Dust - even chemically inert dust (or sand) - would be a pollutant at a high enough concentration. It will get into your lungs, sting your eyes, find its way into bearings and other machinery parts... Not immediately lethal, but something to take into consideration when staying on planet (you'll need filter masks if the dust is fine enough, not to mention having to deal with rapid erosion of machines). The same goes to soot from heavy industry and its likes, though soot is more dangerous on the long run (it is carcinogenic in the same way as cigarette smoke is).
Ozone on sea-level or near it could be quite a pollutant (it is corrosive), especially with an Earth-type atmospheric pressure or more. Many of the smog issues in our world involve ozone, among other gases (such as sulfur compounds).
Oh, and about that high oxygen atmosphere discussed by rust... It has some interesting implications beyond the problems of oxygen toxicity itself. First, it will make everything far more flammable and every fire will be stronger, bigger and faster than on Earth.
Second, it will allow insects to evolve into larger sizes than on our modern earth, as insects use tracheal breathing, which is essentially the unassisted diffusion of oxygen from the atmosphere to the body's tissues through long tubes; the longer the distance the oxygen has to travel, the less efficient the diffusion, and this is one of the limiting factors for insect size. Higher oxygen concentration in the atmosphere makes diffusion easier and thus larger tracheal-breathing organisms could evolve. In the Carboniferous period on Earth (circa 350-300 million years ago) oxygen levels were more than 150% of the current level, resulting in insects evolving to large sizes, including
dragonflies with a wingspan of 75cm!
.
