Answering Your Questions On Aerosol Movement And Virus Transmission

Stan Bunger
July 21, 2020 - 2:22 pm

    As we continue to navigate these unprecedented times, KCBS Radio is getting the answers to your questions about the coronavirus pandemic. Every morning at 9:20 a.m. Monday-Friday we're doing an "Ask An Expert" segment with a focus on a different aspect of this situation each day.

    Today we’re looking at aerosols and how the virus can be transmitted through the air with Dr. Linsey Marr, professor of civil and environmental engineering and expert in airborne disease transmission at Virginia Tech.

    This is such a hot topic right now and I've asked others involved in aerosols research a variation on the same question: did you ever guess when you started studying this as your life's work, that you'd be in the middle of something this crucial?

    No, I mean I knew that aerosols could be important but I thought it would come down to flu, not a more novel virus like this. And I had no idea it would blow up into what we have going on now.

    I'm interested in asking you about this pre-print paper. I'm sure in your community it was headline news; in our community it takes a little while to digest this stuff. But this is a paper that looked at six patients in hospital rooms in April and air samples were taken, they were analyzed and it seems, if I'm reading this correctly, that what the writers of the paper are arguing is that they were able to grow the coronavirus that causes COVID-19 from those air samples. Do I have it right?

    Yes, I think if you're talking about the paper from the group at the University of Nebraska, they collected air samples near the foot of the patients' bed, so not right by their head, so these must have been small enough aerosols to remain in the air traveling to the end of the bed. And they were able to show that the virus replicated. We've known that the virus is there in the air, but the question has been, can it infect? Can it grow and replicate? And this is really the first evidence showing that.

    And to be clear, can you define the difference between droplets and aerosols?

    Yeah, droplets and aerosols - there's a smooth transition between them. So droplets are bigger, aerosols are smaller. An aerosol is really just a small version of a droplet, it's so small that we can't see it. But you know that when people cough or sneeze or sometimes when they talk - I know when I talk, sometimes these large droplets fly out and get on my computer screen. But for every one of those that we see, there are hundreds to thousands more of the microscopic ones, and we call those aerosols because they can stay floating in the air for minutes to hours and we can breathe them in. In contrast, the large droplets, if I'm in a Zoom meeting sitting at my computer, those fly straight from my mouth and hit the screen or if there's someone there they could hit the person, if it lands in their eyes or their nose or their mouth, it can cause infection. But at the same time there's this cloud of aerosols that we can't see that's coming out, too. 

    Okay, let's get to some questions sent in by our listeners to And we have a lot today, there's obviously a keen interest. Let's start with this one: how far can aerosols travel and remain in the air to be infectious?

    Well, the very smallest ones - and the virus was just shown to be infectious in these smallest ones - can stay floating in the air for hours, during which time they can drift around in the room, they can travel all the way across the room. But as they travel farther, if they get out of that room then they become very dilute. Where if you had maybe 1,000 in the room and let's say half of those get out, well now those are spreading out everywhere else - not to say that they're going everywhere - but they become very dilute. Maybe you end up with one or two in the adjacent room, rather than the original 1,000. 

    And that kinds leads to another question on the list: it seems to me that we'd all feel a bit more in control if we knew, a - how much virus it takes to get sick, b - the rate at which infected people give off viruses and maybe c - the distance those virus particles might travel. Are we close to knowing all that?

    Those are all really good questions. I think we are close, there are studies going on about how much virus people who are infected release into the air. We know from physics, if we know the size of these aerosols or droplets I can tell you how far they travel. How much it takes to get infected? Now that is a big question that we don't know yet. There's a group of us that's digging into the literature, into papers that have been published to try to figure that out.

    But despite all those unknowns, here's what we do know: transmission occurs mainly in these close contact situations or if you're in a poorly ventilated room with a lot of people. It's much less likely to occur outdoors. Masks can help reduce the rate of transmission much more than people initially expected. So there are things we do know and that we can do so that we do have control over the situation.

    My neighbors next door invite their friends and their children over in the afternoon. They stay on their deck, nobody wearing a mask. I live in an attached single family home. Could this affect neighbors next door, maybe through the pipes or vents? How can you protect yourself if this is a problem?

    So if you're next door, I'm guessing the windows are closed and you're worried about virus in the air coming into your residence through pipes and vents or something like that. It depends on how far away. For there to be a reasonable chance the people would have to be sitting right next to your house, having a conversation right at the intake to your air handling system. The farther away, the better. I think if you're 20, 30, 40 feet away there's very low risk. Things are going to really spread out into the atmosphere and it's very unlikely that anything would get into the neighboring house.

