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PROFESSOR: So now
that we understand

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the types of droplets
that are emitted

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by different types
of respiration,

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and their evolution
in the environment

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as a function of humidity
and other factors.

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Let's talk about
the different ways

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that transmission
can actually occur

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between two individuals
of a respiratory pathogen.

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So the first is
contact transmission

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through so-called fomites.

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These are residues of those
infectious droplets that

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had been emitted by breathing,
which can build up on surfaces

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such as tables, floors,
even on people's clothing,

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on their hands,
from the breathing

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of an infected person.

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And then as a central
person touches

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those surfaces and then
touches their eyes or touches

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their nose.

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And in various ways gets
the pathogen into their body.

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So that is not likely to be the
dominant mode of transmission

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for COVID-19, as the
evidence is building up

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of other modes of transmission.

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Nevertheless, it is
still recommended

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to be disinfecting
and washing surfaces

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to protect against this
potential mode of transmission.

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Another mode of transmission is
through large ballistic drops.

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So these are the
droplets we talked

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about at the beginning,
some of which

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are large enough to
sediment to the ground.

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Others of which will
eventually settle,

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but which can be
transferred by the momentum

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of a cough, or a sneeze,
or some violent exhalatory

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event to another person.

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And that other
person can directly

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breathe in those droplets.

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So this is especially
important when dealing

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with symptomatic individuals.

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On the other hand,
for COVID-19, it's

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well established that the
majority of transmissions

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are, in fact, asymptomatic.

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So people are not
coughing or sneezing,

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and yet are still managing
to transmit this highly

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infectious virus.

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Which brings us then
to the third mode

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of transmission, which is
through aerosol droplets.

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These are the droplets
which do not settle quickly

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on the timescale of occupancy
of the room, or a ventilation,

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or other factors that
remove those droplets,

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such as deactivation.

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And those droplets are emitted
even in normal respiration/

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normal breathing.

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So just simply the puffs
of breathing, speaking--

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put those aerosol
droplets in the air

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and then they're
carried by air currents

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in the room, which we will
analyze in greater detail later

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in this course, and
essentially fill the room.

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And as a first
approximation, those droplets

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are spread throughout
the room, and is

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a well mixed space of air.

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And within that
context, there are still

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two modes of transmission
we can talk about.

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If people are not wearing
any masks or face covering,

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then those puffs
of respiration can

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be directly impinging upon
a susceptible person, who

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then can breathe them in.

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And those aerosols will be
at a higher concentration

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than the background ambient
air of the well mixed room.

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And we refer to that
as short-range aerosol

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transmission.

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And we will return to that in
the last part of the course.

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But what we're going
to focus on first

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is long-range
airborne transmission.

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So these are the droplets
that end up in the air.

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They become well mixed
throughout the space.

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And anyone in the room,
even very far away,

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can breathe those droplets
in and, over time,

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can inhale an infectious
dose and become infected.

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It's important to recognize
the role of face coverings,

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especially in the context
of these respiratory

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droplet transmission modes.

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So whether it's a
mask or a face shield,

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those facial coverings
can essentially

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eliminate short-range
aerosol transmission.

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Of course, they also completely
eliminate large drop emission

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because the droplets
don't make it

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through those ballistic
protections if you will.

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And even the momentum generated
in the air from the breathing

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is largely eliminated
by shields or masks.

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On the other hand, these
small aerosol droplets

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can pass through masks.

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They certainly can pass
around face shields or even

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plastic shields that you
see in various public spaces

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these days.

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And those droplets then are
quickly spread around the room

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and we're left with the
airborne mode of transmission.

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We will see later, there
is an important distinction

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between a mask and
a shield, however.

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While both of them blocked
the momentum of the fluid that

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leads to puffs and respiratory
jets and plumes of transfer,

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they do provide, in
the case of the mask,

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additionally filtration,
which can actually

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block many of the droplets--
not necessarily all,

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but a significant fraction.

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And that will be
an important aspect

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of understanding
how to make spaces

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safe from airborne transmission.