Six feet: You've heard that measurement everywhere by now—from officials at the Centers for Disease Control and Prevention (CDC); 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, generally 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 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, Randall told Health this spring.
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 explained. 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 at a much greater distance from the source of infection."
What do studies of airborne transmission tell us?
Could the novel coronavirus exist as an aerosol for airborne transmission? A laboratory experiment published March 27 in the New England Journal of Medicine (NEJM) provides 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 produced 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.
Other researchers point to "super-spreader" events, where one person infects many, as evidence that the virus may be transmitted via particles that hang in the air. That's likely what happened to a group traveling by bus in Wuhan, China, resulting in a third of the passengers becoming infected, per the New York Times.
So, is the coronavirus airborne or not?
Let's just say there's been a lot of debate on airborne transmission, and many scientists now believe it's possible that tiny, virus-containing particles can linger in the air. In July, more than 200 international scientists signed an open letter cautioning that airborne transmission is "a real risk" and urging the WHO to update its guidance. WHO has acknowledged "emerging evidence" of different modes of transmission but says "much more research is needed."
In a 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."
On Monday, the CDC finally conceded that "COVID-19 can sometimes be spread by airborne transmission." The updated advisory comes after the public health agency previously published, then retracted, guidance acknowledging the potential for such exposure. The CDC's backpedaling prompted an uproar among scientists who insisted that people need to be aware of this mode of transmission. Now the agency is admitting there's evidence that some people with COVID-19 have infected others who were more than 6 feet away. These incidents occurred within enclosed spaces with inadequate ventilation, it says. "Sometimes the infected person was breathing heavily, for example while singing or exercising," adds CDC.
Infectious disease experts writing in the journal BMJ say the 6-foot rule is based on outdated science. They want the public to know that, in some cases, that may not be enough, especially in poorly ventilated areas, the Washington Post reports.
Experts say the best defense against COVID-19 remains a multipronged approach that includes frequent hand washing, cleaning frequently touched surfaces, avoiding crowded indoor spaces, and, of course, wearing a face mask.
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 CDC, WHO, and their local public health department as resources.
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