WEBVTT 00:00:10.880 --> 00:00:13.940 PROFESSOR: So the transfer of respiratory pathogens 00:00:13.940 --> 00:00:16.970 is primarily accomplished through droplets 00:00:16.970 --> 00:00:19.880 that are emitted by an infected person 00:00:19.880 --> 00:00:24.740 and then either breathed in or ending up on surfaces 00:00:24.740 --> 00:00:28.880 and touched and incorporated into the body in some other way 00:00:28.880 --> 00:00:31.590 by a susceptible person. 00:00:31.590 --> 00:00:35.450 So let's begin by talking about the formation of droplets 00:00:35.450 --> 00:00:37.860 during respiration. 00:00:37.860 --> 00:00:44.600 So these droplets can form in different parts 00:00:44.600 --> 00:00:47.220 of the respiratory tract. 00:00:47.220 --> 00:00:49.700 So the respiratory tract refers to the whole system 00:00:49.700 --> 00:00:52.760 of your breathing apparatus in your body. 00:00:52.760 --> 00:00:55.200 So that includes, of course, your lungs, 00:00:55.200 --> 00:00:59.390 which involves a network of passages 00:00:59.390 --> 00:01:02.210 going from the large bronchus down to the bronchioles 00:01:02.210 --> 00:01:08.240 and ultimately to the alveolar sacs where the air is exchanged 00:01:08.240 --> 00:01:10.730 or oxygen gets into the blood and carbon dioxide 00:01:10.730 --> 00:01:13.670 is picked up, and then you exhale. 00:01:13.670 --> 00:01:18.080 In the upper respiratory tract, we have of course 00:01:18.080 --> 00:01:21.470 the mouth and the nose and the larynx, the voice 00:01:21.470 --> 00:01:24.530 box where sounds are made. 00:01:24.530 --> 00:01:26.300 The nasopharynx is sort of the region 00:01:26.300 --> 00:01:30.470 behind the mouth and the nose where 00:01:30.470 --> 00:01:32.900 the passages are connected. 00:01:32.900 --> 00:01:35.270 And in all of those different regions 00:01:35.270 --> 00:01:37.640 of the respiratory tract, when we breathe in, 00:01:37.640 --> 00:01:40.280 air is coming through in one direction, 00:01:40.280 --> 00:01:42.979 and of course when we exhale, it's coming back out. 00:01:42.979 --> 00:01:45.000 And there is a lot of fluid in the lungs. 00:01:45.000 --> 00:01:49.880 So the airways are lined typically 00:01:49.880 --> 00:01:53.210 with surfactant film and mucus, which 00:01:53.210 --> 00:01:57.039 is a thick substance we're all familiar with. 00:01:57.039 --> 00:02:00.110 It can vary in composition but generally has 00:02:00.110 --> 00:02:03.420 some large macromolecules and in particular proteins 00:02:03.420 --> 00:02:04.970 that are called mucins, which give it 00:02:04.970 --> 00:02:06.860 it's sort of thick consistency. 00:02:06.860 --> 00:02:10.160 There are also ions such as sodium and chloride, which 00:02:10.160 --> 00:02:12.050 are dissolved in the liquid. 00:02:12.050 --> 00:02:16.880 And even liquid such as saliva in your mouth 00:02:16.880 --> 00:02:22.370 have a similar composition but less of the sort of thick mucin 00:02:22.370 --> 00:02:25.370 proteins that I mentioned compared to the deeper 00:02:25.370 --> 00:02:26.540 parts the respiratory tract. 00:02:26.540 --> 00:02:27.950 Of course, when someone gets sick, 00:02:27.950 --> 00:02:31.730 also there can be more of the sort of mucus and phlegm 00:02:31.730 --> 00:02:37.190 that's generated to help the body deal with the pathogen. 00:02:37.190 --> 00:02:38.930 So all those liquids and fluids are 00:02:38.930 --> 00:02:42.000 present in different parts of the respiratory tract. 00:02:42.000 --> 00:02:45.950 And so there are a number of mechanisms which are still 00:02:45.950 --> 00:02:49.550 subject to scientific research and debate by which 00:02:49.550 --> 00:02:53.150 droplets are created and ultimately emitted 00:02:53.150 --> 00:02:55.280 when a person is breathing out. 00:02:55.280 --> 00:02:59.210 So let's begin by thinking about such processes 00:02:59.210 --> 00:03:01.880 in the upper respiratory tract. 