How to effectively ventilate classrooms using fans, HEPA filters or Corsi-Rosenthal box fans and monitors

THE IDEA SEEMS pretty simple, once someone explains it. The virus that causes the pandemic disease Covid-19 passes from human to human on tiny droplets of spittle, through the air. Masks block some of them. But what if—and I am literally spitballing here—you could clean those particles from the air itself?

Researchers who study aerosols and indoor air quality have (mostly) (finally) convinced the scientific establishment—the World Health Organization, the US Department of Health and Human Services—that Covid-19 transmission has an airborne component. And now some of those same aerosol specialists have begun to think this ventilation approach is a good idea. A big, complicated central air system filled with filters and maybe even germ-busting ultraviolet light, like what a hospital or skyscraper might have, would be great. A $500 air purifier could make a real difference in pulling infectious bits out of a room before they can infect a person. But it’s possible, some of them speculate, that even a store-bought filter stuck onto a $20 box fan might do some good too. It’s cheap, and while it wouldn’t cut the risk of infection to zero, it would still pull virus-laden particles out of the air. “It hadn’t occurred to me until two days ago, until someone pointed it out,” says José-Luis Jiménez, an aerosol scientist at the University of Colorado, Boulder, “but I think it’s a brilliant idea.”

On a recent Zoom call, environmental engineer Richard Corsi—dean of the Maseeh College of Engineering and Computer Science at Portland State University and an indoor air quality expert—held up a Folgers Coffee box to illustrate for me how a fan at one end, and filters on three or four other sides, could reduce the “pressure drop” that can come from putting a filter in front of a fan and still move (and clean) room-sized volumes of air. “You’ve got me really pumped up on this right now. I don’t have a lot of free time, but this would be something to build a prototype of,” Corsi says. “If it wasn’t for the fact that, you know, we’re still on lockdown, this would be a great fourth-year team project.”

Here’s some of the theory: People infected with the SARS-CoV-2 virus make more virus. It comes out of their mouths and noses when they talk, sing, breathe, or cough, riding inside blobs of spit and snot that range in size from about the diameter of a human hair to ultra-super-teeny-tiny—like, one could fit inside a single pixel in the sensor of a high-end digital camera. Scientists have long distinguished, somewhat arbitrarily, between larger “droplets” that fling outward ballistically and fall to the ground or some other surface after a couple of meters (unless they get into someone’s eyes, nose, or throat first) and teeny “aerosols”' that float around, vaporlike, borne on winds and breezes. And people infected with Covid-19 but without symptoms—common spreaders of the disease—emit those small, floaty particles.

Masks are a good way to keep one’s own self from being a filthy particle-emitter. Nobody wants to be the main character in a super-spreading event, right? But cleaning particles out of the air altogether has seemed like the provenance of building-wide air handling and climate control systems. A hospital HVAC system might cycle all the air in the building a dozen times a day, stale out and fresh in. But what about the rec room of a care home for the elderly, or a public school classroom? Or 50 public school classrooms? Or your house?

$100 Corsi-Rosenthal box in Mrs. Clasen's room at LFP Elem

A little bit of peer-reviewed science says a homebuilt kludge might work. One study, from Singapore and looking not at aerosolized virus but the soot and smoke of Indonesian wildfires, found that a simple filter and fan mounted in a window to pull air inside reduced particulate matter between 1 to 10 microns by around 75 percent. That’s suggestive. The particles in the study were from smoke and soot, but they were well within the size range of the smaller, virus-bearing particles from respiration, which is what counts here.

Other researchers have looked at room-scaled portable air purifiers and their impact on airborne disease. Back in the mid 1980s, researchers at UC Berkeley found that HEPA filters could significantly reduce “respirable particles” in the air of buildings made more airtight by new construction technologies, without as much fresh-air ventilation. In the mid-1990s, another Berkeley team showed similarly good reductions in airborne particles carrying tuberculosis bacteria, with portable and ceiling-mounted filters of HEPA quality and slightly lower effectiveness. (Here’s a more recent run-through of the available technologies and how they might help.)

