As with most things, even if there is no "magic point," you need to draw the line somewhere as accurately as you can, or you'll be trapped in pedantic bickering and inaction forever.
Drawing an arbitrary line where none logically should exist is the cause of, not the solution to, pedantic bickering and inaction. You're going to get a lot more people to agree "pollution should be lower" than "this is a good amount of pollution."