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4
Measurement of Exposure to
Asbestiform Fibers
N
For more than 50 years, asbestos-containing dust in the workplace has
been measured to characterize occupational exposure to these particles.
These measurements were needed to correlate specific health effects in
workers with their exposure to the dust, to ensure the proper functioning
of dust control equipment, and to evaluate compliance with the fiber
and/or dust standards or guidelines in effect at that time. In
developing these measurement methods, attempts were made to balance and
maximize specificity, sensitivity, and biological relevance for the
different dust components. As measurement technology and knowledge of
agents and disease mechanisms advanced, new sampling and analytical
methods were developed with the goal of obtaining measurements that would
be useful in protecting workers.
Holt, 1957; Walton, 1982. ~
(See reviews by Ayer and Lynch, 1961;
Attempts are now being made to determine the concentration of fibers
in other environments, such as in buildings and in areas removed from
known fiber sources. Techniques useful for the workplace are not always
easily applied to other situations, where concentrations of materials are
likely to be hundreds or thousands of times lower.
In this chapter, the committee describes the measurement methods used
to determine the concentration of asbestos in a given environment.
Although the discussions are focussed on the specific methods used to
measure asbestos, many of these methods may also be used to measure other
asbestiform fibers. The development of the techniques is presented
within a historical perspective.
MEASUREMENT TECHNIQUES
Table 4-l summarizes the principal methods used
and identification of asbestiform fibers (Burdett
the quantification
_ , 1980~. The
earliest methods measured mass. In the gross mass methods, airborne dust
wan collected by filtration, precipitation, or impaction, and the total
dust was determined by simple weighing on conventional balances. X-ray
diffraction techniques were used to identify mineral phases present in
the dust; magnesium analysis was used as an index of chrysotile asbestos
82
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83
TABLE 4-1. Asbestiform Fiber Measurement Methodsa
Measurement Collection
Quant if icat ion Ident if icat ion
~.
Mass, grose Filtcr Gravimetric
Electrostat ic Gravime t ric
prec ipitator ~ piezoe lec tric
Mass, re-
spirable
Count Impingement
Mineral ident if icae ion
by x-ray; chrysot i le
ident if icat ion by
magnesium analysis
Not appl icable
Impac t ion
Hi-vol/filter
Beta-absorpt ion
Mic roscop ic
Horizontal e lu- Gravime t ric
triator/f ilter
Cycloneffilter Gravimetr~
Light micro~cope
Not app 1 icab le
Mineral ident i f icat ion
by x-ray
Mineral ident i f icat ion
by x-ray; chrysot i le
ident if icae ion by
magne ~ ium ana ly ~ i s
Mineral identification
by x-ray; chry~ot i le
itent if icat ion by
magnesium analys is
Identification
by morphology
Impac t ion Light mic rose ope Ident i f icat i on
by morpho logy
The rma 1 Light mic rose ope Ident i f icat ion
prec ipitator by morphology
Hembrane filter Light microscope Identification by
phase contrast morphology; mineral
't ident i f ic at ion by
· dispersion staining
~.
Nuclepore filter TEM,b SEM,C .Hineral identification
image recog- by SAED;d chemical
nit ion composition by
- EDXAe
Nuc lepore f i 1 te r Light sca t t e ring Ident i f icat i on o f
fibers by magnetic
alignasnt
aAdapted from Burdett et al., 1980.
bTEM - Transmiss ion e lectron mic roscope .
CSEM - Scanning electron microscope.
tSAED - Selected area electron diffraction.
eEI:~XA - Energy-dispersive x-ray analysis.
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84
content. When there was a need to collect and measure samples over short
times, such as in the evaluation of controls or brief exposure episodes,
the mass of the small amount of material could be measured by very
sensitive piezoelectric or beta-absorption instruments.
Major drawbacks to these analytical methods were the insensitivity of
the x-ray me thod in the detection of small part ic le a , the nonspecificity
in the resolution of chrysotile from the other serpentine minerals, and
the similar nonspecificity of the magnesium assay. Because much of the
mass measured by gross methods consisted of particles too large to
penetrate into the lung, techniques were often used to remove the large r
particles before assay. The horizontal, parallel plate elutriator was
preferred in the United Kingdom, whereas industrial hygienists in the
United States tended to use small cyclone devices.
