HEPA filters are a good idea, but they can be in short supply and they need to be replaced on a regular basis. Box fans are not good for attaching filters too

HEPA filters are a good idea, but they can be in short supply and they need to be replaced on a regular basis. Box fans are not good for attaching filters too because they are not designed to deal with static pressure from a filter. The gaps between the blades and the case mean if you put much/any obstacle in the way, it will just recirculate unfiltered air.