Findings in a research project conducted at Southwest Research Institute challenge the assertions by some that exhaust emissions from natural gas buses are inherently “cleaner” than those from diesel buses. Natural gas and diesel school buses were tested in three configurations (conventional natural gas and both conventional diesel and new, low-emitting diesel technology, such as that now used in California school buses). Overall, the results demonstrate that low-emitting diesel technology has the lowest emissions of most criteria pollutants and toxic air contaminants [TACs]. Natural gas exhaust had higher levels of six TACs - acetaldehyde, acrolein, benzene, formaldehyde, methyl ethyl ketone, and propionaldehyde - and did not have lower emissions of any TAC, as compared to low-emitting diesel. The low-emitting diesel technology now utilized in California school buses produced much lower emissions than the conventional diesel school bus configuration; it also produced lower emissions than the natural gas bus configuration.
Presented at the Society of Automotive Engineers International Truck & Bus Meeting & Exhibition, November 18, 2002, Cobo Center, Detroit, Michigan, by Charles A. Lapin, Ph. D. , D. A. B. T. , Toxicology Consultant, co-author of a forthcoming SAE paper on the research.