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?
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.