Mining and processing of vermiculite from the
worlds largest deposit near Libby, Montana,
began in the early 1920s and continued for 70
years. In 1963, W.R. Grace acquired the mine
from Zonolite Company and operated it until
it was closed in 1990. These operations
included open-pit mining and on-site milling
of the mineral; transporting the raw material
by truck and rail to two processing plants in
Libby and to plants throughout the United
States and Canada, where it was heated and
expanded (exfoliated); and finally, shipping
the nished product by truck and rail to dis-
tribution centers nationally (Myers 1960).
Vermiculite is a silicate mineral with the
unique property of expanding 8- to 12-fold in
volume upon heating (Meisinger 1980; Myers
1960; Otis 1960). Because of its re-resistant
and absorptive properties, vermiculite is used
commercially in the construction industry as
insulation and ller material, and in agricul-
ture as a soil additive and carrier agent for fer-
tilizers and other chemicals (Lockey 1981;
Meisinger 1980). Vermiculite from Montana
has been shown to be contaminated with
tremolite, actinolite, and other forms of the
amphibole series of asbestos, ranging from
2 to 26% in the raw form (Amandus et al.
1987b; Atkinson et al. 1982; Bank 1980;
Dixon et al. 1985; Moatmed et al. 1986). Thus, mining, handling, processing, and per-
sonal or commercial use of vermiculite have
led to widespread contamination of the Libby
area with asbestos-contaminated vermiculite
(Dixon et al. 1985). Although no serious health effects from vermiculite alone have been reported to date
(Addison 1995), health effects such as pleural
thickening, pleural calcications, pleural effu-
sions, asbestosis, mesothelioma, and lung can-
cer from occupational exposure to asbestos
have been well documented. Early evidence of
pulmonary fibrosis among Libby mine and
mill employees was described in an X-ray sur-
vey of miners in 1959 (McDonald et al.
1986a). Additional evidence came from a
report of 12 cases of pleural effusions over a
12-year period among employees of an Ohio
fertilizer plant that processed vermiculite from
Libby, followed by a cross-sectional study of
workers in the plant that demonstrated a rela-
tionship between cumulative fiber exposure
and radiographic changes and pleuritic chest
pain (Lockey et al. 1984). Two separate but
parallel cohort studies of workers at the vermi-
culite mine in Libby showed excess mortality
from lung cancer, malignant mesothelioma,
and nonmalignant respiratory disease
(Amandus and Wheeler 1987; McDonald
et al. 1986a). These same investigators also conducted radiographic studies of then-current
workers and found that the prevalence of
pleural thickening and small opacities
increased with increasing levels of cumulative
fiber exposure (Amandus et al. 1987a;
McDonald et al. 1986b). Together, these nd-
ings provide substantial evidence that exposure
to the amphibole type of asbestos from the
Libby vermiculite mine results in adverse respi-
ratory health effects similar to those seen with
exposure to other forms of asbestos. Reported cases of mesothelioma among household contacts of asbestos workers led to
a growing concern that asbestos exposure and
the risk of later disease could spread beyond
the workplace to the home and community
(Anderson et al. 1976; Berry 1997; Hansen
et al. 1993, 1998; Magnani et al. 2000, 2001;
Newhouse and Thomson 1965; Wagner et al.
1960). Several studies have reported an
increased prevalence of pleural thickening,
pleural plaques, and calcifications as well as
parenchymal opacities among household
members of asbestos workers (Anderson et al.
