American Scientist

Linsey Marr is a Virginia Tech professor of civil and environmental engineering who focuses on airborne disease transmission. Marr has been featured in the New York Times, The Atlantic, and Wired, and in 2018 she co-wrote an op-ed for the Washington Post on avoiding the flu. Kaisen Lin recently defended his PhD in virus aerosol survival at Virginia Tech. They spoke with Cassandra Hockman, who is a science writer and researcher with the Center for Rhetoric in Society at Virginia Tech. Hockman works with the Sigma Xi (publisher of American Scientist) Virginia Tech chapter and Virginia Tech’s Center for Communicating Science to produce a Science on Tap event series (which was founded and is coorganized by American Scientist digital features editor, Katie L. Burke).

Briefly, what is a virus?

Marr: A virus is a small particle, about 10 times smaller than bacteria, that occupies this gray area between whether it's alive or dead. Viruses cannot survive on their own; they need to infect a host cell that can be from humans, other animals, other bacteria, or even plants. Viruses hijack host cells' machinery to make lots of copies of themselves that then burst out and spread.

Do viruses survive in air and on surfaces?

Marr: Once they're released from the host into air or in feces and saliva, some are able to survive in the environment for long periods of time—hours or days—but some of them fall apart right away and don't last.

How do different surfaces and even different size droplets affect their survival? 

Marr:Our collaborators at the University of Pittsburgh at Seema Lakdawala’s lab have looked at flu virus on different types of surfaces such as copper, steel, and different types of plastics. That virus survives for a long time on steel and plastic and not as well on copper, which is known to be an antimicrobial type of surface.  

Lin: In terms of how different sizes of droplet will matter, it's mainly because there are components in the droplet, such as salts, that inactivate the virus. Droplets will eventually evaporate in the environment. During this process, this component can get concentrated, and the larger droplets basically have a higher initial amount of this component, so the concentrated component will cause more virus decay.

What changes if you wipe a counter down with soap or a chemical such as alcohol?

Marr: Different chemicals break down the virus. The flu virus and the coronavirus have a lipid envelope, which is a fatty layer membrane around them. Soaps, detergents, and alcohol are really good at destroying that envelope. They physically tear apart the virus, so that it can no longer infect cells.

What is known so far about the SARS-CoV-2 virus in terms of its transmission and spread in droplets or on surfaces?

Marr: There was a study that was just published in the New England Journal of Medicine, where they took the virus in the laboratory and they generated these aerosols into a chamber, and they found that there were very small aerosols that can stay floating around in air for hours, or more. It survives for at least three hours during that period, with some of them dying off so they fade away over time.

The virus is able to survive under those conditions, at around room temperature but a higher humidity level than what we typically find, like 65 percent.

And then they also put large droplets, almost like a small puddle, onto different types of materials such as steel, plastic, and copper, and they found that the virus would slowly decay over several hours.

Do we know what dispersal might look like in an enclosed space?

Marr: They should disperse like any other particles, like air pollution particles, dust particles, or cigarette smoke. I've been describing it as like when you see someone smoking and you see that plume dispersing, that's what happens with viruses in the air too. Those that are in the larger droplets, that are large enough to see, are going to fall to the ground pretty rapidly, but there are definitely ones that can stay suspended and floating around.

If our research pans out, then raising the humidity to more of that sweet spot, above 40 percent and below 70 percent, could have an impact on transmission.

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Are there specific locations where we should be more careful, such as in a bathroom where there's a hand dryer that could disperse aerosols?

Marr: You generally want to avoid small, confined spaces because concentrations levels can build up in there. Just like if there's a smoker around and they're in a small space, you're going to be exposed to a lot more cigarette smoke. As for hand dryers in the bathroom, they can blow things around, which can help or hurt, depending on what the exact situation is. If you're using the hand dryer, you probably just washed your hands, so I wouldn't be concerned about someone blowing it around using the hand dryer. It could be a concern if you have someone who's sick, and standing there coughing and breathing heavily at the hand dryer while they're drying their hands. Then the motion of the dryer could spread it around in the bathroom. But at the same time, it also means there's less of a concentrated cloud near the dryer, so the next person who comes along might be exposed to less than they would otherwise. So it's complicated.

What do we know so far about how the warmer temperatures and upcoming summer months might affect spread and transmission?

Marr:It’s a complicated question. In warmer temperatures in general, viruses decay more. In a lot of places, though, we maintain our indoor temperatures at a pretty constant level, but even a few degrees warmer in the summertime than the wintertime could help. But there's actually a bigger effect of humidity; you can have differences in survival of a factor of 100 based on swings in humidity.  

