Sources of genotoxicity and cancer risk in ambient air.

Products of incomplete combustion are identified as a major source of carcinogenic risk in urban areas, especially those from small non-industrial sources. The major ubiquitous emission sources outdoors in populated areas are residential home heating and motor vehicles. Indoors the major combustion source is environmental tobacco smoke. Polycyclic organic matter adsorbed onto the particles emitted from incomplete combustion are estimated to make the largest contribution to human genotoxic and cancer risk. Mutagenic emission factors combined with dispersion modelling indicated that automobiles and heating sources were major sources of mutagens. Ambient air studies to apportion the sources of mutagens in non-industrial areas confirmed this prediction. To apportion and estimate the cancer risk of ambient organic matter from particles in vivo animal tumour data, receptor modelling and human exposure data were combined. Tumourigenicity studies of the source apportioned ambient organic matter provided the relative tumour potencies of two ambient samples of different source composition. The human cancer unit risks were developed based on the comparative potency method using tumour data from these ambient samples. Residential wood combustion accounted for 75% of the exposure to particle associated organics, but only 20% of the estimated cancer risk. The remaining 80% of the risk appears to be associated with the mobile source component and atmospheric transformation products from these source emissions.