00:03:09.980 --> 00:03:12.100 So in the upper respiratory tract, 00:03:12.100 --> 00:03:16.900 we can imagine, first of all, that the passages have 00:03:16.900 --> 00:03:18.430 a little bit larger spacing. 00:03:18.430 --> 00:03:21.070 So for example, your mouth might be open by centimeters 00:03:21.070 --> 00:03:22.480 or millimeters. 00:03:22.480 --> 00:03:24.700 If you go into your nose, there's 00:03:24.700 --> 00:03:28.240 of course various hairs and smaller structures which 00:03:28.240 --> 00:03:33.250 are often covered with mucus and liquids, which as the air 00:03:33.250 --> 00:03:39.370 is passing by, could be leading to some breakup of droplets. 00:03:39.370 --> 00:03:42.670 And then of course also in the voicebox and other areas 00:03:42.670 --> 00:03:44.840 of the upper respiratory tract. 00:03:44.840 --> 00:03:53.640 So the main mechanism here for generating droplets 00:03:53.640 --> 00:03:58.650 would be the breakup of viscoelastic filaments 00:03:58.650 --> 00:04:00.300 in a fluid flow. 00:04:00.300 --> 00:04:07.010 So another word for breakup is fragmentation 00:04:07.010 --> 00:04:15.060 of viscoelastic filaments. 00:04:15.060 --> 00:04:20.829 And by that, I mean that the mucus especially is a fluid. 00:04:20.829 --> 00:04:23.640 So it has a viscosity, a resistance to shear flow. 00:04:23.640 --> 00:04:25.320 But it also can have some elasticity. 00:04:25.320 --> 00:04:27.320 If you pull on it, it can pull back a little bit 00:04:27.320 --> 00:04:29.730 because there are these macromolecules present. 00:04:29.730 --> 00:04:32.610 So in general, we have a somewhat complicated reality 00:04:32.610 --> 00:04:35.900 of that liquid or that fluid. 00:04:35.900 --> 00:04:38.000 And a filament refers to the fact 00:04:38.000 --> 00:04:40.430 that those droplets can be stretched out, 00:04:40.430 --> 00:04:43.110 and as the air is then blowing past those filaments, 00:04:43.110 --> 00:04:44.420 it can start to break up. 00:04:44.420 --> 00:04:46.260 So this is our basic mechanism. 00:04:46.260 --> 00:04:50.450 And this is mainly going to be happening 00:04:50.450 --> 00:04:56.450 while a person is exhaling, at least in terms of emissions. 00:04:56.450 --> 00:04:57.920 It's also possible when you Inhale, 00:04:57.920 --> 00:04:59.710 there'll be some of those droplets created. 00:04:59.710 --> 00:05:03.200 They go into your lungs or get deposited on the surfaces 00:05:03.200 --> 00:05:05.690 and then manage to somehow come back out again. 00:05:05.690 --> 00:05:08.570 But certainly during exhaling, you would imagine more-- 00:05:08.570 --> 00:05:12.480 or you could see actually that more droplets are created. 00:05:12.480 --> 00:05:16.200 So if we think of some examples of that, 00:05:16.200 --> 00:05:20.850 we might have, for example, when I'm speaking or breathing 00:05:20.850 --> 00:05:23.930 and my mouth is a little bit open, 00:05:23.930 --> 00:05:30.770 if I imagine drawing kind of let's say a person's lips 00:05:30.770 --> 00:05:34.080 and mouth might look something like this. 00:05:34.080 --> 00:05:36.810 So I'm kind of exaggerating here, 00:05:36.810 --> 00:05:39.320 but of course there's saliva present, 00:05:39.320 --> 00:05:43.310 and there may be little filaments that form. 00:05:43.310 --> 00:05:45.830 Of course, we can see this. 00:05:45.830 --> 00:05:50.000 And then as we're inhaling, and especially as we're exhaling, 00:05:50.000 --> 00:05:54.860 then these filaments will kind of bend and they can break, 00:05:54.860 --> 00:05:57.110 and some of them will be emitted. 