Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission

Abstract

Globally, health-care authorities are searching for effective measures to prevent community transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Although data on factors related to this transmission are scarce, the spread of SARS-CoV-2 is thought to mostly be via the transmission of respiratory droplets coming from infected individuals. Small droplets, from submicron to approximately 10 μm diameter, produced during speech and coughing, have been shown to contain viral particles, which can remain viable and infectious in aerosols for 3 h. The droplets can be transmitted either directly by entering the airway through the air (aerosols), or indirectly by contact transfer via contaminated hands. The mode of transmission could affect whether an infection starts in the upper or lower respiratory tract, which is thought to affect the severity of the disease progression. Notably, the dose–response relationship of SARS-CoV-2 infection is still unclear, especially with respect to aerosol transmission of the virus. However, aerosols containing a small concentration of virus in poorly ventilated spaces, combined with low humidity and high temperature, might result in an infectious dose over time. To better understand the spreading of respiratory droplets and possible preventive measures, we analysed droplet production due to coughs and speech by measuring the droplet size distribution, travel distance and velocity, and the airborne time in relation to the level of air ventilation. We did a laser diffraction measurement using a spray droplet measurement system (Malvern Spraytec, Malvern, UK) to determine the size distribution of respiratory droplets in a single cough and during speech. In a cough from a healthy volunteer, we found two distinct types of drops, large droplets (100–1000 μm in diameter) and small droplets (1–10 μm), with the small droplets being much more prevalent (appendix p 1). During speech, only the small droplets were found (appendix p 1). Although large droplets have been specifically related to coughs, here we observe that both sizes of droplet are produced by coughing. Next, we used a SprayScan (Spraying Systems, Glendale Heights, IL, USA) laser sheet to track droplets by filming the scattering of laser light by droplets from a cough to determine the speed of the droplets and their trajectory. Large droplets were observed to fall onto the ground rapidly (appendix p 2). We found that although the speed of the drops ranged 2–7 m/s at the start of the cough, the visible large drops (typically 500 μm in diameter) do not travel far before their trajectory bends down due to gravity to rapidly fall onto the ground within 1 s. This observation can be explained by balancing the forces of gravity (F=mg; where F is force, m is mass, and g is acceleration) and air drag (F=6πηRU, in which η is the air viscosity, R is the radius of the droplet, and U is the falling velocity), from which it also follows that the small droplets of typical radius of 5 μm will take 9 min to reach the ground when produced at a height of 160 cm (ie, average speaking or coughing height). These small droplets are of specific interest because they have been associated with aerosol transmission of the SARS-CoV-2. We also investigated droplets coming from the nasal cavity, and found that with normal breathing no droplets are detected above the background noise level (2·3 [SD 1·5] droplets, and 2·6 [1·7] droplets for nasal breathing). From a sneeze, we found mostly very large drops, originating from both the buccal and nasal cavities, that are not persistent. The same laser sheet was used to investigate how long small droplets from a cough will float through the air. We used a specially designed spray nozzle from Medspray (Enschede, The Netherlands) to disperse a controlled quantity of small droplets into the air, reproducing the effect of coughing. The droplets have an average diameter of 5 μm and are dispersed homogeneously by the spray nozzle. We analysed the number of droplets passing through the stationary laser sheet suspended in the centre of the experimental chamber using an algorithm that detects the illuminations caused by the droplets. We repeated this experiment in three rooms with different levels of ventilation: no ventilation, mechanical ventilation only, and mechanical ventilation supported by the opening Published Online May 27, 2020 https://doi.org/10.1016/ S2213-2600(20)30245-9 Dr G ar y Se tt le s/ SP L