The LA basin has always had a smog problem in the Occidental/Pomona area because the prevailing winds push all pollutants up against the mountains (the cities are located at their foot). Everything from the LA area tends to concentrate there, and in the 1970s the air was terrible, and it still may be. But, it has to do with the geography and winds. You could have the same problem near any city given the same atmospheric and geographic conditions.
We had smog alerts due to thermal inversions here in the southeast due to a similar effect, the “Appalachian wedge.” Not so much anymore, but it’s generally hazy in summer anyway, a photochemical haze.