08/28/2020

How Your Indoor Air Ventilation Affects Coronavirus Spread

17:14 minutes

a red air vent inside a building
Credit: Shutterstock

This story is a part of Science Friday’s coverage on the novel coronavirus, the agent of the disease COVID-19. Listen to experts discuss the spread, outbreak response, and treatment.


This year, back-to-school season comes with some major challenges to keeping students and teachers safe. Recently, New York City Mayor Bill DeBlasio announced a plan to give K-12 classes the option to move outdoors; the idea is that an open space, with a fresh breeze, lessens the chance of spreading the coronavirus.

We’ve been brain-storming, too: What if you could bring the benefits of the outdoors inside, by creating better ventilation in the classrooms, akin to outside winds? What would it take to re-design or modify a typical classroom—not to mention your office building or home?

Most modern buildings ventilate space with 80% recycled indoor air, and 20% of fresh outdoor air, to save on energy costs. But Shelly Miller, professor of mechanical engineering at University of Colorado, Boulder says, “In a pandemic, we don’t care about energy efficiency.” Miller explains that to lower the risk of infection, ideally indoor spaces would be ventilated with 100% outdoor air—but most building HVAC systems aren’t strong enough to handle that. 

Miller joins Jose-Luis Jimenez, professor in the department of chemistry and biochemistry at University of Colorado, Boulder to discuss what we know about the coronavirus, and our indoor air space and how we could build safer, healthier indoor spaces for the future.


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Segment Guests

Shelly Miller

Shelly Miller is a professor of Mechanical Engineering at the University of Colorado Boulder in Boulder, Colorado.

Jose-Luis Jimenez

Jose-Luis Jimenez is a professor in the Department of Chemistry & Biochemistry at the University of Colorado Boulder in Boulder, Colorado.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. It’s back to school season. And of course, this year, that comes with some major challenges to keeping students and teachers safe.

This week, Mayor Bill de Blasio of New York announced a plan to let K through 12 classes be held outdoors. The idea being that an open space with a fresh breeze lessens the chance of spreading the coronavirus. We’ve been thinking, what if you could bring the benefits of the outdoors inside by creating ventilation in the classrooms akin to the wind outside? What would it take to design or modify a typical classroom, not to mention your office building or your home?

Here to tell us what we need to know about the coronavirus and our indoor air space is Dr. Shelly Miller, Professor of Mechanical Engineering at the University of Colorado in Boulder and Dr. Jose Louis Jimenez, Professor in the Department of Chemistry and Biochemistry at the University of Colorado in Boulder. Welcome both of you to Science Friday.

SHELLY MILLER: Thanks for having me.

IRA FLATOW: Let me begin with you, Dr. Jiminez. What have we learned over the last five or six months about how the virus is spreading throughout the air?

JOSE-LOUIS JIMINEZ: Well, as you probably know, it’s a controversial issue. We have been told by the CDC and the WHO that the virus primarily is transferred through touching contaminated surfaces or touching other people and then touching our eyes or our nose, or by these large droplets that come out of people when they talk or they cough or sneeze. And then they may impact us on the eyes, the nostrils, or the mouth.

And then there’s this other way, which would go through the air, through aerosols, in which smaller particles come out at the same time when we talk. But they stay floating on the air and then we may inhale them when we breathe, and that can make people sick. The importance of third route has been controversial, but we think that the evidence is increasing and increasing that it is important.

IRA FLATOW: Dr. Miller, what are we talking about when we say ventilation? Is there an exact term for that phrase?

SHELLY MILLER: It generally means bringing outside air indoors to dilute the air that’s inside, that’s been occupied and possibly contaminated by indoor releases, including an infectious virus. And so it’s done in very different ways depending on the type of space that you’re considering, homes versus commercial buildings versus health care facilities, for example, are all ventilated very differently.

IRA FLATOW: How are most modern indoor spaces ventilated? I’ve always– because I’m an engineering geek myself. I like to look at rooms. I like to look at the ventilation. And I notice there are gratings on the ceiling. There are gratings on the floor. How does the air get ventilated in a room?

SHELLY MILLER: Yes. So most commercial spaces have an air handling unit and a very complex, in many cases, or sometimes a little bit simpler, HVAC system, which is heating, ventilating, and/or air conditioning system. Many spaces need to be either air conditioned and heated to make sure that the indoor space is comfortable. And so what is typically done is some outside air is brought directly indoors to dilute the indoor air.

And then, the indoor air is recirculated so that it can be either heated or cool, depending on what it is what is needed in the indoor space. A typical building is ventilated with only 20% outside air and 80% recirculated air commonly. And here in this pandemic, we’re really recommending 100% outside air, which is very difficult for most building systems to achieve.

IRA FLATOW: Would it be just a question of putting a better filter in the system?

SHELLY MILLER: We do recommend putting a better filter in the system. But we haven’t really designed our building systems, our air handling units to have this kind of capacity. Partly that the system doesn’t have the right size. It might need a bigger fan, it might need also different heating and cooling components.

