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Transition from saliva droplets to solid aerosols in the context of COVID-19 spreading

Authors
  • Stiti, Mehdi1
  • Castanet, Guilaume2
  • Corber, Andrew3
  • Alden, Marcus1
  • Berrocal, Edouard1
  • 1 Division of Combustion Physics, Department of Physics, Lund University, Lund, Sweden
  • 2 Université de Lorraine, CNRS, LEMTA, Nancy, France
  • 3 Aerospace Research Centre, National Research Council of Canada, Ottawa, Canada
Type
Published Article
Journal
Environmental Research
Publisher
Elsevier
Publication Date
Sep 23, 2021
Volume
204
Pages
112072–112072
Identifiers
DOI: 10.1016/j.envres.2021.112072
PMID: 34562485
PMCID: PMC8459388
Source
PubMed Central
Keywords
Disciplines
  • Article
License
Unknown

Abstract

To control the evolution of a pandemic such as COVID-19, knowing the conditions under which the pathogen is being transmitted represents a critical issue, especially when implementing protection strategies such as social distancing and wearing face masks. For viruses and bacteria that spread via airborne and/or droplet pathways, this requires understanding how saliva droplets evolve over time after their expulsion by speaking or coughing. Within this context, the transition from saliva droplets to solid residues, due to water evaporation, is studied here both experimentally, considering the saliva from 5 men and 5 women, and via numerical modeling to accurately predict the dynamics of this process. The model assumes saliva to be a binary water/salt mixture and is validated against experimental results using saliva droplets that are suspended in an ultrasound levitator. We demonstrate that droplets with an initial diameter smaller than 21 μm will produce a solid residue that would be considered an aerosol of <5 μm diameter in less than 2 s (for any relative humidity less than 80% and/or any temperature greater than 20°C). Finally, the model developed here accounts for the influence of the saliva composition, relative humidity and ambient temperature on droplet drying. Thus, the travel distance prior to becoming a solid residue can be deduced. We found that saliva droplets of initial size below 80 μm, which corresponds to the vast majority of speech and cough droplets, will become solid residues prior to touching the ground when expelled from a height of 160 cm.

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