TORONTO, CANADA – OCTOBER 22, 2017: TTC subway train at Finch station. TTC operated by the Toronto Transit Commission.
The air quality inside Toronto’s subway has always been poor, particularly on Line 2 that runs East-West along Bloor St. The concentration of tiny particulate matter, known as PM 2.5, is around 10 times higher than what is typically seen outdoors, and is higher than many other subway systems around the world.
“We don’t have regulations for indoor air in Canada, but if this was outdoor air it would be triggering alarms,” says Greg Evans, a member of the Chemical Institute of Canada who studies air pollution at the University of Toronto. “It’s similar to what we’ve experienced in Canadian cities when there are wildfires – we’re experiencing those sorts of concentrations in our subway system in Toronto day in day out.”
Over the past 10 years the air quality in Line 1 was improved somewhat, but Line 2 has gotten worse. So Evans and his colleagues set out to determine what the sources of the pollution are, which will help find solutions to the problem.
They looked at the detailed chemical composition of the particles to see where they were coming from. They eliminated several possibilities, such as residual dust that has accumulated over decades. Instead, they found that most of the particles are coming from the wheels, brake pads, and rails during the braking process as trains come into the station.
“There’s steel, iron oxides, and manganese particles from the wheels and rails, and elements like barium and copper from the brake pads,” he says. The issue is worse on Line 2 because the trains are older and use friction braking rather than the regenerative braking used on the newer trains on Line 1. The work was published in the journal Transportation Research Part D: Transport and Environment.
PM 2.5 are tiny particles of airborne solids or liquids that are smaller than 2.5 micrometres, which means they are small enough to follow the twists and turns of your respiratory system and penetrate deep into the lungs. They are associated with a variety of health problems, including respiratory issues like asthma, increased risk of heart attack, neurodegenerative diseases like Alzheimer’s, and reproductive problems.
“Globally air pollution is the top environmental burden on disease, close to 8 million deaths a year, and most of that is associated with PM 2.5,” says Evans.
Theresa Moreno Pérez, co-ordinator for earth and environment research at the Spanish Research Council, says while the findings are broadly similar to other studies on subway systems worldwide, identifying the various metallic elements involved is an interesting new contribution, as they may have previously unforeseen repercussions on the possible health effects of breathing them.
“As I never tire of pointing out, especially after the lessons learnt from the COVID19 pandemic, as a society we should be expecting and demanding clean air to breathe just as we expect and demand clean water from the tap,” she says.
Evans says it’s not clear whether the metallic particles might have different health effects than the largely carbon-based ones found outdoors, but there is reason to believe they have a greater capacity to generate reactive oxygen species, which could deplete the antioxidants in our lungs.
The findings point to ways that subway air quality could be improved, such as improving ventilation systems, and switching to newer trains with automatic braking systems. Moreno Pérez says that adding full-length platform screen doors is an effective, if expensive, way to reduce air pollution on the platforms. Systems that automatically monitor air quality in the subway and post the results publicly, as in Seoul and Taipei, would also help manage the risk.
“Monitoring would be a big step,” says Evans. “You can’t manage what you don’t measure.”