This transcript has been edited for clarity.
Eric Topol, MD: Hello this is Eric Topol, and I'm really pleased to welcome Dr Linsey Marr, who is the Charles Lunsford Professor of Civil and Environmental Engineering at Virginia Tech. She's been a leader in the pandemic and well before this on interactions of viruses and nanomaterials in the atmosphere. So welcome, Linsey.
Linsey Marr, PhD: Thank you so much for having me. I'm really excited to be here.
Topol: You have been a phenom, and I've learned so much from you during the pandemic. I am so impressed. I thought we would start with the COVID-is-airborne story was blown up for quite some time. You have been on it from the get-go. You are the lead person I follow to learn about this field. Can you review what went off the track and if or how we are going to get back on it?
Marr: We were off track to begin with because the default assumption in medicine is that respiratory pathogens — colds, flus, etc. — were transmitted by large droplets that people coughed out, and either sprayed and landed in someone else's eyes, nose, or mouth. Or maybe they landed on a surface or someone's hands, which became contaminated. People would touch those surfaces and then transfer the pathogen — the virus or bacterium — into their eyes, nose, or mouth.
That was the default assumption for several decades, leading up to this. There were some exceptions to the rule for diseases such as measles and tuberculosis, but otherwise, people just assumed that these types of diseases were transmitted by large droplets.
When COVID-19 (SARS-CoV-2) came about, that was still the assumption. I remember an article in The New York Times published in 2020 asking six key questions about how bad this will get.
One key question was how far does the virus travel in air? And experts said, "Well, the novel coronavirus doesn't travel more than 6 ft, so we don't have to worry about it. In contrast, measles and chickenpox viruses can travel hundreds of feet."
I've been studying flu transmission for 12-13 years now, and I knew the beliefs about SARS-CoV-2 were inaccurate, but nobody listened to what some other aerosol scientists and other experts, including myself, were saying. It didn't matter that much when we had only a background level of disease. But then all of a sudden, it really mattered.
Topol: And it was, of course, not just the World Health Organization but the Centers for Disease Control and Prevention (CDC) who was reluctant to accept data that this was not a droplet story, but rather the virus was truly aerosolized. Is this just a reflection of an unwillingness to be receptive to new ideas?
You knew about this for quite some time. You and your colleagues, and other leading scientists in this space (which the medical community hadn't covered adequately) were clamoring about the need to update our views, but there was great resistance to that, right?
Marr: It was in textbooks that this is how diseases are transmitted, so that's what medical students learned and took to be true. It's hard to change people's minds when they've learned something from a textbook.
There was also additional resistance to the idea of airborne diseases because in healthcare and hospitals, that term carries a lot of weight in terms of precautions required for healthcare workers and the facility. If a disease is "airborne," that means that everyone has to wear N95s. The patient is supposed to be in a negative pressure room called an airborne infection isolation room. These things are resource intensive. There is some reluctance to do that unless you really have to. Furthermore, even if they did acknowledge that the disease was airborne, there simply are not enough resources, not enough of these types of facilities in the world to accommodate the number of patients who would have to be in those types of rooms. That said, it wouldn't have to be all or nothing because if you know that the disease is airborne, then greater use of N95s would have cut down on the transmission of infection. If healthcare facilities said, "Okay, this is airborne," many other things would be necessary to protect healthcare workers.
Topol: That's a great point. I hadn't realized the implications for medical facilities and how that would feed into the resistance. In general, wholesale changes about a concept in medicine lead to reluctance. It's like a sacred cow, and it's hard to debunk.
That brings us to masks because you've already touched on the N95. And I also want to talk about filtration, ventilation, humidity and other features, but first, just as there was a lack of receptivity about the airborne aspect, there's also been tremendous backlash about masks and people saying, "Where's the proof? And I looked at it like, do we need proof for parachutes to know they work?
Obviously, there have been a lot of studies. And every study that comes out is taken apart by anti-mask people. Can you help us understand? Do you really need — for a respiratory virus that's airborne — proof that masks work? There have been simulation studies. What constitutes proof and do we need it?
Marr: The proof you need is to find out whether the mask reduces the amount of virus that they expel into the air? On the other side, the proof you would need is to find out whether they protect people from getting infected by breathing in the virus from the air around them. Do masks reduce the amount of virus that they breathe in enough to reduce their risk for infection? That is the type of proof you need.
There have been calls for randomized controlled trials. Those are really hard to conduct in the proper way when you're trying to look at masking. It's hard for people to wear them all the time, but that's not the question you're asking. You're asking, "Does it reduce your risk?" Not, "Does it completely prevent transmission," but does it reduce your risk for transmission?