00:05:57.110 --> 00:06:01.070 And in fact, these have been recently visualized 00:06:01.070 --> 00:06:04.220 in great detail. 00:06:04.220 --> 00:06:06.710 And anyway, so that's one mechanism. 00:06:06.710 --> 00:06:08.780 So it's these filaments of saliva 00:06:08.780 --> 00:06:15.450 in this case could be forming around-- 00:06:15.450 --> 00:06:17.790 I'll just mention this picture might 00:06:17.790 --> 00:06:19.380 be, for example, the mouth. 00:06:19.380 --> 00:06:22.080 We could also look at the act of speaking. 00:06:22.080 --> 00:06:26.430 We will discuss in detail later in this course 00:06:26.430 --> 00:06:29.100 that the emissions of infectious droplets 00:06:29.100 --> 00:06:32.130 is very strongly correlated with vocalization. 00:06:32.130 --> 00:06:34.860 If you're speaking, there's many more emissions 00:06:34.860 --> 00:06:37.300 than when you're just simply breathing, 00:06:37.300 --> 00:06:39.480 and when you're speaking in a louder volume 00:06:39.480 --> 00:06:42.600 or when you're singing, that rate of emission 00:06:42.600 --> 00:06:44.850 goes up very significantly. 00:06:44.850 --> 00:06:46.980 So there's clearly emissions related to the vocal-- 00:06:49.620 --> 00:06:55.140 the voicebox and to the vocal folds in the glottis, 00:06:55.140 --> 00:06:57.500 which is basically the voicebox. 00:06:57.500 --> 00:07:04.950 So what that looks like is if you take kind of a side view, 00:07:04.950 --> 00:07:06.470 there are these-- 00:07:06.470 --> 00:07:07.890 as a cross-sectional view, there's 00:07:07.890 --> 00:07:10.400 these folds where the air is flowing through, 00:07:10.400 --> 00:07:13.290 let's say, in this direction. 00:07:13.290 --> 00:07:16.290 And these are kind of waving together. 00:07:16.290 --> 00:07:18.090 They're vibrating where the frequency could 00:07:18.090 --> 00:07:19.860 be, for example, 100 hertz depending 00:07:19.860 --> 00:07:24.360 on the tone of your speech and the type of vowels 00:07:24.360 --> 00:07:27.720 you're making or other sounds. 00:07:27.720 --> 00:07:31.130 And again, what we have is that some here 00:07:31.130 --> 00:07:35.420 are saying this might be in the glottis. 00:07:35.420 --> 00:07:38.780 This could be the vocal folds. 00:07:38.780 --> 00:07:43.270 And this is basically the voicebox, 00:07:43.270 --> 00:07:45.490 is more colloquial term. 00:07:45.490 --> 00:07:47.770 And as the air is flowing through there, 00:07:47.770 --> 00:07:49.159 this part is vibrating. 00:07:49.159 --> 00:07:52.450 So there's some kind of maybe motion. 00:07:52.450 --> 00:07:54.280 I'll just kind of indicate like this 00:07:54.280 --> 00:07:57.850 just that this is kind of shaking and vibrating 00:07:57.850 --> 00:07:59.440 and coming together. 00:07:59.440 --> 00:08:01.810 And of course, there's also mucus 00:08:01.810 --> 00:08:05.240 and other liquids that are here lining all these things. 00:08:05.240 --> 00:08:07.600 And when those folds come close together, 00:08:07.600 --> 00:08:11.230 they touch each other, and they can pull apart and again form 00:08:11.230 --> 00:08:14.470 these filaments that can break up and generate 00:08:14.470 --> 00:08:17.950 droplets that will be emitted of different sizes. 00:08:17.950 --> 00:08:21.350 Now, one thing to notice is the length scale, 00:08:21.350 --> 00:08:24.520 so the mouth when it's opening might have a length scale 00:08:24.520 --> 00:08:26.980 obviously on the order of maybe centimeters but more likely 00:08:26.980 --> 00:08:29.180 millimeters in the regions where there could actually 00:08:29.180 --> 00:08:31.150 be emissions of droplets. 