Aerosol/Droplet Expiration

High level of certainty:

1. Humans produce macroscopic droplets and aerosols by natural expiratory activities which include: breathing out, talking, laughing, coughing, sneezing, and singing^2,3,4.
2. Humans emit droplets and aerosols primarily composed of respiratory fluids in a range of particle sizes from the nanometer to millimeter scale ⁵⁶⁷.
3. 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.
4. 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 loudness^8–10.
5. 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).
6. 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.
7. RNA of SARS-CoV-2 has been detected in samples of breath exhaled by persons infected with COVID-19^13–17.

Low level of certainty:

1. Dominant mechanisms for aerosol formation:
   1. vibration of the vocal folds of the larynx where voice loudness may increase the total volume and mass of particles produced¹⁸;
   2. opening of occlusions in the small airways of the lungs (“fluid film burst”)^19,20;
   3. airflow turbulence in the respiratory tract interacting with fluids lining the walls of the airways²¹;
   4. fragmentation of mucosalivary fluid produced from the mouth^2,7
2. 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 groups^7,8,10,18. 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.
3. 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.

Unknown:

1. Current studies have been unable to conclusively determine whether viable SARS-CoV-2 virions are aerosolized during natural expiratory activities.
2. The rates of SARS-CoV-2 release within droplets and aerosols for different expiratory activities other than breathing has yet to be quantitatively measured.
3. 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.

References:

1. Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. (John Wiley & Sons, Inc., 1999). doi:10.1007/s007690000247
2. Bourouiba, L. The Fluid Dynamics of Disease Transmission. Annu. Rev. Fluid Mech. 53, 473–508 (2021).
3. Duguid, J. P. The size and the duration of air-carriage of respiratory droplets and droplet-nuclei. J. Hyg. (Lond). 44, 471–479 (1946).
4. Papineni, R. S. & Rosenthal, F. S. The size distribution of droplets in the exhaled breath of healthy human subjects. J. Aerosol Med. Depos. Clear. Eff. Lung 10, 105–116 (1997).
5. Gralton, J., Tovey, E., McLaws, M. L. & Rawlinson, W. D. The role of particle size in aerosolised pathogen transmission: A review. J. Infect. 62, 1–13 (2011).
6. Fennelly, K. P. Particle sizes of infectious aerosols: implications for infection control. Lancet. Respir. Med. 2600, 1–11 (2020).
7. Johnson, G. R. et al. Modality of human expired aerosol size distributions. J. Aerosol Sci. 42, 839–851 (2011).
8. Bourouiba, L. Fluid Dynamics of Respiratory Infectious Diseases. Annu. Rev. Biomed. Eng. 23, 547–577 (2021).
9. Asadi, S. et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci. Rep. 9, 1–10 (2019).
10. Gregson, F. K. A. et al. Comparing aerosol concentrations and particle size distributions generated by singing, speaking and breathing. Aerosol Sci. Technol. 55, 681–691 (2021).
11. Yan, J. et al. Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community. Proc. Natl. Acad. Sci. U. S. A. 115, 1081–1086 (2018).
12. Fennelly, K. P. et al. Variability of Infectious Aerosols Produced during Coughing by Patients with Pulmonary Tuberculosis. Am. J. Respir. Crit. Care Med. 186, 450–457 (2012).
13. 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
14. 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
15. Ryan, D. J. et al. Use of exhaled breath condensate (EBC) in the diagnosis of SARS-COV-2 (COVID-19). Thorax 76, 86–88 (2021).
16. Feng, B. et al. Multi-route transmission potential of SARS-CoV-2 in healthcare facilities. J. Hazard. Mater. 402, 123771 (2021).
17. 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
18. Asadi, S. et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci. Rep. 9, (2019).
19. Johnson, G. R. & Morawska, L. The mechanism of breath aerosol formation. J. Aerosol Med. Pulm. Drug Deliv. 22, 229–237 (2009).
20. 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).
21. Moriarty, J. A. & Grotberg, J. B. A Mucus – Serous Bilayer. J. Fluid Mech. 397, 1–22 (1999).
22. Leung, N. H. L. et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nat. Med. 26, (2020).