Research A Retrospective Assessment of Occupational Exposure to Elemental Carbon in the U.S. Trucking Industry Mary E. Davis,1,2 Jaime E. Hart,3,4 Francine Laden,2,3,4 Eric Garshick,3,5 and Thomas J. Smith 2 1Department of Urban and Environmental Policy and Planning, Tufts University, Medford, Massachusetts, USA; 2Exposure, Epidemiology and Risk Program, Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA; 3Channing Laboratory, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts, USA; 4Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA; 5Pulmonary and Critical Care Medicine Section, Medical Service, VA Boston Healthcare System, West Roxbury, Massachusetts, USA Background : Despite considerable epidemiologic evidence about the health effects of chronic exposure to vehicle exhaust, efforts at defining the extent of risk have been limited by the lack of his- torical exposure measurements suitable for use in epidemiologic studies and for risk assessment. o Bjectivs : We sought to reconstruct exposure to elemental carbon (EC), a marker of diesel and other vehicle exhaust exposure, in a large national cohort of U.S. trucking industry workers. Methods : We identified the predictors of measured exposures based on a statistical model and used this information to extrapolate exposures across the cohort nationally. hTese estimates were adjusted for changes in work-related conditions over time based on a previous exposure assessment of this industry, and for changes in background levels based on a trend analysis of historical air pollution data, to derive monthly estimates of EC exposure for each job and trucking terminal combination between 1971 and 2000. r esult : Occupational exposure to EC declined substantially over time, and we found significant variability in estimated exposures both within and across job groups, trucking terminals, and regions of the United States. Average estimated EC exposures during a typical work shift ranged from < 1 gμ/m 3 in the lowest exposed category in the 1990s to > 40 gμ/m 3 for workers in the high- est exposed jobs in the 1970s. c onclusi : Our results provide a framework for understanding changes over time in exposure to EC in the U.S. trucking industry. Our assessment should minimize exposure misclassification by capturing variation among terminals and across U.S. regions, and changes over time. k ey words : air pollution, diesel, lung cancer, occupational health, traffic exposure, trucking indus- try. Environ Health Perspect 119:997–1002 (2011). doi:10.1289/ehp.1002981 [Online 29 March 2011] Although there is considerable concern about the health effects of chronic exposure to vehi- cle exhaust, efforts to define the extent of risk have been limited by the lack of historical exposure measurements suitable for use in epidemiologic studies and for risk assessment. There is no standard accepted practice to ret- rospectively estimate exposure levels. Despite significant progress and research on this topic in the 1990s, historical exposure recon- struction remains a demanding and time- consuming process (Ward et al. 2003) that is driven primarily by the available data (Davies et al. 2009; Stewart 1999), which are often incomplete or missing for important histori- cal exposure periods (Seixas and Checkoway 1995). However, without an accurate and historically relevant exposure model, it is dif- ficult to obtain unbiased estimates of lifetime dose, resulting in exposure misclassification and ultimately risk estimates that are likely to be biased toward the null. In previous epidemiologic studies of U.S. railroad workers exposed to diesel exhaust (Garshick et al. 2004; Laden et al. 2006), esti- mates of historical exposures were not avail- able for quantitative risk estimates (Woskie et al. 1988a, 1988b). Studies of truck- ing industry workers reported associations between lung cancer and estimated cumu- lative exposure to diesel exhaust based on vehicle miles traveled by heavy-duty trucks, historic truck emission factors, and fuel con- sumption (Bailey et al. 2003; Steenland et al. 1998). These studies made linear extrapola- tions of historic exposure (spanning ≥ 20–30 years) assuming that the estimated exposures applied uniformly across a spatially dispersed cohort. This approach is limited in its ability to address temporal trends and ignores the impact of spatial variability altogether. A more recent retrospective assessment of exposure to diesel exhaust in U.S. miners used histori- cal measurements of carbon monoxide (CO) along with information on engine horsepower and mine ventilation to reconstruct exposure to elemental carbon (EC) (Vermeulen et al. 2010a). However, the correlation between current CO and EC measurements in this study was modest (r = 0.4; Vermeulen et al. 2010b). The lack of available exposure data is not unique to the miner’s study, but instead highlights the difficultly of obtaining reliable historically relevant exposure data. In this article, we provide a detailed descrip- tion of the methods used to retrospectively estimate exposure to EC in particulate matter (PM) ≤ 1 mμ in aerodynamic diameter (PM1) Environmental Health Perspectives • volume 119 | number 7 | July 2011 in an occupational cohort of U.S. trucking industry workers. We designed the approach to exposure assessment based on specific job duties and work locations, and unlike previous investigators, we identify the predictors of expo- sure in a statistical model. More specifically, we used the baseline exposure model developed from our industrial hygiene assessment of the industry to predict unmeasured on-site worker exposures for the study cohort as a whole in 2000. We then extrapolated these estimates to previous years, adjusting for changes in air pol- lution background and work-related exposure conditions over time, to derive monthly EC estimates from 1971 to 2000 for each job and terminal location. Materials and Methods Trucking Industry Particle Study. The Trucking Industry Particle Study (TrIPS) was designed as a joint effort in exposure assessment and epidemiology (Garshick et al. 2002) in cooperation with the International Brotherhood of Teamsters and four large U.S. trucking companies. As part of this study, we performed a comprehensive exposure assess- ment of the U.S. trucking industry in which > 4,000 environmental samples were collected between 2001 and 2006. During the study period, we visited 36 different trucking termi- nals that were randomly selected to be region- ally representative of the full set of 139 large terminals in operation in 2000 (Smith et al. 2006). For a map of the sampled trucking ter- minal locations, see Supplemental Material, Figure 1 (doi:10.1289/ehp.1002981). This large-scale exposure assessment was conducted in combination with a retrospec- tive epidemiologic cohort study of lung cancer Address correspondence to M.E. Davis, Department of Urban and Environmental Policy and Planning, Tufts University, Medford, MA 02115 USA. Telephone: (617) 627-4719. Fax: (617) 627-3377. E-mail: mary.davis@tufts.edu Supplemental Material is available online (doi:10. 1289/ehp.1002981 via http://dx.doi.org/). This work was supported by a grant from the National Institutes of Health, National Cancer Institute (R01 CA90792) and by a grant from the Health Effects Institute (4705-RFA03-1/04-1). The authors declare they have no actual or potential competing financial interests. Received 16 September 2010; accepted 28 March 2011. 997