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:
From the perspective of transport, it is convenient to define the wide spectrum of droplet sizes exhaled in respiratory releases between small and large droplets 1. The cut-off between these sizes is typically 60–120 μm 1,2,3, varying with droplet composition and ambient conditions such as temperature and relative humidity.
Large droplets tend to travel ballistically, with a trajectory determined by their initial velocity, composition, and ambient conditions 2.
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 time 2.
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 liquid 4.
After evaporation, droplet nuclei are left which remain suspended as aerosol. These have relevance for both long (i.e. ventilation) and short (i.e. puff) range/time scale problems.
The puff emitted by a cough or sneeze can transport droplets further than the ballistic model would predict, due to airflow, buoyancy, turbulence and humidity 5,6,14,15,16.
The maximum travel distance associated with various emission events (e.g. coughing, speaking, breathing) is affected by ambient conditions.
Coughs can transport 10-30 μm droplets at least 2.5 m 5,6,13,14.
Sneezes can result in droplets travelling large distances, e.g. 8 m 7.
Low level of certainty:
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 humidity 3,4.
The ventilation mode (e.g. displacement, mixing) can impact droplet settling time and travel distance8.
The body thermal plume can affect the transport of exhaled/inhaled droplets/aerosol 9,11.
Large-scale vortices within the exhaled puff from a cough can help sustaining 30-100 μm droplets for distances in excess of 2 m, while its effect also enhances the distance travelled by large ballistic droplets. 14
Small-scale turbulence causes a broadening of evaporation times of small droplets, with an overall delay of evaporation due to prevalence of droplets at high humidity regions in the puff which, in turn, increases their travelled distance 15,16.
The use of masks can reduce the distance travelled 12 and the amount 13 of cough-emitted droplets significantly.
SARS-CoV-2 viral load as a function of droplet size. This information is needed to assess the importance of different transmission modes.
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.
The dilution of an emitted droplet/aerosol cloud with distance from the source.
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.
The significance of resuspension of deposited droplets/aerosols due to ventilation and movement of people10.
de Oliveira et al. Evolution of spray and aerosol from respiratory releases: theoretical estimates for insight on viral transmission. Proceedings of the Royal Society A, 477: 20200584. (2021) https://doi.org/10.1098/rspa.2020.0584
Marr, L. 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). http://www.doi.org/10.1098/rsif.2018.0298
Chong, K.L., et al. Extended lifetime of respiratory droplets in a turbulent vapor puff and its implications on airborne disease transmission. Physical Review Letters 126, 03402 (2021). https://doi.org/10.1103/PhysRevLett.126.034502
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