6 feet not enough? Just breathing normally can carry saliva droplets over 7 feet

WASHINGTON — Over the last year, the “six foot rule” has become synonymous with social distancing guidelines during the coronavirus pandemic. Now, a new joint study including scientists from Stony Brook and Harvard Universities finds that simply breathing normally can send particles further than that.

Their results show, in an indoor setting, normal breathing can launch saliva droplets as far as 7.2 feet away in a span of just 90 seconds. While these preliminary findings somewhat call into question the validity of the six foot rule, study authors also have some good news to share.

The team finds wearing a face mask significantly and drastically reduces the distances these potentially viral droplets travel. After nearly two full minutes, saliva droplets coming from someone wearing a mask only traveled a 0.72 meters, or just under 2.4 feet. So, as long as everyone wears a mask, the six foot rule may actually be quite overcautious.

Study authors used extremely realistic computer simulations to produce these findings. While earlier similar studies have focused entirely on the spread of saliva and droplets after a sneeze or cough, this project zeroed in solely on simple breathing. It isn’t something we realize as we breathe in and out, but each breath creates “periodic jet flows” containing saliva droplets. Of course, the velocity of these jet flows is far lower (less than a tenth) than what happens during a sneeze or cough.

Masks drown out saliva’s power to fly through the air

Saliva droplets
Instantaneous simulation results of saliva plume concentration contours (in volume fraction) during normal breathing. Top and bottom images show the case without and with a non-medical face mask. Considering a threshold of 1 part per million, saliva concentrations below 10-6 are cut off. (Credit: Ali Khosronejad)

After analyzing the simulations, researchers report that normal breathing creates a complex field of vortices capable of transporting saliva droplets outward and away from the mouth.

“Our results show that normal breathing without a facial mask generates periodic trailing jets and leading circular vortex rings that propagate forward and interact with the vortical flow structures produced in prior breathing cycles,” says study author Ali Khosronejad in a media release.

Adding a face mask to the breathing equation throws a major wrench into all of this. Researchers explain that a mask “dissipates the kinetic energy of the jet produced by an exhaled breath,  disrupting the vortices and limiting the movement of virus-laden droplets.” Simply put, wearing a mask limits the power particles have to roam and fly around freely.

The influence of evaporation on saliva droplets was considered as well. Without a mask, saliva droplets from exhaled breath only partially evaporate. Within a stagnant indoor setting, those partially evaporated droplets can remain hanging in the air for days. This suggests an indoor environment can remain viral days after a COVID-19 infected individual had been there. However, wearing a mask substantially reduces the chances of this happening.

“To simplify the breathing process, we did not consider the flow of air-saliva mixture through the nose and solely accounted for the flow through the mouth,” Khosronejad concludes. “In future studies, we will explore the effect of normal breathing via both the nose and mouth.”

The study appears in the journal Physics of Fluids.