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, 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 buildings 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 set-up 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 set-up, and use his company’s extensive testing gear to see how much particle reduction he got. He actually used a four-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. And then he used his company’s enviable array of analytic technology 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 one-inch thick MERV-13 filter dropped that to 320 feet per minute. But, in a counter-intuitive twist, the four-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. This is all too informal. “This is not an air purifier and I would not recommend estimating CADR. This is 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.”