Gaseous substances such as nitrogen dioxide (NO2) and sulfur dioxide (SO2) stimulate the process of nitration of polycyclic aromatic hydrocarbons, and the transformation products display a broad spectrum of mutagenicity, genotoxicity, and carcinogenicity. Bacterial mutation by nitroarenes is specific. Tetracyclic nitroarenes are thought to be the most mutagenic compounds in the Salmonella test system, and some are carcinogenic in rats and mice. Furthermore, it was found that the mutational nitroarenes produced mostly DNA damage, which is subject to recombination repair in the rec assay system using Bacillus subtilis. Nitroarenes in the environment seem to be ubiquitous; the majority of the compounds are emitted directly from diesel emissions, kerosene heaters, and gas and liquefied-gas burners or heaters. In nitroarenes induced during incomplete combustion, nitropyrene and nitrofluoranthene derivatives are the most important mutagens/carcinogens for determining the chronic toxicity of nitroarenes overall.