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Critical Reviews in Toxicology, 38:191–214, 2008
Copyright
c
2008 Informa Healthcare USA, Inc.
ISSN: 1040-8444 print / 1547-6898 online
DOI: 10.1080/10408440701845609
An Evaluation of Reported No-Effect Chrysotile Asbestos
Exposures for Lung Cancer and Mesothelioma
Jennifer S. Pierce, Meg A. McKinley, Dennis J. Paustenbach,
and Brent L. Finley
ChemRisk, Inc., San Francisco, California, USA
Numerous investigators have suggested that there is likely to be a cumulative chrysotile exposure
below which there is negligible risk of asbestos-related diseases. However, to date, little research
has been conducted to identify an actual “no-effect” exposure level for chrysotile-related lung
cancer and mesothelioma. The purpose of this analysis is to summarize and present all of the
cumulative exposure-response data reported for predominantly chrysotile-exposed cohorts in
the published literature. Criteria for consideration in this analysis included stratification of rel-
ative risk or mortality ratio estimates by cumulative chrysotile exposure. Over 350 studies were
initially evaluated and subsequently excluded from the analysis due primarily to lack of cumu-
lative exposure information, lack of information on fiber type, and/or evidence of significant
exposures to amphiboles. Fourteen studies meeting the inclusion criteria were found where lung
cancer risk was stratified by cumulative chrysotile exposure; four such studies were found for
mesothelioma. All of the studies involved cohorts exposed to high levels of chrysotile in mining
or manufacturing settings. The preponderance of the cumulative “no-effects” exposure levels
for lung cancer and mesothelioma fall in a range of approximately 25–1000 fibers per cubic
centimeter per year (f/cc-yr) and 15–500 f/cc-yr, respectively, and a majority of the studies did
not report an increased risk at the highest estimated exposure. Sources of uncertainty in these
values include errors in the cumulative exposure estimates, conversion of dust counts to fiber
data, and use of national age-adjusted mortality rates. Numerous potential biases also exist. For
example, smoking was rarely controlled for and amphibole exposure did in fact occur in a ma-
jority of the studies, which would bias many of the reported “no-effect” exposure levels towards
lower values. However, many of the studies likely lack sufficient power (e.g., due to small cohort
size) to assess whether there could have been a significant increase in risk at the reported no-
observed-adverse-effects level (NOAEL); additional statistical analyses are required to address
this source of bias and the attendant influence on these values. The chrysotile NOAELs appear
to be consistent with exposure-response information for certain cohorts with well-established
industrial hygiene and epidemiology data. Specifically, the range of chrysotile NOAELs were
found to be consistently higher than upper-bound cumulative chrysotile exposure estimates
that have been published for pre-1980s automobile mechanics (e.g., 95th percentile of 2.0 f/
cc-yr), an occupation that historically worked with chrysotile-containing friction products yet
has been shown to have no increased risk of asbestos-related diseases. While the debate re-
garding chrysotile as a risk factor for mesothelioma will likely continue for some time, future
research into nonlinear, threshold cancer risk models for chrysotile-related respiratory diseases
appears to be warranted.
Keywords Asbestos, chrysotile, mechanics, threshold
INTRODUCTION
Over the past 30 years, there has been an increasing amount
of research devoted to understanding the relative carcinogenic
potencies of the various asbestos fiber types (i.e., serpentine
chrysotile versus amphibole forms, such as amosite, tremolite,
Address correspondence to Brent L. Finley, ChemRisk, Inc.,
25 Jessie St. Suite 1800; San Francisco, CA 94105, USA. E-mail:
bfinley@chemrisk.com
and crocidolite). Wagner et al. (1965) were the first to note the
apparent differences between crocidolite versus chrysotile po-
tency when they reported that mesothelioma cases were quite
common near crocidolite mines, but were absent in popula-
tions living and working near chrysotile mines. From the mid-
1970s through the early 1990s, numerous epidemiology studies
of asbestos-exposed cohorts described substantially higher dis-
ease rates in cohorts exposed to a mixture of fiber types (or
predominantly amphiboles) versus those observed in cohorts
191
192 J. S. PIERCE ET AL.
exposed to predominantly chrysotile (Enterline and Henderson,
1973; Meurman et al., 1974; McDonald and McDonald, 1977;
Weiss, 1977; Acheson et al., 1981, 1982; Thomas et al.,1982;
McDonald et al., 1983, 1984; Ohlson and Hogstedt, 1985;
Gardner et al., 1986; Newhouse and Sullivan, 1989; Piolatto
et al., 1990). In 1978 the American Conference of Govern-
mental Industrial Hygienists (ACGIH) recommended thresh-
old limit values of 0.2, 0.5, 0.5, 2 and 2 f/cc, for crocidolite,
amosite, tremolite, chrysotile and “other forms” of asbestos,
respectively (ACGIH, 1980). The more stringent recommen-
dations for the amphiboles were “because of their greater po-
tential for disease production” (p. 30). A U.S. Environmental
Protection Agency (EPA) work group recently concluded that
amphiboles are 4 times and 800 times as potent as chrysotile at
inducing lung cancer and mesothelioma, respectively (Berman
and Crump, 2003). Hodgson and Darnton (2000) of the United
Kingdom (UK) Health and Safety Executive estimated that the
risk of mesothelioma is in the ratio of 1:100:500 for chrysotile,
amosite and crocidolite, respectively. In a more recent estimate
of prospective mesothelioma incidence in the United Kingdom
(based on import volumes of different asbestos fiber types), they
assigned chrysotile a value of zero potency (Hodgson et al.,
2005).
It has been suggested that differences in asbestos fiber type
potency are due in part to differences in physicochemical prop-
erties that result in a much higher degree of biopersistence for
amphibole fibers. Chrysotile fibers form large parallel sheets,
while amphibole fibers are arranged in long linearly organized
chains (Bernstein and Hoskins, 2006). The straight-chain struc-
tures are more biologically durable because they are more dif-
ficult to clear from the lung via macrophage engulfment or the
mucociliary escalator. In addition, chrysotile fibers are easily de-
pleted of critical components of their structure (e.g., magnesium
and other cations) at low pH inside macrophages, thereby weak-
ening the fibers, facilitating their destruction, and subsequently
reducing their residence time in the lung (Jaurand et al., 1977;
Roggli and Brody, 1984). Amphibole fibers are far more resis-
tant to this type of leaching, and therefore have a much longer
residence time (Jaurand et al., 1977; Roggli and Brody, 1984;
Hesterberg et al., 1998). As such, the biological half-life of in-
haled amphibole asbestos fiber types is in the range of years
to decades, whereas the half-life of chrysotile is only days to
weeks (de Klerk et al., 1996; Finklestein and Dufresne, 1999;
Bernstein and Hoskins, 2006).
Chrysotile asbestos was historically used in hundreds of con-
sumer products, including joint compound, floor tiles, brakes,
manual clutches, automotive gaskets, mastic coatings, and weld-
ing rods. Although there are dozens of published epidemiolog-
ical studies of asbestos-related diseases (i.e., lung cancer and
mesothelioma) in occupational cohorts exposed to chrysotile
during the manufacture or use of these products, to our knowl-
edge there has been no systematic analysis of the available
exposure-response information to identify a likely range of min-
imum cumulative chrysotile exposures necessary for increased
risk. Browne (1986) provides the only quantitative estimate of a
“threshold” cumulative exposure for asbestos-related diseases.
He examined the relative risk of lung cancer stratified by cu-
mulative exposure to asbestos of mixed fiber types (chrysotile
and amphiboles) in 10 different cohorts and concluded that “the
threshold for increased risk of lung cancer appears to be some-
where in the range of 25–100 f/cc-years” (p. 558).
However, this assessment was not specific to chrysotile be-
cause studies with probable or known significant amphibole ex-
posure were included (Enterline and Henderson, 1973; Seidman
et al., 1979). Also, the Browne (1986) review did not ad-
dress mesothelioma. Dunnigan (1986) also reviewed the avail-
able epidemiological and experimental data and concluded that
“for chrysotile only exposures (without amphiboles), there is
a threshold, below which no adverse health effects can be de-
tected,” but did not offer a quantitative estimate of the threshold
dose (p. 41). Several others have since suggested that a thresh-
old dose for chrysotile-induced disease may indeed exist (Ilgren
and Browne, 1991; Meldrum, 1996; Churg, 1988; Hodgson and
Darnton, 2000) but, like Dunnigan (1986), they have not posited
an actual value or range of values.
Approximation of the cumulative chrysotile exposures as-
sociated with increased lung cancer and mesothelioma disease
would aid in the health risk assessments of chrysotile-exposed
occupations in several ways. First, it would aid in the analysis of
occupations with well-established epidemiological and indus-
trial hygiene assessments. For example, we recently determined
that vehicle mechanics working with chrysotile-containing au-
tomotive friction products in the 1970s experienced median cu-
mulative chrysotile exposures ranging from 0.16 to 0.41 f/cc-yr
(Finley et al., 2007). Since it has been established that vehicle
mechanics are not at an increased risk of developing lung cancer
or mesothelioma, this estimated range of exposures should be
below the chrysotile exposures necessary to cause lung cancer
or mesothelioma. Second, a more informed understanding of
the available chrysotile exposure-response data would improve
the health risk assessments for occupations where chrysotile
exposure information is available, but for which relevant epi-
demiological analyses do not exist and/or are difficult to ob-
tain due to confounding exposures. For example, up until the
1980s, welders often used welding rods that contained low lev-
els (<1% by weight) of chrysotile asbestos in the flux. Some
epidemiological studies report elevated rates of mesothelioma
in welders, yet it is known that welders often experienced di-
rect and indirect exposure to amphibole-containing insulation
(Danielsen et al., 1996; Moulin et al., 1993; Newhouse et al.,
1985; Teta and Ott, 1988). Hence, the potential contribution of
chrysotile-flux exposures to these disease endpoints cannot be
determined directly from the epidemiological data. However, it
should be possible to characterize estimated chrysotile-flux ex-
posures via comparisons to the cumulative chrysotile exposure-
response data and thereby reach a risk assessment conclusion for
welders. In addition to these and other retrospective analyses,
as recently noted by Yarborough (2006), there are emerging
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 193
nanotechnology research techniques involving the use of syn-
thetic chrysotile in microelectronics, and it would therefore be
beneficial to be able to accurately predict the magnitude of any
potential health risks associated with the manufacture and use
of these materials.
In this article, we assemble and summarize the published lung
cancer and mesothelioma information for all chrysotile-exposed
cohorts for which exposure and response data are available. Em-
phasis is placed on those studies where amphibole exposures
are relatively low and stratified exposure-response results are
reported by the authors. A range of cumulative “no-effect” ex-
posure levels (highest estimated cumulative exposure at which
no increased risk was reported) is identified from all studies that
meet the criteria for inclusion (as defined in this analysis). Un-
certainties that are likely to introduce bias are described in detail,
and the upper-bound estimates of cumulative chrysotile asbestos
exposure for U.S. brake mechanics are compared to the putative
no-effect exposure levels to assess consistency with the brake
mechanic epidemiological literature. We also discuss applica-
tions of this analysis to prospective occupational and consumer
settings that might involve chrysotile exposures in the future.
METHODS
Study Selection Criteria
We performed a literature search for asbestos-exposed co-
horts in multiple databases using a variety of search strategies
and keyword combinations. To locate additional studies we sys-
tematically searched the reference lists of all studies identified
by our search, as well as key review papers. We incorporated
into our analysis all of the studies on occupational cohorts that
met the following criteria:
1. Outcomes of interest included lung cancer (variously iden-
tified as “lung cancer,” “respiratory cancer,” “malignant res-
piratory neoplasms” or “malignant neoplasms of the lung”)
and/or mesothelioma.
2. The cohort was predominantly exposed to chrysotile asbestos
(less than 10% of the potential asbestos exposures involved
amphiboles).
3. There were no other known occupational exposures to respi-
ratory carcinogens.
4. Relative risk or relative mortality estimates were provided or
could be calculated and stratified by cumulative chrysotile
exposure.
5. Cumulative chrysotile exposures were stratified into two or
more exposure levels by the authors.
If multiple studies existed on a single cohort, the study with
the most power (i.e., longer follow-up period, larger study pop-
ulation) was selected for the analysis. Wherever possible, we
identify the following study elements for each cohort:
•
Workplace description, type of industry, and location.
•
Cohort demographics (age, duration of employment,
employment initiation date, smoking status, disease
latency).
•
Time period or decade(s) of exposure and follow-up.
•
Diagnostic methods.
•
Source of control population.
•
Follow-up period.
