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The Clean Air Act in the United States identifies diesel-powered motor vehicles, including transit buses, as significant sources of several criteria pollutants that contribute to ground-level ozone formation or smog. The effects of air pollution in urban areas are often more significant due to congestion and can lead to respiratory and cardiovascular health impacts. Life cycle assessment (LCA) has been utilized in the literature to compare conventional gasoline-powered passenger cars with various types of electric and hybrid-powered alternatives, however, no similarly detailed studies exist for mass transit buses. LCA results from this study indicate that the use phase, consisting of diesel production/combustion for the conventional bus and electricity generation for the electric bus, dominates most impact categories; however, the effects of battery production are significant for global warming, carcinogens, ozone depletion, and eco-toxicity. There is a clear connection between the mix of power-generation technologies and the preference for the diesel or electric bus. With the existing U.S. average grid, there is a strong preference for the conventional diesel bus over the electric bus when considering global warming impacts alone. Policy makers must consider regional variations in the electricity grid prior to recommending the use of battery electric buses to reduce carbon dioxide (CO2) emissions. This study found that the electric bus was preferable in only eight states, including Washington and Oregon. Improvements in battery technology reduce the life cycle impacts from the electric bus, but the electricity grid makeup is the dominant variable.