You don't even need a HEPA filter. A lower grade filter will do fine as well for recirculating air: the idea is to reduce particles by passing the air of the ro

You don't even need a HEPA filter. A lower grade filter will do fine as well for recirculating air: the idea is to reduce particles by passing the air of the room again and again through the filter. If you have a air source that has more particulate pollution than the sink a better filter makes more sense.