1976, 1979; Kilburn et al. 1985). Libby area
physicians have reported cases of asbestos-
related pulmonary disease among household
contacts of former mine workers and other
residents of the community who were not
directly associated with the mining or process-
ing operations (Whitehouse 2000). Although household contacts may have been exposed by workers taking home asbestos
on their clothes, shoes, and hair, numerous Environmental Health Perspectives VOLUME 111 | NUMBER 14 | November 2003 1753 Radiographic Abnormalities and Exposure to Asbestos-Contaminated
Vermiculite in the Community of Libby, Montana, USA Lucy A. Peipins, 1,* Michael Lewin, 1 Sharon Campolucci, 1 Jeffrey A. Lybarger, 1 Aubrey Miller, 2 Dan Middleton, 1 Christopher Weis, 2 Michael Spence, 3 Brad Black, 4 and Vikas Kapil 1 1 Agency for Toxic Substances and Disease Registry, Atlanta, Georgia, USA; 2 U.S. Environmental Protection Agency, Denver, Colorado, USA; 3 Montana Department of Health and Human Services, Helena, Montana, USA; 4 Lincoln County Department of Environmental Health, Libby, Montana, USA Address correspondence to M. Lewin, ATSDR/
Division of Health Studies, 1600 Clifton Road NE,
E-31, Atlanta, GA 30333 USA. Telephone: (404) 498-
0607. Fax: (404) 498-0077. E-mail: Mlewin@cdc.gov *Current location: Division of Cancer Prevention and Control, Centers for Disease Control and
Prevention, Atlanta, GA, USA. We thank P. Peronard and D. Strausbaugh for assis- tance in coordinating the medical testing program,
and B. Gottschall and C. Rose for coordinating the
spirometry testing. We also thank S. Wing and
D. Williamson for their helpful comments on early
drafts of the manuscript. The medical testing program in Libby, Montana, was supported by funds from the Comprehensive
Environmental Response, Compensation, and
Liability Act (CERCLA) trust fund through the
Agency for Toxic Substances and Disease Registry,
Public Health Service, U.S. Department of Health
and Human Services. The authors declare they have no conict of interest.
Received 19 March 2003; accepted 1 July 2003. Mining, handling, processing, and personal or commercial use of asbestos-contaminated vermi-
culite have led to widespread contamination of the Libby, Montana, area. We initiated a medical
testing program in response to reports of respiratory illness in the community. The purpose of this
analysis was to identify and quantify asbestos-related radiographic abnormalities among persons
exposed to vermiculite in Libby and to examine associations between these outcomes and partici-
pants self-reported exposures. A cross-sectional interview and medical testing were conducted in
Libby from July through November 2000 and from July through September 2001. A total of 7,307
persons who had lived, worked, or played in Libby for at least 6 months before 31 December 1990
completed the interview. Of those, 6,668 participants 18 years of age received chest radiographs to assess the prevalence of pleural and interstitial abnormalities. We observed pleural abnormalities
in 17.8% of participants and interstitial abnormalities in < 1% of participants undergoing chest
radiography. We examined 29 occupational, recreational, household, and other exposure pathways
in the analysis. The prevalence of pleural abnormalities increased with increasing number of expo-
sure pathways, ranging from 6.7% for those who reported no apparent exposures to 34.6% for
those who reported 12 pathways. The factors most strongly associated with pleural abnormalities were being a former W.R. Grace worker, being older, having been a household contact of a W.R.
Grace worker, and being a male. In addition to being a former W.R. Grace worker, environmental
exposures and other nonoccupational risk factors were also important predictors of asbestos-related
radiographic abnormalities. Key words: asbestos-related disease, medical screening, pleural plaques,
radiographic opacities, radiography, tremolite-actinolite, vermiculite. Environ Health Perspect
111:17531759 (2003). doi:10.1289/ehp.6346 available via http://dx.doi.org/ [Online 2 July 2003] Environmental Medicine Article other sources of environmental asbestos expo-
sure exist for Libby community members. Air
sampling in downtown Libby in 1975 and at
several points in the 1980s detected levels of
asbestos well above the Occupational Safety
and Health Administrations occupational
limit of 0.1 fiber/cm 3 over 8 hr of exposure (Atkinson et al. 1982; Dixon et al. 1985; U.S.
Department of Labor 1994). Residents could
have also been exposed to asbestos through
the use of vermiculite in gardening activities,
in home insulation, as aggregate in driveways,
or through other uses around the home.
