The thought of being in an office building, a restaurant, or on public transportation has never been more frightening as evidence emerges about just how efficiently the novel coronavirus can spread through crowded indoor spaces.
A case report published last month in the academic journal Emerging Infectious Diseases detailed how one pre-symptomatic person infected nine others at a restaurant in Guangzhou, China. The 10 people were spread out across three tables and overlapped in the restaurant for roughly one hour. The researchers suspect that an air conditioning unit recirculating air within that section is to blame since no one else in the restaurant became infected.
Another study published in the same journal detailed an outbreak at a call center in South Korea where 94 people working on the same floor — and most on the same side of the building — all fell ill with the virus. The authors of the study write, “This outbreak shows alarmingly that [SARS-CoV-2] can be exceptionally contagious in crowded office settings such as a call center. The magnitude of the outbreak illustrates how a high-density work environment can become a high-risk site for the spread of COVID-19 and potentially a source of further transmission.”
“Fairly large droplets that are coming from people talking, coughing, sneezing, breathing, singing will float in the air depending on how much air movement we have.”
Finally, a preprint paper from China — which has not yet been peer reviewed — found that every outbreak involving three or more people came from indoor environments, most notably homes, public transportation, and restaurants. The scientists identified only one outbreak linked to an outdoor setting, and that case involved just two people. The study concludes, “The transmission of respiratory infections such as SARS-CoV-2 from the infected to the susceptible is an indoor phenomenon.”
These reports have added to the debate about whether the virus could be airborne and raised concerns about how to open up the economy safely without putting employees and customers at risk. And while it seems like there should be a clear-cut answer to the question of whether SARS-CoV-2 is spread through the air, the situation — like everything with this virus — is complicated.
“I think that a lot of the problem is arising out of confusing terminology,” says Donald Milton, MD, PhD, a professor of environmental health at the University of Maryland. “It’s also coming in part from fear. There’s a lot of fear of this word ‘airborne,’ and what does it mean to say that something is airborne transmitted?”
By some definitions, airborne transmission depends on the size of the particles being spread, with only aerosols, which are less than five microns in size, qualifying as airborne. Particles larger than five microns are classified as droplets, which are heavier and so drop to the ground faster. But most scientists say this cutoff is a false dichotomy that oversimplifies the matter, and in many situations a droplet can also be airborne.
“Aerosol means that it’s something that can float in the air, and how big it can be depends a lot on how much movement there is in the air,” Milton says. “Fairly large droplets that are coming from people talking, coughing, sneezing, breathing, singing will float in the air depending on how much air movement we have.”
The virus itself is only a tenth of a micron, but it travels in wet mucus particles created by breathing, talking, coughing, and sneezing that range in size. The conventional thinking has been that the average respiratory droplet is about 10 microns and travels roughly two meters, hence the six-foot social distancing guidelines. However, several studies have now found evidence of aerosols containing SARS-CoV-2 in hospital rooms and areas around people infected with the virus. Also, droplets can quickly evaporate and shrink in the air, causing them to stay afloat longer.
Another definition of an airborne infection relies on the result rather than the mechanism. “The way it’s defined is that we need to see and make the observation of people at a distance becoming sick,” says Richard Martinello, MD, an associate professor at Yale School of Medicine who specializes in infectious diseases. “What I mean is that if you have somebody who’s infected and contagious, they’re at point A, and then you have a susceptible person who is at a distance — and there’s no specific definition of what that distance is […] — and they then acquire that disease.”
“Are we most vulnerable in the nose or in the lungs? And what difference does it make how you got infected with it? It may make a difference.”
This definition is based on two additional factors besides the size and flight pattern of the viral particles. First, is the virus still “alive” and intact when contained in aerosols and droplets of different sizes? Just because viral RNA is detected on a surface or in a person, it doesn’t mean the virus is still infectious — the RNA could be from “dead” virus that can no longer replicate and infect a cell. While not every virus can survive in tiny aerosols, a heavily cited study from the New England Journal of Medicine found that SARS-CoV-2 does for up to three hours.