Conveniently, the Singapore researchers didn’t use HEPA-quality filters. That stands for “high-efficiency particulate air," and they’re great, grabbing 99.97 percent of particles 0.3 microns or larger. (It’ll actually get even smaller particles, too, because of the crazypants physics of wee small things and filter material).

The thing is, HEPA filters are expensive, and so are the modern heating, ventilation, and air-conditioning systems that use them. Top-line room air purifiers cost several hundred dollars. But a cheap DIY purifier may also cut down Covid-19 risk. The Singapore researchers used a MERV-13 filter (“minimum efficiency reporting value,” duh), because perfection wasn’t necessarily the goal. Reduction is. As Penn State architectural engineer William Bahnfleth has shown, while HEPA filters ring the bell at the top of the MERV scale, an idealized MERV-13 filter picks up nearly half of 0.1-micron-diameter particles, and pretty much everything 1 micron and above. That’s the right size to get the particles carrying the virus too.

Home Depot sells 1-inch thick, 20- by 20-inch MERV-13 filters for about $10 each; Best Buy sells a 20-by-20 box fan for $26.99. Combine them (or the equivalent from some other store) and you have a homebrew filtration fan. Jim Rosenthal, CEO of Tex-Air Filters, a filter manufacturer in Texas, suggests putting the filter on the inflow side of the fan, the side that’s sucking in air, as opposed to the side blowing it out. (The arrow on the filter—you’ll see it—should be pointing at the fan). Room air gets sucked into the filter first, and the fan blows clean air out. That’s “pull-through” instead of “blow-through,” the way residential AC works. Rosenthal says that in the DIY version, that lets the pressure pull the filter tight against the fan. You’ll still probably need tape—choose between gaffer and duct according to your own personal belief system. Either way, air gaps are the enemy here.

If you don’t have cross-ventilation, you can just set the box-fan/filter in the middle of the room. “This type of setup is especially useful for situations in which, basically, you don’t have access to outdoor air. Like, there are schools where you can’t open windows because of shooters or the geometry of the building,” Jiménez says. “That’s where this kind of thing is genius. The one in my kid’s classroom costs around $350, but you can make something like this that would do something similar for $50. I’d have to see how much air it cleaned, but you’re talking about a fifth or a tenth of the cost.”

That’s a good question. How much air is cleaned? Portable air purifiers get described in terms of a number called the"clean air delivery rate" (CADR), a combination of the efficiency of the filter in pulling gunk out of the air and the speed of air pushed through the system. CADR conveys that in cubic feet per minute, and clean-building researchers now recommend five complete change-outs of the air in a classroom every hour. So ideally, you’d use the cubic footage of the room and the CADR of the filter setup to figure out how big a purifier, or how many, you need. (CADR isn’t the be-all/end-all, either. Its calculations assume that a room is what researchers call a “well-mixed box,” which is to say, homogeneously mixed air and pollutants throughout. That’s not how things work with pathogens in the real world.)

When I talked to Rosenthal about all this, he graciously offered to go out, buy a 20-inch box fan, slam together a setup, and use his company’s extensive testing gear to see how much particle reduction he got. He actually used a 4-inch-thick filter (more expensive, but more filtration, and you don’t have to change them as often).

Rosenthal put his fan/filter in the Tex-Air Filters tool room, next to the company’s factory—he wanted a massive particle count. Then he used his company’s enviable array of analytic technologies to take some readings. Running the fan only reduced the overall number of 0.3-micron particles by about 25 percent. But remember, that’s not the relevant range. Viruses are reeeeeal small, but in the air they’re riding inside bigger—but still quite teensy—dessicated balls of protein and salt. The ones hanging around as airborne infectious problems are anywhere from 1 to 50 microns, and in Rosenthal’s informal test, particles between 1 and 10 microns took a huge hit. His rig sucked about 60 percent of the 1-micron particles out of the air, and nearly 90 percent of the 10-micron ones.