All the mass methods yield results stated in terms of mass of dust
per unit volume of air. In occupational environments, the units commonly
used are milligrams of dust per cubic meter of air, whereas the much
lower dust masses found in nonoccupational ambient environments are more
conveniently expressed as nanograms of dust per cubic meter of air.
Counting methods are far more sensitive than mass determinations,
since samples with too little mass to be weighed are usually adequate for
counting.] Furthermore, since small particles far outnumber large
particles, counting emphasizes the respirable dust. Lantly, fibers can
be counted separately from other particles.2
Particles deposited directly on microscope elides by impaction or
thermal precipitation can be counted by light microscopy. However, a
more even dispersion can be obtained by impinging a jet of dust-ladened
air on a surface submerged in a liquid. The liquid is then transferred
from the impinger to a counting cell where the particles are allowed to
settle so they can be seen and counted in the same focal plane. These
methods have low and differing efficiency ant resolving power. The
membrane filter, however, is a very efficient dust collector. After
being rendered transparent, thereby making the f ibers visible, the fi lter
can be examined by phase contrast microscopy. The best resolution is
For example, 1 ng of chrysotile dust would yield 400 fibers 5 Am in
length and 0.5 Am in diameter. A nanogram is about a thousand
times lighter than most analytical balances can weigh with precision and
accuracy.
2As noted in Chapter 2, shape alone does not determine whether a
particle is asbestifonm. In a workplace where asbestos fibers were the
major dust present, the distinction was presumably not of major practical
importances For occupational environments, asbestos fibers are counted
if they are more than 5 Am long and at least three times longer than
they are wide (National Institute for Occupational Safety and Health,
1977~.
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85
obtained by the transmission electron microscope, which can resolve
particles made up of only a few hundred atoms. Somewhat larger particles
may be identified by techniques that reveal their chemistry (a probe
technique) or crystallographic characteristics (by electron diffraction).
Results of impinger counts are usually expressed in millions of
particles per cubic foot; dust concentrations measured by other methods
are typically expressed as particles or fibers per cubic centimeter. In
some electron microscope techniques, fibers or dispersed fibrils are
counted, and the results are then converted to units of mass per volume.
MEASURING ASBESTOS DUST IN THE WORKPLACE
The Impinger Technique
Early investigators of workplace exposures to asbestos fibers in the
United States used the impinger technique, then commonly used in mines.
Dust was collected in an alcohol medium, usually over a short period
(e.g., 20 to 30 minutes), and the suspension was examined by light
microscopy at lOOX total magnification. All particles in the dust were
counted. Very few fibers were seen, partly because of the low resolving
power of that optical system. The counting of large numbers of samples
was tedious, and interob~erver measurement differences led to systematic
bias.
The first asbestos dust "standard" in the United States was based on
measurements made with impingers by Dreessen et al. ( 1938~ . mese
investigators correlated observed health effects with measured dust
exposures in the asbestos text ile industry and tentatively concluded,
with reservations, that limiting exposure to 5 million particles per
cubic foot (5 mppcf) of air may be effective in preventing asbestosis.
No corre let ion with cancer of any type was attempted. They recognized,
as did later investigators, that counts of all particles provided a very
indirect index of disease potential.
The Membrane Filter Technique
In the membrane filter technique, efficient, convenient collection
media are used for assaying the work environment (Edwards and Lynch,
1968; Holmes, 1965; Leidel et al., 1979~. A portion of the filter may be
rendered transparent and then examined with a phase contrast light
microscope. Fibers with an aspect ratio greater than 3 to 1 are counted
on a prescribed, representative area of the filters (National Inst itute
for Occupational Safety and Health, 1977~. This technique is
sufficiently sensitive to allow fibers in workplaces to be counted with
measurable precision and accuracy.
dSee Chapter 2 for a discussion of mineralogical definitions.
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86
Prior to the 1960s, fibers of several lengths were counted separately
and reported (Lynch, 1965) . During the 1960s, the U. S. Public Health
Service followed the counting strategy developed in the British textile
industry, and counted only fibers >5 Am in length--a length that was
later incorporated into the U. S. occupat tonal asbestos standard (U. S.
Occupational Safety and Health Administration, 1971~. The longer
asbestos fibers were believed to be the agents responsible for asbestosis
(Beattie and Knox, 1961~. In addition, when only the "longer" fibers
were counted, greater precision was attained from repetitive fiber counts
on the same spec imen (Addingly, 1966) .
