the waste rock or ever playing on waste piles had ORs of 3.60 (95% CI: 1.71, 7.58) and 3.23 (95% CI: 1.59, 6.52), respectively. When background exposure was added as a covariate, ORs decreased to 2.78 (95% CI: 1.26, 6.10) and 2.17 (95% CI: 0.99, 4.78), respectively. When examined by exposure percentiles, the strength of association increased in the unadjusted models, but a monotonic expo- sure response beyond the 50th percentile was not evident in the adjusted models. The ORs for the third and fourth quartiles, compared with exposures below the median, were also attenuated when the background exposure was accounted for. When all sources of expo- sure were combined, the prevalence ORs were 3.79 (95% CI: 1.76, 8.16) and 1.05 (95% CI: 0.44, 2.50), respectively, for the fourth and third quartiles of exposure compared with exposures below the median. Discussion We observed an association between the prev- alence of pleural abnormalities and estimates of environmental exposure to asbestos fibers attributed to processing Libby vermiculite. These results support the hypothesis that non- occupational community exposure to asbestos fibers from the processing of Libby vermicu- lite produced measurable effects. Associations were most consistent with long-term, lower exposure, as represented by background expo- sure, in contrast to more intense but intermit- tent exposure from specific activities. These results must be considered in con- text with the following limitations of the study. The study is small, which limited esti- mate precision and our ability to explore more complex exposure models. The requirement of attending a clinic for an X-ray, with no explicit benefit to the participant, likely limited study participation, and it is possible that participa- tion was influenced by known health status. However, arguments may be constructed to favor a positive or negative bias in the results. Our metrics of activity-based exposure were derived from self-reported activities as assessed in 2001 when the cohort was enumerated. Although this provided excellent baseline information for the whole cohort, assump- tions about the duration of the activity were needed, which likely introduced some expo- sure misclassification. An air dispersion model was used for retrospective estimation of back- ground exposure based on residential history and proximity to the vermiculite plant. This model assumes an equivalent exposure oppor- tunity for all people, regardless of where they spent their day. Assessment of radiographic pleural abnormalities was accomplished with a single PA chest radiograph, and although this is a standard screening practice, pleural abnor- malities may be missed without evaluating multiple views. More sensitive assessments of Environmental Health Perspectives • volume 120 | number 1 | January 2012 Characteristic Totalb Sex Male Female Birth year ≤ 1940 1941–1950 1951–1960 ≥ 1960 Played on waste piles Moved waste from WM/WRG plant Used waste at home, lawn, or garden Installed/removed vermiculite insulation Any contact with the waste Any activities leading to exposure Any asbestos-exposed job Total exposure, f/cc × months [median (IQR)] Background exposure [median (IQR)] Activity-based exposure [median (IQR)] Model 1 2 3 4 5 6 Parameter Background exposure Total activity exposure Cumulative exposure Background exposure Total activity exposure Pile-playing exposure Pile-playing exposure Background exposure Community asbestos exposure from Libby vermiculite Table 3. Characteristics of study participants with and without pleural abnormalities. pleural abnormalities can be done with com- puted tomography (CT) scans; however, cost, potentially unnecessary exposure to ionizing radiation, and risk of frequent false-positive findings reduce the utility of CT scans for screening low-risk individuals. False-positive misclassification was reduced by accepting pleural abnormalities only when there was con- sensus by two B-readers. The initial assessment of the two primary readers detected 71 pleural abnormalities consistent with pneumoconiosis, whereas the consensus readings only found 49. In a number of the disputed assessments of pleural abnormalities ultimately classified as normal, the clinic notes from the dissenting B-reader postulated the presence of pleural shadows from fat. Associations were attenuated if the 22 participants with disputed readings were classified as having pleural abnormali- ties. For example, the regression coefficient for background exposure was reduced from 0.401 (95% CI: 0.165, 0.637) to 0.146 (95% CI: –0.091, 0.382), and cumulative exposure was reduced from 0.283 (CI: 0.104, 0.463) to 0.168 (95% CI: 0.014, 0.323). An estimated 138,000 tons of Libby ore were processed at the WM/WRG plant in Minneapolis. The resulting emissions and subsequent use of the waste rock contaminated areas up to 2 miles from the plant (Kelly et al. 2006). Our exposure models combined ambi- ent dispersion and activity-based estimates that demonstrate substantial nonoccupational asbestos exposures for residents of this neigh- borhood, albeit with considerable variability by exposure pathway (Adgate et al. 2011). Although median exposure estimates for the cohort were driven by direct plant emissions, contact with waste rock resulted in substan- tial additional exposures to some individuals. Most important, playing on piles of waste rock was the largest contributor to high-end expo- sures for the subpopulation of individuals who engaged in these activities. Previous research in Libby has shown that playing on piles of waste rock is associated with changes in lung function and is a risk factor for asbestos-related disease (Horton et al. 2006, 2008; Peipins et al. 2003). Although these studies of retrospective com- munity exposure to asbestos explored the rela- tionship between Libby asbestos and various health effects, most of them did not provide individual estimates of cumulative exposure. Our estimates of cumulative exposure con- centrations (fibers per cubic centimeter × months) Pleural abnormalitiesa Values are n (%), except as noted. aConsensus by two B-readers. bExcludes 7 cases with X-rays that could not be classified by B-readers. Table 4. Regression coefficients from logistic regression models for associations between prevalence of pleural abnormalities and log concentration of background, activity, and total exposure. aAdjusted for year of birth, history of having an asbestos-exposed job, and sex. Yes 49 (100) 39 (79.6) 10 (20.4) 19 (38.8) 20 (40.8) 8 (16.3) 2 (4.1) 28 (57.1) 13 (26.5) 8 (16.3) 8 (16.3) 35 (71.4) 33 (67.3) 22 (44.9) 0.202 (0.032–0.480) 0.079 (0.023–0.161) 0.013 (0.0–2.72) β-Estimatea ± SE (95% CI) 0.401 ± 0.120 (0.165, 0.637) 0.098 ± 0.036 (0.028, 0.168) 0.283 ± 0.091 (0.104, 0.463) 0.322 ± 0.125 (0.078, 0.567) 0.063 ± 0.039 (–0.013, 0.139) 0.080 ± 0.030 (0.021, 0.139) 0.042 ± 0.034 (–0.025, 0.109) 0.331 ± 0.130 (0.076, 0.585) No 405 (100) 198 (48.9) 207 (51.1) 67 (16.5) 116 (28.6) 140 (34.6) 82 (20.2) 147 (36.3) 37 (9.1) 34 (8.4) 35 (8.6) 182 (44.9) 167 (41.2) 105 (25.9) 0.049 (0.012–0.202) 0.028 (0.008–0.066) 0.0 (0.0–0.079) p-Value 0.001 0.006 0.002 0.009 0.102 0.008 0.221 0.011 47