Three Cheers for the Makers of Corsi-Rosenthal Boxes!

By Lambert Strether of Corrente.

I imagine readers know what this is:

It’s a Corsi-Rosenthal (CR) box — a DIY box fan filtration system that removes particles, including Covid-bearing aerosols, from indoor air, so you and yours don’t breathe in SARS-CoV-2 virions and, if the dice fall the wrong way, end up with pink mush for lungs or brain fog. Cleaning indoor air is important!

Naked Capitalism readers have known about “Corsi Boxes” since April 2021 at the latest:

Of course, we’ve written on “Corsi-Rosenthal” boxes — more on the name change below — also:

If you only read the CDC (“the national public health agency of the United States“), you wouldn’t know about Corsi-Rosenthal boxes under either name:


So, I’ve gotta ask: Who’s really working for the public health, here?

* * *

In this extremely simple, Twitter-aggregating post, I will give a little bit of the histor and cheer on the many CR box makers. I will give a number of examples of their work, allude to the difficulties of producing CR boxes internationally, and remark on mutualism. Since I assume most of you know how to make Corsi boxes, I’ll confine directions to an Appendix.

First on the history and the name change. From (sorry) Wikipedia:

In August 2020, Richard Corsi, an environmental engineer and the incoming Dean of Engineering at the University of California, Davis (UC Davis), spoke with Wired reporter Adam Roger about an idea he had for combining multiple store-bought filters with a box fan to improve the efficiency of home-made air filter designs. Rogers contacted Jim Rosenthal, the CEO of filter manufacturer Tex-Air Filters, who had collaborated with Corsi at the University of Texas and in the Texas chapter of the Asthma and Allergy Foundation of America, to run some tests on a single air filter attached to a box fan. Inspired by Corsi’s idea to use multiple filters, Rosenthal later came up with a 5-filter design. Rosenthal named it after Corsi, although after a New York Times article mentioned the boxes by that name, Corsi tweeted that Rosenthal really deserved the credit, and that he preferred the name Corsi–Rosenthal Box.

And just this week, a laudatory explainer and interview with Jim Rosenthal in the Fort-Worth Star Telegram:

Aerosol experts have increasingly called for more attention on air quality as a way to reduce the number and density of tiny particles of coronavirus floating through indoor air in schools, offices and hospitals. And among the numerous expensive, resource-heavy interventions, an increasingly popular option is a tool that can be built with a box fan, some tape, and a few high-quality air filters for a total cost of about $65.

The Corsi-Rosenthal box, also called the Corsi-Rosenthal cube, takes its name from Jim Rosenthal, a Fort Worth resident and CEO of Tex-Air Filters, which Rosenthal founded in Fort Worth in 1997.As this stage in the pandemic, Rosenthal identifies his main role in the development of the Corsi-Rosenthal box as a cheerleader. He gives feedback on Twitter to people making their own boxes at home, answering questions and encouraging those who are making the boxes. And he’s been following the research happening at universities through the country, including at the school of nursing at the University of Connecticut and at the University of California, Davis. As experts have studied interventions like the Corsi-Rosenthal box and other high-quality air filters, consensus has grown that reducing the amount of virus that can spread via the air is essential in limiting new infections.

“We really should have the National Guard making these by the truckload,” said Jose-Luis Jimenez, an atmospheric chemist at the University of Colorado.

Yes, it’s certainly odd that’s not happening. More:

Jimenez explained that when a person breathes, speaks and shouts, they emit small amounts of spit. Some are larger, and are known as droplets, which come out of someone’s month before quickly falling. The smaller amounts, largely invisible to the human eye, can float in the air, lingering in enclosed spaces for minutes and sometimes hours.

A good way to think about aerosols, Jimenez said, is to think about how the scent of cigarette smoke will linger in an enclosed space long after someone has put out their cigarette.

The same thing can happen with aerosols containing the coronavirus, which is why maintaining a distance of six feet alone is not enough to provide protection in enclosed, crowded spaces with poor ventilation, like schools and office buildings.

Rosenthal and Corsi’s intervention relies on the air filtration provided by MERV 13 filters, which researchers have repeatedly found is capable of trapping small particles that contain the coronavirus or other viruses.

Next, sadly there is no map aggregating all the Corsi-Rosenthal Box (CR Box) projects across the country. Here at least is one article on CR Boxes from in schools from the School Library Journal (it would be the librarians):

Corsi-Rosenthal Boxes are effective, inexpensive, safe, and, by all accounts, easy to build. The DIY air cleaners were created by Richard Corsi, dean of engineering at the University of California (UC) Davis and Jim Rosenthal, CEO, Tex-Air Filters. The open-source design requires one box fan, four MERV 13 filters, and duct tape. Cardboard can help improve efficiency by acting as a fan shroud.

