ARB Research Seminar

This page updated June 19, 2013

Evaluation of Methods to Reduce Exhaust Penetration into School Buses

Arthur M. Winer, Ph.D., Environmental Science and Engineering Program, Department of Environmental Health Sciences, School of Public Health, University of California, Los Angeles

November 27, 2007
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA

Research Project


"Self-pollution," the intrusion of a school bus's own exhaust into the bus cabin, and "leader-follower pollution" lead under some conditions, to high passenger exhaust exposures. This study investigated how and where self-pollution occurs, and tested various methods to mitigate "self" and "leader-follower" pollution.

We first investigated the magnitude and location of exhaust system leaks and found that in a well-maintained bus, exhaust system leaks were insignificant. An attempt was also made to identify specific exhaust entry points into the passenger compartment but this proved infeasible as many entry points were identified. Quantification of overall cabin tightness for 17 buses using the "blower door" method showed lower leak rates in newer buses. The blower door method can be used as a diagnostic test to ensure cabin tightness is maintained as buses age.

The self-pollution mitigation methods evaluated were elevation of the exhaust outlet, power ventilation of the cabin, and a combination of the two strategies. These methods were evaluated for their efficacy in reducing not only self-pollution but also exhaust intrusion from closely following a leading bus (leader-pollution). Tests were conducted in both idling mode and while driving a prescribed route, using four buses representative of the current California in-use school bus fleet. Self-pollution and exhaust intrusion from a leader bus were measured using a dual tracer gas approach to compare elevated versus standard (low) exhaust pipe heights.

The use of power ventilation appeared to reduce exposure to self-pollution and leader-pollution. However, occasionally exhaust plumes reached the blower inlet at low driving speeds or during idling, causing high peak concentrations in the bus's cabin that largely negated the benefits of the blower. Use of an elevated exhaust height significantly reduced self-pollution while producing modest reductions in leader-pollution. Children's exposure to exhaust from self-pollution could be significantly reduced by raising the exhaust outlet to a high position in school buses.

Speaker Biography

Arthur Winer, Ph.D., is Distinguished Professor of Environmental Health Sciences in the School of Public Health, University of California, Los Angeles and a core faculty member and past Director of the Environmental Science and Engineering Program. Over the past 35 years he has published more than 200 peer-reviewed journal articles and book chapters on a wide range of air pollution topics. Dr. Winer's current research is focused primarily on air pollutant exposure measurements, including localized exposures related to diesel and gasoline vehicle emissions; community exposures in the vicinity of southern California ports and refineries; and children's exposures in diesel school buses, portable classrooms, homes and other relevant microenvironments. In addition to his research and teaching contributions, Dr. Winer has worked for three decades at the local, state, and national levels to promote legislation and public policies designed to address a broad range of air pollution and public health concerns.

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