How much distance is enough?

Updated November 26, 2020

Since the pandemic started, people around the world have been asked to practice proper social distancing by keeping at least two meters away from others. Professor Marco E. Rosti, who leads the Complex Fluids and Flows Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), postdoctoral fellow Dr. Stefano Olivieri, and their international collaborators are looking at whether this distance has a scientific grounding. This research was published in Scientific Reports.

The most common route that SARS-CoV-2 – the virus that causes COVID-19 – is transferred between people is via small respiratory droplets that an infected person exhales when coughing, sneezing, or talking. The research team is investigating how far different-sized droplets can travel when someone coughs. 

“When you cough, you expel a jet of air,” explained Professor Rosti. “The transport and dispersal of this jet of air is complicated, and can be influenced by humidity and temperature, but also by fluid turbulence.”

By using historic data, Professor Rosti simulated what the jet of air looked like, both in terms of its turbulence and droplet inertia.

He found that the larger respiratory droplets (those of 100 micrometers or larger) only went as far as one meter. But smaller particles could travel up to seven meters, far beyond the one to two meters recommended by health professionals worldwide. They also evaporated quickly and thus remained airborne for longer periods of time due to the turbulent nature of the flow. 

Diagram of person sneezing which shows range of flying droplets.

In the first few months of the pandemic, experts were undecided about whether the virus was only transmitted via respiratory droplets and direct contact or whether it could remain airborne. 

“There is now evidence to suggest that SARS-CoV-2 can remain airborne, but the science is still unclear on the extent to which this contributes to the spread of the coronavirus,” said Professor Rosti. “Not all droplets contain the virus and not all viral particles can infect you. One meter of space works for droplets that would settle, so it is still worthwhile adhering to the social distancing advice. But, on top of this, wear masks and wash your hands frequently.”

Professor Rosti is now studying the impact of barriers found in offices and supermarkets and the differences between men and women when it comes to how far the jet of air travels. 

* This work used computational resources provided by HPCI under the grant hp200157 of the “HPCI Urgent Call for Fighting against COVID-19”.