The rapid increase in new combustion technologies and new fuels for automobiles, residential and industrial heating, and other energy-related processes poses a particularly unique problem for both the scientific assessment of risk and the regulatory decision-making process that may result in the final management of these risks. These problems arise because the combustion source used to provide our transportation and energy needs result in the emission of incomplete combustion products. There is a large scientific data base that supports the conclusion that all such combustion emissions contain polycyclic organic matter (POM), including polynuclear aromatic hydrocarbons (PNAs) and substituted PNAs. Many of these emissions and individual PNAs produce malignant tumors in animals and are mutagenic in short-term genetic toxicology bioassays. Therefore, since all incomplete combustion emissions are likely to be carcinogenic, a quantitative estimate of cancer risk from a new combustion technology is difficult to assess without comparable estimates of cancer risks from the existing technologies.