Community asbestos exposure from Libby vermiculite Minneapolis. We identified 2,222 members of the original NMCVI cohort who were never employed at the WM/WRG plant, were not a household contact of WM/WRG employ- ees, and were first at risk of exposure to the contaminated vermiculite before 1980. The latter inclusion criterion ensured an adequate latency period for effects of the asbestos expo- sure to become apparent. To obtain a sample representative of the range of community exposures, the popula- tion was stratified into groups to represent three exposure scenarios: a) intense intermit- tent exposure, b) long-term high ambient background exposure, and c) low ambient background exposure. We classified people with a childhood history of playing in the piles of waste rock outside the plant as the group with intense intermittent exposures to potentially high concentrations of asbes- tos fibers. The long-term high and low back- ground exposure groups were selected based on residential history and frequency matched to the age distribution of the intense intermit- tent exposure category. All members of the cohort were classified by cumulative expo- sure experienced before 1 January 1980. The exposure estimates were based on the period- specific air dispersion model developed by the Minnesota Pollution Control Agency and the MDH and on residential history. The initial model estimated exposure based on the amount of time the individual lived in the affected area multiplied by the yearly average plant emissions rates (Kelly et al. 2006; MDH 2003). Excluding persons identified as having intense intermittent exposure and those who did not have appropriate residence data, a total of 1,839 individuals were classifiable by background exposure. The long-term high background exposure group was initially iden- tified as the upper quartile of this exposure distribution. The low-exposure category was selected from the lowest quartile of this distri- bution. When the available samples from the upper and lower quartile were exhausted, the second and third quartiles were entered into the selection protocol. This selection protocol ensured a distribution of estimated exposure. The institutional review board protocol required MDH, as the institution of record for the NMCVI, to initiate contact. MDH forwarded the names and contact informa- tion of individuals interested in learning more about the study to the University of Minnesota. The potential participants were sent a letter and study brochure, along with consent and HIPAA (Health Insurance Portability and Accountability Act) forms. This was followed with a call from a trained telephone interviewer to review the study pur- pose, procedures, requirements, consent, and HIPAA forms and to answer any questions. Those who chose to participate signed and returned the consent and HIPAA forms to the study office. Upon receiving the signed consent and HIPAA forms, a second packet containing a cover letter, clinic information, and question- naire was mailed to each participant. An inter- viewer followed-up by telephone to ensure the instructions and procedures were understood and to encourage the participant to promptly attend a participating clinic and return the completed questionnaire. Radiographic measurements. Participants attended a local contract medical clinic that specializes in occupational health screen- ing and could perform plain-film X-rays for B-reading. The clinics offered evening and weekend appointments to accommodate vari- ous schedules. The prevalence of pleural abnor- malities, parenchymal opacities, and other evidence of asbestosis was determined with a single posteroanterior (PA) radiograph follow- ing the recommendations of the International Labour Organization (ILO 2002). The radiographs were reviewed and scored by National Institute for Occupational Safety and Health–certified B-readers using standard methodology (ILO 2002). The B-readers were board-certified, practicing radiologists from a practice in the Minneapolis–St. Paul metro- politan area. The presence of pleural abnor- malities consistent with pneumoconiosis, both pleural thickening and pleural plaques, was the primary indicator of asbestos-induced changes in the lungs. The final classification of radiographic changes consistent with pneumo- coniosis required consensus by two B-readers. If there were discrepancies between two ini- tial readers regarding pleural abnormalities or the presence of parenchymal abnormalities with an ILO classification of 1/0 or above, the radiographs were reviewed by a third certified B-reader. The clinical interpretations were summarized and sent to the participants and their health care provider if requested. Exposure assessment. We assessed expo- sure to asbestos fibers through ambient expo- sure and activities that brought participants in contact with the waste rock. The details of the exposure assessment are summarized here and reported in full elsewhere (Adgate et al. 2011). Briefly, background exposure was determined by length of residence in the affected commu- nity and estimates of airborne fiber concentra- tion. The MDH study ascertained residential histories within the affected community for all study participants (Kelly et al. 2006). Air dispersion modeling and deposition were used to characterize cumulative potential asbes- tos background exposure for this population (Kelly et al. 2006; MDH 2005). The models evaluating facility emissions and resulting air concentrations and deposition accounted for prevailing weather patterns, emission con- trols installed in the plant, and post-shutdown Environmental Health Perspectives • volume 120 | number 1 | January 2012 emissions due to fugitive dust and construc- tion activities. Ambient fiber concentra- tions during peak production years before installation of air pollution control equip- ment (1938–1972) varied by several orders of magnitude, with concentrations ranging from 0.026 fibers/cm3 (f/cc) within one to two blocks of the plant to 0.0001 f/cc at the margins of the neighborhood. The exposures for this study were estimated for the period of plant operation (1938–1989). Cumulative background exposure (CBG; fibers per cubic centimeter × months) was estimated by sum- ming duration and period-specific airborne fiber concentration for each subject: CBGi = Σ BGijTj , [1] where BGij is monthly average airborne asbes- tos fiber concentration (fibers per cubic centi- meter) for subject i from the air dispersion model during the jth time period, and Tj is the number of months subject i lived in the impact zone of WM/WRG plant emissions. Potential activity-based exposure path- ways were ascertained by questionnaire for the NMCVI study. The interviews solicited self-reports of direct contact from moving waste rock from WM/WRG plant, using waste rock at home (e.g., on their lawns or gardens), installing or removing vermiculite insulation, and playing in or around waste piles at the plant. Exposures were estimated based on probable ranges of the activity durations, the frequency of the activities, and ranges of potential airborne fiber concen- trations. The potential airborne fiber concen- trations were derived from experimental data that reconstructed these activities in the U.S. EPA Libby investigation (Weis 2001a, 2001b). Uniform distributions were assumed based on the following ranges for each activity: moving waste rock from plant, 0.07–0.14 f/cc; using waste rock at home, 0.02–0.227 f/cc; install- ing or removing vermiculite insulation, 0.142–0.568 f/cc; and playing in or around waste piles at the plant, 0.14–1.72 f/cc. Monte Carlo simulation software (Crystal Ball 2000, version 2.2; Decisioneering, Boulder, CO) was used to estimate the probability distribution of asbestos exposures (fibers per cubic centimeter × months) for each activity (k) based on the assumption that during these activities each individual was exposed to a range of asbestos concentrations with varying exposure durations that ranged from 0.25 to 8 hr. The cumulative activity concentration (CAC; fibers per cubic centimeter × months) for each activity (k) was estimated as follows: CACik = (AC × T × F × 12) ÷ (24 × 365), where for each individual i, AC is activity- specific asbestos concentration in air (fibers [2] 45