If you're a regular GO Transit commuter and you're concerned about your health, you may want to start sitting at the back of the train.

New research from the University of Toronto has found that in some circumstances passengers on commuter trains are at risk for exposure to "markedly high levels" of carcinogenic diesel exhaust. Passengers in the car directly behind the locomotive are at particular risk.

The study recommends that "immediate steps be taken to evaluate and where needed mitigate exposure in all diesel powered passengers trains" and that "passengers with existing cardiac or respiratory conditions may as a precaution want to travel near the rear" of trains being pulled by a locomotive.

The study was shared with the Toronto Star in advance of its expected publication this week by the journal Atmospheric Environment. Over the past year its authors showed their findings to Metrolinx, the provincial agency that operates GO, and officials say they have already taken steps to reduce pollution inside their coaches.

"While we don't take these issues lightly, we welcome any findings that can help us run a healthier, safer transit service," said Greg Percy, Metrolinx's chief operating officer.

He stressed that it is safe to travel by GO train. "We want to assure everyone who relies on GO Transit every day that we place the highest priority on their health and safety and we will continue to monitor and report on air quality to ensure we see improvements."

The research was conducted by the Southern Ontario Centre for Atmospheric Aerosol Research at U of T and is one of the first studies to measure pollutant exposure inside commuter trains.

Using portable devices, the researchers tested the air in coaches running on GO's Richmond Hill line for two components of diesel exhaust: ultrafine particles and black carbon.

While diesel exhaust can cause cancer, both of the components the scientists measured are toxic in their own right. Ultrafine particles have been known to cause lung and other health problems, while black carbon (essentially soot) has been linked to damage to respiratory, cardiovascular and nervous systems.

The researchers found that concentrations of the two pollutants were not high when trains were in "push mode," which is when the locomotive is at the back of the moving train. Roughly half of GO trains are pushed.

But when the trains were in "pull mode," with the locomotive at the front, the researchers found that inside the coaches concentrations of ultrafine particles and black carbon were five and four times higher respectively than in air sampled on the streets of downtown Toronto.

The problem was much worse in the front coach directly behind the engine, which showed concentrations of the pollutants that were nine times higher than at street level.

The concentrations in coaches in the middle of the pull-mode trains were three to four times lower than in the front coach, but were still "notably higher" than on the street, the study found.

Study co-author Dr. Greg Evans said that the findings shouldn't discourage people from using public transit. He said he still takes GO from his home in Ajax every day — although he has changed his commuting habits.

"I wouldn't travel in the front car when it's in the pull mode anymore," he said.

He pointed out that pollution inside coaches is likely not a problem isolated to GO. According to the paper, 18 of 26 public transit agencies in Canada and the U.S. that operate commuter trains exclusively use diesel locomotives.

Evans said that the ultimate solution is to electrify the GO network, but in the meantime he recommended interim measures like installing high-efficiency filters in train coaches, using less polluting Tier 4 diesel engines, and closing coach vents when trains pull out of the station. That's because the high power used to get the train up to speed results in greater emissions.

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Percy, the Metrolinx chief operating officer, said that the agency has been working with the researchers and has already put some safety measures in place.

He said Metrolinx is on track to have new high efficiency filters on all of its coaches by the end of March. The new Union Pearson Express fleet is made up entirely of Tier 4 locomotives, and by the end of 2018 the agency expects to have 17 of the cleaner engines as part of its GO fleet.

"Going forward, any new diesel locomotives we add to our fleet will be Tier 4," he said.

While Metrolinx plans to electrify about 80 per cent of the GO network, the project isn't scheduled to be complete until 2024. Percy said that the agency can't move any faster. "If we could we would."

He noted that the $2-billion electrification plan involves a lengthy study and design process, and is complicated by the fact that it will have to be implemented on active rail corridors.

"We're going as fast as all of that can come together," Percy said.

That's unlikely to satisfy critics who will see the study's findings as vindication of their calls to electrify Metrolinx lines sooner.