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Linsey Marr is an environmental engineer at Virginia Tech who studies the behavior of tiny particles in the air, including pollutants and viruses.
As the COVID-19 crisis deepened, she found her chosen line of work relocated from a corner of academia to center stage of governmental, media and public interest. One of only a handful of experts on viral transmission, she was sought after for interviews by new outlets including PBS NewsHour, MSNBC and The New York Times
Marr has a bachelor’s degree from Harvard, a doctorate from the University of California-Berkeley, and postdoctoral training at the Massachusetts Institute of Technology. She was a Fulbright Scholar in 2017-18; received the Virginia Tech Ut Prosim Scholar Award, the highest honor for faculty, in 2021; and received the State Council of Higher Education’s Outstanding Faculty Award in 2022. She serves on the National Academy of Sciences, Engineering and Medicine’s Board on Environmental Studies and Toxicology.
Reporter Randy Walker recently interviewed Marr in Virginia Tech’s sustainable nanotechnology lab in Kelly Hall about her work with COVID-19, and what she’s focusing on next.
Marr: I come at this from the angle of the subfield of environmental engineering, which is in civil engineering, and environmental engineers specialize in understanding and tracking and measuring pollutants in the environment. And that can be the indoor environment or the outdoor environment. And one of the most important types of pollutants in the atmosphere are particles. They’re responsible for an estimated seven million premature deaths per year. And really, a virus in the air is a particle in the air. And so all that knowledge and expertise that environmental engineers apply to particulate air pollution, a lot of that also applies to viruses that are in the air.
And so I think we bring a very different perspective than biologists and medical people in the medical field because we understand how these things move when they’re in the environment, I mean indoor or outdoors. Like, a biologist or medical person just knows that the virus somehow gets from me to you. But I can actually tell you, well, I released it in particles that were 2 microns in size. Those came out of my mouth at high speed when I coughed or when I was talking, but they pretty quickly slowed. And then they were just carried by the air currents. I know that even though the air feels still in this room, there’s continuous movement of air at a certain speed in different patterns. And I know that this 2-micron particle, which carries the virus in it, and also contains salt and proteins and other things from our respiratory fluids — think of mucus — it can float around in the air, and it can reach the area of your breathing zone, and you can breathe it in and it could take many minutes for that to happen. And it’s not going to fall to the ground quickly within 6 feet, like we were told. [But] that 6-foot rule does apply to those really large, wet droplets that you can see.
Cardinal News: What’s the difference between a droplet and an aerosol? And which one is the vector for COVID?
Marr: Droplets we typically think of as being large wet things that we can see. If someone coughs or sneezes, that’s the stuff you can see. And so aerosol particles are tiny, they’re too small to see. And really, for every one droplet that you see, there’s hundreds of tiny aerosol particles that are microscopic that you can’t see. And they float in the air and they can carry virus around. And we know that those carry virus, we know that the virus in those can be infectious, and that other people can breathe them in. And we’ve got all the evidence now for that. It’s been shown.
These large wet droplets could also carry virus, but they’re not going to go very far. They could, if you’re close enough, maybe by chance it could land in your eyes or in your nostril or on your lips. And in theory, you could become infected that way, although that really hasn’t been shown. They could also fall onto the table or something, contaminate a surface. And you could potentially touch that and transmit it into your body that way too. But really the easiest way for transmission to occur is just we’re sharing the air. Like [if I were] a smoker, I’d puff out my smoke plume and you happen to breathe that in. That’s very easy. That happens all the time.
Cardinal: So aerosols are really the way that COVID is transmitted most often.
Marr: We don’t have exact numbers on it. But I’d say it’s likely that aerosol particles are the way that COVID is most commonly transmitted. And another way to think about aerosol particles is cigarette smoke particles, because they’re very similar in size. And they’ll behave in the same way, in that they come out of someone’s mouth. And there’s this kind of dense plume of smoke when you’re close to the person. And then farther away, it’s more diluted. But if you’re in a small room with no ventilation, that smoke is going to build up over time, and everyone in that room will end up being exposed to it and breathing it in.
