It is known that when people sneeze or cough, they can potentially transmit droplets carrying viruses like SARS-CoV-2 to others in their vicinity. But what happens when someone is talking to an infected person? Do the droplets of the saliva in the speech also carry an increased risk of infection?
A research team has carried out computer simulations to answer these questions. The group included researchers from the Department of Aerospace Engineering at the Bengaluru-based Indian Institute of Science (IISc), along with collaborators from the Nordic Institute for Theoretical Physics (NORDITA) in Stockholm and the International Centre for Theoretical Sciences (ICTS) in Bengaluru.
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 saliva droplets from one to another. Their simulations showed that the risk of getting infected was higher when one person acted as a passive listener than when they engaged in a two-way conversation.
Reporting their findings in a research paper published in the science journal, Flow, of the Cambridge University Press, the scientists noted that a two-way conversation seemed to significantly reduce the aerosol exposure compared with a relative monologue by one person and the relative silence of the other because of the ‘cancelling’ effect produced by the two interacting speech jets. The unequal conversation is shown to significantly increase the infection risk in the person who talks less.
The study has also revealed that factors like the height difference between the people talking appear to play an important role in viral transmission. In the simulations, when the speakers were either of the same height or of drastically different sizes (one tall and another short), the risk of infection was found to be much lower than when the height difference was moderate – the variation looked like a bell curve. Based on their results, the team suggests that just turning their heads away by about nine degrees from each other while maintaining eye contact can considerably reduce the speakers’ risk.
Giving details of the study, Sourabh Diwan, Assistant Professor in the Department of Aerospace Engineering and one of the corresponding authors, recalled that though in the early days of the COVID-19 pandemic, experts believed that the virus mostly spread symptomatically through coughing or sneezing, it soon became clear that asymptomatic transmission also leads to the spread of COVID-19. However, very few studies have looked at aerosol transport by speech as a possible mode of asymptomatic transmission. The new study fills the gap.
To analyse speech flows, he and his team modified a computer code they had initially developed to study the movement and behaviour of cumulus clouds – the puffy cotton-like clouds that are usually seen on a sunny day. The code (called Megha-5) was written by S Ravichandran from NORDITA, the other corresponding author on the paper.
A press release from IISc said that 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 discuss with public health policymakers and epidemiologists to develop proper guidelines. “Whatever precautions we can take while we come back to normalcy in our daily interactions with other people, would go a long way in minimising the spread of infection,” Diwan says.
The study team consisted of Rohit Singhal of IISc and Rama Govindarajan of the International Centre for Theoretical Sciences, besides Diwan and Ravichandran.