To bring in all that outside air and then heat it and cool it at the same time expends a ton of energy. And so there is this push-pull issue going on, where we usually try to spend as little energy as we can to heat and cool air in a building but still make it comfortable. And now, in a pandemic, we need a lot of outside air. And we’re saying, oh, well, let’s use the energy that we need to do this right. And then let’s back off when we don’t need so much outside air to save energy.

IRA FLATOW: Dr. Jiminez, is there a good way to visualize how the virus moves through the air in a room?

JOSE-LOUIS JIMINEZ: Yeah. So you have these two ways. The droplets will go in front of the person emitting them. And they will fall to the floor in a couple of meters, or six feet. And then these other aerosols, the better analogy is smoke, like cigarette smoke or vaping smoke. It comes out of the person, but it doesn’t fall to the ground.

It stays in the air and gets diluted. And really depends on how the air is moving in the room. If it’s a room that has an air current, if the windows are open, then it may move very quickly. If it’s a room in which the air is not moving still, then it may accumulate and accumulate over time. And then that room will be more dangerous. We’ll have more smoke, which in this case is just an analogy for the virus.

IRA FLATOW: Now I know that in some well water systems, people have drinking water and they have well water systems, to make sure that there is no bacteria in the water, they install a simple ultraviolet light inside the system. And it kills on contact. It just kills all the bacteria in the water.

Why could we not do that with the air? Put a UV system up in the duct somewhere and just kill the bacteria.

SHELLY MILLER: Oh, yes. We can definitely do that. It is a really effective technology for disinfecting air. You have to make sure it’s designed just like you would for a water system, so that it provides enough contact time with the air and provides enough energy to inactivate the airborne virus. But that is a really useful technology that I think a lot of buildings have been using and are using, especially in this pandemic, to better treat the recirculated air as opposed to increasing a filter.

IRA FLATOW: Have we just solved the whole problem right here, right now?

SHELLY MILLER: Well, we haven’t. Because UV is a technology that’s applicable in certain settings. It’s not applicable in all settings, just like every engineering technology doesn’t work in every setting. And so trying to understand where it will work and where it won’t is a challenge.

IRA FLATOW: Dr. Jiminez, let’s talk a little bit about the theory here. When did we discover that poor ventilation would cause a problem for public health? And why haven’t we tackled this problem earlier?

SHELLY MILLER: Well, I mean, I think this has been known for a long time. And I was actually reading some writings yesterday of Benjamin Franklin, where he was saying that people being in cultures around the city and one of them was sick, and then other people would get sick. So generally, it has been known for a long time.

But as Professor Miller said, there are these tensions. And I think in the last few decades, the tension towards saving energy and the push from climate change and for economic reasons, you know, to ventilate less and recirculate more air has worn over the disease transmission, which brings you in the other direction.

IRA FLATOW: So if you were to design the perfect ventilation system, let’s say you were building a new classroom or a new home or someplace, a mall, where people are going to gather? What would that look like? Would that look like bringing in 100% air?

SHELLY MILLER: So we are moving in new builds to bring in supply air. But we’re applying technology called heat recovery ventilators or energy recovery ventilators as an example of a technology where you can bring in more outside air. But then you can recover some of the energy that you’re losing by expelling out indoor air and exchange it with the outside air. It’s a heat exchanger type technology. And that will allow you to bring in more outside air, actually even filter the air, without expending too much energy.

IRA FLATOW: But as far as the air circulating in the room, I’m looking at a room with duct work in it, from soup to nuts, from the beginning, you have a blank slate of how you would do it.

SHELLY MILLER: So you know, right now we mostly supply air at the ceiling and exhaust air at the ceiling. But there are really important considerations for the airflow patterns that you might have in a room. And that does drive how air is handled.

And so one suggestion, and it’s studied quite often and applied in many situations, is to bring in clean supply outside air at the floor. It rises past the people, which are the main source of contaminants in an environment. And then it is exhausted out the ceiling. So that is a common recommendation.

IRA FLATOW: Dr. Jiminez, some people are worried now that HVACs are spreading the virus indoors, not getting rid of it. Is there any truth to that?

JOSE-LOUIS JIMINEZ: We don’t know, to my knowledge, of any case where this has been shown for these virus. And we don’t think it is very likely. So there are other viruses, like measles, for example, that are very highly contagious. And you see that kind of pattern, what they call long range transmission. This virus, at least most of the time, is less contagious.

And when people get sick, get infected, it’s because they have been talking to someone else closely who has the virus, or when they have been in the same room for a long time. So Professor Miller and I have studied this case of the choir in Washington. A lot of people got infected, but that was after spending 2 and 1/2 hours in a room with low ventilation and singing together. And many other outbreaks are when people were singing in Karaoke or spending a long time in a bar where the music may be loud.

So I mean, we think this virus is not very contagious. And we have to help it infect a little bit. You know, we have to spend a lot of time indoors talking loudly without masks and with a lot of other people. And that’s how a lot of people get infected.

IRA FLATOW: What about a fan blowing in a room, Dr. Miller? Is that mixing of the air helping or hurting? And if I’m concerned, do I want to be by the fan in a restaurant, let’s say?

SHELLY MILLER: Yeah. I am a little bit hesitant to promote that kind of approach. Because we have seen that strong air currents have transported the virus from an asymptomatic infected individual to other individuals in the space. And they have been infected.