Cardinal: If somebody’s standing over there and they’re smoking, within 30 seconds I can smell it. It travels.
Marr: It travels, right, exactly. And viruses can do that, too. They can travel fast. And our noses are very sensitive to the smell of some of the gases in smoke. So even if you can smell that person’s cigarette smoke, it doesn’t mean, oh my gosh, I’m going to get COVID if they have COVID. But it’s an indicator that yeah, it’s possible if they happened to be a super shedder or the type of person who’s infected and releases a lot of virus, then yeah, it’s possible, if you think about the cigarette smoke analogy.
Cardinal: Just one virus particle, if I were to inhale it, that’s not necessarily enough to trigger COVID — or is it?
Marr: Maybe not. But there was a study in the UK where they actually deliberately exposed volunteers, who were compensated, to the virus. And so they expose those people to about 10 to 50 individual virus particles, and half of those people became sick.
Cardinal: Even with that few particles.
Marr: Yes, even with that few, right.
Cardinal: Wow. So it doesn’t take a big viral load.
Marr: No, it doesn’t take a ton, obviously, from that experiment, at least.
Cardinal: You’re not wearing a mask. Why?
Marr: The risk level, or the community level, according to CDC, is green in this area. And I feel like we’ve reached that point where there’s enough population immunity, the current circulating variants seem like they are still risky if you’re older or immunocompromised, but I think for healthy, vaccinated people, it’s a manageable risk that’s probably not bigger than flu and other diseases that we live with routinely.
Cardinal: How well do KN95 masks work?
Marr: If they fit your face well and they kind of seal around the edges, they will block at least 95% of particles from the air. So you’re breathing in 95% less. So let’s say there’s 100 virus particles in the air that you might breathe in. If you’re wearing a good KN95, the amount that you’re really actually going to breathe in is less than five. So that helps. But again, it has to be kind of a genuine one — because there’s a lot of fakes out there — so that the material it’s made out of is a really good filter. And the second really important point is that it fits you well and doesn’t have leaks around your nose or the sides of the cheeks or around the chin, because if leaks, then particles just go through those gaps and they’re not being filtered out.
Cardinal: Is a KN95 much less effective than an N95?
Marr: A genuine KN95 should be made out of similar materials as an N95. I would say the main difference is that an N95 usually has straps that go around your head, rather than your ears. And so you can get a tighter fit with an N95 with those straps that go around your head. But some people don’t like that. So the KN95 is a good alternative.
Cardinal: You work with a wide range of nanoparticles in this lab. Is a virus a nanoparticle?
Marr: We think about viruses as one type of nanoparticle because a nanoparticle refers to something that’s smaller than 100 nanometers in size. Typically, when we think of nanoparticles, we think of engineered things that are like carbon soccer balls, or we think about metal nanoparticles like silver nanoparticles or gold nanoparticles. But how these different particles behave in the environment, a lot of it depends on their size, and viruses are the same size. So we do a lot of our virus work here, use some of the same tools. We only work with viruses that are not harmful to humans in this lab. And then the work with flu virus and SARS-CoV-2, which causes COVID-19, that takes place in other laboratories that have very special biosafety precautions.
Cardinal: In those labs, what does it take to prevent you and your students from getting sick?
Marr: First of all, if there’s a vaccine for it, like for flu, the people working with it have to be vaccinated. They are wearing gloves, usually double gloves, lab coats, and very good masks like N95s when they’re working with flu, and it’s all taking place inside what we call a biosafety cabinet, which is like a fume hood, where all the air is being sucked into it, so nothing can escape from it. And then it gets goes through HEPA filters and then it’s exhausted to the air. So everything gets removed, there’s no escape.
Plus any procedures that potentially can generate aerosols also take place inside their own chamber or glove box or something inside that biosafety cabinet, to have multiple layers of protection. And then, working with the COVID virus, there’s additional things where the researchers are not just wearing a mask, but they’re actually wearing this special hood, and it has a little battery-powered pump unit. It’s delivering filtered air to the people. There’s very specialized procedures for entering and leaving this space, the whole room is under negative pressure also, to keep things from escaping. So there’s a lot of steps that we take to protect people.
