How to build a Corsi-Rosenthal box for wildfire smoke

scienceJul 16, 2026861,996

The Corsi-Rosenthal box is a low-cost DIY air cleaner built from MERV13 furnace filters and a 20-inch box fan to reduce indoor wildfire smoke, virus-laden aerosols, pollen, and dust. Typical builds use four 20x20x1 MERV13 filters arranged as a cube with a 20-inch box fan on top and sealed with duct tape; builders can use 2-inch-deep filters (three 20x20x2 plus two 16x20x2) for greater surface area, quieter operation, and longer filter life. Optional fan shrouds or covered fan corners close gaps and improve efficiency; tests found a 13.5 to 14.25 inch shroud opening works best with a Utilitech fan and a 15 inch opening suits Lasko fans. Sources recommend avoiding some low-performing brands (Filterbuy MERV-13 has not tested well) and note Filtrete 1900 as a commonly available effective choice. Researchers including Marwa Zaatari rate the CR box among the most cost-effective portable air cleaners, and experts including William Bahnfleth say well-constructed DIY MERV13 air cleaners align with ASHRAE guidance for improving clean air delivery. The Corsi-Rosenthal Foundation and Clean Air Crew publish step-by-step instructions, photos, and FAQs to help people build, modify, and distribute these boxes where wildfire smoke is degrading indoor air quality.

Chris Person

@papapishu.bsky.social

Reupping this and the reminder that just a furnace filter on a standard Corsi-Rosenthal box is very capable if you can’t find get an air purifier for your house. aftermath.site/3d-printing-...

69445d ago

Chris Person53

Also, I know people I trust that swear by the Airfanta 3Pro. Collapses down to fit when you don’t need it, super energy efficient.

Hobo Joe5

Seconding cleanairkit for those who don't have a printer or just want something quick and cheap! It gets the job done very well. www.cleanairkits.com

Matatoes Lasagna1

Recently got a K2+ with CFS and the CFS just stopped turning on. They sent me a replacement motherboard and I had to spend 3 hours taking apart the thing to swap it out only for it still fail. Sighhh... probably should have just got the bambu.. This has nothing to do with filters I'm just upset

Joshua Dysart1

This is how they monetize air, by making it unbreathable without the need for their products.

Quoting this

Matt Novak55

This is what my friend did and it brought the AQI inside her house from 250 down to 19.

lydia3

i have built one of these for california wildfire season and it’s shockingly effective. enormously powerful tool that breaks down and stores flat.

National Weather Service

@nws.noaa.gov

Wildfire smoke is impacting air quality across much of the Great Lakes region into the Northeast U.S. Poor air quality can be hazardous to anyone. Before spending time outdoors, check the air quality forecast to ensure that you aren’t doing yourself more harm than good. airnow.gov

Air Quality and Health: Exposure to air pollutants such as particulate matter and ground-level ozone can cause headaches, irritated eyes and sinuses, fatigue, difficulty breathing, chest pains, asthma attacks, irritated throat and increased coughing. Poor air quality can be hazardous to anyone, and it can aggravate health problems such as asmtha, heart disease, and lung disease. Seniors, children and those with compromised immune systems are especially at risk.

11545d ago

Gracu ✨🌈

@prismagic.bsky.social

Hey for people in the northeast who are getting hit by wildfire smoke like me. Wear a mask. N95 or better. @flomask.bsky.social is the GOAT. This smoke is no joke and TERRIBLE for your lungs.

6545d ago

Mira (remarkably woman shaped)

@miranii.bsky.social

genuinely be careful with this wildfire smoke if you live in the affected areas I biked to a restaurant to get lunch today and even though I am back and relaxed in my room I am coughing and still having somewhat of a hard time breathing GDJAGS

9945d ago

Mira (remarkably woman shaped)44

granted I live in one of the spots that's really bad, but still, not to be too preachy but its fucking insane to me that it will only get worse every summer from now on and people will still deny a changing climate

💜evil devilgirl activities💜2

i looked outside and its all just Yellow. rly worrying but i feel bad abt the ppl outside who still have to do construction. genuinely dangerous

Nessa2

if able to folks should ideally stay inside and keep windows and such shut but... not always possible xwx fires this big are scary though, the sheer scope makes it so hard to just sit it out from safety with a lot of folks still forced to work in this condition aaaaaa

Shawa1

dude yesterday it was orange, i thought i went color blind when i woke up

It's Comms OPEN Being a ⋏⟟⋔⏚⌰⟒1

Hope you feel better soon

Ralf W

@raflw.bsky.social

For anyone in the huge smoke plume (MN to the East Coast), I'm intrigued by this DIY Corsi Rosenthal "wedge". I'd seen the CR boxes since the SF Bay area fires, but this seems more space efficient, uses the fan in the expected vertical orientation, and needs 2 v. 4 of the moderately spendy MERV 13s.

Simple pictogram illustration of how to assemble two Merc 13 filters, some cardboard and duct tape into a DIY home filtration system.

4345d ago

Dr. Elizabeth Sawin

@bethsawin.bsky.social

CO2 and Covid-19 virus: We tried to show them that the planet is a unitary whole, with no boundaries, connected by ever-moving invisible gases and currents of air and that they thrive together or not at all, but 🤷‍♀️ Wildfire smoke: Allow me.

