Document B5vrY5mKNbMRRggV59MynVeBE

JournalafToxleofogy and Environmental Health. Part 8,7:25-80,200* Copyright 0 Tijrtof &. Francis Inc. ISSN: I09KM04 print/ 1521-6950online DOi: .KM0SQ/IO9374OO39O22J42I fTsyior&Ffands m healthjdencei ENVIRONMENTAL AND OCCUPATIONAL HEALTH HAZARDS ASSOCIATED WITH THE PRESENCE OF ASBESTOS IN BRAKE LININGS AND PADS (1900 TO PRESENT): A "STATE-OF-THE-ART" REVIEW Dennis J. Paustenbach,1 Brent L Finley,1 Elizabeth T. lu,3 Gregory P. Brorby,1 Patrick j. Sheehan1 'ChemRisk, San Francisco, California, USA 2 Exponent, Santa Rosa, California, USA 3 Exponent, Oakland, California, USA Throughout the history of automobile development, chrysoSk asbestos has been an essential component of vehicle brake linings and pads. Acceptable alternatives were not fully developed until the 1980s, and these were installed in vehicles produced over the past decade. This article presents a "stitie-of-the-an' analysis of what was known over time about the potential environmental and occupational health hazards associated with die presence ofchrysotfie asbestos in brake linings and puds. As part of this analysis, the evolution of automobile brakes and brake faction materials, beginning with the early 1900$, is described. Initial concerns regarding exposures to asbestos among workers involved in the manufacture offriction products were raised as early as 1930. Between 1930 and 1959, eight studies were conducted for which friction product manufacturing workers were putt of the pxspxilaihm assessed. These studies provided evidence of asbestos's among highly exptosed workers, but provided little information on the magnitude ofexpxmre. The US Public Health Service proposed the first ocotfUtkmaf guideline for asbestos exposure 1938. The causal nda&mhip between asbestos exposure and lung cancer was confirmed in 1955 in asbestos textile workers in the United Kingdom, and later, in 1980, in South Africa, mesothelioma was attributed to asbestos exposure to even mlativefy- hw afttome concentrations of ooddofite. Between 1960 and 1974, five epidemiology studies of friction product rmiHifaauring workers were conducted. During this same time period, the initial studies of brake Bnlng wear (dust or debris) emissions were conducted showing that automobile heating was not a substantial contributor of asbestos fibers greater than Spttin length to ambient air. The fist exposure surveys, as weftas preliminary health effects studies, hr brake mechanics were also conducted during this period. In 1971, the Occupational Safety and Health Administration promulgated As first national standards for workplace exposure to asbestos. During the post-7974 time period, most of the information on exposure o/ brake mechanics to aaborne asbestos during brake repair was gathered, primarily from a series ofsampfing surveys conducted by the National Institute ol Occupa tional Safety and Health in the United States. These surveys indicated that the time-weighted average asbestos concentrations (about 7-6 h m duration) during brake servicing were between 0.004 and 0.28 fibers pier cubic centimeter, and the mean timewvtfghted average asncemratkm was about 0.05 fibers pec cubic centimeter. The dm aho showed that brake mechanics were not exposed to time-weighted average concentrations above workplace exposure Unties in effect at the time of the study, from 1975 to 2WZ mote than 25 epid&rMogy studies were conducted examining the risks of asbestos-related diseases in brake mechanics. These studies dearly indicated to brake mechanics were not at increased risk ofadverse health effects due to exposure to asbestos. Spseofkalfy, the stud** found no increased rids of mesothefioma or asbeslosis in brake meeftankx andno evidence that long cancerm lids occupationalgroup can be attributed to exposure to asbestos duringbrake repair. This could be due to one or a number offactors: the airborne cnncmtraifrtn ofdeysatik asbestos and the duration of exposure are too smafi to be s^nfcmt, the ehrysotile fibers are too short Us be bfoh&cally important, that chrysotile fibers am stfostwtialiyfessfiotentlhmamfMKifeftbmmmdvdag fang carnsr and mesothelioma, or other yet-fo-be-undecstood factors, finally, there were 20 stoeftes published during this time period exafoatmg asbestos exposureorasbestos-related health effects m friction product manufacturing workers. These studies indented that these workers were historically exposed to concentrations of chrysolite Biers perhaps 70 to 50 tones greater than those of brake mechanics, bus the risk of asbestos#, mesothelioma, and king cancer, if any, was not apparent, except for those workers who had some degree of exposure to amphibole asbestos during their careers. The authors flank the fafawmg Individuals ior their contributions: Krishna ABamneni, Arnold E. Anderson, Valerie A Craven, Michael Goodman, Renee Kalmes, Amy K. Madl, and Richard O. Richter. TMs research was primarily funded by Ford Motor Company, DaimierChrystef Corporation, and General Motors Corporation who have been involved in litigation related to brake dust. Some of the authors lave served as expert witnesses In liSgatSon regarding the potential health hazards to mechanics historically involved in brake repair. Address correspondence to Dennis Paustenbach, ChemRisk, 100 Spear Street Suite 525 San Francisco, CA 94105, USA. E-mail: dpaustenbadi@diemfisk.com ^"'"exhibit 25 . 26 D.). PAUST6N8ACH ET At. Braking involves the use of a controlled force to reduce speed, stop, or bold an object in a stationary position, and is accomplished by rubbing two surfaces together that produce friction. Friction is the resistance to the relative motion between two surfaces in contact. Brake linings and brake pads* are the sacrificial frictional materials that are placed in contact with the brake drum or rotor to stop the vehicle (Figure 1). Improvements In brake performance have been a necessary part of the history of the automobile. As automobiles became larger and heavier, and were able to achieve higher speeds, it became increasingly necessary to redesign brakes and friction materials to improve automobile transportation and safety (Harper, T 997). The evolution of brakes and brake linings and pads has been largely tied to the physical and chemical properties of the types of materials composing the brake linings and pads. The friction materials used in the earliest automobiles (approximately 1890 to 1910) consisted of camel hair, cotton belting, or elm wood, and later, cotton-based textile materials that were impregnated with various ingredients (Harper, 1997). However, as automobiles became more widely used, it was determined that these materials did not offer adequate driver protection, given their inability to withstand heat or control speed under a variety of conditions (Haqaer, 1997). Heat is an unwanted product of friction and must be dissipated to the surrounding environment as efficiently as possible (Baker, 1986). Early automobile manufacturers were therefore confronted with the need to design a bake that could stop vehicles relatively quickly, but not so Quickly that passengers would be thrown from their seats. Brakes were also required to function under different vehicle speeds and environ mental conditions (e.g., hot, cold, dry, and wet). At the start of the 20th century, it was shown that some chrysotiie* fibers were sufficiently long and flexible to be woven into fabric for brake linings (Harper, 1997). Unlike cotton fibers or hair, tire e c:> FIGURE 1. Diagrams of a brake lining and brake pad. (a! A brake lifting designed to be used with an internal drum brake, (b) A brake pad desgned for use with a disc brake. The term brake "finings" is used when referring to the friction material used with external band or internal drum brakes. The term brake 'pads' is toed when referring to the friction material used in disc brakes. The word 'asbestos" was orignaliy Intended by mineralogists to refer to a specific mineral series. However, "asbestos* has been defined in U.S- courts as 'a generic term for a number of hydrated sificates that, when crushed or processed, separate into flexible fibers made up of fibrils" (ZoStai, 19??). The U-S. Environmental Protection Agency (EPA) and the Occupational Safety and Health Administra tion (OSHA) have used the term to refer to sac once commercially viable, silicate minerals; chrysotiie, which is part of the serpentine mineral family, and amosite, oroddoiite, tremolite, actinoKte, and anthophy*. which are part of the amphibole mineral tally (OSHA, 1986: U.S. EPA, 1994). However, the nonfibrous forms of actinoOte, amhophytiite, and tremofite are not included in the regulatory def inition of asbestos {OSHA, 1992). Unfortunately, many of the studies mentioned in this paper did not indicate the specific type of asbes tos applicable to the study. Where possible, the specific form of asbestos applicable to a study is identified. Otherwise, the term "asbestos" will refer to one or more of the six forms of asbestos recognised by OSHA and the U.S. EPA. HWBUI0011247 ASBESTOS IN BRAKE LININGS 27 chrysotile fibers better maintained their integrity under higher temperatures (such as those generated during braking) and during increased vehicle speeds (Harper, 1997). Chrysolite ore was readily available during this time and, when wiltowed during refining, was easily separated into silky fibers of various lengths. The longer fibers tended to be curly, flexible, and easily span into doth. Chrysolite fibers also had good tensile strength and were heat resistant, durable, and pliable (Skinner et al., 1988). Because of its unique characteristics, chrysotile became a key component of vehicle brake linings starting in the early 1900s, The superiority in performance of the chrysolite fibers to that of other fibers resulted in the continued use of chrysolite through the end of the 20th century, even as vehicular modifications were made to address increasing public demands for improved vehicle performance and faster speeds {Rosato, 1959; Harper, 1997). It was not long after the initial use of chrvsotile fibers in brake linings, however, that exposure to the fibrous forms of asbestos was recognized as a possible occupational health hazard. For example, at least 10 case reports had been published between 1900 and 1930 that suggested exposure to high concentrations of airborne asbestos dust might produce lung disease (Auribatm, 1906; Marchand & Riesal, 1906 (as cited in Merewether, 19331; Murray, 1907; Fahr, 1914; Cooke, 1924, 1927; Pancoast & Pendergrass, 1925; Oliver, 1927; Simson, 1928; Seiler, 1928; Wood, 1929; Wood & Page, 1929; Stewart & Haddow, 1929). By 1930, it was clearly shown that occupational exposures to asbestos fibers in manufacturing settings could cause asbestosis, a potentially debilitating lung disease, at high airborne concentrations (Merewether & Price, 1930). A causal association between occupational exposure to asbestos and two types of cancer (lung cancer and mesothelioma) was later established in the second half of the 20th century based on studies by Doll (1955); Wagner et al. (1960); Manctiso and Coultier (1963}; and Sefikoff et at. (1964, 1965) (IARC, 1976; Levine, 1981 [in a review for the U.5. Department of Health, Education and Welfare!; Enterline, 1991). Due to concerns about disease incidence in asbestos-exposed workers, as well as the hazards posed by other chemicals and substances, occupational guidelines for airborne asbestos and a host of other chemicals were offered in the United States as earfy as the 1940s (Beaetjer, 1984). Federal regulations for asbestos, however, were not established until the early 1970s, when the Occupa tional Safety and Health Administration (OSHA), U.S. Environmental Protection Agency (EPA), and Consumer Product Safety Commission (CPSQ were formed. Over time, occupational health and environmental regulations regarding asbestos exposure became increasingly stringent, and in the Sate 1980$, the federal government banned asbestos in products, including brake linings and pads. This ban was later overturned in court; however, the national effort to remove asbestos from commercial products has continued. Today, virtually all new automobiles produced in the United States contain nonasbestos organic (i.e., nonchrysotiie) brake linings and pads. This article presents a state-of-the-art analysis of the development of dhrysotile-containing brake linings and pads,* the potential health hazards associated with these materials, and the governing regulations over the last 100 yr. This review also considers automobile dutch plates, the other asbestos-containing friction material product widely used in automobiles; however, tire focus is on brake finings and pads because dry dutch plates operate in a dosed housing and have a long wear life. Emphasis is placed on what was known over time about the potential health hazards to friction product manufacturers* and brake mechanics posed by exposure to chrysotile in brake linings and pads. This article represents the most comprehensive review of the asbestos exposure and health effects literature to date on these occupational groups. The review of the history of the automobile braking system and brake linings is based on previously published summaries. * TWs chronology applies only Co U.5. passenger cars and l%ht torcfcs. The development of automobile braking systems m other countries differs from that which occurred in die United States, and as a resuft, die history associated wilh friction material use and devel opment m other countries wi# also vary from that of the United States. rUe term 'friction product manufacturers' s used to refer to those involved in she manufacturing of brake linings and pads. This more generic term is used ever 'brake lining manufacturers,'' because some of the stucSes died refer to workers who manufactured fric tion products other than brake linings or pads. 28 D.M*AUSTE^ACHETAt- This analysis is divided into three time periods: 1900 to 1959, 1960 to 1974, and 1975 to 2002. These were selected based on what were perceived to be seminal events. The first period covers the use of chiysotlfe fibers in brake linings and the growing awareness and eventual recogni tion of asbestos-related diseases in manufacturing workers. The second period is defined by the expanding use of the disc brake by automobile manufacturers; die initial studies of asbestos emissions from automobile braking and occupational exposure of brake mechanics; and establishment of the first federal regulations governing asbestos exposure. The third and final period is marked by the 1975 meeting between researchers from Mount Sinai School of Medicine (Mount Sinai), the National Institute of Occupational Safety and Health (NiOSH), and labor and industry representatives, where the concern regarding brake mechanics and their potential exposure to chrysolite fibers in brake linings and pads was raised and during which most of the exposure surveys and epidemiological studies of brake mechanics were conducted. This section also discusses the search by friction product, brake, and automobile manufacturers for alternatives to chrysolite fibers in brake finings and pads. For each time period, the following topics are addressed in the following order: (1) major developments in brakes and brake linings and pads associated with passenger vehicles and fight trucks used in the United States, (2) relevant brake safety and performance issues, (3) exposure and health effects studies on brake mechanics and friction product manufacturing workers, (4) toxicology studies on asbestosrelated diseases, and (5) guidelines and regulations governing occupational exposure to asbestos that were in effect This approach is intended to illustrate at which time specific scientific knowledge about brakes and brake lining? and pads, exposure to asbestos during brake repair, as well as asbestos toxicity, was gained over the past century. THE EARLY YEARS (1900-1959) During this time period, the use of asbestos in commercial products expanded dramatically. Vehicle brake linings made of chrysotile were initially used in the United States and Europe about 1910. From 1900 to 1959, significant advances were made in the manufacture and design of brakes and brake linings. These developments, as wdf as related events described in later sections, are summarized in Table 1. in 1930, the first epidemiology study was published showing that exposures to high concentrations of asbestos in dusty manufacturing settings resulted in asbestosis (Merewether & Price, 1930). In 1938, the first guidance level for asbestos in the workplace was proposed (Dreessen et ai, 1938). It was two decades later that a causal relationship between asbestos exposures at levels draff produced asbestosis and lung cancer was documented for manufacturing settings (Doll, 1955). It was the next decade before the first epidemiological studies of asbestos workers in the United States showing an association between asbestos exposure and lung cancer (Mancuso & Couftier, 1963; Selikoff et al., 1964) and mesothelioma (Selikoffetal., 1964,1965) were published. During the 1900 to 1959 period, the focus of health effects studies was primarily on asbestosis and other forms of non-malignant lung disease in-dusty industries, little consideration was given to the types of asbestos to which workers were exposed or differences to exposure among occupations. There were three studies during this early time period that specifically looked at asbestosis in friction material manufacturing workers (George & Leonard, 1939; Brachmann, 1940; Stone, 1940). The data provided during this time period were, however, inadequate to describe the dose-response relationship between airborne concentrations of asbestos and the prevalence of asbestosis among friction product manufacturing workers, and no data were provided on the potential exposures of or possible health hazards to brake mechanics who worked to less dusty settings. Developments in Brakes and Brake Linings and Pads In an analysis of the early years of transportation safety, Yanik (1997a) concluded that automobiles were not considered a significant hazard to drivers or bystanders, even with their rudimentary braking and steering apparatus at the turn of the twentieth century. This finding is not surprising gjven that there were only about 8000 motor vehicles in the United States in 1900 (Sundstrom, 1985). While the stopping distances afforded by the brake systems during this time were excessive (he., a distance of 59 ft was necessary to stop a car at 20mph), automobiles were so few and driving speeds were so HWBUI0011249 ASBESTOS IN 8RAM UNINGS 29 TABUE1. Ttmefirse of Key Events Renting to the Development of Slakes, Stake linings, and Stake Performance Standards for Passenger Ore and Light Trucks Woven Cotton to Woven Asbestos Brake linings 1901 Herbert Frood develops and patents cotton-based textile materia! brake lining 1902 Disc brake is patented ibut Is not common!/ used) 1903 introduction of the internal-expanding brake mechanism 1909 foybestos develops woven asbestos and brass wire brake lining 1908 Herbert Frood develops composition brake linings 1920s Molded bake linings wSh Internal drum brakes become popular 1922 U.S. Bureau of Stendaitfe develops standardized test for severe service conditions and long weai/dwabiilty of brake Bntngs tonovatton to the Friction Material and Brake lining Industry 1930s internal-expanding drum brake dominates the market 1946 Fifst asbestos exposure limit is established CS mppd) 1948 introduction of bonded brakes 1959s introduction of sintered metal finings Mid-1950s Tapered, bonded, riveted linings available directly from automobile factory 1960s Introduction of semitnetaMIc brake fining formulations 1965 Introduction of the disc brake in U~S. passenger cars 1966 National Motor Vehicle Safety Act t$ passed establishing the National Highway Safety Bureau Cater renamed the National Highway Safety Administration) 1968 Disc brakes become the norm for front brakes of automobiles Asbestos Substitutes In Brake linings 1970s 1975 1986 1989 1991 1993 2000 Initiation of research for non-asbestos organic brake finings ftat federal standard tFMVSS105) developed--defines stopping distance U.S. EPA proposes banning manufacture of ssbestos-conta Ining goods, inducting friction materials U.S. EPA passes the Ban and Phase Down Rule U.S, Court of Appeals annuls the VS. EPA ban, but companies continue to phase out asbestos-containing brake linings Per the original U.5. EPA ban, phase-out begins for original equipment manufacturers of passenger cars and light trucks Virtually all U.S. passenger cars and light bucks use asbestos-free brake linings and pads low (t.e,, no more than 9-16mph) that collisions were infrequent and often not very serious. Further, because of the openness of the roads, drivers were frequently able to steer out of the way of accidents {Vanik, 1997a). As automobiles became more popular, however, public awareness of brake performance and automotive safely issues increased significantly. Shift from Woven Cotton and Hair to Chrysolite-Based Brake Linings Automobiles in the fate 19th century had either a small brake block applied directly on the tire tread or external band brakes. These latter brakes consisted of a flexible steel band lined with woven frictional material that was drawn tight against tire outside of a steel or cast iron drum on the rear wheels, thereby causing friction and braking action (Ferodo, 1968; Newcomb & Spurr, 1971). Elm wood, camel hair, and cotton belting were initially used as friction materials, but after 1901, these materials were replaced with Herbert Frood's more temperature-resistant cotton-based brake block (Harper, 1997). These cotton-based friction materials maintained their integrity as long as the bulk temperature generated-during vehicle braking did not exceed approximately 250 C (Newcomb & Spurr, 1971). However, when the brake linings underwent severe use with continuous application, such as downhill descents, the heat generated during braking charred the cotton-based materials and rendered them ineffective (Harper, 1997). This failure in performance resulted in increased risks to drivers and passengers, as well as other drivers or nearby pedestrians. Safety precautions became even more pronounced with the introduction in 1903 of an internal expanding braking mechanism that effectively sealed off the lining; from the cooling air (Harper, 1997), and by 1910, there was an increased number of vehicles on the roadway (approxi mately 500,000 vehicles; McGeveran, 2001). At this time, a substitute friction material was necessary to meet the evolving needs of brake systems. 