Grassroots air filters saved lives during COVID-19
One afternoon, a dozen students from Arizona State University got together to spend the morning cutting cardboard, gluing fans and assembling filters in an effort to build 125 portable air purifiers for local schools. That same morning, staff members at a homeless shelter in Los Angeles were installing 20 homemade air purifiers themselves, while in Brookline, Massachusetts, another DIY air purifier quietly hummed in the back of a daycare classroom while the kids were playing.
The technology in all three – an unassuming tape and cardboard construction known as the Corsi-Rosenthal box – plays an important role in the fight against COVID-19. The story of its birth also says a lot about communities as sources of innovation and resilience in the face of disasters.
Simple technology with a big effect
When it became clear that COVID-19 was being spread through the air, people started wearing masks and building managers rushed to upgrade their ventilation systems. This usually meant installing high efficiency HEPA filters. These filters work by capturing virus-laden particles: air is forced through a porous mat, contaminants are filtered out, and clean air passes through.
The efficiency of a building’s ventilation system is governed by two factors, not just the quality of the filters. The amount of air circulating through ventilation systems is also important. Experts typically recommend five to six air changes per hour in shared spaces, meaning a room’s entire volume of air is replaced every 45 minutes. However, many older building systems cannot handle this volume.
Portable air filters are one option for augmenting ventilation systems, but they typically cost hundreds of dollars, putting them out of reach for schools and other public spaces facing budget constraints.
This is where the Corsi-Rosenthal box comes in. It is a cube made up of four to five standard oven filters topped with a standard box fan blowing outward. Once sealed with tape, it can be placed on a floor, shelf or table. The fan draws air through the sides of the cube and out the top. The units are simple, durable and easy to manufacture, and are more efficient than simply placing a single filter in front of a box fan. It usually takes 40 minutes, minimal technical expertise, and $60 to $90 in materials available at any home supply store.
Despite this simplicity, these homemade units are extremely effective. When used in a shared space like a classroom or hospital ward, they can supplement existing ventilation and remove airborne contaminants, including smoke and virus-laden particles. A series of recent peer-reviewed research has shown that portable air purifiers can significantly reduce aerosol transmission. Further pre-print and in-review studies have found that Corsi-Rosenthal boxes perform as well as professional units at a fraction of the cost.
Origins of the Corsi-Rosenthal box
The official story of the Corsi-Rosenthal box began in August 2020, when Richard Corsi, air quality expert and now dean of the University of California, Davis, pioneered the idea of building cheap box fan air filters on Twitter. Jim Rosenthal, CEO of a Texas-based filter company, had toyed with a similar idea and quickly built the first prototype.
Within days, DIYers and air quality engineers were building their own Corsi-Rosenthal boxes and sharing the results on social media. A lively conversation emerged on Twitter, mixing sophisticated technical analysis from engineers with insight and effort from non-specialists.
By December, hundreds of people were making Corsi-Rosenthal boxes, and thousands more had read press articles at outlets like Wired. In different corners of the world, people have been fine-tuning the designs based on the availability of supplies and different needs. Their collective improvements and adaptations have been documented by dedicated sites and blogs as well as reports.
In some cases, design tweaks proved influential. In November 2020, for example, a homeowner in North Carolina discovered an issue with air being sucked into the corners of most commonly used square fans. Subsequent tests by air quality experts showed that adding a shroud to the fan increased efficiency by up to 50%.
Analysis of social media and media coverage gives an idea of the scale of the Corsi-Rosenthal box phenomenon. By January 2022, more than 1,000 units were in use in schools, and thousands more in homes and offices. More than 3,500 people had used the hashtag #corsirosenthalbox on Twitter, and tens of thousands more contributed to the conversation online. News articles and explainer videos on YouTube had collectively racked up more than 1.9 million views.
Communities as sources of innovation
The story of the Corsi-Rosenthal box is part of a larger story of the grassroots response to the COVID-19 pandemic. The early days of the pandemic did more than just weigh people down terribly. They also galvanized a massive entrepreneurial effort, with tens of thousands of ordinary citizens lending a hand in the design and production of essential medical supplies and personal protective equipment that were suddenly needed.
My research team is following these efforts. Through dozens of interviews and months of archival research, we’ve built a database of more than 200 startups – formal and informal, non-profit and for-profit – whose activities range from design from oxygen concentrators to 3D printing face shields to building UV disinfection rooms. The image of innovation that emerges is a far cry from the traditional image of lab coats and middle managers commonly associated with new technologies.
First, few of the innovations we tracked were invented by a single person, or even a single team. Rather, it was the joint project of vast networks of individual contributors from different backgrounds and organizations. This breadth is important because it brings more knowledge and more diverse perspectives. It can also be useful for leveraging existing knowledge. For example, as Corsi-Rosenthal boxes gained traction, the community was able to build on earlier iterations that had been developed to help fight wildfire smoke.
Second, the innovation process lacked hierarchical control. There was no single person directing where and how the technology was used. This lack of control facilitated experimentation and adaptation to local conditions. An example is the development of oxygen concentrators for use in hospitals in India. Realizing that existing Western technologies frequently failed in the more humid operating environment typical of India, teams of innovators came together to develop and share improved open source designs.
Third, these communities shared knowledge online. This allowed individual contributors to communicate directly and share ideas, allowing knowledge to spread quickly through the network. It also meant that knowledge was more easily accessible. Detailed designs and test results from air quality engineers working on Corsi-Rosenthal enclosures were readily available to everyone in the community.
Additionally, most of the organizations we tracked used Facebook, Twitter, and Slack as tools to manage collaboration within and between organizations. As I and others have argued, this gives grassroots innovation tremendous promise, especially in a world where large-scale disruptions like a pandemic are increasingly common.
The pitfalls of grassroots innovation
Despite this promise, there are areas where local innovation communities are faltering. One of the challenges is the lack of technological sophistication and resources. While some of the communities in our study were producing remarkably complex devices, the greatest contribution was to much simpler products like face shields and surgical gowns.
Then there are rules and regulations. Even when grassroots communities can produce safe and effective innovations, existing rules may not be ready to receive them. Some hospitals were unable to accept community-provided PPE during the pandemic due to inflexible supply policies, and today some schools continue to ban Corsi-Rosenthal boxes.
A final problem is maintaining the effort. While grassroots communities were essential to keep hospitals and medical facilities running during the early days of the pandemic, many of the efforts that depended on volunteer labor eventually faltered.
What this means for the future
As we approach the second anniversary of the US declaration of emergency, a key lesson the world has learned is the importance of investing in indoor air quality, for example through monitoring and improved ventilation and filtration. And the value of ventilation as a noninvasive public health tool is even greater as mask mandates decrease.
Another broader lesson is the power of local innovation and citizen engineering to develop these technologies. The story of the Corsi-Rosenthal box, like that of the thousands of other local innovations developed during the pandemic, is fundamentally about people taking the well-being of their communities into their own hands. the most popular tweet shared about Corsi-Rosenthal boxes came from a 14-year-old aspiring engineer in Ontario offering to build and donate boxes to anyone in need.
Douglas Hannah is assistant professor of strategy and innovation at Boston University.
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