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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers 1 INTRODUCTION BACKGROUND MANUFACTURED vitreous fibers (MVF) are fibrous, inorganic materials derived from various minerals. They are also referred to as man-made vitreous fibers, man-made mineral fibers, and synthetic vitreous fibers. MVF are manufactured by melting the raw materials and then forming fibers by several methods, including a drawing process that produces continuous fibers that are used in textiles; a rotary spraying process that produces fibers for insulation; a flame attenuation process that produces specialty fibers such as those used for filtration; a blowing process that is typically used to produce refractory ceramic fibers (RCF); and a wheel-centrifuge process used for RCF, rock wool, and slag wool insulation fibers (TIMA 1993). MVF are designed and developed to meet specific requirements, such as providing insulation at various temperatures, resisting degradation during filtering, and sound proofing. Historically, MVF have been grouped into classes on the basis of the primary materials from which they are made. For instance, glass fibers are made from sand, rock fibers from basalt, slag fibers from smelter residues, and RCF from clay. However, with the introduction of new fibers and new production processes over the last decade, that classification system is no longer adequate. Changes in the chemistry of fibers in all the classes noted above have resulted in overlap of various classes and fibers that no longer appropriately fit into any of the classes. In addition, the types of fibers in a given class can be variable; for instance, glass fibers vary widely in biologic potential (McConnell et al. 1999) as a result of differences in their chemistries and production methods (ACGIH 1997). Little consideration was given to potential health effects of exposures
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers to MVF until the hazards posed by inhaled asbestos—including asbestosis, lung cancer, and mesothelioma—were identified in the early 1960s (Wagner et al. 1960; Bader et al., 1961). Asbestosis was the first disease that clearly defined the potential for inhaled fibers to cause human health effects. The first case of asbestosis was described in 1907 by Murray, who reported that a carding machine operator died of pulmonary injury associated with diffuse pulmonary fibrosis. In 1924, Merewether and Price (1930), reporting on a study of British workers noted the unequivocal relationship between asbestos and pulmonary fibrosis. In the 1950's and 1960's, the association between asbestos exposure and the development of lung cancer (Doll 1955) or mesothelioma (Wagner et al. 1960) was first recognized. Only recently has the latent nature of asbestos-related disease been fully appreciated, particularly in the case of mesothelioma, which can occur more than 20 years after exposure. Gilson (1966) concluded that the average latent interval for the development of carcinoma of the lung was 20 years, whereas the interval between first exposure to asbestos and onset of symptoms of mesothelioma can be 25-50 years, with an average latency of 33 years. During World Wars I and II, there was a need to insulate ships rapidly; as a result, the number of asbestos-exposed individuals increased. Their exposure was correlated with an increased incidence of asbestos-related lung and pleural disease, often manifesting itself 1 or 2 decades after the end of exposure (Kennedy and Kelly 1993). MVF were designed as replacements for asbestos in various applications, including insulation. In light of the asbestos legacy and the absence of a toxicological database on vitreous fibers combined with the latency issue, it is not surprising that concerns have been raised about human health effects related to MVF exposure. Because of the many types of fibers in use in the 1970's, including MVF, a series of studies were conducted to evaluate their pathologic potential. Numerous types of fibers were instilled into the animal's pleural cavities (Stanton and Wrench 1972) or injected into animal's abdominal cavities (Pott and Friedrichs 1972) primarily to evaluate their carcinogenicity. Both groups of investigators found that the dimensions of the fibers were critical to their pathogenicity. Stanton et al. (1981) proposed that although fibers greater than 8 µm in length and less than or equal to 0.25 µm in diameter might be more carcinogenic, the ratio of fiber length to width—the aspect ratio—was more important for carcinogenicity than either dimension alone. Those studies were the original basis of two of the three legs
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers of the “3-D” concept—dose, dimension, and durability. The importance of durability quickly followed when it was recognized that a fiber had to reside in the lung for a relatively long period before it could cause chronic disease. Until 1984, the Navy based its occupational standard for exposure to fibrous glass dust and vitreous fibers on a gravimetric standard chosen with reference to the then operative Threshold Limit Value (TLV®) of the American Conference of Governmental Industrial Hygienists (ACGIH) for such dusts and on the permissible exposure limits of the Occupational Safety and Health Administration (OSHA). The guidelines (ACGIH) and standards (OSHA) treated glass fibers as nuisance dusts that were not otherwise regulated. The standard applied only to the mass of respirable particles, not to the particular toxicological properties of the material. The adequacy of this approach was questioned when some durable fibers became suspected of being animal carcinogens and when the dimensional characteristics of asbestos fibers were implicated in cancer in shipyard workers chronically exposed to asbestos. Those developments suggested that standards would be more soundly based on the number of WHO (World Health Organization)1 fibers, especially those greater than 20 µm in length, rather than on their mass and that differentiation among fiber types based on their toxicity might be warranted. Between 1984 and 1985, the Navy adopted an exposure standard of 2 fibers per cubic centimeter of air (f/cm3) for all MVF. The standard was based largely on the practical difficulty of distinguishing among fiber types when assessing exposures in workplace operations and on the prevailing standard of 2 f/cm3 for asbestos fibers, on the grounds that treating all fibers as though they were as toxic as asbestos would ensure sufficient protection. That was a conservative position, because asbestos fibers were thought to be substantially more toxic than most vitreous fibers in light of responses in animals to intratracheal instillations and intraperitoneal implantations in animals. That approach became increasingly impractical as the OSHA standard for asbestos was lowered from 2 to 1 f/cm3 and then to 0.1 f/cm3. Because of the questionable relevance of this standard to vitreous fibers and the increasing burden posed by adhering to an asbestos-based 1 WHO fibers are defined as having a diameter less than 3 µm, a length greater than 5 µm, and an aspect (length/width) ratio greater than 3:1 (WHO 1985).
