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,-. Assessing Nonoccupational Exposures to Asbestiform Fibers Lack of information about exposure is often the major impediment to assessing health risks associated with environmental substances. In this chapter, the committee defines exposure and explores the sources of asbestiform fibers, both naturally occurring and man-made. It also describes the gene rat movement of fibers in commerce and in the nonoccupational environment, notes the difficulties in determining amounts of fibers and in defining exposure, presents descriptive estimates of exposure levels and of the numbers of people exposed to various fiber types, and discusses the magnitude and significance of uncertainties about exposures to asbestiform fibers. In discussing the various types of fibers, asbestos is described first to provide perspective for the discussion of the other fibers. DEFINITIONS OF EXPOSURE To understand the extent of current and future health risks from exposures to substances of concern, it is necessary to characterize past, current, and projected future exposures. Information on past exposures serves as a guide for interpreting observed health impacts in epidemiological research and as a basis for estimating cumulative exposures. Information on current and projected future exposures provides information useful in making decisions about regulating exposure levels. The goal of exposure assessment is to estimate the distribution of various levels of exposure over a population or subpopulation so that the information can be integrated with data on the substance's toxicity. Figure 3-1 provides one example of a distribution of asbestos exposure for Some urban populations (Suta and Levine, 1979~. In that example , exposure is expressed as units of mass per unit volume of air. Exposure information on asbestos is also often expressed by using the fiber con- centration in air or water (fibers/cm3 or fibere/liter, respectively) -and the duration and pattern of exposure (e.g., 40 hours/week, 48 weeks/year, for 23 years). Attempts at exposure assessment involve many assumptions, complica- tions, and difficulties. To characterize exposures completely, one would like to know: 48
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49 so loo 70_ 60 SO ~0 30 - ~. o UJ m Cot tr: in 9 1° 0 8 ~7- UJ m 5 U' ~4 20 2 _~ O\ 1 · 1 · I ~ 0 10 20 30 40 50 60 70 so 90 loo PE RCENT OF POPU LATION EXPOSED TO CONCENTRATIONS AT OR ABOVE THE INDICATED LEVEL FIGURE 3-1. Distribut ion of exposures to asbestos in ambient air of urban areas. From Buts and Levine, 1979. Who is exposed ? age sex race health status other exposures , e. g., tobacco smoke · To which fibers are they exposed? - type of fiber - dimensions of fibers - other fiber characteristics · How are they exposed? Occupat tonal community (near known sources of material of concern) consumer use of manufactured product general environmental .
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50 · By what routes? - respiratory - oral - other · What pattern? daily peak intake annual fiber intake cumulative fiber intake cumulative exposure, e.g., fibers/cm3 times number of years concentration of exposure, e.g., fibers/cm3 or fibers/liter · How frequently, and how long? - continuously regular, periodic, e.g., ~ hours/workday; once per month irregular, but repeated single incidents age during exposure · Through what chain of events? natural weathering mining and mi 11 ing manufacturing processes transportation storage use industrial discharges waste disposal - environmental transport · How many people are exposed by various routes and under various conditions? - single routes of exposure - multiple routes and types of exposure, e.g., oral and respiratory, occupational and consumer In general, these questions are not easily answered, and fibrous materials such as asbestos pose some special difficulties. For example, fibers remain in the lungs after external exposure has ceased. In addition, there is no consensus on the best way to measure and express toxicologically significant doses, e.g., whether to use mass, fiber counts or fibers with particular characteristics. Measures of total mass are possibly not related to toxicity as reliably as appropriate fiber counts. Moreover, the various methods of collecting and counting fibers often do not correlate well with one another. However, to compare
