Metro Commuter Exposures to Particulate Air Pollution and PM<sub>2.5</sub>-Associated Elements in Three Canadian Cities: The Urban Transportation Exposure Study

System-representative commuter air pollution exposure data were collected for the metro systems of Toronto, Montreal, and Vancouver, Canada. Pollutants measured included PM<sub>2.5</sub> (PM = particulate matter), PM<sub>10</sub>, ultrafine particles, black carbon, and the elemental composition of PM<sub>2.5</sub>. Sampling over three weeks was conducted in summer and winter for each city and covered each system on a daily basis. Mixed-effect linear regression models were used to identify system features related to particulate exposures. Ambient levels of PM<sub>2.5</sub> and its elemental components were compared to those of the metro in each city. A microenvironmental exposure model was used to estimate the contribution of a 70 min metro commute to daily mean exposure to PM<sub>2.5</sub> elemental and mass concentrations. Time spent in the metro was estimated to contribute the majority of daily exposure to several metallic elements of PM<sub>2.5</sub> and 21.2%, 11.3% and 11.5% of daily PM<sub>2.5</sub> exposure in Toronto, Montreal, and Vancouver, respectively. Findings suggest that particle air pollutant levels in Canadian metros are substantially impacted by the systems themselves, are highly enriched in steel-based elements, and can contribute a large portion of PM<sub>2.5</sub> and its elemental components to a metro commuter’s daily exposure.