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers standard, the Navy sought to establish a standard specific to MVF and adopted a standard of 2 f/cm3 in 1995. It was based on the Navy's review and evaluation of published research, on manufacturers' recommendations, and on practical aspects of implementation. In 1997, ACGIH proposed to revise the TLV for synthetic vitreous fibers from 2 f/cm3 to 1 f/cm3 (ACGIH, 1997); in 1998, it adopted the 1-f/cm3 limit. In a laudable desire to keep the exposure of Navy personnel in line with health-protection standards established in other sectors, the Navy adopted a policy of complying with the 1-f/cm3 TLV although this value was never formally adopted as the Navy's exposure standard. The Navy noted, however, that the exposure standards and recommendations of ACGIH, the National Institute for Occupational Safety and Health, and other institutions around the world are not strongly supported by quantitative evidence of potential risks, whether associated with cancer or chronic noncancer effects. There is a substantial amount of mortality data on workers involved in glass fiber—as well as rock and slag wool—manufacturing in the United States and Europe. Mortality data on RCF manufacturing are being collected in the United States and Europe, but the number of cases are insufficient for any definitive analyses. Some morbidity data on mineral wool and glass production workers do exist and are more comprehensive than morbidity data on RCF production workers. There are very few morbidity and mortality data on end users of MVF. Collectively, the data show little evidence of positive effects, and the consistency of the results and their applicability to the evaluation of health-protective inhalation standards in humans are uncertain. Consequently, although it is desirable to base exposure standards for MVF on toxicological and epidemiological data specific to the fibers in question, and quantitative dose-response analyses are preferred for providing confidence that numerical standards are set appropriately, existing standards are not supported by clearly articulated arguments for a causal association based on these data. Several questions arise from those observations. Even if the Navy established a standard chosen to reflect exposure levels established by other organizations, such as the ACGIH 1-f/cm3 guideline, what assurance is there that health protection would be achieved? Is a standard set in this way substantially stricter than necessary? For instance, could the Navy's standard of 2 f/cm 3 be justified as sufficient to protect health? Is there evidence that different standards might be necessary for different categories of MVF?
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers THE SUBCOMMITTEE'S TASK To address the above questions, the Navy Environmental Health Center (NEHC) reviewed the available information on the relevant properties of fibrous glass, rock and slag wools, and RCF, in an October 1997 document, Man-Made Vitreous Fibers (NEHC-TM 6290.91-1 RevA); in its associated Health Hazard Information Summary (NEHC 1997a); and in a further document, Navy Exposure Limit for Man-Made Vitreous Fibers: A Retrospective Look at the Decision History (Krevonick 1998). The Navy requested that the National Research Council (NRC) review those documents to determine whether the Navy's recommended exposure limit of 2 f/cm3 is scientifically valid. NRC assigned the project to the Committee on Toxicology, which convened the Subcommittee on Manufactured Vitreous Fibers. The subcommittee was assigned the following task: A study will be conducted to review the Navy's toxicological assessment of manufactured vitreous fibers (MVF) and evaluate the scientific validity of the Navy's recommended exposure limit of 2 fibers/cubic centimeter of air. The subcommittee will determine whether all relevant toxicity and epidemiology data were appropriately considered in developing the exposure limit. The uncertainty, variability, and quality of data and the appropriateness of the assumptions used in the derivation of the exposure limit will also be reviewed. Deficiencies in the database on MVF will be identified, and where appropriate, recommendations for future research and data development will be made. During its review of the Navy documents, it became apparent to the subcommittee that potential exposure of Navy personnel to MVF were unlikely to be the same as that experienced by workers involved in the manufacture of MVF. The subcommittee subsequently asked the Navy to provide whatever exposure information it had on the nature of exposures of Navy personnel. During the course of the subcommittee 's review of the Navy's 1997 documentation, the Navy changed the process by which it adopts an occupational exposure limit. After the Navy had given the subcommittee the requested exposure information, it requested that the subcommittee comment, if possible, upon the new occupational exposure limit of 1 f/cm3 and the process used by the Navy to select it.
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers ORGANIZATION OF THE REPORT In the remainder of this report, the subcommittee reviews the Navy 's documentation and provides an assessment of the Navy's approach for setting occupational exposure limits. Chapter 2, Chapter 3, Chapter 4, Chapter 5 through Chapter 6 provide commentary on the adequacy and completeness of the Navy's review of the relevant scientific literature and its interpretation. Specifically, Chapter 2 discusses information on manufacturing processes, chemical composition, and classification of fibers; Chapter 3 assesses data on sampling, analytical methods, and dosimetry; Chapter 4 reviews biopersistence of vitreous fibers; Chapter 5 discusses toxicological studies on MVF; and Chapter 6 addresses the epidemiological studies. Chapter 7 reviews the Navy's approach to selecting an exposure limit and offers recommendations on how the Navy could conduct the kind of scientific evaluation and risk analysis necessary to establish a protective exposure limit. Chapter 8 addresses information gaps and topics for future research.
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