Corsi-Rosenthal Box: How to Make an Air Filter With a Box Fan

Indoor air quality is important, since Americans spend most of their lives inside. A good filtration system can help protect you and your family from allergens, indoor pollutants like smoke, and viruses like COVID-19. But quality air purifiers, like HEPA machines, can be pricey.

They don’t have to be, though. Richard Corsi, PhD, of the University of California-Davis and Jim Rosenthal, CEO of Tex-Air Filters, have created a type of homemade air filter that costs less than a third of the price of many HEPA devices.

Some early research suggests these DIY air cleaners, made from cardboard, tape, furnace filters, and a box fan, might be more effective in some ways than HEPA purifiers. Experts say they could offer a simple solution to improve air quality in our homes, classrooms, and workplaces, which aren’t always well-ventilated.

“One night, I just scribbled out this idea on a piece of paper,” says Corsi, who is dean of the College of Engineering at the University of California-Davis. “It wasn't an original idea – people had put a single filter on a box fan before. But not with MERV 13 filters.” (MERV, or minimum efficiency reporting values, is a measure of how well a filter captures particles. The higher the MERV, the more effective the filter.)

Once Rosenthal heard about Corsi’s device, he built the first one. Just like that, the Corsi-Rosenthal box was born.

Share Share on Facebook Share on Twitter Share on Pinterest A Corsi-Rosenthal box is a do-it-yourself air purifier made with 4 MERV 13 air filters and a box fan. Photo Credit: Festucarubra / Wikipedia

How Do Corsi-Rosenthal Boxes Work?

Four filters and a cardboard base make up the box, which is sealed by tape and topped by a fan. The fan pulls the air in through the sides of the box and through the filters, then blows it out into the room.

At first, researchers weren’t sure why this box seemed to clean faster than a high-efficiency particulate air (HEPA) filter. In a single pass of air, a HEPA filter can catch a lot more particles than a MERV 13 filter. Yet in tests, “the Corsi-Rosenthal box outperformed the HEPA filter on every sized particle,” Rosenthal says.

Experts have a theory about why. “You have such high flow from these fans, way more flow than you get out of a commercial HEPA filter,” Rosenthal says. “The air goes through the filter more than once in the same period of time. It may only get 50% of the particles at first. But the second time through, it's getting much more.”

How Effective Are Corsi-Rosenthal Boxes?

One study looked at how well Corsi-Rosenthal boxes removed chemicals from the air in an academic building at Brown University. In the 2-month period when the boxes were used, concentrations of certain PFAS (chemicals found in cleaning products and textiles) went down by 40%-60%. At the same time, concentrations of phthalates (from personal-care products and building materials) went down 30%-60%.

Another small study involving 2 rooms found that the clean air delivery rate (CADR) of the Corsi-Rosenthal box was significantly better than that of the HEPA filter it was compared to. That means it could clean the air in the rooms much more quickly. The researchers also said the “cost per air unit cleaned” was about a tenth of that of a HEPA air cleaner. (Corsi was one of the researchers involved in this study.)

“It’s really quite impressive compared to a lot of the commercial filters on the market,” says Chris Cappa, PhD, a UC-Davis professor and researcher who worked with Corsi to study how well the Corsi-Rosenthal box performed. ”Especially given the price point.”

According to the Environmental Protection Agency (EPA), the best ways to improve indoor air quality are to remove sources of pollutants (like cleaning chemicals or cigarette smoke) and ventilate your space with outdoor air.

The EPA says a portable air cleaner like a HEPA purifier or Corsi-Rosenthal box could help supplement these efforts. But the agency notes that we need more research into homemade air cleaners, which aren’t performance-tested like those you can buy. For that reason, it doesn’t recommend DIY purifiers as a long-term solution.

Experts note that while indoor air cleaners can cut down on viruses that can spread through the air, they’re not enough to protect people from COVID-19. Wearing masks and practicing social distancing are still the best ways to reduce the spread of those viruses.

How Can You Make a Corsi-Rosenthal Box?

You can buy everything you need for a Corsi-Rosenthal box at a hardware store. You’ll need:

• 4 MERV 13 filters
• Duct tape
• A 20-inch box fan
• A cardboard box (you can use the one the fan came in)
• Scissors

“The cost of the materials is less than $100,” Rosenthal says. “The idea is to make it simple. If you can tape a box to ship UPS, you can make one of these.”

Petri Kalliomäki, DSc, a postdoctoral student at the University of Maryland who studies ventilation, is part of a team that has helped students build many Corsi-Rosenthal boxes to use in their dorms. “Everyone has a slightly different technique, but they’re mostly the same,” he says.

Here’s how it works:

• First, make the sides of the box by taping the four filters together. Make sure the air intake direction of each filter is inward. Arrows on the filters show you which way the air flows.
• Cut a piece of cardboard to serve as the base. Tape it to the bottom of the box formed by the filters.
• Place the fan on top, positioning it so air will be blown outward. Securely tape it to form the top of the box. If your fan has rounded edges that keep it from lining up with the sides, cut small wedges of cardboard and tape them over the gaps.
• To prevent backflow through the fan’s edges, cut more cardboard wedges and tape them to create “shrouds” for the fan’s corners. (Or use more tape to make the shrouds.) Leave only the circle created by the fan blades exposed.

In every step, use plenty of tape to seal the edges. “Make sure that there are no leakages,” Kalliomäki says.

Overall, the process takes around 15 minutes.

How to Build a Corsi-Rosenthal Box Richard Corsi, PhD, dean of the UC Davis College of Engineering, explains how to build a do-it-yourself air purifier using a box fan. Share

How Long Does a Corsi-Rosenthal Box Last?

You can leave your Corsi-Rosenthal boxes running as long as you like. But the longer you run them, the more often you’ll need to switch out the filters.

“The amount of time that you can run a Corsi-Rosenthal box depends so much on the environment,” says Corsi. “How often do you switch it on? Are you switching it on just 8 hours a day in your office or in a school classroom? Or are you running it 24 hours at high speed? And how dirty is the environment that the box is in?

“If it's in a house and you're running it 24 hours a day, and you have three dogs with fur that's getting all over it, or if there's just a lot of dirt on the floor, you’ll have to replace them more frequently.”

In a relatively clean environment where you run the device only during the day, you can probably get about 15 months out of the filters. Even when they start to look soiled, there may be some life left in them. When in doubt, check recommendations for the brand of filters you used.

“I've heard people say that they put them in their homes, and within 3 months they’re gunky,” says Corsi. “That doesn't necessarily mean the filters need to be changed. But people think they look dirty enough to be switched. They could have probably gone for 4 or 5 months.”

In certain situations, such as if you’ve had wildfires in your areas, you’ll need to change filters more often.

This is even simpler than building the device. “Keep the base, keep the shroud, and just replace the filters,” Rosenthal says. “Cut the filters off and put new ones on. It takes 10 minutes.”

How Loud Is the Corsi-Rosenthal Box?

At about 51 dBA (a measure of how sound affects the human ear), the boxes are quieter than the average refrigerator. But they’re noisier than some air purifiers you can buy, at least when you turn the fan up high.

“They tend to be a little bit louder than commercial HEPA filters,” says Kalliomäki. “But it's kind of like white background noise. It might be a good thing, depending on the environment.”