conditions, respectively, with higher exposures during cold weather when repair bay doors are closed (Table 1) (Smith et al. 2006). Driver exposures have also changed over time because of alterations in work practices. We estimated driver exposures to be between 1.3 and 2.5 times greater during the late 1980s than during the early 2000s, even after controlling for higher background levels in the past (Table 1). The estimated impact of window status (at 10°C cutoff) for P&D driv- ers was greater in the past, which is likely due to the different mix of traffic sources present and the greater intensity of these sources. We found no sizable window effect in LH drivers, whose truck cabs were equipped with air con- ditioning throughout the entire study period. Table 2 provides a numerical summary of predicted shift-level EC exposures by job group over time. Clerk EC exposures were indistinguishable from background levels, because no immediate exposure source is pres- ent in these indoor work locations. As such, exposure levels for these workers are assumed to be equivalent to background conditions throughout our study. Also, the use of diesel forklifts on terminal docks varied by com- pany, and the summarized occupational expo- sures in Table 2 are not relevant to all periods; Job Background/clerks LH P&D (warm) P&D (cold) Dockworker (propane) Dockworker (diesel) Dockworker (gasoline) Mechanics (warm) Mechanics (cold) 699 388 535 197 632 96 53 0.5 1.1 1.2 1.0 0.9 1.5 4.3 GSD 3.2 2.3 2.3 3.5 2.2 2.4 4.0 23 72 25 31 12 54 9 38 42 Historical exposure to diesel in the trucking industry that is, the diesel dockworker phase is limited primarily to the 1980s. This assumption also applies to dockworkers exposed to gasoline- powered forklifts, because propane-powered forklifts dominated after the 1990s. Although we present the full panel of exposure estimates for each job category here, cumulative expo- sure estimates for the epidemiologic model are based on individual work histories and on the profile of fuel use that was provided by the participating companies. The EC levels for mechanics were sig- nificantly higher than those for both driver groups in each decade noted in Table 2 and were higher than for propane dockwork- ers (p < 0.01). These results are consistent with the measured exposure data observed by both Smith et al. (2006) and Zaebst et al. (1991). EC exposures were also higher among mechanics than among gas dockworkers dur- ing the relevant exposure periods (pre-1990s; p < 0.01). Exposure conditions were similar between diesel dockworkers and cold-weather mechanics during the relevant period of diesel forklift use (before 1990). Figure 1A displays median annual trends in estimated EC exposure levels across select job groups. Because the EC estimates for die- sel dockworkers are nearly four times higher Zaebst studya (1988–1989) GM 1.1 3.8 6.3 2.8 1.3 27.2 5.5 4.8 28.0 GSD 2.0 2.3 1.9 1.6 2.0 1.7 1.6 2.4 2.8 Predictedc Historicald GM multiplier 1.1 NAe 2.3 1.7 2.5 2.5 2.2 1.3 1.36 1.0 20.9f 4.2f 2.0 2.4 7.6 3.7 for 26 separate New Jersey locations for 1971– 2000, and hourly COH data from California for 1980–2000. In a separate analysis, we constructed nationwide models of annual exposure to PM10 and nitrogen dioxide for 1985–2000 (Hart et al. 2009). We decided to use the New Jersey COH data to estimate background EC conditions over time in our cohort because these data provided the longest unbroken panel to reconstruct historical expo- sures and because they corresponded to the available data on background EC conditions during the Zaebst et al. (1991) assessment. A more detailed description of the choice of New Jersey COH and comparison with the other exposure data is provided in the Supplemental Material (doi:10.1289/ehp.1002981). To explicitly incorporate a background trend based on COH into the exposure model, we derived ratios comparing the median annual COH value in each year with the estimate for base year 2000 and used them to adjust annual background EC predictions for 1971–1999. In summary, we first used the baseline exposure model developed from our TrIPS industrial hygiene assessment of the industry to predict unmeasured on-site worker expo- sures for the study cohort as a whole in 2000. Next, we extrapolated these estimates to previ- ous years. To derive monthly EC estimates from 1971 to 2000 for each job and terminal location, we adjusted for changes in air pollu- tion background and work-related exposure conditions over time. Results Table 1 provides data from the TrIPS (Smith et al. 2006) and Zaebst et al. (1991) expo- sure assessments, along with the job-specific historical multipliers that we developed from this comparison and applied to our cohort. Not surprisingly, estimated exposures were significantly higher for 1988–1989 than for 2001–2006 for all job groups. For dock- workers driving propane forklifts, the geo- metric mean of the predicted EC value was approximately equal to the geometric mean reported by Zaebst et al. (1991) (predicted, 1.36; actual, 1.30) after accounting for ele- vated background conditions in the past. However, a multiplier to account for the type of fuel used in forklifts driven by these workers indicated the impact of higher emitting fuels, namely, diesel and gasoline, on past exposures. In our cohort, mechanics experienced the greatest change in duties over time of all the jobs. Previously, mechanics in the truck- ing industry performed most major repair and replacement work, whereas currently they per- form only minor repairs and preventive main- tenance. Even after accounting for elevated background conditions, past mechanic exposure levels were still 2.4–3.7 times higher than were the levels predicted for warm- and cold-weather Environmental Health Perspectives • volume 119 | number 7 | July 2011 Table 1. Historical occupational EC in the trucking industry (μg/m3). TrIPSb (2001–2006) No. of observations GM Measured No. of observations Abbreviations: GM, geometric mean; GSD, geometric standard deviation; NA, not applicable. aZaebst et al. (1991). bSmith et al. (2006) and Davis et al. (2009). cRepresents predictions from baseline exposure model adjusted for background conditions. dHistorical multipliers were generated based on relationship between measured geometric means from Zaebst et al. (1991) and those predicted by baseline exposure model, unless otherwise specified; linear extrapolation applied to years between study periods. eYear-specific background multipliers applied for 1971–1999 are based on archived COH data. fHistorical multipliers for dockworkers driving diesel or gasoline forklifts were devel- oped based on ratio to propane as observed by Zaebst et al. (1991). Empty cells indicate that no diesel or gasoline fork- lifts were observed for Smith et al. 2006. Table 2. Summary statistics of shift-level EC predictions by job per decade (μg/m3). 1971–1980 1981–1990 1991–2000 Job group Mean Median SD Mean Median SD Mean Median Background/clerks 1.79 1.65 0.74 1.25 1.20 0.40 0.80 0.75 Dockworkersa (diesel) 40.80 37.25 17.28 32.06 29.86 12.19 24.73 22.83 Dockworkersa (gasoline) 8.20 7.49 3.47 6.44 6.00 2.45 4.97 2.00 Dockworkersa (propane) 1.95 1.78 0.83 1.53 1.43 0.58 1.18 1.09 Mechanics (all) 19.66 9.72 22.48 15.23 7.66 16.98 7.64 3.86 Mechanics (warm climate) 7.75 6.33 5.05 6.08 5.07 3.75 3.16 2.56 Mechanics (cold climate) 43.23 37.72 24.86 33.57 29.77 18.27 16.43 13.19 LH drivers/hostlers 6.40 5.88 2.64 4.46 4.26 1.45 2.21 2.01 P&D drivers (warm) 10.41 9.59 4.16 7.23 6.97 2.25 3.09 2.77 P&D drivers (cold) 4.56 4.15 1.99 3.18 2.95 1.12 1.79 1.64 aDockworker exposure predictions not relevant to all time periods; based on company reported fuel-use profiles. SD 0.31 9.96 4.59 0.48 9.91 2.33 12.79 1.04 1.64 0.80 999