How to Build a DIY Air Purifier With a Box Fan and Furnace Filters

Published July 17, 2026 by Aldo Giammusso

Dirty indoor air is bad for your lungs, and it is quietly hard on your computers too. One cheap build fixes both.

Dust, wildfire smoke, pollen, and pet dander float around every home. You breathe them in, and so do your electronics. The same fine particles that irritate your lungs get pulled into the intake fans on your desktop, laptop, and game console, where they settle on the parts that keep those machines cool.

A Corsi-Rosenthal box is a do-it-yourself air purifier you build from a 20-inch box fan and a few furnace filters. It costs roughly 50 to 150 dollars in parts, and it moves about as much clean air as purifiers that sell for several times the price. Cleaner air is easier on your body and easier on the machines you have paid good money for. Here is how to build one, which filters to buy, how to do it safely, and why it matters for your computers.

What Is a Corsi-Rosenthal Box?

The Corsi-Rosenthal box was created in August 2020, during COVID, by Richard Corsi, an environmental engineer now at UC Davis, and Jim Rosenthal, the CEO of Tex-Air Filters. The idea was simple: instead of running one filter, build a cube out of four or five of them and let a single box fan pull air through all of the walls at once.

The design is a cube of four or five MERV-13-or-higher furnace filters, with a 20-inch box fan sitting on top pulling air out through the filter walls and a cardboard base sealing the bottom. Air gets pulled in through every filter face and pushed up and out through the fan. Because so much filter surface is working at once, the fan moves a lot of air without straining.

The design has held up under real testing. A UC Davis study of a five-filter build measured effective clean air delivery rising from about 600 cubic feet per minute at low fan speed to about 850 at high speed. An EPA chamber study found it removed about 97 percent of infectious aerosols in 30 minutes and 99 percent within an hour. A Brown University study found it cut indoor levels of PFAS and phthalates. Tex-Air estimates more than a quarter-million of these boxes have been built, and per unit of clean air they run about ten times cheaper than commercial purifiers.

One honest limit up front: a Corsi-Rosenthal box removes airborne particles, things like dust, smoke, PM2.5, and some virus-carrying aerosols. It does not remove gases. It will not clear carbon monoxide or most household fumes, so it is never a substitute for a carbon monoxide alarm.

MERV, MPR, and FPR: Which Filter to Buy

Filters are rated on a scale called MERV, short for Minimum Efficiency Reporting Value, defined by the ANSI/ASHRAE 52.2 standard. For home and commercial filters the scale runs from 1 to 16, and higher numbers catch finer particles.

MERV 8 catches larger dust, pollen, and mold spores. MERV 11 adds finer dust and common allergens. MERV 13 captures a meaningful share of PM2.5, smoke, and viral aerosols, and it is the level the CDC and ASHRAE recommended for indoor air during COVID. For a Corsi-Rosenthal box, MERV 13 is the one to buy.

If you shop for filters you will also run into two store-brand scales. 3M uses MPR on its Filtrete filters, and The Home Depot uses FPR. Both are rough stand-ins for MERV. As a guide, MERV 8 is about FPR 5 or MPR 600, MERV 11 is about FPR 7 or MPR 1000 to 1200, and MERV 13 is about FPR 10 or MPR 1900 to 2200 and up. Treat these as ranges, not exact matches.

One caveat worth knowing: matching labels do not mean matching performance. By 3M's own numbers, a 1-inch MPR 1900 filter captures only about 62 percent of the finest microparticles, less than a true deep-pleat MERV 13. The label says MERV 13, but a thin filter does not perform like a thick one.

There is also a difference between putting a high-MERV filter in your furnace and using one in a box fan. In a furnace, a dense 1-inch MERV-13 filter chokes airflow and raises static pressure, which is why HVAC pros keep furnaces at MERV 8 to 11 on a 1-inch slot, or step up to a 4 to 5-inch deep-pleat filter for MERV 13. A box fan is the opposite situation. Because a Corsi-Rosenthal box spreads the work across four or five filters, the combined surface area keeps resistance low, so MERV 13 is ideal here.

