modeled estimates, exposure was assigned in one of three ways: for terminals still in exis- tence, exposure was assigned using the near- est directly modeled larger terminal (25% of exposure time; 1,041 terminals); for termi- nals no longer in operation but with known region, exposure was assigned using the average exposure of directly modeled terminals in the same region (24% of exposure time; 1,583 terminals), or national annual job averages if the location of the terminal was not known (1% of exposure time; 83 terminals). Although historical data were limited, it was assumed that gasoline forklifts were used before 1971. Exposures for dockworkers and combination workers for 1971 and afterward were assigned based on company-specific information on the use of diesel or propane forklift fuel. Workers were not assigned any EC exposure after retire- ment or before hire. Statistical methods. Proportional hazard regression was used to estimate associations between lung cancer mortality and EC. To closely adjust for age and lung cancer secular trends, risk sets were generated using attained age in 1-year increments as the timeline, and an ordinal variable for calendar year (1985 through 2000) was included in all models. Workers in the cohort were hired over many decades (from before 1960 until after 1980). Each period of work was associated with different unmeasured work practices and vehicle charac- teristics. Furthermore, individuals who entered follow-up (in 1985) at different ages had dif- ferent risks of developing lung cancer during the follow-up period. To meet the assump- tions of proportional hazards, we assigned sepa- rate baseline hazards based on decade of hire Table 2. Descriptive characteristics of the 31,135 male trucking industry workers [mean ± SD or n (%)]. Characteristic Age at hire (years) Age in 1985 (years) Years of work < 10 10 to < 20 20 to < 30 ≥ 30 Race Caucasian African American Other/unknown Census regiona Northeast South Midwest West Unknown Decade of hire < 1960 1960–1969 1970–1979 ≥ 1980 Value 36.0 ± 8.3 49.1 ± 6.0 21.6 ± 8.7 2,950 (9.5) 10,443 (33.5) 12,202 (39.2) 5,540 (17.8) 26,430 (84.9) 2,818 (9.1) 1,887 (6.1) 4,555 (14.6) 11,682 (37.5) 9,818 (31.5) 4,759 (15.3) 321 (1.0) 3,107 (10.0) 8,284 (26.6) 13,745 (44.2) 5,999 (19.3) aBased on last known home address. Environmental Health Perspectives • volume 120 | number 9 | September 2012 (< 1960, 1960–1969, 1970–1979, ≥ 1980) and age in 1985 (40 to < 50, 50 to < 60, 60 to < 70, ≥ 70 years). For example, the baseline hazard for a person 40 years of age in 1985 (born in 1945) who began work in 1975 was the same as that for all workers in their 40s in 1985 who were also hired in the 1970s, and the risk set for that individual would include all workers with the same attained age. All models were also adjusted for race and census region of residence (based on last known home address). We conducted sensitivity analyses with and without total years of employment as a time- dependent covariate (modeled as either con- tinuous or in quartiles) to assess its effect as a potential confounder. Previously in this cohort, Garshick et al. (2008) reported that lung cancer mortality was inversely associated with total duration of employment; this association is consistent with a survivor bias among those who remained at work. Cumulative EC (gμ/m 3-months) and aver- age EC (micrograms per cubic meter) were estimated for each worker from date of hire through the earlier of retirement date or through 31 December 2000. Time-dependent cumulative exposure was modeled as lags of 0, 5, and 10 years, as continuous, and in quartiles. Time-dependent average exposure was mod- eled with lags of 0 and 5 years (continuous and in quartiles). For example, in a 5-year lag model, exposure in the current year and in the previous 4 years is not included in expo- sure calculations to exclude the contribution of more recent exposure to lung cancer mor- tality risk. Linear trend p-values were derived using an ordinal variable that was based on the median of each quartile. Penalized splines were incorporated into regression models to assess possible nonlinearity in mortality risk. Analyses were performed using SAS software (version 9.2; SAS Institute Inc., Cary, NC) or R for Unix (R Foundation for Statistical Computing, Vienna, Austria). Analyses were also performed that excluded workers who were employed for ≥ 1 year as a mechanic (n = 1,811). Mechanics experienced significant historical changes in job duties that weaken the validity of extrapolation of current Lung cancer and elemental carbon exposure to historical estimates. Our expo- sure estimates reflect the current practice of performing minor repairs and maintenance whereas in the past major engine repairs were performed. In addition, the nature of expo- sure is different from that of other workers. Based on our field observations, mechanics are exposed to EC for relatively brief periods when engines are intermittently run inside shops allowing for particle aging. Workers in other jobs on roadways and loading docks have exposures characterized by considerably longer periods of exposure to fresh exhaust from continuously running engines. Results Population. The mean cumulative years of work in the study cohort was 21.6 years, 84.9% were Caucasian, and most lived in the South (37.5%) or Midwest (31.5%). (Table 2). Long-haul diesel trucks were first used dur- ing the 1950s in this industry, and nearly all workers began employment during or after this period. Most workers also began employment concurrent with or after the introduction of diesel P&D trucks first used during the 1970s. There were 4,003 dockworkers or combina- tion workers with at least 1 year of diesel fork- lift exposure (median, 5.8 years; interquartile range, 2.9–9.0 years) in the 1980s and into the 1990s. The average age of hire was in the mid-30s (Table 2), most likely due to hiring policies requiring previous experience. There was a wide range of cumulative EC exposure (Table 3). The 75th percentile of estimated cumulative exposure was > 8 times that of the 25th percentile for a 10-year lag, and almost 4 or 5 times higher for no lag and 5-year lagged exposures, respectively. Lung cancer mortality. Through 2000, there were 779 lung cancer cases (734 under- lying cause) out of 4,306 deaths. In mod- els adjusted for calendar year, race, and census region, we examined cumulative EC in quartiles without and with adjustment for employment duration (Table 4). When adjusted for race, calendar year, and census region only, lung cancer mortality was elevated for the upper three cumulative EC quartiles Table 3. Distribution of cumulative EC (µg/m3-months) from hire date through retirement or through 31 December 2000, whichever occurred first, in the entire cohort, excluding 1,811 mechanics. Entire cohort (n = 31,135) Cohort excluding mechanics (n = 29,324) 5-year lag 10-year lag 5-year lag 10-year lag Percentile Maximum 99 95 90 75 50 25 10 5 Minimum No lag 24,130 8,725 4,829 3,560 2,076 1,061 530 271 167 10 24,130 7,729 4,305 3,131 1,803 860 371 136 48 0 23,106 6,482 3,579 2,595 1,436 596 167 2.0 0 0 No lag 15,242 6,623 4,044 3,057 1,891 990 508 260 161 10 15,074 5,588 3,599 2,723 1,635 801 351 128 44 0 10,341 4,543 2,997 2,283 1,305 553 155 0 0 0 1303