Additionally, Libby residents have described
other activities that involved close contact
with vermiculite, such as playing in piles of
vermiculite at the processing facilities, expand-
ing or popping vermiculite at home by heat-
ing pieces of the ore over the stove, playing at
the baseball eld located adjacent to the ver-
miculite expansion plant, and playing along
Rainey Creek Road that leads from town to
the vermiculite mine. A community-based medical testing pro- gram was initiated in response to reports of
illness among people exposed to asbestos-
contaminated vermiculite in Libby. In this
article we outline the results from radiographic
testing and self-reported exposure pathways
for 7,307 persons who participated in this pro-
gram. The main objectives were to a) identify
and quantify possible asbestos-related pleural
and interstitial abnormalities among partici-
pants and b) examine associations between
these outcomes and the participants exposure
histories. Materials and Methods Participants and data collection. People were
eligible for participation in the medical testing
program if they had resided, worked, attended
school, or participated in other activities in the
Libby, Montana, area for 6 months before 31 December 1990. We identified partici-
pants from local telephone directories and
through paid newspaper, radio, and television
advertisements placed locally and throughout
the northwest region of the United States, and
in the major newspapers in Chicago, Illinois;
Boston, Massachusetts; New York, New York;
and Dallas, Texas. The highest readership days
of each newspaper were targeted. In addition,
word of mouth and medical referrals brought
participants to the testing program. A toll-free
telephone line was established for interested
persons to obtain information about the pro-
gram and to determine eligibility for screening
participants. Telephone screening to deter-
mine eligibility began in April 2000 and con-
tinued through September 2001. Eligible
persons living outside of Libby had to provide
their own transportation to Libby for medical
evaluation. In-person interviews and medical
testing were conducted in Libby in two waves: from July through November 2000 and from
July through September 2001. After informed
consent was obtained, trained interviewers
administered a computer-assisted question-
naire to obtain demographic characteristics,
residential history, occupational history,
household contact history, recreational activi-
ties and other potential pathways for vermi-
culite exposures, cigarette smoking status,
medical history, and self-reported symptoms
and illnesses. The medical screening consisted
of spirometric testing and chest radiographs.
In this article we report on the radiographic
findings of this program. Spirometric results
will be reported separately. Medical evaluation and classification of outcomes. Chest radiographs were offered to
participants 18 years of age and included posterioranterior (P-A), right anterior-
oblique, and left anterior-oblique views.
Women of child-bearing age were informed
that they should postpone their chest radi-
ograph if they were pregnant. The physician
on site assessed the consistency and quality of
each chest radiograph taken and provided a
routine radiologic interpretation, which
included recording asbestos-related changes
on a summary report form. If findings on a
chest radiograph suggested the need for
urgent medical attention, the physician com-
pleted a referral form, and the participant was
counseled and directed to an appropriate
source of medical care. After initial evaluation and interpretation by the on-site physician, the participants chest
radiographs were read and classied indepen-
dently by three B-readers certified by the
National Institute for Occupational Safety and
Health, without knowledge of the subjects
age, smoking history, or exposure category,
according to guidelines developed by the
International Labor Organization (ILO 1980).
Although the P-A view is recommended by the
ILOs guidelines for the classication of radi-
ographs (ILO 1980), we used the lateral right
and left oblique views as well as the P-A view
for evaluating abnormalities. The addition of
right and left oblique views increases the preci-
sion and the sensitivity and specicity of chest
radiographs in finding pleural abnormalities
not evident on the P-A view alone (Lawson
et al. 2001; Levin et al. 2000). A subject was classied as having a pleural abnormality if two out of three B-readers
indicated a) any unilateral or bilateral pleural
calcication on the diaphragm, chest wall, or
other site or b) any unilateral or bilateral
pleural thickening or plaque on the chest
wall, diaphragm, or costophrenic angle site,
consistent with asbestos-related pleural dis-
ease, using the P-A view, the oblique views, or
a combination of those views. A subject was
classied as having an interstitial abnormality
if two out of three B-readers indicated the presence of opacities, applying ILO guide-
lines, with a profusion of 1/0 or greater using
the P-A view only. Exposure characterization. To describe potential exposures to asbestos or vermiculite,
participants were asked if they had ever
worked for W.R. Grace or Zonolite (WRG)
or as a contractor for WRG; if they had ever
been exposed to dust at non-WRG jobs; if
they had ever mixed, cut, or sprayed asbestos;
if they had ever had other occupational expo-
sure to asbestos; or if they had exposure to
asbestos in military service. In addition, partic-
ipants where asked if they had ever had a job
as a pipe or steam tter, plumber, brake repair
person, insulator, dry wall nisher, carpenter,
roofer, electrician, or welder. Some questions
sought information about potential exposures
in daily life, including having been a house-
hold contact of a WRG worker (during the
time the worker was at WRG), length of resi-
dence in the Libby area (categorized by quar-
tiles for analysis: 6 months through 14 years,
1522 years, 2334 years, and > 34 years),
and having asbestos products in the home or
using vermiculite in gardening or insulation
(yes or no). There were questions about recre-
ational activities such as playing in vermiculite
piles, popping vermiculite, and playing on the
baseball field near the expansion plant, or
playing along the Rainey Creek Roadthe
road to the mine (never, sometimes, or fre-
quently). Participants were also asked if they
had any other contact with vermiculite (never,
sometimes, or frequently). Covariates. Age, sex, cigarette smoking sta- tus, a history of chest injury or chest surgery,
tuberculosis, pneumonia, scleroderma, lupus,
rheumatoid arthritis, congestive heart failure,
cancer, or pulmonary disease were ascertained Environmental Medicine | Peipins et al. 1754 VOLUME 111 | NUMBER 14 | November 2003 Environmental Health Perspectives Table 1. Distribution and characteristics of partici-
pants in the Libby medical screening program. Variable No. (%) Age 1017 600 (8.2) 1844 2,519 (34.5) 4564 3,031 (41.5) 65 1,157 (15.8) Sex Male 3,601 (49.3) Female 3,706 (50.7) Smoking history Never 3,711 (50.8) Ex-smoker 2,156 (29.5) Current 1,438 (19.7) Years lived in Libby 0.513 1,864 (25.7) 1421 1,892 (26.1) 2233 1,761 (24.2) 34 1,747 (24.1) BMI 018 234 (3.2) 1924 2,185 (30.2) 2529 2,529 (34.9) 30 2,292 (31.7) from the examiner-administered questionnaire.
