Scholarly Journals: Best Bets
Proquest provides a nice mix of popular and scholarly sources. You can limit your search to scholarly journals (in the "Source Type" menu) and to peer reviewed articles (click the box below the search bars).
You might try a search like asthma AND Philadelphia
You can use the subject menu to include and exclude academic subjects or topics of focus. For this class, "Public Health" is likely a useful filter.
PubMed and Web of Science largely provide scholarly sources.
Try a search like this in PubMed:
Filter with limits on the left:
Click "Similar Articles" to find, well, similar articles.
Try a similar search in Web of Science:
Click "Times Cited" (circled on right) so you can potentially find similar, newer articles.