The other factor is the impact of the particles’ size on seeding an infection. Large droplets can’t get deep into the lungs, so a virus that only survives in larger droplets would have to be able to enter the body through cells in the nose. On the other hand, some viruses infect cells in the lungs first, so they would need to remain viable in smaller aerosols.
Right now, scientists are fairly certain people can become infected with SARS-CoV-2 in the upper respiratory tract, but they don’t know if an infection can be initiated from tiny aerosols that bypass the nose and travel down to the lungs.
“We don’t know where we are most vulnerable,” says Milton. “Are we most vulnerable in the nose or in the lungs? And what difference does it make how you got infected with it? It may make a difference.”
As evidence for airborne transmission of the novel coronavirus grows, strategies are emerging about how best to mitigate the risk of infection. The first two tactics, physical distancing and wearing masks, are already encouraged and even mandated in some places. A third line of defense is less about the person and more about the environment — ramping up indoor ventilation.
A recent study published in Nature found coronavirus RNA in the air in two hospitals in China, although the researchers did not establish if the virus was able to infect people. The concentration of the virus varied dramatically depending on where in the buildings the scientists tested. Aerosol virus levels were virtually undetectable in specially ventilated patient rooms and open-air public waiting areas. However, poorly ventilated rooms, such as patient bathrooms, had high levels of airborne virus. The research raises the issue of the importance of ventilation and what types of environments and filtration systems might be safest.
“The more ventilation clearly the better because it has a filtration system and it’s going to remove something which is airborne,” says Pratim Biswas, PhD, chair of the Department of Energy, Environmental & Chemical Engineering at Washington University in Saint Louis. “An ICU, a critical care unit in the hospital, has excellent ventilation. If I go to a shop or grocery store, it also has ventilation, but because it’s such a large area it’s very difficult to rely on just that.”
At the highest end of filtration systems are “clean rooms,” like in the ICU, that use a very high flow rate with extremely clean air to eliminate all foreign particles. At the other end of the spectrum is the humble filtration step of opening a window. Office buildings, retail spaces, and airplanes fall somewhere in between, with a central air conditioning system that either pulls in air from outside or recirculates air within the building. In both cases, the air is run through a filter to remove the particles, although the quality of the filter can vary widely. The system also breaks down when people enter and leave the building, which brings in outside air that isn’t immediately filtered.
“At this day and age, any indoor environment where there are other people […] the same criteria hold: as much distance as possible and then mask.”
“Mechanical ventilation systems that have appropriately sized cooling and heating capacity can bring air in from outside and dilute the aerosols indoors,” Milton explains. “[However], these systems are often not sized big enough to be able to cool or heat a lot of air, and so they tend to recirculate the air through environments. Then you can do things like have filtration or UV lights inside the ducts to clean the air that has recirculated. In that way, you can break up the amount of virus that accumulates in a room.”
Stand-alone air purifiers can help in a small room, whereas air conditioners with poor filters can make the situation worse by distributing the air more widely without removing the virus particles — as in the case of the Chinese restaurant.
Even with the best filtration system, Biswas still recommends physical distancing as “the first and most effective way to keep yourself safe.” The second way is to wear a mask.
“At this day and age, any indoor environment where there are other people […] the same criteria hold: as much distance as possible and then mask,” he says. “I would strongly recommend everyone to wear a mask until we have better control of the situation.”
What about taking advantage of the natural ventilation of the great outdoors? Battles about shelter-in-place requirements have been particularly contentious around public outdoor spaces like parks, trails, and beaches. The three experts interviewed for this article agree that being outdoors is generally safer than indoors, but again, rules about social distancing and wearing masks still apply.
“Outside, the movement of the air provides us some degree of protection,” says Martinello. “I haven’t seen any data specific to SARS-CoV-2 yet, but we know with other pathogens that sunlight does, in fact, have a disinfecting property.”
“It’s definitely better than an indoor enclosed environment,” Biswas agrees. But, he says, cramming hundreds of people onto a beach is definitely not good.