•
Quantity of chrysotile and amphiboles (if applicable)
processed and best estimate of percent amphiboles
used.
•
Air sampling methods used and the method for calcu-
lating individual worker cumulative exposure.
•
Stratified cumulative exposures for lung cancer and
mesothelioma.
Due to the lack of available information regarding smoking
habits and employment history in most studies, we were unable
to control for smoking and previous occupational exposures to
amphiboles (e.g., shipyard and insulation employment). We also
did not attempt to differentially weight the studies, nor did we
reinterpret any of the authors’ findings.
No-observed-adverse-effect levels (NOAELs) were deter-
mined for each study as the highest exposure group at which
there was no statistically significant increased risk for lung
cancer and/or mesothelioma. If a risk metric (e.g., a mortality
ratio or odds ratio) or confidence interval was not provided
by the authors, when possible it and/or the 95% Fishers exact
confidence interval was calculated based on the available
data using OpenEpi software (available through Emory
University School of Public Health; http://www.sph.
emory.edu/∼cdckms/exact-midP-SMR.html). To avoid confu-
sion and for the sake of consistency, if the risk estimates
were reported in the studies as a percentage, we reported the
equivalent proportion in our analysis; this is noted in the text.
If no increased incidence of cancer was reported in a cohort,
the NOAEL was considered to be the highest exposure group
in the study.
Cumulative exposure measurements reported in units other
than fibers per cubic centimer per year (f/cc-yr; equivalent to
f/ml-yr) were converted to f/cc-yr using the conversion factor
provided by the individual study authors. If cumulative expo-
sure was reported in millions of particles per cubic foot per year
(mppcf-yr) and a conversion factor was not provided, a conver-
sion factor was determined based on published factors for plants
with similar operations.
Throughout this article we use the term “cumulative expo-
sure” in lieu of “cumulative dose” because the degree to which
the airborne asbestos levels measured in these studies actually
resulted in an inhaled “dose” is unknown. Also, we use the term
“NOAEL” instead of “threshold” to emphasize that the high-
est minimum cumulative exposures at which no effects were
observed are simply that, exposures without observed effects;
whether or not these exposures truly represent “thresholds” be-
low which effects do not occur cannot necessarily be discerned
due to study limitations (as described in the Discussion).
194 J. S. PIERCE ET AL.
RESULTS
Cohort/Exposure Study Identification and Determination
of No-Effect Exposures
During our review, over 350 studies were initially evalu-
ated and subsequently excluded from the analysis. Reasons for
study exclusion were primarily lack of cumulative exposure in-
formation, lack of information on fiber type, and/or evidence
of significant exposures to amphiboles. The following stud-
ies met the inclusion criteria: Albin et al. (1990), Berry and
Newhouse (1983), Brown et al. (1994), Dement and Brown
(1994), Dement et al. (1994), Hughes et al. (1987), Lacquet et al.
(1980), Liddell and Armstrong (2002), McDonald et al. (1983a,
1984, 1993), Neuberger and Kundi (1990), Peto et al. (1985),
and Piolatto et al. (1990). These studies examined cohorts ex-
posed to chrysotile asbestos during asbestos mining and milling
or the manufacture of asbestos-containing cement, friction, and
textile products.
No-effect cumulative exposure levels for lung cancer and
mesothelioma for the studies just listed are presented in
Tables 1 and 2. When possible, we provided best estimates of
the fraction of amphiboles present, as reported in Berman and
Crump (2003). In addition, if the NOAEL was in the highest
or the lowest exposure group, and the NOAEL was reported as
“>”or“<,” respectively, the mean and median cumulative ex-
posure of the NOAEL group was reported in Table 1 or 2 if this
information was provided by the authors.
Asbestos Cement Products Manufacturing
Belgium
Lacquet et al. (1980) is a follow-up to Van den Voorde (1967),
and presents x-ray results and updated mortality data for work-
ers in a Belgian cement factory. The factory processed about
39,000 tons of asbestos annually, consisting of 90% chrysotile,
8% crocidolite, and 2% amosite, which were used in the man-
ufacture of building materials and pipes (Berman and Crump,
2003; p. A.29).
The cohort was comprised of male workers who worked in
the factory for at least 12 months within the 15-yr period of 1963
through 1977 (the size of the cohort is not presented). Specific
demographic information for each individual, such as employ-
ment duration, job classification, smoking history, average age
at employment initiation, and latency were not provided. All
causes of death were determined by family doctors and/or so-
cial workers who visited the relatives (Belgian authorities do
not release individual information from death certificates). Ex-
pected mortalities by age group were calculated based on the
yearly rates for Belgium for the years 1965 to 1975; rates for
other years were estimated by the authors.
Fiber counts measured using the membrane filter method
were available from 1970 through 1976; dust concentrations for
the previous years were estimated by the authors using a logistic
decay model with an inflection point at 1960. Fiber concentra-
tions were estimated from 1928 onward, and were thought by
the authors to be much higher than the actual levels measured
post-1970. Lacquet et al. (1980) considered their estimates to
be accurate to within one order of magnitude of the actual con-
centrations in the factory. Individual exposures were calculated
based on the duration of time spent at each of the five general
areas of the plant: Area 4 involved handling of raw asbestos
fibers, milling, and mixing of asbestos cement; Area 3 involved
the finishing of cement products by sawing, drilling, filing, etc.;
Area 2, which was situated between the previous two areas, was
where asbestos-cement pipes and sheets were molded, pressed,
dried, and lifted off the mold; Area 1 represented nonmanufac-
turing locations with very low asbestos concentrations, such as
offices; and Area 0 represented work outside the asbestos indus-
try, with negligible dust levels. The asbestos concentrations in
Areas 4, 3, 2, and 1 were estimated to be 100, 24, 16, and 0.4
fibers/cc, respectively. The authors did not present the individual
time-weighted average concentrations or exposure estimates.
Lacquet et al. (1980) segregated the cohort into seven expo-
sure groups, with a total of 29,366 man-years of observation,
and stated that there were no statistically significant increases
in respiratory cancer deaths in any exposure group, including
those in the highest estimated cumulative exposures of 1600–
3200 fiber/cc-yr. To address the possible influence of the
“healthy-worker effect,” an internal case-control study was also
performed, in which 4 control subjects were selected at ran-
dom per case. The authors reaffirmed that dust exposure did
not significantly affect mortality due to respiratory cancer. Stan-
dardized mortality ratios (SMRs)
∗
and confidence intervals for
the different exposure groups were calculated by us for respi-
ratory cancer, based on the number of observed and expected
respiratory cancer deaths provided by the authors (see Table 8,
p. 790). One death due to pleural mesothelioma was reported
in the highest exposure group (1600–3200 fiber/cc-yr); how-
ever, the expected number of mesothelioma deaths based on the
background incidence in Belgium was not provided. For the
purposes of our evaluation, it was assumed that this single case
represented a true increase in mesothelioma risk for that expo-
sure group. The NOAELs for nonmesothelioma respiratory can-
cer and mesothelioma in this study therefore were 1600–3200
f/cc-yr and 800–1599 f/cc-yr, respectively.
New Orleans
A prospective cohort study was conducted among workers in
two cement manufacturing plants in New Orleans that were in
operation since the 1920s (Hughes et al., 1987; Weill et al., 1973,
1979). Chrysotile was the primary fiber type used in both plants.
Plant 1 consisted of one building in which flat shingles and cor-
rugated sheets were produced. Amosite was used in corrugated
∗
The standardized mortality ratio (SMR) is used to compare the mortality
experience of a study population with a standard population, and is calculated
as observed deaths divided by expected deaths. It is an estimate of the relative
risk.
TABLE 1
Studies of Lung Cancer in Chrysotile Cohorts
Fraction Total Number of Risk Estimate
Amphiboles
a
Minimum Cases for Exposures at
Best Estimate (%) Disease Latency (# of Cases Associated the NOAEL NOAEL
Authors Year Industry (Range (%)) Classification (Years) with NOAEL) (95%CI) (f/cc-years)
Lacquet et al. 1980 Cement
Manufacturing:
(Belgium)
∼10 (NA) Respiratory
cancer
Not Specified 21(0) — 1,600–3,200
d
Berry &
Newhouse
1983 Friction
Materials Man-
ufacturing:
(United
Kingdom)
0.5 (0–2) Lung cancer 10 105(5) OR = 0.88
(0.24–2.72)
100–356
d
McDonald et al. 1983 Textiles:
(Pennsylvania)
8(3–15) Malignant
neoplasms
respiratory
20 53(6) SMR = 1. 60
(0.59–3.48)
120–240
e
McDonald et al. 1984 Friction
Materials Man-
ufacturing:
(Connecticut)
0.5 (0–2) Malignant
neoplasms
respiratory
20 73(1) SMR = 0.55
(0.01–3.08)
≥112
d,e, f
Peto et al. 1985 Textiles:
(Rochdale)
5(2–15) Lung cancer 20 93(6) SMR = 1.06
(0.39–2.31)
85.7–114.3
Hughes et al. 1987 Cement Manu-
facturing: Plant
1 (New
Orleans)
1(0–2) Respiratory
malignancies
20 22(5) SMR = 1.23
(0.40–2.85)
≥140
d,e
(mean =
256.2)
Hughes et al. 1987 Cement Manu-
facturing: Plant
2 (New
Orleans)
5(2–15) Respiratory
malignancies
20 42(4) SMR = 1.56
(0.42–3.94)
≥70
d,e
Albin et al. 1990 Cement Manu-
facturing:
(Sweden)
3 (0–6) Malignant
respiratory
disease except
mesothelioma
20 27 (NA) RR = 1.9
(0.5–7.1)
≥40
d
(mean =
67, median =
88.2)
(Continued on next page)
195
TABLE 1
Studies of Lung Cancer in Chrysotile Cohorts (Continued)
Fraction Total Number of Risk Estimate
Amphiboles
a
Minimum Cases for Exposures at
Best Estimate (%) Disease Latency (# of Cases Associated the NOAEL NOAEL
Authors Year Industry (Range (%)) Classification (Years) with NOAEL) (95%CI) (f/cc-years)
Neuberger &
Kundi
1990 Cement Manu-
facturing:
(Austria)
NA (NA) Lung cancer Not Specified
b
49 (24) SMR = 0.96
(0.64–1.43)
>25
d,g
Piolatto et al. 1990 Mining and
Milling: (Italy)
0.3(0.1–0.5) Lung cancer Not Specified
c
22(10) SMR = 1.1
(0.55–2.11)
>400
d
McDonald et al. 1993 Mining and
Milling:
Asbestos Mine
and Mill
(Quebec)
1(0–4) Malignant
neoplasms of
the lung
20 133(22) SMR = 1.55
(0.97–2.35)
≥942
d,e
McDonald et al. 1993 Mining and
Milling:
Thetford
Mines
(Quebec)
1(0–4) Malignant
neoplasms of
the lung
20 155(28) SMR = 1.05
(0.70–1.52)
314–942
e
Liddell &
Armstrong
2002 Mining and
Milling:
(Quebec)
1(0–4) Lung cancer 20 44(8) SMR = 1.12
(0.48–2.21)
≥1,884
d,e,g,h
(mean =
3,832)
Brown et al. 1994 Textiles: (South
Carolina)
0.5 (0–2) Lung cancer 15 124(7) SMR = 0.65
(0.28–1 .43)
1.4–2.7
e
a = Source: Berman and Crump, 2003; b = In a further investigation on lung cancer mortality, the authors excluded all persons with less than 15 years latency. They reported
that the results did not differ substantially from that provided for the whole cohort, however risk estimates stratified by cumulative exposure were not provided (p. 618); c =
Although there was not a requisite minimum latency, Piolatto et al. reported that 3 deaths due to lung cancer occurred in individuals with less than 20 years from their first exposure
to asbestos, 7 with between 20 and 30 years, and 12 with over 30 years since their first asbestos exposure, d = Indicates that this was the highest exposure group in the study; e =
Converted units to f/cc-years; f = Lack of apparent dose-response; marginally significant increase observed in lowest exposure group, however, no statistically significant increase
observed in higher exposure groups; g = Adjusted for smoking; h = Same principal cohort as McDonald et al. 1993.
NA = Not available.
−=Zero cases reported.
196
TABLE 2
Studies of Pleural Mesothelioma in Chrysotile Cohorts
Fraction Total Number
Amphiboles
a
Minimum of Cases (# of Risk Estimate for
Best Estimate (%) Disease Latency Cases Associated Exposures at the NOAEL
Authors Year Industry (Range (%)) Classification (Years) with NOAEL) NOAEL (95%CI) (f/cc-years)
Lacquet
et al.