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51 dose-response relationships among studies and to predict health effects from exposure or dose measurements, data concerning exposure and dose must be expressed in the same unite, even though there in uncertainty about the appropriate conversion factors (see Chapter 4~. Two major approaches are used for exposure assessment: one is based on measurements of exposure data and the other on calculations from more indirect indicators of exposure. In the first approach, exposure data are gathered as directly as possible. For example, a portable sampler worn by a person may provide good measurements of exposure. Most measurements are less direct, however, and the assessor must relate measured concentrations in air, water, or food to absorbed dose through some model of the exposure, absorption, and elimination processes. The amount of material present in the body of the exposed person provides an additional way of assessing exposure. The measurement approach is founded on real exposure data rather than on a framework of assumptions; however, measurement procedures are expensive and many measurements are usually required if generalizations are to be made for a variety of situations. By contrast, calculation-based approaches begin with less direct measurements of exposure, e.g., measurements of production volumes or chemical and physical properties. Then, ultimate distributions of exposures are estimated through a series of calculations or mathematical models that attempt to represent the behavior of the substance. Although this second approach obviates the need for multiple measurements of fiber concentrations, it must depend on a series of assumptions and mathematical respresentations that may be exceedingly poor descriptions of real phenomena but that must be kept relatively simple to avoid excessive computational costs. The validity of the input data--whether measured or simply estimated--may also be questionable. A conceptual model for determining fiber exposures is discussed in Appendix D. This model is useful for making rough exposure estimates when few or no measurements exist. It incorporates a scheme representing commercial and environmental flows of fibers, including such factors as natural occurrence, imports and exports, disposal, ambient concentrations, and biodisposition. The positive features of both approaches described above could be combined by calibrating the calculations against exposure measurements in known situations and then using the models to extrapolate or interpolate to unknown situations. Ideally, the actual amount of materials entering the human body would be measured for the most common conditions of exposure encountered by humans, taking into account differences in expo- sure both over time and by location. 1
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52 IR SOURCES The major properties of the asbestifonm fibers of concern to this committee are described in Chapter 2. Figure 3-2 shows a simple classification system for fibers with those properties. This classification is based on commercial use rather than on other distinctions among the fibers. Thus, commercially used asbestos and natural nonasbestos fibrous materials such as attapulgite are shown in the figure, whereas fibrous erionite, which is not used in commerce, is not specifically noted. Rather, such fibers are included in the general category "noncommercial natural mineral fibers." Aabeatiform fibers probably account for the vast majority of the msas of moat of these materials. Hugging et al. (1962) indicate that virtually all attapulgate conaiata of asbestiform fibers, even though the fibers are short. The committee was unable to determine whether or not the material commercially exploited as attapulgite is all fibrous. By contrast, the fibrous form of erionite is rarer (T. Zoltai, University of Minnesota, personal communication, 1983~. There are many sources of exposure to asbestiform fibers. In addition to exposures from natural sources, humans are exposed during such activities as mining, milling, manufacturing, use, and disposal of fiber-containing products. Because the committee was asked to study nonoccupational exposures, this report is focussed on environmental discharges or releases, rather than on exposures in the workplace. Naturally occurring mineral fibers are a source of exposure through natural weathering or human disturbance of mineral deposits. Fibers measured in air far removed from known asbestos sources (Thompson and Morgan, 1971) or in drinking water are probably derived largely from such source s . Similarly, mining and milling of asbestos are direct sources of fiber release into air and, occasionally, into water. Manufacturing of synthetic fibers may be considered a process that is parallel to the milling of asbestos. However, the fibers discharged during manufacturing probably represent a substantially smaller portion of the production output than would result from asbestos milling, because of differences in the processes and because cost considerations probably encourage greater efforts to minimize los see through discharge in the production of synthetic fibers. Manufactured fiber products can be divided into two major classes: primary products and Secondary products. Primary products are those made directly from asbestiform fibers (see Table 3-1). The different fibers and their respective primary uses are not completely interchangeable. Secondary products are made from primary products. For example, asbestos paper and cord (primary products) may be used for making electric and