Participants were also asked if they were a) not
at all, b) a little, or c) very concerned that
something in the neighborhood environment
may be harmful. Body mass index (BMI) was
calculated from height and weight measure-
ments obtained during the medical examina-
tion and classified into standard categories
(< 18, 1924, 2529, and 30) for descriptive purposes and into quartiles (< 24, 2427,
2831, 32) on the basis of its distribution in the participants for statistical modeling. Age
was modeled as a continuous variable, and
cigarette smoking status was classied as never
smoked or ever having smoked (ex-smoker and
current smoker). Statistical analysis. We used uncondi- tional logistic regression modeling to esti-
mate, by use of the odds ratio (OR) and 95%
condence intervals (CI), the risk of respira-
tory abnormalities for each of the exposure
pathways while controlling for all other expo-
sure pathways and other established and sug-
gested risk factors for respiratory illness.
Models were estimated separately for the out-
comes of pleural abnormalities and interstitial
abnormalities. We rst tabulated the frequencies of demo- graphic and other participant characteristics to
obtain a descriptive prole of the participants.
We then tabulated prevalence of pleural abnor-
malities and interstitial abnormalities for each
of the occupational, recreational, and house-
hold exposure pathways and for each of the
covariates, as well as for the increasing number
of exposure pathways. Crude ORs and 95%
CIs for pleural abnormalities and interstitial
abnormalities were calculated for each of these
pathways and covariates. We began an uncon-
ditional logistic regression multivariate model
with all potential exposure pathways and
covariates. The presence of effect modication
was evaluated in this model by use of interac-
tion terms between the exposure variables and
demographic characteristics. We modeled age
as a continuous variable and, when appropriate,
included linear and nonlinear terms in the
model. Criteria for inclusion in the nal model
included statistical signicance of the explana-
tory variables (p < 0.10), stability of the esti-
mate, effect on other variables in the model
(confounding), biologic plausibility of the inter-
action, and t of the model. We used a logistic
regression model to describe an exposure
response relationship with the number of expo-
sure pathways while controlling for covariates.
A chi-square test and pairwise contrasts
between pathway levels were used to assess the
statistical signicance of this trend. We used
Procedures in SAS, release 8.01 (SAS Institute
2001) to perform all statistical analyses. Results Description of the participants. Of 12,829
persons screened by telephone, 3,527 did not
meet the criteria for eligibility and 66 had
unknown eligibility status. Of those eligible,
1,689 either did not schedule an appointment
or did not report for medical testing, 231
refused medical testing, 4 had died before they
were tested, and 5 were physically unable to
come in for testing. The remaining 7,307 cur-
rent and former residents of Libby and the
surrounding area participated in the medical
testing program (6,149 in the rst wave and
1,158 in the second wave, resulting in a
78.6% participation rate). Among those who
participated, 81% stated that they currently
lived in Montana, and of those, 80% currently
resided in the Libby, Troy, or Eureka areas.
Most of the participants outside of Montana
came from Idaho, Oregon, and Washington.