00:08:31.150 --> 00:08:34.120 If we look at the vocal cords, that scale is also 00:08:34.120 --> 00:08:36.460 going to be millimeters, but when the vocal cords really 00:08:36.460 --> 00:08:37.990 come together and pull apart, we might 00:08:37.990 --> 00:08:39.820 be looking at scales that are much smaller than that. 00:08:39.820 --> 00:08:41.770 So some of these filaments that are breaking up 00:08:41.770 --> 00:08:44.080 could be significantly smaller, and so 00:08:44.080 --> 00:08:47.760 vocalization may lead to droplets that are quite a bit 00:08:47.760 --> 00:08:48.260 smaller. 00:08:48.260 --> 00:08:50.550 In fact, in the case of the mouth, as I just mentioned, 00:08:50.550 --> 00:08:54.130 the sort of length scale might be of order of millimeters 00:08:54.130 --> 00:08:56.890 for the filaments that are breaking up. 00:08:56.890 --> 00:08:59.680 And the size of the droplets R might 00:08:59.680 --> 00:09:06.140 be on the order of 10 to 100 microns or even bigger, 00:09:06.140 --> 00:09:06.640 actually. 00:09:06.640 --> 00:09:09.880 In fact, it can even go up to-- well, 00:09:09.880 --> 00:09:12.500 maybe not quite millimeters, but in the case of, let's say when 00:09:12.500 --> 00:09:14.960 you're coughing or spitting, certainly you are spitting out 00:09:14.960 --> 00:09:16.630 millimeters, but it could be even-- 00:09:16.630 --> 00:09:18.250 maybe I'll put even here 1 millimeter 00:09:18.250 --> 00:09:21.040 as sort of a kind of upper bound on the types of droplets 00:09:21.040 --> 00:09:22.840 that you could be emitting. 00:09:22.840 --> 00:09:25.930 In the case of the voicebox, our length scale's a bit smaller. 00:09:25.930 --> 00:09:28.930 It might be on the order more like of 100 microns 00:09:28.930 --> 00:09:31.430 for these filaments that are breaking up. 00:09:31.430 --> 00:09:33.920 And the radius of droplets that you're going to form 00:09:33.920 --> 00:09:35.510 are going to be smaller, and they 00:09:35.510 --> 00:09:38.510 might be ranging more in the 1 to 10 00:09:38.510 --> 00:09:41.510 micron range or possibly larger, again, 00:09:41.510 --> 00:09:42.560 depending on the details. 00:09:42.560 --> 00:09:44.750 If you're coughing and there's a lot of mucus here, 00:09:44.750 --> 00:09:49.260 certainly you could get maybe larger than that as well. 00:09:49.260 --> 00:09:52.610 So breakup of filaments is a primary mechanism 00:09:52.610 --> 00:09:56.760 of drop formation, especially in the upper respiratory tract. 00:09:56.760 --> 00:09:58.970 Now what about in the lower respiratory tract? 00:09:58.970 --> 00:10:00.680 So that's really referring to your lungs. 00:10:11.050 --> 00:10:14.890 So in the lower respiratory tract, 00:10:14.890 --> 00:10:21.060 there is significant evidence and also at least 00:10:21.060 --> 00:10:23.010 qualitative theories and to some extent 00:10:23.010 --> 00:10:27.880 quantitative theories showing that the main mechanism is not 00:10:27.880 --> 00:10:31.850 so much the breakup of filaments in a flow, 00:10:31.850 --> 00:10:39.550 but rather the bursting of filaments of mucus 00:10:39.550 --> 00:10:42.760 but in much smaller domains, where it's not 00:10:42.760 --> 00:10:45.310 so much that the fluid is whipping by and breaking apart 00:10:45.310 --> 00:10:47.350 the droplets, but it's simply breaking up due to surface 00:10:47.350 --> 00:10:47.890 tension. 00:10:47.890 --> 00:10:50.830 Just that it's this instability kind 00:10:50.830 --> 00:10:53.490 of like in a dripping faucet or a stream of liquid when you 00:10:53.490 --> 00:10:55.660 start to stretch it out and let surface tension act, 00:10:55.660 --> 00:10:57.160 it kind of squeezes down, eventually 00:10:57.160 --> 00:10:58.190 wants to make droplets. 00:10:58.190 --> 00:11:00.130 So that's kind of the rupture of a film. 00:11:00.130 --> 00:11:03.590 Under surface tension, it's more likely to be the mechanism. 