Review of occupational epidemiology studies of diesel exhaust

After decades of research involving numerous epidemiologic studies, a causal relationship between diesel exhaust exposure and lung cancer has not been conclusively demonstrated. Epidemiologic studies of the transportation industry (trucking, busing, and railroad) show a small elevation in lung cancer incidence (relative risks [RRs] generally below 1.5), but a dose response for diesel exhaust is lacking. The studies are also limited by a lack of quantitative concurrent exposure data and inadequate or lack of controls for potential confounders, particularly tobacco smoking. Furthermore, prior to dieselization, similar elevations in lung cancer incidence have been reported for truck drivers, and in-cab diesel particulate matter exposures of truck drivers were comparable to ambient highway exposures. Taken together, these findings suggest that an unidentified occupational agent or lifestyle factor might be responsible for the low elevations in lung cancer reported in the transportation studies. In contrast, underground miners, many of whom experience the highest occupational diesel particulate matter exposures, generally do not show elevations in lung cancer. To increase statistical power, individual studies have been combined in meta-analyses or pooled analyses. Combining of studies may obscure important variability between studies or disagreements of results and may introduce a quantitative bias in the results.