* Squirrel-Cage Fan - $22
** Air Filter - $8
* Standard rotary fans don't work as well. Plus, it's harder to cut a circular piece of filter.
** You pick how much filtering power you want by choosing the quality of the air filter. You can get different MERV ratings. Spun glass filters don't work; forget those. Pick a filter height that matches the height of the air flow openings in your fan.