Nearly a century — that’s how long Philadelphia Gas Works reported that it would take to replace 1,500 miles of the city’s aging gas pipes. But the state’s Public Utility Commission said that the city needs to quicken its pace in fixing the pipes.

Each day, methane seeps from pipes built, in some cases, centuries ago, wasting gas and contributing to the city’s greenhouse gas emissions. The Mayor’s Office of Sustainability estimates that a year of gas leaks in Philadelphia releases about the same amount of emissions as 120,000 cars.

StateImpact Pennsylvania reported that nearly two-thirds of the gas pipes beneath the city are made of cast iron and steel, which are more susceptible to leaks. PGW is looking to replace those pipes with plastic ones, but every mile of pipe would cost an estimated $1 million. The Pennsylvania Public Utility Commission is investigating what, if anything, can be done to make the process more efficient, including calling on the city to return the $18 million fee it collected annually from PGW.