Mutagenic nitroarenes, diesel emissions, particulate-induced mutations and cancer: An essay on cancer-causation by a moving target

Initial analyses of the lung tumors seen in rats exposed for their lifetime to elevated levels of the emissions of diesel engines suggested that they were due to powerful mutagens and carcinogens (PAHs, nitro PAHS) adsorbed onto the diesel particles. However, further studies showed that carcinogenicity occurred only under conditions that resulted in impaired lung clearance ('overloading') leading to inflammatory reactions and other pathologic sequelae. These observations together with the findings that carbon black, a model for diesel particles devoid of organic mutagens and carcinogens, also induced lung cancers under conditions of overloading led to the suggestion that the cancers resulted from a non-genotoxic mechanism. However, the further finding that inert particulate carcinogens devoid of organics, induced mutations has led to a re-evaluation of the role of mutations in lung carcinogenesis caused by particles and the relevance of the rat model to humans. This is especially timely as epidemiological studies suggest that humans may develop lung cancers following occupational exposure to diesel emissions by a mechanism unlikely to involve lung overloading. Finally, the recent recognition that environmental PM-10 (respiratory size particles) may be responsible for a significant portion of human morbidity and mortality, ensures that the health effect of diesel emissions will continue to receive scrutiny as they contribute to the PM-10 load.