    People might feel better if they actually had a number on this sort of dilution effect; is there a way you can quantify that for us? You know, the difference between being in a room of a given size and being outdoors with a breeze?

    The analogy I use is, let's say you have a drop of dye and you put it into a glass of water, that's your indoor situation, versus if you put it into a swimming pool or even a lake, and that's the outdoor situation where the atmosphere is almost unlimited and things will rapidly spread through the atmosphere. Now there's some very small exceptions to that, but in general that's a good way of thinking about it.

    Regarding the COVID outbreaks on cruise ships earlier this year, has it been determined if those outbreaks were caused by aerosols? Is it the closed quarters, or do they get into the air conditioning system? And if so, could that be mitigated?

    That's a good question, there have been a couple of studies on the Diamond Princess outbreak. And the first study by Professor Yuguo Li at the University of Hong Kong concluded by looking at the patterns of transmission and the air conditioning system that it was not spreading through the air conditioning system. They determined that most of the transmission took place in these close contact situations - imagine people at the restaurants or at social events or shows congregating together - or possibly through contaminated objects that we call "fomites." And then after everyone was quarantined to their rooms, there was almost no transmission except between people within the same room. So that suggests that it was not spreading through the air conditioning system from room to room.

    A follow-up study led by Brent Stephens and Joe Allen, just recently published as a pre-print, looked at different transmission modes and they concluded that aerosols are probably responsible for more than half of the transmission events. But what that means is that even in these close contact situations, people in the restaurants or together at shows can be transmitting via aerosols. It doesn't have to be just long-range room to room. So I think aerosols are important, but again it was these close contact situations or people being together in a crowded room that maybe doesn't have good ventilation.

    Considering how much we learn about this virus, I think I could use a refresher: should we be wearing masks while driving a car with the windows up?

    If you're by yourself or with your own household group I would say no, you do not need to wear a mask. If you are in a taxi, Uber or Lyft, something like that, I would definitely wear a mask and have the windows down if at all possible.

    So really the answer is, it's that outside air that's the win here, not so much that you're going to pick it up from someone riding by on a bike or walking.

    Right, that's correct. It's very unlikely you would pick it up by other people outdoors. Again, you're outdoors, you're moving, the air is moving. Things just become so much more diluted outdoors than indoors, it's really not an issue. We really haven't seen almost any transmission event that can be traced to outdoors. There have been a couple of cases like face to face talking for extended periods of time, but other than that I haven't heard of anything.

    Can the intermediate size virus particles be captured by masks with electrostatic capability?

    Yes, those could in theory. But even without the electrostatic capability, a cloth mask can remove those intermediate sized particles. We just completed some preliminary studies showing even a simple cloth mask, even a bandana can remove at least half if not more of these intermediate sized particles.

    Are fans in the hallway helpful, or might they just help spread the virus?

    Fans, that's a tough one. It really depends on the situation. If the fan is going to help bring in more outdoor air then I would say that's good, because it will promote dilution. If the room is closed then the fan is just blowing things around and probably suspending, lifting things up from the floor that otherwise would have stayed there. You also have to think about where are the people relative to the fan? Are there potentially infected people behind the fan or in front of the fan? So it really depends on the situation.

    I am a dentist and we have always had strict disinfecting and sterilization procedures in the dental environment. One of the new protocols is fogging with hypochlorous acid at a concentration of 200 parts per million. Would your speaker care to comment on the research and effectiveness of this procedure?

    I don't know exactly how effective that concentration is, but certainly hypochlorous acid would inactivate virus. But it depends. You have to have a high enough level, you have to have it for a long enough time.

    I guess this begs the question and maybe this is a dumb layman's question, but is there something you can spray in the air to neutralize virus in these aerosols?

    There's hypochlorous acid. There's other things, hydrogen peroxide is used for disinfecting N95 respirators. The problem is that if it's harming the virus, it's also likely harmful to you. So you would need to do that in a way that doesn't result in exposure to any people.

    I usually go into my big Kaiser facility at this time of year for my routine lab work and my mammogram. How safe is it to go in for those procedures during this surge? How safe are elevators? 

    A hospital, well that's where the sick people are so I would do my best to stay away. If I needed to go, I would wear a very good mask. And then hospitals, of course, are used to infection control, I'm sure they're doing everything possible.

