Traffic-related air pollution (TRAP) is a significant public health concern, linked to various cardiopulmonary diseases, cancers, type II diabetes, and neurotoxicity. In North America, the transportation sector contributes substantially to PM2.5-related mortality. While stringent regulations have reduced ambient TRAP levels, personal exposures during commutes remain a research priority. Commuting, although a small portion of a typical day (approximately 5%), can contribute significantly to daily TRAP exposure. This study focuses on public bus transit, a mode extensively using diesel propulsion, which is a known source of black carbon (BC) and ultrafine particles (UFPs). The Urban Transportation Exposure Study (UTES) aimed to characterize TRAP exposures in Canadian commuting environments, with this paper specifically addressing public bus transit systems in Toronto, Ottawa, and Vancouver. Public bus transit is used by a considerable portion of the Canadian population, making its contribution to air pollution exposure significant. While emission standards have been implemented to reduce diesel exhaust emissions, their impact on in-bus exposures hasn't been fully studied. Furthermore, non-exhaust sources of TRAP, such as brake and tire wear, are increasingly recognized as contributors and are particularly relevant in the context of heavy-duty vehicles like buses. This research sought to characterize TRAP pollutants (NO2, UFPs, BC, PM2.5, PM10, and PM2.5/PM10 elemental content) in these cities' bus systems; estimate the contribution of bus commuting to daily TRAP exposures; identify potential TRAP exposure hotspots in bus stations; and simulate the effects of potential air quality interventions (bus fleet renewal and bus station improvements).

The introduction adequately cites numerous studies linking TRAP to adverse health outcomes, highlighting the importance of reducing exposure. The section also discusses the contribution of the transportation sector to PM2.5 mortality and the impact of emission regulations on ambient air quality. Existing research on the health effects of non-exhaust particles and their increasing contribution to TRAP in urban environments is also mentioned. The review emphasizes the need for specific data on TRAP exposure from public bus transit due to its widespread use and association with diesel emissions.

Air pollution exposure data were collected in Toronto, Ottawa, and Vancouver over three weeks during peak commuting hours in summer and winter seasons. Three researchers in each city collected data using personal sampling backpacks equipped to measure PM2.5, UFPs, and BC continuously. Filter-based PM2.5 and PM10 samples were collected concurrently. GPS and DVRs recorded location and activity (riding or waiting). Data were collected at various bus routes, stops, and stations to reflect the diversity of the transit systems. A microenvironmental model was used to estimate the contribution of bus commuting to daily exposures. For PM2.5 and its elemental constituents, this involved combining personal bus exposure data with fixed-site ambient monitoring data (NAPS network). For BC and UFP, the model accounted for both riding and waiting exposures, using estimated ambient conditions due to limited data availability. The percent contribution of bus commuting was calculated using equations that incorporated the time spent commuting, ambient exposure levels, and correction factors where applicable. Multivariate linear mixed models were used to analyze the impact of bus stop type (bus stop, outdoor bus station, enclosed bus station) and bus type (1983-2003 diesel, 2004-2006 diesel, 2007+ diesel, hybrid diesel/electric, electric) on PM2.5, UFP, and BC exposures. These models adjusted for confounders such as land use and road type within 500m buffers, identified through a directed acyclic graph (DAG). Finally, simulations were performed to estimate the impact of air quality interventions (improving bus station air quality and replacing older buses with hybrid diesel/electric buses) on daily exposures.

Data were collected for approximately 900 (Toronto), 1700 (Ottawa), and 1400 (Vancouver) riding and waiting sessions. A typical 66-min bus commute contributed significantly to daily PM2.5 exposure: 6% (Toronto), 13% (Ottawa), and 11% (Vancouver). The contribution was much higher for certain elements within PM2.5, notably barium and iron (around 57-70% contribution). Enclosed bus stations had significantly higher PM2.5 and BC concentrations than bus stops. Buses with DPFs and hybrid diesel/electric propulsion had significantly lower in-bus PM2.5, UFP, and BC levels compared to older (1983-2003) diesel buses (except for UFP in Vancouver). The simulation showed that improving bus station air quality to the level of bus stops and replacing older buses with hybrid vehicles could significantly reduce daily TRAP exposures.

The results demonstrate the substantial contribution of bus commuting to personal TRAP exposures, particularly for fine particulate matter and certain trace elements. The identification of enclosed bus stations as hotspots underscores the need for targeted interventions to improve air quality in these locations. The significant reduction in pollutants observed in buses with newer technology highlights the potential for fleet renewal to mitigate exposure. These findings have important implications for personal exposure modeling and air quality management, suggesting that interventions focusing on bus station improvements and fleet modernization could effectively reduce health risks associated with TRAP. Further research could investigate the long-term health impacts of these exposures and explore other strategies for reducing TRAP in public transit environments.

This study quantified the contribution of bus commuting to daily TRAP exposures in three Canadian cities, highlighting the impact of bus type and bus stop type on personal exposure levels. The findings support the implementation of strategies like improved bus station ventilation and fleet modernization to reduce health risks associated with TRAP. Future research could focus on investigating the effectiveness of such interventions in real-world settings and exploring other mitigation strategies for reducing personal exposure in public transit.

The study's temporal scope (three weeks in each city) and limited number of sampling locations might not fully capture the variability in TRAP exposures across different times of the year and geographic locations within each city. Reliance on estimations of ambient BC and UFP levels due to data limitations could influence the accuracy of the microenvironmental model for these pollutants. The study focused on bus transit and might not be generalizable to other modes of transportation.