“Look for the Helpers”: Corsi-Rosenthal Box Round-Up

By Lambert Strether of Corrente.

I rarely write a feel-good story; this will be one. As readers know, our ruling class is very strong on keeping their air clean so they don’t catch Covid. Back in 2022, we wrote “How Ashish Jha and Rochelle Walensky of Newton, MA Protect Their Children from Covid (But not Yours)” on ventilation in the schools; and recently, at Davos, our ruling class showed, by their actions, that they know #COVIDisAirborne, and will protect themselves from it (but not you):

#DavosStandard is trending on Twitter right now. What’s this all about? Well, it refers to the high safety standards regarding COVID-19 that are currently being implemented at the 2023 Annual Meeting of the World Economic Forum (WEF) in Davos, Switzerland. What is the WEF doing to help prevent the spread of COVID-19 and to keep its attendees safe? Safety measures include free PCR testing upon arrival, free rapid tests throughout the length of the conference, HEPA air purifiers and good ventilation throughout the meeting spaces, and free masks….

Folks who have long been advocating for similar safety measures for public spaces, schools, workplaces, and more are taking to Twitter to praise the measures in effect at the WEF, and to spread the news that we should all have access to safe places to work, gather, learn, and more.

But what about the rest of us? For us, there’s the “Corsi-Rosenthal” box (named for its inventors, Richard Corsi and Jim Rosenthal). For those who came in late, here’s an image of the Corsi-Rosenthal Box (CR) box, along with its requirements:

As we shall see, these requirements were than fulfilled. The Washington Post gives a good account — unpaywalled, kudos — of the reasoning behind the CR Box and how to build one:

The box uses four common household air filters for the sides — the kind you use for a home HVAC system — a 20-inch fan on top, cardboard, scissors, and duct tape to hold it all together.

Lots of duct tape.

It’s important to get the right kind of filters. The filters need to have a MERV-13 rating, which refers to the filter’s ability to trap particles of a specific size.

When the fan is turned on, air is pulled through the four sides of the box. The filters trap contaminated particles, allowing clean air to flow into the middle of the box and be pushed back out into the environment through the fan. The fan just needs to be plugged into a normal electrical outlet. Not only can the boxes reduce the spread of pathogens such as the coronavirus, they also can reduce other particles, such as those generated by wildfires, as well as dust and pollen.

The supplies to make one filter box cost me $127.

P.S. Making pie crust is harder.[1]

$127 isn’t too bad, though Davos Man is probably trying to figure out how to rent you a CR Box for a lot more money.

Importantly — though oddly, this seems to have made no impact on our public health establishment, already fully committed to the “Ultimate Lockdown” of mass infection without mitigation, vax by injection, and hospitalization and death as the only metrics that matter — the studies on CR Boxes v. Covid are very good. It’s not Godzilla v. Mothra, it’s Godzilla v. Mickey, which is news you can feel good about (at long last).

From Building and Environment, a collaboration of NIOSH and OSHA scientists, “Efficacy of Do-It-Yourself air filtration units in reducing exposure to simulated respiratory aerosols“:

Using two DIY cubes (in the front and back of the room) increased the air change rate as much as 12.4 over room ventilation, depending on filter thickness and fan airflow. Using multiple linear regression, each unit increase of air change reduced exposure by 10%. Increasing the number of filters, filter thickness, and fan airflow significantly enhanced the air change rate, which resulted in exposure reductions of up to 73%. Our results show DIY air filtration units can be an effective means of reducing aerosol exposure.

And from Science of The Total Environment, “Can 10× cheaper, lower-efficiency particulate air filters and box fans complement High-Efficiency Particulate Air (HEPA) purifiers to help control the COVID-19 pandemic?“, and I’m throwing a flag on the Betteridge’s Law violation here:

As we discovered in the test results, lower-efficiency air filtration by combining off-the-shelf components (box fans with heating, ventilation and cooling or HVAC filters) in tested DIY configurations compares favorably in performance (clean air delivery rate, noise) to the tested HEPA air purifiers but at approximately five to ten times lower cost, and can be an affordable, complementary option for rapid aerosol removal indoors in homes, clinics, schools, offices, and other public venues.

Davos Man would have done better to have his people bring some duct tape!

Now let me turn to some case studies (well, tweets) that show how the CR Box has gradually become a thing (which would be why WaPo, three years into the pandemic, decided to cover the story).