Lin: The humidity will mainly affect the evaporation kinetics of these aerosols and droplets. Think about the droplet in a very humid environment. It basically will keep its size. In this way, the harmful stuff for the virus in the droplet won't get concentrated a lot. So that will allow the virus to survive well in the humid environment. But then in the dry environment, the droplet will evaporate very fast, and we believe that the virus will maintain its survivability. So basically, the virus survives well at both low and high relative humidity. But in the intermediate, like between 40 and 70 percent relative humidity, you will find that the droplets still evaporate quite rapidly, but they are able to maintain a little bit of liquid. And at this time there are components in the droplet that are harmful to the virus. So at this intermediate relative humidity range, we have a lot of virus decay. We basically see a U-shaped pattern in terms of virus viability as a function of relative humidity.

In a dry environment, how does the virus survive?

Lin: Their structure is not damaged when they dry out. If someone touches the contaminated surface and then their nose or mouth, the virus can get into the host‘s respiratory system and infect the cells.

Can you describe what your research looks like in the lab?

Lin: We mainly look at relative humidity’s effect on virus viability. The first thing we frequently do is to propagate a virus, so that they are ready for our experiment. The viruses are in liquid solutions. The first type of experiment we run is looking at virus viability in aerosols, small particles suspended in the air for a long time. We use a device called a nebulizer, which can aerosolize the virus from the liquid phase to the airborne phase, and then we introduce the virus aerosols into a drum, in which we can control the relative humidity. The virus will sit in the drum for a certain period of time and then we collect the virus sample so that we know if any virus survived after a certain amount of time. Then for droplets, we basically just use a pipette to spot the droplet onto a surface. We then control relative humidity and compare their survival after the period of time.

How do you work to take that knowledge into practical application?

Lin: There’s a U-shaped effect of relative humidity on the virus viability, so from the practical side, maybe we can maintain indoor relative humidity at that range between 40 to 70 to mostly deactivate the virus. 

Marr: Humidity control is the big question of whether that can help reduce spread, especially during the wintertime when our indoor humidity is often around 20 to 30 percent. In areas where we heat the indoor air when it's cold outside, that ends up resulting in pretty dry environment indoors. Under those conditions, many respiratory viruses survive pretty well, in the air or on surfaces. Humidity is just one of many different factors that affect transmission, but if our research pans out, then raising the humidity to more of that sweet spot, above 40 percent and below 70 percent—you don't actually want to get above 60 because that can promote mold growth—could have an impact on transmission.

There was a small pilot study published last year where they tried this in a school or daycare center in Minnesota. They only had two classrooms, but they humidified one of them and didn't humidify the other, and they found that there were fewer illnesses and maybe less virus found in air and on surfaces in the classroom that was humidified. But there's a lot more research that we need to do before we can really make a recommendation like that.

Do masks help to protect or shield from a virus?

Marr: There was a study on college students who had the flu in Maryland where they sat in a lab for half an hour and just breathed normally, and every 10 minutes they had to say the alphabet. Even those who didn't cough were still releasing virus into the air in very small droplets that can stay suspended and float around for hours. So in terms of barriers like masks, masks are good at trapping small droplets or particles. Filters can also be very effective for small particles because they have a lot of random motion, almost like molecules. Because of the way the air flows around the filter fibers, the moving particles can bump into those filter fibers and be removed. So masks, if they're worn correctly, can definitely help cut down on release of a virus from those who are infected. What's not effective is if people wear a mask that has gaping holes on the sides.

In your op-ed in the Washington Post from 2018, you had some suggestions for ways to protect yourself from the flu. Based on current knowledge of the coronavirus, are the suggestions the same?

Marr: The recommendations for flu are similar for the coronavirus because they spread in similar ways even though the viruses are different. They're both moving around in respiratory droplets in the air.

And how those move around is really determined by the size of the respiratory droplet, it has nothing to do with the virus itself. Where the virus itself comes into play is whether it survives well in those droplets that are floating around. And it may be that one of the things that contributes to the ease of the spread of the coronavirus is that it might survive better than flu, but it also could be that maybe people release more of those viruses into the air. Or maybe it takes fewer to cause infection. There's a bunch of different things that could affect how easy it is to transmit.

Is there one piece of evidence you think right now that we could stand to gain about the coronavirus?

Marr:There have been measurements in a study in China of whether the virus exists in very small aerosols that can stay suspended for hours, but they only looked at whether the virus is present or not. They did not look at whether the virus was still alive and still could infect. I think that's the key piece of information. There is a study showing that laboratory-generated aerosolized virus can survive, but we haven't seen that in the field yet. I would love to see that kind of data.