Air purifiers and Coronavirus - Columbia Professor Q & A

Aerosol scientist Mecneal of Columbia Univ recently answered queries on whether HEPA filters work to purify air of coronavirus.

Q. What’s a HEPA filter and can it catch particles as small as a coronavirus?

A: HEPA is the international gold standard for filtering particles—it means that 99.97 percent of particles 0.3 microns in diameter are trapped. For context, a human hair is 50 microns across. The HEPA standard is more than 100 times smaller because it’s more difficult to trap particles that are smaller or bigger because of the physics of filtration. Masks and filters are not sieves, and snagging a virus is not like panning for gold. Particles either flow past or stick to the fibers in a filter based on their inertia or ability to diffuse toward the fiber.

So yes, HEPA filters can catch particles that contain coronaviruses. People expel droplets of respiratory fluid, saliva, and possibly viruses into the air when breathing, coughing, and talking. Even if the water in the droplet evaporates, the droplets contain salts, proteins, and other material in addition to any virus, which means the remaining particles are typically a few microns in size, making them fairly easy to trap with a HEPA filter.

Q. How important is the EPA’s 0.3 micron HEPA standard, then?

A. HEPA is the gold standard, but filters that aren't HEPA can still help remove particles from indoor air. A homemade air purifier called the “Corsi-Rosenthal Box,” consisting of an HVAC filter and a box fan, are quite effective.

Q. Where’s the best place to put an air purifier?

A. The best place is any room that needs more ventilation—an air purifier is backup support for a space with not enough air flow. I would not push it up against a wall or in a place where airflow is obstructed. Try to put it as close to the middle of the room as possible.

Q. Should you leave the air purifier on when you leave the room? What’s the best setting to use?

A. It depends on the room. If it’s a classroom with a 20-minute break between classes, leave it on, but turn it off at night. The setting depends on the size of the room and the fan’s capacity to pull air through. You should find the right size unit for the room.

Q. Do you use an air purifier at home? What should people look for?

A. Yes, but not for Covid reasons. I have asthma and so do my kids. We live in a dusty old NYC apartment, and the kids seem to breathe a little easier when I run an air purifier in their room. An air purifier consisting of a HEPA filter and fan are all you need. You don’t need ionizers, which create oxidants and particles. It’s counterproductive. If you’re mainly filtering for virus-containing particles, you don’t need PM 2.5 sensors. It’s an energy-saving feature that shuts down the purifier when levels of particles bigger than 2.5 microns in the air go below a certain threshold. But the level is mainly determined by dust and other particle sources, not respiratory aerosols. Go ahead and use the sensors if you’re filtering for pollutants, but you should run the purifier continuously if you’re after virus particles.

Q. How has Covid changed your perspective on particles?

A. I think a lot more about indoor air quality and the critical role of ventilation. It’s hard to believe all the things we used to do indoors without masks.

Q. You’re teaching a required class this fall for chemical engineering majors, “Chemical Process Control.” Do you wear a mask?

A. I teach to a packed classroom, so yes, I wear a mask—a KF94. I’m an aerosol scientist and I’m the person who’s projecting her voice and largely generating the most aerosols in the room. I also have two children at home who aren't fully vaccinated. I talked about it with my students on the first day of class and so far, haven’t found it to be difficult at all. more