30 D. I. RAU5TEN8ACH ET AU Although the use of chrysolite in brake linings* was first conceived in the late 1800s (Hutchings 18961, about tOyr had passed before this idea materialized into a product In 1906, the Raybestos Company introduced a brake fining made from a combination of woven chrysotile and brass wire in the United States (Automobiles, 1922; Stenberg, 1935?, while in 1908, Herbert frood (who is credited with inventing composition brake linings? introduced chrysotile brake linings in the United Kingdom (UK? (Nicholson, 1995). The first test of the adequacy of an asbestos-based lining reportedly occurred in July 1907, where an asbestos brake lining showed no wear, while a leather lining charred, after equal brake usage on a 4000-lb test car (History, 1935?. From that time until the early 1920s, woven asbestos brake finings were predominantly used in the automotive industry (History, 1935?. The asbestos textile fabric used in these linings was woven from tong chrysotile fibers spun into a continuous length and these yams sometimes included wire reinforcement, such as brass, copper, or zinc (Nicholson, 1995}. The process used to make brake linings involved carding, spinning, and weaving of the chrysotile fiber into various textile fabrics and was similar to that used in the woof and cotton industries (Automobiles, 1922). The fabric was then impregnated with various oils and resins to provide the desired frictional properties (Sosils, 1929), and once impregnated, the fabric was drained, dried, calendered (i.e., subjected to a rolling operation to straighten out the linings and Karate constant thickness), and then cut to width. The lining material was then baked for several irs. After this curing, the hardened material was cut to size using abrasive saws or band saws (Nicholson, 1995). It was normal to handle woven materials as far as possible in the process as coils and to control the thickness and width by rolling rather than grinding (Nicholson, 1995). Although different weaving techniques and manufacturing processes were developed over time to improve the performance of the woven chrysotile textile linings, the lack of homogeneity of the woven textile material became a limiting factor as vehicle loads and speeds continued to increase (Sneed, 1929; Soulis, 1929; Harper, 1997). The development of improved versions of die woven fabric material was further limited by the relative scarcity of the long chrysotile fiber, which was needed for the making of yarn used for a variety of other asbestos products (Harper, 1997). Rising Popularity of Internal Drum Brake and Molded Brake Linings Around 1920, a completely new process evolved where shorter chrysotile fibers were mixed with certain resins, oils, and other additives and subjected to pressure in heated dies (i.e., molds) to form the required shape for a lining (Soulis, 1929; Stenberg, 1935; Harper, 1997). The term "molded linings" was, thereafter, used to identify all types of brake linings that were cured in molds under pressure (Rosato, 1959). This new generation of finings included both dry and wet mixed molded formulations, as well as impregnated or nonhmpregnated millboard (Rosato, 1959). The molding process completely revolutionized the industry, enabling the manufacturer to use chrysotile fiber grades that were plentiful and less costly, in combination with whatever resins and additives were required, to produce a material with more homogeneous friction characteristics and better wear properties (Harper, 1997). In addition, the process allowed researchers to vary the composition of the blend to create molded friction materials with different characteristics needed to stop the various types of vehicles on the market (Motorist's, 1968; Ferodo, 1968). Molded linings together with the internal drum brake became popular in the late 1920s due to more severe service requirements for automotive brakes. Because the shoes in internal drum brakes were rigid, the requirements for flexibility in the linings, as was necessary for external band brakes, no longer applied, and molded linings could be used (Motorist's, 1968). Molded linings, unlike woven ones, could incorporate powdered resins that enabled the finings to maintain more consistent friction behavior at hidier temperature ranges (Motorist's, 1968; Harper, 1997). Simply changing the amounts of the different raw materials added to the mixture could readily after the composition of molded linings. This ability to tailor the composition of molded linings for the various types of * Chrysotile asbestos fibers ware the only type of asbestos fibers incorporated into the brake liftings and pads for the passenger cars and light bocks soW in the United States (Rosato, 19S9; A. E. Anderson, personal oomrauntaation, 2002!. The majority of articles reviewed used the generic term `asbestos' to describe chrysotile fibers in brake linings and pads. The authors of this artfcSe have toed to identify (he specific asbestos types where possible without compromising the intent of the source article. 1it m HWBUI0011251 ASBESTOS IN BRAKE LININGS 31 vehicles on the market resulted in an increased market demand for molded linings in lieu of woven ones. By the mid-1930s and over the next 30 yr, the internal-expanding drum brake dominated the market (Newcomb & Spurr, 1989). External brakes continued to be used as the parking brake for select car models until at least the mid-1960s (Bendix, 1966; Anderson, personal communication, 2002). Continued Innovation in the Friction Product and Brake Lining Industries During this time period, there were several additional important developments in the friction product industry. For example, rivets were used to attach brake linings to shoes prior to the 1940s and replacement brake linings were available as prefinished segments, or could be cut and machined from large rolls. It was during these cutting drilling and grinding operations that exposures to chrysotile fibers could occur (Sheeny et at, 1989). By the late 1940s, however, bonded brakes were introduced and mechanics merely exchanged old brake shoes for ones with lining already attached to them (Bonding, 1948). The concept of preattached lining and shoe sets became popular for both bonded and riveted linings (Anderson, personal communication, 2002). Because entire shoes were replaced, the need to cut, drill, or rivet replacement brake linings was eliminated, and the potential for exposure to chrysotile by brake mechanics was reduced. For a limited' period of time in the mld-1950s, when brake shoes were first installed with a fixed anchor, brake mechanics sometimes needed to taper the new brake linings to achieve a proper fit Shortly thereafter, however, tapered, bonded, or riveted linings were available from the factory. After 1960, considerably fewer beveling or grinding operations were performed by automobile mechanics replacing brake linings (Sheehy et al., 1989). In the 1950s, sintered metallic lining were introduced. These lining? operated effectively under severe usage conditions and at high temperatures, such as those experienced in some police cars or taxicabs (McCuen, 1959; Reinsch, 1970). Sintered metallic linings were made using powder metal lurgy techniques and contained combinations of metallic powders, ceramic powders, and "green" binders (i.e., materials that hold the compact together prior to firing). This type of friction material, which could be iron or copper based, was predominantly used in heavy-duty applications. Some sintered ferrous drum brake linings were later released for a few original equipment manufacturer car applications. However, these finings tended to be sensitive to environmental conditions (e.g., tem perature and moisture) and required higher pedal pressures (Anderson, personal communication, 2002). These factors limited their commercial applications such that they were not considered a plausible substitute for asbestos-based lining on autos at that time (ASME, 1988), Brake Safety and Performance Standards In the early years of brake lining development, it was up to the individual manufacturers of friction products, in conjunction with auto manufacturers, to determine the appropriate balance between efficiency in stopping a vehicle versus the wearing and durability qualities of a friction material (Automobiles, 1922). Eventually, the friction product, brake, and automobile manufacturers developed laboratory and field tests to ensure that the braking system of a vehicle would perform up to a minimal standard set by the individual automobile manufacturers. These early internal tests eventually evolved into the laboratory and field performance tests similar to those used today by friction product, brake, and automobile manufacturers to test vehicles prior to their commercial release (e.g.. Pikes Peak, Detroit City Traffic) (Anderson, personal communication, 2002). The labo ratory and field programs were designed to test how well the car and its associated braking system, not just the brake lining, behaved in a variety of operating conditions intended to simulate real-life and nonideal conditions (i.e., hot; cold, dry, and humid environments, steep descents). Over time, as problems or deficiencies in performance arose during actual long-term use of various car models, new field tests were developed and specified by the automotive industry. While the individual auto manufacturers had variations of each test type (i.e., high-altitude environment, hot-temperature environment) tailored to their own specifications, generally all of the U.S. auto manufacturers ran the same types of tests with the philosophy that car models would not be released for use by the public until these were passed (Anderson, personal communication, 2002). For many decades, these internal standards set by the automobile manufacturers were the only requirements of vehicle braking systems; there were no federally mandated safety standards in effect during Shis era. 32 0, |. PAUSTTNBACH FT AL TABLE 2. Timeline of Case Reports Published Prior to the first Epidemiology Study of an Asbestos-Exposed Worker Population 09001929} Decade Year Topic Reference 1900s 1910s 1920s 1906 1906 High mortality In an asbestos textile factory is reported and considered probably due to a pneumoconiosis from asbestos dust "Unusual bodies" noted in asbestos worker's Sungs 190? 1314 1924 1925 1927 1927 1928 1928 1929 1929 1929 "Asbestos spicules' observed in lung of asbestos worker who died of pulmonary fibrosis without tuberculosis (initially observed in 1899} Asbestos workers with pneumoconiosis, including large "crystals" in lungs lung fibrosis in asbestos worker attributed to inhalation of asbestos dust Actual danger In asbestos factories considered comparatively sllgjrt- Authors dispute Cooke's 1924 condusicms. first reviews of asbestosis. "Curious bodies' identified in pulmonary tissues from an asbestos worker. Several asbestos-refated deaths in British asbestos factories likely mfereported as being refated to tuberculosis "Curious golden yellow segmented structures* reported in Song tissue of four asbestos miners So Rhodesia A case of pneumoconiosis attributed to Inhalation of asbestos dust rather than tuberculosis Several cases of pulmonary fibrosis (likelyasbestosis} in subjects who had been exposed to asbestos dost Asbestosis observed In subject who worked as a weaver id an asbestos factory "Asbestosis bodies" recommended to replace the term "curious bodies." Authors identify the present ofasbestosis bodies in sputum samples as well as pulmonary tissues. Auribauit 11996} Marchand and ffltesal {1906} {as cited in Mwewether 19331 Murray 0907} Fahr {5914} Cooke {1924} Paneoastand Pendergrass {1925} Cooke <1927!' Oliver (1 927) Simsctr (1928) Seiler (1928) Wood (1929) Wood and Page (19295 Stewart and Hadckjw(1929! Health Effects Studies During the first 30 yr of the 20th century, there were only a few case reports suggesting that exposure to asbestos might be associated with adverse health effects, either similar to those associated with other dusts or, later, adverse effects unique to asbestos. However, as the use of asbestos became pervasive in society, there was a corresponding increase in the number of case reports. Not surprisingly, additional studies were initiated to understand asbestos-related health hazards. With few exceptions, die workers evaluated in those studies performed activities in manufacturing settings (e.g., among the dustiest environments) and were not end users of asbestos-containing products. Brake mechanics were therefore not included in the earliest study populations. Some of the early studies included friction product manufacturing workers, although they were not the focus of these investigations nor were the data for friction product manufacturing workers sufficient to quantitatively characterize their exposures to asbestos or risks of contracting asbestosis. In general, the focus of many occupational health studies conducted prior to 1930 was on the dusty cades, such as coal mining and stone masonry, it was recognized that long-term exposure to highly dusty environments could lead to lung diseases--generally called pneumoconioses or "dusty lung disease" (Hoffman, 1918; Pancoast & Pendergrass, 1925). During this time period, asbestos was initially considered to be similar to certain other workplace dusts, such as silica and coal cfust, to which persons were exposed in other occupations. As shown in Table 2, the first documented case of lung disease associated with asbestos exposure was observed in 1899 and reported in 1907 (Murray, 1907). Specifically, necropsy revealed extensive and diffuse pulmonary fibrosis and the presence of asbestos spicules in the subject's fonts {Murray, 1907). Testifying before an inquiry at the British Government Commission on Occupational Disability, Murray (1907) connected the workplace exposure to airborne asbestos to the scarring he observed in the worker's lungs (Kilbum, 1992). Around the same time, case reports were published in France and Germany (Auribauit, 1906; Marchand & Rresal {19061, as cited in Merewether, 1933; Fahr, 1914), and factory inspectors in the UK and the United States were reporting about the possible HWBUI0011253 ASBESTOS IN BRAKE IININGS 33 dangers associated with asbestos dust observed during their surveys of manufacturing environments (Anonymous, 1898; Collis, 1911; Hoffman, 1918). More than 10 years later, Cooke (1924) published a description of pulmonary fibrosis in a woman who had worked for 20 years in an asbestos textile factory. Shortly thereafter, the term "asbestosis" was used to describe the asbestos spicules observed in the lungs of case report subjects, and the term "pulmonary asbestosis" was used to describe the pneumoconiosis condi tion observed (Cooke, 1927, 1929; McDonald, 1927; Oliver, 1927). By the late 1920s, addi tional case reports were published in the literature, all of which contributed to a growing awareness that inhalation of asbestos dust might be a potential cause of .disease (Pancoast & Pendergrass, 1925; Seiler, 1928; Simson, 1928; Stewart & Haddow, 1929; Wood, 1929; Wood & Page, 1929). However, these early case reports provided little, if any, information regarding the specific activities of the workers, the concentration of airborne particles, or details about'the dis ease in these workers. Often, these reports were also complicated by the presence of tuberculosis OB), making it unclear whether the lung dysfunction or patnology was primarily caused by asbestos or TB, or whether one disease had to precede the other (Pancoast & Pendergrass, 1925). Further more, these workers bad also often been exposed to other dusts during their careers, were smokers, and may have had lung infections--all factors that made it more difficult to clearly associate asbestos exposure with respiratory disease. For example, it was recognized that "the proneness of workers with dust fibrosis to be affected by pulmonary tuberculosis has been shown to be the main cause of the increased mortality rate from the latter disease in certain dusty occupations" (Merewether & Price, 1930, p. 5). Thus, although there were a number of case reports about asbestos, their usefulness was limited. The purpose of case reports is to describe interesting and unusual observations, but they cannot be uski to determine disease causation (Last, 2001). Rather, an epidemiology study that considers exposure to the agent (and confounders), the back ground rate of the disease, and the rate of the disease irt the exposed population is needed to confirm a causal association. Confirmation of the health hazards specific to asbestos exposure occurred around 1930 after Merewether and Price (1930) conducted the first epidemiology study* of asbestos manufacturing workers, which was brought about by an earlier case report of3 worker with asbestosis who did not appear to have TB. In this cross-sectional study conducted in the United Kingdom, the population was divided into groups representing the following manufacturing processes: (1) crushing, opening, disintegrating, and mixing; (2) carding; (3) spinning, twisting, doubling, and plaiting; (4) insulating mattress making; (5) weaving and associated processes; and (6) miscellaneous processes ana unclassified workers (Merewether & Price, 1930). Merewether and Price (1930) concluded that there was a dear risk of developing asbestosis from inhalation of asbestos dust, and that duration of exposure and amount of dust inhaled were key factors for predicting disease. This study did not provide data on the dose-response relationship between asbestos exposure and disease, but appears to have resulted in the British Asbestos Regulations of 1931 (His Majesty's Stationery Office, 1931), which required monitoring and dust control in industries where workers were exposed to asbestos (Bartrip, 2001). Following their study, the number of asbestos-related health effects studies of worker population published in various journals increased dramatically. These studies considered asbestosrrelafced exposures and nonmalignant disease incidence in a number of occupations, including textile manufacturers, mattress makers, miners, and other manufacturing operations* (Osborn, 1934; Wood & Gloyne, 1934; Fulton et al., 1935; Home Office; 1935; Lanza There ate two types of commonly used observational epidemiological studies: cohort and case-control. In cohort studies, the investigator selects a study population of exposed and rtonexposed individuals and follows both groups to compare disease tnddence rates or disease-specific mortality rates In the two groups. The case-control study divides the study participants into two groups: the first group (cases! includes people who have the disease to be studied and the second group IcontrobJ consists of people who do not have the disease. Epidemiologists then obtain and compare data regarding past exposures by both groups. A separate category of observa tion studies is surveillance studies, which use preexisting data sources such as death certificates to determine cause of death and recorded occupation. Record-linkage studies link disease and occupational registries to determine occupation-disease associations. An adcfitional study design is the ooss-seolonal study that examines the relationship between disease and other factors, such as exposure, at a point in time. 34 0. J. PAUSTENBACH ET AL af., 1935; Page, 1935; Donnelly, 1936; McPheeters, 1936; Windel, 1937 (as cited in Teleky, 1937]; Dreessen et al,, 1938; George & Leonard, 1939; Brachmann, 1940; Stone, 1940; Vigliani, 1940 [as dted in Teleky, 1941 J; Fleischer et al., 1946, WegsBos, 1947; Castrop, 1948; Lynch & Cannon, 1948; Canepa, 1949; Barnett, 1949; Cartier, 1952; Lynch, 1953; Breslow et al., 1954; Knox & Beattie, 1954; Bonser et al., 1955; Cartier, 1955; Doll, 1955; Frost et al., 1956; Williams, 1956; Thomas, 1957; Braun & Truan, 1958; Honai etaL, 1958). With few exceptions, the workers identified in these studies performed activities in manufacturing settings, and were not end users of asbestosbased products. Most studies during this period did not identify the type of asbestos or quantify the concentration of fibers to which workers were exposed. Fulton et at. {1935) did indicate that chrysotile composed about 95% of the asbestos in commerce in the United States during the 1930s. Brake linings for automobiles in North America and Europe are believed to have contained only chrysolite fibers, although amphibole fibers are reported to have been used in some railroad ertjpne brake linings in the UK during this time period (Newhouse et al., 1982), and amphibole fibers, sometimes in combination with chrysolite fibers, were also used during this period in insulation and other products such as asbestos cement pipe (Fleischer et al., 1946; Finkebtein, 1983). The absence of specific information on chrysotife and amphibole fiber concentrations experienced by populations with potential mixed fiber exposures complicates the understanding of the contribution of fiber type to reported health effects for workers during this era. Exposure and Health Effect Studies of Workers in the Manufacture of Asbestos Brake linings Of the more than 30 studies of workers in asbestos-related industries that were published between 1930 and 1959 referenced earlier, eight discussed in varying levels of detail the potential exposures to and/or possible diseases observed in friction product manufacturing workers but none provided a quantitative evaluation of asbestos exposure nor incidence of asbestosis (Merewether & Price, 1930; Osborn, 1934; Fulton et al., 1935; Lanza etaL, 1935; George & Leonard, 1939; Brachmann, 1940; Stone, 1940; Thomas, 1957) (Figure 2). As indicated earlier, Merewether and Price (1930) conducted the first epidemiology study of asbestos workers in primarily the textile branch of the industry, which likely included some workers employed in the manufacture of woven brake linings, although no specific information on this com ponent of the study population was provided. They concluded that there was an increased inci dence of asbestosis in asbestos manufacturing workers in the dusty factory environments evaluated in the UK. No quantitative exposure information was provided in this study, which is not surprising given that the field of industrial hygiene was just emerging. Despite the lack of quantitative information, Merewether and Price (1930) reported that they believed asbestos could be safely handled in the manufacturing setting with the implementation of proper industrial hygiene engineering controls: "From foe consideration of the nature of the processes in the asbestos industry, and other relevant matters, it is felt that the outlook for preventive measures is good. That is to say that in the space of a decade, or thereabouts, the effect of energetic application of preventive measures should be apparent in a great reduction in the incidence of fibrosis" (Merewether & Price, 1930, pp. 17-18). Several years later, Osborn (1934) reported air sampling results from four asbestos plants that produced woven and molded brake lining:, dutch facing, and wire Insulation. Dust measurements collected during these operations ranged from less than 1 to 82 million particles per cubic foot (mppcf), depending on the operation and whether dust control measures were in place (Osborn, 1934).