1/ 3 hours in the late night & a seed tossed into the social media wind. Origin of the #CorsiRosenthalBox.
There were 2 motivations for the concept.
Motivation 1. Make effective air cleaning more accessible to those who cannot afford $300 HEPA air cleaners.
There were 2 motivations for the concept.
Motivation 1. Make effective air cleaning more accessible to those who cannot afford $300 HEPA air cleaners.
2/ Motivation 2. Help schools employ effective air cleaning at a cost that would not be prohibitive given financial constraints. In summer 2020 school districts seemed overwhelmed, confused, & vulnerable to purchasing shiny objects that might not actually be all that effective.
3/ Three criteria were considered for the device:
(1) Effective, removing a reasonable amount of virus-laden aerosol particles from indoor air.
(2) Inexpensive, much lower cost than HEPA air cleaners.
(3) Open source, not a “money maker” for anyone or any organization.
(1) Effective, removing a reasonable amount of virus-laden aerosol particles from indoor air.
(2) Inexpensive, much lower cost than HEPA air cleaners.
(3) Open source, not a “money maker” for anyone or any organization.
4/ Some were promoting a single filter on a box fan w/ limited results posted on social media, often focused on wildfire smoke. The clean air delivery rate (CADR), an important parameter for removal of aerosol particles from indoor air, was generally not calculated.
5/ From what I could glean the CADR appeared to be, perhaps, between 80 and 200 cfm, enough to put a measurable dent in aerosol concentrations in spaces ranging from a small office to a large bedroom.
6/ As an engineer (and someone who had given homework problems on head loss through filters), it seemed obvious that a single filter on a box fan can cause a relatively high resistance to air flow. This resistance lowers air flow rate & the potential for higher CADR.
7/ I also had concern about the impact on fan motor life. So, I spent an evening thinking about how to overcome the resistance, thereby increasing air flow & CADR, lessen wear on the fan motor, and generally produce a better design.
8/ How about spreading the flow over 4 or 5 pleated filters in parallel? This would lead to a reduction in head loss and greater air flow rate. The CADR would also be higher, while at the same time reducing wear on the fan motor.
9/ This could be done by placing filters side-by-side and using a large plenum for air distribution, a design that seemed unreasonable from the start in terms of space requirements. But the same effect could be achieved in a more compact design ...
10/ by arranging filters in parallel as a box. I went to the kitchen in the wee hours of the night and grabbed a box of instant coffee bags as shown here, envisioned one small side as a box fan and the rest of the sides as filters making up the walls of the air cleaner. 
11/ With the fan blowing outward the system would be under negative pressure, thus sucking air through the filters and aiding with the seal around edges. I later used the same box of coffee bags on a Zoom call with a reporter to illustrate the concept.
12/ I remember thinking during the interview that the reporter thought I was crazy!
Since CADR is the product of air flow rate and fractional filter efficiency for different particle sizes, a good box fan and appropriate filters might remove ...
Since CADR is the product of air flow rate and fractional filter efficiency for different particle sizes, a good box fan and appropriate filters might remove ...
13/ a reasonable fraction of aerosol particles in the size range relevant to conveyance of SARS-CoV-2. The box fan was easy. There were so many on-line with relatively high air flow rates w/ costs as low as US $20.
14/ If MERV 13 filters are used, the single pass removal efficiency would be lower than HEPA filters or higher MERV-rated filters, but would keep the cost lower (accessibility!). At high enough flow rates these ought to be able to reach relatively high CADR ....
15/ across aerosol particle sizes relevant to conveyance of the SARS-CoV-2 virus. MERV-13 filters (say 20 inch x 20 inch x 1 inch) generally run anywhere from US $15 to $20, but the price is often reduced for 4+ filters w/ signif’ bulk discounts from manuf’ for larger purchases,
16/ e.g., for schools or other organizations that want to build larger numbers. So, a 4-filter design might cost $80 to $100/box (box fan + MERV-13 filters + relatively low cost of duct tape to seal edges). However, when purchased in bulk the cost can be considerably lower ...
17/ (around $60 to $65). The latter is roughly 15 to 25% of the cost of many good commercially available HEPA air cleaners.
That was basically the genesis of what has become known as the Corsi-Rosenthal box.
That was basically the genesis of what has become known as the Corsi-Rosenthal box.
18/ An old friend from Texas, @JimRosenthal4, took the concept, built a box and posted it on twitter (the next day, I believe!). It was beautiful (image below). That ignited interest, and since then Jim has been generous with this time & a great proponent for the concept.
19/ Others like @DavidElfstrom have helped to evolve the box with addition of a shroud to reduce back-flow in corners caused by a reverse pressure gradient from the exhaust side back into the fan. Others have been amazing advocates.
20/ @kprather88, @Don_Milton, @MarinaC_Dyb, @JehnML, @ashishkjha & so many others
have been strong advocates for the #CorsiRosenthalBox and even put together armies of students to help build them. @Don_Milton even suggested its use during Presidential debates!
have been strong advocates for the #CorsiRosenthalBox and even put together armies of students to help build them. @Don_Milton even suggested its use during Presidential debates!
21/ @kprather88 was instrumental in getting the box featured on CBS evening news. cbsnews.com/video/homemade…. Andy Slavitt gave me time to discuss the box In the Bubble - engineering.ucdavis.edu/news/dean-rich….
22/ Academic researchers have been studying the effectiveness of the #CorsiRosenthalBox and reporting results that exceed the effectiveness of far more expensive HEPA air cleaners. Some results from a team @UCDavisCOE led by @CappaSnappa below.
23/ A major corporation (3M) has joined in by reporting outstanding results and proclaiming the movement to be "legit". news.3m.com/2022-02-24-3M-…
24/ Some have developed great websites that show how to build the #CorsiRosenthalBox and to highlight many examples of success - cleanaircrew.org/box-fan-filter…
25/ The concept has taken off worldwide because of the efforts of so many who promote, share, & disseminate. I feel like I tossed a seed to the wind in the summer of 2020, but many other wonderful folks have used it to grow rich crops that are protecting so many. More to come ..
26/ I cannot underscore how deeply fulfilling it has been to watch so many involved with this movement, if I can be so bold to call it that. I especially get joy out of observing the excitement of K-12 students learning from building the #CorsiRosenthalBox.
27/ The concept of #CitizenEngineering is very dear to my heart. Protecting lives while inspiring children about STEM and the possibilities it offers. How cool is that!?
28/ Aside from building #CorsiRosenthalBoxes in my home for others, the success of the box has led me to re-think my new lab @ucdavis. I've decided to focus it on the design & testing of new CR Boxes, as well as low-cost air cleaners for other pollutants. Stay tuned!
29/ And once again, thank you to all who have been promoting, building & disseminating the CR Box, thereby significantly lowering the inhalation dose of virus-laden aerosol particles in schools, homes, shelters, churches, hospitals, and more. You have all inspired me. You rock!
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