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PROFESSOR: Now,
let's talk briefly

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about different types of
ventilation and different ways

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that they can mix or not
mix the air in a room.

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So the first type, which
is what we've mainly

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been talking about, is
so-called mixing ventilation.

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So this is when the colder
air is forced from above--

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or in other cases,
the warmer air

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is forced from
below-- in such a way

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that there's an unstable
thermal gradient,

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so that the buoyancy forces end
up contributing towards mixing.

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And then you can see that
if a person's in the room,

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the natural thermal plume rising
around that person of warmer

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air, as well as the
respiratory plume

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sketched here in yellow
from all the breathing--

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both of them are kind of rising
and then well mixed in the room

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before they're finally
removed by the outlet

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of the ventilation.

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And so that is the situation
we've been mainly talking

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about of the well-mixed room.

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But it's worth considering
that in some other cases,

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the ventilation specifically
is set up to reduce mixing.

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So especially in rooms
that have high ceilings,

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it can be advantageous to set
up a stratified ambient, where

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the thermal plume from
the body is rising

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as well as the respiratory
plumes and are collecting

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in the upper area of the room
where they're being gently

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sucked out as the colder air is
being pumped in now from below.

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So that's actually a
stable thermal gradient.

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And so at least
the buoyancy forces

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are not leading to any
large scale convection,

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although there certainly are
still boundary layer flows

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and plumes from the people
and from their motion,

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as I have described earlier.

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One advantage here is
that if a mask is worn,

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then the respiratory droplets
coming from breathing, which

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are sketched here in
yellow, are kind of

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brought closer to the thermal
plume and forced to rise more.

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So it basically causes
the little droplet--

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let's say, the
infectious aerosols

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from the breath
to be more likely

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swept to the upper
reaches, where

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they might sit at the
higher levels of the room

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and then be sucked out.

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So that can actually
be a situation

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where a less well-mixed
room can be better.

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On the other hand, the
advantage of a well-mixed room

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is that the pathogen
in the droplets

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is diluted throughout
a larger space.

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And that can be
more advantageous.

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But in any case, this
situation with high ceilings

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and especially with natural
ventilation, where you set up

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a stratified ambient,
is a strategy that's

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been used for a long time.

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In fact, Florence
Nightingale, in the 1800s,

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recommended that
hospitals should

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have high ceilings
and good ventilation

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for protection against
airborne pathogens.