We performed a retrospective assessment of lung cancer mortality in trucking industry workers 1985–2000 and found that lung cancer mortality risk was positively associated with years of work in jobs associated with regular exposure to freshly emitted vehicle exhaust. These included jobs with current and historical exposure from driving in cities and on urban highways (P&D drivers, combination workers) and on terminal loading docks with exposure reflecting movement of freight and vehicles in the yard around the terminal, background levels, and forklift exhaust (combination workers, dockworkers). In LH drivers, who drive primarily on intercity highways, the risk per year of work was also elevated, but after adjustment for differences in smoking, the increase in risk did not achieve conventional levels of statistical significance. Workers in other jobs in the cohort had too few lung cancer cases to reliably model lung cancer risk based on years of work. Based on exposure defined as at least 1 year of work, lung cancer risk in the clerks was statistically significantly reduced and the risk in mechanics, hostlers, and workers in other jobs was not significantly elevated.
Our analytic approach has the advantage of using internal cohort-based reference groups (i.e., trucking industry workers in the same cohort) to account for unmeasured factors related to lung cancer mortality. For example, the reference group for LH drivers was all workers who never served as an LH driver, and the reference group for the P&D drivers was all workers who had never worked as a P&D driver. Applying job-specific smoking adjustment factors to account for variation in smoking patterns between each job and reference group as in produced results similar to unadjusted risks. The pattern of lung cancer risk was also the same as that observed in our previous study in this cohort where lung cancer rates from the U.S. population served as an external comparison (Laden et al. 2007
). In that report, the SMRs among all drivers and combination workers (SMR = 1.10; 95% CI, 1.02–1.19) and dock-workers (SMR = 1.10; 95% CI, 0.94–1.30) were elevated, whereas for mechanics and hostlers the SMRs were not increased, and for clerks the SMR was statistically significantly decreased. Obtaining similar results using two different approaches in our cohort indicates that the findings are robust.
A greater lung cancer risk in truck drivers has previously been attributed to diesel exhaust because diesel engines manufactured before 2007 in the United States had greater mass emissions of respirable particles with an EC core and adsorbed organic and other PAH compounds than did spark ignition sources. Although spark-ignition light-duty vehicles had lower-mass PM emissions per mile, a much greater number of cars on roadways relative to trucks suggest that automobiles do contribute to vehicle-exhaust–related emissions (Allen et al. 2001
; Geller et al. 2005
; Zhu et al. 2002
). The particle size distributions from gasoline and diesel vehicles are similar, with the ultrafine mode accounting for most particles, and have a similar chemical composition (Kittelson et al. 2003
; Kleeman et al. 2000
). However, in contrast to pre-2007 diesel engines, particles emitted from the gasoline-powered vehicles are predominantly composed of organic compounds (Kleeman et al. 2000
). Under a light load and when idling, diesel emissions from pre-2007 diesel engines also have lower EC and higher OC in PM emissions compared with operating with greater load (Kweon et al. 2002
; Shah et al. 2004
; Zielinska et al. 2004a
Therefore, as part of our previous exposure assessment (Davis et al. 2006
; Smith et al. 2006
), we measured the 2001–2005 terminal background, loading dock, and LH and P&D truck cab EC and OC levels. The EC geometric means (SDs) were as follows: 0.31 (3.72) μg/m3
in terminal offices where the clerks worked; 1.12 (1.91) μg/m3
in LH drivers; 1.09 (2.48) μg/m3
in P&D drivers; 0.76 (2.13) μg/m3
in dockworkers; 0.88 (3.04) μg/m3
in hostlers; and 2.00 (3.82) μg/m3
in mechanics. OC showed less variation, with levels ranging from 19.26 (2.30) μg/m3
in LH drivers to 12.40 (1.54) μg/m3
in P&D drivers. EC and OC for combination workers would represent a mix of dockworker and P&D driver exposures. We also used structural equation modeling to assess the contribution of background and terminal work area exposures (i.e., on the loading dock) to personal measurements of EC and OC (Davis et al. 2006
). Distance from an interstate highway and factors indicating greater freight activity at a terminal were significantly associated with greater personal exposure to EC and OC. Determinants of in-cab exposure to EC and OC in LH and P&D drivers were assessed, and was positively related to ambient particle concentrations attributable to background air pollution and regional traffic (Davis et al. 2007
). Given these observations and our exposure measurements, our results suggest that exposure of the drivers comes predominantly from surrounding vehicles and from background air pollution, as well as some exposure related to the driver’s own vehicle, and that loading-dock exposures are related to both background and work area activities.
To identify specific sources of current occupational particulate exposures, we conducted source apportionment studies using particle-phase organic molecular markers in work area samples from seven freight terminals sampled in 2002 and 2003 (Sheesley et al. 2008b
) as well as in personal and work area samples in terminal workers and drivers at one terminal (Sheesley et al. 2008a
). Source apportionment data in the LH drivers, P&D drivers, and dockworkers indicated that most (~ 80% or greater) of the EC was from diesel sources with a smaller percentage from spark-ignition vehicles and lubricating oil (Sheesley et al. 2008a
). OC source apportionment indicated a substantial mobile source contribution that was mostly attributable to lubricating oil. Detailed studies characterizing specific emissions from propane-powered forklift trucks have not been conducted. However, based on measurements obtained in other settings, propane engines produce ultrafine particles (Guo et al. 2004
; Rundell 2003
) that includes little EC compared with gasoline and diesel engines (Zaebst et al. 1991
). Taken together, our results indicate that a variety of mobile sources contribute to ambient vehicle exhaust particles in the trucking industry, with EC representing mainly diesel sources and OC representing a mixture of mobile sources that depend on the work environment.
