Speaking Spreads Exhaled Droplets

High-speed camera footage shows how the COVID-19 virus spreads through speech.
Inside Science Contributor

(Inside Science) -- These days many of us flinch whenever we hear someone cough or sneeze. COVID-19 has most people on high alert. Now, new research helps reveal the risks we might face from a simple chat with an infected person.

Howard Stone at Princeton University says, "When you ask what is speech, it's these individual sounds that you're making, all of which add up if you're continually speaking. And in that sense, it's a packet of puffs or puff packets. 

"The interesting thing, or at least one interesting thing, is you don't often realize how much of the air you disturb around you."

Howard Stone studies fluid mechanics, which is the study of how fluids or gases behave in certain conditions. Understanding how the virus spreads is crucial to managing the pandemic. So Stone wanted to look at the role of exhaled droplets from speaking. 

"We wanted to really understand, what does it mean for transportive droplets. We weren't interested in coughing and sneezing, because we said, well, that's associated with symptomatic people. Those people are now probably staying home. It's all the people, the claimants who are asymptomatic who may be responsible for spreading."

Using a high-speed camera, researchers filmed the movement of tiny droplets from a person speaking different phrases. 

"Every time droplets come out of your mouth, you would see a little speckle."

They found that phrases that use sounds with a "p" or "b" make and scatter droplets at a greater rate than other phrases.

"Now if you say that 'Peter Piper picked a peck,' which we did, and you say that a few times, after 20 seconds of speaking, all the exhalations that have been created have easily propagated 2 meters, if not more, in 20 to 30 seconds of speaking. In this case I think he's saying, 'sing a song of six pence.' Again, the upper image is what we see with a camera that's looking at the laser sheet. And then the big end of the sentence -- the 'sing a song,' you don't see much -- the end of the sentence, 'pence,' you see this big vortex come flying out.

"Our images suggest that, although you don't realize it, a large part of what you exhale is appearing at the head of the other person, so it's what they inhale."
In the past, droplet formation has been thought to come from deep in the lungs or the upper airway, like the throat and mouth. However, the high-speed camera footage shows a thin line of saliva on the lips that breaks into droplets as your mouth opens to speak. Stone thinks steps can be taken to help reduce the quantity of droplets being ejected during speech.

"We believe it might be important and we show that the same features that happen when you open your mouth with wet lips can be one, turned off: If you put lip gloss on your lips, you eliminate those droplets, at least for the time your lips don't have saliva on them. These videos I think, at least for us, were trying to make clear to people what you do when you're speaking to people."

Researchers point out that a 6-foot social distance doesn't always work like a wall to protect people who are talking to each other. Over time, conversations can cause droplets to move farther than 6 feet, particularly inside buildings. Masks, however, reduce the number of droplets formed and how far they travel.