WEBVTT

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PROFESSOR: So we've
discussed the extent

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to which the size of a
droplet can influence

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the infectivity or the
ability of a virion

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to escape from that droplet
and, also, to be transmitted

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to the deepest, smallest
passages in the lungs

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as a function of its size.

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There is also a dependence
on the relative humidity

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of the air, which
is related to size.

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And so, as we've seen,
humidity does vary the size,

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but there's believed to be
also a more direct effect

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of humidity, as I will
now try to explain.

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So I'm relying here
on the recent work

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of the group of Linsey
Marr, two papers cited here.

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So we can distinguish
between two different types

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of pathogens.

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The first are the bacteria.

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And here there's a
monotonic dependence

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of the relative viability of
the pathogen, of the bacteria,

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after a certain time period.

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Let's say one hour.

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And what is found is that,
above a certain threshold

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of humidity, around
80% relative humidity,

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that there's, essentially,
no change in the viability

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of the bacteria.

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They're alive.

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They're infectious.

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But, as the humidity,
relative humidity, is reduced,

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then there's a significant
drop off in viability, which

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depends on the specific
type of bacteria,

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but it's a fairly general
trend that it comes down

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significantly as you
approach more dry air.

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Now what's happening is the size
of the droplets is shrinking.

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In the case of the bacteria, we
can understand, to some extent,

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why this dependence might be
here by thinking about solutes

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that are present, especially
salts, in the system,

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but, also, mucus--

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mucosal proteins that
we've also discussed.

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And, when the particles become
more dry, then what happens

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is that the
concentration goes up,

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and there's an increase
in the osmotic pressure

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of the fluid around the
bacteria relative to the inside.

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And, as with many
other kinds of cells,

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when exposed to such
high osmotic pressures,

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that can cause stress on the
cell and, potentially, even

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rupturing of membranes or other
structures within the cell.

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And, obviously, then it is
not good for the viability

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of that cell and
leads to deactivation.

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The case of viruses is
a bit more complicated.

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So some old data of Harper from
the 1960s on the seasonal flu,

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in particular, human
influenza virus A,

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which was recently
analyzed by Marr's group,

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showed that there was
a viral deactivation

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rate that, essentially,
was scaling linearly

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with the relative humidity.

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So there's a faster deactivation
rate in more humid air,

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less in dry air.

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This is one way we can
understand the seasonal nature

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of the flu in that, in more
dry, wintry environments,

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especially away from--

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in sort of the northern
or southern hemispheres,

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we can expect that
then the virus

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would be deactivating less.

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But, of course, that's
compounded by the effect

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that, in the winter, people
spend more time indoors,

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and so that's also leading to
more seasonal transmission.

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Now, if we convert
the deactivation rate

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into relative viability
again, then we

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see an interesting dependence
in recent experiments, which

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were done using
bacteriophages, which

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are models of different
kinds of human pathogens,

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including the seasonal
flu and influenza viruses.

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And, in particular, there's
a non-monotonic dependence

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where, essentially, there's a
maximum rate of deactivation

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around the range of 70% or
80% humidity, or 60% to 80%.

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And, similarly, the viability
was the lowest in that range.

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And the way the authors
proposed to explain

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that was a hypothesis that there
are solutes that are present,

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which may be, for
example, sodium

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chloride or, in particular,
chloride ions, perhaps, that,

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when we reach the higher
concentration in the shrunken

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droplets, that there is,
again, a stress on the virus,

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but, in this case, regardless
of the details of the mechanism

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of deactivation for these
encapsulated viruses,

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the idea is that
the cumulative dose

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or exposure of those
solutes is what's important.

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So, if the shrinking
happens very fast,

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and we end up with a
droplet nucleus of, mostly,

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bound water, and it happens
over a short period of time,

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the exposure to those
solutes is limited.

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And, hence, we end up with high
viability, low deactivation

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rate in dry conditions.

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Conversely, in very
humid conditions,

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the droplets stay big.

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In fact, they may
even grow because

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of the hygroscopic solutes.

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And, in that case, there's
plenty of solutes present,

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but they're very dilute.

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And so, again, the effect
on the virus is minimal.

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And the greatest deactivation
and, also, the maximum--

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the sort of minimum
viability is actually

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at an intermediate
range of humidities.

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So this tells you
that maintaining

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a comfortable humidity in
the range of 50% to 80%

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may, actually, be the best
for minimizing the viability

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of viral pathogens.