Corsi-Rosenthal Box — DIY Air Purifier That Works

The Corsi-Rosenthal box is one of those things I wish someone had told me about in August 2020, when the sky over Seattle turned that particular shade of apocalyptic orange and I was stuffing wet towels under my apartment door like that was going to do anything. It’s a DIY air purifier you build in about fifteen minutes from box fans and furnace filters, and it genuinely works — not “works” in the way that a candle supposedly “clears the air,” but works in the way that university researchers and the EPA have actually tested and documented. If you live in the Pacific Northwest, where wildfire smoke season now runs from roughly July through October and sometimes bleeds into November, this thing is worth building right now, before you need it.

What It Is and Why It Works

The design comes from two people: Richard Corsi, an air quality engineer who is now the dean of engineering at UC Davis, and Jim Rosenthal, a filter manufacturer in Texas. They landed on the same concept independently in 2020 and started collaborating after connecting on Twitter during the early pandemic. The idea is almost embarrassingly simple — duct-tape four MERV-13 furnace filters into a cube shape, attach a box fan to the top, and let the fan pull air through all four sides simultaneously.

That four-sided geometry is the whole point. Pulling air through four filters instead of one increases the total filter surface area dramatically, which means the fan doesn’t have to work as hard to move air through any single filter. Less resistance means more airflow. More airflow means more air cleaned per hour. A study out of the University of Michigan found the Corsi-Rosenthal box performed 261% better than a single-filter configuration using the same fan. The EPA tested it. UC Davis tested it. The results held.

MERV-13 specifically matters here. MERV stands for Minimum Efficiency Reporting Value — it’s a standardized rating for how well a filter captures particles at different sizes. MERV-13 filters capture particles down to 0.3 to 1.0 microns, which covers wildfire smoke particles, PM2.5, pollen, and most respiratory droplets. MERV-8 filters, which are more common and cheaper, don’t cut it for smoke. The number matters. Don’t substitute down.

The fan orientation is counterintuitive to some people. You want the fan sitting on top of the box blowing air upward — meaning the fan pulls air inward through the filters and exhausts clean air out the top. If you flip it, you pressurize the inside of the box and force air out through the gaps between filters instead of through the filter media. Arrows on the fan and filter frames will tell you which direction air flows. Pay attention to them.

Parts List and Where to Buy in PNW

Probably should have opened with this section, honestly. Here’s everything you need.

The standard build uses four 20x20x2 inch MERV-13 filters. That size is the sweet spot — common enough that most hardware stores stock it, and sized right for a standard 20-inch box fan. You can use 20x20x1 filters and they’ll work, but the 2-inch depth gives you more filter media and longer life between replacements.

• 4x MERV-13 furnace filters, 20x20x2 inches — Home Depot on Rainier Ave, the Northgate location, and the Sodo store all typically stock these. The Filtrete 1500 MPR filters in that size run about $8 to $10 each. Nordic Pure is another brand that shows up consistently at those stores and is well-regarded by the researchers who tested the design. Budget roughly $35 to $40 for all four.
• 1x 20-inch box fan — Fred Meyer stores across the region (Ballard, Capitol Hill, Burien) carry Lasko and Holmes box fans during spring and summer. The Lasko 3733 is a common one, runs around $20 to $25. If it’s late in smoke season, check Amazon — they’re lighter on stock than you’d think once smoke events start.
• 1 roll of duct tape or foil HVAC tape — You probably have this. If not, a dollar or two at any hardware store. HVAC foil tape makes a cleaner seal, but regular duct tape from a junk drawer works fine.

Total cost lands between $55 and $70 if you’re buying everything new. That’s less than a single replacement filter cartridge for most commercial air purifiers. I built my first one for $58 at the Rainier Ave Home Depot and a Fred Meyer run for the fan.

One note for people in Tacoma, Olympia, or further south — ACE Hardware stores in that area tend to stock Nordic Pure filters more reliably than the big boxes. Call ahead during smoke season. These filters move fast once the air quality index starts climbing.

Step-by-Step Build Instructions

Motivated by the 2022 smoke event that trapped a week’s worth of particulate inside my apartment, I finally built one of these using four Filtrete 1500 MPR filters and a Lasko fan I grabbed from the Capitol Hill Fred Meyer. The whole process took about fifteen minutes, and that includes the time I spent reading the airflow arrows wrong on my first attempt.

1. Lay out your four filters flat and find the airflow arrows. Every furnace filter has an arrow printed on the cardboard frame indicating which direction air is supposed to move through it. You want all four arrows pointing inward — toward the center of the cube you’re building. This means when air flows through the filters into the box, it’s moving in the correct direction through the filter media.
2. Stand two filters up facing each other to form opposite walls. Hold them in place or lean them against something while you work. The arrows on both should point toward each other — toward the center of the space between them.
3. Add the remaining two filters to form a square. Now you have a four-sided open cube. The top and bottom are open. Double-check that all four arrows point inward.
4. Tape all four seams where filters meet. This is the step people rush, and it’s the step that matters most. Any gap in the seams is a gap that air will take instead of going through the filter media. Run tape along the full length of each corner seam, press it firmly into both filter frames, and add a second layer if the first doesn’t lay flat. Foil HVAC tape bonds better to the cardboard frames than standard duct tape, but either works if you’re thorough.
5. Place the box fan on top, facing upward. The fan should sit on top of the open top of your filter cube. The exhaust side of the fan — the side that blows air out — faces up, away from the cube. The intake side of the fan faces down into the cube. This pulls air inward through the filters and exhausts clean air upward into the room.
6. Tape the fan to the top of the filter cube. Run tape around the perimeter where the fan frame meets the filter frames. You don’t need to make this look pretty. You need it to be airtight so air doesn’t bypass the filters around the fan’s edges.
7. Plug it in, turn it on, and set it in the room you use most. Start on the highest setting for the first hour in a smoky situation. You’ll smell the difference faster than you expect.

The mistake I made the first time — I got the fan orientation right but didn’t tape the corner seams well enough. I could feel air seeping out of the corners when I ran my hand along the outside of the cube. Ripped the tape off and did it again properly. Takes two minutes to fix and makes a real difference in performance.

How Long Filters Last in Smoke Season

Pacific Northwest smoke season is different from what these filters were designed for. Furnace filters in a normal HVAC system see relatively clean indoor air. Filters in a Corsi-Rosenthal box during an AQI-200 smoke event are working extremely hard, and the visual evidence accumulates fast.

The practical rule is simple — replace when visibly dirty. Pull the filters off and look at the side that faces the inside of the cube. If it’s gray or brown with visible particulate buildup, it’s time. During heavy smoke events, that can happen in a week of continuous use. During normal indoor use between smoke events, monthly replacement is a reasonable baseline.

In the PNW context, I’d suggest buying two sets of filters at the start of summer and keeping one set in reserve. During the 2020 and 2022 smoke events in the Seattle area, Home Depot locations sold out of MERV-13 filters within a day or two of air quality alerts. The people who already had spares didn’t have to drive to four different stores.

One thing worth knowing — a clogged filter doesn’t fail catastrophically. It just reduces airflow as the media gets loaded up, which reduces how much air the unit cleans per hour. You’ll notice the fan seems to be working harder and moving less air. That’s your signal. The unit won’t start blowing dirty air back at you the way a saturated activated carbon filter eventually can. It just gradually loses effectiveness until you swap the filters.

Spent filters from a smoke event are genuinely impressive to look at. Hold one up to a window and you’ll see exactly what didn’t end up in your lungs. That usually convinces people to build a second unit for their bedroom.