Build a Cheap DIY Air Purifier: The Corsi-Rosenthal Box That Really Works

Why spend hundreds on a branded purifier?

The dye is cast: since 2021, hand-built Corsi-Rosenthal boxes have gone viral from classrooms to clinics because independent tests—University of California, EPA, and Harvard engineering—consistently rate them equal to or better than units costing ten times the price.

• Removes up to 90 % of PM_2.5 within thirty minutes in a typical bedroom (Garcia et al., UC Davis 2022)
• Cuts wildfire smoke VOCs faster than many $300 commercial purifiers (ILO Fund, 2023)
• Total build cost: $30–$45 if you already own a box fan

What the Corsi-Rosenthal box actually is

Invented by environmental engineer Richard Corsi and DIY builder Jim Rosenthal, the recipe is embarrassingly simple:

1. Four MERV-13 pleated HVAC filters form a cube.
2. A 20-inch box fan blows out the top, pulling room air through the filters.
3. You’re done.

The science hinges on high-grade furnace filters (MERV-13/14 or FPR-10) acting as a pleated HEPA substitute while the box fan delivers huge airflow—450–500 cubic feet per minute on low.

Tools and shopping list (all big-box store or online)

Total spend: $33–$43, cheaper when household supplies are on hand.

Step-by-step build that survives toddler kicks

1. Test fit

Lay four filters into a square tunnel: two on short sides, two on long. The printed airflow arrows on filters must point inward toward the cube cavity. If corners don’t mate, swap filters—slight sizing differences matter.

2. Tape corners

Run a 4-inch strip of tape vertically along each vertical corner seam, half on one filter, half on its neighbor. Press to a crisp crease. Add a second horizontal strip 3 inches up for rigidity.

3. Create the top plate

Cut a 20 × 20 in. square of double-ply corrugated box. Center the box fan upside-down, trace around the motor shroud, then cut ½ inch inside the tracing. You want the shroud to sink into the hole snugly but leave a 1-inch lip; this stops the fan from falling through and seals gaps.

4. Seat the fan

Flip the box fan face-side down so it blows upward. Tape the cardboard plate to the filter box, then tape the fan rim in three places. Optional: run zip ties through cardboard and fan grilles for grab-and-lift security.

5. Bottom seal

Before final taping, lay the cube on its side, slide a fifth filter (unused) across the open bottom, mark gaps, and seal with a 6-inch wide tape skirt. A sealed bottom forces all intake through the side filters. No fifth filter needed if you leave the cube on a clean floor.

6. Pre-use safety check

1. Inspect fan blade for chips or imbalance shakes.
2. Confirm the cube is level; shim with a folded magazine if not.
3. Plug into a surge strip with 15-amp fuse to prevent cord overload.
4. Run on LOW only—quieter, cooler, and airflow is still punchy.

Placement tricks the pros hide

• Center of the biggest room for whole-house circulation; raise on a coffee table or dining chair for floor-to-ceiling sweep.
• Never against walls closer than 12 inches; you choke the inlet.
• For smoker or cooktop homes, position 2 ft diagonally from kitchen door; the doorway acts as a clean air corridor to adjoining rooms.
• To sleep quietly in a nursery, set cube on dresser, trigger fan with an Alexa smart plug on 2-hour timer.

How long do filters last?

The EPA Office of Air Quality (2023) says MERV-13 pleats lose roughly 20 % efficiency after 2000 hours of household use—three months on 24/7. Red flag drops: strong odor, gray visible dust band, or 25 % louder fan on same speed. Label the intake with a Sharpie date and rotate one filter out monthly to keep load balanced among all four.

Energy cost reality check

Most 20-inch fans draw 70 W on low. Run 24 hours → 1.68 kWh/day. At $0.15 per kWh, that is $7.56 per month—far less than a Starbucks habit and a fraction of a commercial purifier that averages 120–200 W.

Killer upgrades for perfectionists

Smarter fan control

Add a $30 PM sensor kit with an inverter that ramps the fan only when particle count spikes—typical annual energy drop: 45 %.

Higher-grade filters

If smoke is brutal, swap one inlet filter for a true H13 HEPA (same 20 × 20). Attach it opposite the kitchen side; rest stay MERV-13 to maintain airflow. This hybrid captured 94 % of ultrafine smoke (<0.1 microns) in a Lawrence Berkeley Lab test (2022).

Noise killing quilt

Sue, a librarian in Austin, wrapped her box in a breathable outer wrap of poly-wool felt. She slit 3/4-inch intake slots along two sides inside the felt. Result: 8 dB quieter yet zero airflow loss in a phone-meter reading.

Safety myth-busters

• Fan overheating: UL-listed 20-inch fans have built-in internal thermal cutoffs rated 90 °C. University of Illinois real load tests showed steady 52 °C at low speed when duct-taped shut—no burner-risk.
• Filter off-gassing new smell: Run the cube outdoors 30 minutes before moving inside; VOC spike drops below background levels (MIT IEQ lab, 2022).
• Cardboard flammability: Duct tape burns at 600 °F. Normal fan surface stays well below 140 °F. Risk exists only if cube sits against bare-bulb halogen heat source—keep 4-ft separation.

Cleaning and filter swap checklist (takes 6 minutes)

1. Unplug unit, move to balcony, turn fan blade one direction to vacuum lint.
2. Peel tape from one edge, fold filter back 45° to reveal inner cube wall.
3. Vacuum inner pleats with upholstery brush set to low suction.
4. Drop filters back, re-tape with three 3-in. strips per corner.
5. Write new Sharpie date.

Common fail points—how to avoid them

Cost of ownership after one year

• Swap filters every 3×1 months: $24 ($2 filter replacements × 4)
• Energy @ $0.15/kWh: $90.72 (24/7 low)
• Total first-year: $114.72

FAQs real owners ask

Sources

• EPA: Scientists explain building your own portable air cleaner
• University of California: Effectiveness of DIY Corsi-Rosenthal air cleaners
• ILO Fund: Performance against wildfire smoke
• Lawrence Berkeley Lab: Box-fan filter effectiveness study
• University of Illinois: Testing safe fan filter operation
• MIT IEQ lab: Filter off-gassing study