MEASURING ASBESTOS DUST IN THE AMBIENT ENVIRONMENT
The number of fibers >5 Am in length counted on membrane filters by
phase contrast light microscopy is used as an index for exposure in the
industrial workplace. However, these fibers may constitute only a small
portion of the total number of fibers present. When the fibers collected
on membrane filters, which collect particles as small as 0.01 Am in
diameter, are counted by transmission electron microscopy, up to 100
times more fibers may be detected than are visible by light microscopy.
(See Lynch et al., 1970, for accounts of studies in the textile industry,
and Rohl et al., 1976, for measurements in the brake repair industry.)
The ratio of transmission electron microscope fibers to fibers visible in
the light microscope may be a function of fiber type, industry, degree of
manipulation, distance from emission source, and other factors. Fibers
in the ambient environment far from point sources of asbestos emissions
are gene rally much shorter than 5 ~m, thinne r than O. 5 ~m, and, thus,
predominantly smaller'ehan the resolution capacity of the light
microscope (Spurny and Strober, 1981~.
In the ambient environment, electron beam instruments can be used to
measure fiber concentration and to characterize single, isolated fibers
(Lange r and Pooley, 1973~. Other mineral particulates may pose serious
background problems. For example, in areas where rocks ant minerals are
crushed for processing, particles resembling asbestos may be emitted
into the ambient environment (Larger et al., 1979~.
Because chrysotile accounts for more than 90: of the asbestos used in
the United States, the Environmental Protection Agency (EPA) has used it
as an index of asbestos exposure. However, it was considered impractical
to recover the fibers routinely without introducing artifacts or altering
fiber size. Therefore, only the chrysotile mass, as determined with the
electron microscope, has been monitored routinely (Thompson, 1978).
In the standard technique, large volumes of air are pulled through
membrane filters. Portions of the filters are ashed in nascent oxygen at
low temperatures to remove interfering organic matter. This ash is then
dispersed in water by ultrasound, and the residue in filtered onto
another membrane filter. Then, either this filter is "rubbed out" in a
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87
nitrocellulose medium and portions of the nitrocellulose film examined by
electron beam instrumentation or it is directly transferred to an
electron microscope grid for analysis. Both methods reduce the fibers to
unit fibrils, thus enhancing homogenization of the specimen and reducing
scan time, but information about the original nature of the fibers is
lost.
RELATIONSHIPS AMONG VARIOUS EXPOSURE MEASUREMENT METHODS
.
Data on numbers of fibers in the workplace have been used in
corre rating exposure and heal th e f fee t s in various occupat iona 1 studie s
(British Occupational Hygiene Society Committee on Asbestos, 1983; Dement
et al., 1982; Liddell et al., 1982~. In order to be able to compare
diverse studies and to assess health risks from ambient exposures, it
would be useful to establish a relationship among the various methods of
determining exposure. Specifically, what are the relationships among the
several methods used in estimating asbestos dust in the workplace and in
the ambient environment?
Consistent relationships among these methods do not exist. They are
subject to analytical error and subjective bias (Thompson, 1978~. As
examples, the electron microscope technique and its associated sampling
and analytical techniques have an experimental error of approximately 15:
to 30: of the measurement value. Relative standard deviations of 45X are
not unusual in light microscope counts. In addition, measurements made
in a particular environment at different times will vary because the
actual concentrations vary.
The different techniques measure a variety of indices, which often do
not remain in constant proportion to each other from sample to sample .
For example, with the phase contrast light microscope, fibers longer than
5 Am are counted as a single species, whereas shorter fibers are not
counted at al 1. Therefore, a given fiber count obtained by this
technique would undoubtedly represent very different numbers of fibers
and mass concentrations than the same fiber count obtained by electron
microscopy. In some cases, reproducible conversion factors may be
de termined when large numbers of paired samples are analyzed by the
various methods. However, these conversion factors usually cannot then be
applied to samples obtained under a different set of conditions.
Table 4-2 summarizes some reported attempts to determine conversion
factors among the various methods. The ratios in that table are based on
direct, independent estimates, except for those in parentheses, which
were calculated from other ratios. Although the accuracy of these
estimates is not known, an order of magnitude either way would probably
embrace most situations. These ratios are equivalent only in the sense
that they would be expected if side-by-side measurements were made in an
environment similar to that in which the data were originally obtained.