Because participants in the first and second
years of medical testing were similar in demo-
graphic characteristics, exposure pathways,
concern about neighborhood contamination,
and prevalence of outcomes, the data from
both years were combined. Of the 7,307 participants, 6,668 (91.2%) were 18 years of age and therefore were eligi- ble for and received chest radiographs. During the two waves of medical testing, the physician
on site determined that 525 radiographs could
not be read because of poor quality, and these
were later repeated. Characteristics of the participants are pre- sented in Table 1. The participants were
almost evenly divided by sex with 49% male
and 51% female. The majority of participants
were 1864 years of age (76.0%). Almost half
of the participants were former or current
smokers. Roughly 74% of participants had
lived in the Libby area for 14 years. Many of the participants were overweight. A BMI of
2529.9 is considered overweight, and a BMI
of 30 is considered obese; 67% of partici- pants had a BMI of 25, with almost 32% of all participants in the obese category. The 29 exposure pathways used in the analyses and the number of participants report-
ing each pathway are presented in Table 2.
These include occupational, recreational,
household, and other potential exposures.
Participants may have reported one, several, or
none of these exposures. The most common
pathways were recreational activities along
Rainey Creek Road (4,898, or 67.4%), playing
in the baseball elds near the expansion plant
(4,772 participants, or 65.5%) and playing in
the vermiculite piles (2,442, or 33.7%). The
most common occupational exposure was dust
exposure at non-WRG jobs (2,396, or 32.8%),
and the least common occupational exposure
was working in a shipyard or ship construction
or repair (129, or 1.8%). Males were much more likely than females to have reported occupational expo-
sures. Among those who reported having
worked at WRG, 341 (92.2%) were male and
29 (7.8%) were female. Males also were more
likely than females to have been exposed to
asbestos or vermiculite during nonoccupa-
tional activities, although differences were less
pronounced when compared with the occu-
pational exposures. For instance, among the
exposure pathways, males were more likely
than females to report dust exposures at work
(76.8% vs. 23.2%), working at any job with
exposure to asbestos (71.2% vs. 28.8%), ver-
miculite exposure at other jobs (77.2% vs.
22.8%), frequently playing in vermiculite
piles (61.1% vs. 38.9%), or frequently pop-
ping vermiculite (54.4% vs. 45.6%). The
only notable exception was that females were
more likely than males to have been a house-
hold contact of a WRG worker (60.0% of
household contacts were female). Pleural and interstitial abnormalities. Table 3 presents the crude prevalence rates of
pleural and interstitial abnormalities by expo-
sure pathways for 6,668 participants, 18 years of age, who underwent chest radiography. The
pathways presented here are not mutually
exclusive; for example, a participant who
reported having been a WRG worker, playing Environmental Medicine | Exposure to asbestos-contaminated vermiculite in Libby, Montana Environmental Health Perspectives VOLUME 111 | NUMBER 14 | November 2003 1755 Table 2. Number (percent) of participants reporting
each exposure pathway. a Exposure pathway No. (%) Ever work for WRG 370 (5.1) Secondary contractor work 465 (6.4) Dust exposure at non-WRG jobs 2,396 (32.8) Vermiculite exposure at 1,103 (15.2) non-WRG jobs Worked non-WRG job As a pipe tter or steam tter 164 (2.2) As a plumber 168 (2.3) As a brake repair person 249 (3.4) As an insulator 240 (3.3) As a dry wall nisher 221 (3.0) As a carpenter 568 (7.8) As a roofer 362 (5.0) As an electrician 187 (2.6) As a welder 467 (6.4) Mixing, cutting, or spraying asbestos 144 (2.0) In a shipyard or performed ship 129 (1.8) construction or repair Exposed to asbestos 851 (11.8) Worked around anyone performing 804 (11.0) non-WRG jobs (listed above) Asbestos exposure in the military 182 (2.7) Lived with WRG workers 1,418 (19.5) Vermiculite insulation in Lincoln 3,017 (45.8) County homes Asbestos products in Lincoln County 1,155 (17.4) homes Used vermiculite for gardening 3,702 (51.9) Used vermiculite around the home 638 (8.8) Handled vermiculite insulation 3,635 (50.5) Recreational activities along Rainey 4,898 (67.4) Creek Road Played at ball eld near expansion plant 4,772 (65.5) Played in vermiculite piles 2,442 (33.7) Popped vermiculite 2,797 (38.4) Other contact with vermiculite 2,232 (31.0) a Participants may have answered yes to more than one category. on the baseball eld, and using vermiculite for
gardening would be represented in all three
categories. The overall prevalence of pleural
abnormalities was considerably greater than the
prevalence of interstitial abnormalities. Almost
18% of participants who underwent chest radi-
ography had a pleural abnormality, compared
with only 1% of participants having an intersti-
tial abnormality. The exposure pathway with
the highest unadjusted rate for pleural abnor-
malities was being a former WRG worker, with
186 (51.0%) having pleural abnormalities.
