How can air filtration reduce the risk of COVID-19 spreading in your home?
CSE professor and aerosol expert Chris Hogan weighs in
When College of Science and Engineering professor Chris Hogan began studying aerosols in 2003, the majority of researchers in the field were focused on how the small particles impact outdoor air. But with the COVID-19 pandemic, that has begun to change.
“We also have a lot of new, very accomplished people moving into aerosol science,” said Hogan, a faculty member in the Department of Mechanical Engineering. “I never tried to explain what I do to my neighbors or friends or anything, but now they all kind of know.”
Aerosols are known to spread the virus through the air. For the past six months, Hogan, who is also editor-in-chief of the Journal of Aerosol Science, has been studying the impact of recirculating air purifiers on virus aerosols and how they disperse throughout indoor spaces like offices.
We sat down with him via Zoom to learn more about his field—and how we can continue staying safe this winter.
Why are air circulation and filtration important in reducing the spread of COVID-19?
When we say air circulation, we typically mean ventilation, and ventilation doesn't just mean a fan mixing the air around. It means bringing new air into a space and taking the air that we've been inhaling and exhaling out as quickly as possible. And this is important, not just for COVID but for human health in general, because it keeps the CO2 levels low. But if there's anything infectious that can be spread—through what is called an aerosol route, which just means that the particles or droplets expelled are small enough to stay in the air—we need to clear those out. And the quicker we can do it, the more we can mitigate risk of infection. Bringing clean air in and taking air out is where filtration comes in. [Filters] will collect particles because the particles really can't pass through them. They just stick and stay there.
What are some strategies we can use to mitigate COVID in our homes?
The number one strategy is to not bring people into your home because minimizing face-to-face contact is the best way to stop the spread of COVID-19. But, you can try to increase the air change rate in your home, which is the number of times per hour that the air in a room is refreshed. A good modern office will probably have around five, maybe upwards of seven, air changes per hour. Most homes do have some sort of recirculating air system for ventilation, but probably not a great level of control on it, and most homes just don't have the pump capacity to install high efficiency filters directly in the air handling system.
If you're in a situation where you have someone in your home who you're not commonly exposed to, giving that air a good path to enter and exit by opening multiple windows will give you a really high ventilation rate, actually.
It’s hard to beat that. Of course, this is harder to do in the winter, and you may want to look into getting a recirculating air purifier to increase the ventilation rate in a certain room.
How effective are recirculating air purifiers in rooms?
The technology that goes into them is usually very efficient in removing particles and droplets from an aerosol. In general, HEPA filters, or High Efficiency Particulate Air filters, have to undergo certain testing to even have that rating, which means that they are 99.97% efficient or better. With a recirculating air purification system of a high enough flow rate, without leaks and with a good filter, you can increase the air changes per hour in the room you place it in.
Four air change rates per hour means 98% of the particles originally in the room are removed over that hour, so that’s pretty good. If you can get to that level with the air purifier, that helps, and that should be almost additive with the natural air change rate. Where it wouldn't help is if you were in a big open space. What will help there is exploiting that big open space to keep your distance from the person and then minimizing exposure time. The volume there is so big that [using air purifiers] is a drop in the bucket. It’s not going to do very much.
What research have you done on aerosols and air filtration specifically as it applies to COVID-19?
I’m working with Bernard Olson, who is a staff scientist in mechanical engineering, and Dr. Montserrat Torremorell in the [U of M College of Veterinary Medicine] whose specialty is swine influenza and other respiratory viruses that can infect pigs. We have devised a test where we aerosolize different coronaviruses. So, not SARS CoV-2, but coronaviruses that infect pigs or cows and generally cause mild to asymptomatic infections.
We devised a wind tunnel unit in which you can put one of these air purification devices, expose it to virus-laden aerosols, and then look at what got through—and of what did get through, how much of that is still active and viable.
Other technologies, like ultraviolet (UV) light for example, don't just collect particles but inactivate them. All the particles get through, but the particles are no longer infectious after proper UV exposure. So, we look at both—did the technology collect particles, and did it inactivate viruses. I want to give Bernard a lot of credit for designing the system, and Dr. Torremorell’s lab group is really excellent about doing these viability studies of aerosol samples to see how many viable viruses are leftover, which is not easy.
You’re also working with the Mayo Clinic’s Well Living Lab. What are you studying there?
The Well Living Lab is interested in understanding the potential for infection spread by air as well as through surfaces in office settings, first and foremost. We’ve been working with them to devise an aerosol dispersion test where we simulate someone breathing. A lot of HVAC design is more about air change rates per hour. I've said nothing about the way the room is constructed, but that of course is going to play a role.
You could imagine there are spots in a room where particles preferentially deposit. So, we're devising tests to look at that, and to see if there ways of designing the office to eliminate infection.
Can we better design our dwelling spaces to prevent infection or teach people how to even stand relative to one another? The 6-foot rule gets debated, but the point is that it’s a rule. It’s certainly better than the one-foot rule or the no-foot rule. But can we come up with better rules that are backed by particle transport physics, and that’s kind of where we’re going.
Interview by Olivia Hultgren
If you'd like to support this work and other COVID-19-related activities, visit the CSE Response Fund.