Study was conducted to characterize the range of children's exposures to diesel vehicle-related pollutants and other vehicle pollutants during their commutes to school by school buses. It was the most comprehensive school bus exposure study ever conducted. Researchers at the University of California's Riverside and Los Angeles campuses, measured pollutant concentrations inside five conventional diesel school buses while driving actual school bus routes in Los Angeles. For comparison, a diesel bus equipped with a particulate trap and a bus powered by natural gas were also included.
Buses were outfitted with dual sets of real-time instruments, which allowed front versus back and inside versus outside comparisons. The researchers measured multiple diesel vehicle-related pollutants, including black carbon and particle-bound PAHs, as well as many other exhaust pollutants. A tracer gas was used to determine the bus's own contributions to on-board concentrations. The study measured exposures inside the buses and did not include tail-pipe emissions tests.
Measurements indicated that for some buses, significantly higher exposures of vehicle-related pollutants occurred during the bus commutes than roadway pollutant concentrations alone would indicate. The high commute concentrations were a function of several influences:
- the high concentrations of pollutants already present on roadways, especially if traffic was heavy;
- the direct influence of other vehicles being followed; and
- the contribution of the bus’s own emissions. The extent of a bus’s own contribution to these high concentrations appeared to be highest when windows were closed for the older diesel buses, but bus-to-bus variability was high.