The Committee was asked to develop a list of organic chemicals identified in the vapor phase in exhaust from diesel- and gasoline-fueled engines and to explore the availability of toxicologic and epidemiologic information on each of them for use in making a quantitative and qualitative assessment of the human health risk associated with exposure to such exhaust. The committee determined that quantitative risk assessment could probably be applied to only three to five vapor-phase components of diesel exhaust: formaldehyde, benz(a)-anthracene, acrolein, and perhaps 2-nitroflourene and chrysene. Data on the others seem insufficient for detailed quantitative risk assessment, although comparative risk assessment or qualitative risk assessment would be possible. Readers may find interesting the listings in the appendix of specific chemicals in exhaust of engines as summarized from published literature.