DIY Box Fan Air Filter: Cleaning Wildfire Smoke Out of Your House
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The sky went orange yesterday. The air smells like a campfire that got away from somebody, the AQI on your phone reads 187, and the wind is not shifting for four more days. Nobody has told you to evacuate. You are just going to be breathing this, and so is whoever in the house already has asthma.
That is the problem a box fan air filter solves, for roughly the price of a takeout dinner for two. Twenty minutes, no tools. The design is called a Corsi-Rosenthal box, and in published testing it moves more clean air than commercial HEPA purifiers costing several times as much.
One line before we start, because these two situations get confused constantly. Our guide to wildfire evacuation preparedness covers the day you leave — go-bags, trigger points, routes out. This article is the opposite scenario: you are staying, the house is fine, and the only thing wrong is the air.
What the Corsi-Rosenthal Box Actually Is
In August 2020, during a brutal West Coast fire season, environmental engineer Richard Corsi floated publicly that taping furnace filters to a box fan could deliver serious air cleaning for almost nothing. Jim Rosenthal of Tex-Air Filters built and tested it, developing the version that uses filters as the four walls of a cube with the fan on top. Corsi has said publicly that Rosenthal deserves the build credit.
The physics is simple. A box fan moves a lot of air but has almost no static pressure — push it through one restrictive filter and airflow collapses. Four filters’ worth of surface area drops resistance per square inch far enough that the fan keeps moving air near its free-flow rate, all of it filtered.
You need a 20-inch box fan, four 20x20x1 MERV-13 furnace filters, cardboard for the base and shroud, and a roll of heavy-duty duct tape. The 2022 UC Davis study of this design listed total materials at $58.43.
Why MERV-13 and Not the Cheap Filter
MERV rates how much a filter catches across three particle size ranges. A MERV-8 filter — the beige four-dollar one most people put in the furnace — is built for dust, lint, and pollen. In the 0.3 to 1.0 micron range it captures essentially nothing measurable.
Wildfire smoke is PM2.5, and most of that mass sits well below one micron — the size that gets past your upper airway into the deep lung. MERV-13 is the lowest rating specified to capture a meaningful fraction of that range, which is why EPA names it directly in its DIY air cleaner guidance. Below 13 you have built a box that hums. Above 13 is pointless too: higher ratings add resistance faster than capture, and a box fan lacks the pressure for them. For the simpler build, a single 4-inch-deep MERV-13 filter in 20x20x4 has enough pleat area to work respectably alone.
Build Instructions: Box Fan Air Filter in Twenty Minutes
Do it in this order and nothing needs redoing.
- Check the arrows. Every filter has an airflow direction arrow on the frame. All four must point inward, toward the center of the cube.
- Stand the four filters in a square, edges touching, open at top and bottom.
- Tape the four vertical corner seams, inside and out. Any gap is unfiltered air taking the easy route.
- Cut a cardboard square for the base off the filter footprint and tape it on. Seal it completely.
- Set the fan on top, blowing up and out — not down into the box. Tape it to the filter frames all the way around.
- Add the shroud. Cut a cardboard square the size of the fan face with a round hole in the middle roughly the diameter of the blade circle. Tape it over the fan’s output face so only the blade circle is open.
The shroud is the step everybody skips and the highest-return two minutes in the build. Without it, air recirculates backward through the dead corners outside the blade path — Rosenthal measured 462 feet per minute of reverse corner airflow without a shroud, dropping to zero with one. In EPA lab testing, the shroud raised a single-filter build’s smoke CADR from 111 to 156: about 40 percent more clean air for the cost of a cereal box.
Four filters or one? What the testing shows
EPA’s Office of Research and Development measured wildfire smoke CADR — cubic feet per minute of genuinely clean air delivered — across the common configurations.
| Configuration | Smoke CADR (cfm, EPA testing) |
|---|---|
| One 1-inch MERV-13 filter | 111 |
| One filter + cardboard shroud | 156 |
| One 4-inch MERV-13 filter + shroud | 248 |
| Two 1-inch filters + shroud | 263 |
| Corsi-Rosenthal cube, four 1-inch filters | 401 |
A separate 2022 study by Dal Porto, Kunz, Pistochini, Corsi and Cappa in Aerosol Science and Technology tested a five-panel cube and reported effective CADR rising from about 600 cfm on low to about 850 on high. Those figures run higher than EPA’s — methods and chambers differ between labs — so read both as evidence that the cube is in a different league than a single filter, not as one exact number. Use the one-filter build when you need air moving in five minutes; use the cube for a multi-day event or a room bigger than a small bedroom.
DIY Box Fan Filter vs. Commercial HEPA Purifier
The UC Davis team ran their cube against two commercial HEPA units on the same rig.
| Factor | Corsi-Rosenthal box | Commercial HEPA purifier |
|---|---|---|
| Materials cost | About $58 (2022 study) | Commonly $150–$500 |
| Measured CADR | ~400 cfm (EPA) to ~850 (UC Davis, high) | ~129 and ~285 cfm (two units, UC Davis) |
| Noise | Loud — EPA researchers flagged it as the biggest barrier to actual use | Quieter, usually has a sleep mode |
| Footprint | A 20-inch cube, two feet tall, looks like what it is | Compact, living-room friendly |
| Replacement filters | Generic furnace filters, sold everywhere | Proprietary cartridge, often $40–$80 |
The noise line is not a throwaway — a device you switch off because it is annoying delivers zero clean air. If somebody will not sleep next to a box fan on high, buy them a real HEPA room air purifier and put the cube in the common area. Medium speed costs some CADR and buys back a lot of quiet.