Cardinal: Can you break it down to the individual number of viral particles you might be studying in a particular experiment?
Marr: So in one experiment, we might use something that looks like an eyedropper, and drip out a drop of liquid that contains the virus. And so that might be one microliter of fluid. And let’s say that when we grew the virus, we know we had 100,000 virus particles per milliliter. We grow the viruses ourselves, flu and COVID. So we have to have certain special cells to grow them in, because the virus grows or replicates by infecting a cell, and then making lots of copies of itself and then bursting out.
We’d have like 100 virus particles in this droplet because it’s one microliter, 100,000 per milliliter in the stock that we grew. And so we might kind of drip out this small droplet that contains the virus.
So we might put it in this chamber where we hold it at a certain humidity. And then maybe half an hour later, or an hour later, four hours later, eight hours later, or 24 hours later, we go back, and we pull the sample out. And then we see, how much of those original 100 virus particles, how many of those are still infectious, versus how many died? So that’s one type of experiment.
Another type that we’re wrapping up right now is, again, in a chamber, we actually spray the coronavirus. And then we have masks that we’re pulling the air through. And so we’re contaminating the masks intentionally. That type of mask [KN95] and cloth masks and N95s and surgical masks. So we contaminate it as if you were breathing in the virus from the air. We can put the mask into some liquid and see how much infectious virus do we find on there. How much of it actually got on there and survived.
The other thing we’ve done is to take artificial skin and push it against the mask, like if you’re touching it with your fingers, and then we analyze the skin to see if any virus transferred to the skin. So this is to address the question of, let’s say I’ve worn my mask in an area where there’s lots of virus around, there’s virus on the mask. When I’m taking off the mask, if I touch the mask, can I infect myself? So that’s another experiment we’ve done.
Cardinal: This KN95 mask is probably a couple months old. Should I throw it away?
Marr: They last a long time, I think it’s going to keep working. The straps will break first, or it’ll get dirty.
Cardinal: Are there viral particles that are stuck to this thing that are still alive?
Marr: They wouldn’t be alive anymore, no. So we expose the mask to an amount of virus that would be like if you were in a hospital room with an infected patient for half an hour or an hour. Let’s say you’ve got that much virus on your mask. We take the mask, and we set it aside for an hour, we were not able to get any infectious virus off of the mask, even after an hour, for surgical masks and N95 type masks. With cloth masks, we did recover some.
Cardinal: So these particles, once they’re free of their hosts, they don’t live very long.
Marr: Well, it really depends, because they did survive on the cloth masks. And we know that if you just drip some of the virus onto a plastic surface, it can survive for many hours, you can still get an infectious virus off there after days. But in those types of experiments, I think they’re putting in really an unrealistically large amount of virus onto the surface. So you have to be careful when you’re interpreting those results.
Cardinal: At the beginning of COVID, everybody was swabbing down the surfaces. I didn’t worry about that as much as being face to face or in a room with someone.
Marr: You were correct.
Cardinal: Did we really need to shut down society to the extent that we did, where people didn’t come to the office?
Marr: With the shutdowns, that’s kind of a question that’s on the policy side, which is outside my area of expertise. But I think one of the main drivers for that was to protect the health care system to make sure that our health care system was not overwhelmed. Because if the health care system becomes overwhelmed, then I think society does end up shutting down, because then it’s not just the people with COVID, but it’s the people who go to the hospital for other things, and they can’t even get the health care they need. And then it would be complete chaos.
I understand there’s lots of politics around this and disagreements. I would like to think that we can all agree that we should at least try to protect our health care system, and so to do what we need to in order to keep our health care system functioning, and not allowing that to break down. And, what we saw from New York City early on suggested that there was the risk of health care systems across the country to break down, and we need to avoid that. And so that led to the shutdowns. But the details of that, and like, whether we should have done that in a different way, that’s kind of a policy question that’s outside my area of expertise.
Cardinal: Are you working on anything now that you’re really excited about?