A study showed the boxes effectively reduce aerosols in indoor spaces. The aerosol transmission of the SARS-CoV-2, the virus that causes COVID-19, plus the increased transmissibility of the Omicron variant has put a spotlight on the need for proper ventilation and cleaning the air indoors as a key element to cutting down infections.

Across the country, concern about children getting COVID launched building projects and volunteer efforts that put them in classrooms from colleges to K–12.

At Brown University and UC San Diego, students built boxes for their classrooms and labs. Brown is also conducting a study on the effectiveness of the units.

DC Public Schools librarian K.C. Boyd has three boxes donated by Peter Krupa, a parent in the district who started a GoFundMe campaign to raise money for the supplies. In less than a month, he was funded to build and deliver approximately 50 boxes.

By mid-January, a GoFundMe to build Corsi-Rosenthal Boxes for Winston-Salem/Forsyth County (NC) Schools district funded 66 units, put together by 40 volunteers.Elsewhere, college faculty led the way and produced the boxes to bring better ventilation to local public schools.

A project by Arizona State University (ASU) placed 37 units as of mid-January with another 86 scheduled to be built….

Meet “Gus”:

Why in the name of all that is holy isn’t Gus seen as a photo op? Why isn’t Jill Biden on Good Morning America helping some cute kids put together a CR Box with masking tape? A question that answers itself, once asked.

Seems like others are taking steps to make the photo op happen:

Snappy red outfit! Here’s a Scout project not just on making the boxes, but documenting how to make them:

“Hero fac members”:

“Making classrooms safer”:

Theatre majors:

Dance school:

Engineering school:

Party in the Grange Hall:

From Montreal, PQ:

From the Yukon:

From an allergy sufferer:

Sidebar: This is the allergy sufferer’s cat:

End Sidebar.

Internationally, CR boxes may be harder to make. Box fans aren’t always available, the closer to the equator you get the less likely it is there will be furnace filters, and cheap Chinese HEPA filters compete.

Here in the Phillipines, a fan that is not a box fan. But note the qualification on the filter:

Difficulties in Australia:

Success in New Zealand:

* * *

Here endeth the litany on CR Boxes and fun. I find the CR box movement — if I may call it that — hopeful. The movement is marked and informed by what I’ve started to call mutualism (in opposition to the hegemonic libertarianism; “solidarityism” was not euphonious. Readers, if you have a better term, please suggest in comments). Corsi insisting that “Corsi-Rosenthal” was the proper name for the box is a fine example of mutual care. (Indeed, the aerosol thought collective generally is marked by the sort of give and take that I had previously only encountered with the scholars who developed MMT.) Many if not most of the CR boxes not made for families are given away. Many are constructed in group settings: Classrooms, dance schools, engineering schools, the grange. Makers share information on how best to build and place the boxes. CR Box makers aren’t whinging about “freedom.” They’re going out and delivering it, both for themselves and — breathing being a social relation — for society. Perhaps, as a country, we have more social capital than we thought.

Readers, have you spotted CR boxes in the wild? Have you made one yourself?


The Clean Air Crew site has a lot of good data on CR Boxes. From that site, here is a Twitter thread to guide construction:

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About Lambert Strether

Readers, I have had a correspondent characterize my views as realistic cynical. Let me briefly explain them. I believe in universal programs that provide concrete material benefits, especially to the working class. Medicare for All is the prime example, but tuition-free college and a Post Office Bank also fall under this heading. So do a Jobs Guarantee and a Debt Jubilee. Clearly, neither liberal Democrats nor conservative Republicans can deliver on such programs, because the two are different flavors of neoliberalism (“Because markets”). I don’t much care about the “ism” that delivers the benefits, although whichever one does have to put common humanity first, as opposed to markets. Could be a second FDR saving capitalism, democratic socialism leashing and collaring it, or communism razing it. I don’t much care, as long as the benefits are delivered. To me, the key issue — and this is why Medicare for All is always first with me — is the tens of thousands of excess “deaths from despair,” as described by the Case-Deaton study, and other recent studies. That enormous body count makes Medicare for All, at the very least, a moral and strategic imperative. And that level of suffering and organic damage makes the concerns of identity politics — even the worthy fight to help the refugees Bush, Obama, and Clinton’s wars created — bright shiny objects by comparison. Hence my frustration with the news flow — currently in my view the swirling intersection of two, separate Shock Doctrine campaigns, one by the Administration, and the other by out-of-power liberals and their allies in the State and in the press — a news flow that constantly forces me to focus on matters that I regard as of secondary importance to the excess deaths. What kind of political economy is it that halts or even reverses the increases in life expectancy that civilized societies have achieved? I am also very hopeful that the continuing destruction of both party establishments will open the space for voices supporting programs similar to those I have listed; let’s call such voices “the left.” Volatility creates opportunity, especially if the Democrat establishment, which puts markets first and opposes all such programs, isn’t allowed to get back into the saddle. Eyes on the prize! I love the tactical level, and secretly love even the horse race, since I’ve been blogging about it daily for fourteen years, but everything I write has this perspective at the back of it.


