Scientists Tested a DIY Air Cleaner – And It Beat Expensive Filters

Low-cost, DIY air purifiers are proving to be just as effective as expensive commercial units at protecting indoor spaces from wildfire smoke and airborne viruses.

Designed during the COVID-19 pandemic, the Corsi-Rosenthal Box uses simple materials – HVAC filters, a cardboard frame, and a fan – to clean air in homes, schools, and offices.

Wildfire Smoke and Indoor Health Risks

When wildfires threaten communities, their health impact extends far beyond the flames. Smoke lingers in the air, carried by the wind into homes, schools, and workplaces, posing serious risks even miles from the fire. To help people protect themselves indoors, researchers have developed a low-cost, durable do-it-yourself (DIY) air cleaner. This simple device performs on par with expensive HEPA filters, removing pollutants like wildfire smoke from the air and potentially reducing the spread of airborne diseases.

Richard Corsi, dean of the College of Engineering at the University of California, Davis, is presenting findings from his team’s recent studies on DIY air filtration at the ACS Spring 2025 Digital Meeting, hosted by the American Chemical Society.

The Hidden Danger Indoors

Even when people stay inside during smoky days, they’re still exposed to harmful air. This is because most pollution we breathe actually comes from indoor sources, including pollutants that seep in from outdoors. Historically, indoor air quality has received far less attention than issues like water pollution.

That changed with the arrival of COVID-19.

“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.

A Simple, Affordable Solution

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.

Putting the CR Box to the Test

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.

Real-World Respiratory Protection

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 8 feet (244 centimeters) of the people speaking, the respiratory plume dissipates within seconds at a distance from the air-filtering device.

Designing for the Future

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.

Meeting: ACS Spring 2025

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.

The research was funded by UC Davis.

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