Our observations regarding lung cancer mortality in truck drivers are supported by a previous case–control study conducted by Steenland et al. (1990)
and a companion exposure assessment (1988–1989) (Steenland et al. 1992
; Zaebst et al. 1991
). They identified cases and controls from retirement records of teamsters who died during 1982–1983 using the Teamster Central States Pension Fund database and obtained smoking history from next of kin. In contrast to our study, where actual work records were available, these researchers obtained work history from next-of-kin report and from self-report from each worker’s retirement application. Although they observed an elevation in smoking-adjusted lung cancer odds ratios for LH and P&D drivers and a positive trend in lung cancer risk with greater duration of work in LH drivers, the risk estimates were imprecise because the population was relatively small. As we have found in our present exposure assessment, geometric mean EC exposures in these late 1980s measurements were similar for LH and P&D drivers (3.8 and 4.0 μg/m3
, respectively) and approximately 4-fold greater than in the same jobs in our 2001–2005 exposure assessment (Smith et al. 2006
). Measurements made in the late 1980s also indicated that exposures in dockworkers were greater than our present exposures and varied by forklift type, with much higher levels associated with diesel and gasoline forklifts (Steenland et al. 1992
; Zaebst et al. 1991
). However, in contrast to our present results, there was no increase in lung cancer risk among dockworkers in the Steenland et al. study. Unlike our present study, they included relatively few dockworkers, and the category included dispatchers, clerks, and other workers who were unlikely to spend considerable time on a loading dock and would have had little exposure.
The lung cancer risk in the mechanics was not elevated, although they had the highest EC values historically (Zaebst et al. 1991
) and in our exposure assessment. Despite present EC values intermediate to dockworkers and drivers, the mortality risk for hostlers was also not increased. Although the sampling plan did not include a detailed assessment of real-time exposures, our field observations indicate that because truck engines are shut off while in the shop area, mechanics are exposed primarily to aged exhaust and only intermittently to fresh exhaust. In contrast, drivers and dockworkers commonly experience periods of more or less continuous exposure to mobile-source related fresh PM related to vehicle exhaust. Hostlers drive primarily small, specialized tractor units in the terminal yard moving trailers to and from the freight dock. Because yard-related truck activity varies throughout each shift, it is also possible that their intermittent pattern of vehicle exhaust exposure accounts for lack of increased risk. One of the potential mechanisms whereby traffic-related exposures may result in DNA damage is due to short-lived reactive oxygen species (ROS). ROS activity has been found to be associated with the smallest particle size fractions and with traffic-related PM compared with other sources, potentially explaining the greater risk in persons in jobs with more constant exposures (Baulig et al. 2004
; Briede et al. 2005
; de Kok et al. 2006
; Li et al. 2003
). In addition, the cohort included few hostlers and mechanics, resulting in imprecise effect estimates, and we were unable to fully assess their risk by modeling years of work.
A limitation of this analysis is lack of personal information on potential risk factors for lung cancer. Although cigarette smoking is a major risk factor, the degree to which it is a confounder in this study depends on differences in smoking behavior associated with job title within the cohort. To minimize the possible effect of uncontrolled confounding by smoking, we have studied workers of similar socioeconomic class, a known correlate of smoking habits (Brackbill et al. 1988
; Stellman et al. 1988
). Although we were unable to directly survey workers in this cohort, we used a representative sample of active and retired workers in the industry (Jain et al. 2006
) and found that variation in lung cancer risk among exposed workers did not reflect differences in smoking rates, and adjustment did not significantly influence the observed results other than slightly reducing the effect among LH drivers. These findings are consistent with a recent review of the literature where differences in smoking rates did not explain increased lung cancer risks associated with occupational exposures (Blair et al. 2007
). As expected (Thun et al. 1997
), we found that smoking rates in the survey varied by age and birth cohort (Laden et al. 2007
). Although a limitation of this indirect adjustment is that smoking patterns between jobs may have been different historically, we found that birth-cohort–specific smoking rates were similar among drivers and nondrivers (Laden et al. 2007
), suggesting that smoking rates did not vary substantially among jobs. In addition, we tightly controlled all analyses for age and calendar year. It is also unlikely that variation in other potential risk factors for lung cancer (e.g., family history or history of obstructive lung disease) would vary systematically by job title and contribute to confounding.
Another limitation is the lack of job history information before a worker joined one of the four unionized companies in the cohort. The average age of starting work in the trucking industry from the questionnaire was 25 years, which suggests that some workers may have had up to 10 additional years of exposure to mobile-source–related combustion PM in trucking industry jobs. This would reduce the estimated risk attributable to each year of work in .