There are plenty of other analogies — parachutes and seatbelts — where there are physical mechanisms to explain how the equipment is protective. Do we need to put ourselves through a randomized controlled trial of masks to show that? No. In some cases, it's unethical. In other cases, it's simply not needed.
That's some of the background on resistance to masks. Of course, it really originated with the shortage of supplies for healthcare workers, which led even the head of the Centers for Disease Control and Prevention (CDC) at the time to tell the general public not to buy masks. It would have been better for them to be honest and just say, "There's a shortage for healthcare workers. Please use what you can (cloth masks, etc.)," but the message shouldn't have been that masks don't work.
There's another aspect of this about N95s and the way that our country regulates respiratory protection and the way that the Occupational Safety and Health Administration (OSHA) regulates them. If workers require respiratory protection, it has to be a fit-tested N95 to show that it's not leaking and that it is doing its job.
The attitude was "fit-tested N95 or bust." If it's not a fit-tested N95, then it is worthless. So that was the attitude, even though we know that even if it's not perfectly fit-tested, an N95 is still going to provide some protection, probably a lot more than a flimsy cloth mask.
Topol: Let's say you're wearing a KN95 or N95, which a lot of people are still doing. If you're among people who aren't wearing masks, like on a plane or in an airport or other public transport, how much protection does that give you?
It's obviously not 100%, especially, as you say, because most of these people in the public don't have fitted masks. Many do their best to make sure that they have a tight fit, but it's not perfect by any means. Can you give us a sense about two-way and one-way masking and how much we can expect to get from that in terms of reducing the risk, not just of COVID, but perhaps other respiratory viruses?
Marr: Let's say that 100 virus particles are in the air that you breathe in over the course of your plane flight. They're floating around in the air. Now, if you're wearing a mask that's, let's say, 90% efficient at trapping those virus particles, instead of breathing in 100, you're going to breathe in only 10 of them because 90% were trapped by the mask. You can still get sick with 10, depending on the virus and your immune system, but you've reduced your exposure from 100 down to 10. That has to be helpful in reducing your risk.
Now, if everyone is wearing masks, then those 100 viruses wouldn't be there. Let's say that the person who was spewing them into the air is now wearing a mask. And so instead of releasing 100 virus particles when unmasked, they now only release 10 because they are masked. So now 10 viruses are floating around in the air that you are breathing in.
You're wearing your mask, which is 90% efficient, and so now instead of breathing in 10, you're breathing in just one particle. This is how two-way masking helps. It reduces the amount that gets into the air, and it helps reduce the amount that you breathe in from the air around you. That will work for any virus that's in the air, not just SARS-CoV-2, but also flu virus and most respiratory viruses.
People talk about the size of the virus particle itself, but what really matters is the size of the respiratory droplet or respiratory aerosol that carries the virus because we don't have naked viruses floating around out there in the air. They come out in our tiny droplets of respiratory fluids. Some of those evaporate, but there's all this other kind of respiratory goo with the virus.
Topol: What I find interesting is when I suggested that you get a "threefer" by wearing a KN95 or N95 to protect from the tripledemic, people write back on Twitter or social media, "Where's the proof? Where's the proof?"
You would think it would help. Do you need proof? I guess that's one question. You come at it from a different discipline, from engineering. And engineering may not be rife with randomized trials. You depend on different types of standards for proof which you just reviewed. So could you comment about why is there such questioning, if indeed these are aerosolized viruses?
Masks can't hurt, right? They have to help to some degree. And by the way, I love that explanation of the one-way and two-way masking and how that helps, but I just don't get the continued, questioning resistance. Maybe people feel they can't breathe well when they have a mask on tightly or they feel claustrophobic, but comment about this issue because it's just never-ending.
Marr: Yeah. With respiratory syncytial virus (RSV), the default assumption is that these diseases are transmitted by large droplets. They're not airborne, so you don't need a high-quality mask. And some of the thinking about RSV may come from one particular study that's well-known, where they had nurses cuddling with infants who were infected in a hospital. And then they had other people at a distance. They found that the cuddlers were most likely to become infected themselves when cuddling with the infants who had RSV.
So it was presumed that RSV was transmitted by large droplets or close contact, but you have to remember that when you are close to someone, you're also exposed to much higher amounts of any viruses they may be releasing in the air. If that person's smoking — and we know babies don't smoke — but if a person smoking and you're close to them, you're going to be exposed and breathe in a lot more cigarette smoke than if you're farther away.
To me, that study was inconclusive in terms of transmission route because when you are close to an infected person, there are a number of ways for the virus to transmit. People have presumed that RSV is not airborne, but in the cuddler study, it very well could have been.
There was another study where they sampled the air. They found infectious RSV, not just the RNA, the genomic material but actually infectious virus in the air. I think that was in some kind of healthcare facility, where very small particles would stay aloft, floating around in the air for many minutes to hours.