* No analysis of health effects was performed in this study. Around this same time period, Fulton et al. (1935) conducted a cross-sectional study of asbestos factory workers from various industries including woven brake lining manufacturing. Six air samples of dust from two plants in areas where the weaving of brake linings took place showed concentrations of 8.5 to 34.7mppcf. Induces studies published in of translated into English that provide data on the incidence of an asbestos-related disease in a worker population, and is not intended to be a complete fct of asbestos-related pubScadonsduring this era. The impingei- colleefioiVlight field illumination dust counting method was the common method for determining dost concentra tions m afr from the late 1930s to the early ?%0s. Air concentrations reported by this method are in milSon partldes per cubic foot of air {rnppdS and are total dust counts, which are nonspecific for fibers. A thorough evaluation of asbestos sampling techniques is provided in Walton fJ982). Ml HWBUI0011255 1 V 9 35 HWBUI0011256 36 D. |. MUSTEKBACH ET AL Fulton et al. {1935} also reported that 14 of 56 workers {approximately 25%) evaluated from the textile departments of the four fabricating facilities studied had slight or moderate symptoms of asbestosis. Whether any of these workers were involved ip the manufacturing of brake linings was not specified. Lanza etal. {1935) conducted a similar cross-sectional study of 126 workers in asbestos fabricating plants in die United States, including one facility that produced brake bands and clutch plates. Airborne dust samples collected In the molded brake band and clutch department showed concentrations of one-half and three-fifths mppcf with asbestos comprising about 25% of the dust. Lanza et al. (1935) reported that 67 of the 126 workers studied were diagnosed with asbestosis; however, it is again unclear how many, if any, of these were involved in the manufacture of brake lining; and dutch plates or what specific exposures they had experienced. Lanza et al. (1935) concluded it was "not practicable as yet to establish standards for die asbestos dust content of air" (p. 11), and likewise Fulton et al. (1935) noted that it was not "possible from our findings to establish the maximum safe concentration of asbestos dust in the air" (p. 28). Several years later, more specific evaluations of the health effects of asbestos in friction material manufacturing workers were conducted. George and Leonard (1939) and Stone (1940} examined cases of asbestosis in brake lining manufacturing workers as part of disability daims submitted to insurance companies. These researchers diagnosed asbestosis in 13% {George & Leonard, 1939) and 82% {Stone, 1940) of the workers examined, respectively, but neither study provided quantitative information on the concentration of airborne dust to which these workers may have been exposed nor the criteria used to diagnose asbestosis. Furthermore, it was not known whether the effects were caused by handling or processing raw asbestos or due to exposure to dusts associated with fabricating (he molded lining. Similarly, Brachmann (1940) in a cross-sectional study reported asbestosis in a subset (number unspecified) of 151 textile brake lining manufacturing workers involved in grinding and drilling operations in Germany, but no quantitative information on airborne dust levels was provided. In the mld-1950s, Thomas (195 7) conducted a cross-sectional study of asbestos factory workers from various industries in Australia including brake lining manufacturing. Although this researcher observed asbestosis in 15% of the workers, the study (as in previous studies) did not specify whether any of the workers diagnosed with asbestosis were involved in the manufacture of brake linings (Thomas, 1957). In addition, no quantitative information on the concentration of airborne dust to which these workers may have been exposed was provided. Although studies were conducted from the 1930s to the 1950s that investigated asbestosis in asbestos manufacturing workers, including friction product manufacturing workers, most of these studies were not specifically focused on friction product manufacturing workers. These studies did show asbestosis among highly exposed worker populations; however, they often contained inadequate information to determine the asbestos exposures of these workers, and therefore were insufficient to estimate the dose-response relationships for friction product manufacturing workers and other manufacturing workers exposed to asbestos. The tasks of the workers diagnosed with asbestosis were not specified; quantitative exposure information was not provided; and quantitative diagnostic criteria were not applied in identifying asbestosis. Finally, these studies did not address lung cancer. There also were no studies of brake mechanics during this period. Because the studies of friction product manufacturing workers during this era were insufficient to characterize exposures and disease incidence for this occupation, they too provided no insight as to the potential asbestos exposures or risks to brake mechanics. The working conditions as well as the types and duration of activities potentially contributing to asbestos exposures of the two groups of workers were very different from one another, and end users of asbestos such as brake mechanics were generally considered to work in far less dusty environments than friction material manufacturing workers and therefore to be at lower risk of adverse health effects, The Dreessen Study in the late 1930s, the assistant U.S. Surgeon General and several coworkers were the first researchers to quantify exposure to asbestos and relate it to specific health effects (Dreessen et al., 1938). This study was commissioned and conducted by the U.S. Public Health Service, the primary environmental and occupational health agency within the federal government during that time. The study consisted of 541 employees of asbestos textile factories in North Carolina HWBUI0011257 ASBESTOS IN BRAKE tiNINCS 37 and, based on review of the air sampling data and the observed incidence of asbestosis in workers, it was conducted that exposure to airborne concentrations of less than Smppcf of dust containing asbestos would not produce an increased risk of developing asbestosis. These researchers further con cluded that "if the dust concentration in asbestos factories could be kept below 5 million particles,,,new cases erf asbestosis probably would not appear" (Dreessen et al., 1938, p. 1??). Although considered a thorough study for the time (McDonald, 1984), one shortcoming, which Dreessen noted, was that the empfoyees studied were much younger than the average industrial employee and that they had been employed in this industry for relatively short periods of time. Spedfically, about 85% of the employees studied had been exposed for less than lOyr (Dreessen et at, 1938). Little Concern Expressed During This Era for End Users of Asbestos-Containing Products During this time period, thousands of products containing asbestos fibers were being developed with numerous end uses; however, only a few asbestos-related studies were published that identified asbestos-related exposures to end users of asbestos-containing products, as shown in Figure 2 (Fleischer eta!., 1946; Canepa, 1949; Breslow et al, 1954; Frost et at, 1956). The largest epidemiology study involving end users of asbestos-containing products, conducted by Fleischer et al. (1946), indicated that few cases of asbestosis were observed in more than 1000 shipyard pipe fitters exposed to amosite asbestos fibers in pipe-covering material. Based on this observation, the researchers concluded that covering pipe with asbestos insulation, known to be a dusty task, was not a dangerous occupation. Similar to the Dreessen et al. (1938) study, a shortcoming of this study is that only 5% of those examined had worked in this occupation for more than lOyr (Fleischer et al,, 1946). Nevertheless, over die course of the next 15yr, the focus of asbestos-related health effects studies continued to be on workers in manufacturing environments, and not on end users of asbestos products. Not surprisingly, no industrial hygiene or worker health studies were therefore published on brake mechanics (who used finished products) prior to 1970. Confirmed Association Between Asbestos Exposure and Lung Cancer Although there were numerous case reports published in the 1930s and 1940s suggesting that exposure to asbestos might be linked to an increased risk of Sung cancer, it was not until 1955 that a clear epidemiologi cal association between asbestos exposure and lung cancer was confirmed (Doll, 1955). Prior to that time, it was difficult to know whether smoking; asbestos; exposure to other dusts; previous lung ailments, such as tuberculosis; or other factors (individually or combined) were the cause of an increased incidence of lung disease among exposed asbestos workers. Specifically, Doll con ducted a cohort mortality study of 113 men and, on the basis of the results, concluded that asbes tos workers with 20 or more years of experience had 10 times the risk of developing lung cancer compared to the general population. In this cohort, aO 11 men whose deaths were attributed to lung cancer also had signs of asbestosis. Doll further observed that the incidence of both asbestosis and lung cancer associated with asbestos exposure greatly diminished as tire number of years during which workers were exposed prior to 1930 decreased (Le., prior to implementation of dust control measures in the UK). Thus, it was unclear to him whether the increased incidence of lung cancer was due only to very high exposure to asbestos or whether there might also be an increased risk at much lower airborne concentrations. Specifically, he noted that "Whether the specific industrial risk of lung cancer has yet been completely eliminated cannot be determined with certainty; the number of men at risk, who have been exposed to the new conditions only and who have been employed for a sufficient length of time, is at present too small for confidence to be placed in their experience, it is dear, however, that the risk has for some time been greatly reduced" (Doll, 1955, p. 86). it was during the same time period that Breslow etal. (1954) published the first epidemiological study showing occupation and cigarette smoking as factors in lung cancer. With regard to occupa tions using asbestos, Breslow et al. (1954) conduded that "The group of steam fitters, boiler makers, and asbestos workers lies on the borderline of statistical significance when the effect of dgarette smoking is controlled"' (p. 180). In short, during the 1950s, a number of researchers were uncertain whether exposure to asbestos alone, without exposure to dgarette smoke, could increase the incidence of lung cancer. 38 0.). PAUSTENBACH ET At Toxicology of Asbestos At least until the mid-1940s, the field of toxicology was in its infancy. Until this time, very few animal toxicity studies were being conducted, and of those conducted, most were designed to identify whether any health effects occurred within one or two days of exposure (acute toxicity testing), long-term (chronic) toxicology studies that mimicked occupational exposures were gener ally not conducted until the 1950s. Additionally, the Society of Toxicology was not formed until 1960. Despite several early case reports, which raised questions about the health hazards posed by asbestos exposures, few attempts were made during this time to address the severity or dose-response relationship for asbestos diseases using worker population data or animal toxicology studies. In the 1930s and 1940s, a small number of toxicology studies were initiated to better understand the development of asbestos-related health effects, including the relationship between asbestos exposure and TB, with the intent of helping to treat or alleviate symptoms of asbestos-induced fibrotic changes observed in worker populations. These studies evaluated acute, subchronrc, or chronic exposures to asbestos via several routes of administration in various species to replicate asbestos-related fibrosis. These early studies suggested that the severity of asbestosis and the fibrotic response was dependent on the size of the fiber (i.e., length), as well as the animal species studied, with rats and guinea pigs generally identified as the more sensitive species to asbestos-induced fibrosis (Gardner, 1942; King et a!., 1946; Vorwald et ai., 1951). Vorwaid et al. (1951) were the first to publish a comprehensive sales of experiments comparing the fibrotic response of different asbestos fibers of varying lengths in several animal species. They alas reported a difference between lung response and fiber size, indicating the significance of fiber size in the pathogenesis of asbestosis. In general, these early studies identified that longer fibers induce more severe fibrosis than short fibers (King et ai., 1946; Vorwald et a!., 1951). Although asbestos-related lung fibrosis was successfully reproduced in animal models by the late 1950s, the laboratory studies at the time were unable to induce lung cancer in animals via inhalation (Vorwald et ai., 1951; Lynch et al., 1957). Only one study reported lung tumors in mice following chronic inhalation exposure to chrysolite (Nordmann & Sorge, 1941); however, results of the study were later questioned because one of the tumors they identified was an adenomatous type not uncommon as a spontaneous tumor in mouse lungs and the other lesion was questionable as to whether it may have been an area of squamous metaplasia, rather than neoplasia (Smith et al., 1965). It has also been alleged that data supporting the association between asbestos and lung cancer in animals was omitted from the Vorwald et al. (1951) study (Hardy & Egitman, 1991; Ulfcnfeld, 1991). However, as noted earlier, the association between asbestos exposure and lung cancer was confirmed by an epidemiological study (Doll, 1955) during the same time frame as the animal studies discussed earlier. Asbestos is an agent for which epidemiological studies, rather than animal toxico logical studies, provided the clearest evidence of the association of asbestos-exposure with both' asbestosis and lung cancer. Asbestos Guidelines and Regulations For die first four decades of the 20th century, there were no national guidelines or regulations to limit exposure to airborne contaminants in the workplace or outdoors (i.e., ambient air). Various states had offered some guidance values for certain chemicals and substances, but these were largely ignored or were intended for selected industries (Frederick, 1984). This is not surprising given that people Interested in industrial hygiene were first coming together in the early 1930s through organizations such as the American Public Health Association (APHA) and the National Safety Council (NSQ. The American Conference of Governmental Industrial Hygienists (ACGIH) and the American Industrial Hygiene Association (AIHA) were not formed until 1938 and 1939, respec tively. Also in 1938, the American Association of industrial Physicians and Surgeons (MIPS) estab lished a permanent conference dedicated to occupational disease that was composed largely of industrial hygienists. In these early years, it was estimated that there were only approximately 300 industrial hygienists in die United States, by 1950, this number increased to approximately 900 (Clayton & Clayton, 1994). i i an i u r; i tw iiii r in inii imiiii iiiiiitmiiiiiiiiiiiiiinmniiiiiiiramr ruin i i h i ASBESTOS IN BRAKE UNINGS 39 By the early 1940s, ACGSH had formed a subcommittee to recommend exposure limits for chemicals and substances commonly found in the workplace (Beaetjer, 1984). The need for such a committee was recognized as important for bringing order and uniformity to the various state and local industrial hygiene units with their numerous, and often conflicting, recommendations for the protection of workers (Stokinger, 1981). The first list of recommended values was issued byACGIH in 1946. Values were compiled from a list reported during ACGIH's 1942 annual meeting, list published by Warren Cook in 1945, and the published values of the Z-27 Committee of the American Standards Association (ACGIH, 1968; Faustenbach, 2000). The ACGIH guidelines regarding the purpose of occupational exposure limits for airborne chemicals were intended to "represent conditions under which it {was] believed that nearly all workers would be repeatedly exposed day after day without adverse effect" (Beaetjer, 1984} and "should not be regarded as fine lines between safe and dangerous concentrations" (LaNier, 1984). This philosophy is reflected in the terminology that the ACGIH subcommittee used to represent its recommended exposure limits. Initially, the term "maximum allowable concentration" (MAQ was used to describe the recommended exposure limit However, this term erroneously implied that the concentrations above the MAC posed a genuine potential health hazard; therefore, the sub committee changed the term to "threshold limit value" (TLV) in 1948 (LaNier, 1984). After 1953, a prefacing statement was applied to the TLVs, in which the term was defined as the "maximum average atmospheric concentrations to which workers may be exposed for an eight-hour working day without injury to health" (LaNier, 1984). In later years, modifications of the definition of the TLV were developed and discussed within the preface of the annual TLV booklet As discussed earlier, Dreessen et aJ. (1938) recommended an occupational guideline of 5 mppcf for exposure to airborne asbestos. Prior to 1946, severed states had adopted this recommended guideline (Air Hygiene Foundation [Illinois], 1941; Oregon State Board of Health, 1945; State of California Department of Industrial Relations, 1945), and, not surprisingly, ACGIH adopted 5 mppcf as its recommended exposure limit for all types of asbestos in 1946. The report by Dreessen et al. (1938) appears to have served as the primary basis for ACGIH's first TLV for asbestos (ACGIH, 1962). THE MIDDLE YEARS (1960-1974) From 1960 to 1974, additional developments occurred in the manufacturing and design of brakes and brake friction materials. Disc brakes, which offered cooling far superior to internal drum brakes, were introduced in the mid-1960s and later adopted for use in the front wheels of most automobiles. A new friction product formulation, called semimetallic, was developed and incorporated for use with disc brakes. In addition, the National Motor Vehicle Safety Act was passed, which led to the first brake system performance requirements by the federal government Further, concerns were expressed during this time period with regard to potential asbestos exposures and related health effects for a wider variety of worker populations. In particular, studies conducted and published during this period suggested that asbestos exposures could result in chronic diseases other than asbestosis and lung cancer,-such as mesothelioma, a rare cancer (Wagner et al., I960), and that exposure to even relatively low concentrations of asbestos fibers could pose a significant health hazard. The question arose whether the type of asbestos fiber exposure (i.e., chrysolite versus amphibofe) was an important predictor of disease. The importance of fiber length continued to be discussed. The focus of asbestos health effects studies also expanded to indude manufacturing environ ments and end users of asbestos-containing products, induding brake linings and pads. To investigate these Issues further, additional studies were conducted during this period, induding efforts to (1) measure the amount of chrysotile fibers released from brake linings and pads during braking, and (2) quantify the exposure of brake mechanics to asbestos and incidence of observed asbestosrelated health effects. In 1970, the first national health regulations pertaining to asbestos exposures in the workplace were established (OSHA, 1971a). The following sections describe these issues in more detail. Wmrnm................................... ........ ................. .. - - n11111111runnn^unnrmnnr- iimm--r ........ ...................... ............ ............................................................ ............... .... .................................. .................. ......... .......................................... > HWBUI0011260 49 D. J. PMJST6J48ACH ST Al, Developments in Brakes and Brake Linings and Pads introduction of Disc Brakes Although the internal drum brake dominated the automobile market from the 1930s until the 1960s, changes in automobile design (e.g., reduced brake pedal effort, higher horsepower engines, and increased car weight) revealed that these brakes had an important limitation (Rodger, 1950). Specifically, under continuous downhill braking (or quick repeated hard braking from high speeds), the internal drum brake could overheat and lose its ability to stop a vehicle with a reasonable brake pedal effort (Disc, 1969). This problem stemmed from the fact that most of the heat generated during braking had to be transferred through the relatively thick wall of the drum before it could be dissipated to the surrounding air, which was its only coolant The brake drum was also nested within the wheel, which restricted access to airflow that promoted cooling. These design constraints resulted in heat building up within the drum, which led to unac ceptably high temperatures in the brake linings {Disc, 1969). The effect of such continued heat was that brake linings could wear out rapidly and some drivers were unable to apply sufficient brake pressure to stop the car. To address this issue, disc brakes were introduced for the front wheels of passenger vehicles in the 1960s. This type of brake consisted of a caliper containing two flat small friction pads (i.e., brake pads), which were damped onto a revolving disc when the brake was applied. The disc brake was attached to and rotated with the hub, which carried the road wheel. The caliper, which retained the brake pads, was the means for forcing the pads into contact with the disc. Because of its unique design, the disc brake offered cooling that was far superior to that of the internal drum brake. Heat was dissipated directly from the hottest part of the front disc by a greater surface area and cooling airflow. Likewise, when the brake was released, the working races of the brake pads were directly exposed to airflow and were cooled between brake appli cations. Because both faces of the disc were working surfaces, die total area exposed to cooling air was appreciably greater than in an internal drum brake of comparable braining power. The open-to-air arrangement of the disc brake meant that tire brake was self-cleaning, and accumu lations of dust (which occurred in the internal drum brakes) were avoided. The ultimate effect was reduced brake wear and a diminished likelihood of scoring the metal working surfaces (Disc, 1969). Phase-In of Front-Wheel Disc Brakes The effectiveness of the disc brake at high speeds as well as other functional considerations (e.g., fade resistance, cooling rates, and consistency in per formance), led to its widespread development for passenger vehicles during the mid-1950s and early 1960s (Anderson, 1995). Disc brakes were first introduced in U.S. production cars in 1965 (Rinek & Cowan, 1995). The new Federal Motor Vehicle Safety Standards (FMVSS) braking require ments resulted in front disc brakes being installed on virtually all cars by 1974 (Jacko et al., 1984). internal drum brakes, however, remained on the rear of most U.5. cars during this period, because rear disc brakes were readily contaminated by water and particles coming off the front wheels. They were also difficult to design with an integral and dependable parking brake (Anderson, personal communication, 2002). Initially, dhrysotile-based pads were used with the disc brake (Disc, 1969; Harper, 1997), but in tiie 1960s, a new friction material was developed called 'semimetallic" (or '`resin-bonded metallic*). This formulation provided better performance in disc brakes than the traditional chrysotiie formulations with respect to stable friction, improved fade resistance and durability, rotor compatibility, and quieter operations (jacko & Rhee, 1992). The semimetallic pads had little strength, however, so a chrysotile-based backing material was added to the semimetallics during the molding process {Anderson, personal communication, 2002). With the second generation of better performance semimetaliic formulations, semimetallic pads gained general acceptance by the vehicle manufacturers in the mid-1970s, despite their relatively higher costs, which were due to more expensive ingredients, higher specific gravity, and more costly processing requirements (Jacko et al., 1980). Semimetallic pads were used primarily in automobiles that were driven aggressively, such as highway patrol cars and taxis, but for other vehicles driven under more normal conditions, these pads were found not generally to work well HWBUI0011261 Asairros in brake unings 41 Brake Safety and Performance Standards First Federal Standard for Brake Systems By die mid-1960s, there were over 61 million mastered vehicles on U.S. roadways (McGeveran, 2001) and a growing number of annual fatalities associated with automobile use (National Motor Vehicle Safety Act, 1966). In fact, it was estimated that about 1.6 million persons had dial on the roads since the introduction of the automobile (National Motor Vehicle Safety Act, 1966). Afarmed by this feet, the U.S. Congress sought to involve the federal government in the area of vehicle safety (Yanik, 1997b). Prior to this time, automobiles were subject to only .a small number of state regulations, and the federal seat belt and brake fluid laws. Consequently, the National Motor Vehicle Safety Act was passed in 1966, which mandated federal reflations for the design and manufacture of all automobiles in the U.S. (Yanik, 1997b). In the legislative preamble to the National Motor Vehicle Safety Act it was noted that the federal government had the responsibility to ensure the safe performance of private passenger cars (which it had yet to fulfill), and it was quoted that "for too many years, the public's proper concern over the safe driving habits and capacity of the driver (the 'nut behind the wheel') was permitted to overshadow the role of the car itself" (National Motor Vehicle Safety Act, 1966, p. 2710). From this legislation, the National Highway Safety Bureau was formed, which was later renamed the National Highway Traffic Safety Administration (NHT5A) (National Motor Vehicle Safety Act, 1966). Authority was granted to NHT5A to perform a number of functions, including (1) establishing federal motor vehicle safety standards; (2) conducting automotive research, testing, development and training; and (3) providing a mechanism for the recall of vehicles that were manufactured with safety defects or were not in compliance with the federal safety standards (National Motor Vehicle Safety Act, 1966), According to NHTSA, the braking system was one of the most important factors in vehicle performance: "Brakingsystem performance has consistently rated high on the safety criticality list The dominance of the role of braking systems in accident avoidance maneuvers has long been recognized and undisputed. The importance of braking in motor, vehicle safety is evidenced by the fact that of all vehicle defects which cause or contribute to accidents, brake failures leads the list" (National Highway Traffic Safety Administration, 1973) Health Effects Studies in the early 1960s, a significant finding was made within the scientific community in regard to health effects associated with asbestos exposures. Specifically, a number of researchers pointed out the casual relationship between asbestos exposure and risk of lung cancer and mesothelioma even in the absence of asbestosis. The paper by Wagner et al. (1960) represented die first publication to highlight this observation, wherein they reported on 33 cases of mesothelioma identified in three South African groups: (1) croddoiite asbestos miners, (2) residents with no apparent occupational exposure to asbestos but who lived in the vicinity of the croddoiite mine, and (3) workers who were exposed in other asbestos-related industries. This case series study is credited with linking exposure to croddoiite with mesothelioma and highlighted the potential for asbestos-related disease among individuals with relatively low exposure levels. in a later study by Tnomson et al. (1963), 500 consecutive autopsies of the general population in Cape Town, South Africa, were reviewed. Based on the presence of so-caHed "asbestos bodies" in the lungs of the decedents, these researchers concluded that asbestos fibers had been inhaled by tiie urban dwellers, interestingly, these researchers also speculated that brake wear debris (i.e., brake dust) could have accounted for a significant source of asbestos in urban air, although they noted that the concentrations of asbestos inhaled were "not in amounts sufficient to produce pulmonary lesions or disability" (Thomson etal., 1963, p. 31). Based on this finding several studies were initiated in an attempt to estimate (1) the amount of chrysotife in brake wear debris, and (2) the concentrations of chrysotile fibers in the ambient air. Soon after the publication of Thomson et at. (1963), Dr. living Selikoff, who was a researcher at the ML Sinai Hospital in New York Gty, and his colleagues reported on a cohort mortality study of 632 workers who installed asbestos-containing insulation materials (Selikoff et al., 1964). Specifically, Selikoff et al. reported a much higher than expected incidence of lung pleural, and other cancers in 42 O. J. PAUSTEN8ACH IT At. these workers. For example, 45 of the 632 workers died of lung or pleural cancer, which was 6.8 times higher than expected (Selikoff et at., 1964). Prior to this study, the only other large-scale study of insulation workers as end users of asbestos products in the United States was that of Fleischer et al. Cl 946), who observed only three cases of asbestosis among shipyard insulators (a very "dusty" occupation). However, unlike this previous study, the majority of participants in the Selikoff et al. study had worked in the same industry for more than 20 yr (the majority of workers In the Fleischer et al- study had worked for 5 yr or less in die pipe-covering industry). By this time, it was becoming apparent that exposure duration was not the only important predictor erf disease, but that time since first exposure (i.e., latency) needed to be considered as well. Based on these and subsequent data the latency period between first exposure and the development of disease is believed to be about 30yr or more for mesothelioma, 20yr or more for lung cancer, and 15 to 40yr for asbestosis (Lanphear & Buncher, 1992; Murphy et al., 1996; ATSDR, 2001). Therefore, it is now dear that Studies that attempt to evaluate the health of workers need a sufficient latency period to be able to detect an increased incidence of asbestos-related disease. in a later publication, Selikoff et al. (1968) also reported a synergistic effect between exposure to asbestos and smoking. Selikoff's publications of insulation workers eventually (in die period after 1974) led to additional worker population studies largely focusing on end users of other asbestos products such as asbestos sprayers, railroad maintenance workers, bus garage workers, locomotive drivers, construction workers, and electrochemical plant workers (Ohlson et al., 1984; Gustavsson et al., 1990; Hilt et al., 1991; Sanden & JarvhoSm, 1992; Fletcher et al., 1993; Nokso-Koivfcto & Pukkala, 1994; Oksa et al., 1997). These and other subsequent studies served as the foundation for justifying lower occupational exposure limits for asbestos in workplace air. The fact that these worker populations were not studied earlier is not surprising. As noted by Selikoff in 1970, "(WhifeJ the risk of heavy exposure to occupational dusts (in the asbestos mining and manufacturing industry! had been recognized for some years,.. .the extrapolation of that experience to another classification of workers. ..was a more.sophisticated task for clinical medicine and epidemiology" (Selikoff, 1970, p. 163). Concern About Chrysolite Emissions to Ambient Air Oue to Brake Wear The concerns that brake wear debris might significantly contribute to asbestos concentrations in urban air (which were initially raised bvThomson etai., 1963) motivated the U.S. Public Health Service (U.$. PH5), the U.S. EPA, automobile manufacturers, and friction product manufacturers to conduct a series of studies to evaluate potential chrysotile emissions from brake wear (Lynch, 1968; Anderson etai., 1973; jacko et al., 1973). Other researchers also collected data on chrysotile content in brake wear debris, although these data were not the primary focus of their investigations (Hickislt & Knight, 1970; Luxon, 1970; Davis & Coniam, 1973). Table 3 summarizes the amounts of chrysotile measured in these studies, as well as those measured in subsequent studies, which are discussed later.* These studies found that during brake use, some chrysotile fibers released from the brake lining duringbraking are either trapped in the brake housing, fell to the road, or are emitted to tie atmosphere but that the percent asbestos in brake wear debris (i.e., brake dust) is quite small. In die first study, which was conducted by Lynch (1968) of the U.S. PHS, wear debris from brake lining test machines was collected and analyzed for chrysotile fibers using electron microscopy. In this study, generally less than 1 % of the wear debris particles emitted from the brakes was found to be chrysotile fibers, even though the chrysotile fiber concentrations in original (unused) brake linings ranged from 30% to 50% by weight (Lynch, 1968). In the few cases where the percentage of wear debris found to be chrysotile fibers was greater than 1%, the author noted that those tests were conducted at extremely high temperatures for the type of lining tested, and had these linings been subjected to such conditions on a vehicle, the brakes would have failed. Lynch also noted that the wear debris was largely a nonfibrous mineral resulting from the thermal decomposition of chrysotile fibers. A follow-up study co-sponsored by the U.S. EPA and Bendix (a friction product manufacturer) measured an average of 0.23% by weight chrysotile content in ail size fractions of the brake wear debris from the automobiles * Care needs to be taken when comparing the results of the various studies because the methods employed to estimate the per centage of chrysotile fibers in brake dust varied among the researchers. HWBUI0011263 ASBESTOS IN BRAKE UNtNGS 43 tested (jacko et al., 1973). Only 1% of the debris was classified as airborne. The results of this study were considered to be more accurate, because an automobile was used rather than a laboratory dynamometer for testing, and sophisticated sampling techniques separated trapped debris in the drum from that fraction normally dropped on the road from the airborne fraction and analytical techniques included both optical phase-contrast microscopy and transmission electron microscopy (TEM). Another study, which was conducted by researchers at Ford Motor Company using a dynamometer, collecting and analyzing the airborne wear debris (with TEM), confirmed the results of Lynch (1968) and jacko et al. (1973) and concluded that less than 0.05% by weight of airborne brake dust consisted ofchrysotile fibers (Anderson et al, 1973). Anderson and colleagues observed no chrysolite fibers longer than 5 pm in wear debris samples. The observations that only small amounts of chrysolite fiber were emitted from brakes were not surprising. The intense local heating and severe local mechanical action resulted in the decomposition of most of the chrysolite fibers in brake linings or pads during usage to nonfibrous magnesium silicate in both crystalline (forsterite) and amorphous phases {Gafretl & Schreiber, 1967). Because the decomposition products lacked the fibrous characteristics of asbestos, the magnesium silicates, including forsterite, were not thought to pose a health hazard similar to asbestos. Exposure Studies Focusing on Brake Mechanics The first studies to measure the amount of chrysolite fibers released during the replacement of brakes in passenger vehicles were published by researchers from the U.S. PHS and the automobile and friction product industries in 1970 and 1972 (Hatch, 1970; Hickish & Knight, 1970; Dement, 1972) (Figure 3). These studies reported measurements of airborne concentrations from personal samples on mechanics collected during brake repair activities that, prior to 1970, typically involved using compressed air or brushing to remove the brake wear deeds from the drum prior to installing the replacement linings. The publication by Hickish and Knight (1970) was the only study during this time period that reported a daily long-term time-weighted average (TWA) airborne asbestos concentration during passenger car brake repair work, which was 0.68 fibers per cubic centimeter (f/cc).* Based on later surveys, it turns out that the mechanic sampled by Hickish and Knight serviced far more cars during the day he was sampled (11) than mechanics in other studies (generally 3 or fewer per day) (johnson et al., 1979; Roberts & Zumwakfe, 1982). Hickish and Knight also reported a daily long-term TWA asbestos concentration of 1.75 f/cc during brake repair on heavy trucks. Based on these results, Hickish and Knight wrote, "Our investigations show that exposure to asbestos during brake maintenance is not as severe as was anticipated, and in the situations we examined, the personal exposure of the operators was below the limit corresponding to 50-year exposure"* (p. 20). Both of the reported long-term TWA concentrations were also below the proposed ACGIH TLV-TWA of 12 f/cc that was in effect at the time and the soon to be adopted OSHA permissible exposure limit (PEt)-TWA of 12f/cc. it should be noted that these researchers also evaluated.airborne asbestos concentrations generated during different brake cleaning techniques (Knight & Hickish, 1970), as did another researcher in Great Britain (Lee, 197(5). However, the data from these studies were collected over very short time periods (on the order of minutes) and are not considered relevant to evaluating long-term exposure to brake mechanics. Following the initial paper by Hickish and Knight, industrial hygienists within the auto industry in the U.S. became involved in evaluating potential exposure to mechanics during auto brake repair. For example, between 1972 and 1974, Ford Motor Company (Ford) collected a number of By the mtd 1960s, asbestos analyses were shifting tan Ihe impinger dust counting method, which provided total dust measure ments in mmpef, to a fiber counting method using phase-contrast microscopy (PCM), which provided astesas measurements m fibers per ml %t!5 or fibers per cubic eenfimeter Wed}, which am equivalent units fi e, 1 {M is the same as 1 tfccj. This change in analytical technique was in recognition of the fact tot to number and sire of to trtdfvkiuai asbestos fillers versus to total number of particles were more relevant for assessing potential health risk. The membrane ffiter method also allowed for long-term (eg., 8-h! personal sam pling, A total dust meMurement of 1 mppcf is roughly equivalent to 6 f/oc (AOGiH, 59715, * to 1970, Great Britain's occupational exposure limit for asbestos was TOO fibers per cubic centimeter-year ifa-yrl. which repre sents a cumulative dose rather ton an airborne concentration {lane et *1, 19661. Cumulative dose is equal to the concentration of asbestos in air CSte) multiplied by the duration of exposure to that concentration (years}. Therefore, if the exposure duration was SOyr, the airborne asbestos concentration would have to be greater than 2 f/cc to exceed 100 f?a>yr. i HWBUI0011265 ASBESTOS IN BRAKE LININGS 45 brake personal airborne asbestos samples of mechanics conducting various brake repair operations on automobiles (Lick, 1973; Ford, 1974). The measured brake job TWA concentrations were considerably below die prevailing occupational exposure limits. First Health Effect Studies of Garage Mechanics Several studies and case reports were published in the 1960s and 1970s that evaluated the relationship between occupational exposure to asbestos and asbestos-related diseases. Of these, a few included a reference to occupations such as garage mechanics or service station operators (Hueper, 1965; Newhouse & Thompson, 1965; Bentley, 1970; McDonald et al., 1970; Dels et al., 1971; Moertel, 1972; Greenberg & Davies, 1974). However, the health effects studies of primary interest during this and subsequent eras are those epidemiological studies that quantitatively evaluated the association of work as a garage mechanic and asbestos-related diseases. Enteriine and MdOever (1963) published results from a surveillance study of mortality from cancer of the trachea, bronchus, and lung bv occupation in the United States in 1950. The authors reported an elevated death rate for "automooite mechanics and repairmen," but suggested it could be due to exposure to automobile exhaust This study did not control for smoking, which would have been necessary,to properly evaluate an association between occupational exposure and lung cancer. Boilfat and Lob (1973), however, were the first researchers to specifically evaluate the prevalence of asbestosis among garage mechanics (Figure 4). in this study, lung function and chest radiography data from 39 brake lining repair workers in Switzerland were examined, and no cases of asbestosis were observed. No information was provided regarding the airborne concentration of chrysotile fibers in the workplace (Boilfat & Lob, 1973). No New Information on Brake Lining Manufacturing Workers Few studies were conducted between 1960 and the mid-1970s that provided additional information on airborne concentrations of chrysotile fibers in the U.S. friction product manufacturing environment or on asbestos-related health effects to U5. friction product manufacturing workers (Figure 5). Enteriine (1965) and Enteriine and Kendrick (1967) conducted a mortality study of white males between the ages of 15 to 64 employed at some time between 1948 and 1951 in asbestos building, textile, and friction product plants in the United States. Enteriine and Kendrick (1967) provided results explicitly for those 7510 men employed at 11 friction product plants included in the study and reported 29 cases of asbestosis, of which 21 cases were reported with asbestosis as the cause of death. In these workers. The authors also reported a standardized mortality ratio (SMR) of 123.7 for cancer of the respiratory system (which could indude respiratory cancers other than lung cancer, for example, laryngeal cancer) for this group of workers. Enteriine and Kendrick (1967) did consider mesothelioma, noting that "on only one of the 1853 death certificates examined did the term 'mesothelioma' appear, and this was for a worker in the asbestos building products industry." Overall, the authors considered the results for the friction product plant workers an indication of a "modest" response to asbestos exposure and conducted: "these workers had slightly increased death rates for cancer of the respiratory system and a hi^i death rate for abestosis" (Enteriine & Kendrick, 1967). In regard to exposure information available for these workers, the authors stated that "historical data regarding dust levefe proved to be inadequate and this part of the project was abandoned" (Enteriine & Kendrick, 1967, p. 182}.' In an additional study from this time period, McVittie (1965) reported that 247 workers (tnduding four "brake lining workers" who are presumed to be friction product manufacturing workers) were diagnosed by physical exams as having asbestosis between 1955 and 1963, but the diagnostic criteria used to make this determination were inconsistent with the current diagnostic standards, and other sources of asbestos exposure were not considered. Studies published during this time period that presented chrysotile exposure measurements in friction product manufacturing environments typically did not include complete information (e.g., sampling duration) about the conditions under witch sampling occurred. For example, a 1964 U.S. PH5 health hazard evaluation collected asbestos exposure data from personal and area air samples at the Raybestos-Manhattan Friction Products plant in Manbelm, PA (U.S. PH5 1964).