Current research on the cancer risks of complex mixtures of urban air pollutants addresses the identification and comparative assessment of mutagenic and carcinogenic chemicals and emission sources. To identify genotoxic compounds in urban air, micromutagenesis methods are coupled to analytical fractionation and new chemical characterization methods. Among polar mutagenic compounds, many appeared to be nitrated and or oxygenated aromatic compounds. Source apportionment of the mutagenic activity observed in urban air shows that vehicles and residential heating are major contributors to the ambient mutagenicity of the aerosol fraction. Comparative tumorigenesis and DNA adduct dosimetry studies are also being conducted with these same combustion emission sources and urban aerosols. The potential hazard and sources of the gaseous emissions have not been determined, but appear to be significantly increased by atmospheric transformation by ozone and nitrogen oxides.