00:11:03.590 --> 00:11:09.700 And so this is kind of maybe more generally 00:11:09.700 --> 00:11:18.690 can be thought of as an elastocapillarity instability 00:11:18.690 --> 00:11:32.130 of mucosal films, specifically in the deepest 00:11:32.130 --> 00:11:35.940 part of the lungs and in the smallest passages 00:11:35.940 --> 00:11:43.170 during during inhaling in the bronchioles 00:11:43.170 --> 00:11:45.450 and also, to some extent, in the alveola. 00:11:51.740 --> 00:11:58.450 During inhaling, that's when the breakup is happening, 00:11:58.450 --> 00:12:00.400 and then any droplets that are creating, 00:12:00.400 --> 00:12:03.670 some may deposit on the walls of the respiratory tract, 00:12:03.670 --> 00:12:06.490 but some fraction of them will be swept back out again. 00:12:06.490 --> 00:12:08.650 So let me explain this a little bit more detail. 00:12:08.650 --> 00:12:12.400 I should also mention this mechanism is also 00:12:12.400 --> 00:12:21.620 referred to as the bronchial film burst hypothesis. 00:12:24.280 --> 00:12:27.820 And I say it's a hypothesis because despite the fact 00:12:27.820 --> 00:12:30.310 that there's been a lot of study of the droplets 00:12:30.310 --> 00:12:33.190 that are produced by different forms of respiration 00:12:33.190 --> 00:12:35.920 and some theoretical modeling, it's 00:12:35.920 --> 00:12:39.580 difficult to actually observe this process occurring 00:12:39.580 --> 00:12:40.910 in the body. 00:12:40.910 --> 00:12:42.430 And so it's still-- 00:12:42.430 --> 00:12:45.560 it's a hypothesis that people are still studying. 00:12:45.560 --> 00:12:47.350 So what we're thinking of here is 00:12:47.350 --> 00:12:54.760 if we zoom in to a bronchiole, which is a passage that looks 00:12:54.760 --> 00:12:57.880 maybe something like this, it's like basically it's 00:12:57.880 --> 00:12:59.470 a flexible tube. 00:12:59.470 --> 00:13:01.660 And the smallest ones of these now 00:13:01.660 --> 00:13:05.540 are getting down to the scale of 100 microns or so. 00:13:05.540 --> 00:13:07.750 So it is kind of like a typical length scale here of, 00:13:07.750 --> 00:13:13.170 let's say for the radius, might be 100 microns or less. 00:13:13.170 --> 00:13:17.160 And these of course are lined with mucus as well. 00:13:17.160 --> 00:13:19.620 And in some places, there's a bridge. 00:13:19.620 --> 00:13:28.780 So it's kind of like there's almost like bubbles of air 00:13:28.780 --> 00:13:30.490 with sort of little bridges of mucus. 00:13:30.490 --> 00:13:32.470 In fact, you may actually have even some places 00:13:32.470 --> 00:13:36.440 where the passage after exhale has completely collapsed. 00:13:36.440 --> 00:13:38.290 And so maybe some parts of it are touching. 00:13:38.290 --> 00:13:40.150 Others are not touching. 00:13:40.150 --> 00:13:46.510 But there's kind of these little bridges of liquid or films, 00:13:46.510 --> 00:13:51.070 bronchial films that are kind of extending across at least part 00:13:51.070 --> 00:13:53.620 or even all of those channels. 00:13:53.620 --> 00:13:56.470 Now, imagine we start in this situation, 00:13:56.470 --> 00:13:57.700 and we start inhaling. 00:13:57.700 --> 00:14:00.490 And let's just say this is the direction of inhaling. 00:14:00.490 --> 00:14:02.310 Let's imagine that the alveola, which 00:14:02.310 --> 00:14:05.970 is kind of on the end of this tube, and so 00:14:05.970 --> 00:14:08.560 let's see what happens if we start inhaling. 00:14:12.990 --> 00:14:16.120 So for inhaling, then the air is flowing in. 00:14:16.120 --> 00:14:17.580 And so the first thing that happens 00:14:17.580 --> 00:14:21.240 is that these bubbles are going to start essentially-- 00:14:21.240 --> 00:14:23.850 this film is essentially going to be pushed. 