Ever smelled a dogfood factory? BARF! (Though paper mills are pretty bad...) 
There's a Purina plant right next to I-70 here in Denver. I hold my breath for as long as I can when I have to ride past it!
The worst smells I've encountered on the highway were in west Texas driving past the oil fields. Crude oil has to be one of the most disgusting smells in existence, and they would go on for miles at a stretch!
