Day after day, thousands of commuters are breathing high levels of toxic diesel pollution trapped in Chicago's two major rail stations and even inside the trains they ride, a Tribune investigation has found.

Testing by the newspaper found the amount of diesel soot lingering in the air steadily increases as commuters walk deeper into Union Station or the Ogilvie Transportation Center. Levels of the lung- and heart-damaging pollution jump higher on platforms, where acrid blue clouds of diesel exhaust hover between trains, many of them built in the 1970s.

It gets dramatically worse, not better, after boarding a train. As the doors close and the locomotive pulls out of the station, Tribune testing found, the air trapped inside the stainless-steel cars contains levels of diesel soot up to 72 times higher than on the streets outside.

Pollution levels remain high during most of the trips away from the city, the Tribune found. Exposure drops sharply only after getting off the train.

The testing sheds new light on the amount of pollution many people breathe as part of their daily routine. The U.S. Environmental Protection Agency considers diesel exhaust one of the most dangerous types of air pollution. Studies have linked exposure to a variety of health problems, including cancer, heart attacks, respiratory diseases, diabetes and brain damage.

Yet federal and state officials acknowledge they are woefully behind in assessing how breathing highly polluted air for short periods of time every day might affect a person's health.

Air quality on Chicago's commuter lines also isn't expected to improve significantly any time soon. Rather than replacing its disco-era locomotives with newer, cleaner models, Metra is refurbishing a third of its aging fleet to keep them chugging for at least another two decades.

Lack of ventilation at Union Station and the Ogilvie Center also remains a problem, keeping soot and toxic gases concentrated inside stations used by more than 245,000 people every weekday.

"It's horrible sometimes, especially at rush hour when all of those idling trains are backed into the station," said Kurt Kreis, a technology specialist at a Loop investment bank who has been riding Metra to and from southwest suburban Orland Park for more than a decade.

Metra officials told the Tribune they are doing their best on a limited budget. Locomotives push train cars into the station so the engines stay closer to the outside air, they said, and technology soon will allow some locomotives to shut down engines automatically after a certain period to reduce idling and save fuel.

"I'd like to do more, but we just can't with the money we have now," said Richard Soukup, Metra's chief mechanical officer.

Reacting to the Tribune's findings, the agency scheduled a meeting Tuesday with federal, state and local officials.

"It is my intention to fully investigate this matter, and assure you that Metra will remain proactive in this area," William Tupper, the agency's acting executive director, wrote in an invitation to the meeting.

Prized for their power, durability and fuel efficiency, diesel engines power not only locomotives but also long-haul trucks, school buses and construction equipment. Researchers estimate that more than half of people's daily exposure to diesel pollution comes during their commute, even though on average it accounts for just 6 percent of their day.

To take a snapshot of the diesel pollution Chicago-area commuters breathe every day, the Tribune rented a handheld device that measures black carbon, or soot, a key ingredient in diesel exhaust.

The testing device, manufactured by Magee Scientific, of Berkeley, Calif., is similar to ones used by researchers in peer-reviewed studies that pinpointed pollution hot spots near highways, rail yards, shipping ports and quarries.

During walks, drives and train rides, the Tribune found spikes of the pollution that far exceeded normal levels in Chicago and other U.S. cities.

For example, on one afternoon the amount of soot measured on Union Station's south platform was 21 micrograms per cubic meter of air, a tenfold increase from the street. After the doors closed on a train bound for Downers Grove, the figure jumped to 39, and then, shortly before the end of the 23-minute trip, to 72 micrograms per cubic meter.

By contrast, normal levels of diesel soot in Los Angeles, which has long suffered some of the nation's worst air pollution problems, are 1 to 2 micrograms.

Off the train in Downers Grove, the device registered soot levels of less than 2 micrograms per cubic meter. Other than a short burst when an eastbound train pulled into the station, levels stayed low during most of the return trip downtown.

Soot levels varied from trip to trip but always jumped dramatically as outbound trains left the city's crowded stations. Levels spiked as high as 50 micrograms per cubic meter on a train from Union Station to Schaumburg, 46 from Ogilvie to Arlington Heights and 21 from the LaSalle Street Station to Tinley Park. (Levels on the CTA's electric trains were consistently low.)

Pollution from diesel engines is a complex mix of toxic substances such as benzene, arsenic and formaldehyde, many of which can cause cancer. It also is filled with fine particles, commonly called soot, so small that thousands could fit on the period at the end of this sentence.

Studies increasingly are raising alarms about soot, which can lodge deeply in the lungs and penetrate the bloodstream. Breathing even small amounts can inflame the lungs and trigger asthma attacks, researchers have found. Several studies have linked soot exposure with heart attacks and premature death.

California officials estimate diesel exhaust is responsible for about 70 percent of the cancer risk people in that state face from breathing toxic air.

The effects of short-term exposure are still being studied, though scientists at Columbia University recently linked bursts of diesel soot with respiratory ailments suffered by New York City high school students.

"If you can see what you're breathing, it's especially bad for you," said Scott Fruin, an environmental health researcher at the University of Southern California who has studied air pollution during commuting and reviewed the Tribune's findings. "Even when you can't see it, these particles are getting into our bodies and causing damage."