WRG workers also had the highest rate of
interstitial abnormalities (3.8%). Other path-
ways associated with high rates of pleural
abnormalities included history of asbestos
exposure in the military (42.9%), working in a
shipyard or performing shipyard construction
or repair (34.9%), and being a secondary con-
tract worker for WRG (34.8%). The lowest
prevalence of pleural abnormalities (14.4%)
was seen for the sometimes exposure cate-
gory of playing at the baseball field near the
expansion plant. The rate of pleural abnormalities increased from 5.1% in younger adults 1844
years of age, to 20.0% for participants 4465
years of age, to 39.7% for participants 65 years of age. Males had a signicantly higher
rate of pleural abnormalities (26.6%) com-
pared with female participants (9.1%). The
crude OR for pleural abnormalities among
males compared with females was 3.61 (95%
CI, 3.144.15). Current and former smokers
(ever smokers) were twice as likely to have
findings of pleural abnormalities than those
who never smoked (crude OR, 2.18; 95% CI,
1.912.49). Participants with a high BMI
were more likely to have a nding of pleural
abnormalities than were those with a lower
BMI. This risk increased with increasing
quartiles of BMI. Compared with the first
quartile, the crude ORs for pleural abnormal-
ities were 1.80 (95% CI, 1.442.24) for the
second quartile, 2.80 (95% CI, 2.2533.49)
for the third quartile, and 3.71 (95% CI,
2.994.60) for the highest quartile. Increasing
length of residence in the Libby area was also
associated with increasing risk of pleural nd-
ings. Compared with participants residing in
the Libby area for < 14 years, crude ORs
ranged from 0.91 (95% CI, 0.731.13) for a
residential duration of 1421 years to 3.62
(95% CI, 3.004.36) for residential duration
of > 34 years. Table 4 presents the final unconditional logistic regression model in which all expo-
sure pathways, as well as other risk factors and
interaction terms, were assessed for their con-
tribution to the risk of pleural abnormalities.
The model shows that the following factors
were associated with pleural abnormalities:
having been a WRG worker, having been a
household contact of a WRG worker, having been exposed to asbestos in the military, hav-
ing played in vermiculite piles, being male,
being older, having lived in the Libby area for
a longer period of time, having smoked ciga-
rettes, and having a high BMI. We found significant interaction terms between age and being a former WRG worker
and between sex and being a household con-
tact of a WRG worker. Although the odds of
nding a pleural abnormality is higher for for-
mer WRG workers than for nonworkers, the
magnitude of the OR decreases with increasing
age. Table 5 shows that estimated odds of nd-
ing a pleural abnormality ranges from 4.18 for
a 30-year-old former WRG worker compared
with a nonworker, to 1.14 for a 90-year-old
former worker compared with a non-WRG
worker, after adjusting for all other variables in
the model. The model also shows that the esti-
mated odds of nding a pleural abnormality is
almost five times greater for males than for
females (among nonhousehold contacts) after
adjusting for other variables in the model (OR, 4.84; 95% CI, 3.836.11). The estimated
odds of nding a pleural abnormality is 3.62
(95% CI, 2.704.83) times greater for females
who were household contacts of former WRG
workers when compared with females who
were not. The corresponding increased odds of
pleural abnormalities among male household
contacts is 1.71 (95% CI, 1.322.22). As age increases, the odds of finding a pleural abnormality increase, although the
rate of that increase slows. For example,
among non-WRG workers, the estimated
odds of finding a pleural abnormality for a
40-year-old was three times greater than for a
30-year-old (OR, 3.04; 95% CI, 2.693.43).
However, the odds reduce to 2.02 (95% CI,
1.872.18) when comparing a 60-year-old
with a 50-year-old. This effect is slightly
attenuated for age contrasts involving former
WRG workers (OR = 2.45 for a 40-year-old
vs. a 30-year-old; OR = 1.63 for a 60-year-old
vs. a 50-year-old). Among the recreational
exposure pathways, those who played in the Environmental Medicine | Peipins et al. 1756 VOLUME 111 | NUMBER 14 | November 2003 Environmental Health Perspectives Table 3. Number (percent) of pleural and interstitial abnormalities by exposure pathways. Exposure pathway Pleural, all views Interstitial, P-A view All participants 18 years of age (n = 6,668) 1,186 (17.8) 54 (0.8) Ever worked for WRG (n = 365) 186 (51.0) 14 (3.8) Secondary contractor work (n = 463) 161 (34.8) 8 (1.7) Dust exposure at non-WRG jobs (n = 2,390) 527 (22.1) 21 (0.9) Vermiculite exposure at non-WRG jobs (n = 1,100) 254 (23.1) 8 (0.7) Worked non-WRG job As a pipe or steam tter (n = 164) 54 (32.9) 2 (1.2) As a plumber (n = 168) 50 (29.8) 3 (1.8) As a brake repair person (n = 247) 68 (27.5) 2 (0.8) As an insulator (n = 240) 44 (18.3) 1 (0.4) As a dry wall nisher (n = 221) 41 (18.6) 0 (0.0) As a carpenter (n = 566) 129 (22.8) 2 (0.4) As a roofer (n = 361) 79 (21.9) 2 (0.6) As an electrician (n = 187) 48 (25.7) 3 (1.6) As a welder (n = 465) 122 (26.2) 5 (1.1) Mixing, cutting, or spraying asbestos (n = 143) 47 (32.9) 2 (1.4) In a shipyard or performed ship construction or repair (n = 129) 45 (34.9) 4 (3.1) Exposed to asbestos (n = 849) 208 (24.5) 7 (0.8) Worked around anyone performing previous 12 jobs (n = 801) 184 (23.0) 6 (0.8) Asbestos exposure in the military (n = 182) 78 (42.9) 2 (1.1) Lived with WRG workers (n = 1,376) 358 (26.0) 17 (1.2) Vermiculite insulation in homes (n = 2,819) 600 (21.3) 27 (1.0) Asbestos products in homes (n = 1,071) 222 (20.7) 9 (0.8) Used vermiculite for gardening (n = 3,469) 702 (20.2) 29 (0.8) Used vermiculite around the home (n = 586) 120 (20.5) 1 (0.2) Handled vermiculite insulation Sometimes (n = 2,739) 572 (20.9) 25 (0.9) Frequently (n = 732) 191(26.1) 9 (1.2) Recreational activities along Rainey Creek Sometimes (n = 3,134) 546 (17.4) 21 (0.7) Frequently (n = 1,433) 307 (21.4) 15 (1.1) Played at ball eld near expansion plant Sometimes (n = 2,034) 293 (14.4) 10 (0.5) Frequently (n = 2,290) 432 (18.9) 16 (0.7) Played in vermiculite piles Sometimes (n = 1,430) 269 (18.8) 8 (0.6) Frequently (n = 875) 220 (25.1) 5 (0.6) Popped vermiculite Sometimes (n = 2,140) 460 (21.5) 13 (0.6) Frequently (n = 538) 138 (25.7) 9 (1.7) Other contact with vermiculite Sometimes (n = 1,759) 340 (19.3) 14 (0.8) Frequently (n = 323) 73 (22.6) 5 (1.6) vermiculite piles frequently had twice the risk
of pleural abnormalities compared with those
who never played in the piles (OR, 2.02; 95%
CI, 1.592.57). The rate of interstitial abnormalities increased from 0.04% in younger adults
1844 years of age, to 0.5% for participants
4465 years of age, to 3.2% for participants 65 years of age. Males had a higher rate of
interstitial abnormalities (1.2%) compared
with female participants (0.4%). The crude
OR for interstitial abnormalities among males
compared with females was 2.68 (95% CI,
1.474.86). Current and former smokers (ever
smokers) were twice as likely to have ndings
of interstitial abnormalities than were those
who never smoked (crude OR, 2.18; 95% CI,
1.912.49). BMI was not associated with
interstitial abnormalities. Participants residing
in the Libby area for > 34 years had a 4-fold
increased risk of interstitial abnormalities
(crude OR, 4.38; 95% CI, 1.8210.54). We assessed the independent contributions of the exposure pathways and covariates to the
risk of interstitial abnormalities using multi-
variate logistic regression. The strongest predic-
tors of interstitial abnormalities were having
been a former WRG worker (OR, 2.71; 95%
CI, 1.265.87) and being male (OR, 2.48;
95% CI, 1.085.68). Increasing age, having
worked on a ship or done ship repair, and hav-
ing a history of pneumonia were also signi-
cantly associated with interstitial abnormalities. The models above demonstrate the relative importance of various exposure pathways and
covariates in predicting pleural and interstitial
abnormalities. However, the majority of par-
ticipants reported multiple pathways rather
than a single exposure pathway. Only 2% of
the participants reported no to all exposure
pathways, whereas 48.1% of the participants reported six or more exposure pathways. The
prevalence rates for pleural and interstitial
abnormalities among participants with multi-
ple exposures compared with those with no
apparent exposures is displayed in Figure 1.
