Children's School Bus Exposure and Mitigation Studies

Children can be exposed to high levels of air pollutants from buses

Children's School Bus Exposure and Mitigation Studies

In California, about one in eight children take a school bus between home and school. Even though the time children spend in school bus commutes account for less than 10% of a child's day, the commutes contribute 33% of a child's daily exposure to some air pollutants. To understand this issue and reduce children's exposure to air pollutants in school buses, CARB has funded studies to examine these exposures and possible mitigation approaches, discussed below (additional selected references). Please email research@arb.ca.gov to request the related final reports.

Children's School Bus Exposure Study (2003)

In the first study, researchers at UC Riverside and UCLA measured diesel vehicle-related pollutants, including black carbon, particle-bound PAHs, and other exhaust pollutants, inside five diesel school buses driving typical 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 studied.

Major Findings

Measurements indicated that in 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, including:

The high concentrations of pollutants already present on roadways, especially if traffic was heavy. With windows open, diesel particulate concentrations were 2-3 times higher on the urban route compared to the suburban or rural route.
Cleaner buses have lower in-vehicle exposures. The CNG-powered bus and the particulate trap-equipped bus showed significantly reduced on-board concentrations of diesel particulates compared to the conventional diesel buses.
The contribution of the bus’s own emissions. The diesel particulate concentration was up to 2.5 times higher when windows were closed, but bus-to-bus variability was high.

Recommendations from this Report

Reduce school bus-related exposures by assigning the newest and cleanest buses to the longest routes.
Avoid caravanning of buses through staggered departure times.
Replace conventional (uncontrolled) diesel school buses with natural gas-powered or particulate trap-equipped buses.
Maintain diesel school buses to reduce visible exhaust.

Evaluation of Mechanisms of Exhaust Intrusion into School buses and Feasible Mitigation Measures (2006)

In this study, researchers at UC Riverside attempted to clarify how and when self-pollution of school buses occurs and tested various methods to reduce it. The mechanism of self-pollution was investigated by evaluating the magnitude of exhaust system leaks, searching for exhaust entry points using a tracer gas, and determining the overall leak rate of the bus cabin. They also investigated how exhaust enters a school bus from a leader bus that is being driven or idled in front of it.

Major Findings

Exhaust system leaks in a well-maintained system were insignificant.
One method tested for determining the overall tightness of a bus was effective.
Exhaust from a leader bus can be more significant than self-pollution.

Recommendations from this Report

Buses should be checked for tightness as buses age.
Self-pollution of buses could be significantly reduced by placing the exhaust outlet above the bus.
School buses should not caravan close together or follow closely behind other diesel-powered vehicles.

Reducing Air Pollution Exposure in Passenger Vehicles and School Buses (2015)

CARB sponsored a study by UCLA to explore the application of two prototypes of high efficiency cabin air filters to reduce in-cabin exposure to particulate pollution, including PM2.5 and ultrafine particles (UFP). Filter performance was evaluated in twelve passenger vehicles and six school buses.

Major Findings

For passenger vehicles, some filters reduced UFP concentrations by 90%, black carbon by 81%, and PM2.5 by 66%.
For school buses, in-cabin UFP and black carbon levels were reduced by 88% and 84%, on average.
The filtration system achieved 55% reductions on average for PM2.5 and kept levels below 12 μg/m³.

Recommendations from this Report

High efficiency filtration should be used to effectively reduce in-cabin exposure to PM2.5, black carbon, and UFP.
CARB Webinar by Dr. Yifung Wu (UCLA): Reducing Air Pollution Exposure in Passenger Vehicles and School Buses

Other CARB School Bus Programs

School Bus Idling ATCM Program: CARB approved an airborne toxic control measure (ATCM) that limits school bus idling and idling at or near schools to only when necessary for safety or operational concerns.
Lower-Emission School Bus Program: The goal of this program is to reduce schoolchildren's exposure to pollution from school buses by funding new, cleaner school buses.