(1) American kawaii:

This is only on example of many; people seem to have an irresistable urge to decorate their CR Boxes.

(2) A fun project:

Maybe now the WaPo has given the OK, Doctor Jill Biden will go on Good Morning America and build a CR Box with some cute kids?

(3) A STEM project:

Heavy emphasis on schools here, and why not, school ventilation being what it is, and Covid transmission in schools being what it is.

(4) An assembly line:

This one is in the Philippines, but I’ve seen the same idea in America, generally at the college level (or for institutions like churches).

(5) Why not more?

Interestingly, CR Boxes tend to appear in private homes or public spaces like schools or churches. They tend not to appear in public spaces that are businesses, especially restaurants (who could save their business and not infect their staff, a twofer). I think that’s unfortunate, and I wish I knew why. Liability? Decibels? Maintenance? Ignoramce? Hatred of mitigation by management?

A bonus: From Environmental Science and Technology, “Does Using Corsi–Rosenthal Boxes to Mitigate COVID-19 Transmission Also Reduce Indoor Air Concentrations of PFAS and Phthalates?” (another Betteride’s Law violation, come on guys):

Using a quasi-experimental design, we quantified the impact of a relatively low-cost “do-it-yourself” air filter (Corsi–Rosenthal Box; CR Box) on indoor air concentrations of 42 PFAS and 24 other SVOCs. We sampled air before (October–November 2021) and during (February–March 2022) deployment of CR Boxes in 17 rooms located in an occupied Providence, Rhode Island office building… While CR Boxes were deployed, concentrations of seven PFAS (N-EtFOSE, N-EtFOSA, FBSA, PFBS, PFHxS, PFOS, PFNA) were 28–61% lower and concentrations of five phthalates (DMP, DEP, DiBP, BBzP, DCHP) were 29–62% lower.

So the added benefit would be that in a school running Corsi boxes, your kids won’t be breathing toxic “forever chemicals” (or if they do, at lower concentrations).

I generally don’t go all gooey about “the best of America” but that really does seem to be happening with CR Boxes. I see:

(1) People working together. Yes, many individual CR Boxes are built, but there seems to be strong tendency to build them in bulk (“bet you can’t build just one”) together with others.

(2) A Gift Economy. CR Boxes tend to be given away. People don’t build them for schools, and then invoice the school.

(3) Do-It-Yourself (DIY <-- acronym) innovation. The CR Box is part of the maker zeitgeist, but there’s a lot of traffic going on about how to improve them. Classic garage inventor and tinkerer stuff.

(4) Efficiency. Making plans, building assembly lines, developing metrics; America used to be able to make stuff. Maybe we can do that again.

There are a lot of guides on how to build CR boxes, but I thought this one was especially detailed and simple. These are good tips:

The only tools I needed were a Swiss army knife, scissors, a pencil, and a loop of string. (Oh, and a thumbtack.)… I purchased my fan and filters online, and both got shipped in boxes that are the perfect size to provide the cardboard you need… Building it was fun and easy. Recommended!

(Fascinatingly, the above post is from a site called “Conferences that Work.” So maybe this guy an organize the next Davos!). I would welcome tips from readers who have build CR boxes!

* * *

Since DIY on a mass scale brings innovation (really), there have been some changes in CR Box design, and the development of a market for kits.

First, some users complain that the square floor fans used by the CR Box are too noisy. The answer is to use PC fans[2], which are quieter. This makes construction more difficult. Here is an example:

Second, now there are kits:

One rationale for going commerical would be UL certification:

But I’m not so sure UL certification is needed. From Underwriters Laboratories, “An Evaluation of DIY Air Filtration“, although in the context of wildfires, not Covid:

Household use of portable air cleaners has increased in the face of recent wildland fire activity and in response to COVID-19. Some local agencies have begun recommending Do-It-Yourself (DIY) air cleaners (i.e., furnace air filter attached to electric box fans) during smoke events as DIYs offer an affordable and accessible alternative to commercially available air cleaners, which frequently sell out during smoke events…

Chemical Insights (CI) of Underwriters Laboratories with support of the Office of Research and Development at U.S. EPA conducted a study assessing the potential fire risks of operating DIY air cleaners. Specifically, this study evaluated the risks of fan overheating and potential fire ignition resulting from different fan/filter test scenarios..

