The recent COVID-19 pandemic prompted the need for deeper understanding of the transport of fluids and droplets emanating from our respiratory tracts when we cough, sneeze, speak or breathe. The droplets transport will influence the spread of coronavirus and determine the implementation of guidelines about social distancing, mask wearing, crowded gatherings, as well as everyday practices of social behaviour in private, public and business environments.

When sneezing or coughing, larger droplets are formed by saliva, and smaller droplets by mucous coating of the lungs and vocal cords. The smaller droplets are often invisible to the naked eye. Past research has shown that most respiratory droplets do not travel independently on their trajectories. Instead, droplets in a continuum of sizes are trapped and carried forward within a moist, warm, turbulent cloud of air. Although the mechanisms of virus transmission are still under debate, it is widely accepted that aerosol or respiratory droplet transmission is the critical factor for the rapid spread and continued circulation of viruses in humans.

We think that it is likely the dosage and time of exposure which determine whether or not infection will finally occur. Therefore, it is crucial to decide on the scenarios that will allow the transmission to longer distances. Our study aimed at advancing the understanding of the transfer of airborne particle carriers to humans through advanced coupling between droplets and fluid flow modeling and simulation.

The initial modeling configuration of the problem takes into account several parameters that can influence the simulation, including the wind speed in an open environment.

An accurate prediction of the transfer of airborne particle carriers to humans from a cough is governed by the following modeling considerations that must be taken into account:

• The saliva droplets initial size distribution at the onset of the coughing event.
• The human mouth-print of the cough.
• The period of the cough and its intensity (or initial saliva droplets speed).
• The numerical modeling approach to capture the complex varying space and time scales, e.g. both heat and mass transfer considerations, modeling of mass and phase changes due to droplets evaporation, coalescence, break up and turbulent dispersion in interaction with the bulk flow field.

The computational domain in the simulation is a grid in front of the coughing person . The analysis involved running partial differential equations on 1008 saliva droplets and solving approximately 3.7 million equations in total.

Our findings showed that, when a person coughs, the wind speed in an open space environment significantly influences the distance that airborne disease-carrier droplets travel.

• Without surrounding wind speed, the droplets will fall to the ground in a short distance from the person exhaling or coughing. The present analysis shows that the range may not exceed one meter (See Figure 2). A tiny number of particles may travel slightly further longer. Still, their trajectory beyond one meter will already be at a height significantly below half a meter dropping towards the ground. Thus, these droplets may not constitute a risk regarding facial contact of adults at this distance.
• At wind speeds from 4 to 15 km/h, we found that saliva droplets can travel to distances up to 6 meters with decreasing concentrations and liquid droplets size in the wind direction (see Figure 2). Our findings imply that depending on the environmental conditions, the 2 meters social distance may not suffice. Further research is required to quantify the influence of other parameters such as the environment relative humidity and temperature amongst others.
• The droplets cloud will affect both adults and children of different heights. Shorter adults and children could be at higher risk if they are located within the trajectory of falling droplets.
• At a lower wind speed, the droplet total mass reduction occurs more slowly compared to a higher speed, which may prolong the exposure of a human to the droplets if the subject is located within the droplets envelope.

Overall, the results showed that in open spaces, airborne droplets carriers can travel significantly further than the 2 meters recommended distance due to the wind speed. The issues arising from the past and recent pandemic require a holistic approach to elucidate the open scientific questions and address the practical challenges. Such an approach would require closer interaction between bio-medicine, engineering fluid physics, and social sciences.

About the University of Nicosia

The University of Nicosia (UNIC) is the largest university in Cyprus, and the largest university in southern Europe that teaches primarily in English, welcoming 12,000+ students from over 70 countries worldwide. The University of Nicosia is a comprehensive university with 6 schools, 20 departments and over 100 degree programs offered on-campus and online.

The University of Nicosia is best known in the fields of medicine, law, blockchain, accounting, education, forecasting and international relations and for a series of partnerships with other leading European universities. UNIC was most recently ranked #106 in its region by QS and #300 to #400 globally in the Times Higher Education Impact rankings.

To learn more about the University of Nicosia, please see: https://www.unic.ac.cy/

UNIC’s COVID-19 Response

University of Nicosia faculty across a variety of academic disciplines are actively engaged in research and analysis and serving in scientific / public policy roles supporting the global COVID-19 response.

For more information about the University of Nicosia’s work relating to COVID-19 including potential collaboration, please see: https://www.unic.ac.cy/coronavirus/