The short answer
Tape four or five 20x20x1 (or 20x20x2) MERV 13 filters into a cube with a 20-inch box fan on top blowing outward. It moves a lot of air through a lot of filter area; it's louder than a commercial purifier and needs supervision.
What you need
- A 20-inch box fan (a newer model with a safety-listed motor).
- Four MERV 13 filters, 20x20x1 or 20x20x2, plus cardboard for the bottom.
- Duct tape.
Build
- Tape four filters into a cube, arrows pointing inward.
- Close the bottom with cardboard or a fifth filter.
- Tape the fan on top, blowing out and away from the cube.
- Cover the fan's corners with cardboard so air is pulled only through the filters.
Limits and safety
It's noisier than a purpose-built purifier on its quieter settings, bulkier and has no carbon stage.
SafetyDon't leave it running unattended for long periods, keep it away from children, and stop using it if the motor overheats.
Products mentioned
Questions readers ask
Do box fan air purifiers work?
Yes, studies have found they remove a large share of smoke particles in a room; they're louder than commercial units.
Which filter size?
20x20 to match a 20-inch box fan; MERV 13 is the usual grade.