This was the week airborne transmission became a big deal in the public discussion about covid-19. Over 200 scientists from around the world cosigned a letter to the World Health Organization urging it to take seriously the growing evidence that the coronavirus can be transmitted through the air. WHO stopped short of redefining SARS-CoV-2 (the virus that causes covid-19) as airborne but did acknowledge that more research is “urgently needed to investigate such instances and assess their significance for transmission of COVID-19.”
“I honestly don’t know what people are waiting for,” says microbiologist Chad Roy of Tulane University in the US. “It doesn’t take WHO coming out to make a proclamation that it’s airborne for us to appreciate this is an airborne disease. I don’t know how much clearer it needs to be in terms of scientific evidence.”
So what does “airborne” really mean in this context? It’s basically an issue of size. We’re pretty sure that SARS-CoV-2 is spread through tiny droplets that contain viral particles capable of leading to an infection. For a virus to be airborne, however, means a few different things, depending on the expert you’re talking to. Typically it means it can spread via inhalation over long distances, perhaps even through different rooms, of small particles known as aerosols.
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“That’s why when you ask some of the professionals if the virus is airborne, they’ll say it’s not, because we’re not seeing transmission over those sorts of distances,” says Lisa Brosseau, a retired professor of public health who still consults for businesses and organizations.
There is also some debate on what we mean by “aerosol.” The droplets that carry viral particles through the air can come in all sorts of sizes, but while the larger ones will drop quickly to the ground or other surfaces, the smaller ones (just a few microns across) can linger in the air for a while, giving them a chance to be inhaled. The word is mostly used to describe these smaller particles, although Brosseau would prefer the term “aerosol transmission” to cover the entire gamut of inhalable viral particles being expelled into the air—large and small alike.
If SARS-CoV-2 is airborne, it’s far from the only disease. Measles is notorious for being able to last in the air for up to two hours. Tuberculosis, though a bacterium, can be airborne for six hours, and Brosseau suggests that coronavirus superspreaders (people who seem to eject a larger amount of the virus than others) disseminate the virus in patterns that recall the infectiousness of tuberculosis.
The evidence that this type of transmission is happening with SARS-CoV-2 arguably already exists. Several big studies point to airborne transmission of the virus as a major route for the spread of covid-19. Other studies have suggested the virus can remain in aerosolized droplets for hours. One new study led by Roy and his team at Tulane shows that infectious aerosolized particles of SARS-CoV-2 could actually linger in the air for up to 16 hours, and maintain infectivity much longer than MERS and SARS-CoV-1 (the other big coronaviruses to emerge this century).
We still don’t know what gives SARS-CoV-2 this airborne edge. “But it may be one reason this is a pandemic, and not simply a small outbreak like any other coronavirus,” says Roy.
How to stay safe
Whether the virus is airborne isn’t simply a scientific question. If it is, it could mean that in places where the virus has not been properly contained (e.g., the US), the economy needs to be reopened more slowly, under tighter regulations that reinforce current health practices as well as introducing improved ones. Our current tactics for stopping the spread won’t be enough.
Roy would like to see aggressive mandates on strict mask use for anyone leaving home. “This virus sheds like crazy,” he says. “Masking can do an incredible amount in breaking transmission. I think anything that can promote the use of masking, to stop the production of aerosols in the environment, would be helpful.”
Brosseau, however, says that though masks can limit the spread of larger particles, they are less helpful for smaller ones, especially if they fit only loosely. “I wish we would stop relying on the idea that face coverings are going to solve everything and help flatten the curve,” she says. “It’s magical thinking—it’s not going to happen.” For masks to really make a difference, they would need to be worn all the time, even around family.
Brosseau does believe the evidence is trending toward the conclusion that airborne transmission is “the primary and possibly most important mode of transmission for SARS-CoV-2.” She says, “I think the amount of time and effort devoted to sanitizing every single surface over and over and over again has been a huge waste of time. We don’t need to worry so much about cleaning every single surface we touch.” Instead, the focus should be on other factors, like where we spend our time.
One of the biggest questions we still have about covid-19 is how much of a viral load is needed to cause infection. The answer changes if we think it is aerosols that we need to worry about. Smaller particles won’t carry as large a viral load as bigger ones, but because they can linger in the air for much longer, it may not matter—they’ll build up in larger concentrations and get distributed more widely the longer an infected person is around to expel aerosolized virus.
The more people you have coming in and out of an indoor space, the more likely it is that someone who is infected will show up. The longer those infected individuals spend in that space, the higher the concentration of virus in the air over time. This is particularly bad news for spaces where people congregate for hours on end, like restaurants, bars, offices, classrooms, and churches.
Airborne transmission doesn’t necessarily mean these places must stay closed (although that would be ideal). But wiping down surfaces with disinfectant, and having everyone wear masks, won’t be enough. To safely reopen, these spots will not just need to reduce the number of people allowed inside at any given moment; they will also need to reduce the amount of time those people spend there. Increasing social distancing beyond six feet would also help keep people safer.
Ventilation needs to be a higher priority too. This is going to be a big problem for older buildings that usually have worse ventilation systems, and areas with a lot of those might need to remain closed for much longer. The impact of asymptomatic spread (transmission by people who don’t feel ill) and superspreaders only compounds the problem even further. But research conducted by the US Department of Homeland Security has shown that in the presence of UV light, aerosolized particles of the size the Tulane researchers studied would disappear in less than a minute. A number of businesses have begun deploying UV-armed robots to disinfect hospital rooms, shopping malls, stores, public transit stations, and more.
For many places, considerable delays in economic reopening might ultimately be the price of getting the virus under control. Otherwise the kind of thing that happened when a single open bar in Michigan led to an outbreak of more than 170 new cases could become commonplace.
Heading into the fall, "the implications are profound—but not that hard to grasp," says Donald Milton, an aerobiology expert at the University of Maryland and one of the authors who spearheaded the letter to WHO. "We need to subsidize bars and restaurants to stay closed. We need to increase ventilation where we can and start making as widespread as possible use of air sanitation with upper-room germicidal UV and maybe far UV in those places that must be open, like elementary schools. We need to stagger hours of starting work and keep density on public transport low, or open windows. And we need to wear masks."
This post has been updated with additional comments from Donald Milton.