An exposureresponse relationship is apparent
between the number of exposure pathways
and the prevalence of pleural abnormalities.
Among those who reported no to all expo-
sure pathways, 6.7% had pleural abnormali-
ties, compared with an almost 35% prevalence
among those reporting 12 pathways. An exposureresponse relationship is not seen for
interstitial abnormalities, which had a preva-
lence of about 1% regardless of the number of
pathways reported. This trend in increasing
pleural abnormalities with increasing number
of pathways could not be solely attributed to
former WRG workers. This relationship was
apparent, although slightly attenuated, even
after we removed the workers from the analy-
sis (30.1% prevalence of pleural abnormalities
among those reporting 12 pathways). Table 6 presents the results from an uncondi-
tional logistic regression model of this rela-
tionship controlling for age, sex, BMI,
smoking, and residential duration. There was
a statistically significant trend of increased
pleural abnormalities with increasing number
of pathways, with ORs ranging from 1.40 for
one pathway to 3.75 for 12 pathways. Discussion In this analysis we sought to identify radi-
ographic abnormalities and significant expo-
sure pathways among participants in a
community-based medical testing program in
Libby, Montana. Almost 18% of the partici-
pants undergoing radiography had pleural
abnormalities identied by at least two of three
certified B-readers. Interstitial abnormalities were identified in 1% of participants. These
ndings are consistent with clinical reports by
Libby area physicians that patients more com-
monly have pleural abnormalities and that
interstitial disease is generally diagnosed at a
later stage (Whitehouse 2000). Pathologic
effects of asbestos and of asbestos-contami-
nated vermiculite have been established for
both the pleura and parenchyma (Amandus
et al. 1987a; Becklake 1976; Lockey et al.
1984; McDonald et al. 1986b), and the sever-
ity of these effects has been associated with
latency, duration, and intensity of exposures
among workers. Our findings of predomi-
nantly pleural effects suggests less prolonged or
intense exposures and/or shorter latency peri-
ods. Alternatively, the cross-sectional study
design may have removed a disproportionate
number of people suffering or dying from
more severe disease. Furthermore, differential
respiratory health effects observed in a number
of asbestos-exposed populations also may be
due to differences in the physical and chemical
properties of the mineral fiber exposures
(ATSDR 2001). Our analyses further demonstrated a statis- tically signicant increase in the prevalence of
pleural abnormalities with an increasing num-
ber of exposure pathways. Participants report-
ing more pathways might be expected to have
more cumulative exposure than would those
reporting fewer pathways. Those who reported 12 exposure pathways had a prevalence rate
of almost 35% for pleural abnormalities, com-
pared with a prevalence of 6.7% for those who
reported no exposure pathways. No directly
comparable Montana or U.S. population stud-
ies are available to estimate the rate of pleural
abnormalities among those in Libby with no
work-related exposures. Studies of differing
groups within the United States believed to
have no substantive work-related asbestos
exposures have found prevalence of pleural
abnormalities ranging from 0.2% among blue-
collar workers in North Carolina (Castellan
et al. 1985), to 2.3% among patients at
Veterans Administration hospitals in New
Jersey (Miller and Zurlo 1996), to 4.6% among
urban New Jersey residents (Anderson et al.
1979). Subjects in our category of no appar-
ent exposure had a greater rate of pleural
abnormalities (6.7%) than did those in the Environmental Medicine | Exposure to asbestos-contaminated vermiculite in Libby, Montana Environmental Health Perspectives VOLUME 111 | NUMBER 14 | November 2003 1757 Table 4. Risk of pleural abnormalities by exposure pathways and covariates. a Variable Level Beta Adjusted OR (95% CI) Intercept 19.46 Workwr Yes 2.08 b HHWR Yes 1.29 Vermplay Sometimes 0.60 1.82 (1.482.25) Frequently 0.70 2.02 (1.592.57) Sex Male 1.58 Resdur (years) 1421 0.25 1.29 (0.971.71) 2233 0.22 1.25 (0.961.62) 34 0.75 2.12 (1.662.70) ln(Age) 3.86 BMI 2nd quartile (2427) 0.18 1.20 (0.911.60) 3rd quartile (2831) 0.56 1.75 (1.322.32) 4th quartile ( 32) 1.14 3.12 (2.374.12) Smoke Ever 0.30 1.35 (1.141.59) Age