A critical review of the relationship between occupational exposure to diesel emissions and lung cancer risk

In 2012, a working group of the International Agency for Research on Cancer classified diesel exhaust (DE) as a human carcinogen (Group 1). This decision was primarily based on the findings of the Diesel Exhaust in Miners Study (DEMS). The disparity between the results of various methodological approaches applied to the DEMS led to several critical commentaries. An expert panel was subsequently set up by the Health Effects Institute to evaluate the DEMS results, together with a large study in the trucking industry. The panel concluded that both studies provided a useful basis for quantitative risk assessments (QRAs) of DE exposure. However, the results of both studies were non-definitive as the studies suffer from several methodological shortcomings. We conducted a critical review of the studies used by the International Agency for Research on Cancer (IARC) working group to evaluate the relationship between DE and lung cancer. The aim was to assess whether the available studies support the statement of a causal relationship and, secondarily if they could be used for QRA. Our review highlights several methodological flaws in the studies, amongst them overadjustment bias, selection bias, and confounding bias. The conclusion from our review is that the currently published studies provide little evidence for a definite causal link between DE exposure and lung cancer risk. Based on two studies in miners, the DEMS and the German Potash Miners study, QRA may be conducted. However, the DEMS data should be reanalyzed in advance to avoid bias that affects the presently published risk estimates.