* The air A copy of the survey was obtained through the National Technical information Sendee. White authorship is assigned to the National Institute of Occupational Safety and Health (NJOSH), NfOSH was not in existence in 1964. The survey was conducted by die U.S. Public Health Service. HWBUI0011267 47 HWBUI0011268 pflfw f m w w w w ^ w n p i p n*rn w i '* m~r * nn r^ r* nr u n r^ -^ p rjtfr^ m m w w w w ^ 48 0.1- PAU5THN8ACH ET AL sampling data showed asbestos fiber concentrations ranging from less than detection limits to 130.1 f/cc depending on the specific workplace activity. For example, as shown in Figure 6, the asbestos fiber concentration ranged from 1.5 to 75.3 f/cc during grinding. The concentration of total dust ranged from less than detection limits to 3.4 mppcf (U.S. PHS, 1964). Between 1965 and 1972, U.S, PHS collected chrysotiie fiber and total dust in personal, breathing zone, and area meas urements at the American Brake Shoe facility in Winchester, VA (Dement & Shuler, 1972a). Sam pling durations associated with the measurements were not reported. Fiber concentrations ranged from less than detection limits to 17.5 f/cc, whereas total dust measurements ranged from 0.1 to 7mppcf. Fiber concentrations during grinding ranged from less than the detection limit to 8,1 f/cc (see Figure 6). later, in 1972, U.S. PHS collected data from personal air samples measured during brake rebuilding operations at the Genuine Parts Company, Rayloc Division plant in Atlanta, GA. These data showed much lower asbestos fiber concentrations ranging from 0.2 to 1.6 f/cc, and from 1 to 1.6 f/cc during grinding (Dement & Shuler, 1972b; see Figure 6). Similar to U.S. PHS (1964), the U.S, PHS surveys In 1972 (Dement & Shuler, 1972a, 1972b) also did not report sampling durations, making it difficult to interpret whether these results represented peak or longer term average air concentrations. Toxicology of Asbestos Much of the toxicological research conducted in the 1960s and early 1970s was aimed at developing an animal model that would be predictive of the asbestos-related cancers observed in workers. The failure of animal studies conducted in the 1940s and 1950s to show cancer induction via inhalation of asbestos led to the exploration of other more direct routes of asbestos exposure at higher administered doses. Although the Mellon Institute was successful in inducing lung tumors in rate via inhalation in the 1960s (Gross & deTreviffe, 1967; Gross et a!., 1967), most experiments conducted during this time period consisted of injecting asbestos fibers directly into the animal's pleural and peritoneal cavities or trachea. Even these routes of direct application of asbestos to the tar^t tissue, however, were successful only in inducing tumors in some animal models. For example, lung cancer was induced Intrapieuralfy only in rate (Wagner, 1962; Berry & Wagner, 1969; Wagner & Berry, 1969; Donna, 1970) and hamsters (Smith et-af., 19653, the two species identified in the first half of the century as being sensitive to asbestos. Mesotheliomas were induced intrapleurally in rate with chrysotiie and crocfcfolite and in hamsters with amosite (Wagner, 1962; Smith et al., 1965). In general, intratracheal administration was not reliable in inducing lung cancer or mesothelioma in animals (Smith et ai., 1965; Gross et al., 1967; Reeves et al., 1971). These studies usually did not evaluate inhalation exposure, which would allow for various biological protection mechanisms to occur; thus, it is not possible to use them to quantitatively predict the human health hazard (because of particle size, deposition, role of macrophages, and other factors). In the early 1970s, Reeves et al. (1971) suggested that the rat was the superior animal model for predicting asbestos-related malignancies. To date, rt is the only species to consistently develop lung tumors following inhalation and mesotheliomas via both intrapleural and intraperitoneal injection. More recent studies show that the Syrian golden hamster is more sensitive to the induction of mes othelioma from amphibole fiber inhalation exposures than the rat (McConnell et al, 1999). To explain tire mechanism of asbestos-related carcinogenesis in animals, several hypotheses were offered in the 1960s and 1970s. Most of them involved investigation into alterations of the fib ers and the effects of these alterations on fiber size, route of administration, and physical properties as they related to tumorigenicity. For example, Wagner and Berry (1969) compared carcinogenicity in rate for different asbestos fiber types and composition, and found that oil-extracted and natural crocidotite induced mesothelioma at a similar rate when administered intrapleurally. Smith et al. (1965) found that exposure to soft chrysotiie fibers (i.e., less brittle) did not result in animal tumors, regardless of the route of administration, but that exposure to harsh chrysotiie fibers ff.e., more brittle) did result in tumors in exposed animals. Some investigators also found modified (milled) asbestos to be less carcinogenic than raw asbestos fibers (Pott et al., 1972; Wagner et al., 1974). This work sup ported the hypothesis that long fibers (5 to 20 pm) were possibly the carcinogenic fraction or that the shorter fibers (less than 5 pm) were much less carcinogenic. HWBUI0011269 i i I r j*f r*& 5 II IIg.^ w & is (=o/j) sNoavaiNaoNOD soisaasv \ i \ & i i t* p- ; i f l $ f r t I i \ t L HWBUI0011270 so D. 1. PAUSTtNBACH FT At. In the early 1970s, Stanton and Wrench (1972) conducted experiments with asbestos and non asbestos fibers, as well as nonfibrous materials such as talc, to evaluate the hypothesis that all fibers of a certain length and diameter could induce mesothelioma. These test results indicated that, irre spective of the type of fiber, ail fibers of a certain length produced mesotheliomas by implantation, with fibers greater than 8 pm in length and less than 0.25 pm in diameter being the most patent In later animal Implantation studies, chrysotile fibers Jess than 10pm in length were found to induce lung tumors in addition to mesothelioma (Pott et alv 1974). These studies prompted further investiga tion of fiber morphology and its role in the generation of fibrosis (asbestosis) and cancer. Several dif ferent mechanisms were hypothesized to have accounted for the observed difference in the cancer potency of asbestos, including those related to fiber type, size, and iron content. These hypotheses continue to be explored (Govema et a!., 1999; Ghio et at, 1997). Asbestos Guidelines and Regulations From 1960 to 1974, the field of industrial hygiene matured substantially. The federal govern ment began to actively promulgate environmental and occupational health regulations during this period. Asbestos was prominent among the hundreds of chemicals that were regulated. Figure 7 presents the 8-h TWA-PEL for asbestos mandated by OSHA starting in 1971, as well as the changes brought about by subsequent rulemakings. Stricter Occupational Exposure Guidelines In the early 1960s, various legislation such as the Walsh-Healy Act (Department of Labor, 1960) and the Longshoreman's Act (Office of the Secretary of Labor, 1960) adopted the 5 mppcf value as a binding regulatory limit for airborne asbestos fibers in specific industries. In 1964, at the Conference on the Biological Effects of Asbestos, it was discussed . whether the 5 mppcf standard was adequate to provide lifetime protection from exposure to all forms of asbestos (Le., chrysotile, crocidolite, actinolite, anthophyilite, tremolite, and amosite). This conference called attention to the rising rate of lung cancer among asbestos workers and high lighted the notion that end users may also be vulnerable to asbestos exposures (Schalf, 1965; Com, 1986; Nowinski, 1987). Following this conference, in April 1968, ACCIH recommended, in a Notice of Intended Change, lowering the asbestos TLV from 5 mppcf to 12 fibers per milliliter (f/mi, which is the same as f/cc) as an 8-h TWA (LaNier, 1984). The federal government soon followed suit in May 1969 by amending the Walsh-Healy regula tions to require a threshold limit of 12 f/cc for fibers greater than 5 pm in length (Nowinski, 1987). In 1970, in a second Notice of Intended Change, ACGIH recommended further lowering the TLV to 5 f/cc for fibers greater than 5 pm in length. As part of the proposed TLV in 1970, a ceiling limit of 10f/cc for a maximal duration of 15 min was included {ACGIH, 2001). The ACGJH TLV represented a nonenforceabte guideline; therefore, no federal health stan dards governing all workplace exposure to asbestos had been promulgated through 1970. This changed in 1971, however, when OSHA established regulations that promulgated PELs for asbestos, as well as a whole host of other chemicals and substances (more than 400) that were being used in the United States (OSHA, 1971a;-Martonsk et a!., 2001; Stoktnger, 1981). On May 29, 1971, OSHA issued % first asbestos standard for general industry and set the PEL at 12 f/cc as an 8-h TWA based on the 1968 ACGIH proposed TLV for asbestos (OSHA, 1971a; Martonik et a!., 2001). Just 6 mo later, in response to a request from the AFL-CIO (OSHA, 1971b) OSHA issued an emergency temporary standard (ETS) of 5 f/cc for fibers greater than 5 pm for an 8-h TWA and 10f/cc for a 15-min ceiling limit These values were adopted as a final standard in June 1972, with the stipulation that the PEL would be lowered again to 2 f/cc in 1976 (OSHA, 1972). Other agencies involved with chemical regulations during this period included the U.S. ERA and CP5C For the first few years after inception, CPSC had little involvement with asbestos; however, starting in the mid to late 1970s, CPSC became involved in regulating asbestos-containing products (CPSC, 1977,1979). The U.S. EPA, on the other hand, placed asbestos on the first National Emission Standards for Hazardous Air Pollutants (NESHAP) fist in March 1971 (U.S. EPA, 1971). In April 1973, when the U.S. EPA promulgated its national emission standard for asbestos under NESHAP, it noted the following: "Asbestos is too important in our technology and economy for its essential use to be stopped. But because of the known serious effects of uncontrolled inhalation of asbestos min- HWBUI0011271 I !l r~ i HWBUI0011272 I 1 1 J *] JJII. ......L.1JIJJL.JI..IJJ .III . II 111.1 L1 _JHU 1 L1 111J1 1111111] St 52 D. PAUSTENBACH ET AL erais in industry and uncertainty as to the shape and character of the dose-response curve in man, it would be highly imprudent to permit additional contamination of the public environment with asbestos. Continued use at minimal risk to the public requires that the major sources of man-made emission into the atmosphere be defined and controlled" {U.S. EPA, 1973, p. 8820). This U;S. EPA regulation controlled spray application of products containing more than 1% asbestos. THE LATER YEARS (1975-2002) From 1975 to the present day, significant changes occurred in the friction products industry. The first federally mandated brake system performance requirement was implemented in 1975. However, its effect on the friction product industry was minor compared to the increased regulatory efforts to control occupational and public exposure to airborne asbestos. Because of a higher level of concern about asbestos than in the earlier era, which was due to a substantial increase in worker studies in the early 1960s through the 1970s, and the cost of compliance with asbestos standards, asbestos product industries and suppliers began the search for a substitute for asbestos. This effort by the private sector to replace asbestos in consumer products was monumental given the thousands of applications that had been found. For example, researchers estimated that asbestos had been incor porated into as many as 3000 products by the mid-1970s (Levine, 1981). The challenge of finding a substitute to replace cbrysotiie in brake finings and pads was particularly difficult given the unique qualities of cbrysotiie fiber. Further, preliminary findings were reported by Mount Sinai researchers, first in 1975 and again in 1976, which suggested that brake mechanics might be at risk of asbestosrelated health effects (Lorimer et al., 1976). These preliminary findings, although later shown by Mount Sinai researchers to be unconfirmed (Nicholson et al., 1984), spurred a number of exposure and epidemiology studies of brake mechanics over the next 20yr. Manufacturers of friction products, brakes, and automobiles spent over two decades developing an adequate substitute for cbrysotiie fibers in brake linings and pads. By 2000, chrysolite was virtually eliminated in the braking systems of new North American vehicles. The following subsections discuss these issues in more detail. Developments in Brakes and Brake linings and Pads As of the late~1970s, the typical U.S. car had front disc and rear drum brakes. U.S. passenger cars and light trucks with four-wheel disc brakes were rare and, if available, were offered only as an optional feature. Because disc and drum brakes were a standard feature in many vehicles during this period, efforts to develop asbestos-free brake linings and pads posed a real challenge, particu larly for after-market (i.e., replacement) parts. A number of factors lea to significant efforts to research and develop replacement fibers for cbrysotiie in brake linings and pads, including (1) concerns about the toxicity of ail forms of asbestos, (2) increasingly stringent air quality standards and regulations, (3) issues related to the disposal of asbestos-containing wastes, and (4) rising insurance costs. When first contemplated in the 1970s, the only viable alternative for chrysotile-based brake pads on disc brakes was a semimetallic formu lation, which was already used for some disc brakes. However, the challenge was to find an appro priate material to replace the chrysotile-containing backing material used on semimetallic disc pads (Anderson, personal communication, 2002). In addition, an altogether new friction material needed to be developed for the brake linings in the rear internal drum brakes. Over the next 20yr, automobile, brake, and friction product manufacturers expended substan tial effort toward developing drum brake linings that did not contain chrysotile fibers (Anderson, personal communication, 2002). Many of these years were spent attempting to develop functional semimetallic drum brake linings. However, manufacturers had difficulty obtaining uniformity with the raw materials mixtures, handling and forming the in-process materia!, and manufacturing high-quality parts consistently. Modifications to the semimetallic formulation to facilitate processing generally resulted in a friction material that did not achieve the commercially acceptable performance char acteristics required for drum brakes (facko et al., 1980). The nature of semimetallic friction formula tions did not lend itself to molding into the curved segments required for passenger drum brakes, and when cured, they were more brittle and subject to cracking (jacko et ai., 1980). In addition, as m HWBUI0011273 ASBESTOS IN BRAKE. UNINGS S3 She drums became hotter and expanded during use, the semimetallics did not conform to the increasing drum diameters {Nicholson, 199S). Because of these masons, efforts to develop semimetaliic drum brake linings for passenger cars and light trucks ended in the fate 1980s (Anderson, per sonal communication, 2002). Although other friction materials were available at the time, these materials were not appropriate for brakes on passenger cars and light trucks designed in the United States, For example, sintered metallic linings were developed, andafthough these liningp were used in race cars, they were too sen sitive (to prior usage, temperature, and moisture) to be used on a production passenger car or li^ht truck. Carboo-caroon friction materials, invented in the late 1970s, were too sensitive to water and hydrocarbon vapors to be used on passenger vehicles, and also had fight-duty wear problems (Anderson, personal communication, 2002). In addition, the carbon-carbon friction materials were cost-prohibitive for passenger vehicle brakes. These friction materials therefore were used only in military aircraft, race cars, and some commercial aircraft (ASME, 1988). In the 1970s, research was initiated for nonasbestos organic (NAO) fibers that would serve the same function as chrysotile fibers (ASME, 1988). The transition to NAO-based friction products created many additional challenges, in terms of both manufacturing and performance fiacko & Rhee, 1992). Specifically, it was impossible to find a single fiber that had the same characteristics as chrysotiie (e.g., good tensile strength, low wear, easy processing, flexible, multitude of grades available, good insu lator, and thermafstability at temperatures up to 500"O (Nicholson, 1995). In addition, chrysotile fibers were a significant component of brake linings and pads, typically varying in composition between 30% and 70% by weight Oacko & Rhee, 1992). This explains, in pari why it took nearly 20yr to find acceptable asbestos substitutes for all the passenger cars and light trucks available on the market (Anderson, personal communication, 2002), Because friction material performance is greatly influenced by composition and the manner in which the components are misted, fabricated, and cured, thousands of batches of experimental fric tion materials were made before proper NAO formulations were found. As of the early 1990s, more than 1200 different asbestos fiber substitutes had been investigated (Anderson, 1992). These substi tutes included aramid, fiberglass, mineral wool, wollastonite, steel wool, processed mineral fibers, and organic fibers {ASME, T 988). With time, a "fiber cocktail" (blend of several different fibers) was found that provided acceptable performance in an NAO formulation. Beginning in the early 1980s, fight trucks were the first to incorporate NAO-based rear drum linings and semimetallic pads with NAO backing material Oacko et al., 1984). The in-service use of these first-generation NAO materials demonstrated the challenges that still needed to be overcome. Specifi cally, the NAO brake linings tended to be hard and brittle; low in permeability and highly aniso tropic; and prone to developing hot spots, blistering, and cracking in sendee (ASME, 19885However, as these problems were resolved by the mid-1990s, NAOs gradually replaced asbestos-based materials on drum brakes and also replaced some of the semimetallic front disc pads on U.S. vehicles Oacko & Rhee, 1992). By 2000, vehicle manufacturers had eliminated the use of chrysotile-based brake linings and pads for virtually all passenger vehicles and light trucks sold in the United States. Brake Safety and Performance Standards The first federal standard applicable to brake systems was established in 1975 and referred to as FMVSS 105. The purpose of tnis standard was "to Insure safe braking performance under normal and emergency conditions" for hydraulic service brakes and associated parking brake systems, as was found on passenger vehicles (National Highway Traffic Safety Administration, 1978). FMVSS 105 defined service brake and parking brake system performance requirements (e.g., stopping dis tance) over a broad range of conditions (Gillespie, 1992), and these requirements pertained to the foil vehicle, not individual components in the brake system. The establishment of FMVSS 105 did not result in the elimination of internal tests conducted by automobile, brake, and friction product man ufacturers, because the federal standards were considered to be minimal, and not necessarily suffi cient, reauirements by these industries. TTre federal government issued a harmonized regulation (i.e., compatible with regulations from other countries) in 1995, referred toas FMVSS 135 (National Highway Traffic Safety Administration, i ! HWBUI0011274 54 D.