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The 2006 School Bus Pollution Report Card by the Union of Concerned Scientists finds that U.S. school buses are among the oldest vehicles on the road, with an average age of nine years, and emit nearly twice as much soot per mile as tractor-trailer trucks. The report grades states on tailpipe soot emissions, smog-forming pollution, and the success of their cleanup programs. While some states like California and Washington have made significant strides in reducing soot through retrofits and cleaner fuels, national soot pollution from school buses has only decreased by more than two percent. The document highlights the vulnerability of children to diesel exhaust, which is linked to asthma and lung growth deficits, and argues that substantial federal and state investment is needed for bus replacement and the installation of particulate traps.

The report argues that California can significantly reduce its global warming impact by mandating the adoption of existing and near-term vehicle technologies. Modeling shows that currently available improvements in engines, transmissions, and air conditioning could reduce fleet average heat-trapping emissions by 20%, while advanced technologies available within five years could achieve a 40% reduction. These improvements are presented as cost-effective, with fuel savings offsetting initial costs within a few years, and would provide a model for other U.S. states and Canada.