By Jim Rosenthal
Introduction: This is the story of how the knowledge and creativity of a researcher (Dr. Richard Corsi) and the practical experience of the CEO of an air filter company (Jim Rosenthal) combined to develop an extremely effective and inexpensive air cleaner. This open source design has been used extensively in schools, churches, homes, and office buildings to remove particles like SARS-Cov-2 and smoke from wildfires.
Methods: It started with an article in Wired magazine entitled: “Could a Janky, Jury-Rigged Air Purifier Help Fight Covid-19? Indoor-air experts think: Sure, maybe. Why the hell not? We convinced the CEO of an air filter company to give it a try.”
The article described the concept of a DIY air cleaner using a box fan and an air filter. The CEO who tested the concept with 1”,2” and 4” MERV 13 filters was Jim Rosenthal. The article also contained the following:
“On a recent Zoom call, environmental engineer Richard Corsi—dean of the Maseeh College of Engineering and Computer Science at Portland State University and an indoor air quality expert—held up a Folgers Coffee box to illustrate for me how a fan at one end, and filters on three or four other sides, could reduce the “pressure drop” that can come from putting a filter in front of a fan and still move (and clean) room-sized volumes of air. “You’ve got me really pumped up on this right now. I don’t have a lot of free time, but this would be something to build a prototype of,” Corsi says. “If it wasn’t for the fact that, you know, we’re still on lockdown, this would be a great fourth-year team project.”
Jim Rosenthal (Practitioner) decided to create the “Box” air cleaner conceived by Dr. Richard Corsi (Research). The result was the “Corsi Box.” Dr. Corsi and I have been collaborators and friends for over 15 years. He was Chair of the Civil, Architectural and Environmental Engineering School at the University of Texas (and recently appointed Dean of the Engineering School at UC-Davis – his alma matter.) I was honored to serve as an Advisory Director of the innovative and successful UT IGERT program. And in a small way, I was a supporter of research in his Department (primarily through the donation of filters). He served on my Board of Directors when I was President of the Asthma and Allergy Foundation of America – Texas Chapter. We met over an ozone-terpene reaction (long story). The “Box” was his idea. I thought it would be fitting to name it after him.
By utilizing the box design the air cleaner increased air flow from 320 fpm to 580 fpm, decreased noise by 20 dB, and extended the life of the filters to 6 months. With MERV 13 filters it has a calculated CADR of approximately 600. (Actual CADR testing is underway.)
Early on, Dr. Don Milton suggested that two “Corsi boxes” be used for the 2020 Vice-Presidential debate to protect the participants. (as reported in the New York Times). After the article appeared, Dr. Corsi sent the following tweet to Dr. Milton:
“Thanks for the shout out. Great article. One correction. I mentioned the concept of a portable air cleaner with walls made of filters in an interview. But it was actually Jim Rosenthal who built a unit shortly thereafter and deserves credit. (gr8 craftsmanship!)”
Dr. Milton’s response – “So, Corsi-Rosenthal Box is the proper name!”
In October of 2020, two HVAC experts (Neil Comparetto and John Semmelhack) made DIY Box air cleaners with a slight variation. Instead of 5 filters, they used 4 filters with cardboard on the bottom. The new name – “Comparetto Cube” was the result. This “authorized” modification to the Corsi/Rosenthal Box has been very successful as well – even with a “do-it-yourself” segment on the “Your Old House” TV show. Comparetto and Semmelback also did some extensive testing with their version as shown here.
Many other modified designs have been made. All of these have been encouraged. The idea has been to promote widespread use of the concept – the more, the merrier. Literally thousands of people from around the world have built Corsi/Rosenthal boxes. They are inexpensive to build and extremely effective. They work!
One of the most creative variations was proposed during the 2020 football season for coaches who insisted on wearing face shields.
In the Fall of 2020 we identified a problem with the design. Basically, it had to do with a round fan in a square box. While the fan created strong positive air flow, the corners had air flow going in the opposite direction. Fans used to cool automobile engines have the same problem which is solved by using a “shroud” around the outer edge of the fan. The shroud can be made of any material. For the first one we used a cardboard cutout. Tape worked just as well. We found that a properly sized shroud dramatically increased performance. The article explaining the problem and the solution is here:
A measure of the impact of the “Corsi-Rosenthal Box” is reflected in the number of on-line visits to the articles in the www.texairfilters.com website. There have been over 65,000 visits to the articles since August of 2020. Due to the lack of HEPA air cleaners and the heightened awareness that SARS-Cov-2 was airborne, there was a surge in readership when they were first posted on the site. About 1,000 unique visitors read the articles every day. The two biggest surges came when the “Corsi-Rosenthal Box” article was first published (3,026 hits on August 23, 2020) and during the midst of the West Coast wildfires (average of 2,000 hits a day in early September). The combination of low cost (less than $100) and high effectiveness has been very popular.
Here are a few of the “success” stories that have been reported so far:
Dr. Shelly Miller – University of Colorado
“The Corsi filter box reduced PM2.5 by 92% on average! Yep. It works! My student is testing a filter she made in her small apartment, challenged the air cleaner with cooking emissions.”
Dr. Clark Vangilder
“We just finished our second whole house quarantine for a resident who had to isolate therein after testing positive for COVID. Each time a Corsi-Rosenthal box filter fan was part of the air purification puzzle. The four of us who still haven’t caught COVID are grateful.”
“We built 18 of these for a small school here in East Rochester. Classes have been going 5 days a week all school year without any outbreaks.”
Neither Dr. Corsi or I wanted to be known as an adjective describing a DIY air cleaner. But it looks like “Corsi/Rosenthal Box” has become a common term. So be it. People know what it is. If that encourages one more person to build a DIY air cleaner to protect their family, friends, students, businesses or whatever, it’s worth it.