Personal exposures to traffic-related air pollution in three Canadian bus transit systems: the Urban Transportation Exposure Study

The study addresses how much bus commuting contributes to personal exposure to traffic-related air pollution (TRAP) and how vehicle technology and transit infrastructure influence exposures. TRAP is ubiquitous in urban areas and is linked to increased cardiopulmonary disease, certain cancers, type II diabetes, and neurotoxicity. Although commuting accounts for roughly 5% of a Canadian’s day, exposures in this microenvironment can disproportionately affect daily TRAP exposure. The purpose of the Urban Transportation Exposure Study (UTES) bus component was to quantify personal exposures to NO2, ultrafine particles (UFPs), black carbon (BC), PM2.5, PM10, and fine-fraction elements while waiting for and riding buses in Toronto, Ottawa, and Vancouver; to estimate the contribution of bus commuting to daily exposures; to evaluate whether certain bus stop/station types act as exposure hotspots; and to simulate potential air quality improvements from bus fleet renewal and station interventions. This work is important for refining exposure assessment and informing policies on transit system design and fleet technologies that can mitigate health risks from TRAP.

The introduction summarizes evidence linking TRAP to cardiopulmonary morbidity and mortality, cancers, type II diabetes, and neurotoxicity. Transportation contributes a substantial share of PM2.5-related mortality in North America. Regulatory actions since 2004–2007 have tightened diesel engine emission standards and enabled effective after-treatment (e.g., diesel particulate filters) via ultra-low sulfur diesel fuel, contributing to declines in ambient PM2.5. However, the impact of these changes on in-bus exposures had not been well characterized. Non-exhaust emissions (brake, tire, and road wear) are increasingly important components of TRAP and contribute metals to PM, with epidemiologic links to cardiopulmonary outcomes and lung cancer. Prior UTES publications addressed subway/metro and private vehicle exposures; this study fills a gap for public bus transit, which predominantly uses diesel propulsion and is a common commuting mode in Canada.

Data collection: Personal exposure monitoring was conducted in the public bus transit systems of Toronto and Ottawa (three weeks in summer 2010 and winter 2011) and Vancouver (winter and summer 2013) during weekday peak hours (7–10 a.m., 3–6 p.m.). During each 3-hour session, three researchers sampled simultaneously on selected high-ridership routes covering the network. Researchers alternated between riding and waiting by disembarking at regular intervals to capture both microenvironments, logging bus and stop IDs via synchronized digital voice recordings and GPS. Sampling inlets were positioned in the breathing zone. Continuous instruments measured PM2.5, UFPs, and BC at 1-second resolution; data were averaged over each waiting or riding session. Integrated filter-based PM2.5 and PM10 samples were collected concurrently (each representing ~30 hours per week per researcher), yielding 18 PM2.5 and 18 PM10 samples per city. Microenvironmental models: To estimate the weekday contribution of bus commuting to daily exposure, two approaches were used. For PM2.5 mass and fine-fraction elements, integrated bus exposure measurements were combined with city/season-specific ambient 24-hour gravimetric PM2.5 and elemental composition from the National Air Pollution Surveillance network, applying a correction factor for an 11% positive bias between the personal environmental monitor and the NAPS dichotomous sampler. The commuting time was set to 66 minutes. For BC and UFP, high-resolution waiting (mean 16 minutes) and riding (mean 50 minutes) exposures were combined into a commute-weighted value and compared against three ambient scenarios (low, moderate, high) due to limited ambient BC/UFP monitoring data. Bus and stop classifications: Bus IDs were linked to fleet data to classify propulsion and emission control categories: 1983–2003 diesel (referent), 2004–2006 diesel, 2007-and-later diesel (DPF-equipped), hybrid diesel/electric, and electric. Stops were classified as bus stops (outdoor roadside), outdoor bus stations (multi-route hubs, outdoor), and enclosed bus stations (indoor/below grade hubs). GIS verified stop types. Statistical analysis: Nine multivariate linear mixed-effects models (one per city for each pollutant: PM2.5, UFP, BC) estimated the association of bus stop type with waiting exposures and bus type with riding exposures, adjusting for land-use and road network confounders within 500 m buffers identified via a directed acyclic graph. An AR(1) covariance structure accounted for repeated measures within sessions. Effects were expressed as percent changes relative to referent categories. Diagnostic checks included Cook’s distance and residual normality tests. Intervention simulations: Using model-derived effect sizes and observed frequencies, the study simulated two interventions: (1) reducing concentrations at stations to bus-stop levels to estimate improvements in waiting exposures, and (2) replacing 1983–2003 diesel buses with hybrid diesel/electric buses to estimate improvements in riding exposures. Daily means before and after interventions were compared to calculate percent reductions.

- Bus commuting time (66 minutes; ~4.6% of the day) contributed disproportionately to daily exposure to certain fine-fraction elements: • PM2.5-Ba: ~59% (SD 15%) in Toronto, 60% (SD 20%) in Ottawa, 57% (SD 18%) in Vancouver. • PM2.5-Fe: 70% (SD 19%) in Toronto, 64% (SD 15%) in Ottawa, 70% (SD 15%) in Vancouver. - Estimated contribution of bus commuting to weekday PM2.5 mass exposure: 6% (SD 2%) in Toronto, 13% (SD 10%) in Ottawa, and 11% (SD 4%) in Vancouver. - Enclosed bus stations were identified as hotspots with elevated PM2.5 and BC relative to standard outdoor bus stops. - Riding exposures were associated with bus type. Buses equipped with diesel particulate filters (2007-and-later) and hybrid diesel/electric buses had significantly lower in-bus PM2.5, UFP, and BC compared with 1983–2003 diesel buses, with the exception that UFP reductions were not observed in Vancouver. - Simulated interventions suggested that improving air quality at stations (to bus-stop levels) and renewing fleets toward hybrid diesel/electric buses could reduce waiting and riding exposures, respectively (directionally beneficial; city- and pollutant-specific magnitudes not detailed in the excerpt).

Findings show that short durations spent in bus environments can dominate daily exposure to certain TRAP components, notably fine-fraction metals such as Ba and Fe that are markers of non-exhaust sources (e.g., brake and wheel wear). Elevated PM2.5 and BC concentrations at enclosed stations indicate that design and ventilation of transit infrastructure substantially influence commuter exposures. The observed lower riding exposures in DPF-equipped and hybrid diesel/electric buses relative to older diesel buses highlight the effectiveness of post-2007 emission controls and alternative propulsion in reducing in-cabin pollutants, addressing a key objective of the study. Together, these results emphasize that both infrastructure interventions (reducing station concentrations) and fleet modernization can meaningfully lower commuter TRAP exposures, thereby potentially mitigating associated health risks.

This study provides a comprehensive assessment of personal TRAP exposures in three Canadian bus systems, quantifying the contribution of bus commuting to daily exposure and identifying hotspots and technological determinants of exposure. Bus commuting disproportionately contributes to daily exposure to fine-fraction metals and materially contributes to PM2.5 mass, while enclosed stations elevate PM2.5 and BC. Fleet categories with DPFs and hybrid diesel/electric propulsion reduce in-bus PM2.5, UFP, and BC compared to older diesel buses. These insights can inform transit policy and exposure models. Future research should quantify the effectiveness of specific station ventilation or filtration interventions, evaluate exposures across additional seasons and cities, incorporate real-world BC/UFP ambient baselines, and assess health outcomes linked to observed exposure reductions.

- Ambient data for BC and UFP were limited; the contribution of commuting to daily exposures for these pollutants relied on assumed ambient scenarios (low, moderate, high), introducing uncertainty. - Sampling focused on weekday peak commuting hours and selected high-ridership routes, which may limit generalizability to off-peak periods, weekends, or less-traveled routes. - Integrated PM2.5 and PM10 sampling yielded city-level weekly composites; while corrected for sampler bias, temporal variability within weeks may not be fully captured. - The mixed-model analysis adjusts for land use and road types within 500 m buffers, but residual confounding from unmeasured factors (e.g., meteorology, transient traffic conditions) is possible. - Some results and detailed magnitudes (e.g., exact percent reductions from simulated interventions) depend on supplementary materials not provided here.