1
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88
TABLE 4-2. Relationships Among Methods of Measuring Exposure to
Asbestos in the Workplace
Equivalent Values Expected from Various
Measurement Me thods
Phase Contras t Elect ran
Light Microscope Microscope
Impinger (PRIM) ~ >5-~- (E>l) (EM Mass
Base Valueb (mppef) long fibers/cm3) fibers/cm3) (mg/m3)
1 n~ppcf 1 6 (360)d (0.2)
~ impinger) c
1 >5-m long fibers/ O. 17 1 60 O. 03
cm3 (pCLMye
1 Et! fiber/cm3 (O. 0028) 0.017 1 O. 0005
~ EM count ~
1 mg/m3 (5 ~30f 2, OOOg 1
(mass)
aRatios developed by Cook and Ilarklund, 1982; Davis et al., 1978; Dement
et al., 1982; Lynch et al., 1970; Rohl et al., 1976, and the British
Occupational Hygiene Soc iety (Walton, 1982) were used to construct the
table. Some adjustment was necessary to achieve consistency.
bGiven the base value indicated in column 1, the other columns show the
equivalent value to be expected from the indicated method. Thus, 1 mpp`:f
by impinger would be equivalent to 6 >5-m-long fibers/cm3 measured by
PCLM or 360 fibere/cm3 measured by the EM. Numbers have been rounded.
CCollected in an impinger and counted at lOOX light field. mppcf =
millions of particles per cubic foot.
dRatios in parentheses are calculated from other ratios.
eCollected on membrane filters and counted by ACID at 430X.
Ethic rat lo converts to 30 ID fibers/ng versus the nominal 20 fibers/ng
sometimes used.
"This ratio converts to 2,000 total EM fibers/ng.
The data for this table were obtained from workplace dust clouds or other
environmental samples containing high concentrations of asbestos. .
Fiber size/weight relationships are also presented (Table 4-3) to
indicate the ratios that might be expected under various conditions. The
electron microscope (EM) count/mass ratio (2,000 fibers/cm3 per
mg/m3) in Table 4-2 is equivalent to 2,000 EM fibers per nanogram,
OCR for page 89
89
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JO
which corresponds to a distribution of fibers with a mean length of 3 Am
and diameter of 0.3 Am. Such a distribution suggests the presence of
substantial numbers of fibers longer than 5 ~m, which would be visible
under a light microscope.
The ambient air in environments far from asbestos sources have few
fibers longer than 5 ~m. In general, those remote ambient environments
will contain many more, but smaller, fibers in a given mass than would
the workplace clouds on which Table 4-2 was based (Spurny and Strober,
19813. For example, if the fibers in the remote environment had average
lengths of 1 Am and diameters of 0.1 ~m, there would be 70,000 EM
fibers/ng (instead of the 2,000 determined for the workplace) and the
ratios would be altered accordingly.
EXPOSURE TO CHRYSOTILE IN THE AMBIENT ENVIRONMENT
. . .
Chrysotile has been detected in urban air (Selikoff et al., 1972) and
in lunge of urban dwellers (Larger et ale ~ 1971; Pooley, 1972; Pooley et
al. ~ 1970). Ambient levels of chrysoeile asbestos are usually expressed
as mass concentrations (ng/m3~. To estimate health risks from these
ambient exposures, the mass measurements need to be converted to the
equivalent fiber concentrations that are used as dose measurements in
workplaces, for which dose-response curves have been developed. There is
no single way to do this conversion since, as explained earlier, the dust
clouds are quite different, especially in regard to the sizes of fibers
they contain. One approach is to convert the ambient mass data into
numbers of fibers of the shortest length (5 ~m) generally counted in the
workplace, with an assumed diameter of 0.5 ~ and aspect ratio 10:1.
There are 400 f ibers of this ~ ize in 1 ng .
For a mass concentrat ion of 20 ng/m3, which is typical of outdoor
environments not near known sources , this conversion yie Ids a
concentration of 0.0080 fibers/cm3. Even in workplaces, however, most
fibers are shorter than 5 ~m. Assuming that workplace fibers average
3 Am x 0.3 Am (the assumption made in Table 4-2) and applying the
equivalency factors in Table 4-2, a typical equivalent concentration for
20 ng/m3 would be 0.040 fibers/cm3. However, if we were to assume an
average remote ambient fiber size of 1 Am x 0.1 ~m, then a concentration
of 1.4 fiber/cm3 would weigh 20 ng/m3.
