Nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) are formed during incomplete combustion. Sources include emissions from vehicles (mainly diesel vehicles), heating, smoking, certain types of food processing, and incomplete combustion in general. Nitro-PAHs are direct-acting mutagens, and a number of them have been shown to be carcinogens. 2-Nitrofluorene (NF) represents a model substance for the nitro-PAHs. An attempt has been made to calculate the human cancer risk due to exposure to nitro-PAHs by two different models. In the first model, genotoxic lesions were transferred to units of Gray (gamma-irradiation), and in the second model a mega study (24,000 animals) on the carcinogenicity of one metabolite of NF was used to elucidate the risk. Gamma-irradiation of the rat liver gave rise to preneoplastic foci in a dose-dependent manner, which was statistically significant. The Gray-equivalents of chemically (NF) induced foci were calculated, and from the human nitro-PAH exposure, expressed in Sievert, a human risk estimate was calculated. In the second model, an extrapolation from laboratory animals to man was performed because tumor data on 2-acetylaminofluorene (AAF), a major metabolite of NF, were available in the literature. The tumor dose-response data on AAF was linear for tested lifetime doses. The results of both models agreed, with a risk range of 0.15-49 x 10(-6) on human cancer risk for an urban citizen.