The Clean Room Strategy for a Multi-Day Wildfire Smoke Event
You are not cleaning the whole house. Do not try. EPA’s guidance is to build one cleaner air room and concede the rest.
Pick a bedroom, ideally with an attached bathroom. The bedroom wins because that is where the longest continuous block of breathing happens — eight hours, unmasked, at rest. Keep windows and doors closed, but never block anything in a way that makes the room hard to leave in an emergency. Run the filter continuously with the door shut. EPA sizes portable air cleaners at a tobacco-smoke CADR of at least two-thirds of the room’s square footage, so a 400-cfm cube covers 600 square feet.
The AQI driving these decisions comes from the EPA scale, updated in 2024. Current 24-hour PM2.5 breakpoints, per AirNow:
| Category | AQI | PM2.5 (µg/m³) | What it means for you |
|---|---|---|---|
| Good | 0–50 | 0.0–9.0 | Normal activity |
| Moderate | 51–100 | 9.1–35.4 | Unusually sensitive people ease up outdoors |
| Unhealthy for Sensitive Groups | 101–150 | 35.5–55.4 | Start the clean room; sensitive groups stay in |
| Unhealthy | 151–200 | 55.5–125.4 | Everyone limits outdoor exertion |
| Very Unhealthy | 201–300 | 125.5–225.4 | Stay indoors; mask for even brief trips out |
| Hazardous | 301+ | 225.5+ | Emergency conditions for everyone |
A cheap PM2.5 air quality monitor in the clean room earns its price. Outdoor AQI tells you what is coming in; only an indoor reading tells you whether the box is winning. If the number is not falling within an hour of closing the door, you have a leak, a loaded filter, or too much room.
What Else to Do While the Smoke Sits on You
If your HVAC has a fresh-air intake, close it and set the system to recirculate. Leave the blower running so it keeps pushing air through the furnace filter, and upgrade that filter to MERV-13 if the system tolerates it.
Then stop adding particles indoors. EPA’s don’t-do list during a smoke event: no vacuuming without a HEPA machine, no candles or incense, no aerosol sprays, no smoking or vaping, no frying or broiling, no gas, propane or wood-burning appliances. Damp-mop instead of sweeping. A gas stove is a combustion source inside the room you just sealed.
An N95 indoors is not normally needed in a working clean room. It makes sense when you cross the house or step outside, when indoor PM2.5 is still high, or when helping someone with a respiratory condition. Keep a box of NIOSH-approved N95 respirators on hand; cloth and surgical masks do not filter PM2.5 meaningfully.
Smoke events also bring power problems, since utilities cut lines during high fire danger. Sort your backup communications plan first, and know where the first aid kit is without turning on a light.
Filters load fast — replace them
In normal use a MERV-13 filter lasts months. In heavy smoke it can be finished in days, and EPA is blunt: a dirty filter makes the cleaner “almost completely ineffective.” Check yours daily and swap it when the intake face goes gray or the box smells like smoke instead of removing it. Buy the replacement set with the first — shelves empty once smoke arrives. Ignoring consumables is one of the most common preparedness mistakes people make: gear bought once and never fed.
Fan Safety and the Limits of a Box Fan Air Filter
The fire question, stated honestly
The fire concern is real but narrower than the internet makes it. EPA’s guidance is to use a fan built in 2012 or later carrying a UL or ETL mark — those meet current UL 507 requirements for electric fans, which brought in fused plugs and thermally protected motors. On older units EPA is explicit: if you must use a fan built before 2012, do not leave it unattended and do not use it while sleeping.
For newer certified fans the tested evidence is reassuring: EPA’s research with UL’s Chemical Insights Research Institute found DIY air cleaners safe to run even with both sides of the fan blocked, with no dangerous motor heating. EPA does not tell you to shut a modern fan off overnight. Still — buy a new fan rather than pulling one out of the garage rafters, skip the extension cord, keep it off carpet where it can suffocate its own intake, and confirm your smoke alarms work before running it overnight. If the motor smells hot or the fan slows under load, unplug it.
What it does not do
Say this out loud so nobody in the house gets it wrong. A box fan air filter removes particles. It does not remove carbon monoxide, and it does not remove most gases.
CO is a colorless, odorless molecule far too small for any particle filter — HEPA included — to touch. Smoke contains it, and the generator someone rolls out during a fire-driven power shutoff is a much bigger CO source than the fire itself. A running Corsi-Rosenthal box in a room with a CO problem does nothing but make you feel protected. You need a working CO alarm, and generators always stay outside, well away from windows and doors. Headache, dizziness, nausea or confusion in more than one person at once means get everybody outside and call 911 — do not investigate first.
The same limit applies to the acrolein, formaldehyde and other VOCs in smoke. Activated carbon helps somewhat with odors; particle filters do not. If the house smells strongly of smoke despite a good indoor PM2.5 reading, that is the gas fraction, and the honest fix is leaving — which makes it a family evacuation plan problem.
What to Do This Week
- Buy a 20-inch box fan with a UL or ETL mark, dated 2012 or newer, plus eight 20x20x1 MERV-13 filters — four to build with, four in reserve.
- Build the cube tonight. Twenty minutes, no tools. Cut the shroud; do not skip it.
- Pick the clean room now. Bedroom, door closed, attached bath if you have one.
- Find out whether your HVAC has a fresh-air intake and how to close it.
- Test the CO alarm and smoke alarms; replace any unit past its expiration date.
- Store the PM2.5 monitor, N95s and spare filters in that room’s closet so it is one grab.
