9/ For schools without mechanical ventilation but operable windows, open 1 window and put a box fan in it blowing from outdoors in. Open a second window as far from the first one as possible with a fan blowing out (preferably at slightly lower flow rate). Can get by w/o 2nd fan.

10/ To the extent that some recirculation will be used in mechanical systems (even with increased outdoor air supply rates), increase MERV rating of filters to MERV13 if possible. If not, consider MERV12 or MERV11. Inspect filter seating for by-pass regularly.

11/ If a portable air cleaner is used go with a proven technology - HEPA filtration. Look for a system that has a certified clean air delivery rate (CADR) for smoke of at least 250 (cubic feet per minute).

12/ Do a quick calculation for your room.

EqACH = CADR x 60/(A x Z)

A = floor area of room in ft2 and Z = ceiling height in ft.

Here EqACH is the aerosol particle decay rate (/hr) that's equivalent to the same amount of air changes per hour by ventilation, e.g., 3/hr, etc.

13/ Portable HEPA air cleaners can be effective at reducing particles, particularly under conditions with inoperable windows and limited capacity for mechanical ventilation (it happens). Aim for an EqACH of 3/hr or higher if possible to supplement particle removal by ventilation.

14/ Don't worry about add-ons to the HEPA air cleaner. Many are unproven and some are worse. Just use the old workhorse .... HEPA.

15/ Not enough attention has been paid to the value of UVGI for virus inactivation. Done right (proper exposure time and intensity of field; w/ safety in mind) it can be incredibly effective at inactivating viable viruses in particles.

16/ UVGI in mechanical systems requires recirculation and is less effective if greater outdoor air supply is employed with less recirculated air. Upper-room UVGI can be operated continuously & does not suffer from the recirculation constraint.

17/ UVGI can also be employed by in-room recirculation through a drop (false) ceiling. I am aware of some restaurants employing such systems.

18/ Review for reduced inhalation dose. MASKS for all, physical distancing to avoid high near-field inhalation dose by an infector w/ a faulty mask or poor fit, increased ventilation (& activities outdoors as possible), improved filtration where recirculation is employed, ...

19/ portable HEPA air cleaners as supplements in classrooms and other school spaces, e.g., office suites, with appropriate sizing, and consideration of UVGI.

20/ And finally, beware of unproven technologies that sometimes come with slick marketing and that might be relatively ineffective and/or cause unwanted exposures to oxidants & oxidized by-products.

21/ And to end w/ good news. A lot of teachers and parents are becoming educated and serving as champions for effective risk reduction in schools. I have faith that their voices will make a big difference. Kudos to all of those who have been pushing reality. Keep being heard!