Exposure to diesel exhaust increases the risk of developing and dying from lung cancer, according to the results of a large cohort mortality study in more than 12,000 exposed workers and a related nested case-control study.
The Diesel Exhaust in Miners Study confirms prior studies demonstrating a possible causal relationship between diesel exhaust exposure and lung cancer, and suggests that this association represents an important public health burden in urban areas worldwide.
Both studies were published online March 5 in the Journal of the National Cancer Institute.
In the main cohort mortality study, the standardized mortality ratio (SMR) of 1.26 for lung cancer among 12,315 exposed workers was significantly greater than were state-based mortality rates, wrote Michael D. Attfield. Ph.D., formerly with the National Institute for Occupational Safety and Health in Morgantown, W.Va., and his colleagues.
Esophageal cancer and pneumoconiosis mortality also were significantly elevated in the exposed workers (SMRs, 1.83 and 12.20, respectively), the researchers found (J. Natl. Cancer Inst. 2012 [doi:10.1093/jnci/djs035]).
Outcomes were evaluated in workers exposed to diesel exhaust at eight nonmetal mining facilities in the United States using retrospective quantitative estimates of respirable elemental carbon (REC) exposures as a surrogate for diesel exhaust exposure. The data covered the periods from the introduction (between 1947 and 1967) of diesel-powered equipment to 1997.
A dose-response relationship between exposure and lung cancer mortality was not apparent in the overall cohort, but clear evidence of such a relationship emerged after the investigators divided the population, comparing those who had worked underground at some point (ever-underground) with surface workers.
"In contrast to the complete cohort, hazard ratios that increased with level of exposure were seen for ever-underground workers using quartiles of REC exposure. Using the expanded exposure categories, hazard ratios for 15-year lagged cumulative REC exposure rose with increasing exposure, the trend being more pronounced when workers with shorter tenures were excluded," the investigators said.
As for the surface-only workers, no clear exposure-response trend was seen in the quartile analysis, but there was evidence of an increasing trend in risk with increasing exposure, they said.
The overall findings were "essentially unchanged" after the researchers controlled for cumulative exposures to silica, asbestos, nondiesel exhaust polycyclic aromatic hydrocarbons, and respirable dust.
Though limited by factors typical of cohort mortality studies, such as uncertainty in exposure assessments and lack of information on lifestyle factors, this study had strong statistical power and the findings suggest that despite ongoing efforts to reduce diesel engine emissions, diesel exhaust exposure remains a problem.
"Certainly, many workers around the world, in mining and in other industries and jobs, continue to be exposed to REC at levels similar to those observed in this study; in addition, environmental exposures have been shown to reach the levels seen for average REC intensity in surface workers in this study," the investigators concluded.
The findings are bolstered by those from the companion nested case-control study, which also evaluated the relationship between diesel exhaust exposure and lung cancer mortality, but which included adjustment for smoking and other established and hypothesized potential confounders.
For this companion study, Debra T. Silverman, Sc.D., of the National Cancer Institute, Bethesda, Md., and her colleagues compared 198 lung cancer death cases and 562 incidence density–sampled control subjects.
After adjustment for smoking, history of employment in high-risk occupations for lung cancer, and a history of respiratory disease, they also found statistically significant increasing trends in lung cancer risk with increasing exposure (J. Natl. Cancer Inst. 2012;104:1-14 [doi:10.1093/jnci/djs034]).
"Our findings are important not only for miners but also for the 1.4 million American workers and the 3 million European workers exposed to diesel exhaust and for urban populations worldwide," the investigators said.
Citing Los Angeles; the Bronx borough of New York City; Mexico City; Estarreja, Portugal; and nine urban centers in China, they noted that some of the higher-average elemental carbon levels reported in major cities represent potential lifetime exposures that approximate the cumulative exposures experienced by underground miners with low exposures in this study.
"Because such workers had at least a 50% increased lung cancer risk, our results suggest that the high air concentrations of elemental carbon reported in some urban areas may confer increased risk of lung cancer. Thus, if the diesel exhaust/lung cancer relation is causal, the public health burden of the carcinogenicity of inhaled diesel exhaust in workers and in populations of urban areas with high levels of diesel exposure may be substantial," they concluded.
The Diesel Exhaust in Miners Study was funded by the Intramural Research Program of the National Institutes of Health, National Cancer Institute, Division of Cancer Epidemiology and Genetics, and the National Institute for Occupational Safety and Health, Division of Respiratory Disease Studies. The authors had no disclosures to report.