Abstract In this study, the relationship between genotoxic potency and the occurrence of polycyclic aromatic hydrocarbons (PAH), including benzo(a)pyrene (BaP), and nitro-PAH in urban and semi-rural air masses has been investigated. The Salmonella/microsome assay has been used as a measure of genotoxic potency. We find that the ratios of BaP/mutagenicity and PAH/mutagenicity are highly variable. The processes responsible for the variation are formation and degradation of mutagens and transport of polluted air masses from heavily industrialized regions. Air masses from Central Europe are shown to be highly enriched in mutagens as well as in PAH and nitro-PAH. However, the mutagenic activity is much more elevated than the PAH levels when these air masses are mixed with local urban air. Part of the variation in the PAH/mutagenicity ratio can be explained by photochemical transformation. Since BaP has been used in the past as an indicator of the carcinogenic risk of airborne particles, it is suggested that the cancer risk of air pollution has to be re-evaluated.