Metra has hired an independent firm to check out the air quality on its trains and at Ogilvie Transportation Center. That comes after a report showing diesel soot levels on some trains are extremely high.

Metra at this point is just conducting its own tests to confirm what the Chicago Tribune found: that passengers were inhaling high levels of diesel soot, particulates in the air from diesel engines, at certain points along train routes. Also, the pollution levels were bad in the Ogilvie train station where trains may sit idling for hours.

Metra’s investigation comes after U.S. Sen. Dick Durbin, D-Ill., called on the Environmental Protection Agency to look into the pollution levels. Durbin told reporters Metra recently received grant money that would cut down on the amount of idle time.

Diesel soot can aggravate asthma and has been traced to cancer.