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55 thermal insulation (secondary products). The major secondary uses of these fibers, based on total consumption, are shows in Tables 3-2 and 3-3. The & ited States imports approximately 90% of the asbestos it uses, principally from Canada. 0~ the other hand, it produces essentially all the attapulgite it uses and exports approximately 15% of its total production (U.S. Bureau of Mines, 1982~. The products that yield the greatest potential for exposure are not necessarily those produced in the greatest amounts. Conditions of use also influence exposure potential. For example, because the asbestos fibers in asbestos-cement pipe are relatively tightly bound in their cement matrix (as compared to other uses, such as in insulation), they may present less potential for exposure than some other uses. These different exposure potentials are discussed in the fo1 lowing sections for the various classes of fibers: asbestos, attapulgite and other natural fibers, man-made mineral fibers, and other synthetic fibers. EXPOSE POTTY FOR ASBESTOS Types of Exposure Exposures to asbestos fall in the following four categories · occupational · community (near known sources) · consumer (use of manufactured products) · general environmental Occupation&] Exposure. Because the heaviest exposures to asbestos occur in the workplace, they have received the most attention. There has been particular interest in exposures associated with the following activities: · asbestos mining and milling · asbestos product manufacturing · shipyard activities · installation and removal of insulation in buildings · brake lining manufacturing and replacement However, these occupational exposures are not of concern in this study except as they provide reference points and influence total exposure in conjunction with nonoccupational exposures. Communit Es osures. Closely related to occupational exposures are Y ~ community exposures, which encompass exposures of residents in communities where there are significant industrial sources of asbestos or other fibers. Such sources include mills, asbestos product manufacturing facilities, and brake manufacturing plants. These exposures can occur in
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56 TABLE 3-2. U. S. Consumption of Asbestos Fibers in Secondary Produc t ~ during 1982a Consumpt ion Chrysot i le Secondary Product (thousand of metric tons) (I) . Asbestos-cement pipe37.6 57 Asbestos-cement sheet10.8 100 Flooring products49.0 99 Roofing products7.0 100 Packing and gaskets13.6 99 Thermal insulation0.2 0 Electrical insulation0.7 100 Friction products52.9 - 100 Coatings and compounds25.0 100 All other4_ 9 Total246.5 93 l aAdapted from U. S. Bureau of Mines, 1983. r
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. 57 TABLE 3-3. Estimated U. S. Consumption and Production of Se lee ted Nonasbe ~ too Fibe r Produc t ~ 1981 Consumpt ion ~ thousands of Fiber and Use metric tons) References Attapulgite Drilling mud Pert ilizers Filtering (oil + grease) Oil and grease absorbents Pesticides and related produc t ~ Pet waste absorbent Medical, pharmaceutical, cosmetic ingredient All other use Total 113.5 50.2 18.7 178.2 106.5 105.8 0.06 79.5 l 712.46 197 7 Produc t ion ( thousands of metric tons) l U. S. Bureau of Mines, 1982 Fibrous glass Wool 1,100 Kirk-Othmer, 1980 Text i le 340 Kirk~thmer, 1980 Fine fiber 5 J. Leineweber, Manville Total 1,445 Corp., personal co~nmunicat ion, 1983 Estimated Annual Produc t ion ~ thousands of metric tons ~ Mineral wool 200 J. D. Cornell, U. S. Gypsum Co., personal communi cation, 1983 Ceramic fiber ("current") W.J. Breitsman, Carborundum High temperature insulat ion 20 Corp ., pe rsonal .commur~i Al1 others ~cat ion, 1983 Total 21 Carbon fiber ( inc. luding uses U. S. Bureau of Mines, 1982 In aerospace struc ture s, automotive structures, and sporting goods) 0.56
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58 a variety of ways. Fibers may be transported near the source via air and water or liberated from the clothes of a household member who works with the fibrous material. Community exposures are sometimes grouped with general environmental exposures. Suta and Leslie (1979) have identified eight types of facilities that lead to community exposures to asbestos: mines and mills · friction product plants · gasket, packing, or insulation plants · asbestos-testi~e plants · asbestos-cement plants · asbestos vinyl flooring plans n · roofing products plants 0 asbestos paper plants Mines and mills are usually situated close to one another in rural communities. In the United States, all the mills are located within lOO km of the mine. According to the U.S. Bureau of Mines (1983), four active asbestos mining and milling operations existed in the United States in 1980, and there were three in 1982. The other facilities listed above can be located in either urban or rural settings. Those situated in or near urban areas have the greatest potential for exposing large numbers of people. Coons Or BIONIC. These exposures result from the use of specific products outside the workplace. For example, the wear of vinyl asbestos floor tile can liberate detectable levels of