In a Breeze, 6 Feet of Social Distancing May Not Be Enough

New research indicates droplets released by a cough can travel 18 feet in five seconds in a breeze of under 2 ½ miles per hour.

New Yorkers relax in circles marked for proper coronavirus-minded social distancing at Domino Park in Williamsburg, Brooklyn, on Sunday. (AP Photo/Kathy Willens)

(CN) — Current guidelines from the U.S. Centers for Disease Control and Prevention recommend keeping six feet apart from others to minimize the spread of Covid-19. But a new paper suggests that in a light breeze, six feet may not be far enough.

The novel coronavirus that causes Covid-19 is thought to spread from person to person mainly through respiratory droplets produced when an infected person coughs or sneezes. These droplets can land in the mouths or noses of people who are nearby or possibly be inhaled into the lungs. Because spread of the droplets is more likely when people are in close contact, the CDC advises maintaining a distance of six feet from others whenever possible.

But according to a paper published Tuesday in the peer-reviewed scientific journal Physics of Fluids, saliva droplets released by a cough can travel 18 feet in five seconds in a breeze of less than 2 ½ miles per hour.

To study how saliva moves through air, Talib Dbouk and Dimitris Drikakis of the University of Nicosia, Cyprus, created a computational fluid dynamics simulation that examines the state of each saliva droplet moving through the air in front of a coughing person. The simulation considered the effects of humidity, dispersion force, interactions of molecules of saliva and air, and how the droplets change from liquid to vapor and evaporate. The analysis involved running partial differential equations on 1,008 saliva droplets and solving approximately 3.7 million equations in total.

In a model imitating a wind of about 9 miles an hour, droplets from a cough traveled 18 feet in 1.6 seconds and dispersed more widely than the droplets in a lighter wind.

Further, the analysis showed that droplets moving through the air could have different risk factors for different people. 

“The droplet cloud will affect both adults and children of different heights,” Drikakis said. “Shorter adults and children could be at higher risk if they are located within the trajectory of the traveling saliva droplets.”

The paper’s findings emphasize the importance of covering the mouth and nose when coughing or sneezing, whether wearing a mask or not. The CDC recommends covering the mouth and nose with a tissue when you cough or sneeze and disposing of the tissue immediately — or coughing or sneezing into the elbow rather than the hands if no tissue is available.

Because Covid-19 spreads so rapidly through airborne saliva, the CDC recommends that people wear a cloth face covering to cover their nose and mouth in a community setting. A study funded by the National Institute for Occupational Safety and Health found cloth masks can reduce the dispersal of particles by up to 60%. 

But understanding how airborne droplet transmission works may be crucial to limiting the spread of future outbreaks. 

“This work is vital, because it concerns health and safety distance guidelines, advances the understanding of spreading and transmission of airborne diseases, and helps form precautionary measures based on scientific results,” said Drikakis.

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