5945d ago

Avery (they/them) 🔞 DISABILITY PRIDE ♿ | 🇵🇸 🇸🇩 🇨🇩

@averystorm.bsky.social

🔥 ⚠️ 🔥 ⚠️ 🔥 for folks dealing with wildfire (or other) air quality issues!

screenshot of FB post from Jodi LaCroix

Wildfire People!!
Friendly reminder that four MERV-13 furnace filters taped into a square with a box fan on top is a legitimate air purifier. It's called a Corsi-Rosenthal box. It moves more air per hour than a $300
consumer purifier and costs about $50 to build. The filters are $8-12 each at any hardware store and the fan is $20. When the filters get dark you tape on new ones.

4245d ago

Dr. Amy Acton

@amyactonoh.bsky.social

As a doctor, I'm encouraging Ohioans to pay attention to local air quality updates and take simple precautions if you're in an affected area—especially children, older adults, and anyone with asthma or other lung or heart conditions. www.cleveland.com/weather/2026...

5745d ago

Molotov Cats18

You deserve to lose your license, bigots don’t belong in medicine. Or in politics.

Dr. Amy Acton18

I encourage everyone to follow guidance from local officials and take the recommended precautions to protect your health.

CJ Moose 🫎18

“As a doctor” it is morally reprehensible for you to throw trans Ohioans to the right-wing, fear-mongering wolves. You’re either a coward or a transphobic bigot, and therefore unworthy of being elected into a position of progressive leadership. With deepest sincerity, go fuck yourself.

Chris Holly16

Your response about trans athletes in student sports was disgusting. I'm really ashamed that you are throwing trans kids under the bus. They need protection and to be safe and healthy as well. I'm an Ohio Democratic voter who can't believe you're no better than a republican on this issue.

The Dax Symbiont 🏴🏳️‍⚧️🌲🇵🇸11

As a trans person I’m encouraging all Amy Actons to go outside and hyperventilate til your bigoted lungs can’t say anything else bigoted

猫くノ一9

Always have been a transphobic bigot. When you were in charge of health, you denied my trans 🏳️‍⚧️ siblings to update their birth certificates. Vile! お前はトランスフォビア偏見者

MPR News

@mprnews.org

Dr. Dylan Wyatt, chair and medical director of the emergency department at Aspirus St. Luke’s Hospital in Duluth, said wildfire smoke is a different kind of danger. Those who have a heart condition, COPD or asthma are especially at risk.

4245d ago

⭐️ Fio the Wah! ⭐️ 🔜 MFM

@fiofirefox.com

This may help save lives in areas affected by the smoke from wildfires: If you can, make a Corsi-Rosenthal box. It is made with four MERV13 20” furnace filters and a 20” box fan. The assembly is done with duck tape. Also look for N95 masks! Stay safe out there! en.wikipedia.org/wiki/Corsi%E...

2645d ago

ιм єνιℓ иσω :3

@vreni.bsky.social

New York I know it’s hot as fuck out there but you gotta be wearing a mask outside today! Wildfire smoke will make you sick!

4645d ago

The New York Times

@nytimes.com

If wildfire smoke is polluting the air where you live, you might want to consider wearing a mask outside to protect your health. Here’s what experts want you to know.

3045d ago

Wes Allen2

Why say “might want to consider” instead of “should consider”? You can still consider it!

Karolina Muchova Fan Club1

An AQI of 500 where I live and lettuce is going to give me explosive diarrhea. We all died during covid and this is hell

Marcus

@marcusjmerritt.com

For real, though, N95 masks protect you pretty well from wildfire smoke if you have to breathe the air. Highly recommend

3645d ago

Aerinea2

California companies were subjected to conspiracy theories because they had N95s in reserve before Covid. It was because wildfire season is a yearly thing here

S. Kramer1

My father is just going outside in it!

Umm McShruggy1

It surprises me (maybe it shouldn't) how many people aren't wearing masks at all.

MaxFunHQ

@maximumfun.org

If you're one of the unlucky folks currently breathing the air my dad described this morning as "hot, thick, and tan," a C-R box could help. It's (relatively) cheap and easy and the materials should all be at your local hardware store. Made one during the SoCal fires last year and it helped a ton.

2645d ago

Goth Capo

@tinalope.bsky.social

Hey east coast friends. If the smoke is getting inside, you can build a corsi-rosenthal box for fairly cheap to help filter You can find other options here: corsirosenthalfoundation.org/instructions/ Love, A west coast seasonal wildfire veteran

Instructions to build a corsi-rosenthal box
More instructions on https://corsirosenthalfoundation.org/instructions/

2045d ago

3 sources

corsirosenthalfoundation.orgHow to build your own Corsi-Rosenthal Box - Corsi-Rosenthal Foundation engineering.ucdavis.eduScience in Action: How to Build a Corsi-Rosenthal Box cleanaircrew.orgDIY box fan filters – Corsi-Rosenthal box - Clean Air Crew

How to build a Corsi-Rosenthal box for wildfire smokescience National Academies back extreme‑event attribution sciencescience

How to build a Corsi-Rosenthal box for wildfire smoke

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