This study assessed DIY air cleaners for use in indoor environments at room air temperatures near 20 °C. Further analysis may be required for situations where ambient room air may be at higher temperatures (e.g., 40 °C). Should the blade turning be impeded in some way, it is possible that further heating could be created resulting in an additional hazard. However, fans that have been shown to meet the requirements of UL 507 are also required for a test that eliminates this impedance hazard.

So if you need to go before the school board or the vestry, this study may help.

* * *

As we might expect, the the Center for Disseminating Covid (CDC) continues its malevolence toward mitigation. Now that Evusheld is defunct, CDC issued the following advice to wear a gold star so that the immuno-compromised can make their personal risk assessments. From “Information for Persons Who Are Immunocompromised Regarding Prevention and Treatment of SARS-CoV-2 Infection in the Context of Currently Circulating Omicron Sublineages — United States, January 2023.” Of all the blather — plenty of handwashing advice, one might almost think those psychos were projecting — this is the best part:

[S]imple interventions should be used to improve ventilation in buildings and decrease SARS-CoV-2 transmission by improving air flow.

Note lack of agency! Who, exactly, will be performing these simple interventions? Well, you, presumably, since we’re all on our own now, so one would think the CDC would recommend a cheap and highly effective “intervention,” the subject of this post in fact. No such luck, not in 2022 and not now:


Pretty bad for a remedy that’s known to work! What are they trying to do? Kill people?


How many CR Boxes are out there? There’s no census, so we can’t really now.

Working on the old principle from the blogosphere that 1% of readers comment, 500 would be 1% of total CR Box builders, meaning 50,000. That’s small relative to the United States population. However, measured against institutions, it’s pretty good. There are, for example, 128,961 public and private K-12 schools, and all it takes is one parent with a will to organize to protect the school.

Now, I should caveat that I’ve done a bit of cheerleading for CR Boxes in the post. I think the key issue is that we just don’t know a lot about how indoor air circulates, so we need to understand how best to place the boxes, the interaction with HVAC, the interaction with outside air, etc. For nervous school boards, the UL certification issue should be addressed. And the noise issue may be imporant to some. I would especially welcome reader comments on these points.

CR Box makers are helpers. It’s been wonderful to see a CR “movement” seed itself and flourish. Let’s put some honey badgers on this project and go save some lives!


[1] I love “making pie crust is harder.” Good writing matters!

[2] This astonishes me, but I guess either games or Bitcoin mining have created a demand for rugged and powerful PC fans.

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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. tevhatch

    “So the added benefit would be that in a school running Corsi boxes, your kids won’t be breathing (edit: as much) toxic “forever chemicals.”

    I plan to share this, thought I suspect the first five letters of the primary url is going to make a lot of school marms stop even before they click on the link. If they knew where the expression for the entire url came from, then I suspect all of them would shy off. Sad world we live in, but there it is.

  2. Bob "Fan" Blake

    Is there anywhere on the internet with specific clear lay person English with step by step constructing diagram and specific parts list for the PC fan boxes? I have read many many articles where the techno jargon rapidly escalates to non-comprehension for an old guy of 75. I have made numerous big fan Corsi boxes and I have gone with the 5 filter design and then using PVC pipe made cheap easy to assemble and disassemble stands that allow the Corsi box to sit at any angle – 45 degrees, 90 degrees, you pick the angle no problem. I would greatly want to build a PC fan version but lack the ability to translate the techno jargon. Pictures of each step. Bill of materials. Specific this model fan etc. Can anyone help out?

    1. lambert strether

      This all sounds great…. Could you expand on your reasons for adopting the five-filter design?

      Also, if you have any pictures of your stands, I sure other readers would like to see them; you could send them to my address in Water Cooler.

      1. Raymond Sim

        Yes! Want to see the stands! Wish I thought of that!

        I use the five-filter type placed on a screen-topped table (an ironing board) because it gives the best room turnover of anything I’ve tried. The PVC stands would, I bet, let me dispense with one or more of the tower fans I use to kill dead spots.

          1. Bob "Fan" Blake

            I use an Aranet 4 CO2 detector. Admittedly crude but efficient. If the CO2 level according to the detector does not lower then most likely have not done room turnover. Also use a temperature gauge and having heated with wood since 1976 I “see” cold and warm air movement. If one part of the room measures 65 degrees and another 75 degrees and this is consistent – most likely the air is not “moving.” All crude I admit. But neither am I doing a scientific study. In the end I want a warm room or cool room depending on the season. If all parts of the room are relatively similar great. Not about to start buying air flow meters as for my purposes that would be a waste.

