1/ Thanks, Linsey. I have some concerns about public anxiety regarding HVAC systems & recirculation. First, it is just not feasible for many across North America to have supply air be 100% outdoor air. In some systems more outdoor air and less recirculation through effective https://twitter.com/linseymarr/status/1283827710784155649

2/ filtration might actually be counter-productive. Temperature and RH affect rate of virus inactivation and are more easily controlled w/ recirculation through HVAC systems. Further, rates of outdoor air intake may be limited in some systems, whereas multiple passes through .

3/ a good filter may be highly effective at removing particles that contain viruses. Even if some viruses are transported through a mechanical system and are not captured on a filter or otherwise deposited there can be substantial dilution.

4/ For example, take 10 classrooms on one air handling unit with an infector in one of them. The air that is recirculated is effectively diluted 10-fold in the system.

5/ With effectively-seated MERV13 or better filters there might be another five fold or more reduction in particles that contain the virus. That gets us to a 50-fold lower concentration of particles that contain viruses relative to the classroom with the infector.

6/ Is there still risk of transmission in other spaces due to HVAC systems and recirculation? Yes, of course. But it is likely much lower than many think, at least in buildings with air handling units that serve many rooms, e.g., classrooms or offices.

7/ I believe that what we need is a combo of greater outdoor air supply, improved filtration and/or UVGI, and portable air cleaners. These need to be combined with required masks, lower density, and physical distancing. IMHO, none of these should be off the table.

8/ With respect to CO2 monitors, I see some value. Beware however, that CO2 is a good metric when ventilation is the primary control strategy. Filtration and UVGI can reduce quantum concentration (and thus "infectiousness of air") without a change in CO2.

9/ If ventilation is the primary control mechanism, then the level of CO2 required really gets to rebreathed fraction, e.g., in Rudnick-Milton model. The question is not so much what CO2 concentration do we want, but what is the critical rebreathed fraction that leads to ...

10/ a reproductive number less than unity. That will depend on assumed quanta generation rate and time of exposure. Since quanta generation rate will not be known accurately, using a high value will lead to a lower rebreathed fraction, & thus lower acceptable CO2, & vice-versa.

11/ I would thus err on the side of a higher quanta generation rate and longer exposure time to be conservative. Once the critical rebreathed fraction is determined, the target CO2 concentration can be determined as follows:

12/

CO2 (indoor) = CO2(Breath) x f + CO2 (outdoor).

Here, f is the critical rebreathed fraction and CO2( breath) is usually around 36,000 ppm (a little higher for a crowd of vegans).

I hope this makes some sense. It's very late, but I love the discussion.