“When
someone coughs, talks or even breathes, they send tiny respiratory droplets
into the surrounding air. The smallest of these droplets can float for hours,
and there is strong evidence that they can carry
live coronavirus if the person is infected.
“Until
this week, however, the risk from these aerosols wasn’t incorporated into the
World Health Organization’s formal guidance for nations. The WHO
instead suggested that the coronavirus was primarily transmitted by coughing or
sneezing large droplets into someone’s face, rather than being a longer-term threat
that can float in the air.
“It
took pressure from scientists to start to change that. More than 200 scientists
published an open letter to the WHO on July 6 warning about airborne
transmission of COVID-19 via aerosols and urging the organization to recognize
the risks. The WHO responded Thursday afternoon with an
update in which it acknowledged the growing evidence of airborne spread of the
disease, but it did so with hesitation.
“As professors who
study fluid dynamics and aerosols, we believe
it is important for people to understand the risks and what they can do to
protect themselves.
What is an aerosol and how does it spread?
“Aerosols
are particles that are suspended in the air. When humans breathe, talk, sing,
cough or sneeze, the emitted respiratory droplets mix in the surrounding air
and form an aerosol. Because larger droplets quickly fall to the ground,
respiratory aerosols are often described as being made up of smaller droplets
that are less than 5 microns, or about one tenth the width of a human hair.
“In
general, droplets form as a sheet of liquid breaks apart. You’ve probably
experienced this phenomenon by blowing soap bubbles. Sometimes the bubble
doesn’t fully form, but instead breaks apart into many droplets.
“Similarly,
in humans, small sheets and strands of liquid – mucus – often stretch across
portions of the airway. This most often occurs in locations where the airway
opens and closes again and again. That happens deep within the lungs as the
bronchioles and alveolar sacs expand and contract during breathing, within the
larynx as the vocal folds vibrate during speech, or at the mouth, as the tongue
and lips move while talking. The airflow produced by breathing, speaking and
coughing breaks apart these sheets of mucus, just like blowing the soap bubble.
“The
size of the droplets varies based on how and where they are produced within the
airway. While coughing generates the largest quantity of droplets, research
has shown that just two to three minutes of talking can produce
as many droplets as one cough.
“Droplets
that are smaller than 5 microns can
remain suspended in the air for many minutes to hours because
the effect of air drag relative to gravity is large. In addition, the water
content of virus-carrying droplets evaporates while they are airborne,
decreasing their size. Even if most of the fluid evaporates from a virus-laden
droplet, the droplet does not disappear; it just becomes smaller, and the
smaller the droplet, the longer it will stay suspended in the air. Because smaller
diameter droplets are more efficient at penetrating deep into the pulmonary system,
they also pose a much greater infection risk.
“The WHO guidelines suggested that the virus
RNA found in small droplets wasn’t viable in most circumstances. However, early
research on the SARS-CoV-2 virus has shown that it is viable as an aerosol for up to 3 hours.
Do masks protect from aerosol transmission?
“Face
coverings and masks are absolutely necessary for protection against aerosol
transmission. They serve a twofold purpose.
“First,
they filter the air expelled by an individual, capturing respiratory droplets
and thereby reducing the exposure risk for others. This is particularly
important as they are most effective at capturing larger droplets that are more
likely to have larger quantities of viruses encapsulated within them. This
prevents the larger droplets from directly affecting someone, or evaporating
down to a smaller size and circulating in the air.
“They
also reduce the speed of the puff of air that
is produced when sneezing, coughing or talking. Decreasing the velocity of the
expelled air reduces the distance that droplets are initially transported into
the person’s surroundings.
“It
is important to realize, however, that the protection provided by masks and
face coverings varies depending on the material they are constructed from and
how well they fit. Nevertheless, wearing face coverings to decrease
airborne exposure risk is critical.
Is staying 6 feet away enough to stay safe?
“The
recommendation to maintain a 6-foot separation is based on a study by W. F. Wells in 1934 that showed
an expelled water droplet either falls to the ground, or evaporates, within a
distance of roughly 2 meters, or 6 feet. The study did not, however, account
for the fact that following evaporation of the water in a virus-laden
droplet, the droplet nuclei remains, thereby still
posing a risk of airborne infection.
“Consequently,
while staying 6 feet from other people reduces exposure, it might not be
sufficient in all situations, such
as in enclosed, poorly ventilated rooms.
How can I protect myself from aerosols indoors?
“Strategies
to mitigate airborne exposure are similar to strategies for staying dry when
it’s raining. The longer you stay in the rain, and the harder it’s raining, the
wetter you will get. Similarly, the more droplets you are exposed to, and the
longer you stay in that environment, the higher the exposure risk. Mitigating
risk is therefore based on decreasing both aerosol concentration levels and
exposure time.
“Aerosol
concentrations can be reduced with increased ventilation, although
recirculating the same air should be avoided unless the air can be effectively
filtered prior to reuse. When possible, open doors and windows to increase
fresh air flow.
“Decreasing
the number of emission sources – people – within a space, and ensuring that
face coverings are worn at all times can further decrease concentration levels.
“Methods
of deactivating the virus, such as germicidal
ultraviolet light, can also be used. Finally, reducing the amount of
time you spend in poorly ventilated, crowded areas is a good way to reduce
airborne exposure risk” (The Conversation).
This article has been updated with the WHO response. Amir Mofakham, a research associate in
mechanical engineering at Clarkson University, contributed to this article.
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