Marr: I’m really excited about our big flu project where we are working in child care centers in Michigan with collaborators at the University of Michigan and virologists at Emory University. So we’re doing experiments where we’re in these child care centers, collecting swabs and surveys from kids and teachers in there and trying to see when they’re sick, and then we’re also at the same time collecting air samples and surface samples and trying to see if the virus is there. And we’re kind of establishing a baseline right now. And then next season, next year, we hope to go in there and put in some of these interventions. Like we increase the ventilation or we put portable air filtration units in the rooms and we see if there’s fewer infections and less virus than we find in our [baseline] samples.
Cardinal: Is there such a thing as an affordable filtration unit, just plop it down and it’s going to suck air through and pull viruses out?
Marr: So these HEPA air filter units are great. Those filters will remove viruses very effectively. The reason is that viruses are just another type of particle in the air. And those HEPA filters remove 99.97% of particles in the air, at least that fraction of particles. And so they will take the viruses out of the air, they’ll be trapped on the filter, they’re really not going to come back off very easily. So if you’ve got a good portable HEPA air cleaner in your room or office, and it’s the correct size for the space — it needs to be big enough and move enough air through there — then yeah, it can be very effective at removing virus from the air.
The thing to watch out for is making sure it’s the right size for your space, because they make very small ones that are maybe effective for a closet, that costs under $50. But really, if you want something for the size of a bedroom, you’re going to have to spend $200 to $300. And if you’re in a bigger room, and you might want to get multiple ones of those around the room.
Cardinal: Is one of your goals to figure out how we can reduce the transmission of viruses?
Marr: Yeah, absolutely.
Cardinal: In 20 years from now, how is it going to be possible to achieve that?
Marr: I think a lot of it we already know — by increasing ventilation, using more filtration and UV treatment of air. I think what we don’t know is … how much they will help. Or how much ventilation or filtration or UV do I have to add? And how much reduction in transmission will we see? Will it be 50%? Will it be 80%? And so putting some numbers on those things is where we’re going. I work with many collaborators, and we’re looking at this question for flu of, well, if we increase the ventilation by a factor of 10 in a building, so we’re bringing in 10 times more outdoor air, then, how many fewer people might get infected? And then repeating that with adding filtration in there. And we do a combination of experiments, and then also physics-based modeling to kind of come at it from multiple angles so we can understand what’s going on.
We’re also really interested in humidity and like why colds and flus are seasonal. It seems like these types of viruses survive pretty well when the humidity is really low, like below 40%. And so in the wintertime, when we heat the air, we end up with pretty low humidities indoors, maybe around 20%. The types of viruses that we’re talking about survive pretty well under those conditions. And then they also survive when it’s really, really humid, over 90%. We don’t see that much — maybe when it’s raining. But it’s in this middle range, kind of 40% to 60%, where they don’t survive as well. So maybe if we humidify the air in the wintertime, we might reduce transmission. So that’s another big question that we’re looking at. I really want to try to understand the seasonality of infections, and then, is there something we can do about it to reduce those.
Cardinal: When an office is being built nowadays, why can’t it be built with some kind of airflow that would reduce risk of transmission of viruses? Is this something that engineers and designers are thinking about?
Marr: I would say we know how to do it, the knowledge is there, at least among scientists and engineers who do research. In terms of putting that into practice, it’s just hasn’t been a priority. I think cost and thermal comfort are priorities. And so we have not really put into practice all the tools that we have in terms of ventilation and airflow that could really help reduce the risk of transmission of COVID-19, flu, colds and other things.
Cardinal: What kind of excitement or reward do you personally get out of being at the cutting edge of viral transmission, which became a really big thing with COVID?
Marr: When I first started this research, I thought it was really important. But very few people in the world were looking into it, especially from the kind of engineering and aerosol science perspective. But I thought that it would be really important if there were a pandemic. I thought it would be flu, not coronavirus, but yeah, it’s — I don’t want to say satisfying because it’s terrible what’s happened — but I’m glad that I had developed the expertise over the past 10 years or more to be able to help address some of these really critical questions that have come up over the past few years.