The Diesel Exhaust in Miners study: a cohort mortality study with emphasis on lung cancer

Abstract

Background: Current information points to an association between diesel exhaust exposure and lung cancer and other mortality outcomes, but uncertainties remain.

Methods: We undertook a cohort mortality study of 12 315 workers exposed to diesel exhaust at eight US non-metal mining facilities. Historical measurements and surrogate exposure data, along with study industrial hygiene measurements, were used to derive retrospective quantitative estimates of respirable elemental carbon (REC) exposure for each worker. Standardized mortality ratios and internally adjusted Cox proportional hazard models were used to evaluate REC exposure-associated risk. Analyses were both unlagged and lagged to exclude recent exposure such as that occurring in the 15 years directly before the date of death.

Results: Standardized mortality ratios for lung cancer (1.26, 95% confidence interval [CI] = 1.09 to 1.44), esophageal cancer (1.83, 95% CI = 1.16 to 2.75), and pneumoconiosis (12.20, 95% CI = 6.82 to 20.12) were elevated in the complete cohort compared with state-based mortality rates, but all-cause, bladder cancer, heart disease, and chronic obstructive pulmonary disease mortality were not. Differences in risk by worker location (ever-underground vs surface only) initially obscured a positive diesel exhaust exposure-response relationship with lung cancer in the complete cohort, although it became apparent after adjustment for worker location. The hazard ratios (HRs) for lung cancer mortality increased with increasing 15-year lagged cumulative REC exposure for ever-underground workers with 5 or more years of tenure to a maximum in the 640 to less than 1280 μg/m(3)-y category compared with the reference category (0 to <20 μg/m(3)-y; 30 deaths compared with eight deaths of the total of 93; HR = 5.01, 95% CI = 1.97 to 12.76) but declined at higher exposures. Average REC intensity hazard ratios rose to a plateau around 32 μg/m(3). Elevated hazard ratios and evidence of exposure-response were also seen for surface workers. The association between diesel exhaust exposure and lung cancer risk remained after inclusion of other work-related potentially confounding exposures in the models and were robust to alternative approaches to exposure derivation.

Conclusions: The study findings provide further evidence that exposure to diesel exhaust increases risk of mortality from lung cancer and have important public health implications.

Figure 1

Lung cancer hazard ratios against 15-year lagged cumulative respirable elemental carbon (REC) exposure (μg/m³-y) for ever-underground workers, for all tenures, and after excluding workers with less than 2, less than 5, and less than 10 years tenure at time of event (see Table 4 for <5 year exclusion results). Analyses were performed with the Cox proportional hazards model and probabilities determined with a two-sided χ² Wald test.

Figure 2

Lung cancer hazard ratios against 15-year lagged average respirable elemental carbon (REC) intensity (μg/m³) for ever-underground workers, for all tenures, and after excluding workers with less than 2, less than 5, and less than 10 years tenure at time of event (see Table 4 for <5 year exclusion results). Analyses were performed with the Cox proportional hazards model and probabilities determined with a two-sided χ² Wald test.