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PROFESSOR: So now we're
ready to synthesize

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our knowledge of
airborne transmission

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in a well-mixed room and
epidemiological models

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to arrive at a safety
guideline to limit

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the indoor airborne
spread of COVID-19,

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or, more generally, other
respiratory pathogens.

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Existing guidelines
limit one parameter

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that might be important.

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For example, social
distancing limits the spacing

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between people,
which can be defined

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by the average area per occupant
for example, to a minimum

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of, say, 6 feet or 1 meter.

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Other rules, for
example, adopted here

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in the state of Massachusetts,
limit all gatherings

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to be no larger than
25 persons indoors.

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There also are recommendations
from heating and ventilation

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societies that may recommend
we increase the fresh air

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change rate to six or higher.

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In the UK, for example,
the fresh air flow rate

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might be prescribed to 10
liters per second per person

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or other such numbers.

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Also the time that
an infected person

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is in the presence
of susceptible people

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might be limited
to say, 15 minutes.

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That's involved in the
definition of a contact

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here in the United States.

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So what we will see is
that it's really not

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possible to write down a
realistic guideline that

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bounds any one quantity, because
there will always be situations

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where you either
are too conservative

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or are not careful enough
in bounding that quantity,

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because these qualities
are all related.

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So, for example,
25 persons might

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be perfectly safe in
a very large space

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for a very short amount of time
with very high ventilation.

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But take those 25 persons and
seal them into a small tent,

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breathing each other's
air for 24 hours with very

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little ventilation, it's a
completely different situation.

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And the same holds
for the distance,

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or the time, or the flow
rates, everything is mixed.

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And then what about
other variables

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that we don't
explicitly control?

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Like relative humidity
or filtration efficiency,

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if you're using filters
to filter the air.

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Or if you're wearing masks.

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The quality of the masks,
how well does that come in?

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How about the
volume of the space?

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It's not just the flow
rate, but it's also

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the geometry, even the area,
or the length of the space.

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So somehow all these
variables must be related,

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and the goal of this chapter
is to derive that relationship

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for the case of a
well-mixed room, where it's

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a unique universal guideline.

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And to parameterize this
specifically for COVID-19,

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by looking at a variety of
superspreading events for which

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enough data is available
that we can make

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a reasonable and consistent
inference of the infectiousness

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of exhaled breath.