“Factors like the height difference between the people talking and the quantity of aerosols released from their mouths also appear to play an important role in viral transmission,” the researchers, from IISc’s department of aerospace engineering (DAE), and collaborators from the Nordic Institute for Theoretical Physics (NORDITA), Stockholm and the International Centre for Theoretical Sciences (ICTS), Bengaluru, noted.
Having understood that when a person sneezes or coughs, they can potentially transmit droplets carrying viruses like SARS-CoV-2 to others in their vicinity, the questions they wanted to answer were: Does talking to an infected person also carry an increased risk of infection? How do speech droplets or “aerosols” move in the air space between the people interacting?
Pointing out that while in the early days of Covid-19 experts believed that the virus mostly spread symptomatically through coughing or sneezing, the researchers said, soon, it became clear that asymptomatic transmission also leads to the spread.
However, very few studies have looked at aerosol transport by speech as a possible mode of asymptomatic transmission, which prompted them to carry out the new study, which has now been published in journal Flow.
The team visualised scenarios in which two maskless people are standing two, four or six feet apart and talking to each other for about a minute, and then estimated the rate and extent of spread of the speech aerosols from one to another, IISc said.
It added: “Their simulations showed that the risk of getting infected was higher when one person acted as a passive listener and didn’t engage in a two-way conversation. Factors like the height difference and the quantity of aerosols released also appear to play an important role in viral transmission.”
In the simulations, when speakers were either of the same height, or of drastically different heights (one tall and another short), risk of infection was found to be much lower than when the height difference was moderate. Based on their results, the team suggests that just turning their heads away by about nine degrees from each other while still maintaining eye contact can reduce the risk considerably.
Sourabh Diwan, assistant professor, DAE, and one of the corresponding authors, while describing speaking to be a complex activity, said: “…and when people speak, they’re not really conscious of whether this can constitute a means of virus transmission.”
To analyse speech flows, Diwan and his team modified a computer code they had originally developed to study the movement and behaviour of cumulus clouds — the puffy cotton-like clouds usually seen on sunny days.
“The code (called Megha-5) was written by S Ravichandran from NORDITA, the other corresponding author on the paper, and was used recently for studying particle-flow interaction in Rama Govindarajan’s group at ICTS. The analysis carried out by the team on speech flows incorporated the possibility of viral entry through the eyes and mouth in determining the risk of infection – most previous studies had only considered the nose as the point of entry,” IISc said.
Rohit Singhal, the first author and PhD student at DAE said the computational part was intensive, and took a lot of time to perform the simulations. Diwan adds that it is hard to numerically simulate the flow of speech aerosols because of the highly-fluctuating — “turbulent” — nature of the flow; factors like the flow rate at the mouth and the duration of speech also play a role in shaping its evolution.
“Moving forward, the team plans to focus on simulating differences in the loudness of the speakers’ voices and the presence of ventilation sources in their vicinity to see what effect they can have on viral transmission. They also plan to engage in discussions with public health policymakers and epidemiologists to develop suitable guidelines,” IISc said.