Summary: A new study reveals the recommended 6 foot of distance to help prevent the transmission of COVID-19 may not be enough. Researchers report that even with a slight breeze of 4 KPH, saliva and cough droplets travel 18 feet per 5 seconds.
Source: American Institute of Physics
Airborne transmission of viruses, like the virus causing COVID-19, is not well understood, but a good baseline for study is a deeper understanding of how particles travel through the air when people cough.
In a paper published in Physics of Fluids, Talib Dbouk and Dimitris Drikakis discovered that with even a slight breeze of 4 kph, saliva travels 18 feet in 5 seconds.
“The droplet cloud will affect both adults and children of different heights,” Drikakis said. “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. Many factors affect how saliva droplets travel, 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 how saliva moves through air, 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 is a grid representing the space in front of a coughing person. The analysis involved running partial differential equations on 1,008 saliva droplets and solving approximately 3.7 million equations in total.
“Each cell holds information about variables like pressure, fluid velocity, temperature, droplet mass, droplet position, etc.,” Dbouk said. “The purpose of the mathematical modeling 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.”
Further studies are needed to determine the effect of ground surface temperature on the behavior of saliva in air and to examine indoor environments, where air conditioning significantly affects the particle movement through air.
“This work is vital, because it concerns health and safety distance guidelines, advances the understanding of spreading and transmission of airborne diseases, and helps form precautionary measures based on scientific results,” said Drikakis.
On coughing and airborne droplet transmission to humans
Our understanding of the mechanisms of airborne transmission of viruses is incomplete. This paper employs computational multiphase fluid dynamics and heat transfer to investigate transport, dispersion, and evaporation of saliva particles arising from a human cough. An ejection process of saliva droplets in air was applied to mimic the real event of a human cough. We employ an advanced three-dimensional model based on fully coupled Eulerian–Lagrangian techniques that take into account the relative humidity, turbulent dispersion forces, droplet phase-change, evaporation, and breakup in addition to the droplet–droplet and droplet–air interactions. We computationally investigate the effect of wind speed on social distancing. For a mild human cough in air at 20 °C and 50% relative humidity, we found that human saliva-disease-carrier droplets may travel up to unexpected considerable distances depending on the wind speed. When the wind speed was approximately zero, the saliva droplets did not travel 2 m, which is within the social distancing recommendations. However, at wind speeds varying from 4 km/h to 15 km/h, we found that the saliva droplets can travel up to 6 m with a decrease in the concentration and liquid droplet size in the wind direction. Our findings imply that considering the environmental conditions, the 2 m social distance may not be sufficient. Further research is required to quantify the influence of parameters such as the environment’s relative humidity and temperature among others.