Yeah, I mean, how do they identify the causal effect here? It's obviously not easy, because polluted areas are also poor areas, and poor people live in poor areas (and have other problems).

It would be nice if the article had mentioned this issue. A metastudy of lots of bad correlational studies is just garbage in garbage out. So, did they address the issue?

There are ways round it, by the way. As a recent review said:

"it is unclear why federal ISAs that are the input into all regulatory analyses tend not to incorporate the emerging body of evidence on the effects of air pollution on health outcomes from the economics literature despite the additional rigor imposed by the emphasis on causal inference."

https://www.annualreviews.org/docserver/fulltext/resource/15...

It's not surprising that poverty affects life expectancy but what I find hard to believe is that poor air quality shortens life expectancy by a full 15 years.