1980 Cement
Manufacturing:
(Belgium)
10 (NA) Mesothelioma Not Specified 1(0) — 800–1,599
d
McDonald
et al.
1984 Friction Materials
Manufacturing:
(Connecticut)
0.5 (0–2) Mesothelioma 20 0(0) — ≥112
b,c
Albin et al. 1990 Cement
Manufacturing:
(Sweden)
3(0–6) Mesothelioma 20 12(NA) RR = 1.9
(0.2–21.3)
<15
(mean = 3.1,
median = 1.4)
Piolatto
et al.
1990 Mining and Milling:
(Italy)
0.3 (0.1–0.5) Cancer of the pleura 20 2(1) SMR = 10
(0.25–55.7)
>400
b
a = Source: Berman and Crump, 2003; b = Indicates that this was the highest exposure group in the study; c = Converted units to f/cc-years; d = Only one death due to
mesothelioma was reported; however, authors did not indicate the expected number of deaths due to mesothelioma. Thus, we conservatively assumed that this case represented
a statistically significant increase in mesothelioma.
NA = Not available.
−=Zero cases reported.
197
198 J. S. PIERCE ET AL.
siding from the early 1940s through the late 1960s, and crocido-
lite was used occasionally for approximately 10 years beginning
in 1962. Plant 2 consisted of 4 buildings, each manufacturing dif-
ferent products. Shingles were the first product manufactured by
the plant, followed by roofing materials, pipes, and asphalt floor-
ing products. Pipe production, which commenced in 1946, used
crocidolite and chrysotile. All other areas used only chrysotile.
Berman and Crump (2003) estimated, based on the plant history
provided in Hughes et al. (1987), that amphiboles accounted for
roughly 1% of the total asbestos used at plant 1 (range 0–2%)
(see Table 7–16). For plant 2 the best estimate was 5%, with a
range of 2–15%.
The cohort included all men who had been employed for
at least one month before 1970, and for whom a valid Social
Security number was available from company records. The total
number of men in the study population was 6931, of whom 2565
were employed at plant 1 and 4366 at plant 2. Sixty-one percent
of plant 1 workers initiated employment between 1942 and 1949,
and 74% of plant 2 employees started working during the period
1937 through 1949 (Hughes et al., 1987; see Table 3, p. 163). The
mean durations of employment for plants 1 and 2 were both less
than 4 years, with median employments of less than one year.
On average, age upon hiring was higher in plant 1 (31.7 years)
than in plant 2 (26.8 years), and was particularly high in plant 1
during the Second World War (39.0 years). Although smoking
was not controlled for, based on the results of a cross sectional
study of over 95% of the workers employed in these plants in
1968, the authors indicated that there was a comparable smoking
prevalence between the two plants (Weill et al., 1973, 1975). In
addition, they reported that the smoking rates calculated for the
two plants were only slightly less than the estimate for all United
States men in 1969.
Follow-up continued until 1982 or to age 80, whichever was
reached first, and over 96% of the population was traced. Of
the deceased (n = 2143), death certificates were obtained for
2014 (94%). Deaths for which certificates were not acquired
were assigned to categories of causes of death in the same pro-
portion as those with certificates. The mortality experience of
this cohort was compared to Louisiana rates obtained from the
State of Louisiana Department of Health and from Marsh and
Preininger (1980). The authors noted that age-adjusted lung can-
cer rates in Louisiana for the period of 1960–1979 were 29%
higher for Caucasians and 9% higher for African Americans than
rates reported for the country as a whole (Riggan et al., 1983).
Beginning in 1952, air sampling data were collected in both
plants by industry, insurance companies, and government per-
sonnel using the midget impinger. A total of 100 samples were
taken in plant 1 prior to 1970 and 1664 in plant 2. Membrane fil-
ter sampling began in 1969. The estimated exposure concentra-
tions for the years prior to 1952 were based on both air sampling
data and anecdotal information from company management and
long-term employees. Individual exposures were estimated us-
ing the midget impinger data; the authors do not provide details,
but this was presumably done by job classification and duration.
The relatively recent exposures (up to 10–15 years previously)
were not included in calculating the cumulative exposure for
each worker.
Plant 1 employees were classified into the following cumula-
tive exposure groups: <6, 6–24, 25–49, 50–99, and ≥100 mppcf-
yr. The mean cumulative exposure for each of the exposure cate-
gories was 4, 13, 35, 74, and 183 mppcf-yr, respectively. Hughes
et al. (1987) did not observe a statistically significant increase in
deaths due to respiratory cancer in plant 1 employee, 20 years
or more after their initial employment in any exposure category.
For the highest cumulative exposure group (≥100 mppcf-yr),
an SMR of 1.23 was reported (a confidence interval was not
provided). Plant 2 employees with a lapse of 20 years or more
since their initial employment were divided into two groups,
one that included chrysotile-only exposed individuals, and the
other with both chrysotile and crocidolite exposure. Plant 2 em-
ployees with chrysotile-only exposure were divided into the fol-
lowing cumulative exposure groups: <3, 3–5, 6–24, 25–49, and
≥50 mppcf-yr. No increase in death from lung cancer was re-
ported in any of the exposure groups. An SMR of 1.56 (a con-
fidence interval was not provided) was reported for the highest
exposure category (≥50 mppcf-yr); the authors indicated that
this was not significant at the 0.05 level. It is also important to
note that the authors indicate that as a whole, there is an observed
excess in lung cancer in Plant 2 workers. However, based on the
results provided, this increased risk appears to be localized to
employees with exposure to both crocidolite and chrysotile.
Hammad et al. (1979) developed a particle-to-fiber conver-
sion factor based on comparative midget impinger and mem-
brane filter samples collected in various areas of one of the plants.
The authors approximated that 1.4 f/ml was roughly equivalent
to 1 mppcf. We applied this conversion factor to the aforemen-
tioned exposures which yielded cumulative respiratory cancer
NOAELs for plant 1 and plant 2 employees of ≥140 f/cc-yr
(mean cumulative exposure for this group was 256.2 f/cc-yr)
and ≥70 f/cc-yr (mean cumulative dose for this exposure group
was not provided), respectively.
Nine pleural mesothelioma deaths occurred in this cohort.
Seven of these deaths occurred in plant 2 workers, and six of
these deaths occurred in workers who had previously been em-
ployed in the pipe production area where they had known expo-
sure to crocidolite asbestos (Hughes et al., 1987; see Table 12,
p. 169). Cumulative exposure levels for these workers were not
provided, and therefore a mesothelioma NOAEL was not re-
ported in this study.
Sweden
Albin and colleagues (1990, 1996) performed a cohort mor-
tality study among Swedish cement factory workers, as well as a
nested case control study of the workers with mesothelioma. The
asbestos that was handled was mainly chrysotile (>95%), with
smaller amounts of crocidolite or amosite. Crocidolite was used
only in sheet production performed prior to 1966. The amounts
used from 1953 were less than 1%, and purportedly did not
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 199
exceed 3 to 4% of the total amount of asbestos used. Amosite
(maximum <18% total use) was used for a few years during the
1 950s. Extrapolating from the plant history, Berman and Crump
(2003) estimated that the percentage of amphiboles used in this
plant ranged from 0–6%; they reported a best estimate of 3%
(see Table 7–16).
The exposed cohort consisted of all male employees regis-
tered in the company personnel records from 1907 through 1977
who were employed for at least 3 months (n = 2898). Follow
up continued until December 31, 1986 (Albin et al., 1990). The
referent cohort was comprised of 1552 men employed in five
different industries in the region (fertilizer production, slaugh-
ter house, wool and polyester textile, sugar refinery, and metal
industries) that were not known to have processed asbestos, and
who fulfilled the same requirements as the asbestos workers.
Additionally, the referents with suspected previous occupational
exposure to asbestos were excluded from the analysis, resulting
in a referent group comprised of 1233 subjects. Information re-
garding the demographics and smoking status of the exposed
cohort and referent group was not provided.
Death certificates were obtained and recoded according
to the International Classification of Diseases 8 (ICD-8) by
the National Swedish Central Bureau of Statistics. Regional
(1958–1986) and National (1958–1984) cancer registries were
searched, and all available histopathological information was
reviewed for cases of respiratory cancer. Mesothelioma cases
were confirmed using light microscopy and immunohistochem-
ical staining. A minimum latency of 20 years since start of em-
ployment was applied to both cohorts.
Dust measurements existed for the time period from 1956
through 1977; prior to 1969 impinger or gravimetric measure-
ments were available, and after 1969 the membrane filter method
was used. Albin et al. (1990) estimated average dust exposures
for different jobs and periods using data on dust concentrations,
production, and dust control measures. The estimates for the
period 1947 to 1951 were used by the authors for the entire
period before 1942 based on the assumption that the production
process was mainly the same. The authors indicated that the
actual exposure levels before 1942 may have been greatly
underestimated for some tasks, but explained that workers en-
gaged in these operations only accounted for 5–10% of the total
cohort.
Individual exposures were calculated, presumably based
on individual job classifications and work histories (details
were not provided), for 1503 (78%) of the 1929 Swedish
workers. Albin et al. (1990) developed three cumulative ex-
posure groups, <15 f/cc-yr, 15–39 f/cc-yr, and ≥40 f/cc-yr,
with a total of 17028 man-years of observation, and noted
no statistically significant increase in death due to respira-
tory cancer (excluding mesothelioma) in any group, even those
in the highest cumulative exposure group. The authors indi-
cated that the relative risk estimate was adjusted for possi-
ble confounding by age and calendar year. A statistically sig-
nificant increase in deaths ascribed to pleural mesothelioma
was observed in the two highest exposure groups, 15–39 and
≥40 f/cc-yr, and the authors reported a relative risk of 21.2
(95% CI 2.5–178) and 22.8 (95% CI 2.4–212), for the groups,
respectively. For the purposes of our analysis, the no-effect level
identified for lung cancer was ≥40 f/cc-yr (mean cumulative
exposure for this group was 67 f/cc-yr, median cumulative ex-
posure was 88.2 f/cc-yr), and for mesothelioma was <15 f/cc-yr
(mean cumulative exposure for this group was 3.1 f/cc-yr, me-
dian cumulative exposure was1.4 f/cc-yr).
V
¨
ocklabruck
Neuberger and Kundi (1990) conducted a cohort study among
workers of the oldest cement factory in the world, located in
V¨ocklabruck (upper Austria). From 1895 forward, chrysotile
was the predominant fiber type used in the facility. From 1920
to 1977 crocidolite was also used in the pipe factory. Up to 33%
of the asbestos used in pipe production was crocidolite, which
amounted to roughly 4% of the total amount of asbestos used at
the facility (Neuberger, 2006). Amosite (up to 3%) was also used
in certain products from 1970 to 1986; however, according to
the authors, this usage did not contribute to the overall exposure
of this cohort.
The cohort included all persons employed for at least 3 years
from 1950 to 1981. It was comprised of 2816 people, 82% of
whom were employed before 1969, when the dust conditions had
yet to significantly improve. Smoking information was obtained
via interview for cohort members who had left the plant after
1950 and were still alive in 1982. Lung cancer deaths were ini-
tially determined by review of death certificates; a further analy-
sis of the best available information was performed using results
gathered from hospital, pathological institute and social insur-
ance records. Information on mean age at initial employment,
start date, duration of employment, and mean disease latency
was not provided.
Individual cumulative exposures were estimated from per-
sonal records on duration of exposure at different workplaces,
estimations of dust concentrations until 1965, and dust mea-
surements (mainly by the conimeter method until 1975, and by
personal air samples and membrane filter methods thereafter).
The cohort was subsequently divided into two cumulative expo-
sure groups, ≤25 f/cc-yr and >25 f/cc-yr.
The authors observed an overall increased risk for lung can-
cer (SMR = 1.7), when compared to the age- and sex-specific
mortality rate for lung cancer in upper Austria. However, af-
ter controlling for smoking, the authors reported no increased
risk in mortality from lung cancer in either of the two cumula-
tive exposure groups. For the cumulative asbestos exposures of
≤25 f/cc-yr, an SMR of 1.26 was calculated (95% CI 0.83–
1.95); and for >25 f/cc-yr cumulative exposure an SMR of 0.96
(95% CI 0.64–1.43) was calculated. The cumulative lung cancer
NOAEL for this cohort was therefore >25 f/cc-yr.
Five mesothelioma cases were reported; however, the rela-
tive risk of mesothelioma stratified by cumulative exposure was
not reported, and therefore a mesothelioma NOAEL could not
200 J. S. PIERCE ET AL.
be determined from the information provided. It is worth not-
ing that in a subsequent nested case-control study (Neuberger
and Kundi, 1990), the authors found that that the mesothelioma
cases had significantly higher crocidolite exposure than the
controls.