What You Will Need

Here is the parts list. You can find all of it at a hardware store or online.

• Four or five MERV-13 furnace filters, either MERV-13 20x20x1 filters or thicker MERV-13 20x20x2 filters. Four filters plus a cardboard base is the common build; five uses filters on all sides.
• A 20-inch box fan made in 2012 or newer, with a UL or ETL safety mark on it.
• Strong duct tape or another wide, strong tape to seal the seams.
• A box cutter or a good pair of scissors.
• A sheet of cardboard for the base and the fan shroud. A spare moving box works fine.

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How to Build a Corsi-Rosenthal Box (Step by Step)

The whole build takes about 20 minutes and no special skills. Here is the process.

Step 1: Gather Your Materials

Lay out your four or five filters, the box fan, your tape, a box cutter or scissors, and some cardboard on a clear table or floor. Check that all of your filters are the same size so they form a clean cube, and unbox the fan.

Step 2: Check the Airflow Arrows on Each Filter

Every furnace filter has a printed arrow showing which way air is meant to flow through it. On a Corsi-Rosenthal box, those arrows must point inward, toward the fan, because the fan pulls air in through the filter walls. Set each filter with its arrow facing the center of the cube before you tape anything.

Step 3: Tape the Filters Into a Cube

Stand the filters up on their edges and tape them together at the corners to form four walls of a cube, arrows pointing in. Tape every seam so air is forced through the filter media instead of leaking around the edges. Sealed seams are what make the box work, so do not rush this part.

Step 4: Seal the Bottom With Cardboard

Cut a piece of cardboard to fit the bottom of the cube and tape it on, sealing all the edges. This closes off the base so the fan can only draw air in through the filters. In a five-filter build, a filter takes the place of the cardboard on one face instead.

Step 5: Set the Box Fan on Top

Place the box fan on top of the cube facing up, so it blows air up and out, away from the filters. This is what creates the suction that pulls room air in through all of the filter walls. Tape the fan down to the top edges of the cube and seal the gaps around it.

Step 6: Add a Cardboard Shroud

Cut a piece of cardboard the size of the fan with a round hole in the middle matching the fan blades, and tape it over the top of the fan. This shroud covers the dead corners of the fan and keeps air from flowing backward. It can improve airflow noticeably, though the exact gain depends on your fan, and it makes the box a little quieter.

Step 7: Position It and Keep It Maintained

Set the finished box in the room you spend the most time in, away from walls and corners so air can reach every filter face. Run it on a speed you find comfortable. Replace the filters when they look gray and loaded, and more often during heavy smoke, since a clogged filter barely cleans the air at all.

Why This Also Protects Your Computers

Here is the part most air-quality guides skip. The same particles that a Corsi-Rosenthal box pulls out of the air are the ones that wear down computers.

Dust is the most common cause of computer overheating. That is Dell's own guidance. Dust clogs the fans, vents, and the thin metal fins of the heatsink that carries heat away from the processor. When that airflow gets blocked, the chip runs hotter, and modern CPUs and graphics chips respond by throttling: they deliberately slow themselves down to avoid heat damage. A dusty machine is a slower machine, and sustained heat shortens the life of the parts inside. Game consoles like the PS5 and Xbox get hit the same way, since they cool themselves with the same kind of fans and vents.

Wildfire smoke is worse in one specific way. It is fine PM2.5 particulate, and it clogs airflow just like dust but arrives in far higher concentrations, so filters and vents load up fast when the air outside is bad. Its particles can also be mildly corrosive and even slightly conductive over long exposure, which means that over months and years they can attack the metal contacts inside a machine. This is not instant death for your PC. As the balanced take goes, wildfire smoke will not immediately kill your computer, but heavier and longer exposure raises the risk of corrosion and clogging over time.

