Use a fan to draw outside air through a good quality furnace filter. The air gets "forced" into the room creating positive pressure and therefore it won't allow dust to be drawn back into the room. The fan does not need to be powerful, all you are doing is creating positive pressure. Disclaimer: I have never used that method, I read it in a forum many years ago and some of the fellows that used it reported very positive results. I hope that helps.
Chele