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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers 3 SAMPLING, ANALYTICAL METHODS, AND EXPOSURE ASSESSMENT IN this chapter, the subcommittee reviews the air sampling methodology recommended by the Navy for determining fiber concentrations in workplace air and the monitoring studies of ambient and workplace air summarized by the Navy in the “Exposure Data” section of its report, Man-Made Vitreous Fibers. SAMPLING AND ANALYSIS The discussion of air sampling and analysis provided in the Navy's review of manufactured vitreous fibers (MVF) is a straightforward description of standard procedures for industrial hygiene practice. It can be improved and updated with respect to gravimetric analysis and fiber counting. Gravimetric Analysis When the American Conference of Governmental Industrial Hygienists (ACGIH) adopted a gravimetric limit of 5 mg/m3 for continuous filament glass fibers in 1997, it specified that the sampler's inlet should accept the inhalable fraction (ACGIH 1997). Therefore, the National Institute for Occupational Safety and Health (NIOSH) analytical methods, 0500 and 0600 (NIOSH 1994), cited in the Navy's review, for otherwise unregulated total and respirable particles, respectively, are not directly applicable to MVF without modification.
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers Neither a total particle or respirable particle inlet sampler is appropriate for sampling the inhalable fraction of continuous filament glass. The discussion of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in the section on gravimetric analysis appears to be misplaced, in that electron microscopy is more applicable to fiber counting than to mass measurement. A better example of an alternative means of determining sample mass would be x-ray fluorescence analysis based on the metal content of the MVF. Fiber Counting The introductory paragraphs on fiber counting are appropriate, but the purpose and intent of the remainder of the section are less clear. The cautionary notes about the limited resolution of phase-contrast optical microscopy (PCOM) and SEM for fine fibers (diameter less than 0.3 µm) and superfine fibers (diameter less than 0.1 µm) are appropriate. However, although guidance is given on how to describe the concentration of fibers resolvable with SEM and TEM, there is no guidance provided for determining fiber concentrations with PCOM. Should NIOSH B rules apply to SEM and TEM measurements? If not, why not? If so, should the analysis list separately the fibers seen with SEM or TEM that have diameters above and below about 0.3 µm? If that were done, the concentration of fibers with diameters greater than 0.3 µm might be comparable with the concentration obtained with PCOM, whereas fibers smaller than 0.3 µm might indicate the limitations of PCOM analysis for the particular exposure and risk assessment. It might also be appropriate to note that using the resolving power of an electron microscope to count the number concentration of fibers less than 5 µm in length is inappropriate because the hazard is largely confined to fibers longer than 5 µm. While the 5-µm length limit has long been used and is incorporated in the NIOSH and ACGIH Threshold Limit Value (TLV) protocols, there is evidence that a better length limit may be 10 or 20 µm (HEI 1991; Lippmann 1994). In any case, an important need when scanning electron microscopic images of sampled fibers is to measure the length distribution of at least several hundred fibers longer than 5 µm. Furthermore, microscopic fields to be counted should be separated by 10 fields to ensure that the fiber distribution is valid (Miller 1999).
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers EXPOSURE ASSESSMENT Understanding the risks to workers posed by exposure to MVF requires a knowledge not only of the toxicological effects of MVF and the dose-response relationships, but also of the environmental concentrations to which workers are likely to be exposed when performing maintenance or removal that disturbs the fibers or materials that contain them. The Navy document Man-Made Vitreous Fibers contains a short overview of environmental, nonoccupational exposure to MVFs and a discussion of occupational exposures, including ocular, dermal, as well as upper respiratory irritation, and inhalation hazards. The Navy document reviews MVF air sampling data by fiber type during manufacture, installation, use, and removal of MVF. The subcommittee has two concerns regarding the Navy's review of air sampling data from fiber production operations. First, the Navy is not engaged in the manufacture of MVF, so these data collected during manufacturing appear to have little direct applicability to the Navy. Information on concentrations of MVF to which insulation installers or removers can be exposed is relevant to naval workers, but the Navy does not state whether it conducts monitoring of workers who could be exposed to MVF through direct contact, such as the installation or removal of MVF, or through indirect contact, such as proximity to equipment that contains MVF. The Navy documentation of monitoring data on worker exposures to MVF during manufacturing and use is important for linking exposures to the results of epidemiological studies that examined adverse effects of such exposures. At the same time the Navy must clearly determine whether its proposed occupational exposure limits will be protective for its workers. To do that, the Navy should have presented the results of any exposure monitoring it conducted. The Navy provided the subcommittee with very limited information on its exposure monitoring during the preparation of this report (P. Krevonick, Navy Environmental Health Center, Personal Commun., October 7, 1999) but indicated that such monitoring had been conducted since January 1980. The monitoring information that was presented was cursory and difficult to understand and failed to indicate when the monitoring was conducted, how many workers were exposed, and other basic methodological and evaluative information. Since the Navy has been collecting at least some types of monitoring information since 1980 in its Navy Occupational Exposure
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers Database, the subcommittee does not understand why these data, or a summary of them, were not included in Man-Made Vitreous Fibers with some discussion of exposures, operations that result in high ambient fiber concentrations, and when current or proposed occupational exposure limits have been exceeded and by how much. Such information should have been presented in the Navy's original documentation with some correlation between air concentrations and exposures of naval personnel. The Navy should also have discussed the representativeness of the monitoring information in terms of worker categories. For example, if the Navy industrial hygienists monitored fiber concentrations in air at one site where MVF-containing materials were being sanded, are these concentrations likely to be indicative of those at other sanding sites or sites of similar applications? If workers are engaged in sanding at other sites, are there likely to be differences in exposure related to configuration of the work environment or the extent of ventilation-related controls, with respect to the types, ages, thermal stress histories, the size of the objects being sanded, the abrasives being used, or the mechanical energy applied to the sander? Answers to questions like those would enable both the Navy and the subcommittee to determine which workers have the greatest exposure, whether the worst-case exposures exceed the occupational exposure limits, and, if so, what measures can be taken to reduce the exposures. Failure of the Navy to provide the monitoring information it has is a serious flaw in its documentation. At the least, the Navy document should have linked the studies it reviewed to what was known about exposures of Navy personnel. CONCLUSIONS With regard to air sampling for MVF, there needs to be clear and explicit guidance on the collection of representative and biologically relevant samples of particulate fibers longer than 5 µm for analyses of fiber number concentrations. In addition, installation and removal operations create high dust levels, and there is a need for guidance on the sampling of MVF dusts for analyses of mass concentrations of inhalable particulate matter as a nuisance dust. NIOSH recommends PCOM as an appropriate analytical method for fiber counting of conventional glass fibers, rock and slag wools, and RCF with fiber diameters typically greater than 1 µm. However, that recom
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Review of the U.S. Navy's Exposure Standard for Manufactured Vitreous Fibers mendation should be modified to record separately the total number of fibers longer than 5, 10, and 20 µm. The subcommittee makes the same recommendation for determining concentrations of special-purpose MVF, which have a large fraction of fibers less than 1 µm in diameter. For maintenance and removal operations where occupational exposures are likely to include high dust concentrations instead of or in addition to high fiber number concentrations, samplers with inlets that meet the ACGIH-ISO-CEN (American Conference of Governmental Industrial Hygienists-International Standards Organization-Comité Europeen de Normalisation) criteria for inhalable particulate matter should be used, and gravimetric determinations of the dust should be made.
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