“Air quality is a real threat in many countries but not anymore in the US”

Couldn’t be more wrong.

As the author casually admits, we know nothing without experiments. End of story.

What makes your statement so wrong is that it takes decades for the consequences to show up, if they are measurable at all, in non experimental studies. That is exactly the same reason that cigarettes were considered safe for so long. Health consequences take decades to show up and without experiments all we get are loose correlations. Air quality in many places in America is getting worse, the size of the population in these areas is massive. To declare to potential health consequences 60 years in the future without any solid experimental evidence is the same as the doctors of the 50’s telling patients to smoke up, there’s no evidence cigarettes are bad. Exactly the same.