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.
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