Davis et al. than those for the next highest exposed job category (mechanics), we excluded this group from Figure 1A to better illuminate trends in the other job categories. However, it is clear that the use of diesel forklifts on the semienclosed docks contributed greatly to exposure levels of dockworkers during that time period. Estimated EC exposures in the trucking industry trend higher in the past for all job groups, with the lowest levels observed during the base year 2000. These temporal trends in the exposure profile across job cate- gories reflect both changes in work practices and elevated background conditions over time. All annual exposure estimates differed significantly across job categories (p < 0.05), with the exception of exposures among gas dockworkers and P&D drivers, which were not significantly different in 1975 and 1979. When we compared driver groups, exposure estimates were similar during more recent periods, although these small differences were still statistically significant. The driver differ- ences widen over time as P&D exposure levels increase more rapidly than do LH exposures in the past. This increase is due to the larger effect of background pollution on P&D truck cabs in warm-weather conditions (open win- dows), which was also higher in the past. Figure 1B provides evidence of regional differences both within and between the vari- ous job groups across U.S. Census regions. Mechanic exposures were significantly higher in the Midwest and Northeast (p < 0.01), whereas exposures in the other job groups were comparatively higher in the South and West (p < 0.01). Figure 2 illustrates the importance of tem- perature on mechanic exposure trends over time. Estimated mechanic exposures were sig- nificantly lower (p < 0.01) in warm weather than in cold weather. This result is due to the large impact of open-door ventilation in the semienclosed work environment of the mechanic shop and is consistent with meas- ured exposure data from both studies (Smith et al. 2006; Zaebst et al. 1991). Discussion This article outlines the approach we used to estimate occupational exposure to EC, a sur- rogate marker of vehicle exhaust with a signif- icant contribution of diesel exhaust particles, in the U.S. trucking industry between 1971 and 2000. We used SEM techniques to con- struct a quantitative model of job- and termi- nal-specific exposures across the United States in 2000 (Davis et al. 2006). We then retro- spectively applied these estimates after adjust- ing for historic changes in work practices and background conditions to generate monthly EC estimates for each job and terminal loca- tion from 1971 to 2000. Our approach was substantially different from previous efforts because we combined an extensive assessment of current exposure with historical data in a statistical model to extrapolate exposures spa- tially and temporally across the cohort. We observed evidence of a number of important trends across job groups. By far the largest job-related exposure impact was the past use of diesel-powered forklifts on termi- nal docks, and we identified forklift engine type as a major source of variability within the dockworker job group. Diesel-fueled forklifts provided the largest contribution to dock- worker EC exposures, followed by gasoline- fueled forklifts, whereas work-related exposure from propane-powered forklifts was very low. The results also show significant variabil- ity in exposures among mechanics. Although mechanic exposures at rural terminals in warmer climates were similar to regional background levels, workers in the same job category working at cold-climate urban termi- nals experienced the highest exposure levels in the entire cohort because of reduced ventila- tion when terminal bay doors are kept closed. In addition, mechanic exposures in warmer climates were relatively stable within a given year, whereas mechanics working in colder climates experienced wide seasonal exposure shifts, resulting in wider ranges of predicted values for mechanics in the Midwest and Northeast compared with mechanics in the South and West. In contrast, exposures for P&D drivers, which come from outside of the truck cab, are increased during warmer temperatures and in warmer climates because these trucks are not equipped with air conditioning and cab windows are more likely to be open when it is warm outside. This effect was not as evi- dent for LH drivers, whose truck cabs were equipped with air conditioning throughout the study period. Because of the greater con- tribution of background conditions to expo- sures in P&D drivers, P&D exposure levels were higher than LH exposures in the past, consistent with higher background exposures. Overall, both geographic and temporal trends, which have not been evaluated in pre- vious studies, were important exposure modi- fiers for workers in this occupational cohort. In addition, we identified significant variability in exposures within job groups across termi- nal locations. These results highlight the need in this and similar occupational health stud- ies to control for these important sources of variability when estimating cumulative expo- sures. Even relatively minor estimation errors can result in substantial misclassification bias (Stayner et al. 1999; Stewart 1999), ultimately limiting the power to detect exposure–disease relationships (Ahrens and Stewart 2003). Unfortunately, historical exposure reconstruc- tion efforts are often hampered by incomplete exposure information, and failure to account for temporal changes when estimating expo- sures can introduce a significant amount of error in chronic epidemiologic studies (Seixas and Checkoway 1995; Ward et al. 2003). ) ) ( ( n n o o i i t t a a e e c c Mechanic P&D driver Dockworker (gas) LH driver Dockworker (propane) 12 3 m /g 9 µ trn 6 c n o E C 3 0 1970 quartile range. 1000 60 3 m /g 40 µ r t n C E c on 20 0 1980 1990 Year 2000 Midwest excludes outside values Northeast South West Region Figure 1. (A) Median annual EC trends across select job categories. (B) EC trends across select job categories by census region. The boxes indicate 25th, 50th (median), and 75th percentiles. Extreme outliers are excluded for display purposes, and the length of the whiskers are bounded from above by 1.5 times the inter- ovlume 119 | number 7 | July 2011 • Environmental Health Perspectives