Measurements of ambient concentrat ions observed at single sampling
locations may vary over several orders of magnitude. Seasonal changes in
wind direction, especially near emission sources, account for much of
this variability. For example, in studies reported by Thompson (1978),
20 specimens obtained downwind from an emission source had average
asbestos fiber mass concentrations ranging from 0.03 to 8,200 ng/m3.
However, for industrial cities in the continental United States from 1969
to 1970, average airborne asbestos mass concentrations ranged from 0.6 to
95.0 ng/m3, or two orders of magnitude. During 1971 and 1972,
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l
91
44 samples similarly obtained contained concentrations ranging from 0.4
to 27.7 ng/m3.
COMPLICATING FACTORS IN ENVIRONMENTAL ASSAYS
In an asbestos workplace, all the fibers may be assumed to originate
from the fiber being used. However, because remote ambient environments,
by definition, are distant from known asbestos sources, the identity of
fibers there can neither be assumed a priori nor easily determined with
any certainty, especially by light microscopy (Langer, 1979~. The light
microscope specified for analysis of membrane filter specimens (phase
contrast microscopy) yields only size and shape information, which may
allow the analyst to "identify" fibers by morphology alone. With the
increased resolution of the electron microscope, the internal structure
of the elementary chrysotile fibril may be visualized (Larger and Pooley,
1973~. For chrysotile asbestos, morphological information and the
behavior of ache fiber under the electron beam are usually sufficient
information for identification (see discussion in Langer et al., 1974~.
However, other fibers require additional diagnostic procedures. Selected
area electron diffraction (SAED) yields crystal data reflecting
characteristic structural elements that may enable the microscopist to
distinguish among types of fibers. Chemical information may also be
obtained by means of either energy dispersive x-ray analysis or crystal
spectrometry probe techniques.
For fibers in remote ambient samples to be accepted as asbestiform,
accurate fiber identification is needed. For example, Spurny and Strober
(1981) have shown that more than 90: of "mineral fibers" in nonurban
areas sampled in Europe were not asbestos, but, rather, were such
materials as fibrous gypsum and even ammonium sulfate. In a study of the
fibrous content of the lungs of the general population, Churg (1983)
found approximate ly as many nonasbestos mineral fibers as asbestos
fibers. Therefore, proper diagnostic tools are needed to characterize
fibers in remote ambient samples as asbestiform. Furthermore, when
extrapolating health risks from the workplace deco such remote environ-
ments, it music be recognized not only that fiber concentrations and size
distributions are different in the two environments but also that fiber
types may include nonasbestiform varieties. The asbestiform properties
enumerated in Chapter 2 cannot as Yet be measured on microscopic sampler.
FUTURE MEASUREMENT OF EXPOSURE TO ASBESTIFORM FIBERS
Walton ~1982) has noted that "there is no practicable alternative to
the membrane filter/phase contrast optical microscope for routine use in
the occupational environment . " Nonetheless, the method is too
insensitive and nonspecific to yield the information needed to assess
fiber exposure in the nonoccupational environment.
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92
Several basic objectives should guide the development and eventual
selection of a method for measuring fibers in nonoccupational or remote
environments. First, the method should yield data that are useful in
conducting epidemiological studies relating exposure and disease and in
making decisions designed to reduce health risks. The ideal method
should measure a characteristic, parameter, or index with biological
relevance, i.e. , the measurement should be related to the risk of the
disease end point being studied. Possible types of measurement include
fiber number, mass, length, diameter, and surface charge. Because of the
great extent of environmental variability, developing accurate informa-
tion about the concentrations of fibers in the air will be more dependent
on the number of samples collected than on limitations of analytical
techniques.
Current methods for determining ambient concentrations of f ibrous
particles could benefit from substantial improvement. However,
sufficient standardization is needed to allow comparisons of data from
various laboratories 80 that a data bank of ambient concentrat ions can be
established for use by epidemiologists and other researchers.
Sensitivity and specificity improved as the light microscope was
superseded by the electron microscope (EM) with its greater resolving
power. One issue to be considered now concerns the relative merits of
using the transmission electron microscope (TEM) and the scanning
electron microscope (SEM). Other issues concern methods of preparing the
fibers for the EM without disturbing them and development of improved
identification techniques (Hiddleton and Jackson, 1982) .
