New information could mean updated standards for healthcare professionals and recommendations for the general public. 

By Kasandra Brabaw
April 03, 2020
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Six feet: You've heard that measurement everywhere by now—from officials at major health organizations like the Centers for Disease Control and Prevention and the World Health Organization; on coronavirus-themed TV commercials; in literally every COVID-19 article circulating the internet. For the most part, we all know that, while we should be staying home, in those moments we have to go out into the world, we should maintain a 6-foot distance between ourselves and those around us.

That measurement—6 feet—isn't random; it's based on the idea that this novel coronavirus, SARS-CoV2, spreads through droplets of saliva or mucus that get ejected into the air whenever someone who has COVID-19 coughs or sneezes, says Glenn Randall, PhD, a microbiologist and virologist at the University of Chicago. The virus hitches a ride on these droplets, which fall to the ground or other surfaces within a 6-foot radius—thus the rule to keep our distance. (It's also why health officials strongly urge regular handwashing and not touching your face; those virus droplets can also contaminate commonly-touched surfaces, and sometimes stay there for a while.)

But the thing about the novel coronavirus is that it's new, meaning even the most in-the-weeds scientists and researchers didn't know about it until it first emerged in China in December 2019—and that means there's still a lot we don't know about it, since new research is emerging daily.

One of those things we aren't 100% sure about is whether or not the coronavirus could be airborne. As of right now, respiratory droplets seem like the most common way the virus is transmitted, according to officials. But researchers are currently exploring the possibility that this new coronavirus could, on some level, be airborne, too—and that may change both standards for healthcare professionals and recommendations for the general public. 

What exactly does it mean for a virus to be airborne—and how is it different from virus droplets?

When we think of the word 'airborne' it's easy to assume that it means a virus that can be transmitted through the air at any point—which could technically even apply to those droplets associated with the novel coronavirus, since they travel through the air before they land. But the medical definition of airborne is a bit more specific, which can be confusing, says Dr. Randall.

Essentially, "airborne suggests the virus is no longer part of the droplet that falls to the ground and is literally part of the air we breathe," he adds. When something is airborne, it's actually in an aerosol form—a liquid or a solid suspended in gas. While most of us recognize the word 'aerosol' in terms of hairspray, in the case of a virus existing as an aerosol, it means the solid is the virus molecule, and the gas is the air that transports it.

The World Health Organization currently defines aerosol particles smaller than 5 microns in diameter, while droplet particles are technically larger than 5 microns in diameter. Using this information, a 2013 review article in the Journal of Pathogens explains airborne transmission further, defining it as "the transmission of particles that are comparatively smaller in size [than droplet particles] and thus can remain suspended in air for long periods of time." The review adds that, because of this, airborne particles "potentially expose a higher number of susceptible individuals as a much greater distance from the source of infection."

But some research even calls into question those aerosol and droplet particles—and whether the measurements are arbitrary in terms of coronavirus. A new analysis published in the Journal of the American Medical Association (JAMA), postulates that the categories of large and small droplets aren't specific enough when referring to COVID-19 transmission.

"The rapid international spread of COVID-19 suggests that using arbitrary droplet size cutoffs may not accurately reflect what actually occurs with respiratory emissions," wrote Lydia Bourouiba, PhD, an associate professor at Massachusetts Institute of Technology who studies the fluid dynamics of disease transmission. This, she says, may lead to the virus traveling distances greater than 6 feet—instead, up to 23 to 27 feet.

Of course, not everyone agrees with Bourouiba's assessment—including one key voice on the topic: Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Disease, and a member of the White House Coronavirus Task Force was asked about the report’s key finding during the March 31 daily media briefing. He indicated that he was disturbed by the report, which he found to be “terribly misleading," adding that the forcefulness of a sneeze needed for this type of transmission is "not practical."

Is the coronavirus airborne or not?

Experts can't seem to definitively agree on this one—and even the general definition of "airborne" in terms of the coronavirus is up for debate. According to Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, the novel coronavirus is not airborne. He clarified that in a tweet shared on March 6, explaining, "[coronavirus] spreads from person to person through small droplets from the nose or mouth when a person with #COVID19 coughs or exhales."

It's a pretty direct statement, but while more research continues to be published on the topic, experts have begun to consider the possibility that the novel coronavirus could exist as an aerosol for airborne transmission.

A laboratory experiment published March 27 in The New England Journal of Medicine (NEJM) provides the most robust evidence that the virus can remain in the air. A team at the National Institute of Allergy and Infectious Disease used a nebulizer, which creates aerosols from liquids, to spray COVID-19 molecules in the air as well as samples of the virus that caused the SARS epidemic in 2003. They report being able to detect the virus in the air for three hours. Both viruses dropped by half after 1.1 hours. 

While this experiment proves that the virus can remain in the air, conditions in a lab don’t necessarily prove what is happening in the real world. For example, a nebulizer may not perfectly mimic a cough or sneeze. 

To that point, research done in real-world environments has mixed results. For a study published in JAMA this month, researchers took samples from three patients in infection isolation rooms at the outbreak center in Singapore. Samples were collected for five days over two weeks, sometimes after rooms were cleaned and sometimes before. While the virus was found on some surfaces in patients’ rooms, it was never detected in the air. 

Yet, two more studies (both shared on MedRxiv and BioRxiv, but currently preprint and not yet peer reviewed), did find some evidence that the virus could be airborne. In one, researchers from the University of Nebraska Medical Center collected samples from 11 isolation rooms and found some positive samples, indicating that the virus is shed into the air when an infected person coughs or sneezes, when they go to the bathroom, and when anyone touches a surface containing the coronavirus, thus stirring the particles up. (This also added more clout to the ability of the coronavirus, though rare, to spread via fecal transmission.)

In the other preprint study, conducted by researchers from Wuhan University, the Hong Kong University of Science and Technology, and other institutes, 35 air samples were collected in both patient rooms and medical staff areas in hospitals in Wuhan, China during the COVID-19 outbreak. The researchers found only low concentrations of the virus in the air in patient rooms or in hallways, but higher concentrations in the air near patients’ toilets. This evidence, according to the Wuhan scientists, suggests that, “The virus aerosol...is a potential transmission pathway and effective sanitization is critical in minimizing aerosol transmission of SARS-CoV-2.” 

Still, the WHO stands by their initial recommendations of droplet and contact precautions when it comes to COVID-19—and in a scientific brief shared on March 29, the organization stated that studies focusing on whether the coronavirus is actually airborne "need to be interpreted carefully."

The organization reiterates that the NEJM nebulizer study does not reflect real-world circumstances, and says that several preprint articles finding positive air samples in patient rooms may not be detecting viable virus (it's one thing for a virus to be detected in the air and on surfaces, it's another to determine whether the amount of virus detected is able to infect people). “As evidence emerges, it is important to know whether viable virus is found and what role it may play in transmission,” the organization says. 

As of right now, the WHO is recommending airborne precautions only for healthcare workers or those in a healthcare setting. Some procedures done in hospitals can create aerosols of the virus, which is why it’s so vital for healthcare workers to have access to high-quality n95 masks, says Waleed Javaid, MD, Director of Infection Prevention and Control at Mount Sinai Downtown. 

For the rest of us, the six foot rule is sufficient. If you want to take extra precautions (and because even the CDC and WHO are revisiting their mask recommendations for the general public), you can wear a homemade mask—but please leave the surgical masks and N95 respirators for the healthcare professionals.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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