Health outcomes associated with exposure to diesel exhaust were reported in two complementary papers, distributed on 2 March 2012, from the Journal of the National Cancer Institute and posted to the JNCI website today. Both papers reported an exposure-response relationship with higher risks at increased exposure levels.

• The first (Attfield et al.) documented the risk of dying from any cause, with an emphasis on lung cancer, using data from the full study population (the cohort study). The team found that the risk of lung cancer among heavily exposed underground workers was five times the risk observed among workers in the lowest exposure category.

• The second (Silverman et al., the case-control study) reported on the lung cancer deaths in the cohort study. In the case-control study, investigators obtained detailed information on lung cancer risk factors, including smoking, employment in other high-risk jobs, and history of other respiratory diseases.

  When the investigators took into account smoking and other lung cancer risk factors, the data showed a three-fold risk of lung cancer death overall and about a five-fold risk for heavily exposed underground workers, which is consistent with the cohort analysis.

  For never-smokers, risk of lung cancer death increased with increasing diesel exhaust exposure. Although based on small numbers, non-smokers with the highest level of diesel exposure were seven times more likely to die from lung cancer than non-smokers in the lowest exposure category.

In sum, our results provide further evidence supporting a causal effect of diesel exhaust exposure on lung cancer mortality in humans. We observed a statistically significant exposure–response relationship after we adjusted for possible confounding from smoking and other established and hypothesized lung cancer risk factors. The exposure–response curve showed a steep increase in risk with increasing exposure at low-to-moderate levels followed by a plateauing or perhaps a decline in risk among heavily exposed subjects.

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. Some of the higher average elemental carbon levels reported in cities include Los Angeles (4.0 µg/m³), the Bronx (6.6 µg/m³), nine urban sites in China (8.3 µg/m³), Mexico City (5.8 µg/m³), and Estarreja, Portugal (11.8 µg/m³). Environmental exposure to average elemental carbon levels in the 2-6 µg/m³ range over a lifetime as would be experienced in highly polluted cities approximates cumulative exposures experienced by underground miners with low exposures in our 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.

—Silverman et al.

The research, all part of the “Diesel Exhaust in Miners Study”, was designed to evaluate cancer risk from diesel exhaust, particularly as it may relate to lung cancer, among 12,315 workers at eight non-metal mining facilities. The facilities were located in Missouri (1 limestone mine), New Mexico (3 potash mines), Ohio (1 salt mine), and Wyoming (3 trona mines, which process an ore used in soda ash).

The investigators selected underground mines for their study setting because the heavy equipment used in these mines frequently runs on diesel fuel. In the fairly enclosed environments of these mines, exhaust builds up in the air to levels considerably higher than those found in other occupational settings—such as trucking depots or shipyards—and many times higher than the air inhaled by the general population. The investigators selected only non-metal mines because of their characteristically low levels of other exposures that may be related to lung cancer risk, such as radon, silica, and asbestos.

It was vitally important to undertake a large study of diesel exhaust and lung cancer based on a quantitative assessment of historical exposure, taking into account smoking and other potentially relevant factors in order to estimate lung cancer risk.

—Debra T. Silverman, lead author of the case-control study

Silverman is chief of the Occupational and Environmental Epidemiology Branch in the Division of Cancer Epidemiology and Genetics (DCEG) at NCI.

To quantify exposure for each worker, the investigators collected thousands of measurements of constituents of diesel exhaust in the air at each mine and combined those data with historical exposure information. Diesel exhaust levels were quantified by measurements and estimates of one of its key components, respirable elemental carbon, which is considered the best index of diesel exhaust in underground mining. The same exposure measurements were used in both the cohort and case-control analyses. The methods for this effort were published previously in four papers in the Annals of Occupational Hygiene in 2010. A fifth paper—an evaluation of the exposure assessment—was published March 2012 in the same journal.

In an Editorial in JNCI accompanying the two papers there, Dr. Lesley Rushton, Department of Epidemiology and Biostatistics, Imperial College London, noted that:

These studies in miners make an important contribution to the body of evidence about DEE [diesel engine exhaust] and are timely given the forthcoming International Agency for Research on Cancer (IARC) monograph meeting this year at which the current IARC categorization of DEE as a group 2A (probable human) carcinogen will be reconsidered.

...These results indicate that stringent occupational and particularly environmental standards for DEE should be set and compliance ensured to have an impact on health outcomes. In the occupational situation, in addition to lower emission and more efficient engines, reduction in DEE can be achieved through: 1) engineering controls such as improved ventilation and regular maintenance of vehicles; 2) improving worker practices such as limiting the number of vehicles, particularly in closed spaces, and turning off engines when not in use; and 3) as a last resort, the use of appropriate respiratory protective equipment.

Reduction in the general environment presents more of a challenge, although some of the occupational control measures are also relevant. However, the necessity for such reduction is becoming increasingly apparent and is essential if the health of large numbers of people is not to be compromised.

Resources

• Attfield MD, Schlieff PL, Lubin JH, et al. (2012) The diesel exhaust in miners study: a cohort mortality study with emphasis on lung cancer. J Natl Cancer Inst. doi: 10.1093/jnci/djs035

• Silverman DT, Samanic CM, Lubin JH, et al. (2012) The diesel exhaust in miners study: a nested case-control study of lung cancer and diesel exhaust. J Natl Cancer Inst. doi: 10.1093/jnci/djs034

• Lesley Rushton (2012) The Problem With Diesel. (editorial) J Natl Cancer Inst. doi: 10.1093/jnci/djs13

• Patricia A. Stewart, Roel Vermeulen, Joseph B. Coble, Aaron Blair, Patricia Schleiff, Jay H. Lubin, Mike Attfield, and Debra T. Silverman (2012) The Diesel Exhaust in Miners Study: V. Evaluation of the Exposure Assessment Methods. Ann Occup Hyg doi: 10.1093/annhyg/mes020

• “Journals Warned to Keep a Tight Lid on Diesel Exposure Data” Sam Kean, Science Insider, 17 February 2012