By Dave Savini

(CBS) — Dirty vents, diesel fumes — what are Metra riders breathing in when they commute?

CBS 2’s Dave Savini launched an investigation to find out what passengers are being exposed to.

With help from the Illinois Institute of Technology (IIT), and Microtrace Laboratory, two types of testing were conducted and problems for commuters were found, health and science experts say.

Thick smoke and exhaust from commuter trains fill Union Station and seep into passenger cars.

The 2 Investigators found train ventilation systems covered in filth. Vent after vent, train after train, walls and ceilings dirtied by the air blowing into the cabins.

With IIT, CBS 2 conducted field-testing using special monitors to detect levels of dangerously small or ultrafine particles that can penetrate deep in the lungs and cause health problems.

“Four-hundred-thousand per cubic centimeter, that’s huge,” says IIT’s Brent Stephens. “That’s more than you’d find in the middle of a highway.”

Stephens and his students found higher exposure levels on outbound trains being pulled by the engine versus inbound trains being pushed by the engine in the rear, because diesel exhaust particles flow through passenger cars.

“These levels are very alarming,” says IIT’s Akram Ali. “They are very high.”

Ultrafine particles were about 17 times higher when the engine was in front. The fine particles, called PM2.5, were about 100 times higher.

That’s a concern, says Dr. Robert Cohen, professor of medicine at Northwestern University.

“It’s astounding,” he says. “These particles that are really tiny can cause increased heart attacks and strokes.”

The 2 Investigators also took samples from the filthy vents. Seeing what was on the vent concerned passenger Dave Dunwoody, who worries about the long-term effects.

CBS 2 had the samples lab-tested by Chris Palenik at Microtrace and found more ultrafine particles.

“They were composed largely of metal particles, iron or steel metal particles,” Palenik says.

Iron particles can end up in your lungs.

“These exposures are not healthy,” Cohen says. “They are not good, and I think it’s something that should be addressed.”

Michael Gillis, a Metra spokesman, says the dirty vents have not gone unnoticed.

“They clearly should be cleaning the vents and we have addressed that with our crews,” he says.

Metra follows EPA standards and has switched to cleaner fuel and upgraded the filters, he says. However, he admits there are no EPA standards to regulate the ultrafine particles — despite the potential health effects.

“We have no standards on these particular compounds,” said Gillis. When asked if there should be standards, Gillis replied: “The EPA is looking into that.”

Says IIT’s Stephens: “We know that diesel exhaust is a carcinogen.”

The 2 Investigators also found that exposure risk tends to be higher when you stand in the vestibule or sit in the first few cars behind the engine on outbound trains.

Dr. Cohen says simply keeping the vents clean could help protect passengers and their health.

Here is the full response from the U.S. EPA Region 5: