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On April 18, the World Health Organization (WHO) released new guidance on airborne disease transmission that seeks to create a consensus around the terminology used to describe the transmission of infectious pathogens through the air.

Lydia Bourouiba, the director of the MIT Fluid Dynamics of Disease Transmission Laboratory and the Fluids and Health Network, an associate professor in the MIT departments of Civil and Environmental Engineering and Mechanical Engineering, and a core member of the Institute for Medical Engineering and Science, served on the WHO expert team that developed the guidance. For more than a decade, Bourouiba’s laboratory has been researching fundamental physical processes underlying how infectious diseases spread from person to person.

The new WHO guidance puts forth new definitions of key terminology pertaining to respiratory infectious disease transmission. This reflects a new, shared understanding of how respiratory infectious pathogens move from one person to the next: through the exhalations of turbulent “puff clouds” that carry infectious contaminants in a continuum of droplet sizes and can lead to exposure at a range of distances.

Bourouiba’s lab has pioneered this physical picture and worked closely with a range of stakeholders over the years to ensure that public health guidance incorporates the latest science, improving preparedness for emerging respiratory pathogens. Bourouiba spoke with MIT News about the new WHO guidance.

Q: How did you become involved in creating these new guidelines?

A: I have been researching exhalation emissions for more than a decade. After the first SARS outbreak in 2003, I realized that the mechanisms by which respiratory pathogens are transmitted from one host to the next were essentially considered too random and too brief to be amenable to systematic investigation. Hence, the physical act of pathogen transmission was relegated to a black box. However, I also realized the fundamental importance of understanding these events mechanistically, to ultimately be able to mitigate such transmission events in a rational and principled manner. For this, we needed to understand the fluid physics and biophysics of respiratory emissions.

In the Fluid Dynamics of Disease Transmission Laboratory at MIT, we have been investigating these respiratory emissions. Our work showed that prior guidelines — specifically, the dichotomy of “large” versus “small” drops and isolated droplet emissions (essentially from spray bottles) — were not at all what we actually see and quantify when investigating respiratory emissions. We focused on establishing the full physics of such processes, from emission physiology to the fluid dynamics and biophysics of the exhalation flows and the interaction of the exhaled turbulent multiphase flow with the conditions of the ambient environment (air currents, temperature, and humidity).

Since 2015, I have also been working with the MIT Policy Lab at the Center for International Studies to disseminate our findings to public health officials and various agencies. We organized multiple conferences where we brought in scientists, clinicians, virologists, epidemiologists, microbiologists, and representatives from the U.S. Centers for Disease Control and Prevention and other groups, both before and during the pandemic.

In 2022, I was asked to serve on the World Health Organization’s technical consultation expert team, which was tasked with reaching a consensus on a new framework on respiratory infectious disease transmission. That process lasted about two years and culminated so far in the publication of the new guidelines. The process was obviously accelerated by the Covid-19 pandemic and the issues it brought to the fore regarding the inadequate old definitions. The goal of convening the consultation group was to bring together leading experts from around the globe and from very diverse fields — ranging from fluid physics to clinical medicine and epidemiology — to think through how best to redefine terms related to respiratory infectious disease transmission in light of the latest science. These new guidelines are very much a first step in a series of important consultations and efforts.

Q: How did your research change the WHO’s description of how diseases are transmitted through the air?

A: Our research established that these isolated droplets are not just exhaled as isolated droplets moving semiballistically [that will settle out of the air relatively near to the person who released them]. Instead, they are part of a multiphase turbulent puff gas cloud that contains a continuum of droplet sizes, where the cloud provides a comparatively warm and moist — and hence protective — environment for these droplets and the pathogens they contain, with respect to ambient air. One of our first papers establishing this concept was published in 2014. And we have showed since that models that do not include the proper physics of these turbulent puff clouds can dramatically underestimate the ranges of propagation and also completely shift estimates of risk and pathogen persistence in an indoor space.

These turbulent puff clouds are inhomogeneous, with potential for highly concentrated pathogen-bearing droplet load regions that can persist for a comparatively long time while moving very quickly across an indoor space in some of the most violent exhalations. Their dynamics enable potential effective inhalation exposure at a range of distances, long and short. This continuum and physical picture of concentrated packets of droplets and their impact on persistence of pathogen infectivity and exposure are in complete contrast with the notion of homogeneous mixing indoors, and the prior false dichotomy of “large” droplets that fall ballistically and “small” droplets that essentially evaporate immediately to form aerosols assumed to be deactivated. The prior picture led to the belief that only very few infectious diseases are airborne or requiring air management. This dichotomy, with other misconceptions, rooted in science from the 1930s, has surprisingly persisted in guidelines for decades.

The new guideline is a major milestone, not only because these guidelines do not change very often — every 10 or 15 years at best — but also because in addition to the WHO, five national or transnational health agencies have already endorsed the findings, including the U.S. Centers for Disease Control and Prevention, which also acknowledged the importance of the shift. 

Q: What are the biggest implications of these changes?

A: An agreed-upon common terminology is critical in infectious disease research and mitigation. The new guidelines set the foundation for such a common understanding and process. One might think it is just semantics or a small, incremental change in our understanding. However, risk calculations actually vary tremendously based on the framework one uses. We used mathematical models and physical experiments and found that the physical picture change has dramatic implications on risk estimations.

Another major implication was discussed in one of our publications from the very early stages of the pandemic, which stressed the urgent need for health care workers to have N95 masks because of these cloud dynamics and the associated importance of paying attention to indoor air management. Here again, risk calculations without the puff cloud dynamics would suggest that a typical hospital room or emergency department would dilute sufficiently the pathogen load so as to not pose a high risk. But with the puff cloud and dynamic of the droplets of a continuum of sizes within it, and coupled with it, it becomes clear that health care workers could still be exposed via inhalation to significant viral loads. Thus, they should have been provided N95 masks, in most conditions, when entering the space hosting a Covid-19 patient, even if they were not in their immediate vicinity. That article was the first to call attention to the importance of masking of health care workers due to the actual exhalation puff cloud and continuum of droplet sizes, shaping airborne transmission.

It took public health agencies more than six months to start considering shifting their masking guidelines during Covid-19. But this WHO document is broader than Covid-19. It redefines the basic definitions surrounding all respiratory infectious diseases — those that we know and those yet to come. That means there will be a different risk assessment and thereby different decision trees and policies, trickling down to different choices of protective equipment and mitigation protocols, and different parts of health agencies or facilities that might be activated or deployed.

The new guidelines are also a major acknowledgement that infectious disease transmission is truly an interdisciplinary area where scientists, clinicians, and public health officials of different backgrounds need to communicate with each other efficiently and clearly and share their insights, be it fundamental physics or clinical infectious diseases.  So, it is not just the content of these guidelines, but also the way this update unfolded. Hopefully it changes the mindset for responding to such public health threats.