Fluid dynamics of the pandemic

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Respiratory diseases such as COVID-19 are transported on the respiratory droplets exhaled by an infected person. In 2014, with Lydia Bourouiba (now on the faculty in Civil and Environmental Engineering at MIT) we undertook an integrated experimental and theoretical study of the dynamics of coughing and sneezing. Our study made clear that, contrary to the prevailing wisdom at the time, small respiratory drops are transported farther than larger drops, because they are suspended by the gas flows associated with exhalation, which take the form of either turbulent jets (for normal breathing) or turbulent vortex rings (for coughing and sneezing). In more recent work with my colleague Martin Bazant, I have considered the fate of the smallest such pathogen-bearing droplets, that may be mixed uniformly throughout an indoor space by ambient air flows.

The `Six-Foot Rule’, the safety guideline intended to mitigate the transmission of COVID-19, only protects one from the large, millimeter-scale droplets that can travel no farther than 6 feet, and offers no protection from the micron-scale droplets that may be suspended indefinitely and mixed uniformly through an indoor space. It is now widely believed that COVID-19 is spread primarily via airborne transmission, specifically, by inhalation of these micron-scale respiratory droplets. With Martin Bazant, we developed a new Safety Guideline for mitigating such Indoor Airborne Transmission of COVID-19 and other respiratory illnesses. This guideline suggests a limit on the Cumulative Exposure Time, the product of the number of the people in a particular indoor space and the time spent in that space. The manner in which the Cumulative Exposure Time depends on various factors, including ventilation rate, room volume and human activity, is succinctly stated in our Equation (5).

Most recently, we have deduced how to use carbon-dioxide monitors in conjunction with our new Safety Guideline to provide a real-time assessment of the risk of COVID-19 and other airborne respiratory diseases in indoor spaces.

Explore COVID-19 risk

Our work prompted Kasim Khan to develop an online App, which can be used to assess COVID-19 risks in various scenarios. It uses our theoretical model to calculate safe exposure times and occupancy levels for indoor spaces. By adjusting room specifications, ventilation and filtration rates, face-mask usage, respiratory activities, risk tolerance and more, one can see how to mitigate indoor COVID-19 transmission in different indoor spaces.

Learn more

The science behind our Guideline is also taught in a free, self-paced massive open online course (MOOC) developed by Martin Bazant on edX, named 10.S95x – Physics of COVID-19 Transmission.

Additional related information about indoor airborne transmission of COVID-19 and other respiratory diseases may be found on Martin Bazant’s COVID-19 webpage.