Shameless plug for a great source for very quiet versions of the DIY Corse-Rosenthal boxes for clean air filtering for PM 2.5, Covid, smoke, etc.

Please check out NORTH BOX SYSTEMS in Toronto. https://northboxsystems.ca.

They are legit and it's a great product in various versions, as kits, or fully assembled. The nice thing about them besides the low noise compared to the regular Corsi-Rosenthal boxes with the box fan on top, is that they only need two 20x20x1 filters, instead of 4! Also available are small versions that can run on 12V for a small room, traavel trailer, camping van or vehicle Shipping is only in Canada.

They have designed a version of these great air cleaning boxes for classrooms and looking at integrating it into science programs. These boxes should be in every classroom in Canada.