2-metre distance may not be enough:Sir Riko Mahato

Droplets of saliva can travel as far as six metres even in low wind speeds of 4kmph, a new study has found, indicating that current social distancing guidelines of two metres may be insufficient to stop a Covid-19 patient from transmitting the disease.

The study, published in the Physics of Fluids journal under the American Institute of Physics, used a three-dimensional model to investigate the transport, dispersion, and evaporation of saliva particles from human cough.

The scientists found that when the wind speed is approximately zero, the saliva droplets did not travel 2 metres, which is well within the social distancing recommendations.

“Even with a slight breeze of 4 km/h, saliva travels 6 metres in five seconds. Therefore, depending on the environmental conditions, the 2 m social distance may not be sufficient, so crowed places will be affected,” said Dimitris Drikakis, co-author of the study, in an email to HT. If the wind speed is 15kmph, the droplet travelled 6 metres in 1.6 seconds.

To study how saliva moves through air, the scientists created a computational fluid dynamics simulation that examined 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 vapour and evaporate.

“The purpose of the mathematical modelling and simulation is to take into account all the interaction mechanisms that may take place between the main bulk fluid flow and the saliva droplets, and between the saliva droplets themselves,” said Talib Dbouk, co-author of the paper.

The scientists considered an environment of 20 °C for the fluid, 50% relative humidity, 15 °C at the ground, and 34 °C for the human mouth.

The scientists agreed that more studies needed to be done to understand the behaviour of fluid droplets in indoor environments, where air conditioning significantly affects the particle movement through air.

“It is worth investigating the behaviour of saliva in indoor environments, where air conditioning systems may have significant effects on the movement of particles through the air,” Drikakis said.

Transmission of the coronavirus through speech and cough droplets, especially in public spaces and crowded environments, has been a major area of concern for policy makers and governments. Globally, research has shown that the virus spreads easily in an aerosol form, and even through speech.

As a consequence, governments across the world have increasingly mandated the usage of masks to help stop the spread of the infection.