Richard Corsi, dean of the University of California, Davis (UC Davis) College of Engineering, will present results from his team’s recent DIY air filtration studies today at the ACS Spring 2025 Digital Meeting, a meeting of the American Chemical Society.
Despite advice to remain indoors on smoggy days or when the sky fills with wildfire smoke, human exposure to air contaminants is dominated by indoor air pollutants — including pollutants brought in from outside. Yet, research into indoor air quality has historically taken a back seat to other concerns like wastewater pollution.
That changed with the COVID-19 pandemic.
“The media didn’t pay much attention to indoor air quality, and then the pandemic hit and suddenly there was this tremendous opportunity to educate the public about the issue,” says Corsi.
Knowing that not everyone could work from home and that children would have to eventually return to school, Corsi realized that people needed an affordable way to keep indoor air as safe as possible.
“That’s when we came up with the idea of a low-cost, effective air cleaner that people can build themselves,” he explains. Corsi hoped to reach as many people as possible who couldn’t put down hundreds of dollars for a commercial HEPA air filter.
The DIY solution, called the Corsi-Rosenthal (CR) Box, was codesigned with filtration specialist Jim Rosenthal. The CR Box comprises four air filters commonly used in heating, ventilation, and air conditioning (HVAC); a cardboard base; and a box fan to draw air through the filters and toward the ceiling. Because the components are relatively inexpensive, the CR Box is a fraction of the cost of a HEPA air filter.
As Corsi highlights during his presentation, he and his colleagues recently tested the durability and performance of CR Boxes by installing units in four locations across the UC Davis campus. They ran them for 2,500 hours — the equivalent of two K-12 school years.
The CR Box filters performed as well or better than comparable HEPA filters. The DIY system was also more effective at clearing particles 1-3 micrometers in diameter than particles 0.35-1 micrometers. For a sense of scale, typical smoke particles are smaller than 1 micrometer, whereas dust and pollen are 0.5-3 micrometers.
Although HEPA filters are more efficient at removing particles in a single pass than the HVAC filters used in the DIY system, Corsi explains, much more air passes through the CR Boxes, leading to more particles being removed from the air. Although the CR Boxes’ effectiveness diminished over time, they still performed better than new and more expensive HEPA air filters for the larger particle size and performed competitively with HEPA for smaller particles over the extended study period.
To see if the CR Boxes could also remove particulates exhaled by people in conversation or when sneezing, the researchers then tested the systems on respiratory plumes. “We set up a situation where two people are speaking across the table next to a respiratory plume simulator,” Corsi explains. “We then measured the decay of that plume with no CR Box and with boxes located around the room and running at different speeds.”
They found that when the CR Box is within 3 to 4 feet (91 to 120 centimeters) of the people speaking, the respiratory plume dissipates within seconds at a distance from the air-filtering device.
For future enhancements, Corsi would like to redesign the fan blades to make the CR Boxes quieter, and he sees potential to modify the filters to remove other indoor pollutants, such as formaldehyde from furnishings and building materials, and ozone from smog and some office equipment.
On a more personal note, he appreciates the sense of empowerment CR Boxes give people. “One thing that’s been so satisfying to me is the number of people who’ve told me that they get such great personal satisfaction out of knowing that they’re building something that’s helping them and their families or their students,” he says.
The research was funded by UC Davis.
Visit the ACS Spring 2025 program to learn more about this presentation, “Evidence-based performance assessment of cost-accessible open-source air cleaners” and other science presentations.
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Title
Evidence-based performance assessment of cost-accessible open-source air cleaners
Abstract
Do-It-Yourself (open source) filter-based air cleaners have become popular in response to the COVID-19 pandemic, increasing frequency of wildfires in many regions, and an increasing public awareness of the adverse effects of airborne particulate matter in general. These systems provide the benefit of low cost, e.g., about 1/3rd to 1/8th the capital cost of comparable HEPA-based air cleaners, with similar recurring costs. Several research teams have tested one type of open-source air cleaner referred to as a Corsi-Rosenthal Box (CR Box) to determine clean air delivery rate (CADR) and found them to be more effective than HEPA-based air cleaners and with high effective air exchange rates in right-sized indoor spaces.
Recent studies on CR Boxes completed at the University of California, Davis will be presented. One involved longitudinal performance of CR Boxes placed in different university buildings and showed excellent performance across a range of particle sizes over operational times consistent with two K-12 school years. The single pass removal efficiency across all particle sizes generally tracked with or exceeded those expected for a MERV-13 filter. Initial CADR varied from approximately 600 m3/h to 1,500 m3/h for small (0.3 μm) to large (3 μm) particle diameters, respectively, on high flow setting, with some decline in CADR likely due to loss of electrostatic charge on filters. CADR for low and medium flow settings were lower but still exceeded those of commercial HEPA air cleaners.
A second study involves the impact of CR Boxes on room mixing conditions and rapid dispersion of respiratory plumes using simulated cross-table plumes emitted by respiratory system simulator with carbon dioxide as an inert respiratory tracer. CR Boxes are placed in various positions relative to the source and receptor with additional variations in fan flow settings. Results indicate near-infector benefits that complement the removal of far-field virus-laden respiratory aerosol particles.
Collectively, a growing base of data related to DIY air cleaners in general, and CR Boxes specifically, indicate potential for significant effectiveness and sustained performance at lowering building occupant exposures to fine and coarse particulate matter of both indoor and outdoor origin. This presentation will summarize evidence-based knowledge on DIY air cleaner effectiveness and address recommendations for widespread applications and continuing research needs.
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