These DIY air purifiers could make clean air more accessible during wildfires

As wildfires have raged across the country this spring, smoke plumes from scorched structures can pose a major health threat. Sneaky particles can creep through vents and other home openings, but buying high-quality HEPA filters can be costly and inaccessible for many.

That’s why Richard Corsi, the dean of the College of Engineering at the University of California at Davis, and Jim Rosenthal, the chief executive of Tex-Air Filters, created their do-it-yourself filter. Known as the Corsi-Rosenthal Box, the cost-effective air filter was originally created to filter particles containing infectious viruses in indoor spaces.

According to three studies presented by Corsi at the American Chemical Society, the Corsi-Rosenthal Box fared significantly better than commercial HEPA-based air cleaners at trapping pathogens, and performed better at containing particles generated by wildfires.

“We’re doing this for far less cost, which allows many more people to actually have access to clean air,” Corsi said. The boxes cost roughly $85 for its materials. By contrast, comparable HEPA-based air purifiers can cost hundreds of dollars.

Wildfire smoke contains a slew of harmful fine particulates, known as PM2.5, which are 30 times smaller than a single human hair and can easily infiltrate the heart and lungs. According to the Environmental Protection Agency, PM2.5 “is the air pollutant of greatest concern to public health from wildfire smoke.”

As climate-change-fueled wildfires increase the number of incidents and places that experience low-air-quality days, the box could make cleaner air accessible to vulnerable populations.

“Underserved communities and marginalized communities oftentimes live in housing that tends to be leakier,” Corsi said, “so there’s going to be a higher exposure to wildfire smoke.” He said more than 1,000 boxes with a single filter were given out to shelters, senior homes, and schools across Los Angeles after the recent wildfires.

The materials for the box are simple: a 20-inch box fan for the top, four common household filters for the sides, cardboard, scissors and duct tape. The fan pulls in air through the box’s filtered sides and pushes out clear air through the fan on top.

The key to the box’s success are the filters. Boxes with a MERV-13 rating are able to trap a range of particles, but wildfire smoke particles are smaller than virus particles, dust, and mold spores, making them a bit harder to catch in the filter. Even so, research shows the Corsi-Rosenthal Box performs better than other, more expensive air purifiers.

MERV-13 filters have a lower filtration efficiency than HEPA filters, but the box makes up for that deficit with its ability to suck in higher rates of air than other air cleaners, trapping more particles and producing cleaner air faster.

In one study published last July, researchers tested the boxes in 53 classrooms across Arizona and Connecticut for six months. They found that PM2.5 concentrations decreased over 43% on average and that PM10 concentrations were reduced by nearly 31% with the airflow setting on low. Corsi hypothesized that with airflow set at high, fine particulate matter concentrations could be reduced by up to 75%.

Children who sat in classrooms with the Corsi-Rosenthal Box experienced cleaner indoor air environments, said Megan Jehn, the study’s lead author.

“Adding a filter to the space increased the amount of time that kids spent, over a cumulative amount of time, in good air quality as opposed to moderate or unhealthy air quality,” said Jehn, a professor at Arizona State University’s School of Human Evolution and Social Change. Jehn now has a grant with the Environmental Protection Agency to mitigate indoor air exposure from wildfire smoke using the Corsi-Rosenthal Box.

The boxes do lose some effectiveness over time, with filtering efficiency diminishing over 30% for wildfire smoke particles, with slight variation depending on the type of MERV-13 filter used for the box, over 2,500 hours. Even at that point, the researchers say, the box operates better than a HEPA filter, which needs to be replaced every six months to a year.

Marina Creed, a nurse practitioner at the University of Connecticut Health’s Multiple Sclerosis Center, said she became a believer in the Corsi-Rosenthal box after seeing a social media post about it in 2021. She then created a universitywide program to test the efficiency of the boxes.

“Having clean air has long been associated with being expensive,” said Creed, who encouraged an entire school district in Connecticut to use Corsi-Rosenthal boxes and was recently awarded more than $11 million by the state to give filters to classrooms all over. “I felt very strongly about this prototype as a very significant intervention because it lowers the price for access to clean air.”