asbestos into room air, as can disturbance of old installed asbestos insulation (Sebastian et al., 1982~. Other sources have included hair dryers and other electrothermal appliances, which have been known to release asbestos in breathable form (Organization for Economic Cooperation and Development, 1982~. Rice coated with talc has been reported to contain fibers that were apparently asbestos (BleJer and Arlon, 1973), presttmably because the talc contained such fibers. Asbestos fibers have also been reported in beer and wine (Cunningham and Pontefract, 1973) as well as in drinking water as a result of migration from asbestos-cement pipe (American Water Works Association, 1974~. Because both natural and waste asbestos can also reach drinking water through contamination of its source, drinking water is usually classified as an exposure from the general environment. The Asbestos Information Association (1975) has reported more than 3,000 uses for asbestos. Many of these are probably hypothetical, many others entail very amal1 quantities of asbestos and negligible potential for exposure, and yet other uses have disappeared over time. Nevertheless, there are scores and possibly hundreds of significant uses, most of which relate to the properties listed below:
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71 EXPOSURE TO MAN-MADE FIBERS - Man-Made Mine ral Fibe rs _ . , Man-made mineral fibers (+MFs)--sometimes called man-made vitreous fibers--are glassy and amorphous rather than crystalline. me MMMEs inc. lude fibrous glass , mineral wool ~ i . e ., rock wool and ~ lag wool), and ceramic fibers. ~ MFs compete with asbestos; in some markets and have replaced it in others. Because they are amorphous, Ml1MFs do not split longitudinally; they do sometimes break transversely, yielding shorter, but not thinner, part ic les . Fibrous Glass. Fibrous glass consists of monofilaments of silicate or borosilicate glass usually produced by melting amorphous silicates and forcing the melt through an orifice, followed by air, steam, or flame at tenuat ion. The current proce ~ se s al low produc t ion of re let ive ly narrow ranges of fiber size., depending on the commercial need. The three main classes are textile fiber, woo! fiber, and fine fiber, and there are many subclassifications within these broad classes. Textile fiber is the coarsest, typically 10 to 15 Am in diameter,6 but may range from 6 to 20 Am (JRB Associates, Inc. , 1981~. Wool fiber usually ranges from about 3 to 10 Am in diameter (Konzen, 1982), but can be ~ to 25 Am (JRB Associates, Inc. , 1981). Fine fiber is usually considered to be ~ Am nominal diameter or less.6 Fibrous glass accounts for approximately 80Z of all ~ Fs. At least 90: of the fibrous glass is produced as wool fibers, which are used primarily for thermal or acoustical insulation and for filtration. The largest category by far is thermal insulation, most of which is used to insulate buildings. These fibers are also used as duct linings, as insulation for pipes and appliances, and in ceiling tiles for acoustical insular ion. 6 Textile grades of fibers are used extensively in reinforcing resinous materials, e.g., in "fiber glass" automobile bodies or boat hulls (Watts, 1980~. They are also used in various cloths (especially for draperies), papers, electrical insulation, and cording. Textile grades account for 5: to 10% of all fibrous glasa.6 Approximately 0. 5Z by weight of fibrous glass produced falls within fine fiber size ranges. Because these fibers are expensive to produce, they are found only in specialized markets. Their two major uses are thermal insulation for aerospace vehicles and filtration, mostly to reduce the particulate content of air going to sensit ive areas , such as the clean rooms of semiconductor plants.6 The aerospace insulation is usually made as a fiber blanket sandwiched between metal or woven fiber cloth (Health and Safety Commission, 1979) and is installed between the inner and outer shells of the vehicle. The filter material is usually incorporated into a paperlike matrix with a small amount of binder. 6 6J. Leineweber. Manville Corp., personal communication, 1983.
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72 Lesa than lob of all glass fibers are smaller than 3 Pm in diameter. Fine fiber diameters are generally smaller than 3 um (Konzen, 1982) and cluster around 1 ~m, but may range from 0.2 Em to 5 Em, with typical airborne fiber lengths ranging from 5 to 20 lam (Esmen, 1982~. Less than 2: of all other fibrous glass categories are smaller than 3 Am in diameter. Very fine or superfine grades of fibrous glass have diameters predominantly less than ~ Em. bus the majority of the respirable fibers produced are probably in fine fiber grades; however, the uses for fine fibers to not appear to offer great opportunities for exposure. For example, the few measurements that have been made indicate that few fibers escape into the air during air filtration applications; otherwise, the utility of the filters would be compromised. 