PROFESSOR: So we've discussed the extent
to which the size of a droplet can influence
the infectivity or the ability of a virion
to escape from that droplet and, also, to be transmitted
to the deepest, smallest passages in the lungs
as a function of its size.
There is also a dependence on the relative humidity
of the air, which is related to size.
And so, as we've seen, humidity does vary the size,
but there's believed to be also a more direct effect
of humidity, as I will now try to explain.
So I'm relying here on the recent work
of the group of Linsey Marr, two papers cited here.
So we can distinguish between two different types
of pathogens.
The first are the bacteria.
And here there's a monotonic dependence
of the relative viability of the pathogen, of the bacteria,
after a certain time period.
Let's say one hour.
And what is found is that, above a certain threshold
of humidity, around 80% relative humidity,
that there's, essentially, no change in the viability
of the bacteria.
They're alive.
They're infectious.
But, as the humidity, relative humidity, is reduced,
then there's a significant drop off in viability, which
depends on the specific type of bacteria,
but it's a fairly general trend that it comes down
significantly as you approach more dry air.
Now what's happening is the size of the droplets is shrinking.
In the case of the bacteria, we can understand, to some extent,
why this dependence might be here by thinking about solutes
that are present, especially salts, in the system,
but, also, mucus--
mucosal proteins that we've also discussed.
And, when the particles become more dry, then what happens
is that the concentration goes up,
and there's an increase in the osmotic pressure
of the fluid around the bacteria relative to the inside.
And, as with many other kinds of cells,
when exposed to such high osmotic pressures,
that can cause stress on the cell and, potentially, even
rupturing of membranes or other structures within the cell.
And, obviously, then it is not good for the viability
of that cell and leads to deactivation.
The case of viruses is a bit more complicated.
So some old data of Harper from the 1960s on the seasonal flu,
in particular, human influenza virus A,
which was recently analyzed by Marr's group,
showed that there was a viral deactivation
rate that, essentially, was scaling linearly
with the relative humidity.
So there's a faster deactivation rate in more humid air,
less in dry air.
This is one way we can understand the seasonal nature
of the flu in that, in more dry, wintry environments,
especially away from--
in sort of the northern or southern hemispheres,
we can expect that then the virus
would be deactivating less.
But, of course, that's compounded by the effect
that, in the winter, people spend more time indoors,
and so that's also leading to more seasonal transmission.
Now, if we convert the deactivation rate
into relative viability again, then we
see an interesting dependence in recent experiments, which
were done using bacteriophages, which
are models of different kinds of human pathogens,
including the seasonal flu and influenza viruses.
And, in particular, there's a non-monotonic dependence
where, essentially, there's a maximum rate of deactivation
around the range of 70% or 80% humidity, or 60% to 80%.
And, similarly, the viability was the lowest in that range.
And the way the authors proposed to explain
that was a hypothesis that there are solutes that are present,
which may be, for example, sodium
chloride or, in particular, chloride ions, perhaps, that,
when we reach the higher concentration in the shrunken
droplets, that there is, again, a stress on the virus,
but, in this case, regardless of the details of the mechanism
of deactivation for these encapsulated viruses,
the idea is that the cumulative dose
or exposure of those solutes is what's important.
So, if the shrinking happens very fast,
and we end up with a droplet nucleus of, mostly,
bound water, and it happens over a short period of time,
the exposure to those solutes is limited.
And, hence, we end up with high viability, low deactivation
rate in dry conditions.
Conversely, in very humid conditions,
the droplets stay big.
In fact, they may even grow because
of the hygroscopic solutes.
And, in that case, there's plenty of solutes present,
but they're very dilute.
And so, again, the effect on the virus is minimal.
And the greatest deactivation and, also, the maximum--
the sort of minimum viability is actually
at an intermediate range of humidities.
So this tells you that maintaining
a comfortable humidity in the range of 50% to 80%
may, actually, be the best for minimizing the viability
of viral pathogens.