Aerosols are any solid and/or liquid particles suspended in a gas (e.g. air) ranging in size from ~1 nm to 100 μm1.
High level of certainty:
Humans produce macroscopic droplets and aerosols by natural expiratory activities which include: breathing out, talking, laughing, coughing, sneezing, and singing2,3,4.
Humans emit droplets and aerosols primarily composed of respiratory fluids in a range of particle sizes from the nanometer to millimeter scale 567.
A warm and moist cloud of air is also produced during expiratory activities which affects the forward momentum and transport of the droplets and aerosols emitted. Momentum of the emitted cloud depends on the expiratory activity and increases broadly in order: speaking, breathing, singing, coughing, and sneezing8.
The number and volume of particles measured depends on the expiratory activity and increases broadly in order: breathing, speaking, and coughing and is known to increase with voice loudness8–10.
Typically, more mass and volume of emitted particles are associated with larger droplets or aerosols (of order 100 micrometers to millimeters in diameter) whereas a higher number of particles is associated with aerosols (of order 1 micrometer).
Pathogens (e.g. influenza virus6) are known to be emitted by expiratory activities and subsequently detected in aerosols (including aerosols smaller than 5 µm in diameter)11,12.
RNA of SARS-CoV-2 has been detected in samples of breath exhaled by persons infected with COVID-1913–17.
Low level of certainty:
Dominant mechanisms for aerosol formation:
vibration of the vocal folds of the larynx where voice loudness may increase the total volume and mass of particles produced18;
opening of occlusions in the small airways of the lungs (“fluid film burst”)19,20;
airflow turbulence in the respiratory tract interacting with fluids lining the walls of the airways21;
fragmentation of mucosalivary fluid produced from the mouth2,7
The number, volume or mass concentrations and size distributions from healthy or infected adults producing droplets and aerosols during expiratory activities (e.g. breathing, speaking, coughing, sneezing) has been characterized by a number of groups7,8,10,18. Typically, number concentrations detected are of order 1 particle per cubic centimeter of air with significant variability between subjects and expiratory activities. Low concentrations of droplets and aerosols, variation in instrumentation, correction for dilution and variations due to environmental and sampling conditions contribute to uncertainty in the measurements and limit direct comparability between studies8.
There is limited data for viral loading as a function of size of droplets and aerosols for different pathogens (e.g. influenza, SARS-CoV-2)6,11,22. In one study, samples of influenza in exhaled breath resulted in geometric mean RNA copy numbers of 3.8 × 104/30-minutes for aerosols (< 5 µm) and 1.2 × 104/30-minutes for droplets (> 5 µm) compared with 8.2 × 108 per naso-pharyngeal swab11. One study detected a mean of 2.47× 103 gene copies of SARS-CoV-2 RNA per 20 exhaled breaths from persons carrying mean viral load per oronasopharyngeal swab of 7.97× 106 gene copies17.
Current studies have been unable to conclusively determine whether viable SARS-CoV-2 virions are aerosolized during natural expiratory activities.
The rates of SARS-CoV-2 release within droplets and aerosols for different expiratory activities other than breathing has yet to be quantitatively measured.
Though several studies have reported concentrations of gene copies detected in ambient air, few studies have reported concentrations of SARS-CoV-2 as a function of size of the droplets and aerosols directly emitted from expiratory activities16.
Note: Other possible forms of droplet/aerosol generation from humans (e.g. diarrheal, vomit) are not within the scope of this review.
The statements above are intended to be reviewed regularly as more information and new references emerge. If you have queries about the content of the above and wish to discuss these please contact the editors.
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Asadi, S. et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci. Rep.9, 1–10 (2019).
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Ma, J. et al. Coronavirus Disease 2019 Patients in Earlier Stages Exhaled Millions of Severe Acute Respiratory Syndrome Coronavirus 2 Per Hour. Clin. Infect. Dis. 14–16 (2020). doi:10.1093/cid/ciaa1283
Zhou, L. et al. Breath-, air- and surface-borne SARS-CoV-2 in hospitals. J. Aerosol Sci. 4–10 (2020). doi:10.1016/j.jaerosci.2020.105693
Ryan, D. J. et al. Use of exhaled breath condensate (EBC) in the diagnosis of SARS-COV-2 (COVID-19). Thorax76, 86–88 (2021).
Feng, B. et al. Multi-route transmission potential of SARS-CoV-2 in healthcare facilities. J. Hazard. Mater.402, 123771 (2021).
Malik, M., Kunze, A.-C., Bahmer, T., Herget-Rosenthal, S. & Kunze, T. SARS-CoV-2: Viral Loads of Exhaled Breath and Oronasopharyngeal Specimens in Hospitalized Patients with COVID-19. Int. J. Infect. Dis. (2021). doi:10.1016/j.ijid.2021.07.012
Asadi, S. et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci. Rep.9, (2019).
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Malashenko, A., Tsuda, A. & Haber, S. Propagation and breakup of liquid menisci and aerosol generation in small airways. J. Aerosol Med. Pulm. Drug Deliv.22, 341–353 (2009).
Moriarty, J. A. & Grotberg, J. B. A Mucus – Serous Bilayer. J. Fluid Mech.397, 1–22 (1999).
Leung, N. H. L. et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat. Med.26, (2020).
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