          2. Raymond Sim

            Edit: Posted below before reading Bob’s post.

            In practice, pretty much as Bob describes, using the intuitive sense I’ve developed of how the air moves in the house. To gain that I used an Aranet (It would have been easier with two meters.), a stopwatch, and pieces of yarn or feathers held in my fingertips or taped to the end of a chopstick to work out how quickly the air was refreshed under various ventilation schemes, where it was moving, where it wasn’t etc. Dead spots, where the air didn’t move, or was trapped in an eddy, turned out to be a bigger deal than I had anticipated.

            I’ve settled on a five-filter Corsi Box, elevated two or three feet above the floor, flow directed towards the ceiling and ‘bottom’ filter unobstructed. This establishes a roughly toroidal pattern of airflow that fills our small 8′ high rooms and even scours out corners and around and under furniture and other obstacles pretty well. And, as with Bob’s basement, the air in a room with a Corsi Box is suprisingly nicer to breath.

      2. Bob "Fan" Blake

        Pictures of stands will be sent tomorrow Monday. There is nothing really special about them. None of the joints are glued. Every stand is totally take apart able. Given that there is almost no vibration, no need to glue joints. Also not having glued joints makes modify the angle of the box super simple.

        Went with 5 filter Corsi box specifically because we have heated with wood since 1976 up until about 5 years ago when we switched to pellets as you don’t need to split pellets. Heating with wood makes you become extremely aware of how hot and cold air moves in a house. While I cannot “see” the air currents, having spent years with a hand held temperature gauge sort of sees the places where hot and cold currents are. The 5 filter box with the filter on the bottom while on a stand is ideal for placing in a spot where you want “floor” air to move up. So in our domicile the Corsi boxes do dual duty. They move hot and cold to where we want them and in addition to cleaning the air of gunk. This leads into why I made furniture grade PVC stands. There are places where having the Corsi box fan pointed straight up would be a waste as what we would want in that spot is to move air in a room or out of a room or down the hall etc. The Corsi box if tilted say at a 45 degree angle would be more keeping the goal of moving the air to another location. There are also places where the 90 degree orientation works to achieve our “heat/clean” goal. And then the summer came. With the A/C on having the Corsi boxes on stands moves the coolness where needed. I do a great deal of work in the basement and that is also where all our exercise stuff is. There are two Corsi boxes down there 24/7 and the difference in how the air feels is amazing. No more stuffy basement.

        I did find what I hope is a parts list of PC fans and the adapters needed for them to get them to work when plugged into a wall outlet. Already figuring out a configuration.

  3. Carycat

    I am surprised nobody has chimed in so far. l am in the middle of building box-8. box-0 was a lasko box fan with a furnace filter taped on to help with seasonal allergies way before COVID -19. i started building more and experimented with different form factors and deployed them around the house. I have a classic 4 panel C-R / lasko humming along 3 feet from where i am sitting now. Mrs Carycat is a light sleeper and my last 3 builds are 12cm x6 PC fans with 2 20×25 MERV-13 filters (smaller footprint overall than classic 4 panel) powered by recycled laptop power bricks. l started with a 12cm x2, then a 12cm x4 PC fan box because i have 12cm fans from very old desktops. Even buying the PC fans and filters, my out of pocket for my last pair of 12cm x6 boxes was under USD $120. They are definitely much quieter than the classic C-R, especially when I under volt them for lower noise (trade off for lower air flow) and they consume 6watt as measured at the wall outlet! The Carycats take social distancing seriously and have been masking from day one, switching from surgical masks with mask brace to N-95 as the threat level escalated. So far, nobody in the immediate family got hit and l am trying hard to keep it that way. These help with seasonal allergies too.

    1. lambert strether

      This sounds great, too. Could you expand a bit on your experimentation with form factors? Also, what’s under-volting?

