COVID-19 typically turns deadly when the virus infects the lungs. Hence, how exactly SARS-CoV-2 gets deep into the respiratory tract has been a pressing question since the pandemic started early last year. One pathway is well known: Most people catch the virus in their nose or mouth and then accidentally suck a droplet of their own saliva or nasal drip into their lungs, often while they are asleep. A recent research letter published in the Journal of Internal Medicine proposes a second possible pathway: self-infection to the lungs via one’s own speech.
The idea, says coauthor Adriaan Bax, a biophysicist at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) at the NIH in Bethesda, MD, is that a cloud of infectious droplets spews into the air and hangs around a person’s face for minutes every time they speak. And through sheer chance, because speaking emits a lot of droplets, the possibility of breathing a droplet into the lungs essentially becomes “Russian roulette,” he says. Previous research in 2020 found that masks reduce severe disease, and this latest study suggests that the reason is a reduction in self-infection after initially catching the virus in the upper respiratory tract.
To begin to test this hypothesis, Bax turned to the Deaf community because it “spans the broadest possible spectrum of vocalization,” he explains, ranging from people who rely exclusively on American Sign Language to those who use a substantial amount of vocalized speech. Bax contacted coauthor Poorna Kushalnagar, who is deaf, in the autumn of 2020, wondering if there had been many studies on the extent to which deaf people who prefer not to speak were impacted by COVID-19. “There were none,” says Kushalnagar, a public health research scientist at Gallaudet University in Washington, DC, which specializes in educating deaf students. Intrigued, she agreed to help gather data using surveys of participants.
Kushalnagar designed surveys of 102 participants, all of whom had recovered from COVID-19, and all of whom were born deaf or lost their hearing early in life. Surveys included a number of questions to quantify the severity of each person’s COVID-19 symptoms, such as how much time they spent in bed and whether they were hospitalized, as well as how often each person spoke and if they wore a mask regularly.
Regression analysis of the survey responses confirmed that participants who spoke less also got less sick, as did those who regularly wore a mask. But both of those relationships were individually weak. It wasn’t until the coauthors combined the two factors in the regression that infrequent vocalization and regular mask wearing together showed a strong correlation with reduced disease severity. That these two factors can’t be uncoupled suggests that masks catch speech droplets, preventing self-infection deeper in the respiratory tract, Bax says.
A process called nucleated condensation may be one key to these results, Bax points out. The speech particle flies out of the mouth large and wet and into the hot, wet environment between one’s face and a mask. As the particle crosses the mask, the particle gets even larger because the atmosphere cools, but the absolute humidity stays the same. Large droplets are trapped in the cloth mask and so can’t be breathed back in.
While a “provocative” and “very creative” idea that intuitively makes sense, the behavior of these tiny airborne droplets, and how exactly they swell, shrink, are caught, or pass through a mask, needs further characterization, says Donald K. Milton, a physician and aerobiologist specializing in infectious airborne particles at the University of Maryland in College Park. Chris Cappa, an environmental engineer at the University of California (UC), Davis, agrees that it’s an interesting hypothesis but notes that rebreathing one’s own speech particles may sweep in a cloud of virus-laden droplets that just sweep right out again. Because of the physics of particle motion at such small sizes, some of the droplets may collide with a mucus surface and infect it, but some may not. “I’d like to see some of the more physics-based modeling,” Cappa says—this could help demonstrate, he adds, whether inhaling speech particles actually lets them get deeper in the lungs than they otherwise would.
William Ristenpart, a chemical engineer at UC Davis, questions some of the study’s assumptions, noting it’s “unclear why the authors assume that wearing a facemask will reduce inhalation of self-emitted expiratory particles.” Exhaling naturally pushes speech droplets away from the face, where they’re diluted in the air, he notes. A facemask would trap them and make them more likely to be inhaled on the next breath. What’s more, Ristenpart notes that while most people speak in conversation with a partner, this study didn’t control for the chance of lung infections by a partner’s speech droplets.
Bax disagrees, noting that speech droplets naturally hang around the face in a cloud initially less than 0.5 meters from the speaker’s mouth, and that masks easily trap and hold these droplets, which become glued to the microscopic threads of the cloth, so that the drops aren’t inhaled. He adds that the medical community widely accepts that COVID-19 tends to begin in the nose, mouth, or throat, and then migrates to the lungs and other organs. The odds of someone else’s speech droplet getting into one’s lungs are small. Self-exposure through one’s own speech is a lot more likely, Bax says, which is evinced by the correlation between less vocalization and less severe symptoms.
To date, about 56% of the US population has received at least one dose of a COVID-19 vaccine. Hence, many people have cast aside their masks. But if this hypothesis is correct, people who wear masks aren’t just protecting others, they’re protecting themselves as well, Cappa says. Even vaccinated people, Bax adds, could, in principle, benefit from wearing a mask in the immediate aftermath of a known exposure. Mask-wearing would reduce the odds that any virus loitering in the nose, mouth, or throat could spread into the lungs and cause even mild symptoms.
Bax now hopes to expand this study, which had a relatively small sample size. He’s aiming to go from about 100 participants to thousands. One avenue he’s exploring now is surveying monks at Buddhist Monasteries in India and Nepal, many of whom take a vow of silence. “A survey of that population,” Bax says, “potentially could provide definitive answers on how effectively silence or mask wearing can prevent severe disease.”
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