Metro Commuter Exposures to Particulate Air Pollution and PM^sub 2.5^-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 PM^sub 2.5^ (PM = particulate matter), PM^sub 10^, ultrafine particles, black carbon, and the elemental composition of PM^sub 2.5^. 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 PM^sub 2.5^ 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^ 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^ and 21.2%, 11.3% and 11.5% of daily PM^sub 2.5^ 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 PM^sub 2.5^ and its elemental components to a metro commuter's daily exposure.