Toronto's subway stations and trains had the highest levels of air pollution of Canada's three major rapid transit systems, a newly released study co-authored by a university engineering professor has found.

The study, which was initiated by Health Canada and published in the journal Environmental Science & Technology, compared pollutant levels on Toronto's subway lines with Vancouver's SkyTrain and Montreal's Metro systems.

Conducted in 2010-2011, It found pollutants on trains and platforms in Toronto were up to 10 times higher than outside air, and around three times higher than levels in Montreal's Metro. Vancouver's levels were lowest among the three systems.

"The findings have revealed some opportunities whereby we can really improve the air quality in the Toronto system," study co-author Greg Evans, a University of Toronto engineering professor, told CBC Toronto.

​Along with researchers from Health Canada and McGill University, Evans and his students collected data on Toronto Transit Commission (TTC) trains and platforms over three weeks in the summer of 2010 and the winter of 2011. The students each had knapsacks with small portable instruments that measured tiny particles in the air.

"They would travel on the subway and either travel a route continuously, or get out at each station and make measurements there, then wait for the next train," Evans said.

On Toronto subway platforms and trains, the study, conducted in 2010-2011, found an average of 100 micrograms of pollutants per cubic metre — comparable 'to a typical day in Beijing.' (Canadian Press)

Researchers used a metric called PM2.5, which are airborne particles smaller than 0.00025 centimetres. The particles are easily inhaled and can damage lung tissue.

Health Canada recommends indoor PM2.5 levels should be kept "as low as possible." The World Health Organization designates the safe level for the tiny particles at 25 micrograms per cubic metre over 24 hours.

Pollution compares with 'typical day in Beijing'

Evans said Toronto's outdoor air quality on a typical day would measure around 10 micrograms of pollutant particles per cubic metre of air. That may rise as high as 30 micrograms on a day of poor air quality.

On Toronto subway platforms and trains, however, the study found an average of 100 micrograms of pollutants per cubic metre — comparable to "a typical day in Beijing," said Evans.

The U.S. Department of State, which monitors air quality in Beijing, classifies a reading above 101 micrograms to be unhealthy for sensitive groups.

04-24-2017 16:00; PM2.5; 36.0; 102; Unhealthy for Sensitive Groups (at 24-hour exposure at this level) —@BeijingAir

So what was to blame for these high levels? The research found the pollutants were made up of high concentrations of metal.

"We believe a lot of the particles that we detect in the air come in because of the abrasion of the wheels on the tracks," Evans said.

In above-ground stations including Rosedale and Davisville, the study found pollutant levels were about half of those at underground stations. Montreal's system measured around 36 micrograms per cubic metre, which Evans said could be because of its rubber tires, concrete rails and wood-based braking system.

Vancouver's system measured around 17 micrograms, on average.

TTC response

Responding to the study, the TTC said in a release that the system remains safe and plays a "vitally important role in reducing pollution caused by vehicle emissions."

"This study was not intended to assess impacts on overall health; rather it looked at levels of certain commonly occurring particulates and pollution," the statement says. "This research was done in 2010 and 2011 at a time we had already started taking steps that will improve air quality on the trains and reduce certain pollutants in the underground stations."

The TTC said it is planning a broader subway air quality study, because the information it currently has is from 1995. (CBC)

That includes the introduction of Toronto Rocket trains on Line 1 — The Yonge-University-Spadina line — which have newer HVAC systems. Those trains began running in 2011.

"We will continue to work with Health Canada to monitor the steps we are already taking to improve air quality including the impact our mitigation measures have had. That work begins this summer," the statement reads.

The TTC also said it is planning a broader subway air quality study, because the information it currently has is from 1995.

Evans said he plans to do a followup study using data from the newer trains.

"While people may be interested in the fact that the levels are quite high in the subway, the thing that interests me is the opportunities that this presents to actually make the system better for all of us," he said.

Evans noted the average amount of time commuters spend on the TTC is about an hour. He said that means a person's daily overall exposure to pollutants wouldn't increase by a significant amount — in general, it would go up by around 20 per cent.

Still, he said, any small steps which can be taken to improve air quality would be "good progress."