1
00:00:10,940 --> 00:00:12,610
PROFESSOR: So we've
discussed the extent

2
00:00:12,610 --> 00:00:15,640
to which the size of a
droplet can influence

3
00:00:15,640 --> 00:00:19,200
the infectivity or the
ability of a virion

4
00:00:19,200 --> 00:00:22,510
to escape from that droplet
and, also, to be transmitted

5
00:00:22,510 --> 00:00:26,440
to the deepest, smallest
passages in the lungs

6
00:00:26,440 --> 00:00:28,600
as a function of its size.

7
00:00:28,600 --> 00:00:31,660
There is also a dependence
on the relative humidity

8
00:00:31,660 --> 00:00:33,790
of the air, which
is related to size.

9
00:00:33,790 --> 00:00:36,340
And so, as we've seen,
humidity does vary the size,

10
00:00:36,340 --> 00:00:38,590
but there's believed to be
also a more direct effect

11
00:00:38,590 --> 00:00:42,000
of humidity, as I will
now try to explain.

12
00:00:42,000 --> 00:00:43,930
So I'm relying here
on the recent work

13
00:00:43,930 --> 00:00:51,380
of the group of Linsey
Marr, two papers cited here.

14
00:00:51,380 --> 00:00:53,560
So we can distinguish
between two different types

15
00:00:53,560 --> 00:00:54,400
of pathogens.

16
00:00:54,400 --> 00:00:56,800
The first are the bacteria.

17
00:00:56,800 --> 00:00:59,740
And here there's a
monotonic dependence

18
00:00:59,740 --> 00:01:04,060
of the relative viability of
the pathogen, of the bacteria,

19
00:01:04,060 --> 00:01:05,260
after a certain time period.

20
00:01:05,260 --> 00:01:06,860
Let's say one hour.

21
00:01:06,860 --> 00:01:10,480
And what is found is that,
above a certain threshold

22
00:01:10,480 --> 00:01:13,539
of humidity, around
80% relative humidity,

23
00:01:13,539 --> 00:01:16,270
that there's, essentially,
no change in the viability

24
00:01:16,270 --> 00:01:17,170
of the bacteria.

25
00:01:17,170 --> 00:01:17,860
They're alive.

26
00:01:17,860 --> 00:01:19,750
They're infectious.

27
00:01:19,750 --> 00:01:22,850
But, as the humidity,
relative humidity, is reduced,

28
00:01:22,850 --> 00:01:25,539
then there's a significant
drop off in viability, which

29
00:01:25,539 --> 00:01:28,890
depends on the specific
type of bacteria,

30
00:01:28,890 --> 00:01:31,120
but it's a fairly general
trend that it comes down

31
00:01:31,120 --> 00:01:33,789
significantly as you
approach more dry air.

32
00:01:33,789 --> 00:01:38,420
Now what's happening is the size
of the droplets is shrinking.

33
00:01:38,420 --> 00:01:41,930
In the case of the bacteria, we
can understand, to some extent,

34
00:01:41,930 --> 00:01:46,460
why this dependence might be
here by thinking about solutes

35
00:01:46,460 --> 00:01:49,970
that are present, especially
salts, in the system,

36
00:01:49,970 --> 00:01:51,530
but, also, mucus--

37
00:01:51,530 --> 00:01:54,710
mucosal proteins that
we've also discussed.

38
00:01:54,710 --> 00:02:00,020
And, when the particles become
more dry, then what happens

39
00:02:00,020 --> 00:02:01,700
is that the
concentration goes up,

40
00:02:01,700 --> 00:02:04,310
and there's an increase
in the osmotic pressure

41
00:02:04,310 --> 00:02:07,700
of the fluid around the
bacteria relative to the inside.

42
00:02:07,700 --> 00:02:10,460
And, as with many
other kinds of cells,

43
00:02:10,460 --> 00:02:14,210
when exposed to such
high osmotic pressures,

44
00:02:14,210 --> 00:02:17,540
that can cause stress on the
cell and, potentially, even

45
00:02:17,540 --> 00:02:20,660
rupturing of membranes or other
structures within the cell.

46
00:02:20,660 --> 00:02:23,300
And, obviously, then it is
not good for the viability

47
00:02:23,300 --> 00:02:26,220
of that cell and
leads to deactivation.

48
00:02:26,220 --> 00:02:29,590
The case of viruses is
a bit more complicated.

49
00:02:29,590 --> 00:02:36,090
So some old data of Harper from
the 1960s on the seasonal flu,

50
00:02:36,090 --> 00:02:39,600
in particular, human
influenza virus A,

51
00:02:39,600 --> 00:02:43,140
which was recently
analyzed by Marr's group,

52
00:02:43,140 --> 00:02:45,329
showed that there was
a viral deactivation

53
00:02:45,329 --> 00:02:47,820
rate that, essentially,
was scaling linearly

54
00:02:47,820 --> 00:02:50,250
with the relative humidity.