}. PMJSTEN8AOH ST At. 1995). Like its predecessor, FMVSS 135 required testing of the entire braking system, not of the Individual components, such as brake linings or pads. The requirements listed in FMVSS 135 were also a minimum (and not sufficient} standard for performance. Consequently, the internal labora tory and field tests required by the individual automobile manufacturers, in conjunction with brake and friction product manufacturers, remained the primary means of ensuring that automobile brakes were able to perform under a variety of operating conditions, in other words, although NAO-based material formulations may have satisfied the "minimum" requirements of the federal government soon after development, these new friction material formulations were not incorpo rated into vehicles until the vehicle brake system met the internal laboratory and field tests required by the friction product, brake, and automobile manufacturers. Exposure and Health Effects Studies With the exception of the study by Boillat and Lob (1973), no epidemiological studies to charac terize asbestos-related health hazards to brake mechanics had been conducted prior to 1975. Although employee exposures to asbestos had been evaluated at some friction products manufac turing facilities as early as the 1930s, it was not possible to correlate these exposures with those of brake mechanics (primarily due to differences in duration of exposure, number of iinlngs or pads handled per day, as well as the composition and particle size of the dust). Between 1975 and 2002,10 surveys of exposure and 25 studies of health effects of brake mechanics were published along with 11 surveys of exposure and 15 studies of health effects of friction material manufacturing workers. These more recent studies focused on quantifying exposure levels for wotkers performing specific activities and the relative risk of asbestos-related disease in these workers. Studies Suggesting That Brake Mechanics Were at Risk to Asbestos-Related Diseases It was not until 1975, when researchers from Mount Sinai presented preliminary results on the risks to brake mechanics at a meeting with NIOSH, automobile company, union and other representatives (later published in Lorfrner et al. {1976} and Rohi et at. {1976,1977}), that significant attention was focused on the question of whether brake mechanics might be at risk of asbestos-related diseases (NIOSH, 1975). The concerns raised were threefold. First, the Mount Sinai researchers reported chrysolite concentrations in brake wear debris that were significantly higher than reported in previ ous studies (Lynch, 1968; Hickish & Knight, 1970; Anderson et ai., 1973; jacko et at., 1973). Sec ond, foe reported short-term (i.e., peak) airborne concentrations of chrysolite fibers in brake repair facilities primarily associated with the compressed air blowout of the brake drum were sometimes higher than the OSHA 15-min ceiling limit of tOf/cc (Rohl et al., 1976). Third, a preliminary review of x-rays from a select group of mechanics suggested a higher than expected incidence of x-ray and respiratory function abnormalities (Lorimer et ai., 1976). Some of these same concerns were also raised by Castieman et ai. in 1975. Based primarily on Mount Sinai's preliminary results, NIOSH (Lloyd, 1975) issued an information bulletin for mechanics who might be involved in specific brake servicing operations. In this bulletin, NIOSH acknowledged that brake mechanics, as an occupa tional group, had "not been studied systematically up to now." One of NIOSH's recommendations was to conduct additional exposure and epidemiology studies specific to brake mechanics (NIOSH, 1975). Low Asbestos Content in Brake Dust As part of Mount Sinai's study of brake mechanics, the chrysolite content in brake dust was evaluated using an x-ray diffractrometry technique. These results, published by Rohl et al. (1976), indicated an average chrysolite concentration in brake wear debris of approximately 4.5%, and up to a maximum of 15% by weight for samples collected in the U.S. (Rohl et ai., 1976). In 1977, Rohl et al. re-published these results, along with the results for samples collected in Europe and Australia (Rohl eta!. 1977). Based on samples from all countries, the average chrysotite concentration in brake wear debris was approximately 2.4%. These estimates were as much as an order of magnitude higher than those reported by earlier researchers using JEM (Lynch, 1968; Hickish & Knight, 1970; Anderson et a!., 1973; jacko etal, 1973). Perhaps of greater importance, these estimates were not replicated in later studies by automobile manufacturers (Williams & Muhlbaier, 1982; average of 0.03% by weight; maximum of 0.19% by weight) and the U.S. EPA (Cha et al., 1983; average of 0.02% by weight; maximum of 0.14% by weight) (Table 3). Additional it HWBUI0011275 i n n . ..ffliu iin ii i ASBESTOS IN BRAKE IIN1NGS 55 data provided by other researchers also showed the percent chrysotile in brake dost to be less than 1%by weight (Rowson, 1978; Sheehy etal., 1989}. Care needs to be taken in comparing the results of the various studies, however, because they all used slightly different methods to determine the percentage of chrysotile fibers in brake dust The differences in measurements reported by Luxon (1970) and Rohl et at. (1976,1977) and those reported in the other studies may be a result of the use of quantitative x-ray diffraction to measure chrysotile in brake dust by the two former groups of researchers, whereas "OEM was used in the other studies. One shortcoming of x-ray diffraction is that it does not distinguish between the respirable chrysotile fibers and the nonrespirable fragments nor between encapsulated fiber and free fibers, which is why NIOSH began recommending TEM for identifying asbestos fibers after about 1980 (NIOSH, 1980; Lemen, 1984). This difference in what is being measured likely accounts for the higher estimates of chrysotile content in brake wear debris reported by the Mount Sinai researchers, as compared to the results from other researchers. This conclusion is supported by the fact that researchers estimating the chrysotile content in brake linings and pads observed that most of the brake wear debris samples had a chemical composition similar to chrysotile, but that the form was amorphous and nonfiorous (i.e., forsterite) (Lynch, 1968; Anderson etaL, 1973; Davis & Coniam, 1973; jackoetak, 1973; Rohl etal., 1976). Rohl etal. (1976) did use TEM to evaluate the fiber size distribution in wear debris and found mostly short fibers (80% shorter than 0.4 pm in length), which was consistent with earlier findings. Estimates of Brake Mechanics' Exposure to Chrysotile Fibers As stated earlier, the 1975 meeting between researchers from Mount Sinai and NIOSH representatives resulted in additional studies to measure the concentrations of chrysotile fibers that brake mechanics were exposed to during brake servicing activities (Figure 3). In the 1975 meeting, the Mount Sinai researchers reported airborne chrysotile concentrations ranging from 1.3 to 29.8 f/cc However, these were peak exposure concentrations measured during the cleaning of brakes using compressed air and dry brushing; and all breathing-zone air samples were generally collected for periods of 3 to 8 min in duration {Rohl et ah, 1976). Although these samples suggested that peak concentrations might exceed the OSHA ceiling level if exposure were to last as long as 15 minutes, they were not particu larly helpful in characterizing 8 hr TWA concentrations for mechanics, which were needed to assess the cumulative lifetime dose (i.e., necessary to understand the potential health risk; U.S. EPA, 1986a). Because the preliminary studies of Mount Sinai addressed only peak exposures, additional stud ies that characterized brake mechanics' short-term and long-term exposures to chrysotile needed to be conducted. Most subsequent studies of the short-term chrysotile exposure of brake mechanics were conducted during die removal of wear debris from toe brake drum using compressed air, a dry brush, a vacuum deaner, or some other technique (Johnson et at, 1979; Roberts, 1980a, 1980b; Roberts & Zumwalde, 1982; Nicholson et al,, 1984; Cheng & O'Kelly, 1986; RScSeisperger et a!., 1986; Kauppinen & Korhonen, 1987) (Figure 3). The airborne concentrations of chrysotile measured during compressed air cleaning usually ranged from 0.01 to 15.00f/cc, and those during the dry brush removal ranged from 0.01 to 2.62 f/cc (Johnson et at,, 1979; Roberts, 1980a, 1980b; Roberts & Zumwalde, 1982; Nicholson etal., 1984; Cheng & O'Kelly, 1986; Rodeispergeretal., 1986; Kaup pinen & Korhonen, 1987). Airborne concentrations of chrysotile during other methods of removing brake wear debris (i.e,, wet brush, vacuum) ranged from noodetected to 2.62f/cc (Roberts & Zumwalde, 1982). With the exception of a few samples, the sampling durations for the deaning activ ities (0.5 to 3 min) were all less than the 15-mln sampling period (or a shorter sample corrected for exposure during toe remainder of the time period) needed to assess compliance with the OSHA ceiling limit (Martonrk et al., 2001). Thus, it is not appropriate to characterize these airborne concen trations as being above or below the ceiling limit that was in place at toe time. This is also the case for short-term samples collected on mechanics involved in grinding, sanding, or beveling brake linings, a short-duration task performed relatively infrequently on moldedautomobile brake linings (Rohl et al., 1976; Johnson etal, 1979; Kauppinen & Korhonen, 1987; Weir etal., 2001a, 2001b). There were also several surveys conducted that measured longer term (e.g., during br ake work) and day long airborne concentrations (TWAs) of chrysotile fibers for brake repair workers. NIOSH L r i (- p I- HWBUI0011276 mmm mil mrifi nuirnifiFi inimiinirm iffi* i ri!!r* S6 D. J. PAUSTENBACH T AL researchers evaluated 19 garages servicing brakes of passenger cars, generally those that were active at conducting brake jobs. Each garage performed as few as two to as many as 45 brake jobs per week (Johnson et al., 1979; Roberts, 1980a, 1980b; Sheehy et a!., 1989). In their investigations, the NIOSH researchers coiiected more than 200 air samples on more than 50 mechanics as they per formed brake cleaning and repair operations. According to the NIOSH findings, the bake job TWA airborne concentrations of dvysotile for automobile brake mechanics ranged from less than 0.004 to 0.28 f/ce for fibers greater than 5 pm in length. Based on these data, the mean brake job TWA concentration was about 0.05 (foe (Paustenbach et al., 2003). As shown in Figure 8, these measured values were all below OSHA PEls {daily TWAs) in effect at the time of the studies (Johnson et al., 1979; Roberts, 1980a, 1980b; Roberts & Zumwalde, 1982; Sheehyetal-, 1989). Results from other studies conducted in the U.S. and abroad during this period fell within the range of concentrations measured In the NIOSH studies (ROdelsperger et al., 1986; Moore, 1988; Plato et al., 1995; Yeung et al., 1999) (Figure 3). These studies all used phase-contrast microscopy (PCM) to analyze samples for fibers greater than 5 pm in size and reported airborne concentrations that were below the OSHA standards applicable at the time the studies were conducted. Based on the weight of scientific evidence, these studies indicated that brake mechanics were exposed to rela tively low airborne concentrations of chrysolite fibers, as defined by OSHA regulations. A recent ana lysis of the available data indicates that 90% of the 8-h TWA concentrations experienced by mechanics servicing brakes in the 1970s were less than 0.1 f/cc and in the 1980s, with improved dust control measures, less than 0.004f/cc (Paustenbach et al., 2003). In addition, a recent simulation of brake repair work in the 1960s provided similar estimate of 8 hr TWA concentrations (Blake et al., 2003). The auto industry in the U.S. continued to evaluate potential exposure to mechanics conduct ing auto brake repair activities during this time period. Consistent with other studies, brake job TWA concentrations measured by Ford, General Motors Corporation (CM), and Chrysler Corporation (now Daimler Chryiser) between 1976 and 1989 were below the prevailing occupational exposure limits (Krebs, 1976; O'Brien, 1981, Fischer, 1989a, 1989b). No Increased Asbestosis Risk for Garage Mechanics Epidemiology studies focused on asbestosrelated health effects of garage/brake mechanics were initiated during this era, with some likely as a result of NIOSH's recommendation for further study of this worker population. It is also dear that most studies of occupations handling asbestos products at this time, induding brake mechanics, were focused on identifying the increase in relative risk associated with exposures. Figure 4 presents a timeline reflecting when studies were published that focused on asbestosis, mesothelioma, and lung cancer inddence in garage/brake mechanics. Following Boiiiat and Lob's (1973) observations, only three studies had suffident data to evaluate foe prevalence of asbestosis among populations of bake mechanics (i.e., where both lung function and chest x-rays were evaluated) (Nicholson et al., 1984; Efliehausen et al., 1985; Marcus et al., 1987). The first of these studies, Nicholson et a!. (1984), was a follow-up to Mount Sinai's preliminary research on brake mechanics, which had been discussed with NIOSH in July 1975 and later pub-, fished by Lorimer et al. (1976). At that time, ML Sinai had evaluated the lung function and chest radi ography data for 90 brake repair maintenance workers in New York Gty. Five workers were reported to have restrictive lung function and abnormal x-rays consistent with parenchymal fibrosis that was suggestive of asbestosis. The report suffered from a significant number of shortcomings induding: a limited number of workers, reliance on volunteers, foe lack erf a control group, and no correction for smoking or prior exposure to dusts (induding asbestos). Later, in an attempt to confirm foe lorimer et al. observation, Nicholson et al. (1984), also with Mount Sinai, performed a more com prehensive study consisting of 450 brake repair workers, 124 mechanics with no brake repair experi ence, and 205 nonexposea controls. They compared tire lung function and chest radiograph results of garage mechanics who performed brake repair work with foe results of those who did not and with nonexposed controls. No statistically significant differences were observed for the prevalence of abnormal x-rays among car garage workers with or without brake repair experience. Specifically, the prevalence of x-ray abnormalities among brake repair mechanics (18.5%) was greater than con trols (15.3%), but les than garage workers without brake or auto body experience (20.7%). While Nicholson et al. (7984) detected a decrease in lung function among auto preparation and body HWBUI0011277 S01S38SV M1 SOf"33IH8 S7 E5 HWBUI0011278 58 D. J. PAUSTENBACH ETAi. repair workers without brake experience, no decrease in lung function was observed among brake repair workers as compared to the controls. In short, Nicholson et al/s foflow-up study failed to confirm the initial impressions of Loriner et ai. (1976) that exposure of these mechanics had pro duced an increased risk of adverse health effects. Other studies have also evaluated the incidence of asbestosis has not been observed among Sje mechanics who performed brake repair work. For example, Elliehausen et al. (1985) pered a dintcal examination of 205 auto mechanics and concluded that (1) no significant differ ence could be found in the frequency of suspected radiographic asbestos-related changes in comparison with a reference group, and (2) both the frequency of small irregular opacities in chest x-rays and restrictive changes of lung function were not dependent on the inhaled cumulative fiberdose. Similarly, Marcus et al. (1987) examined the x-rays and lung function of 925 brake mechanics and conducted that asbestos exposure of mechanics does not impair lung function. Other'studies of garage mechanics evaluated either chest radiography or Sung function, but not both (Raithel et ai., 1989; Dahkfvistetal., 1992; Plato etal., 1995). No Increased Cancer Risk for Garage Mechanics Due to Asbestos Exposure Between 1975 and 2002, more than 20 epidemiology studies examined the possible risks of mesothelioma and lung cancer in different types of workers, induding vehfde and brake mechanics. Six of these studies are case-control studies that evaluated mesothelioma (McDonald & McDonald, 1980; Teta et al., 1983; Spirtas et al., 1985,1994; Woitowitz & RSdelsperger, 1994; Teschke et al., 1997; Agudo etal., 2000). McDonald and McDonald (1980) evaluated mesothelioma cases in the United States and Canada. This study identified 480 confirmed mesothelioma deaths. Controls were selected from patients at the same hospitals who had pulmonary metastases from non-putmonary malig nancies. Out of a total of 156 cases and 156 controls for workers not employed in occupations with a recognized mesothelioma risk, die occupation "garage" was reported to have 11 cases and there were 12 cases in the controls. The authors did not calculate a relative risk*; however, based on this finding they concluded that "no increase risk was found in garage workers certainly exposed to chrysolite from brake linings" (McDonald & McDonald, 1980, p. 1655). From these data, Wong (2001) reported a relative risk estimate of 0.91, with a 95% confidence interval (Cl) of 0.39-2.13. Teta etal. (1983) identified 220 cases of mesothelioma and other malignant tumors of the pleura in Connecticut Controls were selected among decedents from Connecticut death certificate files. For subjects employed in the industry "automobile repair and related service," the authors reported a relative risk estimate of 0.65, with a 95% confidence interval (Cl) of 0.08 to 5.53 for mesothelioma. Spirtas et at. (1985, 1994) evaluated 208 cases of mesothelioma identified by the Los Angeles County Surveillance Program, the New York State Cancer Registry (excluding New York City), and 39 large Veterans Administration hospitals. Controls included patients who died of causes other than cancer, respiratory disease, suicide, or violence. Although the authors did not evaluate the risk of mesothelioma in their journal publication (Spirtas et al., 1994), they did report a relative risk estimate of 1.0 (95% Cl: 0.6-1.6) for subjects engaged in "brake lining installation or repair" in their earlier analysis of largely the same data (Spirtas et al., 1985). Woitowitz and Rdaeisperger (1994) evaluated 324 cases of mesothelioma in Germany. The control group included 497 persons from two groups: hospital controls selected among patients who underwent lung resection and population controls. Sixteen cases, 16 hospital controls, and 12 popu lation controls were listed as "motor vehicle repair workers." Seven cases and 12 controls were characterized as definitely engaged in brake service. Based on these data, Wong (2001) reported a The concept of relative risk is used to measure the strength of an association in an observational study and equals the incidence rate erf disease in an exposed group divided by the incidence rate of disease in an unexposed group. The magnitude of rhe relative risk reflects the strength of the association (i.e., the greater the relative risk, the stronger the association). A relative risk of 3.0 or more indi cates a strong association, of 2.0 indicates a moderate association, and between 1.0 and 1.5 indicate a weak association. Relative risks may also be less than 1.0 in value, which would suggest a protective effect from exposure to a factor (Ulienfeid & Sttrfley. 