Stay six feet apart — that’s the rule of thumb when it comes to social distancing. However, a new study suggests we need to take speech into account in addition to physical distancing when creating Covid-19 transmission mitigation strategies. By assessing the physics of saliva droplet formation and subsequent spray while a person speaks, researchers have shown the words we say play a role in how many droplets we spread — and how far they go.
The paper, published in Physical Review Fluids, explores the mechanics behind transmission of droplets through speech. Manouk Abkarian of the University of Montpellier, France, and Howard Stone of Princeton University used high-speed videos to study how a talking person forms saliva droplets.
“Since there are many excellent studies on the size of droplets formed in some of these activities, we decided to study the airflows that would carry small droplets and aerosols from one person to another in casual interaction such as speaking,” says Stone, a professor of Mechanical and Aerospace Engineering with research interests in fluid dynamics.
Previous studies have also explored the role of speech in Covid-19 transmission. Earlier this year, researchers from the National Institutes of Health used laser light to observe droplet production. Their findings revealed that a person talking loudly even for just a second will emit thousands of droplets. Ironically, they found the phrase “stay healthy” is especially effective in sending saliva spraying, due to the pronunciation of “th.”
Similarly to the NIH study, Abkarian and Stone’s research suggested the length a person speaks is a more important factor in droplet spread than has been discussed in social distancing conversations thus far. “We were basically discovering that a ‘distance of security’ made no sense without introducing time in the problem,” says Stone.
They found plosive consonants spread the most droplets the fastest. Plosive sounds refers to the consonants which require a stopping of air flow to pronounce, such as “p” or “d” or “b.” Abkarian and Stone took high-speed videos of volunteers speaking, which revealed that a thin film of saliva forms between the lips as plosive consonants are spoken. Creating these sounds also requires a strong airflow, which then breaks the film into tiny droplets. The longer the speaker talks, the more droplets hang in the air, and the farther they go.
“In brief, our experiments basically show that talking is like producing ‘micro-sneezes,’” explains Abkarian, a researcher of biophysics and microfluidics. “A small amount of droplets is produced with each utterance, which does not seem bad initially, but when accumulated in a conversation could lead to transmission of a significant amount of exhaled material.”
The findings run contradictory to the idea that a physical distance of six feet between people is a universally “safe” distance. Within just a second of talking, phrases with plosives can cause spray reaching as far as one meter. In 20 to 30 seconds of speaking the range of droplets spray can affect a region stretching two meters, possibly even three.
However, it turns out there is one thing which can reduce how much a person sprays it while they say it — lip balm.
“We reasoned if there was an oil film on your lips it would be more difficult to form a liquid film in the first place,” explains Stone. This hypothesis led the pair to test out how lip balm changed droplet production. For one study volunteer, the balm turned out to be effective in reducing the number of droplets sprayed by four times.
Still, Abkarian cautions that running out to buy lip balm from the drug store is not a quick fix to transmission, and not a definite solution. Even if researchers created a balm with a physio-chemcial design meant to stop droplet spray, it would probably only reduce production rates for sounds formed near the lip closure like “p” and “b” and not for other plosives like “t” and “d.”
Currently, the duo is continuing to explore the fluid dynamics of droplet spread in a study with musicians from the MET Orchestra. Through the study, they hope to develop guidelines for safe practices and performances.
Abkarian and Stone are optimistic that a better understanding of the fluid dynamics of droplet transmission will help create better transmission mitigation strategies in conjunction with masks and other social distancing rules, until a vaccine is rolled out.
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