“…New evidence suggests six feet
of distance may not be enough. If SARS-CoV-2 is airborne, as scientists think
it may be, people could be infected simply by inhaling the virus in tiny
aerosol droplets exhaled by someone talking or breathing.
“What’s actually safe is
unknown. It may depend on many factors, including whether people are inside or
outdoors, how loudly people are speaking, whether they are wearing masks, how
well-ventilated a room is, and how far the virus can really fly.
“When people exhale, talk, sing,
cough or sneeze, a cloud of droplets of various sizes leaves the mouth or nose,
says Lydia Bourouiba, a fluid dynamicist at MIT. Most simulations of droplet
behavior have considered big and small droplets separately. Researchers have
worried mainly about bigger droplets — 5 to 10 micrometers in diameter or
larger — as vehicles for transmitting viruses, bacteria or other contagious
organisms.
“Bigger droplets can pack in
more infectious organisms, giving a greater chance of infection if someone
comes into contact with them. But the bigger the droplets are, the heavier they
become, dropping fairly quickly to the ground. Such droplets are thought rarely
to travel more than a meter or two before hitting the ground or another
surface.
“Those droplets might infect
people by direct contact, such as when someone coughs or sneezes right in your
face. But researchers think indirect contact is the main way people catch
viruses, says Qingyan Chen, a mechanical engineer studying how infectious
diseases spread at Purdue University in West Lafayette, Ind.
“Indirect contact might involve
an infected person using their hand to cover a cough or a sneeze, then touching
a cup or another object. If an uninfected person handles the object, the virus
could transfer to that person’s hands. An unwitting nose scratch, eye rub or
finger food snack could then infect that person. That’s why handwashing is so
important.
“Breathing in smaller droplets,
known as aerosols, exhaled or coughed up by an infected person may also cause
infection. Tiny droplets have a hard time overcoming drag from air and are
thought to hang around a person, within a meter (a few feet).
“Hence the six-foot rule: It was
thought to be far enough to be safe from both occasional long-range spit
bullets and invisible clouds of smaller particles. But droplets spewed from
people’s lungs come in a continuum of sizes, from those big enough to see with
the naked eye to microscopic droplets churning through the air as a turbulent
cloud, Bourouiba says.
“‘This cloud, in fact, changes
everything about the dispersal of the drops that you don’t really see,’ she
says. The warm, moist exhaled air within the turbulent cloud has forward
momentum from breathing, coughing or sneezing, carrying droplets of all sizes
much farther than previously thought. In the case of a sneeze, droplets can travel up to eight meters (23 to 27 feet),
Bourouiba reports March 26 in JAMA. That means even small droplets may
spread throughout a room. And if droplets fly that far, the virus may,
too. ‘There’s no reason to believe that the virus only stays in those
[droplets] that fall close by,’ she says.
“Coughs also can propel aerosol
droplets beyond six feet, evidence suggests. Over three flu seasons, fluid
mechanics engineer Eric Savory at the University of Western Ontario in London,
Canada, and colleagues persuaded sick people to cough into a large box that
allowed the researchers to measure how fast and far respiratory viruses travel.
The volunteers coughed while they were sick with influenza, RSV or cold-causing
coronaviruses. Some came back after they were feeling well to cough for science
again.
“Even a meter away from the
mouth, coughs are still traveling at about a meter per second, the researchers discovered. ‘It’s not a speed you can
avoid by turning your head away,’ Savory says. Volunteers who were either ill,
convalescent or healthy all coughed at about the same velocity. Results of the
study will appear in an upcoming issue of Indoor Air.
“The small
droplets do slow down as they get farther from the mouth, Savory says. But his
data don’t suggest what’s a safe distance. ‘A good guidance is you’re lessening
your risk [of infection] the farther you are away from someone.’
Singing to the choir
“The smallest airborne droplets may be more of
a worry than scientists previously recognized. Aerosol droplets containing
infectious SARS-CoV-2 particles can hang around in the air for hours, a March 17 study in the New England Journal of Medicine found.
The experiment, conducted under lab conditions, measured air samples for only
three hours, but found still-infectious viruses. Some researchers have
criticized that study because the virus-laden droplets were made using a
medical machine, not by methods that more closely mimic breathing.
“But people experience the
spread of aerosol particles every day, says William Ristenpart, a chemical
engineer at the University of California, Davis. If someone on one side of a
large room lights a cigarette, puts on perfume or opens a box of chocolate chip
cookies, the smell eventually reaches the other side of the room. ‘It’s not
because [the smoker/perfume wearer/cookie eater is] coughing,’ he says.
Turbulence created by air mixing carries aerosol droplets around the room.
“Ristenpart investigates whether
influenza and other respiratory diseases can spread by airborne particles. The
coronavirus’s contagiousness is a clue that it might, he says. Researchers at
the U.S. National Academies of Science, Engineering and Medicine also concluded
in an April 1 report that the virus might spread through aerosol
particles (SN:
4/2/20).
“Add to that the fact that
people can spread the virus before they
develop symptoms, or without ever
developing symptoms (SN: 3/13/20). In
fact, COVID-19 may be most contagious one to two days before symptoms
appear, when people don’t even know they
are infectious (SN: 4/15/20).
Almost half of people in Iceland who tested positive for COVID-19 didn’t
have symptoms when diagnosed,
researchers report April 14 in the New
England Journal of Medicine. Many would probably develop symptoms
later, says Kári Stefánsson, founder of deCODE Genetics, a Reykjavik-based
company screening anyone who wants a test. By definition, people without
symptoms aren’t coughing and sneezing. But they are talking and breathing.
