Philadelphia’s push for clean air started in 1979 after the EPA put the metro region—which at the time included the city, surrounding counties, and nearby locales—into its “severe non-attainment” category. Regulations implementing changes in the 1980s and 1990s are still in force and will likely be enough to get the city back into attainment within three years, Garrow said.
https://www.bna.com/philadelphia-air-quality-n57982093013/
By Leslie Pappas
The Collected Works of Jim Garrow 