Can COVID-19 Spread Through the Air?
On July 15, the WHO announced that the coronavirus can potentially spread through the air. While the organization made clear that it still believes droplets are the virus’s primary means of spreading, "under some circumstances, airborne transmission may occur." The WHO were prompted to make a statement after a letter signed by 239 scientists was published calling on the WHO to acknowledge growing evidence that the virus can spread through aerosols — tiny airborne particles — as well as droplets of mucus or saliva.
Many scientists view the statement as an important step in recognizing how the coronavirus spreads. Even so, the WHO was reluctant to describe the virus as "airborne" in part because the term doesn’t always mean the same thing to the public that it does to scientists — or even different groups of scientists. This is what you need to understand about the WHO’s statement and how the coronavirus can spread through the air.
The WHO maintains that the coronavirus spreads primarily through droplets — tiny flecks of saliva or mucus of around a millimeter or so that contain the virus and exit the body when an infected person breathes, coughs, sneezes or uses their voice. These droplets usually travel up to six feet, which is why guidelines to keep six feet apart are so common for the coronavirus. However, they can be expelled up to 20 feet or more during a sneeze or particularly bad cough. Like COVID-19, the flu, common cold and pertussis (whooping cough) are all spread by droplets.
To spread a virus (or bacteria, in the case of something like pertussis), droplets usually need to land near or on the eyes, nostrils, mouth or other vulnerable areas. Additionally, droplets that land on a surface can also transfer a virus to anyone who touches it as long as the virus survives. An object covered in virus particles as a result of droplets is known as a fomite. Fortunately, the coronavirus seems to spread through fomites only occasionally, although you should still take care to wash your hands.
Droplets tend to fall to the ground because they’re heavier than the air around them. However, when the fleck of mucus or saliva in question is small enough — just a few microns in diameter — something different happens. The moisture in the droplet may evaporate before it can fall to the ground, leaving the germs inside free to float like dust in the air. In that situation, a virus can survive in the air for hours after leaving a person’s body, which in turn puts anyone entering that space at risk. It’s much rarer for a disease to spread through aerosols compared to droplets, but those that do tend to be highly infectious. Examples include tuberculosis, measles and chickenpox.
The complicated thing about aerosols is that they don’t always mean the same thing to different people. While droplets and aerosols are not the same thing in medical fields, in others, an aerosol is any particle in the air at all, solid or liquid, making droplets technically a kind of aerosol. On top of that, the distinction between aerosols and droplets itself isn’t always accurate. When droplets are produced by coughing or sneezing, they can persist in the air for some time like aerosols. Despite previous thinking that size was the most important factor in determining if something became a droplet or aerosol, more recent science shows that the composition of a droplet cloud, the speed at which it moves and the temperature and humidity of the air all play a big role in determining how long and far germs can spread through the air.
The coronavirus hasn’t shown the ability to consistently spread in aerosol form, meaning it isn’t as infectious as diseases like measles. That means it isn’t airborne in the sense that many doctors use the term — if it’s not creating the specific medical definition of an aerosol, it doesn’t qualify as being airborne. However, the possibility that it might be able to sometimes form aerosols and the likelihood that droplets containing coronavirus can linger in the air anyway is why scientists and the WHO are changing their minds about how they discuss whether or not the coronavirus is airborne.
What This Means for You
Droplets continue to be the main vector for the spread of the coronavirus. However, scientists have identified several incidents where the virus behaved as if it was airborne and transmitted by aerosols. Among other examples, these include an individual in Washington state who spread the virus to 45 other people, many of whom were more than six feet away, as well as a patron at a restaurant in Guangzhou, China who may have infected nine other people as far as 20 feet away as the result of an unfortunately placed air conditioner.
What do these situations have in common? They’re indoors, so the air is often dry and poorly circulated, allowing the coronavirus to survive and stay suspended in the air for a long time. Coughing, sneezing or singing (there have been several so-called "superspreader" events involving choirs) in such spaces likely plays a large role in the spread of the virus. The more people are packed into such an environment and the longer they stay there, the more likely the air is to be saturated with coronavirus.
Masks are still useful for keeping people from spreading the coronavirus (and to a lesser degree from catching it), and handwashing is always useful. At the same time, though, we may need to rethink some of our safety guidelines. While staying six feet apart is useful, more is better, and additional efforts must be made to limit indoor gatherings, especially in buildings with poor ventilation. People who are in such spaces should open windows to help disperse droplets or aerosols that might form.
The knowledge that the coronavirus can spread through the air is something that officials and ordinary people alike should take seriously. Even so, that information also puts us in a better place to take appropriate action to slow the spread of the virus until a vaccine is found.