It’s not as bad as it used to be though.

I was in Pittsburgh for a while. Excellent town, great people. When I was there they were power-washing some of the buildings and it was amazing how different the buildings looked afterwards.

Then of course there was the nearby 1948 Donora smog disaster:

Hydrogen fluoride and sulfur dioxide emissions from U.S. Steel’s Donora Zinc Works and its American Steel & Wire plant were frequent occurrences in Donora. What made the 1948 event more severe was a temperature inversion, a situation in which warmer air aloft traps pollution in a layer of colder air near the surface. The pollutants in the air mixed with fog to form a thick, yellowish, acrid smog that hung over Donora for five days. The sulfuric acid, nitrogen dioxide, fluorine, and other poisonous gases that usually dispersed into the atmosphere were caught in the inversion and accumulated until rain ended the weather pattern.[3]

The fog started building up in Donora on Wednesday, October 27, 1948. By the following day it was causing coughing and other signs of respiratory distress for many residents of the community in the Monongahela River valley. Many of the illnesses and deaths were initially attributed to asthma. The smog continued until it rained on Sunday, October 31, by which time 20 residents of Donora had died and approximately one third to one half of the town’s population of 14,000 residents had been sickened.

https://en.wikipedia.org/wiki/1948_Donora_smog