MANUFACTURED vitreous fibers (MVF), also known as synthetic vitreous fibers, are generally considered to be less hazardous than asbestos to human health. They are used in a variety of thermal- and acoustical-insulation applications, frequently as an asbestos substitute or as a filtration medium. The Navy uses MVF in a number of shipboard and onshore applications.
To protect Navy personnel from potentially harmful exposures to MVF, the U.S. Navy Environmental Health Center (NEHC) developed occupational exposure standards. They are contained in the Navy's Occupational Safety and Health Program Manual and are supported by documentation in NEHC technical manuals. The documentation for the MVF occupational exposure standards assists industrial hygienists, occupational medicine physicians, and other Navy health professionals in assessing and controlling the health hazards associated with exposure to these fibers. Occupational exposures might result from such activities as construction, maintenance, operation, and decommissioning of naval ships and facilities. The exposure standards cover civilian and military personnel, but they do not apply to Navy contractors, who are regulated by the Occupational Safety and Health Administration (OSHA) and applicable state regulatory agencies.
In 1997, the National Research Council (NRC) was asked to conduct an independent review of the Navy's toxicological assessment of MVF and to evaluate the scientific validity of its exposure standard of 2 fibers per cubic centimeter of air (f/cm3). The NRC assigned the task to the Committee on Toxicology, which convened the Subcommittee on Manufactured Vitreous Fibers, a multidisciplinary group of experts, to determine whether all relevant toxicological and epidemiological data were
appropriately considered in developing the exposure standard; and to examine the uncertainty, variability, and quality of data and the appropriateness of assumptions used in the derivation of the exposure standard. The subcommittee was also asked to identify deficiencies in the MVF database and, where appropriate, to make recommendations for future research and data development.
THE SUBCOMMITTEE'S APPROACH
The Navy provided the subcommittee with its document Man-Made Vitreous Fibers, which is part of NEHC's Technical Manual (NEHC-TM6290.91-1 Rev. A) (NEHC 1997b), and its associated Health Hazard Information Summary: Man-Made Vitreous Fibers (NEHC 1997a). A background document, Navy Exposure Limit for Man-Made Vitreous Fibers: A Retrospective Look at the Decision History (Krevonick 1998), was also submitted to the subcommittee. During a public meeting, the subcommittee heard presentations on those documents from the Navy and a presentation on the toxicology of MVF from a manufacturer.
The subcommittee reviewed the Navy's documentation supporting the 2-f/cm3 standard. The documentation addressed an array of topics related to MVF, including production and use, chemical and physical properties, sampling and analysis, standards and recommendations, exposure, and toxicological and epidemiological data. The subcommittee began its review with an overview of the manufacturing processes, chemical composition, and classification of MVF, including newly developed fibers. Changes in fiber chemistry resulting from use and thermal stress were also discussed. The subcommittee reviewed the Navy's supporting documentation regarding sampling techniques, analytical methods, and toxicological and epidemiological studies. In the case of several of those topics, the subcommittee identified data gaps or issues that needed to be more thoroughly addressed by the Navy, including dosimetry and fiber biopersistence. The process used by the Navy to derive its occupational exposure standard was also discussed.
In January 1999, the Navy revised its Occupational Safety and Health Program Manual (CNO 1999), changing the occupational exposure limit for MVF to the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV) of 1 f/cm3. The subcommittee subsequently expanded its review of the Navy's occupational exposure
standard to include the new value. Finally, the subcommittee made recommendations on future research needs.
THE SUBCOMMITTEE'S EVALUATION
In general, the subcommittee found that the Navy made a good start in assessing the health effects of MVF. Further work is necessary, however, to ensure that military and civilian personnel are adequately protected when exposed to these fibers. Recent life-cycle-analysis studies for refractory ceramic fibers (RCF) have generated substantial knowledge on exposures of workers engaged in the manufacture, installation, and removal of RCF, but the data on after-manufacture exposures to other types of MVF are considerably less robust. Most of the monitoring data and epidemiological studies are based on workers in the MVF manufacturing sector. Although exposure data related to the installation and removal of RCF are available in the open literature, such data are generally lacking for other types of MVF in the published literature and in the Navy's documentation. The Navy provided little monitoring information on the types of fibers and concentrations associated with naval operations.
Manufacturing Processes, Chemical Composition, and Classification
The subcommittee determined that the Navy had described the fiber types and their chemical composition appropriately. The three fiber types discussed by the Navy are fibrous glass, rock and slag wools, and RCF, all of which have silica backbones but are produced with different technologies from different materials. The subcommittee noted, however, that the Navy included very little information on the physical or chemical degradation of MVF or on the effects of time and stress on their composition. The subcommittee recommends that the Navy be alert to the potential risks posed by degraded fibers and by new types of fibers.
