The Anatomy Of A Splash

IRA FLATOW: And now, yes, ever wonder if that’s sneezing person next to you on the bus is going to get you sick or maybe you stand back as soon as you flush that toilet to avoid the flying bacteria. I’m with you on this one. My next guest can tell you that’s probably a good idea.

She appears deep into the spatter of tiny droplets to model how disease is spread. That’s sneeze turns out it goes higher in the air than we thought. And you might not want to know about all those drops from the toilet. Lydia Bourboubia is an assistant professor of civil and environmental engineering and mechanical engineering at MIT in Cambridge. Welcome to Science Friday.

LYDIA BOUROUBIA: Hi. Thanks for having me.

IRA FLATOW: Well, what do you know about sneezes that the rest of us don’t?

LYDIA BOUROUBIA: I guess I looked at many more with the type of lighting and imagery that most people don’t use.

IRA FLATOW: And what did you see? I noticed that in the sneezes that we saw in our video that they travel in a pattern that we really don’t expect them to.

LYDIA BOUROUBIA: Exactly. So when I started looking at this phenomena, what I discovered that I guess other people didn’t see before, is that a cloud was really coming out during sneezes and coughs– so these violent expirations– and that it was not just isolated drops that were emitted, which was our prior physical picture of the event. So that turbulent cloud is carrying these drops and has very high momentum.

So basically, it can travel quite fast and is in some sense entrapping these drops and carrying them away in the room, further away from the sneezer horizontally, but also allowing them to travel upward in room.

IRA FLATOW: They go up, because they’re warmer than the air. So they rise. And you point out in the video that that’s kind of the worst thing that can happen for us with the sneeze droplets, because a lot of the air conditioning intake vents are the ceiling, where the sneezes going right up there.

LYDIA BOUROUBIA: That’s right. So in fact, the turbulent cloud is multi-phase cloud. So there’s a gaseous phase and the droplets. And then eventually, these drops are also evaporating into droplet nuclei, which contain basically dried up pathogens.

But the cloud itself, this air, is hot and moist as you mentioned. And that’s what makes it eventually turn up when that momentum forward, that velocity starts reducing. And indeed, it is the worst case scenario in modern buildings, where most of the suction ventilation is upward.

IRA FLATOW: Wow. So we’re just recirculating the sneeze.

LYDIA BOUROUBIA: That’s right. Basically, we’re dispersing that. And most ventilation systems indoors do not have filters between rooms. So whatever is remaining in that cloud that reaches the ceiling can be redistributed.

IRA FLATOW: I think everybody in our audience is looking at the ceiling. But it gets worse than this, because you’ve also turned your camera onto flushing toilets. Let me ask you about that after I tell everybody that this is Science Friday from PRI Public Radio International. Do I want to know what organisms those can spread and how they’re being spread?

LYDIA BOUROUBIA: That’s right. So actually– I mean, I want to reassure also the audience, sneezes are not always– they could be a reaction of an allergic, you know, stimulus and all of that. So in the worst case scenarios, we’re talking about [INAUDIBLE] of hospitals and in times of outbreaks or epidemics pandemics. And that’s when we really need to know what are the risks of transmission and basically, motion of these pathogens from room-to-room. and similarly, for the high-pressure flushes,

Initially, when I started looking at that problem, it was also in the context of c.diff, so clostridium difficile propagation and transmission in hospital settings, where, in fact, the design of the facilities have included a voluntary choice of not actually including lids and choosing high-pressure toilet flushes for efficiency, energetic efficiency, but also efficiency in cases where patient turnout is high. And in those cases, those choices for not having, a lid, for example, where we’re taken too many biofilm formations, so contamination and attachment of other type of organisms on surfaces. And therefore, the question was, well, given that these are high-pressure flushes toilets, to which extent can we explain the recirculation of certain strains that seem to linger in the hospital that individuals sometimes come in without and leave with the emission of potential aerosols from these devices, these modern high-pressure toilet flushes?

And indeed, it was quite surprising to see how much of the droplets were much smaller than what I expected initially. And therefore, these droplets would fall in the same group as the small droplets from the sneeze and coughs in the sense that they can travel further, linger in the rooms, because they’re not going to really settle on surfaces to be cleaned in with the current surface sanitizing protocols. That’s why really–

IRA FLATOW: Are our home flush toilets also putting out a spray like this?

LYDIA BOUROUBIA: So when I tested the more gravitational toilet flushes that are more common in household, there’s a lot less coming out. And in fact, most households also have lids. So the advice would be to lower that lid and make sure that it’s clean, obviously, rigorously for minimizing biofilms. But the real concerns are in these public spaces and common spaces, where for energy efficiency, water efficiency, and turnaround efficiency, we really need to have that optimal high-pressure flush, but yet by optimizing for that, we could be creating second effects, which are very small drops submitted.

IRA FLATOW: So put the lid down–

LYDIA BOUROUBIA: At home, yes.

IRA FLATOW: At home. Does your work make you a bit of a germophobe?

LYDIA BOUROUBIA: I have to say that’s the case. However, I work hard to not go too far on that tendency and stay on the normal. But initially, after having worked on these two problems and other problems, as well, that relate to droplets contaminants circulation indoors, I did start to be a little bit too aware of all these things.

IRA FLATOW: How could you not be? Of course, the idea for you though is to come up with some mathematical descriptions of how all these things all travel in the air.

LYDIA BOUROUBIA: That’s right. So when we’re trying to do is not just have empirical data. So first of all, really shedding light and putting videos, in some sense shedding light on this phenomena allow us to see them, which is great for public awareness, but also measure precisely.

So we actually developed different image processing algorithms to try to extract the data in terms of droplet sizes, temporal evolution. So with time and space, where is all these going? But then also develop mathematical models, basically, fluid dynamics models based on equations to understand better how could we more efficiently target these processes to changed them, minimize them. So how do we– you have so many parameters that you could be changing to try to address this problem, but that would be years and years of really sort of empirical research. But if you understand the process, the physics that govern those emissions, you can narrow down that list and start having predictive models and interventions that are more targeted.

IRA FLATOW: Sorry we’ve narrowed down our interview. That’s it. Lydia Bouroubia assistant professor of civil and environmental engineering mechanical engineering at MIT. Thank you for taking time to be with us today. And she’s also starring on our latest video, our Breakthrough Series, which is produced with Howard Hughes Medical Institute. It’s also there up on our website at sciencefriday.com/splatter. Kudos to Emily Driscoll and Luke Groskin for work on that.

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