Children’s School Bus Exposure and Mitigation Studies

This page updated November 3, 2016

ARB has funded 3 studies to examine children’s exposures in school buses and possible mitigation approaches.  The three studies are discussed and linked below.  A list of published literature on the topic is also provided below in the section titled “Other Publications.”  

The first Children's School Bus Exposure 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.

Major Findings:

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.

Recommendations from this report include:

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

Downloadable Documents

Mitigation Studies

ARB has sponsored two mitigation studies.  First, in 2006, ARB sponsored a study titled "Evaluation of Mechanisms of Exhaust Intrusion into School Buses and Feasible Mitigation Measures." The researchers in this study attempted to clarify how and when self-pollution of school buses occurs and tested various methods to reduce it. They also investigated how exhaust enters a school bus from a leader bus (one bus that is being driven or idled in front of another bus).

The results of the study showed that:

  1. One method tested for determining the overall tightness of a bus was effective and should be used to check tightness as buses age.
  2. Self-pollution of buses could be significantly reduced by placing the exhaust outlet above the bus.
  3. Exhaust from a leader bus can be more significant than self-pollution and thus, this research further strengthened the recommendation that school buses should not caravan close together or follow closely behind other diesel-powered vehicles.

In 2012 ARB sponsored a study entitled "Reducing Air Pollution Exposure in Passenger Vehicles and School Buses" to explore the application of high efficiency filtration to reduce in-cabin exposure to particulate pollution, including PM2.5 and ultrafine particles (UFP).  Automobiles and school buses were equipped with high efficiency filters rated MERV (Minimum Efficiency Reporting Value) 16 or higher, or HEPA (High-Efficiency Particle Arrestance) filters. 

The results of the study showed that high efficiency filtration is an effective mitigation strategy for reducing in-cabin exposure to particles in both cars and buses.  Filtration reduced ultrafine particles in school buses by 88%, black carbon by 84%, and PM2.5 by 55%, and filters were even more effective in cars.

Other ARB School Bus Programs

  • School Bus Idling ATCM Program - The California Air Resources Board has 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 - This program is for the acquisition of clean, safe, school buses.

Other Publications

  • Adar, S.D., D’Souza, J.D., Sheppard, L., Kaufman, J.D., Hallstrand, T.S., Davey, M.E., Sullivan, J.R., Jahnke, J., Koenig, J., Larson, T.V., Liu L-J.S., 2015. Adopting Clean Fuels and Technologies on School Buses: Pollution and Health Impacts in Children. Amer. J. Respir. Crit. Care Med 191, issue 12, 1413-1421.  
  • Behrentz, E., Fitz, D.R., Pankratz, D.V., Sabin, L.D., Colome, S.D., Fruin, S.A., Winer, A.M., 2004. Measuring self-pollution in school buses using a tracer gas technique. Atmos. Environ. 38, 3735-3746.
  • Ireson, R.G., Ondov, J.M., Zielinska, B., Weaver, C.S., Easter, M.D., Lawson, D.R., Hesterberg, T.W., Davey, M.E., Liu, L.J.S., 2011.  Measuring in-cabin school bus tailpipe and crankcase PM2.5: a new dual tracer method. J. Air & Waste Management Assoc. 61, 494-503.
  • Lee, E.S., Fung, C-C.D., Zhu, Y., 2015. Evaluation of a High Efficiency Cabin Air (HECA) Filtration System for Reducing Particulate Pollutants Inside School Buses. Environ. Sci. Technol. 49, 3358-3365.
  • Li, F., Lee, E.S., Liu, J., Zhu, Y., 2015.  Predicting self-pollution inside school buses using a CFD and multi-zone coupled model. Atmos. Environ. 107, 16-23.
  • Liu, L-J.S., Phuleria H.C., Webber, W., Davey, M., Lawson, D.R., Ireson, R.G., Zielinska, B., Ondov, J.M., Weaver, C.S., Lapin, C.A., Easter, M., Hesterberg, T.W., Larson, T., 2010.  Quantification of self pollution from two diesel school buses using three independent methods.  Atmos. Environ., 44, 3422-3431.   
  • Marshall, J.D., Behrentz, E., 2005. Vehicle Self-Pollution Intake Fraction: Children’s Exposure to School Bus Emissions. Environ. Sci. Technol. 39, 2559-2563.
  • Rim, D., Siegel, J., Spinhirne, J., Webb, A., McDonald-Buller, E., 2008. Characteristics of cabin air quality in school buses in Central Texas. Atmos. Environ. 42, 6453-6464.
  • Sabin, L.D., Behrentz E., Winer, A.M., Jeong, S., Fitz D.R., Pankratz, D.V., Colome, S.D., Fruin, S.A., 2005a. Characterizing the range of children’s air pollutant exposure during school bus commutes. J. Expos. Anal. and Environ. Epidem. 15:377-387.
  • Sabin, L.D., Kozawa, K., Behrentz, E., Winer, A.M., Fitz, D.R., Pankratz, D.V., Colome, S.D., Fruin, S.A., 2005.  Analysis of real-time variables affecting children's exposure to diesel-related pollutants during school bus commutes in Los Angeles. Atmos. Environ. 39, 5243-5254.
  • Trenbath, K., Hannigan, M.P., Milford, J.B., 2009. Evaluation of retrofit crankcase ventilation controls and diesel oxidation catalysts for reducing air pollution in school buses. Atmos. Environ. 43, 5916-5922.
  • Zhang, Q.F., Zhu, Y.F., 2010. Measurements of ultrafine particles and other vehicular pollutants inside school buses in South Texas.  Atmos. Environ. 44, 253- 261.
  • Zhang, Q., Zhu, Y., 2011. Performance of School Bus Retrofit Systems; Ultrafine Particles and Other Vehicular Pollutants.  Environ. Sci.Technol. 45: 6475-6482.
  • Zhu, Y., Lee, E., 2015.  Application of a High Efficiency Filtration System for Air Pollution Exposure Reduction inside School Buses, Final report to South Coast Air Quality Management District, Grant no. A-0090914-0.  
  • Fitz, D.R., Winer A., 2006. Evaluation of mechanisms of exhaust intrusion into school buses and feasible mitigation measures. Final report to the California Air Resources Board, contract no. 03-343.

For more information about the Children's School Bus Exposure Study, you may contact Kathleen Kozawa at (916) 323-2999