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

Abstract

Throughout the history of automobile development, chrysotile 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 “state-of-the-art” analysis of what was known over time about the potential environmental and occupational health hazards associated with the presence of chrysotile asbestos in brake linings and pads. As part of this analysis, the evolution of automobile brakes and brake friction materials, beginning with the early 1900s, is described. Initial concerns regarding exposures to asbestos among workers involved in the manufacture of friction products were raised as early as 1930. Between 1930 and 1959, eight studies were conducted for which friction product manufacturing workers were part of the population assessed. These studies provided evidence of asbestosis among highly exposed workers, but provided little information on the magnitude of exposure. The U.S. Public Health Service proposed the first occupational guideline for asbestos exposure in 1938. The causal relationship between asbestos exposure and lung cancer was confirmed in 1955 in asbestos textile workers in the United Kingdom, and later, in 1960, in South Africa, mesothelioma was attributed to asbestos exposure to even relatively low airborne concentrations of crocidolite. Between 1960 and 1974, five epidemiology studies of friction product manufacturing workers were conducted. During this same time period, the initial studies of brake lining wear (dust or debris) emissions were conducted showing that automobile braking was not a substantial contributor of asbestos fibers greater than 5μm in length to ambient air. The first exposure surveys, as well as preliminary health effects studies, for brake mechanics were also conducted during this period. In 1971, the Occupational Safety and Health Administration promulgated the first national standards for workplace exposure to asbestos. During the post-1974 time period, most of the information on exposure of brake mechanics to airborne asbestos during brake repair was gathered, primarily from a series of sampling surveys conducted by the National Institute of Occupational Safety and Health in the United States. These surveys indicated that the time-weighted average asbestos concentrations (about 1–6h in duration) during brake servicing were between 0.004 and 0.28 fibers per cubic centimeter, and the mean time-weighted average concentration was about 0.05 fibers per cubic centimeter. The data also showed that brake mechanics were not exposed to time-weighted average concentrations above workplace exposure limits in effect at the time of the study. From 1975 to 2002, more than 25 epidemiology studies were conducted examining the risks of asbestos-related diseases in brake mechanics. These studies clearly indicated that brake mechanics were not at increased risk of adverse health effects due to exposure to asbestos. Specifically, the studies found no increased risk of mesothelioma or asbestosis in brake mechanics, and no evidence that lung cancer in this occupational group can be attributed to exposure to asbestos during brake repair. This could be due to one or a number of factors: the airborne concentration of chrysotile asbestos and the duration of exposure are too small to be significant, the chrysotile fibers are too short to be biologically important, that chrysotile fibers are substantially less potent than amphibole fibers in inducing lung cancer and mesothelioma, or other yet-to-be-understood factors. Finally, there were 20 studies published during this time period evaluating asbestos exposure or asbestos-related health effects in friction product manufacturing workers. These studies indicated that these workers were historically exposed to concentrations of chrysotile fibers perhaps 10 to 50 times greater than those of brake mechanics, but the risk of asbestosis, mesothelioma, and lung cancer, if any, was not apparent, except for those workers who had some degree of exposure to amphibole asbestos during their careers.

The authors thank the following individuals for their contributions: Krishna Allamneni, Arnold E. Anderson, Valerie A. Craven, Michael Goodman, Renee Kalmes, Amy K. Madl, and Richard O. Richter. This research was primarily funded by Ford Motor Company, DaimlerChrysler Corporation, and General Motors Corporation who have been involved in litigation related to brake dust. Some of the authors have served as expert witnesses in litigation regarding the potential health hazards to mechanics historically involved in brake repair.

Notes

* The term brake “linings” is used when referring to the friction material used with external band or internal drum brakes. The term brake “pads” is used when referring to the friction material used in disc brakes.

* The word “asbestos” was originally 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 silicates that, when crushed or processed, separate into flexible fibers made up of fibrils” (Zoltai, 1977). The U.S. Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) have used the term to refer to six, once commercially viable, silicate minerals: chrysotile, which is part of the serpentine mineral family, and amosite, crocidolite, tremolite, actinolite, and anthophyllite, which are part of the amphibole mineral family (OSHA, 1986; U.S. EPA, 1994). However, the nonfibrous forms of actinolite, anthophyllite, and tremolite are not included in the regulatory definition of asbestos (OSHA, 1992). Unfortunately, many of the studies mentioned in this paper did not indicate the specific type of asbestos 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 recognized by OSHA and the U.S. EPA.

