The identification and assessment of airborne carcinogens has been significantly advanced by the use of short-term genetic bioassays. Bioassay-directed fractionation coupled with new organic characterization methods has provided the tools needed to more efficiently identify potential carcinogens in complex combustion emissions and urban air samples. These studies have shown that a significant portion of the mutagenicity is found in fractions more polar than polynuclear aromatic hydrocarbons (PAHs). New mass spectrometry techniques have been developed to identify and quantify the more polar mutagens in these samples. The comparative mutagenicity and carcinogenicity of a series of combustion emissions has been assessed using dose-response studies in bacteria, mammalian cells and rodents. This data base has been used to develop a comparative potency risk assessment methodology for a series of combustion emissions.