Alexander et al. do not directly translate to the existing health- based standards. The current Occupational Safety and Health Administration (OSHA) permissible exposure limit, which has been reduced con- siderably in the last 40 years (ATSDR 2001), is 0.1 f/cc based on an 8-hr time-weighted average (OSHA 1995). The estimated median asbestos concentrations in U.S. cities range from 10–8 to 10–4 f/cc (ATSDR 2001). At 10–4 f/cc the range of cumulative exposure from 20 to 40 years would be 0.024–0.048 f/cc-month, which is in the lower end of the exposure distribution we describe. The median concentration for the participants with pleural abnormalities was an order of magnitude greater. Thus, exposure esti- mates developed for this study were well above urban background. When converted to a scale of fibers per cubic centimeters × year, the upper end of the exposure distribution for this study was within the lower quartile exposure ranges in which pleural abnormalities were observed in workers in Libby (Horton et al. 2008; Rohs et al. 2008). The impact of Libby asbestos exposure on the health of employees of the Libby ver- miculite mines and mills has been well doc- umented. Mortality studies of W.R. Grace mine workers in Libby have found signifi- cantly increased mortality from nonmalignant respiratory disease and lung cancer (Amandus and Wheeler 1987). A more recent update of an occupational cohort of 406 vermiculite miners employed before 1963 and followed through 1999 reported an excess of lung can- cer deaths [44 deaths; standardized mortality ratio (SMR) = 2.40], nonmalignant respira- tory deaths (51 deaths; SMR = 3.09), and all-cause deaths (285 deaths; SMR = 1.27), which increased with cumulative exposure (McDonald et al. 2004). A total of 12 meso- thelioma deaths had occurred by the end of 1999, representing 4.2% of 285 deaths in the cohort since July 1983. These associations continue to be corroborated through addi- tional follow-up of this working population (Sullivan 2007). Health effects from occupa- tional exposures to Libby asbestos in opera- tions beyond Libby are emerging. Workers at a vermiculite expansion plant that produced material primarily for lawn care products were found to have increased risk of pleural abnor- malities at exposures lower than those seen in Libby (Rohs et al. 2008). Beginning in 2000, ATSDR began inves- tigating the effects of occupational and non- occupational exposure to Libby asbestos in the Libby community. A study of death cer- tificates from 1979 through 1998 found a 40- to 60-fold elevation in asbestosis deaths in Libby (Horton et al. 2006). These deaths occurred primarily among former employees. A respiratory health screening investigation Table 5. Number of pleural abnormalities (n = 49) and prevalence OR estimates by categorical exposure level for background exposure, activity-based exposure, and total exposure [OR (95% CI)]. Unadjusteda Adjustedb Yes Yes Exposure Any contact with waste No (referent) Ever played on piles No (referent) Activity exposure (f/cc × months)c Pile-playing exposure (f/cc × months)c Background exposure (f/cc × months)d < 0.082 ≥ 0.422 0.082 to < 0.422 < 0.158 ≥ 0.549 0.158 to < 0.549 < 0.034 ≥ 0.077 0.034 to < 0.077 Total exposure (f/cc × months)d < 0.0523 0.0523 to < 0.245 ≥ 0.245 Sex Female Male Exposed job No Yes 48 n 14 35 21 28 32 9 8 36 6 7 16 8 25 17 11 21 10 39 22 27 3.60 (1.71, 7.58) 3.23 (1.59, 6.52) 2.04 (0.83, 5.00) 2.36 (0.92, 6.05) 1.45 (0.53, 3.98) 2.49 (1.75, 9.41) 0.91 (0.36, 2.31) 3.18 (1.53, 6.59) 1 1 1 1 1 — — — — — — — 1 1 1 1 1 1 1 1 2.78 (1.26, 6.10) 2.17 (0.99, 4.78) 1.34 (0.52, 3.44) 1.37 (0.49, 3.83) 0.97 (0.34, 2.77) 1.31 (0.44, 3.85) 0.85 (0.32, 2.11) 2.52 (1.15, 5.50) 1.05 (0.44, 2.50) 3.79 (1.76, 8.16) 3.8 (1.6, 8.9) 1.31 (0.63, 2.73) conducted in Libby, Montana (Peipins et al. 2003), found radiographic pleural and inter- stitial abnormalities associated with occupa- tional exposure among former W.R. Grace employees. Among 365 employees screened, 51% had pleural abnormalities and 4% had interstitial abnormalities as determined by at least two B-readers. Among commu- nity members in Libby, the rates of pleu- ral abnormalities were higher among males and increased with age, smoking history, and body mass. Increased prevalence of pleural abnormalities was also found among female household contacts of W.R. Grace employees (OR = 3.62; 95% CI: 2.70, 4.83) compared with females who were not household con- tacts. Participants living > 34 years in the Libby area also were at increased risk (OR = 2.12; 95% CI: 1.66, 2.70) compared with persons living < 14 years in the area. People who played on vermiculite waste piles fre- quently had twice the prevalence of pleural abnormalities as those who never played in the piles (OR = 2.02; 95% CI: 1.59, 2.57). Persons exposed through multiple exposure pathways were also at increased risk. Further evidence (Whitehouse et al. 2008) of poten- tial nonoccupational health effects in Libby has recently been documented and includes 31 cases of mesothelioma that had no docu- mented occupational exposure to asbestos but were exposed to emissions from the plant in Libby. A report (Whitehouse 2004) of 123 Libby patients with occupational and non- occupational exposure indicated progressive loss of pulmonary function associated with pleural changes in 94 individuals. Environmental and secondary occupa- tional exposures to asbestos have been associ- ated with malignant mesothelioma and benign pleural disease in several settings (Ferrante et al. 2007; Reid et al. 2008, 2009). Although these results support the notion that non- occupational asbestos exposure has hazard- ous consequences, the exposures encountered were much higher than seen in our study. Low-level persistent exposure to asbes- tos in other populations also has been associated with measurable changes in the lungs. “Endemic pleural plaques” have been described in areas of the world where min- eral fibers occur naturally in soil or are used locally for whitewashing (Ates et al. 2010; Karakoca et al. 1998; Metintas et al. 2005; Yazicioglu 1976; Yazicioglu et al. 1980). We report a stronger association between the prevalence of pleural changes and esti- mated cumulative exposure acquired over many years compared with higher intermit- tent peak exposures. The reason for this is not clear. It may speak to the scale of expo- sure estimates from activity-based exposure or an overestimate of the frequency of expo- sure through activities, such as pile playing. aUnadjusted for corresponding exposure covariates. All models are adjusted for year of birth, sex, and history of an asbestos-exposed job. bAdjusted for corresponding exposure. Activity-based exposure is adjusted for background exposure, and background exposure is adjusted for total activity exposure. cCut points are based on the 50th and 75th percentile of those with any activity. All participants without activity are included in the referent category. dCut points are at the 50th and 75th percentile of the exposure distribution. ovlume 120 | number 1 | January 2012 • Environmental Health Perspectives