00:14:23.850 --> 00:14:27.180 As that liquid is being pushed, we have some flows occurring. 00:14:27.180 --> 00:14:28.950 There's some recirculation flows in there. 00:14:28.950 --> 00:14:32.430 Also, there's interaction with the elastic or stretchy walls, 00:14:32.430 --> 00:14:35.980 which are soft, of the bronchiole, 00:14:35.980 --> 00:14:37.710 and so it can expand. 00:14:37.710 --> 00:14:40.200 So if you go to the next step, you 00:14:40.200 --> 00:14:45.590 may find as you continue inhaling that now this tube has 00:14:45.590 --> 00:14:46.460 expanded a bit. 00:14:46.460 --> 00:14:47.840 So it might look more like this. 00:14:51.360 --> 00:14:57.290 And then this, to some extent, this film 00:14:57.290 --> 00:14:58.790 would start to get stretched. 00:15:05.690 --> 00:15:09.590 And then at some point, as this thing is trying to open, 00:15:09.590 --> 00:15:13.190 and also it's under some flow, but it's going to burst. 00:15:13.190 --> 00:15:15.350 And this bursting again is not quite the same 00:15:15.350 --> 00:15:17.970 as this situation because the flows are much slower. 00:15:17.970 --> 00:15:21.500 So here, these flows are often at so-called high Reynolds 00:15:21.500 --> 00:15:24.170 number, as we'll talk about later in this class. 00:15:24.170 --> 00:15:27.940 High Reynolds number refers to the tendency of the flow 00:15:27.940 --> 00:15:31.120 to become unstable and for inertial effects and momentum 00:15:31.120 --> 00:15:32.330 of fluid to become important. 00:15:32.330 --> 00:15:35.370 At the scale of the mouth or the nose or even the vocal cords, 00:15:35.370 --> 00:15:37.460 there can be significant inertial effects and very 00:15:37.460 --> 00:15:39.680 complex flows. 00:15:39.680 --> 00:15:41.180 On the other hand, when we get down 00:15:41.180 --> 00:15:43.040 to the smallest channels in the lungs, 00:15:43.040 --> 00:15:44.420 and especially when we reach kind 00:15:44.420 --> 00:15:47.030 of the dead end, these sort of the alveolas, which 00:15:47.030 --> 00:15:49.610 is basically a bunch of little sacs, 00:15:49.610 --> 00:15:55.200 they're kind of at the end here, then it's kind of a dead end. 00:15:55.200 --> 00:15:58.930 There can't be any like very fast flow through that system. 00:15:58.930 --> 00:16:01.840 And so it's actually a low Reynolds number situation. 00:16:01.840 --> 00:16:06.390 So we're not talking about turbulent flows or sprays 00:16:06.390 --> 00:16:07.950 of liquid at high Reynolds number. 00:16:07.950 --> 00:16:10.080 Instead, we're talking about films 00:16:10.080 --> 00:16:12.630 that are getting stretched out, and then they simply break up 00:16:12.630 --> 00:16:15.330 under the effect of capilarity, which 00:16:15.330 --> 00:16:16.580 refers to surface tension. 00:16:16.580 --> 00:16:18.210 So basically, when you expose a surface 00:16:18.210 --> 00:16:19.790 and stretch out a liquid film, it just 00:16:19.790 --> 00:16:22.470 tends to break up into little droplets, basically 00:16:22.470 --> 00:16:25.000 in order to minimize its energy. 00:16:25.000 --> 00:16:27.360 So what we'll see here is that maybe one 00:16:27.360 --> 00:16:28.860 of these films over here has already 00:16:28.860 --> 00:16:32.370 burst and will lead to some droplets that 00:16:32.370 --> 00:16:34.120 are being created. 00:16:34.120 --> 00:16:42.650 So this bursting of the film is what leads to the droplets. 00:16:42.650 --> 00:16:44.630 And when you're inhaling, those get swept 00:16:44.630 --> 00:16:46.010 a little further downstream. 