It's not bad comparatively to when we have an inversion but by no means is it good. We triple EPA toxicity standards sometimes during an inversion but sometimes even when there isn't we are right below the standards.
It all depends on the day, atmospheric stability, wind, humidity, solubility of gases, time of season, cloud cover, albedo, lots of things affect the concentration in the valley. It just becomes visibly noticeable and the worst in the winter.
Whatever though, I have talked way too much in this thread. SLC is still a great place and the skiing is awesome.