      1. Carycat

        PC fans are 12volt dc devices. If you feed them less than 12v (on average), they will still run but with lower air moving capability but less noise. Modern PC have circuitry on the mainboard to use temperature readings from built-in sensor to lower the average voltage fed to the fans when the machine is not at maximum load. A DIY C-R box is not going to have this. I have 19.5v (dell) and 16v (old lenovo) power bricks. When i
        I connect 2 fans in series, i can power each of them wih about 10v or 8v. I you have to buy a power supply, instead of getting a fixed 12v power adapter, you can get a “universal” unit that let you dial in a lower voltage.
        I didn’t do anything exotic with the shape except for one box where i put 3 filter panels in a triangle with 4 PC fans on top. the others are variations on fan placement (box fan vs PC fans, top exhaust or side exhaust which let me use the top panel to hold a book or two).
        My favorite is two 20x30x2in filter which makes the whole assembly about 33in tall and 9in wide with cardboard feet to raise the intake off the ground and a stack of 6 2cm fans on the narrow side which leaves about an inch on the top to mount a power switch although I normally run them 24×7.
        I started off with duct tape but quickly switched to hot gluing the filter panels to the plain cardboard sides. Structurally stronger and gives a perfect air seal. Cardboard can be rescued from your recycle pile, so when it comes time to change the filters, just salvage the fans and attach them to a new filter/ panel assembly

          1. Max

            A normal laptop power supply has the part that plugs into the wall (the input), the “brick” part, and then the part that plugs into the laptop (the output). The output is actually two separate wires – one positive, one negative. The “brick” part will have a label on it that says its output voltage, likely 12.0V DC, but possibly something different.

            The fan will also have two wires – one positive, one negative. (If there are more than two wires, you’ll have to look up which is + and which is -).

            Connect the power supply + to the fan +, and power supply – to the fan -, and it will spin. If you hook them up backwards, it will spin backwards (usually).

            The fan will have a label that says its voltage. In general, you want your power supply voltage to match your fan voltage. If your power supply voltage is lower, the fan will run slower and quieter. If the power supply voltage is higher, the fan will run faster and may even burn out.

            Adding additional fans gets complicated but using the above method, connect each additional fan straight to the + and – of the power supply and you should be okay.

            Hope that helps!

  4. ian

    I made a similar air filter for my lab/workshop. I had several 12V PC case fans and a 120V-12VDC power brick lying around. I made the air box with some scrap lumber and used a HEPA furnace filter. According to the CFM ratings on the fan data sheet, the air in the workshop gets exchanged about every 5 min. I measured the power draw and it is a little under 10W (so I can leave it on all the time). One other suggestion is to stack the filters with a coarser pre-filter (HEPA filters are expensive).

    BTW, for pie crust, Martha Stewart’s recipe for pate brise’ is simple and excellent. The key to success is working with all ingredients cold and being ultra-careful not to use too much water (it will turn ‘just right’ into cardboard if you do).

    1. Jen

      Well, since you brought up pie crusts…I use my mom’s recipe which is equal parts flour and butter, and just enough water so the dough holds together when pinched. She was old school and made it by hand. I use a cuisinart and it’s the only reason I keep the thing.

  5. GF

    How often does one need to change the filters in a 4 – five sided CR box? Is it strictly a time frame or using visual evaluation? If visual, what to look for that says “Change the filter Now”?


    1. ian

      Oddly, the filtering gets better as the filter gets clogged. I just put my hand over the exhaust and change it when the airflow is noticeably weaker.

      1. Cassandra

        But the effectiveness as an air cleaner is related to the number of air exchange equivalents per hour, so if your filters are clogged enough to slow the flow noticeably, you will potentially have more virus left floating in the air. Something to consider…

        1. Lambert Strether Post author

          I would research this. I have a vague memory, and I could be wrong, that the behavior of filters is counter-intuitive (at least within reasonable limits). Hence the “oddly”?

          1. Bob "Fan" Blake

            Take some surveyor tape – very bright colors and extremely light – tape it to the center of the fan. Note how the tape flutters with new filters. Watch the flutter eventually limp out. Change filter. And I have had filters in the bedroom that look almost black but the tape still vigorously fluttering. The color of the filter is only a partial indicator of flow level. Or you could buy a flow meter, take a reading with new filters and then periodically take readings. I like my short one foot flutter tape. It is cheaper.

            1. Raymond Sim

              I would add that a shorter flutterer could mislead. If flow is truly obstructed air will be traveling in through the edges of the grill. A foot-long length of tape will flap about and show that.

          2. Raymond Sim

            The processes by which particles are removed from the airstream and retained are more complex than, for instance, straining rice, and thinking of them as simple strainers can in fact lead to misconceptions.

            I visualze their functioning as a beneficial and amazingly thorough version of the way grease builds up on the grill of a kitchen vent fan.