Friction Products Manufacturing
United Kingdom
A retrospective cohort mortality study was conducted on
workers in a factory producing friction materials in the United
Kingdom (Berry and Newhouse, 1983; Newhouse et al., 1982;
Newhouse and Sullivan, 1989). The plant, founded in 1898, man-
ufactured a variety of friction materials, such as brake blocks,
and brake and clutch linings. Chrysotile was the only fiber type
used in the facility, with the exception of brief periods from
1929 to 1933 and from 1939 to 1944 during which crocidolite
was used to manufacture railway blocks. During both of these
time periods, the blocks were made in a well-defined area of
one of the workshops, and only a minority of the workforce was
exposed. Small amounts of crocidolite were also used sporadi-
cally in an experimental workshop. Berman and Crump (2003)
estimated that 0.5% (range 0–2) of the total asbestos used in this
plant was amphiboles (see Table 7–16).
The initial study group consisted of individuals whose em-
ployment began in 1941 through 1979 who were identified by
factory personnel files, resulting in 13460 subjects, of whom
about two-thirds were men. Over two-thirds of the population
began employment by 1960, and less than 6% of the cohort
began work prior to 1941 (Berry and Newhouse, 1983). The
follow up period was later extended to 1986 (Newhouse and
Sullivan, 1989). The duration of employment ranged from less
than 1 year to over 30 years. Approximately one-third of the men
and women left before completing one year of service, but 27%
of the men and 14% of the women remained at the factory for
10 years or more. Overall cohort mortality information was ob-
tained from death certificates from the National Health Service
Central Registrar and the Department of Health and Social Se-
curity, and was restricted to the period following 10 years after
first employment in the factory.
Beginning in 1967, regular measurements of airborne dust
levels were taken throughout the factory using the membrane
filter method; personal sampling began in 1968. Airborne fiber
concentrations in the earlier years were approximated by the
authors by simulating earlier working conditions, using detailed
knowledge of when processes were changed and exhaust ventila-
tion introduced. Based on knowledge of the historical industrial
hygiene practices and for purposes of quantifying asbestos con-
centrations, the authors divided the factory into four exposure
periods: (1) pre-1931: before the Asbestos Regulations and when
all operations were carried out in an open-plan area; (2) 1932–
1950: when exhaust ventilation was implemented in most ma-
chining operations and there was increased separation between
the stages of production; (3) 1951–1969: gradual improvement
in air quality and application of exhaust ventilation to machines
not included in the Asbestos Regulations; and (4) 1970–1979:
following the introduction of the 2 f/ml threshold limit value
(TLV) (Berry and Newhouse, 1983). In general, fiber concentra-
tions in period 1 exceeded 20 f/ml. In period 2, most operations
had exposures of under 5 f/ml with the exceptions of grinding
(5–10 f/ml) and fiber preparation (10–20 f/ml). In period 3 all
operations were below 5 f/ml, and in period 4 all exposures were
generally in compliance with the TLV. The simulation studies
employed the basic materials and original equipment operated
in the appropriate work setting for the given time periods. Per-
sonal samples were collected in the workers’ breathing zones for
periods of 4 to 5 hours in order to calculate 8-h TWAs, and static
area samplers were mounted nearby at head height to provide
information on general atmospheric concentrations of asbestos
fibers.
A case-control study nested within this cohort evaluated the
association between asbestos exposure and lung cancer mortal-
ity (Berry and Newhouse, 1983). This study was restricted to
men who entered the workforce between 1941 and 1960 and
who had survived for at least 10 years after starting work at this
factory. Although follow-up on this population continued until
1986 (Newhouse and Sullivan, 1989), risk estimates stratified
by cumulative exposure were only available for a follow-up to
1979 (Berry and Newhouse, 1983). The mean year of initiation
of employment for the lung cancer cases was mid-1949, and the
mean year of death due to lung cancer among the cases was
the end of 1970. Three controls were selected for each case,
matched on (1) the year they started working in factory, (2) year
of birth, and (3) survival up to the time of death of the case.
The study population was divided into 4 exposure categories:
0–9, 10–49, 50–99, and 100–356 f/cc-yr. The authors observed
no increased risk of lung cancer in any of the cumulative expo-
sure groups with the exposure level-specific odds ratios of 1.0,
0.79, 0.86, and 0.88. Individual confidence intervals for the risk
estimates were not presented by the authors. The NOAEL for
lung cancer observed in this cohort was therefore taken to be
100–356 f/cc-yr.
Ten deaths due to mesothelioma were observed. The authors
did not estimate the cumulative exposures for these cases, nor
did they calculate a risk estimate for mesothelioma, and there-
fore a mesothelioma NOAEL could not be identified for this
study group. It is worth noting that in a subsequent internal
case-control study (Berry and Newhouse, 1983), the authors re-
ported that 80% of the mesothelioma deaths occurred in people
who worked on the crocidolite contract, compared with only 8%
of the controls.
Connecticut
McDonald and colleagues (1984) studied a Connecticut fric-
tion products and packing manufacturing facility as part of their
investigation into the effects of fiber type on asbestos-related
disease. Until 1957, chrysotile, mainly from Canada, was the
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 201
only mineral type used in the plant; some anthophyllite was
subsequently added in making paper discs and bands. In addi-
tion, between 1964 and 1972 approximately 400 lb crocidolite
was handled experimentally in the company laboratory. Based
on the data provided in McDonald et al. (1984), it was estimated
that only 0.5% (range 0–2) of total asbestos used in this factory
was amphiboles (see Table 7–16).
McDonald et al. (1984) analyzed the mortality of men who
had worked for one calendar month or more before January 1,
1959 and who had a Social Security number and name match-
ing the data in the U.S. Social Security Administration (SSA)
records. Of the 3513 men who were traced until the end of
the study period (December 31, 1977), 1267 (36%) had died,
and death certificates were obtained for 1228 (96.9%). Cohort-
specific information such as mean employment duration and age
was not provided.
Information on exposure was available from surveys con-
ducted by Metropolitan Life Insurance Company in 1930, 1935,
1936, and 1939. There was little additional information on ex-
posure conditions until the 1970s. Estimates of exposure by
process and period were made by an industrial hygienist who
reviewed information related to process and jobs in the plant,
as well as records on environmental conditions and dust con-
trol measures. Before 1970, measurements were made using the
impinger method; in later years membrane filters were used. In-
dividual work history records were obtained; these indicated the
department in which the employee worked, but seldom speci-
fied a job description or the processes involved. Due to vary-
ing dust levels generated by tasks within a single department,
all processes were taken into account when estimating airborne
asbestos concentrations. The authors indicated that this strategy
could lead to overestimation of exposure for many of the employ-
ees in these departments, and underestimation for a few. A con-
version factor was not provided by the authors to convert mppcf-
yr to f/cc-yr. For cement products manufacturing a factor of 1.4
(f/cc:mppcf) has been recommended (Hammad et al., 1979), and
the U.S. EPA used a factor of 1.5 for asbestos products manufac-
turing (Nicholson, 1986); a conversion factor for friction prod-
ucts manufacturing was not found. For the purposes of our anal-
ysis, and for the sake of conservatism, we have used a factor of
1.4 (f/cc:mppcf) to convert dust measurements into fiber levels.
The authors classified male deaths 20 years after first employ-
ment into 5 cumulative exposure groups, the lowest being <10
mppcf-yr, and the highest being ≥80 mppcf-yr. The authors ob-
served a significant increase in respiratory cancer in the lowest
exposure group (SMR = 1.67) in comparison to age-, sex-, race-,
and year-specific death rates in Connecticut (CI 1.26–2.19;
∗
not reported by authors). However, no statistically significant
increase was observed in the four higher exposure categories.
Likewise, there was an inverse relationship between duration of
service and the calculated SMRs for respiratory cancer for ex-
∗
The SMR in McDonald 1984 was reported as a percentage (167.4). For
consistency with the other studies, we have converted it to a proportion.
posures <10 mppcf-yr. The authors suggested that the lack of an
apparent dose-response relationship could be explained by the
selective employment of men in relatively poor health or with
unhealthy habits, such as heavy smoking, in low-exposure jobs
where they often remained for a fairly short time. They also con-
sidered the likelihood that the short-term employees had worked
in other hazardous industries prior to or after their employment
at the friction products plant. Following a further review of the
occupational histories of low-exposure employees, the authors
indicated that the increase in respiratory cancer was most likely
the result of some form of selection bias. Due to the lack of
increased risk in the four higher exposure groups, we have as-
sumed that the highest exposure group represents the NOAEL
for this cohort. No deaths due to mesothelioma were reported in
any of the exposure groups in this study. Using the previously
stated conversion factor of 1.4 (f/cc:mppcf), the NOAEL for this
group was taken to be ≥112 f/cc-yr for respiratory cancer and
mesothelioma.
Asbestos Textile Manufacturing
Pennsylvania
A cohort mortality and subsequent case-referent study were
conducted among workers at a Pennsylvania plant producing a
variety of textiles and friction products (McDonald et al., 1983a).
Chrysotile obtained primarily from Canada and Rhodesia was
the predominant type of asbestos used at this facility, with be-
tween 3000 and 6000 tons processed annually. From 1924 on,
both crocidolite and amosite were incorporated into insulation
blankets for turbines, as well as equipment for chemical facto-
ries and paper mills. Based on the data provided in McDonald
et al. (1983), it was estimated that the percentage of amphiboles
used in this facility was 8% (see Table 7–16).
The cohort consisted of men (n = 4137) and women (n =
998) employed in the factory before January 1, 1959, for at
least 1 month with a verified Social Security number. Survival
status was determined through local inquiries and from infor-
mation provided by the SSA as of December 31, 1977. Tracing
was completed for 97% and 94% of the men and women, re-
spectively, and of those traced, 35% of the men and 18% of
the women had died. Death certificates were obtained for 97%
(n = 1354) of the men who had died and 97% (n = 165) of
the women. The authors chose to exclude females from further
analysis, and noted that relevant information would be reported
separately. The final study group consisted of 1392 male deaths.
The process for determining the cause of death (n = 38) in those
without death certificates was not disclosed. The average age at
the start of employment was 28.92 and the average duration of
service was 9.18 years. Roughly 31% of the study population
was employed for less than 1 year, and slightly more than 25% of
the population remained at the factory for 20 years or more. The
authors indicated that of the men born between 1910 and 1919
included in this cohort, 75% smoked or had smoked cigarettes
in their lifetime.
202 J. S. PIERCE ET AL.
Air samples were taken in the factory by the Metropolitan
Life Insurance Company from 1930 to 1939, by the U.S. Public
Health Service in 1967 and 1970, and collected routinely by the
company from 1956 onward. Until 1967, measurements were
made by the midget impinger method. An industrial hygienist
(A. J. Woolley) estimated dust levels for each department over
time. The process used for estimating individual cumulative ex-
posure was not discussed.
The authors classified male deaths 20 years after first em-
ployment into 5 cumulative exposure groups, the lowest being
<10 mppcf-yr, and the highest being ≥80 mppcf-yr. The authors
observed a significant increase in respiratory cancer in only the
highest exposure group (SMR = 4.16) in comparison to death
rates in Pennsylvania prevalent at that time.
∗
The authors did
not provide a factor to convert particle counts to fiber counts;
however, the ratios recommended for textile manufacturing have
ranged from 1 mppcf = 3 f/cc to 1 mppcf = 6 f/cc (Ayer et al.,
1965; Dement et al., 1982). In this analysis the conversion factor
derived in the South Carolina textiles studies (1 mppcf-yr = 3
f/cc-yr) was applied to the results of McDonald et al. (1983).
This factor was selected because (1) it is in the middle of the
range of recommended factors in the published literature, and
(2) there were some similarities in operations at the two plants.
The respiratory cancer NOAEL for this cohort was therefore
between 40 and 80 mppcf-yr (120–240 f/cc-yr).
Ten deaths due to pleural mesothelioma were identified from
death certificates. Although specific exposure-response infor-
mation was not provided for these cases (and hence, a mesothe-
lioma NOAEL could not be identified), the authors indicated that
they observed “the special risk of mesothelioma associated with
exposure to even quite small proportions of amphibole,” in this
case predominantly amosite (McDonald et al., 1983a, p. 373).
Rochdale
Employees of a Rochdale asbestos textile factory were traced
until June 30, 1983 (Peto et al., 1985). Chrysotile had always
been the predominant fiber type used in the factory, although
from 1932 to 1968 roughly 10322 tons of crocidolite was pur-
chased, which accounted for approximately 2.6% of the total
amount of asbestos purchased over that time period, and for
roughly 5% of the amount used in the manufacture of textiles.
