Transport of Droplets and Aerosols in Respiratory Activities (10/36)

Talk by Lydia Bourouiba, Associate Professor at MIT

Since the beginnning of the century we have been through a number of infectious disease outbreaks: SARS 2003, H5NI (1997) 2005, H1N1 2009, MERS 2012, Ebola 2014, and now COVID-19 2019.

We need to build resilience for COVID-19 as well as against emerging pandemic threats for the pandemics we will see throughout this century.

How are diseases actually transmitted in space and time?

There is a missing link between the microbial/cellular level (what we understand at the microbial/immunology/pathogenesis scale), and the population/indoor-environment level (large/population/epedimiology scale). Fluids are everywhere and dominate this missing link at the intermediate scale.

Why does it matter?

The various preventive measures being implemented focus on this intermediate scale. The question of droplets vs aerosols focuses on this scale. However, modeling these categories as binary is an incorrect dichotomy that does not reconcile with the data we have.

What actually happens?

There is a continuum of particle sizes that exist that travel within a cloud, with only a small portion existing as isolated, large droplets. The cloud is turbulent, moist and hot, and contains both gas, liquid, and eventually solid phases after evaporation leads to a pathogen-bearing nuclei. Modern imaging techniques allow us to see what happens during a cough.

Comparison of gas cloud and droplets from a cough

Current WHO and CDC statements fail to take this modern understanding into account.


There is no need for external air flow since the initial impulse is enough to give the cloud momentum. In this cloud, the dynamics of the cloud have a larger impact on evaporation than the initial particle sizes.

The WHO's incorrect model led to their initial recommendation of social distancing of only a single meter. Clouds have been shown to continue moving with liquid droplets at distances that are an order of magnitude farther than this.

The continuum of droplet particle sizes is present across all exhalations, including talking, breathing, singing, coughing, and sneezing. The first 2 have Reynolds numbers (inertial/viscous forces) approximately on the order of 100-1000, whereas the last 2 are approximately above an order of 10,000, with singing in the middle. With higher Reynolds numbers, the momentum of the turbulent gas phase increases.

Particle Sizes

Various studies have shown similar trends, but a wide range of outcomes in droplet size distributions due to variations in instrumentation, even within a single exhalation category. There is a need for standardization. Additionally, there is inherent variation within the generation process due to variability across patients, and in health vs. infection.

Air-surface-people integrated management

In the initial seconds to minutes at a scale of a few meters, indoor air hygiene management is dominated by directed disperal of the exhalation momentume cloud that carries a concentrated payload. At a scale of an entire room, over hours, air ventilation (dilution and slow advection) dominates. Both parts of the process need to be considered for re-opening. In the near-field interactions it is possible to focus on source control. With the larger scale effects, increasing air changes per hour, or considering winter vs summer HVAC conditions becomes important.

Masks are effective for blocking large droplets, but the gas cloud still escapes from the mask. The presence of the mask redirects the momentum, which helps eliminate high concentration zones, so only the longer time-scale ventilation measures are needed to eliminate the final risk.

Next: What Size Droplets/Aerosols Are Generated by People and How Do They Spread in Air? CQ1 Panel (11/36)

Previous: Transmission Modes of Respiratory Viruses (9/36)

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Lydia Bourouiba is an Associate Professor at the Massachusetts Institute of Technology, where she founded and directs the Fluid Dynamics of Disease Transmission Laboratory. Her research leverages advanced fluid dynamics experiments at various scales, biophysics, applied mathematics to elucidate interfacial flow and fluid fragmentation processes driving mixing, transport, and persistence of particles and microorganisms driving multiscale epidemiology and disease transmission. Dr. Bourouiba founded the Fluids and Health Conference, to be expanded into a Gordon Research Conference that she will chair in 2022, creating an international forum for exchange on frontier research and challenges in health, where fluid dynamic concepts are at the core, including infectious diseases disease transmission and related policy. Dr. Bourouiba is the recipient of many awards, including the Tse Cheuk Ng Tai’s Prize for Innovative Research in Health Sciences, the Ole Madsen Mentoring Award, and the Smith Family Foundation Odyssey Award for high-risk/high-reward basic science research. Dr. Bourouiba received her PhD from McGill University.

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