One afternoon, a dozen Arizona State University students gathered to spend the morning cutting cardboard, taping 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 setting up 20 homemade purifiers of their own, while in Brookline, Massachusetts, another DIY air purifier was whirring quietly in the back of a day care classroom as children played.
The technology in all three cases – an unassuming duct tape-and-cardboard construction known as a Corsi-Rosenthal box – is playing an important part in the fight against COVID-19. The story of how it came to be also reveals a lot about communities as sources of innovation and resilience in the face of disasters.
A simple technology with a big effect
As it became clear that COVID-19 was spread through airborne transmission, people started wearing masks and building managers rushed to upgrade their ventilation systems. This typically meant installing high-efficiency HEPA filters. These filters work by capturing virus-laden particles: Air is forced into a porous mat, contaminants are filtered out, and clean air passes through.
The efficacy of a building’s ventilation system is governed by two factors, though, not just the quality of the filters. The amount of air moved through the ventilation systems matters as well. Experts typically recommend five to six air changes per hour in shared spaces, meaning the entire volume of air in a room is replaced every 45 minutes. Systems in many older buildings can’t manage this volume, however.
Portable air filters are an option for augmenting ventilation systems, but they typically cost hundreds of dollars, which puts them out of range for schools and other public spaces that face budget constraints.
This is where the Corsi-Rosenthal box comes in. It’s a cube consisting of four to five off-the-shelf furnace filters topped by a standard box fan blowing outward. Once sealed together with tape, it can sit 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 make, and are more effective than simply placing a single filter in front of a box fan. It usually takes 40 minutes, minimal technical expertise and US$60 to $90 in materials that are available from any home supply store.
Building a Corsi-Rosenthal box portable air filter comes down to duct-taping together a set of furnace filters and a box fan.
Douglas Hannah, CC BY-ND
Despite this simplicity, though, 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 raft of recent peer-reviewed research has found portable air purifiers can dramatically reduce aerosol transmission. Other preprint…