There's a difference in saying "It's dirty" versus "It's cleaner than it was 100 years ago". Is that really the lesson our kids should take from this? It's like saying the water in my pool is not cold, because it's warmer than it was 2 weeks ago. Believe me; it's cold. Hey kids: jump in!
I would think that removing the pollutants that people are accountable for could give you an idea of what clean air is or should be. Regardless of history, if your society is still putting crap in the air, feeling that it's "clean enough" is just inviting the same behavior that increases pollution to begin with.
Is "cleaner" good enough?
"Cleaner"? Maybe. It is all a trade-off. If we want it to be as clean as nature would let it be, that is one thing, perfectly clean? Then we would need processing plants to take naturally polluted air in and clean it.
It becomes then an issue of definition and measurement (what pollutants are unacceptable? which are acceptable?). Definitions of "clean" are different in different parts of the planet; not just for air, but for water, food, medicines, etc.
Then there is the cost. If it took the GNP of the entire world to make the air and/or water perfect I doubt that the humans would accept this. Somewhere there is a value for effort/money proposition that has to be made and that value must be acceptable to the society it would apply to.
As to your pool analogy, yes, if the kids found it acceptable because it is now warmer than 2 weeks ago, then fine jump in. It is after all what the kids would find acceptable, not what you might think is acceptable. They have to take the consequences of their actions, not you.
In my view, you tell your children that life is a series of propositions, problems, that get sent to them. They need to see that for any decision they make they should understand the consequences of that decision.
Phrases like "dirty" or "clean" add a non-measurable, emotional aspect to making the decision. Emotion is not a good thing to base decisions on.
Is this what I did with my children? Yes. It is exactly what I did. It has worked out well for our family.
I know I'm different in this view than many, perhaps most. I am, however, an engineer.