00:16:46.010 --> 00:16:49.070 Some of them may deposit on the walls and go back into the film 00:16:49.070 --> 00:16:51.680 and coalesce into the film, but others will 00:16:51.680 --> 00:16:53.540 remain suspended in the air. 00:16:53.540 --> 00:16:56.750 And now when you exhale, you start 00:16:56.750 --> 00:16:58.190 pushing back the other way. 00:16:58.190 --> 00:17:04.500 The tube is more open now, and we 00:17:04.500 --> 00:17:07.440 may have a situation like this where there's 00:17:07.440 --> 00:17:09.410 no more sort of spanning films left, 00:17:09.410 --> 00:17:11.250 but there's some fraction of these droplets. 00:17:11.250 --> 00:17:14.640 A few of them may have deposited and coalesced on the surfaces, 00:17:14.640 --> 00:17:17.069 but they're going to start getting blown out the other way 00:17:17.069 --> 00:17:17.569 now. 00:17:21.420 --> 00:17:23.730 And so these are the droplet emissions right here. 00:17:29.520 --> 00:17:31.150 I'll just say it'll eventually do that. 00:17:31.150 --> 00:17:33.000 So some fraction of these will make it all the way out. 00:17:33.000 --> 00:17:34.850 Of course, those droplets can deposit anywhere 00:17:34.850 --> 00:17:35.900 in the respiratory tract. 00:17:35.900 --> 00:17:38.040 In fact, some of them, if you're breathing through your nose, 00:17:38.040 --> 00:17:40.080 may end up getting caught in your nose, actually. 00:17:40.080 --> 00:17:42.000 And so there's an exchange of fluids 00:17:42.000 --> 00:17:44.250 between the different parts of the respiratory tract, 00:17:44.250 --> 00:17:46.620 but some fraction of those droplets will get out. 00:17:46.620 --> 00:17:49.230 And then ultimately, when you're finished exhaling, 00:17:49.230 --> 00:17:50.800 now the pressure is released. 00:17:50.800 --> 00:17:56.760 And this tube kind of relaxes back to its original state 00:17:56.760 --> 00:17:59.730 where there's some mucus here and there's some places maybe 00:17:59.730 --> 00:18:05.310 where it's closed and there's these possibly spanning films 00:18:05.310 --> 00:18:07.860 in some places where it's almost touching. 00:18:07.860 --> 00:18:10.650 So these are some of the basic processes 00:18:10.650 --> 00:18:13.650 by which droplets are emitted. 00:18:13.650 --> 00:18:16.380 As you can see by the range of different processes 00:18:16.380 --> 00:18:20.970 that are possible in the human physiology 00:18:20.970 --> 00:18:22.350 that we've just described, we can 00:18:22.350 --> 00:18:24.030 see there's a range of droplet sizes 00:18:24.030 --> 00:18:27.420 that will depend on the respiratory activity. 00:18:27.420 --> 00:18:30.230 Are you breathing lightly because you're sleeping? 00:18:30.230 --> 00:18:32.040 Are you breathing heavily at high speeds 00:18:32.040 --> 00:18:33.600 because you're exercising? 00:18:33.600 --> 00:18:35.670 Are you vocalizing and generating 00:18:35.670 --> 00:18:38.730 droplets in a different way in the larynx? 00:18:38.730 --> 00:18:40.680 All those activities play a role. 00:18:40.680 --> 00:18:43.540 And also there are variations between individuals. 00:18:43.540 --> 00:18:46.980 And finally, if a person is sick and all these fluids 00:18:46.980 --> 00:18:50.940 I've sketched here as mucus contain pathogens such as virus 00:18:50.940 --> 00:18:54.600 or bacteria, then of course the degree of infection, 00:18:54.600 --> 00:18:57.780 the viral load or the total amount of pathogen, 00:18:57.780 --> 00:19:00.600 the total amount of bacteria plays a role 00:19:00.600 --> 00:19:02.880 as well in sort of how infectious 00:19:02.880 --> 00:19:04.980 the emissions are from breathing. 00:19:04.980 --> 00:19:06.870 But these are some of the basic principles. 00:19:06.870 --> 00:19:10.200 And now we'll move on to ask, what happens to those droplets 00:19:10.200 --> 00:19:15.230 after they leave the mouth of the infected person?