    2. Raymond Sim

      In our case one of the Corsi Box’s main duties has turned out to be clearing woodstove smoke from the outdoor air we bring in for ventilation. As a practical matter, I change the filters when I can’t keep the Aranet reading in the mid 500’s without smelling our neighbors’ odors. During our recent stagnant air event that took less than two weeks. Current filters have been in for two months and are still doing fine, though they’re about the ugliest shade of gray I’ve ever seen. I may replace them for esthetic reasons.

  6. Cassandra

    We have built a couple of C-R boxes for home use. At least when we bought our materials, big box home stores were having a special on one inch MERV-13 equivalent filters, less than $11 each in multiples of 4… one has to assume someone in marketing was trying to promote DIY air cleaners. Our boxes have a bonus effect of removing any smoke/ash leaks when stoking our woodstoves. The filters are beginning to look a bit grubby, but nothing compared to what I’ve pulled out of a furnace in the past. In a public setting, I might change filters more frequently.

    1. Lambert Strether Post author

      > one has to assume someone in marketing was trying to promote DIY air cleaners.

      This being America, that’s a very good sign. (My thought was duct tape sales, but fan sales are obviously a better proxy.) I suppose what would be the ultimate sign of acceptance is leaflets on CR Box manufacture, plus a special deal on the materials, at Home Depot or wherever. Why indeed not?

      > Our boxes have a bonus effect of removing any smoke/ash leaks when stoking our woodstoves.

      Awesome! Year round sales, at least where there’s snow!

      1. John Zelnicker

        Lambert – See the email I sent a little while ago for a picture.

        “plus a special deal on the materials, at Home Depot”

        When I went to buy materials for a C-R box in July, HD was selling 4 20×20 filters at a 25% discount, so I think they were aware.

        Total expense was about $70, time expended was about an hour. The box is in my home office where I spend most of my time.

        I haven’t changed the filters yet and they have turned a light shade of brown because I’m a smoker. However, I don’t think the efficiency has changed. I’m still getting good airflow.

        One of my clients is highly sensitive to cigarette smoke, so last fall I asked her to come inside (I usually go out to meet them in their truck) and tell me if the box was effective. She was impressed and said she couldn’t smell the smoke at all.

  7. Biologist

    I absolutely love this – but how to convince parents and school administrators?

    I would love to build this, and I’m sure I could get other parents and even teachers enthusiastic enough to help or chip in for the materials – if they could be convinced that running these weird looking boxes held together with duct tape will not get them into trouble or lead to electrical (or other) hazards.

    Does anyone have ideas on how to convince and constructively engage with (sceptic) parents, teachers and school administrators to allow this into their classrooms in the face of massive Covid minimisation propaganda and active gaslighting and lack of support from official channels?

    I live in the UK, where the government, NHS and Public Health England have completely abandoned any mitigations, apart from the usual ‘wash your hands more often’, and mass media is completely onboard with ‘let er rip’.

    This country has ‘health and safety’ rules (COSH in USA?) for electrical supplies, aircon, fire safety, kitchen appliances, etc – especially for public buildings like schools and nurseries. These rules are often strict and bureaucratic, and their compliance is often just a paper excersise of filling in endless forms with little enforcement in practice (see Grenfell Tower).

    Needless to say, none of those health and safety rules say anything about clean indoor air. Instead, they have lots to say about electrical hazards, and I think this is the biggest stumbling block for having DIY boxes running on the mains electricity in a space with children. I know these CR boxes have been safety tested in the US but that doesn’t really work as an argument here.

    Any ideas how to convince parents and teachers who have good hearts but are afraid of breaking rules and have been barraged with endless Covid minimisation propaganda?

    1. lambert strether

      > I absolutely love this – but how to convince parents and school administrators?

      People have done it…. But this is something I am not good at in real life. Readers?

    2. ChiGal

      jessica wildfire (her substack was previously linked here) has a post explaining how she and her husband convinced first other parents and then the school to do mitigations. the post includes links to research providing evidence for clean air, masks, etc.

  8. Samuel Conner

    Thank you, Lambert.

    I just forwarded this to a contact in a local community development non-profit for discussion with contacts in local schools.

  9. Chuck Harris

    Thanks for this article. I made a CR box about 6 months ago for use in our house. I used the directions from Rick Corsi’s video. Got a Lasko fan at Target for $11 and ordered 4 2″ filters, which cost $65. I’m pleased with it so far. Seems to move lots of air, which is handy when we’re using the wood stove.

