A group of academic scientists has put together a long paper with scientific information intended for the public on Covid-19 aerosol transmission. I think this is pretty nice science communication. It is not dumbed down, but it avoids jargon. The graphics they include are mostly helpful. Here are a few takeaways:

• Secondhand cigarette smoke is a useful analogy to think about. If you are around smokers outside, you are inhaling much less of their poison than if you are around them inside. The amount of time you are around them makes a huge difference – however, this group says the 15 minute CDC guidance is not supported by good evidence. Outside, distance makes a big difference. Inside, being closer is probably worse, but if you are in an enclosed space with them for any period of time you are at pretty high risk. Opening a window should help, but not as much as being outside.
• Scientists disagree on the relative importance of the three pathways – surfaces, droplets, and aerosols. In the face of uncertainty, it is probably prudent (this is my opinion) to treat them as roughly equal and take precautions against each. Someone coughing or sneezing in your face is a big problem – stay 6 feet away for that reason alone, especially from anyone un-masked.
• Aerosols probably persist for 1-2 hours. (My thought – this suggests staying in a hotel should be relatively safe. The room has been cleaned, hopefully the maids were wearing masks, and hopefully they cleaned the room in the morning and you are checking in in the afternoon.)
• Sun and wind tend to reduce risk. All other things being equal, low temperatures and low humidity seem to aid transmission. (Don’t count on the opposite helping you in a sealed room, though. But I am a proponent of humidifying in the winter anyway.)
• The time it takes air in your house to turn over varies widely – “30 minutes to 10 hours”. For commercial buildings, 12 minutes to 2 hours. Hospitals around 5 minutes!
• A carbon dioxide concentration of 800-950 ppm is indicative of good ventilation indoors. A carbon dioxide meter costs about $150.
• Air filters should help, and yes you can tape a furnace filter to a box fan. (I knew it!)
• “There is no evidence that COVID-19 has been transmitted when people walk past each other outdoors. (But I’m using the bandanna system just because people are scared and confused out there.)
• Taxis and rideshare are not zero risk, but reasonably probably, maybe reasonably low risk if everyone is masked and windows are open. If it is too cold to open windows, it is better to be drawing in outside air than just recirculating air.
• Airplanes have very good ventilation, so it is a myth that one infected person on an airplane can infect everyone. If they are sitting right next to you, not wearing a mask, and/or coughing/sneezing, they can infect you. The airport itself is also probably higher risk than the plane. (But let’s remember people are working in all these places.)
• They say “schools should operate in person only if the levels of infection in the community are low.”
• Elevators are also actually quite well ventilated, and you are not in there for very long. Again, you don’t want people unmasked and/or coughing/sneezing on you. No singing allowed in elevators.
• The dental office is suspect. Technology exists to ventilate them safely (but I didn’t see anything obviously new or high tech at my dentist recently.)
• Masks still help with aerosols. Even though the particles are tiny, they are still inside droplets, which are tiny but not as tiny. Nothing in the air moves around in straight lines, it is turbulent and random, so even if particles are smaller than the openings in the fabric many of them will hit the sides and the risk will be significantly reduced. (Also suggests one reason having multiple layers is better.)
• Masks work better if they fit well. (I’m a little tired of this, my family has about 100 masks now and not one of them fits well. If there are 1 or 2 I think fit pretty well, they are always in the dirty laundry when I need them. The same gremlins that steal one of each of my favorite socks also steal masks on occasion.)
• Face shields and plexiglass barriers don’t help a lot with aerosols. You need a mask.