Berman and Crump (2003) estimated that 5% (range 2–15%) of
the asbestos processed was amphibole (see Table 7–16).
The cohort consisted of the following two subcohorts: (1) men
first employed in 1933 or later, who had completed 5 years of
total employment by the end of 1974, and who had ever worked
in scheduled areas or in maintenance, and (2) a 1 in 10 sample
of all male employees first employed between January 1, 1933,
and December 31, 1974, irrespective of where or how long they
had worked. Workers with Asian surnames (due to difficulty
∗
The SMR in McDonald 1983 was reported as a percentage (416.1). For
consistency with the other studies, we have converted it to a proportion.
in tracing) and those with known previous occupational expo-
sures were excluded, resulting in a principal cohort of 3211
men. Cohort demographics and ranges of occupational tenure
were not specified. The company was aware of most suspected
mesothelioma cases; however, a few additional cases were iden-
tified from the national mesothelioma register and from a review
of death certificates. All obtainable diagnostic information was
also reviewed by Sir Richard Doll, and each case was labeled
as “established” on the basis of postmortem evidence with or
without histological confirmation, “presumptive” on the basis
of death certificate information alone, “uncertain” due to con-
flicting medical evidence, or “incorrect.” Lung cancer deaths
were obtained from company records. The mortality experience
of the principal cohort was compared to the national mortal-
ity rates for selected causes in the United Kingdom, as well
as those observed in Rochdale County Borough from 1969 to
1973.
Area dust measurements in particles per milliliter were taken
in 23 locations with a thermal precipitator between 1951 and
1961 and later with static membrane filters. These exposure es-
timates were reevaluated and adjusted by industrial hygienists to
account for advances in technology and knowledge regarding the
conversion from particles/ml to fibers/cc; the authors suggested
that 35 p/ml was equivalent to 1 f/cc. Dust levels prior to 1951
were assumed to be equivalent to those measured from 1951
to 1955 for departments in which no major changes had been
made. For areas that underwent significant industrial hygiene
improvements, higher values were assigned for the pre-1951
period. Cumulative exposure estimates were calculated for each
of the subjects, allowing for a 5-year lag time between exposure
to asbestos and any observed increase in mortality. Jobs were as-
signed average dust measurements for each 5-year period from
1951 onward. Although details are not provided, it appears that
individual exposures were calculated based on the duration of
time spent performing each job.
The authors classified men with 20 or more years since their
first employment into 6 cumulative exposure groups that ranged
from <1000 p/ml-yr (<28.6 f/cc-yr) to ≥5000 p/ml-yr (≥142.9
f/cc-yr). The cohort was further segregated by year of first ex-
posure (1933 and 1951). No increase in lung cancer risk was re-
ported at exposures up to 3000–3999 p/ml-yr in either subcohort
(risks were elevated at higher exposures for both subcohorts).
Accordingly, for the purposes of this analysis, the NOAEL for
lung cancer risk is 3000–3999 p/ml-yr (85.7–114.3 f/cc-yr).
Fourteen men in the principal (post-1933) cohort died of
mesothelioma. The authors did not develop exposure-related
estimates of mesothelioma risk, and therefore a mesothelioma
NOAEL could not be identified from this study.
South Carolina
A retrospective cohort mortality study was conducted among
workers at a South Carolina textiles plant (Brown, et al., 1994;
Dement and Brown, 1994, 1998; Dement et al., 1982, 1983a,
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 203
1983b, 1994; McDonald et al., 1983b). According to company
personnel, 6 to 8 million pounds of chrysotile was processed
annually. Small amounts of crocidolite yarn (less than 2000
pounds) were woven into tape or made into braided packing
beginning in the 1950s until approximately 1975. The authors
indicated that crocidolite processing was done using wet meth-
ods, resulting in very low exposures (Dement et al., 1983a).
Berman and Crump (2003) provided a best estimate percentage
of amphiboles as 0.5% (range 0–2), based on data reported in
Sebastien et al. (1989) (see Table 7–16).
The cohort consisted of workers employed for at least 1
month between January 1, 1940, and December 31, 1965. In
several analyses the cohort was limited to white male employees
(Dement et al., 1982, 1983a, 1983b; Dement and Brown, 1998);
however, parallel studies expanded the population to include
white male (n = 1247) and female (n = 1229), as well as black
male workers (n = 549) (Brown, et al., 1994; Dement et al.,
1994). A nested case-control analysis was also undertaken on
the expanded cohort to eliminate possible confounding effects
due to mineral oil exposure in the authors’ assessment of lung
cancer risk (Dement et al., 1994).
Participants were initially followed until December 31, 1975,
and subsequent tracing extended through December 31, 1990.
Mortality information was based on SSA files for deaths oc-
curring from 1976 to 1978, and records kept by the National
Death Index (NDI) from 1979 to 1990. If a worker was known
to be alive in 1975, he/she was assumed to be alive as of 1990
if his/her information could not be located in either the SSA or
NDI files. The average number of years of observation was 35
for white females and black males, and 43 for white males. By
December 31, 1990, 41.7% of the expanded cohort was known
to be deceased; this was true for 48.7%, 29.5%, and 53.0% of
white male, white female, and black male participants, respec-
tively. For the overall cohort mortality study, age-, race-, sex-,
and calendar-time specific death rates for the U.S. population
were used to calculate expected deaths and SMRs.
The demographic variables for the entire cohort were not
available; however, this information was provided for partic-
ipants in the nested case-control analysis. Since age at death
was the incidence density matching variable, cases and controls
were nearly identical for this parameter. In addition, the authors
reported that the mean year of birth (range cases: 1913–1917;
controls: 1909–1911), mean employment initiation dates (range
cases: 1941–1944; controls: 1941–1942), and mean time since
first employed (range cases: 34.1–38.7; controls: 3 1.1–35.1)
were similar for cases and controls (Dement et al., 1994, see
Table 7). The mean and median exposure levels experienced
by the cases and controls varied according to race and sex, and
were thought to reflect the difference in job assignment pat-
terns. Among lung cancer cases, the reported mean exposure
level among black males was the highest (12.0 f/cc), followed
by white males (5.5 f/cc) and white females (4.9 f/cc). A sim-
ilar trend was observed for the controls. Mean cumulative ex-
posures for black males, white males, and white females were
38400, 24500, and 13200 f/cc-day, respectively. Cases also ex-
perienced higher mean cumulative asbestos exposures than con-
trols; mean cumulative exposures for black male, white male,
and white female cases were 16400, 14600, and 11900 f/cc-day,
respectively.
Linear statistical models were used for reconstructing his-
toric exposure levels, taking into account textile processes, dust
control measures, and job assignments, based on data from 5952
environmental samples that were collected from 1930 to 1975.
Prior to 1965, all samples were taken using the impinger method,
from 1965 to 1971 both impinger and membrane filter samples
were collected, and from 1971 on, only the membrane filter
method was used. Based on 120 side-by-side particle and fiber
counts, a f/cc to mppcf ratio of 2.9 (95% CI 2.4–3.5) for all jobs
except fiber preparation was derived (Dement, 1980). For fiber
preparation, a conversion factor of 7.8 was calculated (95% CI
4.7–9.1). Unit conversions were previously made using a fac-
tor of 3 for all operations except fiber preparation, for which a
factor of 8 was employed
∗
(Dement et al., 1983a). Cumulative
exposure estimates were made for each worker based on these
estimated exposure levels in conjunction with detailed work his-
tories. Notably, the cumulative exposures reported in this cohort
were on the order of 10 to 10000 times lower than in any of the
previously described studies.
The authors classified members of the cohort into expo-
sure groups ranging from <500 to >100000 f/cc-day. With re-
gards to the total cohort, lung cancer mortality, incorporating a
15 year latency period but not controlling for smoking, was sig-
nificantly increased in the 1000–2500 f/cc-day (2.7–6.8 f/cc-yr)
exposure group (SMR = 1.95, p < .01) and all higher exposure
groups (Dement and Brown, 1994; Dement et al., 1994). There
was no increase observed at exposures of 500–1000 f/cc-day
(1.4–2.7 f/cc-yr) and lower. When examining the relationship
between cumulative exposure and lung cancer mortality by race
and sex, it is apparent that the white male and female popula-
tions were mostly responsible for the overall increased cohort
risk estimates. White males showed a statistically significant
increase in lung cancer at 1000–2500 f/cc-day (2.7–6.8 f/cc-
yr)
†
and white females had increased deaths due to lung cancer
in the lowest exposure group (<1000 f/cc-day; <2.7 f/cc-yr).
‡
This increase was not observed for white females in the second
∗
It is important to note that in analyzing the same exposure data McDonald
et al. (1983) reported a particle to fiber ratio that ranged from 1.3 to 10, with an
average of roughly 6 f/cc per mppcf.
†
A statistically significant increase in lung cancer is observed for white males
in the 1000–2500 f/cc-day exposure group in Dement et al. (1994), Brown et al.
(1994) and Dement and Brown (1998). However, the SMRs reported for lung
cancer in this exposure group are inconsistent and are reported as 2.59 (p < 0.01)
in Dement et al. (1994) and 2.42 ( p < .01) in Brown et al. (1994) and Dement
and Brown (1998), although the cohort composition and follow up periods are
identical.
‡
This increase for white females in the lowest exposure group was not
reported in Brown et al. 1994, although the cohort and the duration of follow up
appear to be identical.
204 J. S. PIERCE ET AL.
lowest exposure group (1000–2500 f/cc-day, 2.7–6.8 f/cc-yr),
but was present in all of the exposure groups at and above 2500–
10000 f/cc-day (6.8–27.4 f/cc-yr). The authors indicated that
the inconsistent exposure-response relationship among the fe-
males may be the result of the unequal distribution of those lost
to follow up (Dement et al., 1994, p. 440). Further, among the
280 females lost to follow-up, 36% worked less than 3 months,
18% worked 3 to 6 months, and 17% worked 6 months to 1
year. The authors also reported that if it was assumed that all
females lost to follow-up were alive at the end of the study,
the dose-response would be altered for the white females, low-
ering the risks in the lowest exposure group, and resulting in
a statistically significant increased risk due to lung cancer in
only those groups with exposures above 2500 f/cc-day. Lastly,
black males showed a statistically significant increase in lung
cancer in only the highest cumulative exposure group (>40000
f/cc-day, >109.5 f/cc-yr). For the purposes of this analysis,
the NOAEL for lung cancer for the expanded cohort is 500–
1000 f/cc-day (1.4–2.7 f/cc-yr)(Brown et al., 1994).
Two deaths were attributed to mesothelioma, both of which
had a latency of >30 years. Information on cumulative exposure
was not provided; thus a NOAEL for mesothelioma could not
be identified for this cohort.
Asbestos Mining and Milling
Balangero
A cohort mortality study was conducted on miners in
Balangero (northern Italy) (Piolatto et al., 1990; Rubino et al.,
1979). Examination of the chrysotile from the mine did not de-
tect measurable concentrations of amphiboles. However, a fi-
brous silicate (balangeroite) accounted for 0.2–0.5% of the total
mass of the samples. A series of recent publications has indi-
cated that based on its chemical composition, form, and durabil-
ity, balangeroite is most similar to crocidolite (Gazzano et al.,
2005, Groppo et al., 2005, Turci et al., 2005). A best estimate of
the fraction of asbestos that consisted of amphiboles was 0.3%
(range 0.1–0.5) (Berman and Crump, 2003; see Table 7–16).
The cohort consisted of males who had worked for at least
1 year at the factory between 1946 and 1975, and was later
expanded to include employment through 1987 (n = 1058).
Follow-up began on January 1, 1946, and ended on December 31,
1975, in the initial study, and was subsequently extended through
December 31, 1987. Cohort-specific demographics were not
provided. Vital statuses following termination of employment
were ascertained through population registers, and death certifi-
cates were obtained from municipal registration offices.
Cumulative exposures were estimated from environmental
measurements carried out from 1969 onward, and from simu-
lated working conditions for earlier periods. The factory archives
were examined for information on daily production, the equip-
ment used, the nature of the job, and the historical numbers
of hours worked per day. Additionally, four workers with con-
tinuous employment since 1935 helped to reconstruct the ap-
propriate conditions, after which fiber counts were carried out
by membrane filter collection and phase-contrast microscopy
(Rubino et al., 1979).
Mortality from lung cancer and mesothelioma was reported
for the following cumulative exposure groups: <100, 100–400,
and >400 f/cc-yr. Lung cancer mortality was compared to age-
and calendar-year-specific rates for Italian men. Statistically
nonsignificant SMRs of 0.8, 1.3, and 1.1 were reported for lung
cancer mortality for the three groups, respectively; confidence
intervals were not provided.
