"Self-pollution," the intrusion of a bus's own exhaust into the bus cabin, leads under some conditions to very high exposures. This study attempted to elucidate how and where selfpollution occurs, and to test various methods to mitigate this phenomenon. 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. Comprehensive detection of leaks in the exhaust system using SO2 from the exhaust as a tracer gas and a survey of leak potential using back pressure measurements showed that exhaust system leaks in a well-maintained system were insignificant. However, identifying specific exhaust entry points into the passenger compartment using tracer gas was found to be infeasible due to the large number of potential entry points. To quantify overall air tightness of cabins, the leak rate of 17 buses was evaluated by pressurizing them with an air blower with a constant flow rate and measuring the pressure differential between the inside and outside of the bus ("blower door method"). Pressure differentials ranged over a factor of five, but in general, newer buses showed lower leak rates.

The primary self-pollution mitigation methods evaluated consisted of elevating the exhaust outlet, power ventilating the cabin, or a combination of the two methods. Because following other buses is also a major source of high bus cabin concentrations, these methods were evaluated for their efficacy in reducing not only self-pollution but also pollution from a leader bus. Comparisons were made both in stationary mode and while driving a prescribed route, using four test buses representative of the current in-use school bus fleet. Exhaust intrusion into the cabin was measured using a dual tracer gas approach to allow for a direct comparison between the mitigated and unmitigated scenarios. Two separate, non-interfering tracer gases were metered into the exhaust in proportion to engine intake flow rates to maintain near-constant tracer gas concentrations in the exhaust. Real-time analyzers were used to monitor the concentration of each tracer gas inside the cabin of the test bus. The concentration data were used to calculate the volumetric fraction of air inside the bus that originated from each tracerlabeled exhaust.

Evaluation of the high-exhaust mitigation strategy used a split exhaust (half of the flow released above the roof and half released at the normal low position) with a separate tracer gas metered into each half. When evaluating exhaust intrusion from a leader bus, both tracers were similarly released on a leader bus while measurements were taken on a follower bus. A second set of leader-follower experiments involved metering one tracer gas in the leader bus exhaust and metering the other tracer gas in the follower bus exhaust. This allowed comparing the magnitude of self-pollution versus exhaust intrusion from a leader vehicle. The effects of power ventilation were evaluated by comparing the above test outcomes with the blower on versus off. While results showed the blower reduced the exposure to self-pollution and leader-pollution most of the time, occasionally exhaust plumes reached the blower inlet at low speeds or during idling, causing high peak concentrations that largely negated the benefits of the power ventilation. Using an elevated exhaust outlet significantly reduced the exposure due to self-pollution, but resulted in only modest reductions in leader-vehicle pollution. Our overall recommendations are to employ elevated exhaust outlets on school buses and to minimize exposure to leader vehicle exhaust by avoiding close caravanning of diesel school buses.