Six Feet Is Not Always Enough: How Saliva Droplets Spread Through the Air

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In Maryland, restaurant patrons stand inside bumper-style tables to keep six feet apart. In New York, sunbathers maintain distance by lounging in white chalk circles painted on a grassy field.

As the United States slowly begins opening public spaces, organizations are getting creative about how to encourage social distancing. But two new studies on the airborne spread of saliva droplets, which can harbor virus particles from respiratory diseases like COVID-19, suggest those six feet alone are not always enough.

Using laser light scattering, a team at the NIH’s National Institute of Diabetes and Digestive and Kidney Diseases, found that loud speech produces thousands of saliva droplets per second, and those droplets can linger in enclosed spaces for up to 14 minutes. Using computational modeling, a team at the University of Nicosia in Cyprus found that a mild cough outside in a light breeze can carry droplets up to 18 feet away.

“Two meters does not suffice if you are in an open space,” says Dimitris Drikakis, senior author of the second study, published today in the journal Physics of Fluids, and a professor at the University of Nicosia, Cyprus, studying computational fluid dynamics. “Two meters is okay if there is no wind or very little, but beyond that situation, saliva droplets can travel a considerable distance.”

Drikakis and colleague Talib Dbouk created a computational model of 1,008 saliva droplets of various sizes moving through the air in front of a coughing person. Their model, totaling 3.7 million equations, incorporated variables such as humidity, force of coughing, interactions of saliva with the air, the evaporation of droplets, and more.

They found that a cloud of saliva droplets released by a cough falls to the ground quickly if there’s no wind. But in even a slight breeze, those same droplets can travel 18 feet in 5 seconds. An even stronger wind will propel droplets the same distance in less than 2 seconds, the authors found.

“Wind in an open space will influence the propagation and distance that saliva droplets can travel,” says Drikakis. “Both citizens and policy makers should be aware.” He and his team are now studying saliva droplet transmission under other conditions.

The NIH study, published in the journal PNAS, examined how speech—no coughing necessary—works as a mode of virus transmission. Unlike saliva droplets from coughing and sneezing, speech droplets are very small, which can make them hard to detect. Philip Anfinrud and colleagues at the NIDDK Laboratory of Chemical Physics repurposed their lasers to visualize these tiny droplets.

Complementing earlier work in the New England Journal of Medicine, Anfinrud’s team set out to quantify the droplets generated by loud speech and detect how long those droplets linger in the air.

In the experiment, with air filters on to keep dust at bay, a person repeatedly spoke phrases, including “Stay healthy,” into a sheet of intense laser light. Particles produced from the mouth generated tiny bursts of light, which the team recorded and analyzed.

They found that speaking loudly generated thousands of small droplets per second. Some of those droplets dried out and shrank from 12–20 microns down to about 4 microns, then lingered in the air. Overall, droplets remained airborne in a stagnant space for an average of 12 minutes, the authors report.

“For asymptomatic people, saliva droplets created from simple speech likely account for the majority of droplets a person expels,” a spokesperson from the NIDDK wrote in a statement to IEEE. The NIH declined to make Anfinrud nor any member of the research team, funded by taxpayer dollars, available for interviews. The results suggest that wearing a cloth mask in public could help slow the spread of disease, the spokesperson wrote.

Neither study examined the transmission of actual virus particles through the air. A key remaining question during this pandemic is how much coronavirus it takes to cause infection, called the infectious dose. So far, researchers don’t know the size of that dose, but suspect it is low, considering the virus spreads easily through casual contact.