  10. Kevin Smith MD

    You don’t HAVE to make a “box”.
    I just taped a single filter right over the front of the fan, and that works just fine [for example, in our dining room]. I think the complexity of building and maintaining a “box” is not very great, but it might be enough to deter some people; and a “box” takes up a lot more space in a room, which could be another deterrent to use in some settings.

    1. Schnormal

      This is the issue where I work. There are six (sometimes more) of us who share a small, nearly windowless space. My boss would surely have vetoed putting a big cube in there, so I just made the single-filter version (filter on the back and shroud on the front).

      Seems to work great, not just for Covid but also for all the other junk we’ve been inhaling — the filter was totally filthy after just one month’s use.

      I used the directions David Elfstrom posted here: https://twitter.com/davidelfstrom/status/1429526273009889286?s=46&t=mQP8Hw_L9m8Y1RATh-McsQ

      Thanks, Lambert! :)

  11. Etrigan

    i had been meaning to build a Corsi Box ever since I heard about them, but escalated it when a household member came down with Covid in autumn. we’re on the second filter change now and watching the side panels turn positively charcoal colored over months has us wandering past it in the living room occasionally commenting “that kind of thing used to be in our lungs…” etc.

    The first time you build one is an annoying hassle that but every rebuild after swapping filter panels is so much easier. I’m not sure if it’s an innovation or not but a wedge of space where a gap was created by the fan’s cord, I stuffed in an n95 before sealing the crack with duct tape.
    CleanAirStars or Joey Fox’s Twitter account also link to the correct aperture size to cut a circle of cardboard to tape over the top of the fan to improve efficiency, looks like they vary slightly by fan model. That people have done the work measuring this with such specificity fills my heart with a steely hope about this one corner of our current collective nightmare.

    I just used the box the fan came in to make the base and aperture since it’s already sized. If I were to make a second full CUBE I would also cut one for the faint to rest upon to make the first base structure easier to manage.
    When you’re cutting the cardboard, a box cutter (cut away from yourself!! Sped it up considerably but I have a large self healing cutting mat, the green kind, from quilting.
    I’m experimenting with foam for replaceable side brackets but at this point it’s mostly for fun, the base model just works. No one else here caught Covid during convalescence (we also masked with N95’s the entire time) and I built a portable model out of ten inch panels for less used rooms. I’ve offered to build one for any friend for base cost but so far no takers.

    My two big experiments coming up: attempting the newer computer fan build (don’t place them in a corner!) and making a truly portable model to bring to other places that could fit in a knapsack.

  12. KidMD

    My choir rejected the donated regular corsi box (after relegating it down the hall briefly), due to the noise level. They did not reject the more expensive, but much quieter, Medical grade HEPA filter (donated).

    Size was not the problem, just the noise. If anyone knows of a reliable and effective, quiet fan for use in corsi filters, please share.

    1. Raymond Sim

      The main reason Corsi Boxes are boxes is the coincidence in size between box fans and a widely available filter type which, fortuitously, makes for enough filter area that the relatively feeble fan can pull useful amounts of air through.

      We have a couple Bionaire window fans which are much quieter than our box fans and I suspect they’d do just fine pulling through a Corsi Box, though I think they might need the filters renewed more frequently, and it would probably take some designing to frame them into one. I’ve seen lots of fans listed on retail websites that seem to use similar motor/blade combos and boast of quietness.

      A Corsi Box can also be louder or quieter depending on the sound reflecting qualities of any surface it might be pointed at. I can personally attest that the reflected sound can be much louder than that reaching an observer along a straight line path. Something as simple as a towel or blanket hung on the wall can make a big difference.

    2. Etrigan

      The newer builds using computer fans and a power adapter are a little more annoying to build but literally an order of magnitude quieter.

