Two-metre distance may not be enough: Study
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 a human cough.Updated: May 21, 2020 02:42 IST
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 a 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.
However, at wind speeds varying from 4 km/h to 15 km/h, the saliva droplets were seen to travel up to 6 metres with a decrease in the concentration and liquid droplet size in the wind direction.
“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 analysis involved partial differential equations on 1,008 saliva droplets and solving approximately 3.7 million equations.
“The purpose of the mathematical 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,” 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.
“We aimed to approximate as much as possible a real situation in winter/spring seasons. Further investigation is required to quantify the effects of ground surface temperature as well as relative humidity and ambient air temperature,” the study said.
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.
“We need to understand the droplet evaporation more deeply, especially at different environmental conditions. It is work in progress,” said Drikakis.
“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,” he added.
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.
Experts said that this study added to earlier literature that under normal conditions, micro droplets can easily travel up to a distance of about 6 meters.
Ajit Mohan Srivastava, a professor at the Institute of Physics, Bhubaneswar, said that normal outside conditions with motion of vehicles, especially during summers in India, have winds changing directions, even having upward drifts resulting from the hot ground. “Such drifts will make the droplet cloud fly higher, and hence travel to larger distances (even larger than 6 metres),” he said.
“One should realize that these droplets resulting from coughing, sneezing, or even talking or breathing are micro droplets quite like the droplets in clouds, fog and mist (size varying from tens to hundreds microns). We all have seen how fog, mist, or cloud, can keep floating in the air, even rising higher with slight upward wind drift. Imagine that all these droplets are carrying infection for the case of coughing from an infected person,” he added.
One important distinction for India, he said, was that with much higher temperatures (until rains start), droplet evaporation will be much faster than in the study of the paper. “Thus smaller droplets will evaporate away, hopefully making transmission less effective. In places like Delhi, where humidity is much lower than the 50% considered in the study, again droplet evaporation will be stronger, hence transmission less effective. But in coastal places, where humidity is generally very high, evaporation will be much less effective, so transmission will be more effective,” he said.
The most important distinction, according to him, was that when people are walking around and vehicles moving, wind pattern is never uniform or horizontal. “Any change in wind direction, especially upward drifts may dramatically increase the droplet transmission distance. In this situation, I will not rule out the possibility that droplet clouds can float in the air for very long time, basically travelling every where inside the room.”