Normal speech by individuals who are asymptomatic but infected with coronavirus may produce enough aerosolised particles to transmit the infection, according to aerosol scientists at the University of California-Davis. Although it's not yet known how important this is to the spread of COVID-19, it underscores the need for strict social distancing measures – and for virologists, epidemiologists and engineers who study aerosols and droplets to work together on this and other respiratory diseases.
Aerosols are particles small enough to travel through the air. Ordinary speech creates significant quantities of aerosols from respiratory particles, said William Ristenpart, professor of chemical engineering at UC Davis Ristenpart is co-author on an editorial about the problem.
These respiratory particles are about one micron, or one micrometre, in diameter. That's too small to see with the naked eye, but large enough to carry viruses such as influenza or SARS-CoV-2.
Last year, Ristenpart, graduate student Sima Asadi and colleagues published a paper showing that the louder one speaks, the more particles are emitted and that some individuals are "super-emitters" who give off up to 10 times as many particles as others. The reasons for this are not yet clear. In a follow-up study, they investigated which speech sounds are associated with the most particles.
Calculating just how easily a virus like SARS-CoV-2 spreads through droplets requires expertise from different fields. From virology, researchers need to know how many viruses are in lung fluids, how easily they form into droplets and how many viruses are needed to start an infection. Aerosol scientists can study how far droplets travel once expelled, how they are affected by air motion in a room and how fast they settle out due to gravity.
"The aerosol science community needs to step up and tackle the current challenge presented by COVID-19, and also help better prepare us for inevitable future pandemics," Ristenpart and colleagues conclude.
Other authors on the editorial are Asadi; Professor Anthony Wexler, UC Davis department of mechanical and aerospace engineering; and Nicole Bouvier, Icahn School of Medicine at Mount Sinai.
Abstract 1
Mechanistic hypotheses about airborne infectious disease transmission have traditionally emphasized the role of coughing and sneezing, which are dramatic expiratory events that yield both easily visible droplets and large quantities of particles too small to see by eye. Nonetheless, it has long been known that normal speech also yields large quantities of particles that are too small to see by eye, but are large enough to carry a variety of communicable respiratory pathogens. Here we show that the rate of particle emission during normal human speech is positively correlated with the loudness (amplitude) of vocalization, ranging from approximately 1 to 50 particles per second (0.06 to 3 particles per cm3) for low to high amplitudes, regardless of the language spoken (English, Spanish, Mandarin, or Arabic). Furthermore, a small fraction of individuals behaves as “speech superemitters,” consistently releasing an order of magnitude more particles than their peers. Our data demonstrate that the phenomenon of speech superemission cannot be fully explained either by the phonic structures or the amplitude of the speech. These results suggest that other unknown physiological factors, varying dramatically among individuals, could affect the probability of respiratory infectious disease transmission, and also help explain the existence of superspreaders who are disproportionately responsible for outbreaks of airborne infectious disease.
Authors
Sima Asadi, Anthony S Wexler, Christopher D Cappa, Santiago Barreda, Nicole M Bouvier, William D Ristenpart
Abstract 2
Previously, we demonstrated a strong correlation between the amplitude of human speech and the emission rate of micron-scale expiratory aerosol particles, which are believed to play a role in respiratory disease transmission. To further those findings, here we systematically investigate the effect of different ‘phones’ (the basic sound units of speech) on the emission of particles from the human respiratory tract during speech. We measured the respiratory particle emission rates of 56 healthy human volunteers voicing specific phones, both in isolation and in the context of a standard spoken text. We found that certain phones are associated with significantly higher particle production; for example, the vowel /i/ (“need,” “sea”) produces more particles than /ɑ/ (“saw,” “hot”) or /u/ (“blue,” “mood”), while disyllabic words including voiced plosive consonants (e.g., /d/, /b/, /g/) yield more particles than words with voiceless fricatives (e.g., /s/, /h/, /f/). These trends for discrete phones and words were corroborated by the time-resolved particle emission rates as volunteers read aloud from a standard text passage that incorporates a broad range of the phones present in spoken English. Our measurements showed that particle emission rates were positively correlated with the vowel content of a phrase; conversely, particle emission decreased during phrases with a high fraction of voiceless fricatives. Our particle emission data is broadly consistent with prior measurements of the egressive airflow rate associated with the vocalization of various phones that differ in voicing and articulation. These results suggest that airborne transmission of respiratory pathogens via speech aerosol particles could be modulated by specific phonetic characteristics of the language spoken by a given human population, along with other, more frequently considered epidemiological variables.
Authors
Sima Asadi, Anthony S Wexler, Christopher D Cappa, Santiago Barreda, Nicole M Bouvier, William D Ristenpart
[link url="https://www.sciencedaily.com/releases/2020/04/200403162709.htm"]University of California–Davis material[/link]
[link url="https://www.nature.com/articles/s41598-019-38808-z"]Scientific Reports abstract[/link]
[link url="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0227699"]PLOS One abstract[/link]
[link url="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1749229"]Aerosol Science and Technology editorial[/link]