  1. none

    HEPA air filters also work, and are usually quieter and smaller. The CR box is great for doing a big space (like a classroom) on a low budget. For e.g. a small bedroom, a lower volume machine suffices. Also, as far as I can tell, for a DIY model it’s not really important to seal things up, since air passes through the unit multiple times. So any that escapes filtration will get filtered on the next pass.

    This is a really good page about classroom ventilation in general, that also has a link at the bottom about how to make CR boxes:

  2. Geoff

    Interesting slant on our national mindset. Just as in the rush to get over COVID we mostly forget the other line of defence-air filtration, we rush to print money mostly forgetting to balance that out with taxes. I am sure there are other examples that make you wonder.

  3. Jason Boxman

    It does make you wonder, what purpose all the layers of government in the United States serve, if any.

    1. XXYY

      I was going to post about this. I made a similar one with 12″ filters that I’m putting into my office if and when we return to work. These would also be great for bedrooms and other small, self-contained spaces that don’t need (or have room for) a 20″ version.

      Note that I believe the preferred arrangement is away from walls with the fan facing up, so the filtered air hits the ceiling and spreads out through the room fairly evenly. A lot of the photos shown here have the fan on the side (maybe because of a tip-over switch?). I don’t think this is a deal-breaker, but if you have the flexibility that’s what to shoot for.

      Also note that the goal is six changes of air per hour in the room. Or more. If you have an anemometer or something to measure airspeed, and a tape measure to calculate the diameter of the fan and the volume of the room, the math is actually not too difficult.

      Lambert deserves a huge award for spreading the word on this simple yet life-saving technology.

  4. Even keel

    I put one in my individual office. I get some comments, but not too many. Some people think it is strange, some cool. Not sure what it does for my reputation. I know for sure that the air in my own office is fresher than the rest of the space.

    The larger office is more high end, so not sure how to get it into common spaces. Some of these ones look very nice (I like the red fan and red tape particularly).

    Nobody I’ve met yet has ever heard of these by name.

  5. Bill Carson

    I made something like this nearly two years ago by taping one air filter to a box fan. I thought about ways to make bigger boxes by adding more filters, but I never took it that far.

  6. Steve

    I built one for our living room. Our daughter lives with us and has a job at a hotel–lots of exposure there. I did not see the bit about the cardboard on the bottom, this is important to keep the corners from falling apart.

    Home Depot, in their infinite wisdom, does not use the term MERV 13 on their filters; in HD terminology, you want FPR 10 filters.

    1. Copeland

      Cardboard on the bottom? I didn’t see that. And this is the first time I’ve seen the upgraded version with the cardboard shroud on the outlet side with a circle cut out. How big should that circle be in relation to the fan blade diameter? I didn’t see any specific guide for this in the post or links.

      1. John Zelnicker

        From all of the pictures I have seen it appears that the shroud opening is just a bit larger than the fan blade diameter.

      2. Copeland

        Oh yes I see now, cardboard on the bottom, way better than just open to the floor.

        But what fan speed is suggested?

        1. Steve

          As “none” commented above, the air is repeatedly filtered. I agree with them that a shroud isn’t needed; we keep ours on low speed so it’s quiet. If you have a larger room, maybe a shroud and higher fan speed will be of benefit.

      3. Copeland

        Looks like they have tested for optimal opening size for various fan brands. For my Lasko 20″ they are suggesting a 15″ dia. opening.

      4. steve

        Fan shroud opening should equal fan blade diameter and if not obvious, centered. Closer the better but not critical. Fan shrouds increase a fans efficiency by eliminating pull back at the corners of a typical box fan, where the air forced out is immediately pulled back through the fan resulting in significant decreased flow through the filters. Fan shrouds also greatly reduces air turbulence which improves perceived flow.

        It’s also a good idea to use a cheap washable foam air filter media on the intake side (outside) of the HEPA filters to help prevent fouling and prolong usability. Clean as needed.

    2. steve

      FPR 10 rated filters are not equivalent to HEPA or MERV 13 filters. Despite what HD might compare them to, particle filtration size is not certified. It takes a close reading of their claims to catch all the weasely words.

      1. XXYY

        HEPA filters are MERV 17 or higher.

        MERV 13 is equivalent to an N95 mask.

        I have seen the argument that MERV 13 is best in this application because you can move more air through it per unit time and so achieve more air changes per hour (and also because it’s cheaper). MERV 17 is harder to get air through and the extra 5% filtration is not important since you’ll be running the room air through the box multiple times anyway.

