Airborne transmission of viruses is not well understood, but a good baseline for study is a deeper understanding of how particles travel through the air when people cough.
Talib Dbouk and Dimitris Drikakis from the University of Nicosia, Cyprus, did just such an experiment and found that even in a slight breeze of 4km/hr, saliva droplets travel around six metres in five seconds. At 15km/hr, it takes as little as 1.6 seconds.
“The droplet cloud will affect both adults and children of different heights,” Drikakis says. “Shorter adults and children could be at higher risk if they are located within the trajectory of the traveling saliva droplets.”
Saliva is a complex fluid, and it travels suspended in a bulk of surrounding air released by a cough, the researchers write in a paper in the journal Physics of Fluids.
Many factors affect transmission, including the size and number of droplets, how they interact with one another and the surrounding air as they disperse and evaporate, how heat and mass are transferred, and the humidity and temperature of the surrounding air.
To study movement more closely, Dbouk and Drikakis created a computational fluid dynamics simulation that examines the state of every saliva droplet moving through the air in front of a coughing person.
Their simulation considered the effects of humidity, dispersion force, interactions of molecules of saliva and air, and how the droplets change from liquid to vapor and evaporate.
The computational domain in the simulation, they say, is a grid representing the space in front of a coughing person. The analysis involved running partial differential equations on 1008 saliva droplets and solving approximately 3.7 million equations in total.
Each cell holds information about variables such as pressure, fluid velocity, temperature, droplet mass and droplet position, Dbouk says. The purpose of the modelling and simulation is to take into account all the real coupling or interaction mechanisms that may take place between the main bulk fluid flow and the saliva droplets, and between the saliva droplets themselves.
“Our findings imply that considering the environmental conditions, the two-metre social distance may not be sufficient,” the authors say in their paper.
“Further research is required to quantify the influence of parameters such as the environment’s relative humidity and temperature among others.”
Related Reading: Is COVID-19 spread through the air?
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