55
00:02:50,250 --> 00:02:54,240
So there's a faster deactivation
rate in more humid air,

56
00:02:54,240 --> 00:02:55,710
less in dry air.

57
00:02:55,710 --> 00:02:58,230
This is one way we can
understand the seasonal nature

58
00:02:58,230 --> 00:03:02,020
of the flu in that, in more
dry, wintry environments,

59
00:03:02,020 --> 00:03:03,490
especially away from--

60
00:03:03,490 --> 00:03:06,840
in sort of the northern
or southern hemispheres,

61
00:03:06,840 --> 00:03:09,150
we can expect that
then the virus

62
00:03:09,150 --> 00:03:11,010
would be deactivating less.

63
00:03:11,010 --> 00:03:13,260
But, of course, that's
compounded by the effect

64
00:03:13,260 --> 00:03:16,150
that, in the winter, people
spend more time indoors,

65
00:03:16,150 --> 00:03:20,140
and so that's also leading to
more seasonal transmission.

66
00:03:20,140 --> 00:03:21,810
Now, if we convert
the deactivation rate

67
00:03:21,810 --> 00:03:25,260
into relative viability
again, then we

68
00:03:25,260 --> 00:03:28,320
see an interesting dependence
in recent experiments, which

69
00:03:28,320 --> 00:03:30,300
were done using
bacteriophages, which

70
00:03:30,300 --> 00:03:35,310
are models of different
kinds of human pathogens,

71
00:03:35,310 --> 00:03:40,440
including the seasonal
flu and influenza viruses.

72
00:03:40,440 --> 00:03:42,690
And, in particular, there's
a non-monotonic dependence

73
00:03:42,690 --> 00:03:45,840
where, essentially, there's a
maximum rate of deactivation

74
00:03:45,840 --> 00:03:51,090
around the range of 70% or
80% humidity, or 60% to 80%.

75
00:03:51,090 --> 00:03:55,510
And, similarly, the viability
was the lowest in that range.

76
00:03:55,510 --> 00:03:57,930
And the way the authors
proposed to explain

77
00:03:57,930 --> 00:04:01,950
that was a hypothesis that there
are solutes that are present,

78
00:04:01,950 --> 00:04:04,230
which may be, for
example, sodium

79
00:04:04,230 --> 00:04:07,830
chloride or, in particular,
chloride ions, perhaps, that,

80
00:04:07,830 --> 00:04:10,740
when we reach the higher
concentration in the shrunken

81
00:04:10,740 --> 00:04:15,180
droplets, that there is,
again, a stress on the virus,

82
00:04:15,180 --> 00:04:20,310
but, in this case, regardless
of the details of the mechanism

83
00:04:20,310 --> 00:04:23,400
of deactivation for these
encapsulated viruses,

84
00:04:23,400 --> 00:04:25,410
the idea is that
the cumulative dose

85
00:04:25,410 --> 00:04:28,020
or exposure of those
solutes is what's important.

86
00:04:28,020 --> 00:04:30,000
So, if the shrinking
happens very fast,

87
00:04:30,000 --> 00:04:32,250
and we end up with a
droplet nucleus of, mostly,

88
00:04:32,250 --> 00:04:35,340
bound water, and it happens
over a short period of time,

89
00:04:35,340 --> 00:04:37,590
the exposure to those
solutes is limited.

90
00:04:37,590 --> 00:04:41,100
And, hence, we end up with high
viability, low deactivation

91
00:04:41,100 --> 00:04:43,470
rate in dry conditions.

92
00:04:43,470 --> 00:04:45,630
Conversely, in very
humid conditions,

93
00:04:45,630 --> 00:04:46,740
the droplets stay big.

94
00:04:46,740 --> 00:04:48,480
In fact, they may
even grow because

95
00:04:48,480 --> 00:04:50,670
of the hygroscopic solutes.

96
00:04:50,670 --> 00:04:53,250
And, in that case, there's
plenty of solutes present,

97
00:04:53,250 --> 00:04:54,250
but they're very dilute.

98
00:04:54,250 --> 00:04:57,150
And so, again, the effect
on the virus is minimal.

99
00:04:57,150 --> 00:05:00,780
And the greatest deactivation
and, also, the maximum--

100
00:05:00,780 --> 00:05:03,450
the sort of minimum
viability is actually

101
00:05:03,450 --> 00:05:05,400
at an intermediate
range of humidities.

102
00:05:05,400 --> 00:05:07,350
So this tells you
that maintaining

103
00:05:07,350 --> 00:05:10,320
a comfortable humidity in
the range of 50% to 80%

104
00:05:10,320 --> 00:05:14,490
may, actually, be the best
for minimizing the viability

105
00:05:14,490 --> 00:05:17,480
of viral pathogens.