The SEM has been used extensive ly to examine environmental f ibers and
has produced some dramatic photographs of fibers In situ. Although the
SEM direct preparation method provides little opportunity for
contamination, the image resolution, contrast, and x-ray resolut ion of
the SEM have not been sufficient for precise mineralogical
identification. With an energy-di~persive x-ray attachment, the SEM can
now provide analytical information for identifying minerals, but it still
does not provide structural data. Because of its high resolving power,
the TEM has been more generally applied to studies of environmental
fibers, especially when confidence in fiber identification is required
(Chatfield, 1979, 1982; Chatfield and Dillon, 1978~.
Researchers do not agree on the best method of preparing
representative and quantitative EM samples. The fibers in air or water
must be deposited evenly and unaltered on the flat surface of an EM grid
and spaced far enough apart to be readily counted and examined, yet not
so far apart that there are too few to count. In most modern methods,
the sample is collected either on mixed cellulose eater Hillipore filters
or on polycarbonate Nuclepore filters. To transfer the deposit directly
onto an EM grid, the fitter must be dissolved, usually by washing gently
with solvent. With uncoated Hillipore filters, as much as 80X of the
fibers is lost.
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93
The most satisfactory direct transfer preparation technique involves
the carbon coating of particles on the surface of a l!tuclepore filter.
This technique is part of the current EPA inte rim procedure ~ Samud ra
et al., 1977) . Nuclepore filters are preferred because, unlike the
Millipore filters, they have a smooth, featureless surface. Because of
this property, vacu~m-coating with carbon produces a replicate that
surrounds and traps the particles, holding them in their original
position as the filter dissolves. The large amount of surface detail on
Millipore filters makes them unsuitable for carbon coating.
Rigorous fiber identification is not always necessary, especially in
occupational or other defined environments. Morphology alone is often
adequate, especially for chrysotile. For environmental samples, which
may contain many fiber-shaped particles of different minerals, selected
area electron diffraction (SAEI)) and energy dispersive x-ray analysis
(EDXA3 may be used to obtain crystallographic and chemical information
for more precise identifications.
RECOMMENDATIONS
Concentrations of asbestiform fibers in urban and rural loca~cions,
and at various distances from known sources, should be routinely
monitored so that fiber levels and population exposures can be determined
with respect to time and location. Fiber characterization is also
needed. If feasible, these data should be used in conjunction with
health studies to determine any effects on the exposed populations.
Characterization of f ihrous dusts should inc lude to the extent
possible the length, diameter, quality, and type of all fibers present
and their concentrations, both as mass and number. Direct transfer
techniques and TEM examinat ion of the preparer ions, or other techniques
that allow examination of particles as they existed in the aerosol,
should be used.
Fiber monitoring techniques for use in nonoccupational environments
should be standardized so that results from various studies are
comparable. Automated instrument techniques are needed to permit
analysis of the large number of samples required to monitor exposure of
different segments of the U. S. population over time.
REFERENCES
Addingly, C. G. 1966. Asbestos dust and its measurement. Ann. Occup.
Hyg. 9: 73-82.
Ayer, H. E., and J. R. Lynch. 1961. Motes and fibers in the air of
asbestos processing plants and hygienic criteria for airborne
asbestos. Pp. 511-522 in C. N. Davies, ed. Inhaled Particles and
ilapours, Pergamon Press, Oxford.
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94
Beattie, J., and J. F. ~ox. 1961. Studies of mineral content and
particle size distribution in the lungs of asbestos textile workers.
Pp. 419~433 in C. N. Dairies, ed. Inhaled Particles and Vapours,
Pergamon Press, Oxford.
British Occupational Hygiene Society Committee on Asbestos. 1983. A
study of the health experience in two U.K. asbestos factories. Ann.
Occup. Hyg. 27 :~-55.
Burdett , G., J . M. LeGuen, A. P. Rood , and S . J . Rooker. 1980. Com-
prehensive methods for rapid quantitative analysis of airborne
particulates by optical microscopy, SEM and TEM with special
reference to asbestos. Stud. En~riron. Sci. 8:323-328.
Chatfield, E. J. 1979. Preparation and analysis of particulate samples by
electron microscopy with special reference to asbestos. Scanning
Electron Microac. I: 563-578.
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Representative terms from entire chapter:
light microscope
