Blood type may affect your COVID-19 risk, studies suggest (1:26)
UPDATE: This article has been updated with new information on September 12, 2020, 6:00 a.m.
In July, a group of 239 scientists released a manuscript entitled “It is Time to Address Airborne Transmission of COVID-19,” in which they advocated for more preventive measures to stop the spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In response, three days later, the World Health Organization (WHO) updated its scientific brief to include more information about how the virus spreads. The question everyone is trying to address: Is the virus that causes COVID-19 airborne?
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The reason there’s been some back and forth on this issue is because it’s a tough question to answer — partly because of the lack of good evidence pointing one way or the other. SARS-CoV-2 has been found in the air, but mostly in laboratory (not real-world) settings. And we’ve seen certain situations in which the virus may have been transmitted through the air, but in those situations other methods of transmission were possible as well. The most likely answer to this question is yes, SARS-CoV-2 is airborne — but probably only in limited circumstances. To understand this, let’s talk a little more about how the virus spreads.
As of now, scientists generally recognize that SARS-CoV-2 spreads through two main routes: droplets and fomites.
Droplets: Droplets (sometimes called respiratory droplets) are tiny particles that are expelled from the mouth and nose when a person coughs, sneezes, or talks. After leaving the body, droplets fall to the ground, typically within a few feet from where they started.
Fomites: Fomites are inanimate objects that may have virus particles on them. For instance, if somebody with COVID-19 coughs on their hand and then touches a door handle, that door handle is a fomite that may be covered in SARS-CoV-2. If you were to touch the door handle and then touch your mouth, nose, or eyes, you could potentially become infected.
So far, government and public health recommendations have primarily been based on these two modes of transmission. We wear cloth face coverings and stay at least six feet apart to avoid transmission by droplets. And we wash our hands frequently to avoid transmission by fomites. But now, scientists and the WHO are considering another possible player: aerosols.
Droplets and aerosols may both sound like a form of “airborne transmission,” but there’s actually an important difference between the two. Droplets are larger (greater than five microns) while aerosols are smaller (less than five microns). Both of these are still extremely small — approximately 1,000 microns can fit across a pinhead — but the size difference leads to an important difference in behavior. Because they are larger, droplets are more affected by gravity and less affected by the air, meaning they fall to the ground after traveling a short distance. On the other hand, aerosols are able to remain in the air for much longer instead of falling to the ground — perhaps up to 16 hours. When talking about disease transmission, droplets are considered a form of direct contact, since you need to be within a certain distance of somebody to be exposed. Aerosols are considered a form of indirect contact, since the particles linger in the air even after a person may have left. This makes aerosols more difficult to avoid.
Common diseases that spread through aerosols include tuberculosis, measles, and chickenpox. And if you’re wondering where aerosols come from, there are two answers — aerosols can be directly created at the same time as droplets, or they can form when droplets partially evaporate and become smaller.
Supporters of the idea that SARS-CoV-2 can be aerosolized point to a few examples in which there’s evidence airborne transmission occurred — namely, at a gym, a restaurant, and in choir practice. However, these venues also have something else in common: there were opportunities for the virus to be passed through droplets or fomites as well. So, while it may technically be possible for SARS-CoV-2 to be airborne in limited circumstances, it isn’t confirmed that this is actually happening. To quote the WHO, “short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out.”
Outside of these situations, there’s one other place researchers recognize SARS-CoV-2 may be airborne: in certain medical settings during so-called “aerosol generating procedures.” Common aerosol generating procedures include things you might see happening in an emergency room (ER) or intensive care unit (ICU), such as airway suctioning, performing CPR, inserting/removing breathing tubes, using breathing machines, and bronchoscopy (a procedure involving putting a camera into the lungs). Outside of these circumstances, there have been no reports of healthcare workers getting infected when properly following droplet precaution measures (a.k.a. wearing personal protective equipment that protects against droplets, but not necessarily aerosols).
One reason aerosol transmission may not be too common is because just the presence of SARS-CoV-2 in aerosols isn’t sufficient. There also needs to be 1) enough aerosols formed when a person coughs/sneezes/talks, 2) enough virus particles in each aerosol that are actually viable and capable of replicating. One scenario is that SARS-CoV-2 is airborne but it doesn’t really matter because it can’t effectively be transmitted that way. Only time — and more research — will tell what the true answer is.
In the meantime, it’s important to focus on doing what we can to prevent the modes of transmission we are sure about. Namely, washing hands (to protect against fomites), wearing a cloth face covering (to protect against droplets and fomites), and practicing social distancing (to protect against droplets). As new information comes out, those steps should be the core preventative measures you take daily against the novel coronavirus.