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OCR for page 48
,-.
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
OCR for page 49
49
so loo
70_
60
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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
OCR for page 53
53
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OCR for page 55
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.
OCR for page 74
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
OCR for page 76
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.
OCR for page 77
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.
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Representative terms from entire chapter:
carbon fibers
