A new University of Toronto study might make GO train riders hold their breath.

The research found elevated levels of certain airborne pollutants in diesel train cars — particularly the ones being pulled right behind a locomotive.

In those cars, researchers recorded levels of ultrafine particles and black carbon that were nine times higher than those found on a busy Toronto street.

"It's similar to travelling on the 401 — but if you travelled on the 401 in an open convertible, behind a large diesel truck, for the entire trip," said researcher Greg Evans.

"These are larger concentrations than most people would encounter any other time during their days."

University of Toronto researchers Greg Evans (left) and Dr. Cheol-Heon Jeong (right). (Tyler Irving/University of Toronto)

Diesel exhaust a known carcinogen

Evans, the director of the Southern Ontario Centre for Atmospheric Aerosol Research, and Dr. Cheol-Heon Jeong, a senior research associate at SOCAAR, conducted their research during 43 trips on GO trains travelling back and forth from Union Station to Richmond Hill.

The team used portable instruments to measure for ultrafine particles — which are formed when gases in the exhaust condense into microscopic particles — and black carbon, which is basically soot.

Both have been linked to negative health impacts and are only two elements of diesel exhaust, which is a known carcinogen that could lead to respiratory, cardiovascular, and reproductive impacts.

The researchers also found a higher concentration of pollutants in the front coach of pull trains than in other forms of public transit — like the subway, for instance.

Middle coaches, however, are a safer bet, with a three-to-four times lower concentration of measured pollutants than in those cars right behind a diesel engine.

Metrolinx installing new filters

While the study's results might be alarming at first glance, Evans said his team worked alongside Metrolinx, the agency which manages GO Transit, to proactively address potential health risks.

"The first thing we did, as soon as we were aware there could be an elevated risk to our customers, was to look at the filtration system," said Greg Percy, chief operating officer at Metrolinx.

Already, new filters installed in the ventilation systems of some train cars have removed 80 per cent of the black carbon and 25 per cent of the ultrafine particles, Percy said. He added those filters will now be rolled out "to the entire fleet."

Down the road, Metrolinx will also be switching much of its train network from diesel to electric, but that won't be completed until 2024.

Metrolinx worked alongside the researchers to proactively address the study's findings. (Andrew Lupton/CBC )

'The closer to the engine, the higher the levels'

Dr. John Molot, staff physician at the environmental health clinic at Women's College Hospital, praised the new research for being careful and cautious with measurements.

He also said it backs up prior findings on air pollution inside different forms of transportation.

"The closer to the engine, the higher the levels," he said. "You see that in buses, you see that in cars."

The peer-reviewed findings will be published on Tuesday in the journal Atmospheric Environment.