    Elevators, so far we think that elevators are not a huge risk because you don't spend a lot of time on there. At the same time, I would try to avoid getting on a crowded elevator and definitely wear a mask on an elevator.

    I read that COVID virus has been detected at high levels in sewage/solid waste and possibly far exceeds the actual number of detected positive cases. How safe is it to use restrooms in public? And hot air hand dryers versus paper towels?

    My group has actually studied emissions from toilets, we were looking into this during the Ebola virus outbreak.

    Were you involved in the toilet plume stuff?

    Some, probably not the recent one but we had some studies about five years ago on toilets. The total amount of aerosols released by those toilets are large in terms of number but really very small in terms of the total volume of droplets and aerosols that come out - smaller than released by talking. So I would say if there's a lid, sure, put it down. But mainly I would wear a mask and avoid talking in the restroom. And wash your hands, of course.

    Naturally. And then the question of the paper towel versus the hot air hand dryer?

    Yeah, hot air hand dryers can blow things around so I would opt for the paper towels or shake your hands off and wipe them on your pants.

    If a COVID-positive patient has an MRI at the hospital, how long does the next patient wait to be scanned and what kind of cleaning should be done in between?

    You know, I don't work in hospitals so I don't know. A more general approach to this would be, you want long enough for the air in that room and the MRI machine to changeover. Hospitals typically have a very high, what we call "air exchange rate" and so at least every 10 minutes the air is changing over. Maybe a few of those air changes, half an hour, should be enough to change everything out. 

    And what role would something like a HEPA air purifier play in these enclosed environments?

    HEPA air purifiers can be very helpful if they have a high flow rate, or what we call a "clean air delivery rate" because the air passes through them, that filter removes almost everything that's in the air and gives you then clean air that's free of virus and other particles that might be in the air. So those can be very helpful. Those will help lower the background level of virus that might be in the air. It doesn't help with the close contact situations because there's that exchange between two people that hasn't had a chance to go through the HEPA air purifier.

    Thinking of how one walks past a person on the sidewalk or in a store aisle, especially if they're not wearing a mask. Do these aerosols tend to travel down? Float up? Travel out? Or all of the above?

    In the absence of any other disturbance they will travel down. The large ones will fall down faster. But usually there's some kind of airflow in the building and plus there's airflow introduced by the people walking, so there's gonna be some horizontal movement close to people. People are warm and so they have what we call a "thermal plume" so very close to the body things kind of rise above us. So it's complicated.

    If aerosols are this dangerous regarding COVID-19 transmission, how come people are not giving importance to eye protection? I ride public transit a lot, how can I better protect my eyes? 

    And I guess that begs the question of whether aerosols and the eyes are an issue.

    I think eye protection is a good idea. We know that for flu, flu can transmit through the eyes. I would say the biggest concern is eye protection in these close contact situations where if someone coughs or is talking right in front of your face and you want to protect those large droplets from flying into your eyes. Aerosols, the small ones are less likely to end up in your eyes. But still I would say it's a good idea.

    Is it dangerous to go to a beach and go swimming?

    No, if you know how to swim.

    (laughs) That may be the single best answer we've heard in months of doing this segment. 

    Again, I would avoid crowds.

    (laughs) Gotcha. 

    If aerosols - that is, sub-micron spread - were a major problem, how could mask wearing make a significant difference? Especially non-N95 masks.

    Yeah, good question. We think of aerosols as being really tiny and that a homemade mask isn't going to do anything against them. But what we have found, and others have found, is that yes, these homemade face coverings, fabric masks are not very good at stopping aerosols smaller than a certain size, let's just say one micron. But for things that are larger than one micron, they actually are decent at blocking those. And it's quite possible that most of the transmission is mediated or occurs via these droplets or aerosols that are larger than one micron. I think they can be helpful and what we've seen in terms of their efficacy with slowing transmission in different countries and different states if they've implemented masks rules is that masks work. They help slow transmission.

    Can you get COVID from touching the outside of your own mask that may have picked up the virus, and then touching your eyes or nose?

    In theory. You'd have to have that virus be on your mask, get onto your mask somehow. It has to survive. When you touch your mask it has to transfer from the fabric to your finger; some of it's going to stay on the mask. And then it has to survive on your finger, there has to be enough on there and then you have to stick that into your eye and it has to transfer from your finger into your eye. And again, there has to be enough to cause infection. So in theory it could happen. I don't know how likely it is. 

    This interview has been edited for clarity.