I've been collaborating with a team of experts, virologists, and epidemiologists. We're looking at daycare centers, and we find RSV in the air more often than we find it on surfaces. We need a paradigm shift. It would probably be more correct that the default assumption is that these respiratory pathogens are transmitted through the air. They are respiratory, so it doesn't seem like it should be that big a leap, but it's hard to change things.
For a long time, people thought that the sun revolved around Earth. It took a real effort, and it was painful, and people thought Galileo was crazy for suggesting that Earth revolved around the sun.
Topol: That's well put. Now, there are modifiers in our space, particularly indoors, where humidity is a factor. You posted earlier about pH and other factors that have to do with ventilation and filtration. Can you give us the skinny on what we can do? What are the optimum or the worst scenarios for infections in indoor spaces?
Marr: One way of thinking about it — because we can't see these things in air — is you have a glass of water, and it's dirty. Are you going to drink that? No. You're going to treat that water, do something to remove the dirt and other pathogens that might be in there before you drink it. You want it to be nice and clear.
That's how we should be thinking about our air. If there's lots of infected people around, they can be releasing virus into the air. The air can be contaminated. There are several different ways to clean the air. One is through ventilation, Imagine being in a sealed room where there is no ventilation, and people are smoking. The smoke just builds up over time. If you open the windows or use your mechanical HVAC system, you can blow some of that smoke out and bring in cleaner outdoor air to replace it. You're diluting it, really, just flushing it out. So that's one way.
Another way is to run better filters in the room. You can buy a portable HEPA air cleaner for a bedroom that passes the air through a very high-quality filter and removes almost all particles from the air, not just viruses, but other things. People with asthma and other sensitivities to particle air pollution can benefit from this. That would be like taking your glass of water and pouring it through a great filter and getting it to be nice clean water.
Other techniques are used in hospitals, for example, germicidal ultraviolet (UV) radiation, which doesn't physically remove the virus from the air, but it will actually kill them off. That's not something that people are going to have in their homes, but hospitals have it. It would be a good idea for crowded bars and restaurants to have that type of treatment too.
Topol: If we could have a magic wand and fix our schools and buildings, which are not well-equipped to handle the issue of filtration, what would we do to increase our protection, not just in the current pandemic, but for future respiratory viruses that inevitably we're going to see?
Marr: That's a great question. One of the first things we should be doing is ensuring that our existing mechanical HVAC systems are working the way they should be. I know it sounds very basic and boring, but we have these HVAC systems that were designed and installed 20 or 30 years ago and have barely been maintained.
They should be checked every few years. And they're probably, in many cases, not being checked to make sure that they are operating as designed. We should do that first. There are ways to adjust the settings to bring in more outdoor air or to run it through more filters. We talk about ventilation rates in terms of air changes per hour. So one air exchange per hour means you take all the air out of a classroom, for example, and replace it with cleaner outdoor air once per hour.
If you do that three times per hour, that means once every 20 minutes or so, you are exchanging that air out. And so things that might build up in the air are being removed. I've been on several committees, and we've recommended aiming for six air exchanges per hour, which is the minimum in hospitals, in patient areas.
That means that six times every hour, or once every 10 minutes, you are flushing that air out. You can see how that would really reduce the amount of virus and other pollutants that can build up in the air over time. That's one basic recommendation.
It's going to cost money, and there are energy considerations too because that's going to affect heating and cooling costs, but there are many benefits to be gained here, not just for the current pandemic, but also for the other respiratory pathogens that we talked about: flus, colds, RSV, and even other types of air pollution that people are exposed to and one of the leading causes of death around the world.
Exposure to particles in the air is estimated to cause 7%-8% of premature deaths per year. A lot of that happens indoors. Some happens outdoors too, but cleaner indoor air can help with that. This comes down to not just cardiovascular disease. Absences and asthma attacks and academic performance are all linked to air quality.
We're getting a little farther afield here. We're not just talking about viruses and respiratory infections, but if you are trying to attack those, there's a lot of co-benefits to having cleaner indoor air.
I'm really glad you brought up these bonus factors because it's impressive. We've underestimated the effects of air pollution on health. There have been some recent important studies on that, just as you mentioned.
Now, what about humidifiers? What if we enhance the humidity in indoor space? Does that help much?
Marr: The evidence suggests that it could. It's not a sure thing. My group really specializes in trying to understand how well viruses survive in indoor air under different humidity conditions.
And it's a fascinating topic for me, of course, because now we're talking about looking at an individual aerosol particle and what's going on as the water evaporates. You mentioned the pH can change. Other things with the respiratory goo can change. And so that can affect how well the virus survives.