Of course, that only gives you single-pass removal efficiency, not volumetric airflow through the device. Rosenthal didn’t have that, but he did use his anemometer—not, as you might suspect, a device for measuring sea anemones, but a windmill-like airspeed sensor—on the outflow. Without the filter, the fan’s medium setting gave him 780 feet per minute. A 1-inch thick MERV-13 filter dropped that to 320 feet per minute. But, in a counter-intuitive twist, the 4-inch thick filter yielded an airspeed of 460 feet per minute. That’s because the fan generates positive pressure, a push. “The positive pressure is distributed over the area of the filter. The face area of the filter is one thing; the media area of the filter is another,” Rosenthal says. “If you have a pleated filter, you have substantially more media area, so the pressure is distributed over the greater media area, and consequently you have greater air flow.”

In other words: A thicker filter gives you more filtration and better airflow, letting you refresh the room as if more outside air was getting in to replace whatever possibly virus-laden particles are there.

Rosenthal wouldn’t venture to calculate a CADR, though, for such an informal device. “It's just a high-efficiency filter on a box fan,” he says.

This isn’t science yet. These aren’t peer-reviewed studies. This hasn’t been tested under ideal conditions multiple times. No one has run these stats. Caveat experimenter. Still, though—seems worth it. Right? “Sure, absolutely. It’s an air purifier. It’s not a HEPA air purifier, but it’s a reasonably good air purifier,” Rosenthal says. “If that’s the only option, I’d rather see people do that than have nothing.”

Keep in mind, this is all with a basic filter literally rubber-banded to the most basic fan. It seems likely that someone could enhance the effect. Jim Wells, a retired economist in Maryland who has been tinkering on fan-filter builds with his physicist brother, has a setup with two units to circulate air through the room. Why not? They’re cheap.

Physics poses some obstacles here. A fan trying to push air through a thick medium like filter-stuff (or pull it through) can start to overheat, or get too noisy for a church choir rehearsal or a classroom. (I built one with a HEPA filter a couple years back to deal with smoke from a California fire season, and I didn’t think either of these were much of a problem.) The Wellses attached a step-down transformer to their fans to reduce the standard 120-volt power supply to just 100 volts. That slowed and quieted them. They put a 5-inch MERV-13 filter on the front (not the back, as Rosenthal does; there is in this matter some conflict), and then measured levels of particulates 2.5 microns or larger—that’s “PM2.5”—in micrograms per cubic meter. “Good air, fresh air, in Annapolis runs 5 to 7,” John Wells says. “In a 10-foot by 12-foot bedroom, I got it down to 0.01 on the meter.” That’s obviously not peer-reviewed proof over time, but it’s a real difference.

“There’s a lot of attention on portable air cleaners now for classrooms. They might be 650 or 700 square feet, 25 kids in them. Oftentimes they’re under-ventilated. In these cases, portable air cleaners might drop the particle levels in air by 50 percent. I like the idea of the makeshift one, because some school districts are so poor they just don’t have the money to buy 1,000 portable air cleaners.”

Home marijuana growers, surely you’ve come up with some clever air-purification and handling tech. Makers, furloughed engineers, tinkerers, where you at? Remember how people wanted to build hospital-grade ventilators, and that turned out to be way too complicated? This isn’t. The components here are basic—the electric motor at the heart of a fan, maybe stepped down with a transformer to not blow so hard, fan blades, a metal frame to hold a panel of filter media, or a bigger box with panels on three or four sides, and a mount for the fan parts. That’s it. Make sure it all clips together with a good seal, and it’ll do … something.

We shouldn’t have to, of course. The money should be there for schools to be well-ventilated and have room for kids to be physically distant. They should be able to afford air purifiers, or have them provided. People shouldn’t risk their lives to go to work. But even if we wish it were otherwise, as a president said once, it is what it is. All of us have to do something, because if no one else is coming to help, we’re it.

By Adam Rogers