19941. HWBUI0011279 ASBESTOS IN BRAKE LININGS $9 relative rfek estimate of 0.87 (95% Cl: 0.46-1.74) for all motor vehicle mechanics and 0.76 (95% Cl: 0.28-2.05) for motor vehicle mechanics definitely engaged in brake service. Teschke et af. (1997) evaluated 51 incident cases of pleural mesothelioma reported to the British Columbia Cancer Agency. Controls were randomly selected from the provincial list of voters. The authors reported relative risk estimates of 0.8 (95% Cl: 0.2-2.3) for "vehicle mechanics" and 0.3 (95% Cl: 0.0-1.4) for "brake lining installation or repair" for analyses with and without latency, con cluding that "brake installation and repair did not appear to be associated with mesothelioma" {Teschke eta!. 1997, pp. 166-167). Agudo et al. (2000) identified 132 cases of mesothelioma from hospitals in two Spanish prov inces. Controls included hospital patients with conditions not related to asbestos. The authors did not report a relative risk estimate for motor vehicle mechanics; however, they did indicate that their study group included 3 cases and 14 controls who were employed as "mechanics, motor vehicles." From these data, Wong (2001) reported a relative risk estimate of 0.62 (95% Cl: 0.17-2.25}. Based on the consistent results of these six studies of vehicle mechanics from four countries, there is no evidence that garage mechanics are at an increased risk of mesothelioma (see Figure 9). The results from three cohort mortality studies OarvhoSm & Brisman, 1988; Hansen, 1989; Custavsson etaL, 1990) and five surveillance proportionate mortality studies (Olsen & jensen, 1987;Coggon eta!., 1995; Hodgson et al., 1997; Milham & Ossiander, 2001; NIOSH, 2002) also support this conclusion. Figure 10 presents a comparison of the relative risks of mesothelioma across various occupations, including vehicle mechanics (Teschke et ai., 1997). The data presented suggest that vehicle mechanics' risk of mesothelioma is similar to that of workers in other occupations not thought to be exposed to airborne asbestos above background concentrations. Of the several epidemiology studies that have evaluated lung cancer, six studies should be con sidered most informative because they were able to adequately adjust their results for smoking (Williams et ai., 1977; Lerchen et al., 1987; Benhamou eta!., 1988; VineisetaL, 1988; Morabia et al, 1992; Hrubec, 1992, 1995). Specifically, Williams et af. (1977) published a summary of the results of the Third National Cancer Survey that examined 7518 incident cancer cases. The report investigated risks associated with specific occupations and industries while controlling for age, sex, race, education, smoking, alcohol use, and geographic location. Cases with various types of cancer were interviewed and case-control analyses were conducted for individual cancer sites using those with other types of cancer as controls ("inter-cancer comparisons"). For the industry category "car repair services," the authors reported a relative risk estimate of 0,85 (confidence interval not reported but relative risk not statistically significant) for lung cancer. Lerchen et al. (1987) evaluated 333 White and Hispanic male lung cancer patients from New Mexico. Controls were selected either through random-digit dialing or from the Health Care Financing Administration records. For "auto mechanics," the adjusted (for age, ethnicity and smoking relative risk estimate was 0.9 (95% Cl: 0.5-1.9) based on 15 cases and 25 controls ever employed for 1 yr or more in this occupation. Benhamou et al, (1988) evaluated 1260 lung cancer cases from France. Controls were selected from hospital patients with other diseases not related to tobacco exposure. A total of 65 cases had ever been "motor vehicle mechanics," compared to 96 controls. The authors reported adjusted (for smoking) relative risk estimate of 1.06 (95% Cl: 0.73-1.54) for this occupational group. Vtneis et al. (1988) evaluated 2973 male lung cancer cases compiled from 5 case-control studies, each conducted in a separate U.5. state. Controls for one study were population based, whereas tire other four studies used hospital controls or deceased controls identified from death certificates. Ninety-eight cases and 90 controls were ever employed as "automobile brake workers," yielding an adjusted (for age, birth cohort, and smoking) relative risk estimate of 1.2 (95% Cl: 0.9-1.7). Hrubec et al. (1992, 1995) conducted a cohort study to examine the mortality experience of 248,046 U.S. veterans from 1954 through 1980 by occupation and industry, to evaluate the associ ation between occupational exposures and risk of various cancers. The cohort consisted of veterans who served in the U.S. Armed Forces at some time between 1917 and 1940, held active U.S. govern ment life-insurance policies, and responded to one of two questionnaires sent in 1954 ana 1957, providing information on smoking history and occupation and industry of employment. For cancers ! i HWBUI0011280 jS' 1 I a s 1 s * C aivwnsa mm mmnm 68 I 1.1 ^ o i HWBUI0011281 HWBUI0011282 62 D. J. PAUSTfNBACH ET AL of the respiratory system, the aufoors reported a smoking-adjusted relative risk estimate of 1.1 (90% Cl: 0.89-136) for the occupational group "automobile mechanics and repairmen" (Hrubec et ah, 1992} and a smoking-adjusted relative risk estimate of 0,9 (90% O: 0.69-3 .17} for the Industry type "automobile repair services and garages" (Hrubec et ah, 1995}. Finally, Morabia et ah (1992} evaluated 1793 mate lung cancer cases identified from hospital records in 9 U.S. metropolitan areas. Controls included cancer or noncancer hospital controls admit ted for conditions not refated to tobacco. The adjusted (for age and smoking) relative risk estimate for the usual occupation of "mechanics and repairmen--automobile" was 0.7 (Cl not reported). The data from these six studies do not indicate that lung cancer risk in this occupation is related to exposure to asbestos from brake repair work (see Figure II). Furthermore, the results from four cohort mortality studies (Rushton et ah 1983; jarvhofm and Brisman 1988; Hansen 1989; Gustavsson et ah 1990} found no clear evidence that lung cancer in this occupational group can be attributed to exposure to asbestos during brake repair. Wong (2001) recently reviewed the key mesothelioma and,lung cancer epidemiology studies reporting the relative risk of mesothelioma or lung cancer in garage mechanics, and conducted a metaanalysis* in an attempt to improve the precision of relative risk estimates (he., a narrower 95% confidence interval). Based on this analysts, the author concluded that "with respect to strength of association, the meta-analysis of the combined data from all six [case-control] studies [focused on mesothelioma! shows a summary relative risk estimate of 0.90, meaning no increased risk" (p. 174). Further, he noted, "the narrow 95% confidence interval (0.66-1.23) of the summary relative risk is indicative of the large underlying database and the precision of the summary risk estimates" (p. 174). Similarly, Wong's meta-analysis of three cohort studies that focused on iung cancer reported a relative risk estimate of 1.01 with a 95% confidence interval of 0.8 to 1.26 (Wong, 1993, 2001). In short, having looked at the published data using several different techniques, Wong (2001) concluded that the results of the epidemiology studies indicate that auto mechanics were not at an increased risk of mesothelioma or lung cancer as a result of working with asbestos-containing brakes. The weight of scientific evidence regarding the epidemiology data from more than 25 studies of motor vehicle mechanics using a variety of study methods and evaluating populations from nine countries were consist clearly indicates that brake mechanics are not at increased risk of asbestosrelated adverse health effects, including lung cancer and mesothelioma, due to exposure to asbestos from brake repair work. This could be due to one or a number of factors, including that the air borne concentration of chrysolite asbestos and the duration of exposure are too small to be significant, the chrysotile fibers are too short to be biologically important, that chrysolite fibers are substantially less potent than amphibole fibers in inducing lung cancer and mesothelioma, or other yet to be understood factors. The airborne concentrations and duration of exposure during brake repair have already been discussed in detail. The biological importance of short fibers continues to be debated; however, additional support for this hypothesis has recently been offered by Raggli et a!. (2002) and Butoor et al. (2003). In the first study, the authors attempted to correlate the type and content of asbestos fiber in lung tissue samples with type of occupation in 1445 cases of mesothelioma. For cases identified as automotive brake repair workers, the lung burden analyses reflected either a normal range of asbestos content or elevated commercial amphtbofes attributable to asbestos expo sure in other occupations. These findings, in combination with the nature of brake dust and the results of epidemiology studies, fed the authors to conclude that brake dust is unlikely to cause mes othelioma (Roggfi et at., 2002). Butnor et al. (2003) conducted a more in-depth analysis of these same cases, also concluding that, "friction product exposure, such as that encountered by auto mechanics, is unlikely to contribute to the development of MM [malignant mesothelioma]," (p, 329) 'The completion of many randomized dinical trials of common agents in she past (wo decades has ted to the use of 'meta-ana lysis,* kj which dab hom similar studies am pooled in a statistically rigorous manner. The putposes of meta-analysis are fourfoW: fil to improve the statistetS power for primary outcomes for subgroups, 121 to resolve unosrtalrsly when reports disagree, 01 to improve esti mates of effect site, and W) to answer questions not posed at the start of the Individual trials (Sacks et al., T 987). Underlying these alms is the assumption that one has access to all of the relevant data from all randomized clinical trials involving a given agent Conversely, metaanalysis obscures differences among trials (Sacks et a!,, 1987). < i i HWBUI0011283 63 HWBUI0011284 S4 D.PAU$TN8ACH ET At, Exposure Concentrations for Friction Product Manufacturing Workers Between 1975 and 2002,11 surveys were published for workers in friction product manufacturing facilities, including two conducted by NIOSH. In 1982, NIOSH collected samples for workers manufacturing brake pads in the Raybestos Friction Materials Company in Crawfcrdsville, IN (Zev & Klemme, 1982). TWA concentrations for raw material mixers (0.03-0,04 f/ec), machine finishers (0,40-0.45 f/cc), dritlers G.56f/cc), and slab cutters {0.67f/cc) were below the OSHA PEL of the time, 2.0f/cc. The NIOSH survey of workers at the Nuturn Corporation in New Castle, IN, provided similar results {Rube & Lipscomb, 1985). For example, the TWA concentrations during grinding ranged from 0.1 to 0.4 f/cc (see Figure 6). Newhouse and colleagues evaluated workers manufacturing friction mate rials in the UK (Newhouse et aL, 1982; Berry & Newhouse, 1983; Skidmore & Dufficy, 1983). They estimated workplace concentrations for four periods, pre-1931, 1932-1950, 1951-1969, and 1970-1979, for several job categories including mixing, forming, and grinding. They reported con centrations of >2Of/cc for all workers pre-1931, 1-2 f/cc to 10-201/cc for workers during 19321950 with the highest concentration experienced by workers handling raw asbestos during mixing and by workers grinding cured linings, and 1-2 free to 2-5 f/cc for all workers during the period 1951-1969. After 1970, concentrations were estimated to range between 0.5 and 2 f/cc for all workers. Newhouse et al. (1982) also reported that croddoiite was used at this facility but only during two well-defined periods before 1945, This reported use of amphibdes in friction products is rare, and in this case, the extent of croddoiite use was apparently limited to railroad engine brake linings. McDonald et al. (1984) reported a similar concentration pattern for workers employed been 1930 and 1969 in a friction products plant in Connecticut; however, concentrations are reported in mppcf. Concentrations for all worker categories had ranges of 1-24 mppcf (1930-1939), 1-10 mppcf (1940-1949), 0.2-7.5 mppcf (1950-1959), and 0.1-5 mppcf (1960-1969). The two reports on friction material workers in China report primarily total dust concentrations (Cheng & Kong, 1992; Yano et al, 2001), although Yano et al. provided a concentration range for raw material handlers of 5.8-58 f/cc. Menichini and Marconi (1982) provide short-duration sample concentrations for workers at a brake lining and dutch plate manufacturing facility in Italy, and Kogan et a!. (1993) provide similar data for workers at a facility in Russia. Chrysolite Friction Products Manufacturers Not at High Risk of Asbestos-Related Diseases Fifteen studies were conducted between 1975 and 2002 that evaluated the risk of developing asbestos-related diseases due to the manufacture of brake linings and pads (see Figure 5). Dumoftier et al. (1990) and Chen et al. (1992) both examined the inddence of asbestosis in friction product manu facturers. Neither study reported an increased risk of asbestosis in the study population. Specifically, Dumortier etal. (1990) did not observe any radiological evidence of asbestos-related disease (asbestosis, pleural thickening, or plaques) in 15 friction product manufacturing workers exposed only to chrysolite. Similarly, Chen et al. (1992) did not observe gross or unequivocal changes of asbestosis In chest x-rays of459 workers exposed to asbestos. However, the specific types of asbestos to which foe workers were exposed were not identified. The major studies evaluating the mortality and cancer inddence of employees working in the manufacture of chrysolite friction products also reported no significant increase in adverse health effects among foe exposed .workers. Spedfically, Newhouse et al. (19825, Berry and Newhouse (1983), and later Newhouse and Sullivan (1989) analyzed the 1941-1986 mortality data for more than 13,000 workers first employed between 1941 and 1979 at a UK factory that produced fric tion products primarily using chrysolite. The authors observed three cases of asbestosis, all in workers who had long periods of employment at foe factory. The study found no detectable excess deaths due to lung cancer. Thirteen deaths were attributed to pleural mesothelioma; how ever, 11 of foe cases had known contact with croddoiite during 2 well-defined periods before 1945 in the manufacture of railroad blocks, one case's diagnosis of mesothelioma was uncertain, and foe last case had been employed at the plant for only 2 wk as a fitter mechanic (i.e., occupa tional history was not well established) (Newhouse & Sullivan, 1989). Overall, the authors stated that "{this study] confirms our previous conclusions that under good environmental conditions chrysolite asbestos products can be manufactured with no detectable excess mortality" (New house & Sullivan, 1989). HWBUI0011285 ASBESTOS !N BRAKE IfNiNGS 65 in the late 1970s, McDonald and Fry (1982) undertook three parallel cohort studies of asbestos factory workers to investigate the effects of mineral fiber type and industrial process on malignant mesothelioma, respiratory cancer, and asbestosis. One of the studies focused on a cohort of 3641 men employed between 1938 and 1958 in a U.S. friction products and packing plant that, with few exceptions, used chrysolite (McDonald et a!., 1984). The authors reported that in no case was asbestosis listed as the cause of death on the death certificate. A significant excess of deaths due to respiratory cancer (which included lung cancer, laryngeal cancer, and "other" respiratory cancers) was observed in the cohort, which was mainly due to high relative risk estimates in men employed for less than one year with minimal accumulated dust exposure. There was no evidence of increasing risk with increasing duration of employment or cumulative exposure, and the author^ postulated that "some selective process may have led to the employment of men of relatively poor health or health habits.,. into low exposure jobs on which they often remained for a fairly short time" (McDonald et at., 1984, p. 155). Based on these finding, the authors concluded: "if we accept that the high mortality from... respiratory cancer... in men employed for less than one year was prob ably due to some form ofselection, our results suggest that the adverse health effects of employment in this chrysolite friction products plant were small" (McDonald et al, 1984, p. 156). McDonald et al. (1984) also reported that there was no mention of mesothelioma on any death certificates. However, Teta et al. (1983) identified three cases of mesothelioma, using tumor registry data, among individuals who had been employed at the same location. These three cases were missed by McDonald et al. (1984) because the cause of death was not identified on the death cer tificate or the worker was employed outside the study's observation period, when the facility was used as a textile plant The discrepancy between mortality and incidence findings is not unex pected. For example, foe National Occupational Mortality Surveillance (NOMS) database maintained by NIOSH reports an age-adjusted mesothelioma mortality rate of between 1 and 2 deaths per million per year for U.S. residents (males and females combined) for the years 1987-1996 (NIOSH, 1999). By contrast, the Surveillance, Epidemiology, and End Results (SEER) cancer incidence database reports an annual incidence of mesothelioma of 14 cases per million per year in U.S. males and 3 cases per million per year in D.S. females for the years 1988-1992 (IARC, 1997). The implications of these kinds of discrepancies with respect to the results of epidemiology studies are not always clear; however, it is important to make sure that foe comparison of observed and expected rates of disease is based on the same source of data (e.g., mortality or incidence), as was done in both the McDonald et al. (1984) and Teta et al. (1983) studies, to avoid biasing the relative risk estimates. Finkelstein (1989) investigated mortality rates among 1657 employees at two Ontario factories manufacturing chrysotile friction products. The study population consisted of workers employed for at least 1 yr after January 1, 1950, and were followed until the end of 1985. These factories were unique in that brake shoes and linings were not produced at the plant; they were simply assembled. Thus, exposure to foe relatively small number of employees working in the two small brake assembly areas that contained friction materials may have been from drilling, grinding, and riveting brakes. While foe authors recognized that only a small portion of the employees worked directly with fric tion materials, the study included all the employees at foe plants because the employees believed that they had been exposed to asbestos from dissemination of fibers throughout the plant The Finkelstein (1989} study reported that no cases of asbestosis were diagnosed. He observed a significant increase in mortality from laryngeal cancer and an elevated death rate due to lung cancer. However, an analysis showed "no association between the risk of laryngeal or lungcancer arid the total duration of employment (a surrogate for the extent of ambient exposure to asbestos or other work place toxic substances) or employment in departments where asbestos had been used" (Finkelstein, 1989, p.129). He reported two possible cases of mesothelioma, although the cause of death was recorded as lung cancer and the diagnoses were not confirmed. Overall, he concluded, "An associ ation between risk of death and occupational exposure Is uncertain" (Finkelstein, 1989, p. 125). This study also provided little quantitative information about actual worker exposures, but did report "The First air-sampling survey was performed by government hygienists in 1975; from that time until the factories were dosed in 1980 foe concentrations of asbestos were less than 2 f/cc of air* (Finkelstein, 1989, p. 126). t r b HWBUI0011286 m 0. f. PAUSTEN8ACH ET AL latsenko and Kogan (1990} published a limited study in Russian on a small cohort of 130 persons involved in brake production in the former USSR. The authors reported one case of asbestosis, but this person had worked elsewhere handling "free asbestos." The authors concluded that despite high levels of dust, there was no significant excess mortality, with no cases of lung cancer or mes othelioma. Kogan et al. (1993} examined mortality rates of two cohorts of asbestos friction product workers from the former USSR. The first cohort consisted of 156 persons from a plant in Ural and was followed for 20yr. The second cohort consisted of 2834 persons from a plant in Yaroslavl and was followed for 40yr. The authors did not report any cases or asbestos's or mesothelioma in either cohort. In the first cohort, no deaths from lung cancer were recorded, and in the second cohort, two deaths from lung cancer were recorded. The second cohort was subdivided into three subcohorts where the main exposures were to chrysotife, vulcanization and/or polymerization vapors and gases/and dusts consisting mainly of asbestos fibers coated with rubber and resin layers from asbestos bakelite or asbestos rubber, respectively. The only excess in cancer noted was that of stomach cancer deaths in the first subeohoft. Thus, Kogan et al. (1993) concluded: "Cancer mortality among asbestos friction product workers was much lower than the expected values with the exception of the increased stomach cancer risk in the subcohort exposed mainly to asbestos dust" (p. 294). Robinson et al. (1979), Cheng and Kong (1992), Pang et al. (1997), and Yano et al. (2001) also observed mortality patterns of friction product manufacturing workers; however, data for this worker population was combined with those from other occupations exposed to asbestos (e.g., building arid textile products). In a review of the risks associated with working in friction material man ufacturing plants. Berry (1994) concluded that "if there were any effects on mortality due to working in the manufacture of friction products using chrysotife, these effects were small and much less than the risks due to working in die dhrysotile textile industry" (p. 545). Toxicology of Asbestos Due to increased knowledge regarding the toxic effects of asbestos, and innovations in science and technology allowing for better analyses, research into the mechanism of action, kinetics of asbestos transport after inhalation, and asbestos-related diseases continued to be conducted during this time period (1975-2002). For example, with the advent of the electron microscope, an increased number of studies were conducted to characterize the effects of asbestos fibers in the lung and to better understand the influence of fiber kinetics on the development of fibrosis and malignant disease (Haque et al., 2001). Also spurring research during this time period was the U.S. EPA1986 evaluation of the carcinogenic potency of asbestos (U.5. EPA, 1986a). Although the U.S. EPA acknowledged the potential impact of fiber length and type on carcinogenic potency, it ulti mately treated all types of asbestos fibers greater than 5 pm equally in its evaluation. .The debate about the potency of long fibers in producing asbestos-related health effects, which began in the late 1960s, developed' full-scale by the 1980s (Crapo et al., 1980; lemaire et al., 1985; Piateketal., 1985; Pinkerton eta!., 1986; B6gtnetaL, 1986; Davisetal, 1986; Adamson & Bowden, 1987a, 1987b; Davis & Jones, 1988). While some early studies evaluating the fibrogenicity of different sized asbestos fibers reported fibrosis in some animals exposed to short fibers (less than 5 jim length), a greater fibrotfc response was always observed with longer fibers (greater than 20 pm length). Subsequent