Our results are not conflicting, but we've had a lot of different types of results out that we haven't put completely together. I will say though that there's overwhelming evidence that these types of viruses survive very well at very low humidity. So we need to think about the indoor humidity.
During the winter, we heat our air. Maybe not so much in San Diego, but in the rest of the country, we heat our air, and the indoor humidity ends up being 10% or 20% in some cases. That is the perfect type of condition for these viruses to survive a long time. So, if you can increase that humidity up to 40% or 50%, the viruses don't survive as well under those conditions. That might help explain some of the seasonality of these respiratory viruses. Why do we see them more in the winter? I'm sure many factors can explain that, but this could be one of them.
These are laboratory studies. There are other epidemiological studies looking at relationships between the number of people who are getting sick and what the weather is like. And they find, also, this relationship where if you take your indoor wintertime humidity and you increase it a little bit, you'll get less transmission.
Finally, one small pilot study was done in Minnesota, where they humidified some classrooms. They found fewer cases of influenza-like illnesses in those classrooms and less virus in the air and in surface samples in that classroom. That was only two classrooms, but I think it offers a hint of or suggests a direction that we should go for further study because if you keep the humidity higher, maybe that will help reduce transmission in your household or in these other environments.
Other factors too, not just the ability of the virus to survive, but our immune response also seems to be better when the air is a little more humid rather than just bone dry.
Topol: Another critical point there. You touched on this earlier, but it's such an important practical point. If you can smell the cigarette smoke or the perfume someone is wearing, that's aerosol content. We have basically said, "Don't worry about outdoor transmission. You can't transmit these respiratory viruses outdoors." Can you get us straight on that question?
Marr: I think transmission can happen outdoors. I've seen some pretty good evidence of it. I would say that it is mainly occurring when people are in close conversation and more than just hello, but standing there, talking with someone for a few minutes.
That's not surprising because as I mentioned, using the cigarette smoke analogy, even if you're outdoors, if you're close to a smoker, right in front of them, you're going to be breathing that smoke, but you also know that if you're farther away, that rapidly dilutes in the atmosphere. I like to say if you're indoors, think about putting a drop of dye in a glass of water — it can't go very far — vs putting a drop of dye into the ocean. That's like outdoors. If you happen to be right near that drop in the ocean, you are going to be exposed, but if you're a little farther away, there's almost nothing because it just becomes very diluted.
I'm sure transmission is happening outdoors on occasion, but the risk of it is much, much lower outdoors than indoors because of this huge dilution that happens in outdoor air. If that's something people are concerned about, I would say, avoid the close, extended, face-to-face conversations.
I suppose it could also happen if you're at a restaurant outdoors, and there's a table upwind of you, and someone's infected there, and you're sitting there for an hour. Somebody is a super-emitter there. They're releasing a lot. And maybe if you're right downwind of them, you could end up being exposed and catching the disease. But I'm sure it's much, much rarer than indoors.
Topol: I know that point. Our son got COVID from outdoor restaurant where he had dinner with a woman for over an hour, where they were face-to-face, talking. It can happen, but that was extended contact.
We've reviewed some really important material here that's not just practical but emphasizes the science, the work that you do, which is really quite extraordinary. And it never has become more important. This, to me, exemplifies why when you have such a crisis that we faced for now a few years, you need transdisciplinary expertise. We don't do that enough. You are, I think, one of the best explainers I've had the chance to meet over the course of the pandemic, not just today, but, of course, on your posts. I really want to encourage people, if they want to keep up with the latest science in this domain, that Dr Marr is the go-to person in my view. I don't know anyone who's more lucid in explaining and also keeping up with the ever-changing new papers that are getting published in this space.
Any further comments that you want to make to the medical community before we wrap up, Linsey? Obviously, we've only scratched the surface. We've touched on some key practical issues and tried to put this in perspective, but I'm sure we've missed some things that you think are worth emphasizing.
Marr: All the research that I've done with my collaborators over the past decade plus has been very transdisciplinary and would not have happened without the cooperation and collaboration of virologists, medical doctors, and epidemiologists. Let's do more of that. We need to respect each other's fields and expertise and see how we can contribute to help each other understand our own fields better.
Topol: Absolutely. That deserves the utmost highlighting because had we listened more to people of your expertise, we might have gotten ahead of this pandemic earlier.
Thank you so much for joining us today. We'll continue to follow you. You're a gem, and thanks for giving the time to us to be explaining critical aspects that are still not emphasized enough. Hopefully, someday, they'll sink in, and we won't keep demanding randomized trials for masks for respiratory viruses. Thanks, Dr Marr.
Marr: Thanks so much for having me. It's been a real pleasure.
All images: Linsey Marr, PhD
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Cite this: To Stop COVID, We Must Clean Up Our Air - Medscape - Dec 22, 2022.