Comparison of PM2.5 Elemental Composition and Oxidative Potential in Two Canadian Subway Systems: Toronto and Montréal

Commuters are exposed to higher PM2.5 levels in subway systems than in other modes of transportation because the underground portions of a subway system are confined spaces, where limited ventilation promotes pollutant accumulation [1].

Toronto and Montréal have some of the busiest public transit systems in North America. Most of the PM2.5 in Toronto’s subway system comes from braking and abrasion of wheels and rails, while PM2.5 concentrations in Montréal's are lower than in Toronto’s subway system [2]. Daily commuting on Toronto’s subway can increase daily PM2.5 exposure by 20% and contribute over 80% of daily exposure to metals such as Cu, Ba, and Fe despite the short time spent in the subway [2]. While the mass concentration of PM2.5 in the Montréal system is lower, commuting in this system still contributes highly to metals such as Cu and Fe. However, there is a knowledge gap regarding whether subway PM2.5 may have more adverse health effects than ambient PM2.5.

Numerous studies have found that exposure to elevated ambient PM2.5 levels can cause adverse health effects, raising concerns about the iron-rich PM2.5 levels found in subway systems [3,4]. These particles may influence the oxidative potential of PM2.5, a metric used to assess PM's ability to generate reactive oxygen species and potential toxicity [5].

Here, we compare the oxidative potential and elemental composition of subway PM2.5 collected across the Toronto and Montréal subway systems, as well as roadside ambient PM2.5 measured in Toronto. By comparing two subway systems with different train technologies and ventilation systems, this study provides new insights into the factors influencing the potential toxicity of subway PM and informs strategies to improve air quality in public transportation systems. In addition, by contrasting these measurements with outdoor environments, this study investigates similarities between non-exhaust emissions in subway and roadside environments.

References
[1] Moreno, T., and Amato, F. Field Actions Science Reports 2020, Special Issue 21, 24–27.
[2] Van Ryswyk, K., Anastasopolos, A. T., Evans, G. J., Sun, L., Sabaliauskas, K., Kulka, R., Wallace, L., and Weichenthal, S. Environ. Sci. Technol. 2017, 51, 5713–5720.
[3] Loxham, M., and Nieuwenhuijsen, M. J. Particle and Fibre Toxicology 2019, 16, 12.
[4] Nieuwenhuijsen, M. J., Gómez-Perales, J. E., and Colvile, R. N. Atmospheric Environment 2007, 41, 7995–8006.
[5] Bates, J. T., Fang, T., Verma, V., Zeng, L., Weber, R. J., Tolbert, P. E., Abrams, J. Y., Sarnat, S. E., Klein, M., Mulholland, J. A., and Russell, A. G. Environ. Sci. Technol. 2019, 53, 4003–4019.