Can you elaborate? It seems counterintuitive unless local smog is trapped by an inversion layer or there is smog-producing industry nearby.

It's at the southern end of the box canyon that is the Willamette Valley (the end that abuts the mountains), and to the north is Linn-Benton County "the grass seed capitol of the world" ( https://www.google.com/search?q=grass+seed+capital+of+the+wo... ). If you have grass-pollen allergies, then Eugene really sucks. For everyone else it is fine except when the farmers burn their fields (which they aren't supposed to do, but enforcement is patchy).