We've long known about the transmission of the coronavirus via respiratory droplets from coughs and sneezes, which is why everyone is encouraged to wear masks and stay six feet away from each other. The question of airborne spread has been contentious for months, with some scientists arguing for preventive guidance, but public health agencies delayed in recognizing airborne transmission. However, we now know that six feet isn't far enough to prevent inhalation of aerosolized particles.
This acknowledgement, and the fact it took so long, has led to some confusion about the way the novel coronavirus spreads, reinforcing the need for precautionary measures. Learn what experts have to say about the airborne spread of COVID-19 and what it means for you.
"From what we currently know, the preponderance of the evidence is that transmission is mainly through respiratory droplets and aerosols, with contamination of surfaces playing a limited role in transmission," says Dr. Davidson Hamer, professor of global health and medicine at the Boston University School of Public Health and School of Medicine.
According to the CDC, the coronavirus mainly spreads through direct and close contact, such as talking to someone without a mask in close quarters. It sometimes spreads through airborne transmission and occasionally spreads through indirect contact, such as touching infected surfaces and then touching your nose, mouth or eyes.
What does it mean when a virus is airborne?
According to the World Health Organization, "airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time."
In other words, when a virus is airborne, it spreads through the air via microscopic particles that can be inhaled.
Dr. Joseph Allen, director of the Healthy Buildings program at Harvard and an assistant professor of exposure assessment science at the T.H. Chan School of Public Health, says the public simply needs to understand that this means our "safe zone" of six feet doesn't necessarily exist.
"Of course, it's a bit more nuanced than that," he says, "but the public has been told that exposure happens within six feet." The truth is, Allen continues, we generate particles that can travel further than that. And because of their small size, they also stay in the air longer.
This is where the confusion starts, says Dr. Philip Tierno, professor of microbiology and pathology at New York University School of Medicine. The term "respiratory droplets" refers only to where the particles come from. A respiratory droplet -- something that comes from your respiratory tract and is expelled from your nose or mouth -- can "be micro or macro in size," Tierno explains.
Every time you sneeze or cough, you release large and small particles. The larger particles travel a short way (six or so feet) and then settle to the ground, falling because of gravity. The smaller particles remain suspended in the air, traveling much farther and resisting the effect of gravity, Tierno says.
Both types of particles are still respiratory droplets, Tierno says, so yes, technically, some respiratory droplets are truly airborne.
What's the difference between aerosols and droplets?
The confusion continues. "The problem is that people use these terms interchangeably," Tierno says, "when in reality they mean different things."
You may have seen several terms floating around the internet, including droplet, aerosol and microdroplet. Microdroplets and aerosols are synonymous: These terms both refer to fine particles that can exist in the air for long periods of time and travel long distances. Droplets, on the other hand, are larger and do not travel as far.
There's a longstanding (circa 1930s) standard in the medical and scientific communities that five microns serves as the "fence" between airborne particles and non-airborne particles. Anything larger than five microns is thought to settle to the ground within six feet -- this belief informed the six-foot social distance barrier that's now commonplace.
However, a letter from researchers published on Oct. 5 urges the scientific community to change this definition. A standard of 100 microns would be more appropriate, the researchers wrote, because in confined spaces, viruses in aerosols smaller than 100 microns can live for long periods of time.
Respiratory particles exist on a continuum, Allen says. "The reality is that [people] release particles of many different sizes, from less than five microns to way more. The medical community has long thought that a five-micron particle settles to the ground in less than six feet, but this is not always the case."
Other factors, like ventilation, environment and velocity can affect how quickly a particle of any size settles, he says. Cigarette smoke might help you visualize this -- if you stand 15 feet away from someone smoking a cigarette outdoors and the wind is still, you probably won't notice the smoke. But with a breeze, the particles of cigarette smoke will quickly travel to you, even with that distance of 15 feet.
"The point for the public is this: There are a range of sizes [of particles], some of which can travel longer than six feet," Allen says.
Many scientists and doctors began lobbying the CDC as early as February 2020 in an attempt to get the public health agency to classify SARS-CoV-2 as an airborne virus. In July 2020, nearly 250 scientists and doctors wrote an open letter to public health agencies urging them to address airborne transmission.
