A study of the air quality of Toronto's subway system has found "elevated" levels of air pollutants — but the city's medical officer of health is urging Torontonians to continue using the TTC.

"Taking the subway is associated with benefits to people's health and wellbeing and is a health-supportive way to travel, especially as an alternative to personal vehicle use," Dr. Eileen de Villa wrote in her report, released Monday morning.

The study, conducted by Toronto Public Health, found that levels of air pollutants on the subway are higher than those in outdoor air and contain "high levels of some metals," particularly on Line 2.

"Air quality data collected in the Toronto subway system shows that, as is the case for other similar subway systems, levels of fine particulate matter air pollution (PM2.5) are elevated," De Villa wrote.

But despite the findings, both De Villa and some Toronto councillors are touting their support for the TTC, saying improvement is inevitable.

"In the wake of this study, I know that some Torontonians will worry about whether it is safe to commute by TTC," said Jaye Robinson, who serves as chair of the TTC and is also the councillor for Don Valley West.

"Our professional public health and transit staff have unequivocally confirmed that the TTC is a safe and environmentally-friendly mode of transportation."

'Riding the TTC is a healthy choice'

Ward 10 Coun. Joe Cressy is also urging commuters to continue using the subway.

"Toronto Public Health conducted a comprehensive health impact assessment of Toronto's subways. The conclusion — riding the TTC is a healthy choice. Like subway systems around the world, with more work we can still improve air quality."

Toronto Public Health conducted a comprehensive health impact assessment of Toronto’s subways. The conclusion - riding the TTC is a healthy choice. Like subway systems around the world, with more work we can still improve air quality. <a href="https://t.co/jbAtZf0pAQ">https://t.co/jbAtZf0pAQ</a> —@joe_cressy

De Villa recommended a series of improvements — or "mitigation measures" — that she says should be taken immediately, including awareness and training programs for employees.

She also recommends the TTC board review its operational systems, such as automatic train control, train frequency and ventilation systems, and review plans for "future line modernization."

De Villa says city council should ask both the board and Metrolinx to continue monitoring and mitigating levels of the pollutant and consider air quality in future budgets and projects.

Similar issues faced in big cities, TTC says

Toronto Public Health launched this study in 2017 after a previous Health Canada study found elevated levels of pollution in the subway when compared to outdoor air.

In her report, De Villa said that although the study found particulate matter, there's no way knowing the exact risks it poses to passengers.

"There are currently no health-based standards or guidelines for assessing passenger risk from subway PM2.5," she wrote.

"The human health risk assessment (HHRA) identified that the PM2.5 levels measured in the TTC subway are expected to be associated with both short-term and long-term health impacts, including elevated risk of mortality, respiratory and cardiovascular disease, and transient respiratory symptoms."

Line 2 rate 115% higher than Line 1

The biggest area of concern for health officials is the Bloor-Danforth line, where riders are being exposed to 183 micrograms of particles for every cubic metre of air.

That's 115 per cent higher than what passengers might breathe in on Line 1, according to the report.

"The fact that the Toronto Public Health department is recommending short, as well as mid and long-term mitigation measures indicates there are obvious risks," Carlos Santos, president of the union representing more than 12,000 TTC workers, said in a news release Monday.

The union said Monday that the report does not address exposures to people working on the subway line.

"It is extremely disturbing that Toronto Public Health wasn't directed to look at this issue, knowing that the risks are obviously far greater for employees," Santos said.

Charges against TTC over air quality levied in November

This news comes just over a month after a TTC worker took the rare step of launching a private prosecution case against the transit agency over what he alleges was a discharge of contaminants in the city's subway system that violates the Environmental Protection Act.

The charges were initiated by Thomas Donnelly, who is a member of the Amalgamated Transit Union (ATU) Local 113.

Donnelly is alleging that he experienced adverse health effects related to exposure from airborne contaminants in the subway system, according to a news release from the union and his lawyer.

Despite the potential health impacts, De Villa says Toronto is "demonstrating leadership" in addressing its subway air quality.

"Subway air quality testing shows we are well below provincially mandated occupational health regulations," The TTC said in a statement monday.

"The TTC will be part of an information exchange and best practices discussions with other large transit systems in New York and London where similar issues are faced."

Subway replaces thousands of vehicles each rush hour

The TTC estimates that during rush hour, each subway train replaces 900 to 990 personal vehicles that would otherwise be on the road.

Toronto Mayor John Tory said making the subway safe for passengers and employees is his "number one priority."

My statement on Toronto Public Health's report on health impacts of Toronto's subway system. <a href="https://t.co/ga7v3sXn3R">pic.twitter.com/ga7v3sXn3R</a> —@JohnTory

"Back in 2017, I said that it was not acceptable that the TTC hadn't conducted a new test on air quality since 1995 and welcomed this new study so we could confirm that we are on the right track and have current information to help guide future work," he said in a news release Monday.

"I am committed to making sure the TTC continues this work in the existing system and in all future subway projects."