No mesothelioma deaths were observed in the lowest expo-
sure category, and one was noted in each of the higher categories.
The expected number of deaths due to mesothelioma in the 100–
400 and >400 f/cc-yr exposure groups was 0.1, yielding non-
significant SMRs of 10.0 for both groups (95% CI 0.25–55.7).
Both mesothelioma deaths occurred in individuals for whom at
least 20 years had elapsed since their first asbestos exposure.
For the purposes of this analysis the NOAEL was assumed to be
>400 f/cc-yr for lung cancer and mesothelioma.
Quebec
Multiple analyses have been conducted on a cohort of
Quebec chrysotile miners and millers (Liddell and Armstrong,
2002; McDonald et al., 1993; Liddell et al., 1977, 1997, 1998;
McDonald et al., 1971, 1973, 1997, 1980). Males born between
1891 and 1920 who were employed in the Quebec chrysotile-
producing industry for at least 1 month comprised the study
population. Follow-up began for each individual after 20 years
from first employment; 9780 men were traced to 1992 (Liddell
et al., 1997). Death certificates were obtained for 98% of the co-
hort, and according to the authors, “adequate information was
collected on most of the rest” (p. 16). For mesothelioma deaths,
a “best diagnosis” was made after all available clinical, biopsy,
and necropsy records were analyzed.
Members of the study population were described according
to the location at which they were first employed; additional co-
hort specific information was not provided (e.g., mean start date
and mean duration of employment). Although nine locations
were identified, companies 5–9 were excluded from the anal-
ysis, leaving 9244 men in the cohort. Company 1 (n = 4195)
was the mine and mill in the town of Asbestos. Company 2
(n = 758) was a factory in the town of Asbestos that in addi-
tion to processing chrysotile had also processed some crocidolite
and amosite. The amount of amphiboles used at this facility was
not included in any of the studies on this cohort. However, it
appears that crocidolite was only used for a short duration in
the 1940s (McDonald et al., 1973). The authors mention that
some employees moved between the Asbestos mine and mill
(company 1) and the Asbestos factory (company 2). Company 3
(n = 4032) was a large mining and milling complex (13 mines)
near Thetford Mines, and company 4 (n = 259) consisted of a
number of smaller mines and mills in the vicinity of Thetford.
Based upon an extrapolation from the air data in Sebastien et al.
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 205
(1986), Berman and Crump (2003) reported a “best estimate”
fraction of amphiboles of 1% (range 0–4) for companies 1, 3, and
4; an estimate was not provided for company 2 (see Table 7–16).
Estimates of dust concentrations at companies 1, 2, 3, and
4 have been made by year for each of the more than 5000 job
classifications up to November 1966 by Gibbs and Lachance
(1972) (McDonald et al., 1993). These estimates were based on
roughly 4500 midget impinger dust counts from annual surveys
conducted from 1948 to 1966. Estimates of the past and present
dust conditions were made after interviews with employees of
long service in collaboration with superintendents or others with
special knowledge of past conditions. These estimates were later
adjusted by Liddell et al. (1997, 1998) to account for new in-
formation on hours worked per week. The authors assumed that
the dust level for 1967 was equal to that of 1966, and for each
subsequent year calculated the annual dust concentration as pro-
portion of that level in accordance with the average trend of fiber
concentration for each worker’s specific mine or mill. An aver-
age conversion factor of 3.14 (f/cc:mppcf) was calculated from
side-by-side midget impinger and optical microscopy measure-
ments (McDonald and McDonald, 1980). A subject’s exposure
for a particular year was calculated as the product of (the fraction
of the year worked in a specific job) multiplied by (the dust level
for the year for that job) times (an adjustment for the length of
the working week).
Exposure accumulated to the age of 55 was determined for
the entire cohort (companies 1, 2, 3, and 4), and each subject
was subsequently grouped into one of the 10 following cate-
gories: <3, 3–<10, 10–<30, 30–<60, 60–<100, 100–<200,
200–<300, 300–<400, 400–<1000, and ≥1000 mppcf-yr. A
statistically significant increase in deaths (after age 55) due
to cancer of the trachea, bronchus, and lung was observed in
the second to highest exposure group, compared to age-specific
mortality rates for Quebec males (SMR = 1.84, 95%CI 1.48–
2.27). The authors noted that at exposures over 300 mppcf-yr,
the excess of lung cancer was 80.4 deaths, one-fifth of which
was probably attributable to smoking. Consequently, Liddell and
Armstrong (2002) analyzed the effects of smoking on lung can-
cer risks in this population. Of the initial 9780 men included
in Liddell et al. (1997), 7279 met the follow-up study crite-
ria. The SMRs for lung cancer for both nonsmokers and ex-
smokers were not elevated even in the highest exposure group
(≥600 mppcf-yr). Therefore, the lung cancer NOAEL for the
entire cohort, when controlling for smoking, was ≥1884 f/
cc-yr. The mean cumulative exposure for the entire ≥600-
mppcf-yr exposure group, including nonsmokers, ex-smokers,
and current smokers, was 1220.4 mppcf-yr (3832 f/cc-yr).
In a previous study with follow-up until 1988 (McDonald
et al., 1993), standard mortality ratios were stratified by cumu-
lative exposure (accumulated to age 55) for company 1, com-
pany 2, and companies 3 and 4 combined. There was no in-
crease in lung cancer in the employees of the Asbestos Mine
and Mill (company 1) even at the highest exposure category
(≥300 mppcf-yr), while employees of the Thetford Mines (com-
panies 3 and 4 combined) and the Asbestos Factory (company 2)
demonstrated an elevated risk for lung cancer at this cumulative
exposure level with SMRs of 1.89 and 7.00, respectively. The
increased risk reported for these two subcohorts was likely the
result of amphibole exposures (McDonald et al., 1997). Com-
pany 2 does not meet our selection criteria due to the failure to
characterize amphibole contamination, and thus it is not con-
sidered further in this analysis. Cumulative NOAELs for lung
cancer at the Asbestos Mine and Mill (company 1) and Thetford
Mines (companies 3 and 4 combined), were ≥942 f/cc-yr and
314–942 f/cc-yr, respectively. Due to the difficulty in weighting
one study more heavily than the other, values from both Liddell
and Armstrong (2002) and McDonald et al. (1993) are included
in Table 1.
Thirty-eight deaths due to mesothelioma (all companies
combined) were classified into exposure groups and no clear
exposure-response relationship was observed (Liddell et al.,
1997). However, the authors did not provide the expected num-
ber of mesothelioma deaths, and therefore a mesothelioma
NOAEL could not be derived from this study.
Summary of Reported Chrysotile No-Effect Levels
Fourteen lung cancer NOAELs were taken from 12 published
studies. The majority of the studies did not observe increased
risk even at the highest chrysotile exposures; NOAELs in these
studies ranged from >25 f/cc-yr (Neuberger and Kundi, 1990)
to 1600–3200 f/cc-yr (Lacquet et al., 1980). NOAELs in those
studies where increased lung cancer risks were reported ranged
from 1.4–2.7 f/cc-yr (Brown et al., 1994) to 314–942 f/cc-yr
(McDonald et al., 1993).
Four cohorts were identified in which pleural mesothelioma
risk was stratified according to cumulative chrysotile exposure,
two of which did not observe an increased risk at the high-
est cumulative exposures. NOAELs from these cohorts were
>400 and ≥112 f/cc-yr for Piolatto et al. (1990) and McDonald
et al. (1984), respectively. The mesothelioma NOAELs taken
from Lacquet et al. (1980) and Albin et al. (1990) were 800–
1599 f/cc-yr and <15 f/cc-yr, respectively.
DISCUSSION
Identifying and cataloging the cumulative exposures at which
no increased lung cancer or mesothelioma risk was reported in
the studies considered here was a fairly straightforward exercise.
Nonetheless, we are unaware of any other published paper that
has attempted to summarize these data, even though the poten-
tial insight to be gained could be substantial. We recognize that
none of the studies examined in this analysis were conducted
for the strict purposes of identifying a NOAEL cumulative as-
bestos exposure. It is also understood that the studies cover a
very broad range of industries and occupational practices, in ad-
dition to having large differences in air sampling methods and
exposure estimation techniques. There are also known differ-
ences in latencies, cohort size, and percent amphibole exposure.
As discussed next, where possible, we identify the limitations,
206 J. S. PIERCE ET AL.
uncertainties and potential biases, and their influences on the
reported NOAELs.
Variability in Study Quality
We did not attempt to differentially weight the studies in this
analysis; however, as would be expected, there is some degree of
variability in the quality of the data collection and interpretation
methods, particularly with respect to air sampling techniques,
latency, use of an appropriate control population, cohort size,
adjustment for smoking, length of follow-up, and loss during
follow-up. Goodman et al. (2004) recently conducted a meta-
analysis of 11 epidemiological studies concerning lung cancer
and mesothelioma risk in vehicle mechanics. In that analysis, a
scoring approach was used to classify the studies into three tiers
based upon characteristics similar to those mentioned here. We
did not employ a scoring system when evaluating the quality
of the studies which met our inclusion criteria; however, it is
apparent that some studies are indeed more informative than
others. Perhaps, in a subsequent analysis, the technique used
by Goodman et al. (2004) may be utilized to perform a similar
analysis of the cohorts discussed in this study.
Consistency of Findings
With respect to characterizing the exposure-response rela-
tionship for lung cancer or mesothelioma, some general obser-
vations can be made which apply to all the cohorts considered
in this analysis: 1) all of the studies reported a NOAEL (i.e.,
none of the studies reported increased risk at all exposures),
2) the studies did not report an increased risk at an exposure be-
low its respective NOAEL, and 3) all of the studies that reported
a LOAEL (lowest exposure at which effects occurred) also ob-
served significant risks at all exposures above the LOAEL. There
are two exceptions to these general observations. McDonald
et al. (1984) observed a significant increase in respiratory can-
cer in the lowest exposure group of <14 f/cc-yr (SMR = 1.67,
95% CI 1.26–2.18). This “effect level” of <14 f/cc-yr conflicts
with the lung cancer NOAELs observed in the other studies
(which range from >25 to 1600–3200 f/cc-yr) and, more impor-
tantly, no statistically significant increase in respiratory cancer
was observed in the 4 higher exposure categories (up to ≥112
f/cc-yr, which we took to be the NOAEL from this study) in
McDonald et al. (1984). The authors suggested that this incon-
gruity might be explained by the selective employment of men
of relatively poor health or health habits (e.g., heavy smokers)
into low-exposure jobs where they often remained for a fairly
short time.
Similarly, Brown et al. (1994) observed significant increases
in lung cancer risk at a cumulative exposure range of 2.7 to
6.8 f/cc-yr, but found no increase at a higher cumulative ex-
posure range of 6.8 to 27.4 f/cc-yr (nonetheless, we took the
NOAEL from this study to be 1.4–2.7 f/cc-yr). Therefore, like the
McDonald et al. (1984) cohort, the low-exposure increased risk
reported in Brown et al. (1994) (2.7–6.8 f/cc-yr) is internally in-
consistent, and is also inconsistent with the lung cancer NOAELs
reported in the other cohorts. It is not possible to determine from
the information reported in Brown et al. (1994) whether the
selective employment issues noted by McDonald et al. (1984)
might also be applicable to or explain the incongruous low expo-
sure effects that they reported. However, it is noteworthy that the
methods used to estimate expected mortalities in this particular
study have been previously criticized by other investigators. Ex-
pected mortalities in Brown et al. (1994) were developed from
yearly mortality rates in the United States. Yet it was known
at the time that the local, age-adjusted county rates were 75%
higher than those reported for the United States as a whole (Ma-
son and McKay, 1974). As noted by the U.S. EPA (Nicholson,
1986) and McDonald et al. (1983b), the increase in local rates
was possibly the result of nearby shipyard employment (and per-
haps by the study plant). It is unclear whether use of local lung
cancer rates would yield a significant change in the findings of
Brown et al. (1994). In short, the internal inconsistencies noted
in McDonald et al. (1984) and Brown et al. (1994) are likely a
result of study design issues, but a definitive conclusion cannot
be reached from the available data.
The LOAEL for mesothelioma reported by Albin et al. (1990)
(15–39 f/cc-yr) was not consistent with the findings of the other
studies, which reported mesothelioma NOAELs ranging from
>400 to 800–1599 f/cc-yr. This inconsistency may simply be
a result of the inherent variability in the design and interpreta-
tion of the various cohort studies, but it may also be the result
of significant amphibole exposure. Specifically, in Albin et al.
