Alexander et al. per cubic centimeter ), T is exposure duration (hours/day), and F is frequency the activity occurred (total days). The expression ACk × Tk × F k has units of fibers per cubic centi- meter × hours. The remainder of the expres- sion [12 months/year ÷ (24 hr/day × 365 days/year)] converts the result to fibers per cubic centimeter × months. The total cumulative asbestos exposure (TCE; fibers per cubic centimeter × months) for each individual is the sum of the estimated background levels and the median values of the simulation output distribution for each individual: TCEi = Σk CBGi + CACik. [3] To focus on the community exposure, the population for this study explicitly excluded employees of WM/WRG and their household members. However, other occupations also have the potential for asbestos exposure. To characterize this potential exposure, we used responses from the NMCVI questionnaire that asked about ever employment in 15 occupa- tions that have potential for asbestos exposure. Statistical analysis. The prevalence odds ratios (ORs) of pleural changes were modeled with multivariate logistic regression for the following exposure metrics: background expo- sure, total activity-based exposure, exposure acquired as a child when playing on the waste piles, and total exposure. We used cumula- tive exposure metrics, expressed as fibers per cubic centimeter × months. The exposure data were log-normally distributed, and model fit, as determined by likelihood ratio statistics, was improved by using the log-transformed variable. We categorized the exposure data as < 50th, 50th–75th, and > 75th percentiles for background, total exposure, and activity exposure for those with any activity, respec- tively. The models were adjusted for sex, year of birth, and ever holding a job with potential asbestos exposure as reported on the NMCVI questionnaire. Missing exposure data were imputed with a multiple imputation algo- rithm (SAS MI procedure, version 9.2; SAS Institute Inc., Cary, NC) based on the expo- sure data for all eligible persons (n = 2,222), creating 18 replicate imputed data sets. We imputed missing data for activity-based expo- sures for only those individuals reporting the activity. For descriptive purposes, we sum- marized the imputed data with the median value for each parameter. Logistic regression models using the 18 imputed data sets were summarized using the MIANALYZE proce- dure in SAS (version 9.2). Prevalence ORs are reported to describe the associations, and 95% Wald’s confidence intervals (CIs) are presented to characterize the precision of the estimates. Results From the original 2,222 persons eligible, 1,765 were randomly selected from the exposure groups and were eligible for contact by the MDH, and 1,133 assented to be contacted by the University of Minnesota. Of the remain- ing 632, 68 were deceased, 367 declined, and 197 could not be reached. A total of 698 con- sented to participate, and 461 completed the clinic visit. Compared with nonparticipants, those who attended the clinic were more likely to be male, born after 1940, and nonsmokers and to have activities that would lead to expo- sure (Table 1). The percentage of participants and nonparticipants who reported having an asbestos-exposed job was essentially the same. Playing on the waste piles was reported by 180 (39%) of the 461 participants, and 223 (48%) reported at least one activity resulting in expo- sure to the waste material. Prevalence of pleural abnormalities con- sistent with pneumoconiosis reported by any B-reader was 15.4% (Table 2). A consensus determination of pleural abnormalities was Table 1. Characteristics of the original eligible study population (2,222 persons) who did and did not attend the clinic [n (%)]. Attended clinic Characteristic Total Sex Male Female Birth year ≤ 1940 1941–1950 1951–1960 ≥ 1960 Ever a regular smoker Played on waste piles at plant Moved waste from WM/WRG plant Used waste at home, lawn, or garden Installed/removed vermiculite insulation Any activities leading to exposure Ever held any asbestos-exposed job 46 made for 10.6% of the participants. Seven participants had X-rays of insufficient quality for B-reads; thus, the final prevalence of con- sensus abnormalities was 10.8%. Consensus readings of diffuse pleural thickening and pleural plaques were reported in 5 and 45 par- ticipants, respectively. Parenchymal abnormali- ties equal to or exceeding ILO category 1/0 were recorded by at least one B-reader in 18 participants, and 9 of these had the consensus of two readers. One of these had a film coded as unreadable by one B-reader, so 8 were clearly ILO ≥ 1/0. There were too few parenchymal abnormalities to statistically evaluate, but of the 8 with a consensus of ≥ 1/0, all had some evidence of exposure through an activity, 6 of which reported playing in the waste piles. The 8 individuals were distributed across the back- ground and total exposure estimates, with 3, 2, and 3 in each of the upper three quartiles. Half of the parenchymal abnormalities also reported having a job with potential asbestos exposure. Participants diagnosed with pleural thick- ening or pleural plaques were older and more likely to be male, to have held an asbestos- exposed job, and to report activities associ- ated with contaminated vermiculite contact (Table 3). The exposure distribution for back- ground exposure and activity-based exposure followed a pattern of higher exposure in those with pleural abnormalities. In models adjusting for age, sex, and history of any asbestos-exposed job, the log-transformed measures of background exposure, exposure from pile playing, total activity exposure, and total exposure were positively associated with pleural abnormali- ties (Table 4). When both background and activity-based metrics were in the model, the background exposure maintained the stron- gest association with pleural abnormalities. The OR for a difference equivalent to the interquartile range (IQR) of total exposure (IQR = 0.014–0.245 = 0.231 f/cc-months) was 2.22 (95% CI: 1.34, 3.68). When evaluated categorically, the associa- tion between pleural abnormalities and back- ground exposure appeared limited to exposure at or above the 75th percentile (Table 5). Persons reporting ever having contact with Table 2. Summary of radiographic results for all participants attending the clinic (n = 461). Radiograph interpretation Any pleural abnormalitya Pleural abnormality,a consensus of two readers Diffuse pleural thickening, consensus of two readers Any pleural plaques observed Pleural plaques, consensus of two readers Any parenchymal abnormalityb Parenchymal abnormality,b consensus of two readers aConsistent with pneumoconiosis. bAt ILO ≥ 1/0. n (%) 71 (15.4) 49 (10.6) 5 (1.1) 70 (15.2) 45 (9.8) 18 (3.9) 9 (2.0) ovlume 120 | number 1 | January 2012 • Environmental Health Perspectives Yes 461 241 (52.3) 220 (47.7) 89 (19.3) 139 (30.2) 148 (32.1) 85 (18.4) 242 (52.5) 180 (39.0) 51 (11.1) 44 (9.5) 44 (9.5) 223 (48.4) 129 (28.0) No 1,761 860 (48.8) 901 (51.2) 654 (37.1) 374 (21.2) 385 (21.9) 348 (19.8) 1,040 (59.1) 401 (22.8) 126 (7.2) 109 (6.2) 129 (7.3) 569 (32.3) 479 (27.2)