7 Occupational exposures to fibrous glass have tended to be considerably lower than those to asbestos, mainly because of innate processing differences (JRB Associates, Inc., 1981) and the higher cost of producing fibrous glass. Typical levels in workplaces have been approximately 0.1 fiber/cm3 as measured with a light microscope (Balser, 1976; Corn, 1976; Esmen, 1982; Health and Safety Commission, 1979; Johnson_ al., 1969; Shannon et al., 1982), although__ concentrat ions may exceed 10 fibers/cm3 in areas where f ine f ibers predominate (JRB Assoc late ~ , Inc ., 198 1 ~ . Concent rat ions were probate ly higher before the use of oils and binders to suppress dust (Hartung, 1982), which were used relatively early in the industry. The fact that atmospheric concentrat ions of fibrous glass in the workplace are lower than those for asbestos by about an order of magnitude suggests that plant emissions might be lower by about the same factor on a pound-for-pound production basis. Although total production is currently greater for fibrous glass than for asbestos products, the portion of fibers in the respirable range is lower. Thus, total emis- sions of fine glass fibers are probably considerably lower than asbestos emissions. Balzer (1976) reported that ambient concentrations of fibrous glass in California air were approximately 0.002 fibers/cm3 and that the average diameter of the fibers was 4 Am; 2/3 of the fibers were detectable by optical microscopy. Although the significance of this isolated report is uncertain, the reported concentration, which amounts to 2,000 fibers/m3, is much higher than any reported for asbestos in ambient air, even in urban areas. If manufacturing emissions are lower for fibrous glass than for asbestos, as suggested earlier, some other explanation would be needed if the Balser counts prove to be accurate and representat ive . For example, a point source might have been nearby. One possible nonmanufacturing source for glass fibers is in-place building insulation, which contains the fibrous material in relatively loose form (albeit with binders) . In addition to being exposed from outside air, a majority of the U.S. population is probably exposed to some extent by living or working in 7J. Leineweber, Manville Corp. ~ personal communication, 1983 e
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73 buildings with fibrous glass insulation. The exposures would probably be highest shortly after installation or disturbance of the insulation. The committee was unable to locate reports of measured concentrat ions of glass fibers in buildings. Many products contain fibrous glass. Smith ( 1976) has reported 35,000 individual product applications. Even if the east majority of them are hypothetical or trivial, many possible sources of nonoccupational exposure still exist. However, fibrous glass production and its use in building insulation are likely to be the major sources. Mineral Wool. Two types of fibers fall under the general rubric of mineral wool. Rock wool is the term for glass fibers made by melting natural igneous rocks ant then drawing, blowing, or centrifuging the melt into fibers. Slag wool is made by similar processes, except that the feedstock is the aiready~melted slag from iron blast furnaces or other metal-slagging processes. Total mineral wool production in the United States is estimated to be approximately 200, 000 metric tons. ~ Because of the generally less elaborate processes for manufacturing these two types of mineral wool, their diameter distribution tends to be broader than that of fibrous glass, and the product contains relatively large amounts of "shot" or residual unfiberized droplets of the molten material (Pundsack, 1976~. Rock wool and s lag wool can serve many of the same purposes as fibrous glass. Most of it is used for either building insulation or specialty "technical" insulation for industrial processes. It is applied primarily as thermal insulation, but some is used for sound tampering. 8 Applications include power plants, chemical processes, and other heavy industrial manufacturing. Much smaller amounts are used in commercial buildings and even less in residence. In current practice, binders are added to the mineral wool so that it can be supplied in the form of blankets or other shaped forms, rather than as loose fiber. The reported measurements of slag and rock wool fiber concentrations in the workplace fall between those found for asbestos and those for fibrous glass, as one might expect from the processes involved and the re let ive cos to of produc t ion. Most of the reported fiber concentrations range from 0.2 to 0.5 fibers/cm3, but concentrations as high as 2 fibers/cm3 have been observed (Esmen, 1982; Health and Safety Commission, 1979; Ottery et al., 1982~ . Per pound of throughput, fiber emissions could be expected to be intermediate between fibrous glass and asbestos. Because the production is lower than that for fibrous glass and the user are somewhat more likely to be industrial than residential, it is likely that population I. D. Cornell, U.S. Gypsum Corp., personal communication, 1983.
