Re: The Diesel Exhaust in Miners Study: A Nested Case–Control Study of Lung Cancer and Diesel Exhaust

In a recent article in the Journal on diesel exhaust exposure and lung cancer mortality among workers in US non–metal mining facilities, Silverman et al. (1) reported a statistically significantly increasing trend in lung cancer mortality with increasing exposure to cumulative respirable elemental carbon (REC) and average REC intensity, lagged and unlagged 15 years. The major advantage of this nested case–control study over previous epidemiological studies is the ability to obtain lifetime diesel exhaust exposure (represented by REC) of individual workers by incorporating historical industrial hygiene measurements with specific job titles and the calendar year. The overall results regarding the exposure–response relationship between diesel exhaust and lung cancer are generally plausible; however, we have questions about the results and interpretation of the interaction between smoking status/intensity and diesel exhaust exposure. The authors observed an attenuation of the effect of cigarette smoking among workers who were exposed to high levels of diesel exhaust, after adjustment for history of respiratory disease at least 5 years before date of death/reference date, history of a high-risk job for lung cancer for at least 10 years, and mine location (surface only vs ever underground work). The authors had proposed several mechanisms to explain the observed attenuated interactive effect, such as hypotheses about enzyme saturation and enzyme suppression (eg, reduced activity of cytochrome P450, subfamily IIB [CYP2B1]); however, it is possible that the attenuated smoking effect in the presence of high levels of diesel exhaust exposure is the result of a negative residual confounding effect of smoking. We derived the smoking prevalence separately for the surface and underground workers from the data provided in the article and found that the underground workers were more likely to quit smoking compared with the surface-only workers (33.9% vs 42.3%). We suspected that underground workers may have smoked less and had also quit smoking earlier than surface workers because smoking is likely to be prohibited in underground working environments where a high level of diesel exhaust exposure is expected, as is the case in the study of Silverman et al. (eg, ≥304 μg/m