How Our Exhalations Help Spread Pathogens Such as SARS-CoV-2
How Our Exhalations Help Spread Pathogens Such as SARS-CoV-2

How Our Exhalations Help Spread Pathogens Such as SARS-CoV-2

Lydia Bourouiba, an expert in fluid dynamics and disease transmission at MIT, explains how the physics of sneezes and coughs leads to the spread of respiratory pathogens such as COVID-19.  

Amanda Heidt
Amanda Heidt
Jul 31, 2020

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Lydia Bourouiba first began thinking about how pathogens travel during the global SARS outbreak that began in November 2002. SARS, a close viral cousin of SARS-CoV-2, the virus causing the current COVID-19 pandemic, leapt from person to person via contaminated surfaces, but even then there was discussion among epidemiologists that airborne transmission may be an understudied mechanism contributing to the spread of the virus. As a physical mathematician with an interest in fluid dynamics and public health, Bourouiba knew that pathogens exist in fluids—in the body, in water, or in air—and she also knew a bit about how fluids move through the world. Despite what she calls the “many gaps in our understanding” of how diseases pass from person to person, these two points made her curious enough to pivot from a career in mathematics and physics to one in epidemiology.

Now, as director of the fluid dynamics of disease transmission laboratory at MIT, Bourouiba and her colleagues merge fluid dynamics and epidemiology to study the transmission of SARS-CoV-2 and other pathogens that dwell in respiratory fluids. With more than 17 million COVID-19 cases and 667,000 deaths worldwide to date, understanding how SARS-CoV-2 and other pathogens spread is critical. Scientists around the world recently petitioned agencies such as the World Health Organization to acknowledge airborne transmission in the current pandemic, prompting the agency to admit that, at least indoors, it “cannot be ruled out.”

Lydia Bourouiba
LILLIE PAQUETTE, MIT SCHOOL OF ENGINEERING

In her lab, Bourouiba uses a combination of theoretical approaches and experimental tools, such as flow visualization, high-speed imaging, and microscopy, to build a more holistic view of the air we exhale as dynamic and complex “clouds” of droplets that can travel more than 20 feet from where we’re standing. Her findings break away from the traditional study of airborne emissions as a dichotomy between large droplets and small aerosols. Instead, she says, our coughs and sneezes project “a continuum of droplet sizes,” a fact that has important implications for modeling and assessing disease transmission.

The Scientist spoke with Bourouiba about the importance of studying pathogens transmitted through the air and how her work might inform our public health decisions during the COVID-19 pandemic.

The Scientist: How does the physics of coughs and sneezes play into the transmission of diseases such as COVID-19?

Lydia Bourouiba: Right now, we believe [SARS-CoV-2 is] transmitted mostly through the respiratory tract. The infection occurs in the respiratory tract and emissions from the tract are the propulsion mechanism of the virus into the environment and toward others. It’s very important, therefore, to understand not only the distances but also the timescales of exposures. The virus is never emitted in the air on its own. It’s always in the mucosalivary secretion that is emitted from either breathing out, coughing, or sneezing. In all cases, you’re creating a cloud, but the drop-size distribution and the energy of the gas will be different.

TS: What happens with our fluids when we do something like sneeze or cough?

LB: The exhalation is very much like a cloud . . . but it’s a [cloud] with a much higher momentum for coughs and sneezes. It . . . is moving around like you see when you have condensation on top of a pot. And it also contains this payload of droplets, like real clouds outside, [with] swelling and motion and turbulence inside. 

See “Gesundheit

The difference between different events [e.g., talking, sneezing, coughing] is simply the volumes [of the clouds] that are emitted, the timescales over which they’re emitted, and therefore also the energy or momentum that they have. That momentum is what is driving the contamination range and exposures of others. 

TS: Do you feel that airborne transmission is being properly addressed during the pandemic?

LB: Yes and no. It has been discussed, and the paper that I published in the JAMA in March [outlining the need for better models to track airborne transmission of COVID-19] has also generated a lot of discussion. But I think that there’s still a very fragmented landscape of research and views about the topic that at this point really should be unified but are still not. 

And it’s because the research hasn’t been done consistently. Particularly in terms of pandemics, there’s a lot of different fields looking at these questions, and although cooperation and information transfer should be our goal, there hasn’t always been a dropping of the barriers between areas of research or even awareness of each other. I don’t think we are at the point yet where we’ve seen a unified thrust forward that could bring all this together.

TS: Why is it so hard to study airborne diseases in particular?

LB: A lot of the information is gathered during a time of crisis in a very rudimentary way. Historically, there have been a lot of times where the questions should continue to be the focus after the pandemic to prepare for the next one [but that often doesn’t happen]. It always leaves us in this vulnerable position of being in a reactive mode during a pandemic, which is not ideal to get down to the mechanisms of problems. Research done in a rush is never really going to lead to deep insights.

TS: What can we do to protect ourselves and others from the risks associated with COVID-19?

LB: Wearing a mask, even if they’re not the hybrid respirators, is critical to minimize the range of that emission from one individual and to provide some level of protection to others. But that does not necessarily replace the fact that we need to be careful about how close we are to others, how long we are in that space, how many individuals are in that space, and how the maintenance and protocols of contamination for both the air and surfaces are being deployed. Instead of thinking about one aspect, we really need to think holistically.

Editor’s note: The interview was edited for brevity.