A new study of commuter exposure to fine particulate matter in Canada’s three biggest rail transit lines has found air quality in Toronto’s subway line to be the worst.
The study compared air quality in the subway lines in Toronto, Montreal and Vancouver and found that for a daily commute of 70 minutes, commuters in Toronto were exposed to an average of 95 micrograms per cubic metre of fine particulate matter (PM2.5), an amount roughly ten times that of outdoor air readings in Toronto and comparable to an average day’s exposure in Beijing.
“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,” say the study’s authors, whose results are published in the journal Environmental Science & Technology.
Researchers from Health Canada, the University of Toronto and McGill University rode subway lines in Canada’s three biggest cities during morning and evening rush hours for three-hour periods, collecting samples with portable pollution metres. Samples were taken both in winter and summer periods and analyzed for differences between exposure during time waiting on subway platforms versus time spent travelling in train cars, along with differences between above-ground transit and below-grade transit.
The results showed that median fine particulate matter and coarse particulate matter were roughly three times higher along Toronto’s subway line than Montreal’s and over five times higher than Vancouver’s.
The study’s authors say that the amount of above- to below-grade transit on the three lines makes up part of the difference. The three systems are of comparable length (all three between 68 and 69 kilometres) but whereas Toronto’s metro is predominantly below-grade and Montreal’s is entirely below-grade, Vancouver’s Skytrain is mostly at-grade and elevated outdoors.
But a further variable at play is differences in system design, with both Toronto’s and Vancouver’s trains using steel wheels on steel tracks whereas Montreal’s uses rubber wheels on concrete rollways. Steel wheel systems are known to produce “rail dust,” friction-generated metal-heavy particulate which gets blown into the air each time a train passes.
“What happens is as the train moves down the tunnel, it’s like a piston that’s pushing all the air in front of it,” says Greg Evans from the Department of Chemical Engineering and Applied Chemistry at the University of Toronto and study co-author, to the Toronto Star. “What that does is any dust that’s in the tunnel gets re-entrained, basically gets blown around, so the concentrations in the station increase,” he said.
Iron, barium and manganese particulate were among the most common sampled on Toronto’s subway, while exposure to copper was noted in Montreal and molybdenum in Vancouver.
The study found that the average TTC commuter’s exposure to fine particulate matter increased by about 20 per cent per day due to travelling by subway. The study’s authors say that improved ventilation, in-car filtration and regular rail dust cleaning are all measures that could improve air quality.
“Our results suggest that metro system features can significantly influence PM2.5,” say the study’s authors. “These findings may be used to guide decision-making processes by transportation planners in Canada and elsewhere to help reduce PM exposures for the commuting public.”