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PROFESSOR: So now, we can
summarize the safety guideline

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in a very simple formula, using
the steady state transmission

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rate as a more
conservative estimate

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than worrying about the
transient buildup of aerosols

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in the room, and using the
simplification of introducing

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an effective size
for particles, R bar,

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that includes and encodes
all the information

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about the radius dependent
mass penetration factor

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or sedimentation rate and
infectivity, et cetera,

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in droplet size distribution.

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And the safety guideline
looks like this.

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It basically bounds the
cumulative exposure time,

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which is n minus 1, the
number of susceptible people

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when one person enters a room of
occupancy n, maximum occupancy,

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and stays for a time tau.

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And I should mention
the time tau does not

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need to be continuous.

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So if you think of, say,
a classroom or an office,

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you will be adding up
the time, total time.

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So if this safe
time is 100 hours

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and you spend 10
hours a day, you

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might get 10 days, according
to this calculation.

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And so that relationship is
plotted as this yellow curve.

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And if you're below
that curve, you're safe.

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So you have either smaller time
than that or a lower occupancy.

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And that's how you
can decide the safety.

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Again, this is a safety
exposure guideline

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for indoor airborne
transmission.

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And we will talk about other
types of transmission later.

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What's nice about
this formula is

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it allows you to
immediately see the effects

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of the scaling, some
of which we've already

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talked about before.

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So if we would like to
move it in this direction,

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obviously, we would like the
curve to be up here somewhere.

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So this is obviously
making the room more safe.

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If I can push it
in this direction,

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I can either get more
people or more time

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or whatever combination
that I'd like.

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How can I get there?

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So Cq is a disease
specific parameter.

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But we will see
that Cq does depend

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on the type of respiration
and activities,

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such as singing
or heavy exercise

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or loud speaking are much worse
than resting or breathing.

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So to get in this
direction, we could

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do smaller Cq, which
could be resting

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versus speaking or singing.

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And we'll come back
to characterizing that

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more carefully.

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So we can at least change that
type of activity in the room.

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Well for physical
parameters, we can

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increase V, the room volume.

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So if you take everything the
same, same number of people,

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and you make the
room much bigger,

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obviously the air
gets more diluted.

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And hence, there's less
chance of transmission

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if people are sort of
scattered about the room.

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What else can we do?

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We can lower Qb, which means
breathe slowly or in a more

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relaxed way.

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So again, coming
back to the activity

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in the room, if you have
heavy physical exercise,

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your Qb might increase, although
it doesn't increase that much.

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Resting breathing is around
0.5 meters cube per hour.

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And even heavy exercise
doesn't get much higher

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than about 3 meters
cubed per hour.

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And so that's a total
of a factor of 6 or so.

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But it comes in squared.

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So that could actually
be significant.

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So definitely heavy exercise
is not as good as resting.

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And what else do we have?

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We have masks.

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Well, obviously,
we can wear masks.

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And that is actually
very helpful,

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because as we've already
discussed, it comes in squared.

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And that is something which
is sort of very simple fix.

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And if the mask
penetration factor

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is 10% or even possibly
less, it comes in squared.

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So that can have a very big
effect of pushing this over,

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as we will see.

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So this is basically
lower P mask.

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So when there's no
mask, Pm is equal to 1.

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Finally we also
have an lambda C,

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because we had written
that out earlier,

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that also includes
several effects,

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such as better ventilation.

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So increase ventilation, so
the flow rate of outside air,

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so the air change rate
should be increased.

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And that gives us
a larger lambda C.

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We can introduce air filtration.

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And that's sort of
buried within lambda C.

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That's PF lambda F. Although
as we discussed earlier,

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because you have to have a
certain amount of fresh air

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coming into the room, air
filtration doesn't give you

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as much benefit
as you might hope,

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but can still give you a
factor of 5 or something

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like that if you have 20%
outdoor fresh air coming in.

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And so those are some
of the key scalings

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that we can understand.

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And just simply by plugging
into a formula like this,

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we can get a sense of how
different mitigation strategies

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can be compared and also
how different types of rooms

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can be compared in terms
of the occupancy time

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and the number of
people that could be

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allowed as a maximum occupancy.

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And what remains now is the
parameterize this for COVID-19

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specifically, which boils down
to understanding this parameter

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Cq.