  13. dougie

    I had hoped to be able to report on my order from @CleanAirKits. The first 2 were ordered Jan 9, and shipped on Jan 26. I ordered 2 more on Jan 10, and have not heard any shipping info yet. Somewhat disappointing, I think they are scrambling to fill orders, and hoping to be a bigger player by fall when schools reopen (per a conversation I had with the company)

  14. Rod

    Thank you Lambert for this straight forward and simple exposition on the CR Box.
    You are, with this, a proven Public Service again—IMO.
    No regulation needed either.
    I plan on copying your article to accompany the CR Box I am building and gifting to my Medical Oncologist’s facility—the one where I have observed some of the Staff wears Surgical Masks and some of the very numerous Patients wear a variety of masks.
    Last week I introduced some of the Staff to the previously unknown CR Box concept and they seemed pretty interested.
    It’ll be a positive and productive outlet—at least for me, and a good use of my SS income too.
    Ain’t it a crazy ol’ World?

  15. Dave Backer

    I believe the Philadelphia chapter of the DSA is doing a safe air campaign and builds CRBs as the tactic, placing them in schools. They fundraise the materials and organize builds at school buildings. Other chapters in NYC and Austin are following the model.

  16. LAS

    I really like this concept, the initiative it shows. However, the references cited as supporting efficacy expressed in their conclusion that results could not be guaranteed.

    “The DIY air filtration units reduced aerosol exposure up to 73%
    depending on the design, filter thickness, and fan airflow. In general, our
    results show that the performance of the DIY air filtration units is largely
    a function of the total airflow rate through the filters. Constructing DIY
    air filtration units with more filters (DIY cube), thicker filters (which
    have a greater surface area), and a higher airflow fan, and the use of two
    DIY units instead of one, all added 2 or more effective ACH to the overall
    room air change rate, which led to a corresponding reduction in aerosol
    concentration. However, the amount of variability in performance seen
    with different DIY configurations, fans, and filters shows that DIY air
    filtration units must be used with caution since individual units’ performance will be unique. In addition, potential problems with construction quality such as leaks and gaps could substantially affect the
    performance of DIY air filtration units. Unfortunately, there is at present
    no simple way for the do-it-yourselfer to verify that their DIY unit is
    performing as expected. DIY air filtration units may be effective for
    temporary use until commercial portable air cleaners with known performance characteristics can be secured or used in areas that cannot
    obtain portable air cleaners. However, the EPA does not recommend the
    DIY units as a permanent alternative to products of known performance
    (such as commercially available portable air cleaners).”

    Just saying. Don’t want to be a total Debbie Downer.

    1. Lambert Strether Post author

      > Unfortunately, there is at present no simple way for the do-it-yourselfer to verify that their DIY unit is performing as expected.

      If that’s true now, I doubt it will be. There’s a ferment of development, with lots of people working in parallel. I outlined some of the work I’d like to see done at the end of the post:

      I think the key issue is that we just don’t know a lot about how indoor air circulates, so we need to understand how best to place the boxes, the interaction with HVAC, the interaction with outside air, etc.

      I do think, however, that for home use, a lot of precision is not necessary, in the same what that it’s not necessary to have a lot of scientific instrumentation* to, say, cook. Follow the directions!


      > potential problems with construction quality such as leaks and gaps

      Remind me of the kind of argument anti-maskers make. Blah blah blah, fit may be imperfect. Well (a) follow the directions and (b) instead of whinging, devote energy to training people instead. None of this is hard, especially if the fundamental attitude is to help people.

      NOTE * For example, in sealing the house against drafts, I discovered that walking around with a candle or a stick of burning incense was really, really helpful, although no doubt there is expensive instrumentation to accomplish the same end.

  17. RA

    As usual I am very late on this reply.

    I made a CR box in 2021 but I made some minor changes in my version. I have finally written up what I did and have posted it as a pdf file here:


    Here is the quick outline of what I did differently from a normal Corsi-Rosenthal (CR) filter box.

    1) Use Gaffer tape rather than duck tape.

    2) Use filters a bit larger than default 20×20″

    3) (The big one) Tape the corners differently to make hinges so the filters can be collapsed flat for moving or storage.

    4) (Optional) Put a UVC Germicidal lamp inside my CR box for additional virus suppression.

    Also some comments about sound noise.

    I hope this post is not too late to reach some people who might be interested.

  18. thoughtful person

    Had a production room, about 2000 sq ft. We had a couple corsoi boxes (filtrete 2200 and utilitec fan) along with a few fans with a single filter. Wore masks until the CDC claimed they weren’t needed. No one got covid.

    A few complaints about noise. Using them at home, way too noisy unless on low. Got a rated quiet standing Pelonis, but on low and acceptably quiet settings hary pulls air through a single filter.

    Got some computer fans on order, rated nice and quiet, we’ll see!

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