And so generally speaking, strong air currents that whip things around and could even suspend the virus and then blow it into people is not a good idea. We do recommend, if you need a little bit more outside air in your space and you’re not getting it, to open a window and put a fan in the window. And if the space is occupied, blow air out. Because if you’re blowing air out, then air will come in to replace it. But do not blow air around in a room.

IRA FLATOW: Dr. Jiminez, why haven’t we heard any guidance from the WHO or the CDC regarding ventilation the way we have with masks and handwashing?

JOSE-LOUIS JIMINEZ: Well, you have. Or you should have. So the WHO has an ad hoc advisory panel in ventilation to which we belong. And we have helped them draft some recommendations for ventilation, which includes some of the things that we have discussed today.

But the problem is nobody is paying attention to them. Because they basically say that transmission of the virus through the air is very difficult. They still take that position.

So then, you know, as Dr. Miller was describing, all these issues with ventilation and adjusting these systems, they are costly. They require effort. They require money. They require work. And unless it is a high priority because people are getting sick that way, these people are not going to pay attention to their recommendations

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. In case you are just joining us, we’re talking about air circulation in buildings and how to make them as safe as possible with Dr. Shelly Miller, Professor of Mechanical Engineering at the University of Colorado in Boulder, Dr. Jose-Louis Jimenez, Professor in the Department of Chemistry and Biochemistry at CU-Boulder.

So I guess the simplest thing to do is have your classroom outside, if possible.

JOSE-LOUIS JIMINEZ: That will be the best thing. And that actually does not depend on any of these debates about whether the virus is going one way or the other. That’s an empirical fact.

There is, for example, a contact tracing study in Japan where they show that it was 19 times more likely, if you talk to someone, that you get infected if you are indoors than if you are outdoors. And there are databases of superspreading events that have more than 1,000 events. And they think there is two outdoors and more than 1,000 indoors.

So it is very, very clear that being outdoors greatly reduces the chance of infection. And that’s what we are recommending, that as many activities as possible should be moved outdoors. And in terms of a silver bullet to prevent infection, I think, of course, you can do a lockdown and then [INAUDIBLE] throughout [INAUDIBLE] or any other way.

But if you want to be a little smarter, if you meet outdoors with the six feet distance and with masks, I would bet that it is really, really difficult to get infected that way, that that’s very safe. And then indoors, there is always more risk. We should use these layers of protection. But you’re always having more risk.

IRA FLATOW: Dr. Miller, you agree?

SHELLY MILLER: Yes. I could add an important point, is that the highest risk is when you’re talking loudly, generating virus containing particles if you’re infected, and close to each other. Because it’s this close contact that drives this transmission. And so if you’re outdoors talking close to each other and talking loudly or talking for a long time, then the risk is higher in those situations, even though you’re outside.

And so I do want people to understand that the close contact for a long time, talking loudly, even outdoors, could possibly put you at higher risk. I mean, it’s much safer outdoors. But move apart. And maybe walk as you’re talking, and face into the wind. And then you might be feeling more comfortable. But at the same time, if you’re going to talk all that much, you might as well wear your mask.

IRA FLATOW: I can see people jostling for the upwind position in a conversation. Have you seen that? I’ve seen that already.

SHELLY MILLER: Oh, I fight for the upwind position on the trail when I’m hiking here in Boulder all the time. But I do have my bandanna on.

IRA FLATOW: I’m guilty of that, also. Since you’re both at the University of Colorado in Boulder, do you anticipate any problems this semester?

SHELLY MILLER: Sure. What we have done is an extensive analysis and overhaul of our building systems. And so we are providing the best ventilation and air cleaning that any university in the country can. And we have incredible social distancing rules and masks.

And on campus, the adherence to our guidance is great. And I cannot imagine that we’re going to have an outbreak on campus. We do have, you know, situations where we might have parties off campus with students who unmask, and close talking loudly.

I haven’t observed one. But it’s common here. So we’re working closely with the landlords in the cities to help us make sure that no transmission off campus also occurs due to our returning students.

IRA FLATOW: Dr. Jiminez, you agree?

JOSE-LOUIS JIMINEZ: Yes. I mean, I think that the campus has been made very safe and CU has listened to Dr. Miller and myself and other experts exceptionally well. And in fact, I always joke that I wish my family listened to what I tell them about preventing contagion as much as my university does.

IRA FLATOW: I’d like to thank our guests, Dr. Shelly Miller, Professor of Mechanical Engineering at the University of Colorado in Boulder, Dr. Jose-Louis Jiminez, Professor in the Department of Chemistry and Biochemistry, also at the University of Colorado in Boulder. Thank you both for taking time to be with us today.

SHELLY MILLER: Thank you for having me. Science Friday is one of my favorite shows.

JOSE-LOUIS JIMINEZ: Same here. Actually, I listen to Science Friday all the time. So I’m proud to be here.

IRA FLATOW: Well, thank you very much. We’re wishing you a safe school year.

SHELLY MILLER: Thank you.

JOSE-LOUIS JIMINEZ: Thank you.

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