not Remain in Area Unless Your Work Requires It Breathing Asbestos Dust May Be Hazardous to Your Health 35 2. Dust shall first be cleaned from brake drums, brake backing plates, brake assemblies, and clutch assemblies using an industrial type vacuum cleaner equipped with a high efficiency particulate air filter system (HEPA-greater than 99% efficiency for 0.3 pm diameter aerosols). After vacuum cleaning, any remaining dust shall be removed using a water dampened cloth or rag. Under no circumstances shall compressed air or a dry brush be used for cleaning. If vacuum cleaning equipment is not available the wet brush cleaning method may be used until a vacuum cleaning system in obtained. Where wet brushing is necessary for cleaning, a NIOSH certified respirator approved for asbestos shall be worn. 3. During brake pad grinding, riveting, and punching operations local exhaust ventilation and dust collection systems shall be designed, installed, and maintained in accordance with the American National Standard Fundamentals Governing the Design and Operation of Local Exhaust Systems, ANSI Z9.2 - 1977 to meet the asbestos airborne exposure standard. 4. During clutch servicing, a NIOSH certified respirator approved for asbestos shall be worn during the removal and cleaning of the clutch, pressure plate and housing assembly, and during installation of the new clutch assembly. Whenever possible, cleaning shall be performed with an HEPA vacuum system as described in (2) above. 3. All table and floor cleaning in areas where brakes and clutches are re paired shall be done with the HEPA vacuum cleaner as described in (2) above. Grinding and riveting machines shall also be cleaned with such a 36 vacuum cleaner and Che remaining dust wiped with a water dampened cloth. A NIOSH certified respirator approved for asbestos shall be used during this cleaning. 6. If not in effect, a respirator program shall be established in accordance with the Occupational Safety and Health Administration (OSHA) Standards, Title 29, U.S. Code of Federal Regulations (CFR), Part 1910.134. (Attachment 2). 7. HEPA vacuum cleaner filters containing asbestos dust, cloths or brushes used for wiping, brake and clutch assemblies, and all liquid used for wet brushing shall be disposed of in accordance with U.S. Environmental Protection Agency (EPA) regulations. These regulations state that the asbestos waste shall be disposed of in sealed impermeable bags or other containers at a disposal site which meets EPA criteria for asbestos dis posal. Also, the waste containers shall display the following warning label or tag printed in letters of sufficient size and contrast to be visible and legible: Breathing CAUTION Contains Asbestos Fibers Avoid Breathing Dust Asbestos Dust May Cause Serious Bodily Harm The EPA regulations for proper asbestos waste disposal are detailed in Title 40, CFR, Part 61, Subparts A and B. 37 8. A NIOSH certified respirator approved for asbestos shall be worn during removal of vacuum bags which contain asbestos dust. 9. Consumption of food and beverages shall not be permitted in work areas where asbestos exists. An area designated for food consumption shall be separate from the work area so as to provide maximum protection against asbestos dust contamination. 10. If the employee is exposed to airborne concentrations of asbestos fibers which exceed the OSHA ceiling level, the OSHA requirement regarding special clothing, change rooms, locker, etc. as detailed in Title 29, CFR, Part 1910.1001 (D) shall be followed. The current OSHA asbestos standard is as follows: the 8-hour time-weighted average (TWA) airborne concentrations of asbestos fibers to which any employee may be exposed shall not exceed 2 fibers, longer than 5 micrometers in length per cubic centimeter of air (fibers >5 ym/cc).The ceiling airborne concen tration to which no employee may be exposed shall not exceed 10 fibers >5 ym/cc. OSHA in 1975 proposed an 8-hour TWA of 0.5 fibers >5 ym/cc with a permissible ceiling exposure of 5 fibers >5 ym/cc for any period not exceeding 15 minutes. NIOSH currently recommends that the TWA exposure to asbestos be 0.1 fibers >5 ym/cc with a ceiling exposure of 0.5 fibers >5 ym/cc for any 15-minute sampling period. , 38 NOTE: Strict adherence to the above procedures should minimize exposures to employees during brake and clutch servicing. These recommendations are based on the results of research conducted by NIOSH. Prepared By: Division of Surveillance, Hazard Evaluations, and Field Studies National Institute for Occupational Safety and Health Cincinnati, Ohio 39