Do-it-yourself box filter clears the air of indoor pollutants

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.