Cleaner indoor air means less dust and smoke floating around to get pulled into your electronics in the first place. That reduces how often your machines need cleaning and how hard they have to run to stay cool. It is not a guaranteed performance boost, and it does not replace opening up a dusty machine and cleaning it out. What it does is cut down the grime going in, which is the honest, useful benefit.

Safety and Honest Limits

This build has been tested, and it is safe when you follow a few rules.

Start with the fan. Use a box fan made in 2012 or newer that carries a UL or ETL safety mark. The EPA specifically recommends this, because newer fans have fused plugs and thermal cutoffs that older ones lack. Do not use a pre-2012 fan for this.

In 2021, UL and the EPA tested this exact design. They ran it with clean, smoke-loaded, and dust-loaded filters, and even blocked both the front and back of the fan for seven hours straight. No fire started in any scenario, all temperatures stayed within the UL 507 safety standard, and the surfaces a person might touch stayed cool.

Even so, follow the EPA's guidance. Do not block the airflow. Do not leave the box running unattended. Keep working smoke detectors in the home. Do not run it on an extension cord, do not use a damaged fan, and keep children away from the running fan.

A few honest limits to be clear about. This is not a medical device, and it does not diagnose, treat, or prevent any disease. It removes airborne particles, not gases, so it is not a carbon monoxide detector or a substitute for one. And a dirty, clogged filter is close to useless, so replace your filters on a regular schedule, and more often when smoke is heavy.

The Bottom Line

A Corsi-Rosenthal box is cheap, proven, and genuinely useful. For roughly 50 to 150 dollars in parts, you get an air cleaner that moves as much clean air as machines costing far more, and cleaner air is easier on both your lungs and the electronics you rely on.

Use a modern box fan with a UL or ETL mark, build the cube from MERV-13 filters, seal the seams, and keep the filters fresh. That is the whole job.

And if the dust and smoke have already taken a toll and a computer in your home or office is running hot or slow, get in touch and we will take a look.

Frequently Asked Questions About DIY Air Purifiers

What is a Corsi-Rosenthal box?

A Corsi-Rosenthal box is a DIY air purifier built from a 20-inch box fan and four or five MERV-13 furnace filters taped into a cube. The fan pulls room air in through the filter walls and pushes clean air out the top. It was created in 2020 by Richard Corsi and Jim Rosenthal, and it removes airborne particles like dust, smoke, and pollen for a fraction of the cost of a store-bought purifier.

What MERV rating should I use for a DIY air purifier?

Use MERV 13. It captures a meaningful share of fine particles, including PM2.5, wildfire smoke, and virus-carrying aerosols, and it is the level the CDC and ASHRAE recommended for indoor air during COVID. In a box-fan build the large combined filter area keeps airflow high, so MERV 13 works well here even though it would choke a one-inch furnace slot.

Does a DIY box fan air filter really work?

Yes. A five-filter build measured by UC Davis moved roughly 600 to 850 cubic feet of clean air per minute depending on fan speed, and an EPA chamber study found the design removed about 97 percent of infectious aerosols in 30 minutes. Per unit of clean air it runs about ten times cheaper than a commercial purifier of similar output. It removes particles, not gases, so it is not a carbon monoxide substitute.

Is a DIY box fan air filter a fire risk?

With a modern fan the risk is low. A 2021 UL and EPA study tested this design, including blocking both sides of the fan for seven hours, and found no fire, with all temperatures within the UL 507 safety standard. Use a box fan made in 2012 or newer with a UL or ETL mark, do not block the airflow, do not run it unattended, and keep working smoke detectors in the home.

Can a DIY air filter protect my computer from dust and smoke?

It helps. Dust is the most common cause of computer overheating, and wildfire smoke is fine particulate that clogs vents and fans the same way. Cleaner indoor air means less dust and smoke pulled into your computers, laptops, and game consoles, which reduces how often they need cleaning and how hard they run hot. It is not a guaranteed performance boost, but it lowers the wear that heat and grime cause over time.