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In 2005, more than 60 percent of the 9.7 billion transit passenger trips in the United States were provided by buses, helping to reduce petroleum consumption, decrease emissions and reduce vehicle miles traveled. Currently, 84 percent of the nation’s buses and trolleys are powered by diesel. Further, more than half a million school buses, mostly powered by diesel, transport approximately 24 million children in the U.S. Diesel exhaust contains a number of harmful pollutants such as nitrogen oxides and volatile organic compounds that form smog, air toxics and fine particulate matter (PM). Exposure to diesel exhaust is associated with a number of chronic and acute health effects. While buses offer many environmental, energy security and economic benefits, it is important to ensure that cleaner, advanced technologies are being incorporated into transit systems across the country. Transit agencies nationwide are making efforts to deploy cleaner, more efficient buses in their systems. Alternative fuels such as compressed natural gas, liquefied natural gas, biodiesel, propane and hydrogen fuel cells are becoming more prevalent in fleets. In addition, hybrid-electric buses have begun gain popularity with over 700 vehicles in service and 400 on order. These cleaner and buses are helping not only to improve air quality and protect public health, but also reduce public transportation’s reliance on oil-based fuels.