The fun part is when you realized that agricultural worlds can have tainted atmospheres, but they can't be overly populated in order to leave enough habitat to devote to farming and husbandry for agricultural export. Having an untainted atmosphere is just a bonus.
I think Occam's Razor would suggest that, out there in the real universe at least, the most common atmospheric 'taint' in an otherwise breathable atmosphere would be carbon dioxide. For most of our Terra's history, in fact, CO2 levels were well above 1,000 ppm (about 3.5 times modern preindustrial levels), and often ranged from 2,000 to 4,000 ppm (most recently in the Triassic). I believe it was even higher than that at the start of the Carboniferous.
These are levels that humans would find unpleasantly tolerable in the lower ranges (1-2,000 ppm),
but downright problematic at the higher ones. Plants, on the other hand, would find those levels darned enjoyable, up to a point.
Agricultural + Rich is certainly doable
Industrial tends to be rather exclusive of most other trade codes, however.
As a side note: It only has just occurred to me, right now, that the Ag designation should really be tied to a planet's size. After all, if tens of millions can reside on a world half the diameter of the Moon and still make it work as an Agricultural world (with the right atmosphere, of course -- and, yes, I know, I know ...), then hundreds of millions could easily make a go of it on a Terra sized world, and even a few billion on someplace like Shiwonee (the Size A Geonee homeworld).