studies conducted up to the 1980s-showed that these early studies observed fibrosis from exposure to short fibers because the short-fiber formulations were contaminated with long fibers (greater than 20 pm length) (Crapo et al., 1980; Lemaire et al., 1985; Pfatek et al., 1985; Pinkerton et a!., 1986; 84gin et al., 1986; Davis et af., 198$; Adamson & Bowden, 1987a, 1987b; Davis & Jones, 1988). By the 1990s, scientists generally believed that fiber biopersistence in the lung;, which is dependent on efficiency of degradation of macrophages, deposition location; fiber dimension, and chemical composition, was one of the primary determinants of fibrogenesss (Hesterbefg et al., 1998a, 1998b; Sear! et a)., 1999). The belief that fiber size and/or persistence has a significant impact on comparative carcinogenic potency of various asbestos types was explored in the 1970s and 1980s and continues to this day. ILU.JIII IItill, i i 1... I I . ..i LULL! 11II 111 J i l l 1 UIIIIIIII.LLI ML ...I..IUU.LLU ASBESTOS IN BRAKE UNINCS 67 Researchers observed that the shape of asbestos-fibers longer than 2 to 3pm is the decisive factor for induction of tumors, but that shorter fibers may be a less but not unimportant factor (Pott et al., 1972). Some scientists believed that the cancer-inducing potential of asbestos was attributed to its aspect ratio (Le,, length to diameter ratio) and that any fiber of a certain length and diameter could produce mesothelioma (Stanton et at., 1981). This belief was partially supported in the early 1970s through the work of Stanton and Wrench (1972) and Pott et at. (1974), who indicated that, irre spective of the type of fiber, all fibers of a certain length (greater than 8 or 10pm) produced mesothe liomas when implanted. Due to growing concerns about occupational hazards in the late 1970s and 1980s, the specific characteristics of asbestos fibers in asbestos-exposed workers were examined in several studies (Wagner et al., 1982; Davis, 1989). The results of these studies revealed that many of the workers exposed to serpentine fibers had a significant number of amphibole fibers retained in the lungs, while the chrysotile fibers were cleared from the lungs. In addition, chrysolite miners were found to have developed fewer lung cancers and mesotheliomas when compared to miners exposed to other types of asbestos (Churg, 1988). Therefore, it was postulated that amphiboles were a more potent asbestos type than chrysotile (Wagner, 1991,1997). Over the next decade, a number of animal studies were conducted in an attempt to explain the differences in potency between chrysotile and amphibole fibers (Davis et al., 1978; Bolton et al., 1982; Monchaux et al, 1982; Suzuki & Kohyama, 1984; jaurand et al, 1987; Muhle et al, 1987; Minardi & Maltoni, 1988; Davis et al, 1991; Adadht et at., 1992; Carthew et al, 1992). While some investigators reported that chrysotile induced the most lung tumors (Reeves et al, 1971; Wagner et al, 1970, 1974; Wagner, 1972; Davis et al, 1978), other researchers reported that amosite was more tumorigenic than chrysotile (Donna, 1970; Suzuki & Kohyama, 1984; Minardi & Maltoni, 1988; Adachi et al., 1992). Other studies reported that crocidolite was a more potent car cinogen when administered intraperitoneally (Carthew et al., 1992), rather than via inhalation (Muhle et al, 1987). The study by Davis et al (1991) suggested a dose-response relationship for mesotheliomas and all forms of asbestos, amosite, chrysotile, and crocidolite, whereas the occur rence of lung cancer in animals appeared to be dependent on the route of asbestos fiber administration (Reeves et al, 1971; Wagner, 1972; Muhle et al., 1987), as well as the fiber length (Berman et al, 1995). Specifically, the inhalation of fibers was found to induce lung cancer in rats, white direct intrapleural or intraperitoneal administration of doses was found to result in pleural or peritoneal mesotheliomas, respectively. In contrast, McConnell etal (1999) reported a dose-related incidence of mesothelioma but no pulmonary neoplasms in hamsters exposed via nose-only inhalation to amosite fibers. Due to conflicting results in animal models, the results of ongoing lung burden studies, and a lack of evidence that only chrysotile contaminated with tremolite can cause cancer, the hypothesis that only amphibole asbestos induces cancer has been questioned (Stayner et al., 1996). These animal research studies, as well as in vitro studies and epidemiological data, on the influ ence of fiber type and size on cancer potency are currently being evaluated by the U.S. EPA to better characterize the risks posed by asbekos at lower doses. One of the initial U.S. EPA efforts in this area was the Asbestos Health Effects Conference, held in May 2001 (U.S. EPA, 2001). More recently, new models for assessing potential human health risks have been proposed (Berman & Crump, 2001, 2002) to update the models initially used by the U.S. EPA in their original 1986 cancer evaluation. Their approach dearly recognizes the impact of fiber size and the potential for greater potency of amphiboles compared to chrysotile fibers. In doing so, the authors recommend a protocol where future airborne asbestos samples be collected in a manner that accurately delineates fiber size and type, induding use of membrane filters for analysis by TEM. The U.S. EPA subsequently con vened a workshop to discuss a proposed protocol to assess asbestos-related risk (U.S. EPA, 2003). The peer consultation panel strongly endorsed the conceptual approach of developing an updated cancer risk assessment methodology that takes into account fiber type and fiber dimension (Eastern Research Group, 2003a). The panelists agreed that there was considerably greater risk for developing cancer with fibers longer than 10pm and that the risk for fibers less than 5 pm is very low and may be zero. The AT5DR also recentiy sponsored an expert panel on the influence of fiber length on asbes tos health effects (ATSDR, 2002). The expert panel also agreed that fibers less than 5 pm pose a _ L J...I.. IlLiLlLLU HWBUI0011288 m 0, j. paustenbach rr At. negligible risk of cancer (Eastern Research Croup, 2003b). Langer (2003) recently re-evaluated the available information on the biologic activity of chrysolite fibers subjected to thermal and mechanical shear stresses that approximated conditions experienced during braking (Langer 2003). He con cluded that these studies show thatchrysotife subjected to these severe conditions does not retain its natural properties due to alterations in its surface and structure, thereby greatly reducing its biologic activity, langer further concluded that complete transformation to forsterite is not required to result in this loss of activity. Therefore, "{ejxposyre to brake wear debris... may be associated with little to no risk of asbestos disease" (Langer 2003). New risk assessments, conducted using more sophisticated methods that account for fiber size and type, biologic activity and better sampling data for mechanics, should help explain why the available epidemiology studies consistently show that there is no increase in relative risk of asbestos disease in mechanics who serviced vehicle brakes with chrysolite linings. This new work on fiber length, as well as chemical form (serpentine vs. amphibole) and biologic activity, should help explain the apparent discrepancies between the lung cancer and mesothelioma rates predicted by the U.S. EPA risk model and that observed in various populations. For example, in 1994, OSHA estimated that there is a theoretical increased excess cancer risk of between 3 and 4 in 1000 at the current PEL of 0.1 f/cc based on an extrapolation using a linear, no threshold model that is analogous to the model used by the U.S. EPA (US. EPA, 1987), However, as reported by Camus et al. (2002) and by others, such model estimates appear to overe$timate,foe actual risks by a factor of 10-100 and, at very low doses, may well be predicting risks that are not present due to the existence of a practical, if not real, threshold. Asbestos Guidelines and Regulations In October 1975, OSHA proposed to lower the asbestos PEL from the existing 5 f/cc to 0.5 f/cc forall industries, but foe agency never issued a final rule on this proposal (OSHA, 1975). OSHA did, however, lower foe 8-h TWA airborne concentration of asbestos dust to 2 Vcc for fibers longer than 5 pm effective July 1, 1976 (OSHA 1972) (Figure 7). The 15-min ceiling limit of 10f/cc was still in effect at this time. In 1978, foe ACGIH adopted separate TLVs for amosite (0.5 f/cc), crocictolite (0.2 fee), and tremolite (0.5 f/cc) while maintaining the OSHA 2f/cc value for chrysotile fibers work place exposures (ACGIH, 2001). Seven years later, OSHA (1983) issued a second emergency temporary standard (ETS), which lowered the asbestos PEL to 0.5 f/cc for fibers longer than 5 pm, in response to a petition from the international Association of Machinists and Aerospace Workers. The ETS was overturned by the U.5. Circuit Court of Appeals in 1984, because it was determined that the quantitative risk assessment used by OSHA to justify the ETS required a more thorough review (Martonik et al, 2001). In response to the court, OSHA proposed two alternative values for the 8-h TWA for asbestos--0.2 f/cc or 0.5 f/cc for fibers greater than 5 pm in length--and proposed to revise te ceiling limit to an unspecified value (OSHA, 1984). On June 20, 1986, OSHA lowered the asbestos PEL to 0.2 f/cc for fibers greater than 5 pm in length as an 8-h TWA for all industries, and removed the ceiling limit requirement. This fatter requirement was eliminated for several reasons. First, it was noted that foe sizeable reduction in foe TWA PEL (from 2 to 0.2 f/cc) effectively reduced the de facto ceiling limit from foe 10f/cc level to 6.4 f/cc. Second, not designating a ceiling level corresponded to OSHA's use of cumulative dose models to derive lung cancer risk and mesothelioma risk values (neither model attributed additional risk to peak ceiling exposures). Third, OSHA believed that there was Sittie biological evidence in tire record that supported a dose-response model based on peak or ceiling exposures (OSHA, 1986). In response to the lawsuit filed in 1984 by the Asbestos Information Association (A1A) and AFL-CIO challenging foe 1986 standard. Asbestos Information Assodation/North American, et al. v. Occupa tional Safety and Health Administration, OSHA added a 1 -f/cc 30-min excursion limit to the asbestos standard in 1988 (OSHA, 1988). OSHA lowered the 8-h TWA for asbestos to 0.1 f/cc for fibers greater than 5 pm in length in 1994, but foe 30-mtn excursion limit of 1 f/cc established in 1988 remained unchanged (OSHA, 1994). Other U.S. agencies were also active in regulating asbestos use during this time period. In the late 1970s, CPSC took regulatory action to fan asbestos in consumer products, focusing on asbestos-containing HWBUI0011289 ASBESTOS IN BRAKE UNINCS 65 patching compounds,, artificial emberizing materials for fireplaces, and later, hairdryers (CPSC, 1977, 1979}. In 1979, CPSC joined the U.S. EPA in issuing an Advance Notice of Proposed Rule making, stating their joint intention to explore the use of the Toxic Substances Control Act {TSCA} to reduce risk to human health from exposure to asbestos (U.S. EPA and CPSC, 1979). In September 1984, the Natural Resources Defense Council (NRDQ submitted a petition to the VS. EPA requesting a prohibition on the continued use of chrysotile in automobile brakes (Warren et at, 1984). The U.S. EPA granted the petition by a notice published in the Federal Register of December 18, 1984 (U.S. EPA, 1994), and in 1986, included chrysotile brake linings and pads in its proposal to ban production and importation of asbestos and asbestos-containing products (U.S. EPA, 1986b). When the U S. EPA issued this latter proposal, it acknowledged that replacement of chrysotile in fric tion materials was more difficult than other asbestos product categories because of its unique characteris tics. In addition, the agency acknowledged that a ban on chrysotile use in brake friction materials might lead to decreased brake performance, and a lack of effective substitutes for aftermarket friction materials. Despite these uncertainties, the U.S. EPA included friction products in its 1989 final ruling on the ban (also known as the Ban and Phase Down Rule) (U.S. EPA, 19895. According to the schedule put forth by the UJL EPA, the phase-out of asbestos use in friction products was to occur by 1993 for original equipment manufacturers of passenger cars and light trucks, and by 1996 for original equipment manufacturers of heavy vehicles and the aftermarket linings and pads for all vehicles. The extension granted with respect to after market brake linings and pads was in response to comments to the U.S. EPA that it was easier to develop replacement asbestos-free brake linings and pads for use in vehicles that were initially designed to use such materials than to develop asbestos-free brake fining? and pads as aftermarket replacement products in older vehicles (that had brake systems designed to use asbestos brake linings and pads). Some commentors also believed that a ban on asbestos use in the aftermarket for brake systems designed for asbestos friction materials would compromise the performance of brake systems designed for asbestos brakes (U.S. EPA, 1989). In 1991, the.U.S. Court of Appeals, Fifth Circuit, in Corrosion Proof Fittings eta!, v. The Environ mental Protection Agency and William K. Reilly, Administrator, annulled most of the U.S. EPA's 1989 ban. In its commentary, the court noted the following: Despite the credible record evidence, by a study specifically commissioned by the EPA, that substitute products actually might cause more deaths than those asbestos deaths predicted by tee EPA the agency did not evaluate the dangers posed by the substitutes, including cancer deaths from the other Fibers used and highway deaths occasioned by less effective, non-asbestos brakes. This failure to examine the likely consequence of die EPA's regulation renders the ban of asbestos friction products unreasonable. (US Court of Appeals, Sth Circuit, 1991, p. 1224} The ban remains in effect for those products that were no longer being manufactured, processed, or imported (i.e., pipeline wrap, vinylfasbestos tile, millboard, asbestos clothing, asbestos-cement corrupted sheet, and asbestos-cement flat sheet) as of the date the Ban and Phase Down Rule came into effect Despite this court ruling, friction product manufacturers, in conjunction with brake and automobile manufacturers, continued to proceed with the elimination of chrysotile asbestos from brake linings and pads. This was due, in part, to the belief that the momentum toward integrating chrysotile-free brake linings into the braking systems of new cars had reached the point of no return, for example, a signific ant investment had already been made to develop chrysotile-free brake finings and pads on the part of the automotive, brake, and friction product industries. Chrysotile-free brake linings and pads were also found to perform satisfactorily In vehicles, and had already been incorporated onto some vehides. Rnaily, friction product, brake, and automobile manufacturers were concerned about having to comply with possible bans on asbestos in the future both in the United States and abroad. SUMMARY Over the fast lOOyr, brake linings have evolved considerably, due to increased public expecta tions about automobile performance and safety. To meet these challenges, the original woven 70 D. J, PAUSTEN8ACH ET AL materials used on external brakes were replaced with chrysolite-based molded finings attached to internal drum and disc brakes. As a better understanding of asbestos-related disease continued into the early 1970s, and environmental regulations became increasingly more stringent, friction product, brake, and automobile manufacturers began to research and develop chrysolite fiber substitutes. Although nonasbestos organic brake linings and pads may have satisfied the minimum FMV5S requirements soon after development, these new friction material formulations were not incorpo rated into vehicfes until they met all of the internal laboratory and field tests required by friction product, brake, and automobile manufacturers to meet the public's expectation of brake performance. It took several more years to develop the proper formulation for non-asbestos brake linings and pads that were equal in performance to ehrysatite-based brake linings and pads that had been in place for more than 70 yr. 8y the 1980s, most U.S. vehicles had incorporated nonasbestos semimetallic front disc pads, but the technology to replace chrysolite rear drum brakes linings was not fully developed until the mid 1990s. Early in the last century, questions were raised about the health hazards posed to workers manu facturing asbestos-containing products. In 1930, the first epidemiology study was published showing that exposures to high concentrations of asbestos in dusty manufacturing settings resulted in asbestosis (Merewether & Price; 1930). In this study, initial concerns regarding exposures to asbestos among workers involved in friction product manufacturing were raised. Between 1930 and 1959, seven studies were conducted where friction product manufacturing workers were part of the study population assessed. These studies provided evidence of asbestosis among highly exposed workers, but provided Me information on toe magnitude of exposure. Findings from early studies of manufacturing facilities eventually served as the basis for the first asbestos exposure limit of 5 mppcf, which was adopted by ACCIH in 1946 and continued to be used as a guideline by ACGIH and others for several decades, it was in 1955 that a causal relationship between asbestos exposure and lung cancer was documented for manufacturing setting: (Doll, 1955). During this same time, animal studies were being developed that used high doses of asbestos in an attempt to replicate asbestos-related worker diseases. The data provided during this time period were, however, inadequate to describe an asbestos dose-response relationship for friction product manufacturing workers, and no data were provided on the potential exposures of brake mechanics. In 1960, mesothelioma was attributed to asbestos exposure in cracidolite miners, as well as individuals living near the mine, leading to an increased focus on low-level exposure to asbestos and concerns about the lung cancer and mesothelioma risks. Simultaneously, several animal models were being developed to better understand these dose-response mechanisms, despite toe high doses required to induce cancer in animals. The speculation that brake wear debris could account for a significant source of asbestos in urban air, the creation of several regulatory agendes, including OSHA and the U.S. EPA, and toe lowering of recommended asbestos occupational exposure levels resulted in toe first series of studies (conducted in toe late 1960s and early 1970) to evaluate the contribution of brake dust to the atmosphere. These studies, conducted by the U:S. EPA, NIOSH, and the automobile industry, suggested that brake wear debris was not a significant source of atmos pheric asbestos as was originally postulated. In addition, studies conducted on toe amount of asbestos released during brake repair operations of passenger vehicles indicated that airborne concentrations of asbestos for brake repair.workers were below the occupational standards of the time. Finally, the first preliminary health effects studies of brake mechanics, as well as five additional health effects stud ies of friction product manufacturing workers, were conducted during toe 1960 to 1974 time period. it was during the post-1974 time period that most of the information on chrysolite exposure of brake mechanics was generated. In 1975, the question about the asbestos-related hazard to brake mechanics came to the forefront with the release of a preliminary study by researchers at Mount Sinai Hospital that, tn part, provided conflicting results to previously published studies. Specifically, they reported that chrysotile concentrations in brake dust were higher than previously thought, and that a preliminary review of x-rays from a select group of mechanics suggested a higher than expected incidence of x-ray and respiratory function abnormalities. The results of this report spurred decades of work to evaluate the exposure of garage mechanics and epidemiology studies of brake workers. ASBESTOS IN BRAKE UNINCS 71 More than 25 epidemiology studies were conducted over the next 30 yr after issuance of the Mount Sinai report on vehicle and brake mechanics. In addition, there have been more than 15 studies of either asbestos exposure or asbestos-related health effects in friction product manufactur ing workers. This body of information indicates that mechanics have not been exposed to harmful concentrations ofchrysotile fibers as a result of their work with brakes. Although the levels of exposure to chrysolite fibers occurring during friction materials manufacturing are substantially higher than those that occur during motor vehicle repair, the studies of friction materials manufacturing workers have shown that an increased risk of asbestos-related disease in this occupational group, if any, is not dearly detectable- Only the friction materials manufacturing workers in the UK who were exposed to crocidolite while making railroad engine brake linings were found to have an increased relative risk of mesothelioma, an example of the differences in the relative potency of amphibote and chrysotife fibers. Despite these finding;, over the years, various regulatory actions have attempted to ban or phase out the use of chrysotife fiber in brake lining? and pads. For example, the U.S. EPA banned the manufacturing, importation, and use of asbestos-containing products, including brake linings and pads, in 1989, due to perceived health risk issues. 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