I strongly suspect that most of the things we now know to be problematic were also known to be problematic to the ancients, but were thought still to be worth it for their rewards. That’s pretty much where we still are today. Nobody likes breathing pollution, everybody likes modernity.

Yes, some things would have just stood to reason in a general respect, even if there wasn't hard science to back it up.

I suppose even then it would have been obvious to anyone traveling outside London that, hmmm, the fog/smog goes away out here. Only in major cities… What could it be?

seems likely. but without a scientific method to back their claims it just becomes a 'common sense' thing.