  7. grayslady

    I didn’t build the box, because it wasn’t suggested 18 months ago, but I did construct a hepa filter on the back of a box fan and it works beautifully.

  8. anon y'mouse

    not a thought about the box, but a friend of mine with severe allergies long ago took replacement air filters and installed them on all of her open windows during the correct season and stopped sneezing (and dusting) as often.

  9. ScottB

    Sadly, a year + ago as a school board member I found out about these too late, after we ordered thousands of commercial filters at something like $1,500 a pop.

  10. Gavin

    Being lucky enough to have allergies to dust mites and cat dander, I get to carry a HEPA filter anywhere I want to fall asleep — even hotel rooms stuff up the nose in moments. And I’m blessed with a wife who is happy to only carpet her office. I’ve had good luck with the Coway AP-1512HH and Kenmore 4283234 – I’ll never know for sure, but my lack of covid [so far] is likely somehow correlated to those running 24/7 in my office and bedroom.
    And they’re pretty durable – my Kenmore is over 12 years old. Refills are cheapest from a site called “filtersnow” for the Kenmore; for the Coway, there’s zero non-manufacturer functional refills. If you’re running it flat-out all day long, replacement is about 5 months for the HEPA and 2 weeks for the “pre”filter in the Kenmore; the Coway is every 5 months or so for the prefilter and maybe 9 for the HEPA because it has a 3rd filter you can clean manually to capture the visible dustbunnies. Also the Coway is really quiet.

    Just FYI: A “UV purifier” emits ozone. Ozone even at small levels can cause chest pain, shortness of breath, coughing, throat irritation. If UV light was going to accomplish the purifying of a microbe, the microbe would have to be exposed to the light for minutes-to-hours — not the seconds each molecule of air is made available in a typical purifier.

    1. steve

      Not all “UV purifier” units emit ozone. Cheap Amazon/ebay/bangood ones probably do because they use 184 nm wavelength lamps and not UVC 254 nm wavelength ones which produce near zero O3.

      Concerning UV sterilization of air, it is very effective and used extensively. Your concern about exposure times is misplaced.

  11. erichwwk

    I have built several CR boxes. In testing airflow, I discovered the fan is not powerful enough to pull air through much of the filter, ignoring the bottom half entirely, and pulling considerable air in around the edges of the fan itself. I’m interested to know what others have learned, and how they test for air flow. My technique is simple, narrow strips of long facial tissue clearly indicates air flow. Is the box of four filters “really” better at filtering than a single 20×20 filter? Has anyone found more powerful 20X20″ box fans than can force air through the filter rather than enter through the top, at the outside edges of the fan itself ?

    1. XXYY

      I think having more filters means the filter elements last longer, as well as reducing the pressure differential needed to get a given volume of air through.

      You can get an anemometer for about 30 bucks and use that to sort of probe how and where the air is moving (see “handheld anemometer on Amazon, e. g.) It’s also lets you do calculations about how often your room air is being changed.

  12. The Rev Kev

    There is almost a MacGyver element to these things. I can just imagine where they got their inspiration from- (4:05 mins)

    It is a shame but I think that governments have become too sclerotic to build and deploy these things even though they are elegant in design. If they tried, they would cost $5,000 a box, be six months late, be under-engineered, have blue tooth and wi-fi connectivity and you would be trying to control these boxes using an app on your mobile which would have problems “talking” to it. Colour me cynical.

  13. Louis Fyne

    amazing, in a bad way, that the Corsi box mentioned only here. not my local newspaper, NPR, local TV, national US media, European media, (but indoor air filtering machines were already widespread in East Asia).

  14. dbk

    Re: “mutualism”: I think it sounds fine and is a good descriptor of what people are doing. By coincidence, I listened to the BBC In Our Time’s recent episode on Peter Kropotkin yesterday, where there was a discussion of his book The Conquest of Bread. Kropotkin came up with the term “mutual aid” to describe this behavior (sometimes equated with empathy), which he had witnessed many times in the animal kingdom.

  15. Roger Blakely

    The screenshot of the CDC Web Site offering nothing about filter boxes was worthwhile. American public health officials are simply not up to the task.

  16. Stan

    Order a 20″ box fan from WM -$25 – 4 merv 13 filters @ ~$12ea = ~$50
    Roll of duct tape = $5

    I have a single filter setup and a 4 filter setup – both seem to function based on the amount of dust they collect.

    Also install a merv 13 furnace filter and run the fan constantly.

    1. XXYY

      Obligatory warning that your furnace setup may not be designed to run MERV 13 filters (this should be an immediate change in all building codes). I always see people warning other people about this, but I don’t know how serious the problem is. In theory, your furnace fan may overheat or be under too much strain.

      If it’s working for you without problems, that’s probably the best and most reliable evidence.

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