Fluid dynamics of human expiratory events can help understand transmission dynamics of COVID-19.

Sourabh S. Diwan, IISc

The primary cause for the spread of COVID-19 infections is believed to be through human-to-human or “droplet transmission”, in contrast to the “aerosol transmission” which largely happens through air, (aerosols are small particles, typically less than 10 microns in size, suspended in the air, a micron is 1/100th the thickness of a human hair). In the droplet transmission, droplets containing the pathogen, released by an infected person through expiratory events such as coughing, sneezing, etc., come in contact with a susceptible person and spread the infection. The WHO has recommended maintaining a distance of 1-2 meters from an infected person, as this is the distance up to which the large expiratory droplets (greater than 100 microns) travel before they settle down. However, recent studies suggest that this distance could be an underestimate and the pathogen can get transported over much longer distances of 7-8 meters in the worst-case scenario, especially through sneezing. The reason for this behavior is that the medium-sized droplets (about 10-100 microns) can get trapped in the “turbulent” jet generated by a cough/sneeze event and therefore get carried by this flow over much longer distances. Recent investigations on this problem (including experiment, theory and computation) have provided useful leads on the spread of the cough/sneeze flows from an infected person, but much work is needed to understand and model these flows better. The ongoing COVID-19 pandemic has provided further impetus to these efforts.

It is interesting to note that the spread of COVID-19 infection through coughing/sneezing is relevant primarily for symptomatic patients. New data and research that is emerging everyday point to another possible route of transmission, which is caused by asymptomatic (or pre-symptomatic) people. Such people could spread the infection just by breathing or speaking which are also examples of expiratory flows, although not as violent as coughing/sneezing. There is another important dimension to the problem. Considering the high transmissibility of the SARS- CoV-2 virus and its rapid spread across the world, scientists are now wondering if the aerosol transmission could also be relevant for this contagion. If this is true, the virus surviving on tiny aerosols could get transported over very long distances by wind or stay circulated in a confined space long after the infected person has left the place. These infected aerosols can also enter into the air-conditioning or ventilation systems in large buildings, e.g. a hospital or mall, potentially causing large-scale infections. The principles of fluid dynamics are clearly relevant for a better understanding of the dynamics of aerosol transmission.

[Last update 16 June 2020]