Aerosol/Droplet Exhalation

Concentration, Size and Viral Load

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:

  1. Humans produce droplets and aerosols by exhalations which include: breathing out, talking, laughing, coughing, sneezing, and singing2,3,4.
  2. Humans exhale droplets/aerosols in a range of particle sizes from the nanometer to millimeter scale5,6,7. The mass and volume of exhaled materials is dominated by large droplets (100 micrometers to millimeters in diameter) whereas the number is dominated by aerosols.
  3. Infectious pathogens (e.g. tuberculosis and influenza virus6) are known to be carried in aerosols (including those smaller than 5 µm in diameter) produced by exhalations including breathing8.

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 produced9;
    2. opening of occlusions in the small airways of the lungs (“fluid film burst”) 10,11;
    3. airflow turbulence in the respiratory tract interacting with fluids lining the walls of the airways12;
    4. fragmentation of mucosalivary fluid produced from the mouth2,7.
  2. The number concentration and size distributions from healthy or infected adults producing aerosols during exhalations (e.g. breathing, speaking, coughing, sneezing) has been studied by a number of groups7,9, but low concentrations of droplets/aerosols, correction for dilution and variations due to environmental and sampling conditions contribute to uncertainty in the measurements and limited comparability between studies.
  3. Viral loading as a function of size of aerosol for different pathogens (e.g. influenza)8 are still uncertain for many viruses and conclusions for one virus do not necessarily translate to other viruses. 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 swab8.
  4. Variability in particle or aerosol exhalation among healthy or sick individuals is not well quantified and models using exhalation should note uncertainty in exhalation source terms.


  1. Current studies have been unable to conclusively determine whether viable SARS-CoV-2 virions are aerosolized during exhalations.
  2. The rates of SARS-CoV-2 release within droplets and aerosols for different exhalations has yet to be quantitatively measured.
  3. Concentrations of SARS-CoV-2 in the aerosols produced and concentrations of SARS-CoV-2 as a function of size of aerosols produced.


  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. 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).
  9. Asadi, S. et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci. Rep. 9, (2019).
  10. Johnson, G. R. & Morawska, L. The mechanism of breath aerosol formation. J. Aerosol Med. Pulm. Drug Deliv. 22, 229–237 (2009).
  11. 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).
  12. Moriarty, J. A. & Grotberg, J. B. A Mucus – Serous Bilayer. J. Fluid Mech. 397, 1–22 (1999).
  13. Ma, J. et al. Exhaled breath is a significant source of SARS-CoV-2 emission. medRxiv 2020.05.31.20115154 (2020). doi:10.1101/2020.05.31.20115154