How to Build a Corsi-Rosenthal Box for Wildfire Smoke
Last updated 2026-07-15
A Corsi-Rosenthal box (CR box) is a do-it-yourself air cleaner that tapes a common box fan to four furnace filters, forming a cube that pulls smoky air through a huge amount of filter surface. It rivals a mid-size purifier for a fraction of the price — typically about 70 dollars in parts.
During a wildfire event, fine-particle PM2.5 is what harms your lungs, and a CR box is one of the cheapest ways to strip it out of a room. Below is a full build, or use the CR box builder to size one for your room.
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How a CR box clears smoke
A single furnace filter taped to a box fan works, but it chokes airflow. The CR design uses four filters as four walls of a cube, so the fan pulls through four times the surface area at once. More area means less resistance, more airflow, and more clean air delivered.
A well-built 20-inch CR box delivers an estimated smoke Clean Air Delivery Rate (CADR) of roughly 300 cubic feet per minute on high — comparable to a mid-size purifier that costs several times more. That is enough to give a typical bedroom or living room the 4 to 5 air changes per hour that experts recommend during heavy smoke.
What you need (about $70)
Everything is available at a hardware store or online. Expect to spend around 70 dollars total.
- One 20-inch box fan — a standard 20-inch box fan. Choose a UL-listed model (see safety below).
- Four 20x20x2-inch MERV-13 filters — a four-pack of MERV-13 furnace filters. MERV-13 captures fine smoke particles; anything lower lets too much PM2.5 through.
- A roll of packing tape or duct tape.
- A piece of cardboard (the filter box works) for the shroud, plus a marker and a box cutter.
Two-inch filters are the sweet spot: they move more air than one-inch filters and cost far less than four-inch ones.
Step-by-step build
The build takes about 15 minutes and needs no tools beyond a box cutter.
- Check the airflow arrows. Every filter has an arrow showing airflow direction. All four must point inward, toward the middle of the cube, because the fan pulls air in through the sides and blows it out the top.
- Tape the four filters into a cube. Stand them on edge and tape adjacent filters together at the seams to form four vertical walls. Tape every seam fully so air cannot leak around the media instead of through it.
- Seal the bottom. Set the cube on cardboard, trace the base, cut it out, and tape it on as a solid floor. Air should only enter through the four filters.
- Add the fan on top. Place the box fan on the open top so it blows upward and outward. Tape it down all the way around with no gaps.
- Cut a cardboard shroud. Cut a square of cardboard the size of the fan, cut a circle in it matching the spinning blades, and tape it over the fan face. This stops air from short-circuiting off the corners and can raise output by 20 to 50 percent.
Safety first
A CR box is safe when built and used sensibly, but it is a modified appliance. Follow these rules:
- Use a UL-listed (or ETL-listed) fan. Newer box fans include a thermal fuse that cuts power if the motor overheats — a genuine fire-safety feature. Avoid old fans that lack one.
- Do not run it unattended overnight or when you leave home. Some safety guidance recommends against leaving any modified fan running while you sleep or are away; run it while awake and present, and switch it off otherwise.
- Keep airflow clear. A clogged filter forces the motor to work harder. Do not cover the top or push it against furniture.
- Keep it on a hard, flat surface away from curtains and bedding.
When to replace the filters
Filters are the only consumable. In clean air a set can last several months, but heavy wildfire smoke loads them far faster.
- During a dense smoke event, expect a set to darken within a few weeks of heavy running; light everyday use may stretch a set to a few months.
- Replace when the filter faces look visibly gray or when airflow noticeably drops — a clogged filter quietly loses CADR.
- Vacuuming the intake side can buy a little time, but replacement is what restores performance. Keep a spare four-pack before smoke season, when filters sell out.
Want a store-bought unit instead? Compare options in our air purifier guide, and before heading out check Can I go outside?
FAQ
Is a CR box as good as a HEPA purifier?
A good 20-inch CR box delivers a smoke CADR around 300 cubic feet per minute, which rivals or beats many mid-size HEPA purifiers at a fraction of the cost. HEPA media captures a slightly higher share of the smallest particles, but the CR box makes up for it by moving far more air. For most rooms it clears smoke just as effectively.
Why MERV-13 and not a cheaper filter?
MERV-13 is the lowest rating that reliably captures fine PM2.5, the particle size that dominates wildfire smoke. Lower filters like MERV-8 are made for dust and let much of the smoke pass through. MERV-13 balances strong particle capture with airflow the box fan can still push.
Is it safe to leave a Corsi-Rosenthal box running all night?
Use a UL-listed fan with a thermal fuse, and follow the common guidance not to leave any modified fan running unattended while you sleep or are away from home. Run it while you are awake and present, and turn it off otherwise. Keep it on a hard surface away from bedding and curtains.
How long do the filters last?
It depends on how smoky the air is. In ordinary indoor air a set can last several months, but during a heavy wildfire event filters can gray out and lose airflow within a few weeks. Replace them when the faces look dirty or airflow noticeably drops, and keep a spare set on hand before smoke season.
Check the air where you are
Live AQI on the 2024 EPA standard, with the reporting monitor and observation time on every reading.
Where this guidance comes from
We paraphrase published guidance rather than inventing our own. The primary sources for everything on this page:
- AirNow — Air Quality and Health (U.S. EPA): the AQI scale and its activity guidance by level.
- EPA — Particulate Matter (PM) Basics: what PM2.5 is and why it reaches deep into the lungs.
- CDC — Wildfire Smoke Safety: who is at higher risk, and when to seek care.
- EPA — Wildfires and Indoor Air Quality: clean rooms, filtration and what actually works indoors.
- AHAM: the CADR standard and the two-thirds room-sizing rule.
Our own AQI arithmetic is documented on the methodology page.
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Some links are affiliate links — see disclosure. This guide is general health information and is not a substitute for medical advice — see disclaimer.