Healthcare, engineering, other white-collar pursuits, and even government (to some extent) can certainly be classified as "industries." The term is not, or at least should not be, limited to old-fashioned "smokestack" indusutries, and a large city which serves as a focal point for these major economic engines can very plausibly be classified as a "metrepolis" AND as "industrial." I think it's more of an economic description than an air-quality rating.

Even those barbeque joints (they're quite excellent ~ I know) belong to an industry, the "hospitality industry."