It's unlikely anything fundamental -- like the mode of transmission -- has changed about the novel coronavirus since it began spreading in early 2020. It's more likely that now, seven months in, the evidence is clear enough to definitely say COVID-19 can spread through airborne particles.
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Why didn't the CDC tell us the coronavirus was airborne?
Some say the CDC was trying to avoid adding to public fear or anxiety about the coronavirus, but this logic is faulty, Allen says. "This is risk communication 101," he says. "You don't hold back information. You have to be transparent about what's happening to establish trust and allow people to act accordingly to protect themselves and others."
Allen, who first wrote about airborne transmission of the coronavirus in February, says he doesn't know what took the CDC so long to acknowledge airborne spread. "We [doctors] were excited a few weeks ago that they acknowledged it, and then they walked it back," he says.
"The result is a confused public," Allen says. "The science is what the science is," and people can't make informed decisions without knowing the truth. Allen says he supposes many more people would've taken basic precautions early in the pandemic had public health officials declared the virus airborne.
Others say the CDC's lack of acknowledgement was of the presidency's accord. "The CDC unfortunately is affected by the White House," Tierno says. "Anything the CDC does can be politically infused. They may not have done this had they had no pressure on them."
Does this mean the coronavirus is more infectious?
No, the identification of airborne transmission doesn't mean the novel coronavirus is more infectious than it already was.
"There's a fundamental misunderstanding that all airborne viruses are highly infectious through airborne transmission," Allen says. "Not all airborne viruses are like tuberculosis or measles," both of which have high and rapid infection rates.
It does mean, however, that the standard of six feet isn't always enough to prevent infection, especially in poorly ventilated areas.
It's still not clear how many cases have occurred due to airborne transmission, and without a solid contact-tracing infrastructure, that's something we may never know, says Allen.
How long does the coronavirus live in the air?
There's no finite number of minutes or hours known yet. Estimates range from just a few hours up to 12 hours or more. Tulane University, for instance, reported that COVID-19 can remain in the air for up to 16 hours.
"'Hours' is typical, but remains largely undefined," Tierno says, "which is an important consideration."
Dr. Roshni Mathew, associate medical director of infection prevention and control at Stanford Children's Health, says it's important to remember that finding the virus's RNA in air doesn't automatically equate to transmission.
"Just having aerosols or finding virus particles does not equate to transmissibility, as there are other factors to consider," she says, notably whether or not the virus is actually viable, meaning able to infect you. The WHO reports that in several studies that found virus particles in the air, the researchers did not find viable particles.
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"The virus that causes COVID-19 is still under intense research," Hamer says, "but it is understood that [larger] respiratory droplets from infected individuals can travel at least a few feet through the air to other persons within close contact."
Aerosolized particles are lighter, so they are able to travel further through the air, Hamer continues, noting that some evidence has shown aerosols containing viruses can travel up to 18 feet. One study conducted in China suggests that aerosolized SARS-CoV-2 can spread up to four meters, or about 13 feet. Another report from April estimates the virus can spread up to 10 meters, or about 32 feet.
Again, environmental factors must be considered. Wind can carry particles, even larger ones, farther than six feet.
What this means for you
Most importantly, everyone should be aware that airborne transmission of COVID-19 means six feet isn't a magic number. The novel coronavirus can spread farther than that, and it's important to keep that in mind, especially when indoors.
The current best practices for preventing the spread of COVID-19 are still our best protection, Hamer says. "The same personal protective measures should be adhered to, including wearing of face masks, good hand hygiene and practicing social distance measures," he says, emphasizing that social distancing means at least six feet apart.
Knowing the novel coronavirus is airborne, people should pay more attention to the ventilation and air quality of their homes and other environments they frequent, Allen says.
"This reinforces the need for masks; it reinforces the fact that we shouldn't be spending time indoors in crowded conditions or unventilated areas," Allen says. "And it matters that the CDC said this."
"It matters," Allen emphasizes, "because before, it was just scientists saying it. It wasn't official. Now it's official, and going against this is going against [CDC] guidance."
The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.