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TV exposures to mineral wools are generally tower than those for fibrous glass, although the proportion of fine fibers may be greater. me nominal diameter for mineral wools appears to be similar to that for glass woole--approximately 6 to ~ Am. However, there is a greater tendency for these wools to contain fine fibers. In the United seater, as much as two-thirds of the fiber count may be less than 3 Am in diameter (Eamen, 1982~. In some European rock woo! plants, however, the portion of respirable fibers may be considerably lower (Ottery et al., 1982). Ceramic Fiber. Ceramic fibers are produced by melting kaolin clay or a combination of alumina ant silica to form aluminosilicate glasses and then blowing the melt to form the fibers. Most of these fibers are used for high temperature insulation. Some alumina and zirconia fibers are produced for even higher temperature applications; these are the fibers most often referred to as refractory (Health and Safety Commission, 1979~. Total annual production is approximately 20,000 metric tons, but there is a capacity deco manufacture at least double that figure.9 Ceramic fibers are used mostly for high temperature insulation. Smaller quantities are used for expansion joint stuffing. Approximately 85X of the fibers produced are sold in the form of blankets or modular building blocks. Bulk fiber, paper, and textile forms are also marketed. The principal industrial purchasers of ceramic fibers are manufacturers of steel and other metals, ceramics, petrochemicals, and catalytic converters for automotive vehicles. Typical uses include insulation for kilns, furnaces, ovens, other types of heaters, and, to a lesser extent, consumer appliances. Virtually all the fibers produced are encapsulated or incorporated into structures. The target range of diameters is 2 to 3.5 ~m, but the diameters can range from less than 1 Am to 12 ~m. Fiber lengths are often several centimeters, but many fibers a few micrometers in length are also produced (JRB Associates, Inc., 1981~.10 In general, occupational exposures to ceramic fibers seem to fall within the same range as those for mineral wools, i.e., usually well under ~ fiber/cm3, but they occasionally exceed that figure (Esmen et al., 1979; Fowler, 1980; Health and Safety Commission, 1979~. Airborne fibers have a median diameter of about 1 pm and a median length of about 10 ~m. Thus, many of the airborne fibers appear to be respirable (Esmen, 1978~. Given the relatively low production volume, the moderate workplace concentrations, and the specialized applications, however, ceramic fibers are probably responsible for rather low general populat ion exposures. 9~. J. Breitsman, Carborundum Corp., personal communication, 1983. OInformation also received from W. J. Breitsman, Carborundum Corp., personal communication, 1983.
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75 Exposure to Other Man-made Fibers In comparison to the MIMFs, other fibers that might be considered asbestiform are produced in relatively small quantities. Among these are fibers of carbon, graphite, alumina, boron, potassium titanate, silicon carbide, and a variety of organic fibers such as Aramid or PTFE (polytetrafluoroethylene, or Teflon) . The organic f ibers usually enter ache same genera ~ markets as the textile grade glass f ibers and are correspondingly thick in diameter. Because these f ibers are not of respirable size, exposures to them are not examined in this report. Even if they were despicable, it would be difficult to classify them an asbestiform under this committee's definition. Most of the inorganic man-made fibers are marketed principally as reinforcement for various kinds of composite materials used in fabricating structures or equipment that must be strong but lightweight. For example, alumina fibers can be incorporated in an aluminum melt to produce f iber-reinforced metal (Chemical and Engineering News, 1980~. Potassium titanate was marketed for reinforcing plastic friction materials in brakes, f ilters, and high temperature insulation, but was withdrawn from the market in the mid-1970s (C. F. Reinhardt, E. I. duPont, personal communication, 1918~. Such fibers are also generally too large in diameter to be respirable, and they are presumed to have a very low exposure potential because they are sealed rather permanently in their matrix. Except for the carbon fibers, inorganic fibers are not discussed further in this report. Ad present, their production is limited, and their uses would not be expected to lead to substantial exposure. In this document, the term carbon f iber is used to describe both the carbon and graphite fiber classes, although such fibers may be manufactured by different processes (Beardmore et al., 1980; Zumwalde and Harmison, 1980~. These fibers are typically of the same general sizes as man-made wools, i.e., approximately 7 Am nominal diameter, and at least in normal use, they fall mostly within a guise narrow range of diameters (Johnson, 1982~. They may exceed 2 or 3 mm in length. Less than 25: of them are shorter than 30 Am and have diameters less than 3 Am (Delmonte, 1981; Zumwalde and Hdrmison, 1980~. Mere is some evidence of longitudins 1 c lesvage after these f ibers have been burned or worked (e.g., after sawing a composite that contains them) (Wagman et al., 1979) Like the other mineral fibers mentioned in this section, most carbon f ibers enter the reinforced materials market, e.g., in aerospace, automotive, and sports products such as golf club shafts. The matrices for the fibers are typically epoxy or polyimite resins (Kear and Thompson, 1980~. The fibers are often treated first with another plastic product such as tetrafluoroethylene to make them less brittle (Harben, 1980~. The U.S. Bureau of Mines (1982) reported that only about 250 metric tons of high-modulus (i.e., with the high-strength properties most like an asbestiform f iber) carbon fibers were produced in the limited
