Metro Commuter Exposures to Particulate Air Pollution and PM2.5-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 PM2.5 (PM = particulate matter), PM1.0, ultrafine particles, black carbon, and the elemental composition of PM2.5. 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 PM2.5 and its elemental components were compared to those of the metro in each city. A microenvironrnental exposure model was used to estimate the contribution of a 70 min metro commute to daily mean exposure to PM2.5 elemental and mass concentrations. Time spent in the metro was estimated to contribute the majority of daily exposure to several metallic elements of PM2.5 and 21.2%, 11.3% and 11.5% of daily PM2.5 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 PM2.5 and its elemental components to a metro commuter's daily exposure.