Eric Schildge is terrified to go back to school in a few weeks. An 8th grade English teacher in Newburyport, the fear of in-person learning during a pandemic is familiar to him. He was in the classroom all last year. He remembers being upbeat and positive around the students but secretly scared.

Schildge has channeled his fear into learning everything he can about clean air and ventilation for his classroom. He went back and forth on Twitter with renowned experts. When waiting to pick up his daughter from daycare, he’d pull up studies online.

“Reading tiny little words on the screen of my phone, trying to just squeeze in a few minutes that I had here and there to try to find out as much information as I could,” Schildge said.

He was looking for something simple and effective — something he could do himself.

It was that same goal that got Don Blair excited about a DIY air purifier, one that’s designed by a renowned air quality expert but can be built by a total amateur. Blair, a citizen scientist in the Boston area, has been spending his spare time pulling together the resources and pro tips teachers like Schildge need to build this homemade air purifier for their classrooms. (GBH News had experts vet Blair’s open-source designs, step-by-step guide and the tips on his website, and they gave it their blessing.)

“It looks like a janky box that has four sides made out of these standard air filters. And the top of the box is a 20-inch box fan,” said Blair, standing next to the mini-fridge-sized air purifier which is held together with colorful duct tape.

“It probably takes about 10 or 20 minutes really to just assemble these things and tape them up,” he said. “And if you do goof up, then no problem, it’ll take you 30 minutes.”

The materials cost roughly $120, and the box should last an entire school year.

The idea is simple: “The fan on top is blowing up and that means it's sucking air in through the filters on the sides,” explained Blair, a former physics grad student and an open-source enthusiast. As the air is pulled through the filters, the vast majority of virus-laden particles are removed.

The original idea for the DIY air filter comes from a man named Richard Corsi, the incoming dean of engineering at the University of California, Davis.

One day, relatively early in the pandemic, Corsi was brainstorming how to make highly effective (but also highly expensive) HEPA-based portable air cleaners accessible to marginalized communities. He had a thought of how to make a good air purifier at home.

“I just threw out this idea on Twitter,” Corsi recalls. “Within 24 hours of suggesting it, Jim Rosenthal in Texas had built this beautiful-looking box with a fan attached.”

The idea took off. Rosenthal, the owner of Tex-Air Filters, and Corsi settled on an official name ‘The Corsi-Rosenthal Box.’

Corsi has had a 31-year academic career, with many millions of dollars in research and tons of publications. And still, he says, this simple idea is perhaps the most significant thing he’s done. He says a lot of people have been testing it and getting really good results.

“People are now reporting 600 cubic feet per minute in clean air delivery rates. That’s phenomenal. That’s actually better than a lot of the more expensive HEPA-based portable air cleaners,” he said.

That means a lot since Corsi is a big advocate for HEPA-based air filters, which you can get at your local hardware store or online then just plug in at your desk or in your classroom. The only issue is that the price tag is often well over $500.

When Corsi thinks about schools starting up this fall, he says he worries about the school districts that can’t afford HEPA filters, as well as those being sold a bill of goods.

“We have almost no information about a lot of technologies that are being heavily marketed to school districts right now. They haven't been rigorously tested and reviewed in peer-reviewed literature,” he said. “There's lots of reasons to be skeptical of their ability to be effective.”

But with this DIY device, Corsi said, “We have a real winner.”

While Eric Schildge’s school district has bought HEPA filters, he says the delta variant and the lack of a mask mandate in schools has meant his anxiety has never been higher. He’s sent the instructions to his daughter’s daycare and had the idea of getting teachers together at his school to build Corsi-Rosenthal boxes before school starts.

“Just sitting in somebody's classroom, putting together five of them and putting them in every one of the rooms in our grade,” Schildge mused.

Corsi himself loves that idea and says he’s been imagining high school shop classes building these boxes for their schools and their communities.


Both Blair and Corsi have a few suggestions for teachers, parents or others who are thinking about building these air filters:

The Filters
Air filters are categorized by Minimum Efficiency Reporting Value, or MERV. A higher MERV number means the filter is better at removing smaller contaminants. For COVID, the ideal filter is MERV 13 or higher.

These filters are easy to get online or in a store, but they are the most expensive component of the device. So, Corsi said, you could save money by putting filters on three sides of the box and just cover the fourth side with cardboard. Or you could use the somewhat cheaper MERV 12 filter. It will still be good, just not quite as good at filtering out the particles that carry the virus.

And, Blair said, make sure you tape the filters together with the correct side in and the correct side facing out. There’s usually an arrow on the edge of the filter showing which way the air should be flowing. For this box, you want the air to flow into the box through the filters and then out the fan on top — so arrows on the filter should be facing in.

The Fan
The fan should be oriented such that the air is blowing straight up, out of the box. But something wacky happens with the air direction in the corners of a box fan. To make sure that air doesn’t flow the wrong direction, it’s best to cut a circular hole in a square piece of cardboard such that the fan blades are visible but the corners of the box fan are covered by the cardboard and prevent air from flowing the wrong direction.

The Duct Tape
Air will find the easiest way in, so make sure that this cube is well-sealed around the edges. That way, the air is forced to go through the filters and get cleaned.

The Classroom Placement
There are a few places in a classroom to avoid placing the box: don’t put it in the corner and or right next to a wall. And if there’s an open window where air is flowing out, don’t put the box right near the window because then you’ll lose lots of the clean air.

Corsi also said you can put multiple boxes in a classroom and it has an additive effect.

“If [the classroom] didn’t have windows that were operable, I would probably try to put two boxes diagonally apart in the classroom,” he suggested.

Also, he said, make sure that the box isn’t in a location where it will get jostled by students rushing past since that could send some of the particles that have collected on the filter back into the air.

The Lifespan And Disposal
Corsi thinks that, because the air is spread out over multiple filters, this device could last an entire school year. But being conservative, he says, he might build a new device every six months.

It’s also important to be careful when disposing of the old filters.

“Just to be extra cautious, make sure the person is wearing a decent mask. And when they remove the filters and bag them in a big plastic bag and toss them into a dumpster, wash your hands afterwards. Just to be safe that you're not resuspending any particles from that filter that might have recently deposited with viruses on them,” Corsi said.