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The separation distance recommended2 to lower the risk of being infected by SARS COV-2 virions via airborne transmission3 appears to be largely based on data from large-droplet experiments in quiescent environments.6 Here, we investigate how far exhaled airflow during normal speech can transport smaller particles5 into quiescent air and into an environment with ambient air motion.
We modeled expiratory flow during normal conversational speech as a jet of constant velocity of 1 m/s and 10 s in duration4 through an elliptical orifice. We seeded the expiratory airflow with SARS COV-2 droplet nuclei (≈ 4 μm in diameter5) and considered the jet’s development in quiescent air (Fig. 1a) and in an environment with a low-speed tailwind 1 (Fig. 1b). In both cases, the simulation showed that while the jet was smooth, axisymmetric, and fully laminar at the mouth, it eventually became unstable and turbulent; however, significant differences in the details of the flows are evident. In the quiescent case, transition occurred a few orifice diameters from the mouth, and once turbulent, further forward motion of the jet was impeded (Fig. 1a). In the tailwind case, the laminar region persisted much longer, and the turbulent cloud was transported farther by the ambient air motion (Fig. 1b). (also, please see accompanied animations.)
While the quiescent case (Fig. 1a) supports the social distancing recommendation of six feet2 (≈ 1.8 m), much caution should be applied. In practice, the air is unlikely to be still, and our investigation reveals that the reach of the virus-laden3 exhaled air is strongly influenced by the ambient airflow. Also, we modeled normal speech (with ~1 m/s expiratory flow rate) but the further people are from each other, the louder they tend to speak. Speaking more loudly or choiring results in an increase in both exhaled and inhaled air volume, and both could promote an increase in virus transmission.3
It is also important to differentiate between outside and indoor gathering. Although it is likely that any virus-laden3 exhaled air is quickly diluted by the ambient air in the case of outside gatherings, indoor gatherings could be fertile grounds for virus transmission. We conclude that the distance virus-laden3 exhaled air travels during speech depends strongly on the motion of the ambient air. While the ambient air motion considered here would be barely perceptible,1 we have shown that even this small air current can double the reach of the virus-laden3 air.
References
ANSI/ASHRAE Standard 55-2017 - Thermal Environmental Conditions for Human Occupancy.
Centers for Disease Control (CDC). 2020 CDC Guidelines on Social Distancing. https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/social-distancing.html.
Centers for Disease Control (CDC). Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission. https://www.cdc.gov/coronavirus/2019-ncov/more/scientific-brief-sars-cov-2.html.
Fuchs, S., U. D. Reichel, and A. Rochet-Capellan. Changes in speech and breathing rate while speaking and biking. 18th Int’l. Cong. Phonet. Sci., Glasgow, United Kingdom, 2015. https://doi.org/10.5282/ubm/epub.25254https://doi.org/10.5282/ubm/epub.25254.
Stadnytskyia, V., C. E. Baxb, A. Baxa, and P. Anfinrud. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proc. Natl. Acad. Sci. USA 117(22):11875–11877, 2020.
Wells, W. F. On air-borne infections: study II. Droplets and droplet nuclei. Am. J. Epidemiol. 20(3):611–618, 1934.
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Kenjeres, S., Henry, F.S. & Tsuda, A. Is Current Social Distancing Enough?. Ann Biomed Eng 49, 1973–1974 (2021). https://doi.org/10.1007/s10439-021-02741-y
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DOI: https://doi.org/10.1007/s10439-021-02741-y