(1990), the exposure-response relationship for pleural mesothe-
lioma was evaluated in a nested case-referent study. For each of
the cases (n = 14), 5 controls were selected based on the follow-
ing factors: same nationality, alive at the time of the diagnosis of
the case, and within 4 years of year of birth and first employment.
The authors reported a significant relationship between cumula-
tive exposure 40 years or more before diagnosis, and calculated
a multiplicative risk of 1.9 for each f/cc-yr. Following an ex-
amination of lung tissue from seven of the mesothelioma cases,
the authors found “much higher crocidolite and also higher to-
tal asbestos and tremolite counts when compared with matched
nonexposure cases from the cohort” (p. 609). The authors sug-
gested that exposure to amosite and crocidolite may have oc-
curred in all of the mesothelioma cases. In short, it is difficult
to determine whether the mesothelioma NOAEL of Albin et al.
(1990) conflicts with (is lower than) the mesothelioma NOAELs
from other studies due to methodological issues or uncertainties,
or whether this simply reflects the inherent variability in these
cohort studies.
The respiratory cancer NOAEL from Lacquet et al. (1980)
also deserves mention. The authors reported no increased risk at
estimated cumulative asbestos exposures of 1600–3200 f/cc-yr.
Aside from the Brown et al. (1994) results discussed earlier, this
is well beyond the cumulative exposures reported to be asso-
ciated with the NOAELs reported in McDonald et al. (1983a)
(120–240 f/cc-yr), Peto et al. (1985) (85.7–114.3 f/cc-yr), and
McDonald et al. (1993) (314–942 f/cc-yr). This could very well
be a result of an insufficient observation period (up to 15 years)
accounting for the long latency for disease.
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 207
It is admittedly difficult to determine any degree of “consis-
tency” when a majority of the studies reported no increased risk
at any cumulative exposure. Further, most of the studies did not
develop an estimate of a mean cumulative exposure that can be
considered representative of the NOAEL [exceptions are Hughes
et al. (1987), Albin et al. (1990), Liddell and Armstrong (2002);
see Table 1]. Therefore, for example, the observation that the
>25 f/cc-yr NOAEL from Neuberger and Kundi (1990) is “con-
sistent with” the ≥140 f/cc-yr NOAEL from Hughes et al. (1987)
is constrained by the fact that there is no information regarding
the SMRs in the 25–140 f/cc-yr and ≥140 f/cc-yr exposure
ranges from Neuberger and Kundi (1990). This also makes it dif-
ficult to identify a discrete exposure range from these studies that
can be considered a NOAEL for chrysotile-related lung cancer
or mesothelioma. At best, one can observe that in chrysotile-
exposed cohorts where amphibole exposure was thought to be
relatively low, the preponderance of the cumulative exposure
NOAELs for lung cancer and mesothelioma fall in a range of
approximately 25–1000 f/cc-yr and 15–500 f/cc-yr, respectively.
Limitations, Uncertainties, and Potential Biases
General Limitations and Uncertainties
One of the greatest sources of uncertainty in the chrysotile
studies is the potential misclassification in the cumulative expo-
sure estimates. None of the studies provided exposure informa-
tion specific to the individuals in the cohort, such as job classifi-
cation, airborne asbestos concentration, duration, or cumulative
exposure estimate. Therefore, it was not possible to evaluate the
accuracy of the exposure estimates.
Also, in many cases the asbestos levels were derived from
total dust measurements, not asbestos fiber counts (see Table 1).
This uncertainty is minimized somewhat by the fact that in most
studies the measured dust levels were compared to fiber levels
based on side-by-side samples, which were then used to derive
a plant- or operation-specific conversion factor. However, as
noted by Berman and Crump (2003), the “correlation between
fiber counts and total dust is sometimes poor within a plant
and generally poor between plants” (p. 5.2). To date, no
universal conversion has been established to compare earlier
dust measurements and current fiber counts, although as
described previously, several “manufacturing-specific” con-
version factors have been reported in the literature. McDonald
et al. (1983a, 1984) collected dust data without developing a
specific conversion factor, so for these analyses we applied the
conversion factor derived in the South Carolina textiles studies
(1 mppcf-yr = 3 f/cc-yr) to the estimated cumulative exposures.
If the lowest conversion factor reported in the literature
(1.4 f/cc/mppcf) were used instead, the calculated no-effect
exposure range would be reduced by a factor of 2 (60–
120 f/cc-yr); if the highest conversion factor reported in the
literature (6 f/cc/mppcf) were used, the calculated no-effect ex-
posure range would increase by a factor of 2 (240–480 f/cc-yr).
We also recognize that the measured concentrations in some
of these studies may not correlate well with specific work prac-
tices or even temporally with the cohort’s tenure in the facility.
In many cases, the air concentrations were measured years af-
ter the exposures began (Berry and Newhouse, 1983; Hughes
et al., 1987; Lacquet et al., 1980; Neuberger and Kundi, 1990).
In general, this measurement would likely result in an underes-
timate of exposure if the asbestos concentrations declined over
time (e.g., due to changes in processes and/or hygiene controls
and greater awareness of the asbestos hazard). In a majority of
the studies, the investigators did, in fact, attempt to “correct”
for possibly higher concentrations in previous years, although
the accuracy of these corrections is difficult to determine. Also,
many of the samples were “area” samples that may not repre-
sent exposures for specific occupations that might experience
higher or lower exposures. Individual worker exposures were
generally calculated using job descriptions described in factory
records in conjunction with the duration of time spent in each
job category. Within a company, specific jobs and processes
were assigned expected asbestos concentrations over time, an
approach that does not take into account variability in the ways
that tasks were performed by different workers in different fac-
tories or locations within a single factory. Cumulative exposure
estimates are therefore also dependent upon the accuracy of the
work histories documented in the factory records.
Fiber length information was not reported in any of the studies
evaluated in this paper, yet it is known that inhaled fiber size is
directly related to respiratory disease potential (risk of disease
generally increases with increasing fiber length). It is likely that
the mining cohorts, and perhaps many of the manufacturing
cohorts, were exposed to unprocessed fibers with average fiber
lengths greater than those associated with handling finished end
products that were made primarily from “short” (<5 μm) fiber
chrysotile (e.g., most joint compound and friction products).
Lack of fiber size information may therefore introduce a degree
of uncertainty in the reported NOAELs, particularly if they are
used to characterize exposure and risk associated with shorter
fibers.
A majority of the studies utilized national or state age-
adjusted mortality rates as reference values. While these rates
are easily accessible and a more appropriate comparison may
not be feasible, it is understood that such standard populations
contain both unhealthy and healthy individuals, while working
populations are generally comprised of those healthy enough to
work. As a result, the calculated SMRs for total mortality are
sometimes lower than expected (the so-called “healthy worker
effect”). Although most of the studies considered here did use
state or national mortality rates as a comparison group, in many
cases a healthy worker effect was explicitly evaluated and de-
termined to have no influence on the results (Brown et al., 1994;
Lacquet et al., 1980; Liddell and Armstrong, 2002; McDonald
et al., 1984; Neuberger and Kundi, 1990; Piolatto et al., 1990).
Peto et al. (1985) is the only study to have concluded the likely
presence of such an effect; the others did not evaluate the in-
fluence of a healthy worker effect (Berry and Newhouse, 1983;
Hughes et al., 1987; McDonald et al., 1983a). Conversely, use
208 J. S. PIERCE ET AL.
of national or state mortality rates for reference values can lead
to an overestimate of worker risk if regional asbestos exposures
contribute significantly to disease, vis-`a-vis the aforementioned
critique of Brown et al. (1994), Mason and McKay (1974), and
Nicholson (1986).
Frequently, the diagnosis of lung cancer and/or mesothelioma
in asbestos cohort studies is based primarily on death certifi-
cates. Information on the causes of death is then commonly
supplemented with additional material from hospital records,
pathology reports and autopsy data. Due to the regular discor-
dance between death certificate diagnoses and diagnoses made
after reviewing all relevant clinical and histopathological data,
if the same diagnostic procedure is not adhered to for both the
study and the reference populations (i.e., if the diagnoses were
based solely on death certificate data for the control popula-
tion), differential misclassification could result (Selikoff and
Seidman, 1992). As described by Enterline (1976), “Supple-
menting death certificates with other information and, in effect,
changing causes of death in the study populations (but not in the
control populations) invalidates comparisons and the calculated
relative risks” (p. 152). Differential classification likely did not
have a large effect on the estimates reported in these cohorts
because the investigators retrieved and relied upon death certifi-
cates for both the cases and noncases. An exception to this may
have occurred in Albin et al. (1990) due to the rate of necrop-
sies performed on the mesothelioma cases compared to those
performed on referents. The percentage of necropsies in the ref-
erents was not reported due to the fact that they were found in re-
gional and national cancer registry databases; however, it is very
likely that necropsies were not performed for this group. Sim-
ilarly, asbestos-related diseases may have been preferentially
diagnosed in asbestos-exposed workers due to increased rates
of necropsies as a result of worker’s compensation packages.
Loss due to follow-up can also play a critical role in the uncer-
tainty of epidemiology studies. When addressing this matter, the
U.S. EPA has noted that “Generally, 10 percent to 30 percent of
an observation cohort will be deceased (sometimes even less). If
10 percent of the group is untraced and most are deceased, very
large errors in the determination of mortality could result, even
if no person-years are attributed to the loss-to-follow-up group”
(Nicholson, 1986, p. 46). The loss-to-follow-up was minimal in
the studies included in this analysis. With few exceptions, the
tracing was complete for upwards of 95% of the populations in
each study.
Insufficient latency was a general limitation of many of the
studies evaluated. The latency between first exposure and the
development of disease is believed to be at least 30 years for
mesothelioma, and at least 20 years for lung cancer (ATSDR,
2001; Lanphear and Buncher, 1992). Six of the 11 lung cancer
cohorts evaluated in this analysis allowed for at least 20 years of
latency (Table 1), while the others ranged from 10 to 15 years.
∗
∗
Three of the cohorts had unspecified latencies (Lacquet et al. 1980;
Neuberger and Kundi 1990; Piolatto et al. 1990). See Table 1 for more details.
Three of the four mesothelioma cohorts had a minimum of a 20-
year latency period (McDonald et al., 1984; Albin et al., 1990;
Piolatto et al., 1990), while the other’s was not specified (Lacquet
et al., 1980). As with any epidemiology study involving a chronic
disease with a long latency, it is possible that an asbestos-related
disease was diagnosed in one or more individuals; however, their
death occurred after the study was completed. Since the risk es-
timates were based on deaths within the cohort during a given
follow up period, this may have resulted in an overestimation of
the NOAEL. However, depending on the distribution of cases
throughout the study period, insufficient latency may have un-
derestimated the NOAEL. As seen in Tables 1 and 2, there was
no clear relation between minimum disease latency and the risk
for lung cancer or mesothelioma (i.e., the risk estimates reported
do not increase with increasing minimum latency).
Lastly, as noted earlier, in many studies increased risks were
not observed for any of the cumulative exposure groups, and thus
the no effect level for these studies was defined as the highest
cumulative exposure group in the study. For certain studies the
highest exposure group was reported by the authors as greater
than (“>”), greater than or equal to (“≥”), or less than (“<”) a
certain cumulative exposure. In these instances, if reported by
the authors, the mean or median of the NOAEL was included in
Tables 1 and 2. This limits the ability to accurately quantify a
NOAEL, as it could be slightly or substantially higher than the
highest cumulative exposure group reported.
Potential Factors That Could Underestimate
the Reported NOAELs
There are several potential or known biases that could result
in underestimation of a cumulative chrysotile NOAEL reported
in these studies. For example, smoking is by far the leading cause
of lung cancer in the world, yet smoking-adjusted risk estimates
were only reported for 2 of the 11 lung cancer cohorts included
in this analysis (the Austrian cement workers and the Quebec
miners and millers). This lack of reporting is particularly impor-
tant because the percentages of blue-collar workers and trades-
men who smoke exceed the national averages (Bang and Kim,
2001; Blair et al., 1985; Hall and Rosenman, 1991). Limited
evidence suggests that smoking may in fact have contributed
to elevated lung cancer rates in these studies. Specifically, Neu-
berger and Kundi (1990) calculated smoker-adjusted and nonad-
justed SMRs for lung cancer, stratified by cumulative exposure
for the Austrian cement worker cohort. The nonadjusted SMRs
for lung cancer for both cumulative exposure groups (≤25 and
>25 f/cc-yr) were significantly elevated; however, after adjust-
ing for smoking, the SMRs for both groups were close to the
null value (see Table 2, p. 617). In addition, when controlling
for smoking, Liddell and Armstrong (2002) found no increased
lung cancer risk at any dose in the Quebec millers and miners.
