Dr. John Campbell (on Youtube) has been talking about using CO2 monitors as a proxy for understanding how recirculated (and therefore potentially full of fomites) indoor air is.
It's a good concept, since indoor CO2 is primarily generated by exhaling humans. It's important to keep your windows open!
There has been a lot of discussions here (Switzerland) regarding this and offices, shops etc having recirculating units without filters or UV sanitizers.
I would think that anything that spreads air without sanitation is an issue.
My default assumption at this point is the longer you breath air contaminated with aerosols carrying the virus the higher your chance of being infected. But the actual statistics behind this are basically unknown. If there a real threshold effect or is that a statistical artifact. No one knows but there are hints that severity is related to exposure.
Also suspect one large droplet likely can infect you by itself.
I'm going with, avoid sharing confined spaces with people outside my household. And stay far away from people not wearing masks.
Did the diamond princess and other cruise ships not provide a good example of transmission through have system as everyone stayed on board but isolated and continued to get sick for more than 14 days ?
It's a good concept, since indoor CO2 is primarily generated by exhaling humans. It's important to keep your windows open!