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What does it mean when a virus is airborne? It turns out it's more than just what comes out when you sneeze

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COVID-19 airborne lifetime (2020)
Transmission of COVID-19 virus by droplets and aerosols (2020)
Recognition of aerosol transmission of infectious agents: a commentary (2019)
Opinion letter
Measles info for CDC
Human cough as jet propulsion
Couple of downloadable open access videos of droplet visualization experiments that accompanies the 2020 PNAS paper.
WHO Transmission of SARS-CoV-2
Natural Ventilation for Infection Control in Health-Care Settings. Annex C.
Indirect Person to Person Transmission
Is the coronavirus airborne? Experts can’t agree
Aerosol Transmission of Infectious Disease
Estimation of airborne viral emission: Quanta emission rate of SARS-CoV-2 for infection risk assessment
Factors involved in the aerosol transmission of infection and control of ventilation in healthcare premises
Transmission of Ebola Viruses: What We Know and What We Do Not Know
Airborne bacteria and viruses
DoH Measles Info
Transmission of SARS-CoV-2
This video was filmed on August 28th, 2020.

For up-to-date information on the COVID-19 pandemic, check out our playlist linked in the description [ intro ]. It can be confusing to try and figure out what public health experts mean when they say a virus is airborne.

We're told that viruses travel on the droplets we exhale -- and that wearing a mask or face covering should help limit those droplets. We're also told that public health experts don't know if a virus is “airborne” or not. But wait… isn't it flying through the air on gobs of spit?

Yes -- but that's not what airborne means, according to formal definitions. Well, some definitions, anyway. It turns out not everyone even agrees on what airborne means.

And though the distinction seems bizarre, it does matter -- because it affects how we protect patients, health professionals, and one another. A more accurate and nuanced definition requires us to dive deeper into aspects of spit physics, biology, and epidemiology. And by the way -- we're only talking about viruses, since when you bring in other diseases, things get even more complicated.

So let's clear the air regarding this term. When you open your mouth to talk, sing, cough, or even breathe, you expel a mist of respiratory droplets. These droplets are one way that for viruses to reach new hosts -- by breathing them in, touching a surface they've landed on, and so forth.

The droplets can be all different sizes, ranging from one one thousandth of a millimeter to half a millimeter in diameter. And their size influences how far they can spread. Big droplets fall to the ground faster and stay closer to the person who breathed them out, while smaller droplets can hang on the air longer and spread farther.

And even some of those can evaporate into smaller, gel-like particles called droplet nuclei that can hang in the air for hours — complete with viable viruses. If that little droplet is small enough and hangs out in the air for a long time, it's called an aerosol. We might be used to seeing that term on hairspray or paint cans, but it applies to anything suspended in the air.

Different organizations have used this relationship between droplet size and longer airtime to classify what counts as an airborne disease or not. But here's the problem: Just because someone's respiratory droplets are small enough to become aerosols doesn't make the virus airborne. Because while size does matter, to truly define an airborne disease, you need to consider the biology of the virus and the clinical data of the disease in addition to the physics of the airborne droplets.

See, in order to actually get someone sick, there has to be a high enough concentration of the virus present in those droplets or droplet nuclei. That might be a single virus particle in some cases, but that infectious dose varies from virus to virus. Which is why not all viruses can be airborne.

What's more, we have to actually observe a virus spreading in this fashion. So for the remainder of this video, we'll use a definition from a 2015 article written by public health researchers that's more centered around the clinical outcome. To be considered airborne, an infected person must expel the virus from their lungs, the virus must be viable in those droplets, and the droplets have to reach tissues the virus can infect.

Measles, for example, is a viral disease that's notoriously airborne. Studies from the 1980s showed measles spreading from child to child in pediatricians' offices, despite hours between visits. We know it's expelled in respiratory droplets.

We know it's viable in the air. And when it's breathed in, it can reach immune cells in the airway, which it infects. So it checks off all three criteria.

On the other hand, HIV is certainly, definitely, one hundred percent not spread through the air. HIV is only present in certain bodily fluids, and respiratory droplets aren't one of them. In other words, it fails the very first requirement for being airborne.

However, other viruses — like the flu — are far less clear-cut in their behavior. Which at long last brings us to the coronavirus in the room. At the time we're making this video, we don't conclusively know whether COVID-19 is airborne.

So what do we need to know to find out? We know that, like with measles, many people have tested positive for COVID-19 after being in the same confined space as other people with the virus, without any direct contact with the sick person. In an article filed to a preprint server in August 2020, researchers from the University of Florida were able to culture SARS-CoV-2 found in the air in a hospital.

But as we're writing this, that article hasn't been peer reviewed or published yet. And even if it gets published, there's a piece still missing. We still need to know how much of the virus it takes to get someone sick.

We haven't nailed down the infectious dose for SARS-CoV-2 yet. So even if it's in the air, that's not the same as actually showing that it can lead to infection clinically -- the last part we'd need to call it airborne. Experts and public health officials are keeping an eye on this evidence, and some say we should already be acting like it's airborne -- better safe than sorry.

That's because airborne transmission changes what kind of protective equipment healthcare workers need to wear when dealing with confirmed or suspected cases of COVID-19. So most hospitals are playing it safe. Plus, it might change how we behave in our day to day to keep ourselves and others from getting sick.

Although it's worth remembering that no one person or organization makes The Call whether COVID-19, or any viral disease, is airborne or not. It's more of an ongoing scientific conversation that takes a broad spectrum of evidence into account. All of which has one ultimate goal — to keep us all safe and healthy.

Thanks for watching SciShow, which is produced by Complexly. If you're a US citizen and you want to not only keep imagining the world complexly with us but also have a voice in your government, check out How To Vote In Every State. The rules for voting are different depending on where you live, so we've made a whole series of videos explaining how to vote in each state, as well as some advice for special cases like territories and overseas voters.

Check out your 2020 guide at [ outro ].