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76 States in 1980, but the rate of production growth is high, having doubled almost every year for the past 5 years (U. S. Bureau of Mines, 1982 ~ . One unusual new use of these fibers is in surgical implants that are reported to improve the healing of torn ligaments and tendons (Arehart-Treichel, 1982). me committee found little information on exposures to carbon fibers. Both the small quantities produced and their applications primarily in composites suggest that little exposure occurs. The greatest opportunity for human exposure probably arises when the composite is accidentally or intentionally burned. Although the matrix is often decomposed under those conditions, the fibers remain relatively intact (Wagman_ al., 1979~. In fact, the first concerns about carbon fibers involved their potential effects on electronic systems after a fire had released them as conducting "wires" in semiconductor circuits. Overall, nonoccupational exposures to carbon fibers are probably extremely low in comparison with those to most of the other asbestiform fibers discussed in previous sections. The potential for such exposures could change, however, if the use of carbon fibers continues to grow and diversify. SUMMARY AND RECOMMENDATIONS . In assessing exposures to asbestiform fibers that could cause adverse health effects, the committee considered ~ I) synthetic and natural f ibers that are used extensively in commerce and (2) natural fibers that are widely distributed by natural processes. As examples of commercial fibers, the committee assessed exposure to chrysotile, crocidolite, and other asbestos fibers; attapulgite; fibrous glass, mineral wool, and ceramic fibers; and carbon fibers. Fibrous erionite was chosen as an example of a noncommercial, naturally produced ant distributed asbestiform fiber. Asbestos and attapulgite fibers are also released by natural processes, as are other natural fibers found in ambient air. Many types of information would be helpful in assessing population exposures to various materials. However, because the most readily available information usually pertains to production or consumption levels, use patterns, fiber dimensions, and populations exposed, this type of information was used for the exposure assessment described in this chapter. Current chrysotile consumption in the United States is approximately 230,000 metric tons per year. Attapulgite production is greater, ant fibrous glass production apparently greater still. Mineral wool, crocidolite , ceramic fibers, other types of asbestos , and carbon fibers are produced or used in smaller quantities, approximately in that descending order.
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77 Al1 types of asbestos have fibers within the respirable range, i.e., leas than approximately 3 Am in diameter, as do attapulgite and erionite. However, the nominal dla~etera of Boat of the synthetic fibers exceed the reapirable range. Exceptions are some types of ceramic fibers, Which are near the upper limit of the Despicable range, and the fine grades of fibrous glass. With the possible exception of carbon fibers, most synthetic fiber products include some fibers of respirable size. Carbon fibers may split to finer, Despicable fibers upon mechanical or thermal stress. Asbestos fibers can be hundreds of micrometers long, although most of them detected in the ambient environment far from production sources are lesa than 3 Am long. Attapulgite fibers are generally lesa than 20 Am long. Target lengths for synthetic fibers are often measured in centimeters rather than in micrometers, but many shorter fibers are also produced. Many of the commercial fibers are used only in binding matrices such as in reinforced plastics or paper products. Fibrous glass, mineral wool, attapulgite, and to some extent ceramic fibers are sometimes used unbound as relatively loose fibers. Because of their limited applications and present low production volumes, ceramic and carbon fibers probably have a relatively low exposure potential. Because of its limited natural occurrence, the same iB true of fibrous erionite. Increased production and diversification of use is likely to be a significant factor for future exposures to carbon and ceramic fibers. The use of asbestos in the United States has declined in recent years. As with most materials, lack of information on exposures to the various fibers limits the ability of investigators to identify the adverse health effects resulting from such exposure. Thus, there is a need to improve this information base and to establish correlations between exposures and health effects. REFERENCES American Water Works Association. 1974. A Study of the problem of asbestos in water. J. Am. Water Works Assoc. 66:~-22. Arehart-Treichel, J. 1982. A healing scaffold. Sci. News 122: - 219. Artvlr,1i, M., and T. I. Baris. 1982. Environmental fiber-induced pleuro-pu~onary diseases in an Anatolian ~riliage: An epidemio- logic study. Arch. Environ. Health 37:177-~81. Asbestos Informatlon Association. 1975. Asbestos~General Informa- tion. Asbestos Information Association of North America, Washington, D.C. Balzer, J. L. 1976. Environmental data; airborne concentrations found in various operations. Pp. 76-151 in Occupational Exposure to Fibrous Glass. Proceedings of a Symposium. Pub. No. 76-151. National Institute for Occupational Safety and Health, Cincinnati.
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Representative terms from entire chapter: