Covid warning: Indoor conversations quickly create ‘jet-like’ spray of potentially viral droplets


Princeton study shows that certain sounds, such as words with a strong “P” in them, create puffs of air that can easily send droplets across a room.


PRINCETON, N.J. — A new study from Princeton University finds a normal, indoor conversation creates a “jet-like” flow of air that quickly sprays tiny droplets all over the room for a number of feet. Of course, these findings strongly suggest a typical indoor conversation can spread the coronavirus over a significant portion of a given room.

“People should recognize that they have an effect around them,” says Howard Stone, a professor of mechanical and aerospace engineering at Princeton, in a release. “It’s not just around your head, it is at the scale of meters.”

Prior studies show that asymptomatic COVID-19 carriers can spread the virus to others while speaking, sneezing, or laughing (via tiny droplets released by those actions). The team at Princeton set out to determine just how quickly and how far droplets are spread out during a normal, indoor conversation.

“Lots of people have written about coughs and sneezes and the kinds of things you worry about with the flu,” Stone says. “But those features are associated with visible symptoms, and with this disease we are seeing a lot of spread by people without symptoms.”

All in all, the research team concludes that a normal, indoor conversation is capable of spreading potential viral droplets at least as far as, if not even farther than, most social distancing recommendations (the six-foot rule).

Ventilation, masks keys to preventing virus transmission

This work focused on indoor environments with poor ventilation.

“It certainly highlights the importance of ventilation,” Stone notes. “Especially if you have an extended conversation.”

What can be done about all this? Researchers say face masks do a very good job of stopping the “jet-like” air flow produced while speaking. That said, a mask isn’t going to 100% stop all aerosols, but it will stop droplets larger than 30 cm.

“Masks really cut this flow of tremendously,” Stone adds. “This identifies why (most) masks play a big role. They cut everything off.”

To come to these conclusions, researchers used a high-speed camera to film tiny laser-illuminated droplets as they traveled from a speaker’s mouth. That speaker was told to recite a number of phrases, such as, “We will beat the coronavirus,” and “Peter Piper picked a peck.” The authors wanted to see how different sounds would incite different air flows.

Researchers note that the “P” sound creates a puff of air in front of the speaker. Moreover, a full-on conversation creates a “train of puffs.” Each single puff produces a vortex of air in front of one’s mouth, and the constant interplay between those vortices over the course of a conversation is what ultimately creates the cone-shaped jet-like airflow from speakers’ mouths.

This jet airflow is quite capable of sending droplets and tiny particles shooting out away from the speaker.

Conversation more powerful than you might believe

Even just a short single phrase can spray some particles well past the three-foot social distancing recommendation from the WHO — within just a few seconds.

That being said, a lot does depend on the length of the conversation. The longer a person talks, the farther they’re going to spread their particles.

“However, more extended discussions, and meetings in confined spaces, mean that the local environment will potentially contain exhaled air over a significantly longer distance,” the study reads.

Scientists say that a six foot distance from others isn’t going to act as an impenetrable fortress against the coronavirus. Within indoor environments, viral droplets appear to be quite capable of traveling that distance, and even further.

“If you speak for 30 seconds in a loud voice, you are going to project aerosol more than six feet in the direction of your interlocutor,” Stone says.

The study is published in Proceedings of the National Academy of Sciences.

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