Simple anti-allergen air filtration system at a fraction of the cost of commercial units. It's not pretty but it does a great job. All parts readily available at your local hardware store or department store.

Step 1: Purchase or Acquire the Following:

- One Cheap 20" box fan ($6-8)
- One very good DISPOSABLE PLEATED 20" furnace filter (I like the 3M ones) ($8-$15)
- Good quality duct tape ($2-$4)

Notes-
1) The brand of fan doesn't really matter. A cheap one works as well as a good one, and price is no indication of how long it will last either.

2) DO NOT buy some goofy expensive electrostatic permanent filter or a filter that isn't pleated. Good quality Disposable Pleated filters pull far more out of the air. If you don't believe me, prove me wrong.

3) Don't skimp by buying cheap duct tape. You'll end up using twice as much and not get as good of a seal.

Step 2: Assemble the Fan According to the Directions That Came With It, Take the Filter Out of Its Packaging.

Step 3: Align the Filter on the Fan

Most cheap fans will be set up to push air in one direction. Likewise, most disposable pleated furnace filters are designed to handle airflow in one direction. They even have an arrow directing you as to the right way to place them in the airflow.

You want to align the furnace filter on the fan so that air exiting the fan will go the right way through the filter.

Step 4: Attach the Filter to the Fan

Using the duct tape, seal the filter to the face of the fan.

Step 5: Shroud the Back of the Fan.

Since you have added a restriction to the front of the fan, air will want to blow out the back of the fan instead of through your filter. Since the fan is round and the corners are square, the air will tend to try to exit the back of the fan through the corners.

Tape over the corners of the fan on the inlet side, covering the outermost 6" or so of each corner. This will prevent air from exiting the back of the fan, allowing the filter unit to work far more efficiently.