Waiting a long time for a subway can be annoying but a new study suggests it could do more than test your patience — it might also expose you to potentially harmful pollution.

According to a study published Tuesday in the journal Environmental Science & TechnologyEnvironmental Science & Technology, concentrations of fine particulate matter on the Toronto subway system are roughly 10 times the level found outside TTC stations. At 95 micrograms per cubic metre, researchers say the levels are typical of an average day in pollution-choked Beijing.

The study also found that concentrations measured on the TTC subway system were almost three times greater than those discovered on Montreal’s Metro and five times higher than those on Vancouver’s SkyTrain.

The lead author of the study, Keith Van Ryswyk, said the research didn’t measure the health effects of the pollution and the findings shouldn’t deter anyone from taking public transit.

But under some conditions, the kind of particulate matter that was measured, known as PM2.5, has been associated with lung problems, and Health Canada guidelines advise that indoor concentrations “should be kept as low as possible.”

“That is to say, there isn’t a safe level of PM2.5, so reducing it in any environment where people spend their lives every day is a good idea,” said Van Ryswyk, a researcher at Health Canada’s air health science division.

In a written statement, TTC spokesperson Stuart Green said the subway “remains a safe system for our customers and employees” and said the transit agency has been working for years to improve air quality underground.

The study was a collaboration between Health Canada, McGill University, and the University of Toronto, and was part of ongoing work to measure air quality in commuter environments, which represent a significant daily source of pollution exposure for millions of Canadians.

Over several weeks between 2010 and 2013, researchers used portable air samplers to measure the air quality on the platforms and inside the trains of rail systems in Toronto, Montreal, and Vancouver.

The most abundant sampled element in the particulate matter on the TTC was iron, and manganese was also present. The ratios of the two substances led researchers to conclude that the source of the particulate was likely friction between the subway’s steel wheels and steel tracks.

The two surfaces rubbing together produces a metal-heavy “rail dust” that is pumped allover the platform every time a train enters a TTC station, said Greg Evans, a U of T engineering professor and one of the co-authors of the study.

“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. 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.

Concentrations of fine particulate were much higher on TTC platforms than on the trains themselves, at 140 and 80.8 micrograms per cubic metre, respectively. Evans said the trains’ ventilation systems mitigate the exposure.

The researchers concluded that the unique features of the three cities’ subway systems accounted for their differing levels of pollution.

Vancouver’s SkyTrain has steel wheels and tracks just like the TTC, but about 80 per cent of the system is exposed to the air. Concentrations of fine particulate matter on the SkyTrain were just 19 micrograms per cubic metre.

Montreal’s Metro is entirely underground, but its trains have rubber wheels that run on concrete “rollways,” and their brakes are operated with wooden shoes. The researchers determined that was likely why concentrations of fine particulate matter were found to be only 35 micrograms per cubic metre, about a third of those on the TTC.

The authors estimated that while commuters spent just 4.9 per cent of their days on the subway system, doing so contributed 21.2 per cent of Toronto riders’ daily exposure to fine particulate matter. In Montreal and Vancouver the number was just over 11 per cent.

“Considering the combined daily ridership of these three metro systems, a significant portion of Canada’s population is being exposed to these particulates on a daily basis,” the study said.

The authors recommended that in order to reduce exposure, transit systems should improve ventilation and in-car filtration systems, and conduct regular rail dust cleaning.

Green, the TTC spokesperson, said the agency was already taking steps to address air quality at the time the measurements were collected, including refurbishing HVAC systems on its older trains, introducing new Toronto Rocket subway cars with better HVAC systems, and purchasing a tunnel vacuum car equipped with a HEPA filtration system.

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Green added that the TTC plans to carry out a study this year to determine whether air quality is affecting subway workers’ health.

“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,” he said.