Completely made-up, non-scientific guidelines, bearing in mind that all these numbers are just arbitrary lines in the sand.
Under 50 - Let it rip
50 - 80 - Maybe not ideal for harder running all the time, but shouldn't be too much acute short-term effect. Easy/long runs no problem.
80 - 110 - Probably won't kill you (in the short-term; long-term who knows). Maybe think about pushing harder workouts back to a later day with potentially better air quality and just run easy, but if it's a critical session for an upcoming race, can probably get away with some faster running and be okay.
110 - 150 - If you're able to get your hands on an N95 mask with a valve, go for it. Best to stick to easy running. Wouldn't try anything too fast or high volume with any sort of frequency in this range.
150 and above - nope.