Stretching and break-up of saliva filaments during speech: A route for pathogen aerosolization and its potential mitigation

Abstract

Speech is a potent route for viral transmission in the COVID-19 pandemic. Informed mitigation strategies are difficult to develop since no aerosolization mechanism has been visualized yet in the oral cavity. Here we show with high-speed imaging how phonation of common stop consonants, found in most of the world's spoken languages, form and extend salivary filaments in a few milliseconds as moist lips open or when the tongue separates from the teeth. Both saliva viscoelasticity and airflow associated with the plosion of stop consonants are essential for stabilizing and subsequently forming centimeter-scale thin filaments, tens of microns in diameter, that break into speech droplets. Moreover, these plosive consonants induce vortex rings that drive meter-long transport of exhaled air, tying this mechanism to transport associated with speech. We demonstrate that a similar mechanism of aerosolization occurs during the vibration of reeds in wind instruments and may occur during the flapping of the glottis folds. Finally, our research suggests a mitigation of droplet production during speech by using a lip balm.

How the language we speak determines the transmission of COVID-19

“…For our analysis, we only selected languages that are represented in our initial list. We selected the consonants /b d p t/ as there is recent evidence that they can produce a lot of droplets during speech [ 9 ]. Note that these consonants are very common in world languages with their frequency to span from 60 to 80% [ 15 ].…”

“…There was a small correlation for /p/ and upon conducting another analysis with different data, we found a large correlation. According to Abkarian and Stone [ 9 ], who investigated stop consonant saliva productions, although both /b/ and /p/ consonants produce a lot of droplets during speech, /p/ surpasses /b/ in terms of droplet emission; this took place when the speaker produced the “Ba-aBa-aB” and “Pa-aPa-aP” sequences. The aforementioned findings can be explained from the fact that /b/ is a voiced consonant, and thus there is vibration of the vocal folds, which leads to rapid pressure modulations in the airflow.…”

“…The results showed that some vowels (e.g., /i/) produce more particles than others (e.g., /ɑ/), and voiced plosive consonants (e.g., /b/) produce more particles than voiceless fricatives (e.g., /f/). Abkarian and Stone [ 9 ] provide novel evidence about the mechanisms that create droplets in the mouth. The researchers recorded a high-speed video of a volunteer who produced various sounds.…”

“…This is because aspiration involves a puff of air and, subsequently, more droplets might be emitted from the mouth compared to non-aspirated productions. Second, we assume that the frequency of occurrence of specific consonants that are said to produce a lot of droplets during speech (see [ 9 ]) will positively correlate with the transmissibility of COVID-19, which can be reflected in the R 0 of the disease.…”

Towards Improved Social Distancing Guidelines: Space and Time Dependence of Virus Transmission from Speech-driven Aerosol Transport Between Two Individuals

“…When an asymptomatic subject exhales or speaks, small droplets carrying virus can spread to a receiving interlocutor. To characterize the important aspects, we note that the drop size distribution [6, 21, 22, 25–27], the influence of loudness and phonetic features on droplet production rates [28–30], and viral densities in saliva [31] have been reported in the literature. Here, we use these measurements, together with the flow field characteristics sketched in Section II.2, to quantify the amount of virus that will reach the receiver in a poorly ventilated space, and so provide a measure of the risk of infection.…”