When McDonald et al. (1993) examined subgroups of this co-
hort, they did not control for smoking and reported increased
risk at exposures lower than the NOAEL reported in the Liddell
and Armstrong (2002) study (>1884 f/cc-yr).
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 209
Exposure to amphiboles (amosite and crocidolite) is likely to
have occurred to some degree in all cohorts. On average the co-
horts experienced exposure to chrysotile asbestos that contained
over 3% amphiboles (Berman and Crump, 2003). As noted ear-
lier, amphiboles have been estimated to be 100–500 times as
potent as chrysotile for producing mesothelioma (Hodgson and
Darnton, 2000). Thus, in some cases amphibole exposures alone
might have been sufficient to induce lung disease, particularly
mesothelioma. Furthermore, because the previous exposure his-
tories of the individuals are unknown, it is not possible to de-
termine whether significant amphibole exposure may have oc-
curred in workers prior to their employment at the facility studied
(e.g., in shipyards).
The presence of other known or suspected respiratory car-
cinogens, such as crystalline silica, which was often used in
cement production, and mineral oil, which was frequently used
to suppress airborne asbestos during manufacturing processes,
could also have biased the no-effect levels for lung cancer to-
wards lower values. There is no clear consensus regarding the
risks of lung cancer with respect to silica exposure in cement
production workers (Jakobsson et al., 1993; McDowall, 1984;
Smailyte et al., 2004; Vestbo et al., 1991), and none of the studies
in our analysis examined this issue. It has been suggested that ex-
posures to mineral oil are responsible for the elevated lung cancer
risk seen in textile workers (but not in other similar chrysotile-
exposed cohorts) (McDonald, 1998). However, a nested case-
control study evaluating the potential effect of mineral oil ex-
posure on lung cancer risk in the South Carolina textile workers
concluded that “mineral oil exposure does not appear to be a
significant confounder in the risk estimates associated with cu-
mulative asbestos exposure (Dement et al., 1994, p. 442). Based
on the results of this analysis, it appears that exposure to other
carcinogens (besides possible amphibole exposure) did not con-
tribute to the increased lung cancer risk observed in this cohort.
As noted earlier, in most cases the original investigators often
attempted to account for higher airborne asbestos concentrations
that likely existed prior to sampling events. However, in some
instances it was not feasible to account for certain activities that
were likely to generate very high concentrations. For example,
according to McDonald et al. (1983b), high-exposure tasks that
were performed at the South Carolina textiles facility were not
considered in the exposure estimates. In particular, during the
years 1937 through 1953, the facility’s dust filtration system
(receiving dust from ventilation inflow in the preparation and
carding departments) consisted of burlap bags stretched across
wooden frames. The baghouse operators would beat the burlap
bags with whips on a daily basis to dislodge the accumulated
dust, resulting in extremely high exposures. Tasks such as this
were often carried out on weekends or as optional overtime,
and were performed by anyone who volunteered. In addition,
from 1945 to 1964 the mixing of fibers, which until that time
was subject to varying degrees of control, was transferred to an
alternate location in the plant (the mezzanine), where asbestos
was moved around by men with pitch forks without any form
of dust suppression. As noted by the authors, “these mezzanine
and baghouse exposures, which could neither be assessed nor
identified in any analysis, have not been included in any analysis”
(p. 363). Clearly, failure to incorporate such high exposure tasks
into the cumulative exposure estimates can lead to a significant
underestimate of the NOAEL.
Potential Factors That Could Overestimate
the Reported NOAELs
Overestimation of worker exposure may have biased the
NOAELs toward higher values in some cases. For example, such
overestimation could occur if samples taken in high dust- or
asbestos-producing operations were subsequently used to char-
acterize exposures to workers involved in lower-exposure tasks.
In addition, the NOAELs could have been overestimated if the
workers with the highest exposure were lost to follow-up.
The primary factor that could bias the reported NOAEL in
any given study toward an artificially high value would be lack
of statistical power. Indeed, it is entirely possible that in many
of these studies a power analysis would indicate that statistically
significant risks could exist at (or below) the reported NOAEL,
but that the increased risks were simply not measurable due to
small cohort size, insufficient number of f/cc-years, or other fac-
tors (it is primarily for this reason that we have chosen not to refer
to the NOAELs in this article as “thresholds,” since that term of-
ten implies a known exposure or dose below which effects do not
occur). While it is beyond the scope of this article to conduct a de-
tailed power analysis of all of these studies, a preliminary review
suggests that the confidence with which the NOAELs can truly
be considered “maximum exposures at which no measurable ef-
fect was observed” varies considerably from study to study. For
example, the McDonald et al. (1993) lung cancer cohorts (com-
pany 1 and companies 3 and 4) appear to be sufficiently powerful
to detect an increased risk of disease. Specifically, at 95% con-
fidence, the power is 100% for detecting a minimum SMR of
2.0 for company 1’s 363,000 person-years and companies 3 and
4’s 607,000 person-years at the NOAEL of 100–300 f/cc-years.
However, for the Brown et al. 1994 lung cancer cohort, the power
to detect a minimum SMR of 2.0 for the 21,901 person-years
associated with the <1000 f/cc-day NOAEL is only 29.8%. The
minimum SMR detectable for this study at the NOAEL with a
power of 80% is 3.5.
Hence, we believe that the NOAELs summarized in this ar-
ticle cannot be taken as true “thresholds” unless and until a
thorough statistical analysis supports such a conclusion. Along
these lines, it is worth noting that Berman and Crump (2003) re-
cently evaluated exposure-response data from several asbestos-
exposed cohorts, including many of those summarized in this
article. For both lung cancer and mesothelioma, they found that
a nonthreshold, linear model provided an “adequate” descrip-
tion of the cumulative exposure–cancer response results. How-
ever, to our knowledge there has been little effort to determine
whether one or more “threshold models” might also provide a
210 J. S. PIERCE ET AL.
reasonable fit to the exposure-response data, and the use of such
models warrants future research.
All of the studies considered here were cohort studies wherein
relative risks were determined by comparing disease rates in an
exposed versus nonexposed (or general) population. This study
design is usually appropriate for diseases with fairly high inci-
dence, such as lung cancer. However, a case-control study design
is more appropriate for rare diseases such as mesothelioma, par-
ticularly if the size of the cohort is fairly small (Wong, 2001). Of
the four cohorts in the mesothelioma analysis, three reported two
or fewer cases of mesothelioma in total (Lacquet et al., 1980;
McDonald et al., 1984; Piolatto et al., 1990) and one reported no
cases (and therefore no risk at any dose) (McDonald et al., 1984).
It is unknown whether a case-control study or an alternate study
design, with a larger cohort, would have yielded a significantly
different outcome. While only four of the mesothelioma studies
considered in this analysis stratified risk by cumulative exposure,
it is important to note that many of the other studies reported
cases of mesothelioma in workers (Berry and Newhouse, 1983;
Hughes et al., 1987; McDonald et al., 1983a, 1993; Neuberger
and Kundi, 1990; Peto et al., 1985; Dement and Brown, 1998;
Liddell et al., 1997). In most of these instances, the authors sug-
gested that amphibole exposure was more likely responsible for
the mesothelioma cases than chrysotile.
Comparison of Chrysotile NOAELs to Vehicle Mechanic
Cumulative Exposures
Finley et al. (2007) recently developed estimates of cumu-
lative chrysotile exposures experienced by vehicle mechanics
FIG. 1. Comparison of upper bound cumulative chrysotile exposures for vehicle mechanics to reported Lung Cancer No-Effect
Levels.
1
∗
Presented in Finley, 2007
1
see Table 1 for the cumulative NOAEL presented in each study.
working with friction products in the 1970s. Automotive fric-
tion products (brakes and manual clutches) in this time frame
typically contained chrysotile, and the numerous published
industrial hygiene surveys of vehicle repair garages in the 1970s
permit a fairly thorough analysis of these historical exposures.
Finley et al. (2007) reported that the 95th percentile and 99th
percentile cumulative exposures for vehicle mechanics in the
1970s were 2.0 and 5.7 f/cc-yr, respectively. As shown in
Figure 1, with the exception of the studies of South Carolina
textile workers (Brown et al., 1994), all of the reported cumula-
tive chrysotile NOAELs reported for lung cancer were far above
the 95th percentile and 99th percentile cumulative vehicle
mechanic exposures. As shown in Figure 2, the cumulative
chrysotile NOAELs reported for mesothelioma are all well
above the 95th and 99th percentile cumulative asbestos expo-
sure for vehicle mechanics. These results are consistent with
the epidemiology literature showing that vehicle mechanics are
not at an increased risk of developing asbestos-related diseases
(e.g., Goodman et al. 2004).
Recent Research on Chrysotile Exposure
and Mesothelioma Risk
The question of whether or not chrysotile exposure is a risk
factor for mesothelioma is a matter of ongoing debate, and
there are some relatively recent published papers that have
reviewed the epidemiological evidence and reached conclu-
sions on this issue. Some researchers support the proposition
that chrysotile exposures theoretically might cause mesothe-
lioma, but that the epidemiological weight of evidence is
NO-EFFECT LEVELS FOR CHRYSOTILE ASBESTOS 211
FIG. 2. Comparison of upper bound cumulative chrysotile exposures for vehicle mechanics to reported Mesothelioma No-Effect
Levels.
1
1
see Table 2 for the cumulative NOAEL presented in each study.
lacking (Doll, 1989; McDonald and McDonald, 1991), while
others believe the evidence clearly demonstrates that only
amphiboles, not chrysotile, can induce mesothelioma (Ilgren
and Chatfield, 1998; Yarborough, 2006; Dunnigan, 1988). For
example, Yarborough (2006) recently analyzed the results of
71 asbestos-exposed cohorts studies, and concluded that “Epi-
demiological review of cohorts does not support the hypothesis
that exposures to chrysotile fibers, uncontaminated by amphi-
boles, cause mesothelioma” (p. 180). It should be noted that the
“chrysotile-only” cohorts considered by Yarborough suffer from
the same study design limitations as those considered here (i.e.,
lack of case-control methodology for a relatively rare disease).
Conversely, others have concluded that the evidence is clear
that chrysotile alone can cause mesothelioma. For example,
in an analysis conducted by Smith and Wright (1996), 25 as-
bestos cohort studies were examined, and the authors stated that
“Since asbestos is the major cause of mesothelioma, and because
chrysotile constitutes 95% of all asbestos used worldwide, it can
be concluded that chrysotile asbestos is the main cause of pleural
mesothelioma in humans” (p. 252). In a more recent analysis, Li
et al. (2004) reviewed the evidence from 26 different cohorts and
concluded that chrysotile asbestos exposure alone can cause both
mesothelioma and lung cancer. According to the authors, “Only
cohort studies on cancer mortality among workers exposed to
chrysotile alone were incorporated in to the meta-analysis” (Li
et al., 2004, p. 460). However, at least half of the cohorts in-
cluded in this analysis were known or suspected to have some
degree of amphibole exposure (Dement et al., 1994; Hughes
et al., 1987; McDonald et al., 1983b, 1984; Peto et al., 1985;
Piolatto et al., 1990; Newhouse and Sullivan, 1989; Liddell et al.,
1997; Germani et al., 1999; Raffn et al. 1996; Gardner et al.,
1986; Thomas et al., 1982; Ohlson and Hogstedt, 1985).
While the exposure-response summary described in this arti-
cle cannot directly address the general question “Is chrysotile a
risk factor for mesothelioma under any circumstances?” due to
the presence of amphiboles in most of the mesothelioma cohorts
considered here, it does seem to indicate that low occupational
exposures to chrysotile (e.g., exposures historically experienced
by vehicle mechanics) are unlikely to cause mesothelioma. Our
findings suggest that a thorough understanding of chrysotile ex-
posures that might occur in a given setting (e.g., estimated expo-
sures that might occur during manufacture or use of microelec-
tronics with synthetic chrysotile fibers) will provide assistance
in reaching conclusions regarding the relative safety of such
activities.
ACKNOWLEDGMENTS
The research supporting this analysis and the time
needed to write the article were funded almost entirely by
Chrysler Corporation, Ford Motor Company, and General
Motors Corporation, which have been and are currently involved
in litigation involving brake dust. These three funding sources
(and their counsel) did not provide editorial comments or review
212 J. S. PIERCE ET AL.
the manuscript prior to submission to the journal. Some of the au-
thors have served and may continue to serve as expert witnesses
regarding the historical exposures of mechanics to asbestos dur-
ing brake repair.
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