Sampling, Analytical Methods, and Exposure Assessment
In reviewing the Navy's sampling procedures and analytical methods
for determining airborne fiber concentrations, the subcommittee concurred with the Navy's decision to use a standard based on fiber count rather than one based on gravimetric methods. In reviewing the Navy's exposure assessment, the subcommittee concluded that the Navy's documentation should provide specific guidance on fiber-counting methods that are considered most appropriate for monitoring particular fiber lengths and diameters. Furthermore, a more up-to-date review of the effects of fiber morphology on deposition in and clearance from the lung would be helpful to assist the Navy in determining which MVF are likely to pose greater risks to workers and therefore require more stringent exposure limits.
The Navy's documentation included a review of environmental exposures and numerous studies on worker exposure; however, most of the latter studies focused on the manufacture of MVF. The subcommittee did not consider the studies reviewed by the Navy to be particularly relevant for determining whether the Navy's occupational exposure standard would be protective of military and civilian personnel. Although the Navy apparently monitors MVF in the workplace, it made no attempt in its documentation to link measured air concentrations to exposures of workers who install, use, maintain, or remove MVF.
Biopersistence of Vitreous Fibers
The ability of vitreous or asbestos fibers to cause disease is based on biopersistence (the time that an intact fiber remains in the lung), which in turn depends on the fiber's chemical composition and physical dimensions. The subcommittee concluded that the Navy's documentation does not provide an adequate assessment of the role of fiber biopersistence in health effects. To address that issue, the subcommittee presents several studies that have been conducted on fiber solubility and biopersistence in the lung. Longer fibers, in general, are more biopersistent, more toxic to cells, and more mutagenic.
Although the Navy reviewed many of the acute and chronic toxicity studies conducted on animals, the subcommittee concluded that the
reviews were not sufficiently critical of the animal models or the methods used to dose the animals. For instance, the Navy did not comment on the relative merits of the cited studies. The subcommittee notes that inhalation studies provide better toxicological information because they qualitatively (although not quantitatively) most closely resemble human exposures. Studies that use intratracheal instillation might be valuable for a preliminary assessment of a fiber's toxicity, but they should not be used for establishing exposure standards. Similarly, intracavitary dosing studies should be used with caution because they might yield false-positive results on the development of lung cancer and mesothelioma.
The Navy was thorough in its summary of epidemiological studies of workers manufacturing MVF, focusing primarily on three large cohort studies in North America and Europe. However, the Navy did not cite several recent studies that re-examined those cohorts or studies that identified new cohorts, such as populations of workers manufacturing RCF. Such omissions are important because recent animal studies suggest that RCF and other very biopersistent fibers might have carcinogenic potential and therefore might warrant special consideration when an occupational exposure standard for MVF is developed. In addition, the Navy did not present a critical assessment of the limitations of the epidemiological studies. For instance, potential confounding factors, such as smoking, can affect the conclusions that can be drawn from the large studies of workers who manufacture MVF.
Evaluation of the Navy's Exposure Standard
The subcommittee reviewed the process and rationale used by the Navy to adopt the occupational exposure standard of 2 f/cm3 for MVF in 1995. That standard was higher than the 1992 proposed OSHA permissible exposure limit (PEL) of 1 f/cm3, but it was lower than the 1977 National Institute for Occupational Safety and Health (NIOSH) recommended exposure limit (REL) of 3 f/cm 3. The Navy acknowledged that it had no scientific justification for selecting the value of 2 f/cm3 instead of the NIOSH or OSHA limits and that the choice was essentially the
average of the two values. However, given the Navy's stated goal of having a reasonable yet protective fiber standard, the subcommittee concludes that the adoption of the OSHA PEL of 1 f/cm3 would have been more conservative. Alternatively, the Navy could have derived its own occupational exposure standard by conducting a risk assessment based on a rigorous scientific review of existing toxicological and epidemiological data.
In January 1999, the Navy policy for establishing occupational exposure standards was changed. That resulted in the adoption of the ACGIH TLV of 1 f/cm3. The subcommittee believes that lowering the standard from 2 to 1 f/cm3 is appropriate on the basis of available toxicological and epidemiological data and is in accord with other national and international occupational health limits for MVF.
The subcommittee concludes that 1 f/cm3 might not be protective for RCF or for some other MVF that are particularly biopersistent. The subcommittee recommends that the Navy consider setting separate exposure standards for the more biopersistent fibers.
Information Gaps and Recommendations for Future Research
Future research efforts should focus on identifying and monitoring the health effects associated with different fiber types. Animal models that are most appropriate for human health risk assessment of MVF should be identified. The development of short-term screening assays to predict long-term effects should encourage the testing of new fibers for health effects. Epidemiological studies should look not only at workers engaged in the manufacture of MVF, but also at those involved in their installation, maintenance, and removal. Data on the latter types of exposure are available for RCF but are less robust for other MVF, especially with regard to exposures during MVF removal. Monitoring studies of workers exposed to new, used, or stressed fibers would add considerably to the understanding of the health effects of both short-term and long-term exposures to these fibers.