About 90% of school bus fuel consumption is diesel, say the researchers. They note that diesel particle matter "has been estimated to cause a majority of the ambient air pollution" in the area studied (California's south coast air basin).
Children are believed to be especially susceptible to air pollution, the researchers say. — As a major form of children's transportation, school bus emissions represent a potentially important source of children's exposure to vehicle pollution.
Scientists monitored the air inside six empty school buses while driving the buses on routes in and around Los Angeles. The highest pollution levels inside the buses were seen with the oldest models (circa 1975) with high diesel emissions and closed windows.
"Self-pollution is substantial for all six buses," write the researchers, who included Julian Marshall, a PhD candidate with the Energy and Resources Group at the University of California, Berkeley.
"The school bus microenvironment contributes significantly to children's estimated total inhalation intake of DPM [diesel particulate matter]," says the report. Primary data came from a study sponsored by the California Air Resources Board, says the journal.
John Corr, president-elect of the National School Transportation Association, says, "The average age of school buses is much newer than some of the extremes that were used in that test." — Corr is also president of The Trans Group, a school bus fleet contracting company based in Spring Valley, N.Y.
Corr says the average age of buses in privately owned fleets is closer to eight years. Buses built from 1995 have significantly improved diesel engines that use electronics to help the engines burn more cleanly and efficiently.