* This chronology applies only to U.S. passenger cars and light trucks. The development of automobile braking systems in other countries differs from that which occurred in the United States, and as a result, the history associated with friction material use and development in other countries will also vary from that of the United States.

^† The term “friction product manufacturers” is used to refer to those involved in the manufacturing of brake linings and pads. This more generic term is used over “brake lining manufacturers,” because some of the studies cited refer to workers who manufactured friction products other than brake linings or pads.

* Chrysotile asbestos fibers were the only type of asbestos fibers incorporated into the brake linings and pads for the passenger cars and light trucks sold in the United States (Rosato, 1959; A. E. Anderson, personal communication, 2002). The majority of articles reviewed used the generic term “asbestos” to describe chrysotile fibers in brake linings and pads. The authors of this article have tried to identify the specific asbestos types where possible without compromising the intent of the source article.

* There are two types of commonly used observational epidemiological studies: cohort and case-control. In cohort studies, the investigator selects a study population of exposed and nonexposed individuals and follows both groups to compare disease incidence 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 (controls) 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 observation 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 additional study design is the cross-sectional study that examines the relationship between disease and other factors, such as exposure, at a point in time.

^† Includes studies published in or 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 list of asbestos-related publications during this era.

* The impinger collection/light field illumination dust counting method was the common method for determining dust concentrations in air from the late 1930s to the early 1960s. Air concentrations reported by this method are in million particles per cubic foot of air (mppcf) and are total dust counts, which are nonspecific for fibers. A thorough evaluation of asbestos sampling techniques is provided in Walton (1982).

* Care needs to be taken when comparing the results of the various studies because the methods employed to estimate the percentage of chrysotile fibers in brake dust varied among the researchers.

* By the mid 1960s, asbestos analyses were shifting from the impinger dust counting method, which provided total dust measurements in mmpcf, to a fiber counting method using phase-contrast microscopy (PCM), which provided asbestos measurements in fibers per ml (f/ml) or fibers per cubic centimeter (f/cc), which are equivalent units (i.e., 1f/ml is the same as 1f/cc). This change in analytical technique was in recognition of the fact that the number and size of the individual asbestos fibers versus the total number of particles were more relevant for assessing potential health risk. The membrane filter method also allowed for long-term (e.g., 8-h) personal sampling. A total dust measurement of 1mppcf is roughly equivalent to 6f/cc (ACGIH, 1971).

^† In 1970, Great Britain’s occupational exposure limit for asbestos was 100 fibers per cubic centimeter-year (f/cc-yr), which represents a cumulative dose rather than an airborne concentration (Lane et al., 1968). Cumulative dose is equal to the concentration of asbestos in air (f/cc) multiplied by the duration of exposure to that concentration (years). Therefore, if the exposure duration was 50yr, the airborne asbestos concentration would have to be greater than 2f/cc to exceed 100f/cc-yr.

* A copy of the survey was obtained through the National Technical Information Service. While authorship is assigned to the National Institute of Occupational Safety and Health (NIOSH), NIOSH was not in existence in 1964. The survey was conducted by the U.S. Public Health Service.

* The concept of relative risk is used to measure the strength of an association in an observational study and equals the incidence rate of disease in an exposed group divided by the incidence rate of disease in an unexposed group. The magnitude of the 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 indicates a strong association, of 2.0 indicates a moderate association, and between 1.0 and 1.5 indicates 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 (Lilienfeld & Stolley, 1994).

* The completion of many randomized clinical trials of common agents in the past two decades has led to the use of “meta-analysis,” in which data from similar studies are pooled in a statistically rigorous manner. The purposes of meta-analysis are fourfold: (1) to improve the statistical power for primary outcomes for subgroups, (2) to resolve uncertainty when reports disagree, (3) to improve estimates of effect size, and (4) to answer questions not posed at the start of the individual trials (Sacks et al., 1987). Underlying these aims is the assumption that one has access to all of the relevant data from all randomized clinical trials involving a given agent. Conversely, meta-analysis obscures differences among trials (Sacks et al., 1987).