Well,....anyone who drove through L.A in the '80's could tell you the reason they have those emission standards,.....Especially on one of those weather inversion days.......
Living here near Pittsburgh, I know a thing or two about smog. There's a town about 10 miles south of PGH. called Donora that actually had a cloud of sulfuric acid form and move through the town killing more than 20 people. Sounds like science fiction, but it's true. Of course, that was back in the '50's when all the steel mills in the Mon valley were running full blast, as it were.
We don't have that problem now......