Aerosol/Droplet Transport


Key statement: Aerosols and droplets can travel a range of distances, affected by their initial conditions (e.g. velocity, size, and composition) as well as ambient conditions (e.g. temperature, humidity, ventilation conditions).

High level of certainty:

  1. From the perspective of transport, it is convenient to define the wide spectrum of droplet sizes exhaled in respiratory releases between small and large droplets1. The cut-off between these sizes is typically 60–120 μm1,2,3, varying with droplet composition and ambient conditions such as temperature and relative humidity.
  2. Large droplets can travel ballistically, with a trajectory determined by their initial velocity, composition, and ambient conditions2.
  3. Small droplets are subjected to the motion of airflows (e.g. those caused by ventilation and heating systems). These droplets can remain suspended in air for a long time2.
  4. Due to evaporation, exhaled droplets decrease in size with time, eventually reaching an equilibrium condition (commonly referred to as “droplet nuclei”) due to non-volatile content in the exhaled liquid4.
  5. After evaporation, small droplets are left which remain suspended as these have relevance for both long (i.e. ventilation) and short (i.e. puff) time scale problems.
  6. The puff emitted by a cough or sneeze can transport droplets further than the ballistic model would predict, due to airflow, buoyancy, and humidity5,6.

Low level of certainty:

The maximum travel distance associated with various emission events (e.g. coughing, speaking, breathing) is affected by ambient conditions.

  1. Coughs can transport 30 μm droplets at least 2.5m5,6.
  2. Sneezes can result in droplets travelling large distances, e.g. 8 m7.
    1. The transport of an evaporating droplet depends on its final equilibrium size. Current data suggests that sputum droplets can evaporate to 20–50% of their initial size, depending on composition and humidity3,4.
    2. The ventilation mode (e.g. displacement, mixing) can impact droplet settling time and travel distance8.
    3. The body thermal plume can affect the transport of exhaled/inhaled droplets/aerosol9.


  1. SARS-CoV-2 viral load as a function of droplet size. This information is needed to assess the importance of different transmission modes.
  2. Droplet composition as a function of droplet size and exhalation mode (e.g. coughing, speaking). This prevents an accurate account of the droplets’ equilibrium size, increasing the uncertainty of transport models3,4.
  3. The dilution of an emitted droplet/aerosol cloud with distance from the source.
  4. The combined effect of buoyancy, droplet composition, ambient conditions, and ventilation strategy on typical settling times and distances travelled by droplets, with quantification of associated variability/uncertainty.
  5. The significance of resuspension of deposited droplets/aerosols due to ventilation and movement of people10.
  6. The effect on droplet trajectories of PPE (e.g. masks, face shields) and/or the human thermal plume11.

Key references:

  1. Wells, F. On Air-Borne Infection: Study II. Droplets and Droplet Nuclei. American Journal of Epidemiology 20, 611–18. (1934)
  2. Xie, , et al. How Far Droplets Can Move in Indoor Environments. Indoor Air 17, 211–2256 (2007)
  3. de Oliveira et Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission. (2020) [MedRxiv preprint]
  4. Marr, C., et al. Mechanistic Insights into the Effect of Humidity on Airborne Influenza Virus Survival, Transmission and Incidence. Journal of the Royal Society Interface 16, 1-9 (2019).
  5. Bourouiba, L., Dehandschoewercker, E., & Bush, J. Violent expiratory events: On coughing and sneezing. Fluid Mech., 745, 537–563 (2014)
  6. Chong, L. et. al. Extended lifetime of respiratory droplets in a turbulent vapour puff and its implications on airborne disease transmission. (2020) [MedRxiv preprint]
  7. Bourouiba, L. Turbulent gas clouds and respiratory pathogen emissions: potential implications for reducing transmission of COVID-19.” Jama 18 (2020): 1837-1838.
  8. Liu, , et al. Short-Range Airborne Transmission of Expiratory Droplets between Two People.” Indoor Air 27, 452–62. (2017)
  9. Ge, , et al. Numerical Study of the Effects of Human Body Heat on Particle Transport and Inhalation in Indoor Environment. Building and Environment 59, 1–9. (2013)
  10. Liu, , et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 582, 557–560. (2020)
  11. Bhagat, K., Davies Wykes, M.S., Dalziel, S.B. & Linden, P. F. Effects of ventilation on the indoor spread of COVID-19. Journal of Fluid Mechanics 903 (2020).

List of contributors:

Megan Davies Wykes (UCAM), Pedro M. de Oliveira (UCAM), Savvas Gkantonas (UCAM)