TORONTO -- Toronto subway riders travelling on Line 2 are exposed to about twice as much fine particulate matter than those who use the Yonge-University Line, a new report has found.

A report set to be presented to the Toronto Board of Health on Jan. 13 found that the levels of fine particulate matter air pollution (PM2.5) in the subway system are “high enough to warrant mitigation.”

Those particles can include aerosols, smoke, fumes, dust, ash, pollen and metals.

According to the report, mitigation efforts on Line 2 are “especially important” given the higher level of air pollution in the tunnel. The data shows that commuters are exposed to 115 per cent higher levels of particulate matter than those who use Line 1.

“Given this difference, Line 2 needs to be a priority for a range of mitigation measures, including future line modernization,” the report says.

At next week’s board of health meeting, the medical officer will be recommending that the city take action to reduce the levels of PM2.5 in the subway system, as well as look into employee awareness programs and review operational systems such as air filtration and ventilation.

The report says there are no health-based standards or guidelines for assessing passenger risk for PM2.5 in subway systems, however symptoms of exposure to outdoor PM2.5 include heart and lung disease, emergency room visits for respiratory and cardiac issues, reduced life expectancy and elevated mortality rates.

In a statement released Monday morning, Toronto Mayor John Tory said that the Toronto Transit Commission (TTC) has been working to improve air quality in the subway system for the last 30 years.

"The health and safety of all Toronto residents is my number one priority. This includes making sure our subway system is absolutely safe for all those who use it – whether they are transit riders or TTC employees,” he said.

“The TTC has been working to improve air quality in the subway system over the last 30 years by introducing new vehicles along with improving ventilation and filtration systems. I know TTC CEO Rick Leary and TTC Chair Jaye Robinson are seized with this issue and will work to continuously improve air quality in our subway system.”

This is the first comprehensive review of subway air quality since 1995.

The union representing TTC workers has expressed concern about the report, saying the fact that Toronto Public Health is recommending mitigation measures "indicates there are obvious risks."

"If the conditions in the subway require many kinds of mitigation for passengers, any reasonable person has to be deeply concerned about what this means for anyone who works there for their living," said union president Carlos Santos. " 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."

However, TTC spokesperson Stuart Green told CTV News Toronto that despite the report, the TTC "is safe to ride."

“The air quality in our subway, it’s not ideal, but it’s certainly well below levels set by the provincial government for employees, so that’s a good thing. But we’re certainly we’re not just comfortable with that. We want to improve on that."