Build a DIY Indoor Air Purifier

Your step-by-step guide to building your own (hatched-in-Portland) Corsi box

By Julia Silverman Illustrations by michael novak November 25, 2021

If anyone has profited from this miserable COVID era, it is the filtration industry. We are swimming in products meant to purify the air we breathe, to neutralize any stray infected particles, not to mention wildfire smoke. You can splash out for a sleek, high-end air purifier, which can run several hundreds of dollars, or you can DIY one for far less. The result will almost certainly be, as Wired magazine puts it “janky” and “jury-rigged” but will absolutely get the job done.

Turns out, the most popular design out there has Portland roots, via Richard Corsi, the former dean of Portland State University’s College of Engineering, who decamped in September for the University of California, Davis. Corsi was one of the first academics in the country to grasp that COVID transmission was airborne; home-made air filters everywhere follow his specifications and bear his name. In case you haven’t already done so, here’s how to build your very own Corsi box.


Required materials:

Four 20-inch MERV-13 air filters, generally sold in packs of six for about $65 at your local hardware store

20-inch box fan, around $20

Roll of duct tape

Two pieces of cardboard, cut to 20-inch squares


1. Put one piece of cardboard on a table or a floor. This is your base.

2. The air filters are the four sides of a 3-D box. Arrange them so that the air flow is pointing inwards, and then use the duct tape to attach them.

3. Place your box fan on top, with the blades pointing up. Attach it to the filters with the duct tape, making sure it is tightly sealed, though leave space for the electrical cord.

4. Take the second piece of cardboard, and cut out a hole the size of the box fan’s blades. Place it on top of the fan to act as a “shroud” so air doesn’t leak back through the corners of the fan.

5. Plug in at least three feet from a wall and enjoy the clean air! In a 500-square-foot room with eight-foot ceilings, this should provide up to five air changes per hour.


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