There is an increased chance of SARS-CoV-2 infection in the presence of even a light breeze, according to a study by researchers at the Indian Institute of Technology (IIT) Bombay who recommend wearing face masks outdoors, particularly in breezy conditions.
The research, published in the journal Physics of Fluids on Wednesday, found that when a person coughs outdoors, wind flowing in the same direction can transmit the virus faster over longer distances than in calm conditions. “The study is significant in that it points to the increased infection risk that coughing in the same direction as the wind could bring about,” said study co-author Amit Agrawal from IIT-B.
“Based on the results, we recommend wearing masks outdoors, particularly in breezy conditions,” Agrawal said.
Other guidelines, such as coughing in an elbow or turning the face away while coughing, should be followed to reduce transmission when socialising outdoors, the researchers said.
The researchers noted that most studies model cough flow using puffs of air or a simple pulsating profile. However, a real cough is more complicated, exhibiting turbulent flow with prominent vortical structures swirling like mini whirlpools, they said.
To investigate these vortices, Agrawal and IIT-B researchers Sachidananda Behera and Rajneesh Bhardwaj used a large eddy simulation, a numerical model that simulates turbulence.
They modelled cough jets in breezy conditions and in calm conditions representing a typical indoor environment.
The researchers said these simulations show even a light breeze of about 5 miles per hour (mph) extends effective social distancing by around 20 per cent, from 3-6 feet to 3.6-7.2 feet, depending on cough strength.
At 9-11mph, spread of the virus increases in distance and duration, they said. The researchers found that the vortices enable bigger droplets to persist in the air longer than has been typically assumed, increasing the time it takes to adequately dilute the viral load in fresh air.
They noted that as the cough jet evolves and spreads, it interacts with the wind flowing in the same direction.