I think it can be explained by combining a number of things:
-even though electricity comes from fusion plants, probably not all countries have them. And smog doesn't stop at borders.
-even if the electricity is clean, not all industrial processes are. Add to that a lack of controls due to balkanization and you can have quite a lot of pollution.
-the pacific ring of fire had a lot of vulcano eruptions (and still does). That too can give a lot of smog, or at least something that looks like it. And it can also give acid rain due to the high sulphates in the air from the eruption.
-don't underestimate the amount of pollution you get from burning wood, as probably a lot of SINless have to do. Especially treated wood (painted for instance) can give a lot of pollutants
-Maybe (and this is purely speculation) leakage through a gate to some metaplane.
all on their own they will probably not give the amount of smog mentioned, but taken together it is possible.
