Carcinogenicity of diesel-engine and gasoline-engine exhausts and some nitroarenes.

In June 2012, an IARC Monographs Working Group reevaluated the carcinogenic hazards to humans of diesel and gasoline engine exhausts, and of some nitroarenes that are found in these emissions. Diesel engine exhaust was classified as “carcinogenic to humans” (Group 1) and gasoline engine exhaust as “possibly carcinogenic to humans” (Group 2B). The most influential epidemiological studies on cancer associated with diesel-engine exhausts were conducted among non-metal miners, railroad workers, and workers in the trucking industry in the USA. The miners study included a cohort analysis [Attfield et al., 2012] and a nested case-control analysis that was adjusted for tobacco smoking [Silverman et al., 2012]. Both showed positive trends in lung cancer risk with increasing exposure, with a 2-3-fold increased risk in the highest categories of cumulative or average exposure. There were few potential confounding exposures in these underground mines, and high exposures were well documented in current surveys. A 40% increased risk for lung cancer was observed in railroad workers exposed to diesel exhaust [Garshick et al., 2004]. Indirect adjustment for smoking suggested that differences in smoking could not have influenced this excess risk substantially. When extended with more accurate exposure assessment, the study showed a 70-80% increase in risk for exposed workers positive trends were observed with duration of exposure, but not with cumulative exposure [Laden et al., 2006]. A large cohort study in the US trucking industry [Garshick et al., 2008] reported a 15-40 % increased lung cancer risk in truck drivers and dockworkers with regular exposure to diesel exhaust. There was a significant trend of increasing risks with longer duration of employment, with 20 years of employment roughly doubling the risk after adjusting for tobacco smoking. These trends were more pronounced when adjustment for duration of work was included [Garshick et al., 2008]. These findings were supported by those in other occupational groups and by casecontrol studies including various occupations involving similar exposures. An increased risk for bladder cancer was also noted in many case-control studies, but not in cohort studies. The Working Group concluded that there was “sufficient evidence” in humans for the carcinogenicity of diesel-engine exhaust. In experimental animals, whole diesel-engine exhaust caused an increased incidence of lung tumours in rats [Heinrich et al., 1995], diesel-engine exhaust particles instilled intratracheally caused benign and malignant lung tumours in rats [Pott and Roller, 1995], and extracts of the particles also caused lung carcinomas in rats and sarcomas at the injection site in mice [Kunitake et al., 1986 ; Grimmer et al., 1987]. The Working Group concluded that there was “sufficient evidence” in experimental animals for the carcinogenicity of these three agents, which also induced, in vitro and in vivo, various forms of DNA damage. Gasphase diesel-engine exhaust – without particles – did not increase the incidence of respiratory tumours in any species tested. Positive genotoxicity biomarkers of exposure and effect were also observed in humans exposed to diesel engine exhaust. The Working Group concluded that there is “strong evidence” for the ability of whole diesel-engine exhaust to induce cancer in humans through genotoxicity.