When a person sneezes or coughs, they can potentially transmit droplets carrying viruses like SARS-CoV-2 to others in their vicinity. 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?
As the emphasis on the importance of wearing masks returns amidst rising number of COVID-19 cases, a research team carried out computer simulations to analyse the movement of the speech aerosols. The team included researchers from the Department of Aerospace Engineering, 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.
According to an IISc. release, the team visualised scenarios in which two mask-less 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. In the study, published in the journal Flow, their simulations showed that the risk of getting infected was higher when one person acted as a passive listener and did not engage in a two-way conversation. 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.
“Speaking is a complex activity, and when people speak, they’re not really conscious of whether this can constitute a means of virus transmission,” Sourabh Diwan, Assistant Professor in the Department of Aerospace Engineering, and one of the corresponding authors, was quoted as saying. He added that in the early days of the pandemic, experts believed that the virus mostly spread symptomatically through coughing or sneezing, and soon it 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, he said.
To analyse speech flows, the team modified a computer code they had originally developed to study the movement and behaviour of cumulus clouds that are usually seen on a sunny day, the release explained. The analysis carried out on speech flows incorporated the possibility of viral entry through the eyes and mouth in determining the risk of infection, as most previous studies had only considered the nose as the point of entry, it added.
“In the simulations, when the speakers were either of the same height, or of drastically different heights (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,” explained the release.
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 for the speakers considerably. 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, but add that 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.