“And singing. Aerosol particles
released when people sing may have led the coronavirus to spread to 45 members
of the Skagit Valley Chorale in
Washington. Two died. Some choir members met for practice on March 3 and 10
before Washington state issued a stay-home order and before Skagit County had
any known COVID-19 cases. Choir members reportedly kept six-foot distance from
each other. But in belting out tunes, whistling and talking to one another,
infected choir members may have propelled the virus into each others’ safety
zones.
“‘A good singer knows how to use
all the air in their lungs,’ says Donald Milton, an infectious disease
specialist at the University of Maryland School of Public Health in College
Park. That may lead to exhaling lots of coronavirus, or breathing it deep into
the lungs during breaths between refrains.
“Even just talking
face-to-face with an infected but
asymptomatic person may be enough to spread the virus, Ristenpart and
colleagues propose April 3 in Aerosol
Science and Technology. Standing six feet apart might cause people
to raise their voices to be heard, and people produce more aerosols and larger droplets
the louder they speak, Ristenpart
and colleagues reported in February 2019 in Scientific
Reports. ‘There’s a compelling case that one should be suspicious
of conversation as a possible vector for transmission,’ he says.
“Speech generates hundreds
of big, wet drops, researchers
report April 15 in the New England Journal of Medicine.
‘Stay healthy’ is, ironically, a phrase that sprays a lot of saliva droplets.
But that study was not capable of measuring droplets smaller than 20
micrometers across, says Matthew Meselson, a biologist at Harvard University
who wrote a comment on the study, also appearing April 15 in NEJM.
Other studies have determined that talking produces thousands of aerosols from
the lungs for every saliva droplet from the mouth, he says.
“It’s too early to tell whether
aerosols and big droplets produce different severities of infection, he says.
The pandemic may change doctors’ and researchers’ view of how respiratory
viruses in general spread, Meselson says. ‘I think we’ll find we really were
behind the curve when it comes to thinking about how disease is transmitted
through the air.’
Masks to the rescue?
“...Studies
of medical grade masks have demonstrated that they can block the forward flow
of large particles, but aerosols still shoot out of the sides and top of
surgical masks. Few studies have addressed the effectiveness
of homemade masks (SN: 4/9/20). And no one knows exactly how many
coronavirus particles are necessary to start an infection. For some viruses, as
few as one virus may be enough. But even if a cloth mask filters out only 10
percent of virus particles, ‘in a pandemic, maybe it’s worth doing,’ says
Milton, the University of Maryland infections disease specialist.
“A
mask of any sort may help the mask wearer release fewer droplets into the air,
which helps to protect people around the wearer. Masks reduce the momentum of
the exhaled cloud of droplets, diverting flow and reducing the range the
particles can travel. But a mask ‘does not replace social distancing,’
Bourouiba says. ‘It is not high-grade protection that people should feel
overconfident about.’
Going with the flow
“Even with a mask, six feet may not be a safe
distance. Airflow is a big factor in determining how far is safe enough, says
Chen, the Purdue mechanical engineer. Indoors, heating and air conditioning
units may draw virus-laden air toward certain parts of a room. But the systems
may also bring in fresh air and cycle out stale air containing viruses and
other particles. Ceiling fans and other fans may also blow the virus around a
room. In such settings, it may be hard to figure out where best to evade the
virus. Just as with indoor smoking, Milton says, ‘you’ll be trapped in that
atmosphere.’
“It’s a good idea to keep six
feet between grocery store patrons, but whether it’s really safe is another
matter, Meselson says. ‘There’s no magic about six feet,’ he says. ‘It’s better
than two feet, and 10 feet is better than six, but for aerosol, I don’t know
what to say.’ Aerosol particles can linger long after a shopper has gone home,
potentially infecting workers or people who visit the store later. His advice:
‘Try not to go into any enclosed space if you have any reason to believe there
was an infected person inside.’
“Outside, six feet is probably a
safe distance. Six feet may be OK when asymptomatic individuals are talking
outdoors, Bourouiba says, particularly if both are wearing masks. Savory, the Western University
engineer studying coughs, agrees. ‘The advice that is being given is really
sound. It’s a good compromise to be two meters away from someone,’ he says.
‘This distance dramatically reduces the transmission capability of this virus
and indeed any virus. If you want to be more safe, you should stay away as
far as you can.’
“Six
feet may also be OK when it comes to the huffs and puffs of exercise. People
exercising outdoors run the risk of stepping into another person’s exhalation
cloud, but then outdoor airflow may quickly dissipate the cloud.
“‘When
you’re walking, you’re walking through your exhaled breath with every step, but
it’s quite well diluted,’ says Julian Tang, a virologist and fluid dynamicist
at the University of Leicester in England. And there is little side-to-side air
movement, so stepping off the path to walk by someone else carries little risk,
he says. ‘It’s quite transient. They’re coming towards me, and then they’re
gone.’
“But
the idea of a ‘safe’ distance outdoors depends on if, and which way, the wind
is blowing. A breeze may carry virus-laden breath farther than six feet, Milton
says. ‘If there’s a strong breeze blowing, probably nobody should be downwind
from each other’”(Science News).
Tina
Hesman Saey is the senior staff writer and reports on molecular biology. She
has a Ph.D. in molecular genetics from Washington University in St. Louis and a
master’s degree in science journalism from Boston University.
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