While it's possible for someone to become infected by inhaling aerosolized particles, the dose required for transmission remains unclear, says the World Health Organization.

By Kasandra Brabaw
Updated July 10, 2020

Six feet: You've heard that measurement everywhere by now—from officials at major health organizations like the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO); on coronavirus-themed TV commercials; in literally every COVID-19 article circulating the internet. For the most part, we all know that when 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, Glenn Randall, PhD, a microbiologist and virologist at the University of Chicago, explained earlier this year. 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 was first recognized 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 the things we hadn't been 100% sure about is whether or not the coronavirus could be airborne. Are respiratory droplets the most common way the virus is transmitted? Or could this new coronavirus 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.

WHO defines aerosol particles as smaller than 5 microns in diameter, while droplets are technically larger than 5 microns in diameter. (One micron is one-millionth of a meter or 0.000039 inches.) 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. An analysis published in JAMA (the Journal of the American Medical Association) 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.

Asked about the report's key finding during a March 31 White House Coronavirus Task Force briefing, Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Disease, indicated that he found it to be “terribly misleading," adding that the forcefulness of a sneeze needed for this type of transmission is "not practical." Dr. Fauci still maintains that while airborne transmission isn't thought to be a major source of infection, "it's something that we can't completely rule out," according to video posted by NBC News.

Is the coronavirus airborne or not?

Until recently, Tedros Adhanom Ghebreyesus, the WHO's director-general, insisted that the novel coronavirus is not airborne, a point he clarified in a tweet shared on March 6, explaining that the virus "spreads from person to person through small droplets from the nose or mouth when a person with #COVID19 coughs or exhales."

But this week the agency acknowledged that airborne transmission, in fact, may play a role. Under pressure from a group of more than 200 scientists describing airborne transmission as "a real risk," the WHO updated a scientific brief on SARS-CoV-2 to reflect the potential that very small particles, or aerosols, can be a source of infection even outside of medical settings. Someone could become infected "if the aerosols contain the virus in sufficient quantity to cause infection within the recipient," WHO stated. However, the dose required to infection someone is not known, it added.

In an updated Q&A, the WHO pointed to reports of COVID-19 outbreaks in closed settings, such as restaurants, nightclubs, places of worship, or places "where people may be shouting, talking, or singing." It conceded that "aerosol transmission, particularly in these indoor locations where there are crowded and inadequately ventilated spaces where infected persons spend long periods of time with others, cannot be ruled out."

In fact, there is other research suggesting 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 in March, 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.” 

In its scientific update, the WHO also acknowledged  that infected people can transmit the virus even if they don't have symptoms. Taken together, these revelations about COVID-19 underscore the importance of wearing face masks in public places and where social distancing isn't possible. Until there's an approved vaccine, abiding by the 6-foot rule, avoiding crowds, wearing a non-surgical mask or cloth face covering, and washing your hands frequently remain some of the best ways to slow the spread of the virus.

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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