Avoid going in places where ventilation systems recirculate unfiltered inside air…. (un-aired homes, etc)
Why? – Unfiltered inside air still potentially has the aerosol from people infected with COVID, taking air from outside or filtering can avoid this by diluting the amount of the virus you’ll encounter.
Evidence – https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1270/5898577