A CR box, or a Corsi-Rosenthal box, is a very efficient, low-cost, make-it-yourself air filter that was developed by two environmental engineers, Richard Corsi and Jim Rosenthal.

The standard is a box fan taped to four MERV-13 furnace filters (like this: https://youtu.be/aw7fUMhNov8 ), but people have adapted them to use computer fans and one or two filters to fit in their space (examples here: https://mobile.twitter.com/hashtag/corsirosenthalbox?src=hashtag_click&f=image and specifically here: https://mobile.twitter.com/robwiss/status/1596940686049554434 ).

Because of the high volume of air a box fan is able to pull through the large surface area of filters, it ends up being pretty similar to or better than a hepa filter in terms of particles filtered, and they’re cheaper and have less plastic and electronics waste. Testing shows that they’re also quieter than many standard air filters: https://mobile.twitter.com/marwa_zaatari/status/1499072595404206090.