"then are we saying all air pollution is an international crime?"

We are maybe not saying that in one voice, but many people, including myself are saying exactly that since quite a while. Same with polluting the groundwater, or the ocean.

One has to differentiate, of course and it is scale that matters. One coal fire is neglectible, but powering the economy with (maybe even unfiltered) coal plants is not. Yet most nations still do it, so it will take a while, before this will be seriously considered a punishable crime.