25 November 2020

The importance of ventilation to prevent spread of Covid-19

Using laser light techniques, University of Amsterdam physicists and medical researchers have found that small cough droplets, potentially containing virus particles, can float in the air in a room for many minutes, especially when the room is poorly ventilated. Good ventilation in public spaces (e.g. public transport, nursing homes) is therefore crucial to slow down the spread of the coronavirus. The results were published in The Lancet Respiratory Medicine on 28 May 2020.

The research was carried out by physicists Daniel Bonn, Stefan Kooij and Cees van Rijn from the UvA Institute of Physics, together with medical researchers Aernout Somsen (Cardiology Centers of the Netherlands) and Reinout Bem (Amsterdam University Medical Centers).

The researchers asked healthy test persons to speak and to cough, and used laser light to analyse the droplets that were produced. Both during speech and coughing, large amounts of small droplets (between roughly 1 and 10 micrometres in size) were observed. During coughing, larger droplets (up to 1 millimeter in size) are also produced. Those droplets fall to the ground within one second, however, and therefore have a much smaller probability of transmitting viruses.

The small droplets only move very slowly to the ground due to the large amount of air drag they experience. The researchers found that such droplets can stay in the air for several minutes. After a single cough, it takes about five minutes for the number of small droplets in the air to be halved. These tiny droplets are therefore much more dangerous when it comes to possible transmission of the coronavirus.

Ventilation
When the same measurements were repeated in a well-ventilated room, the results improved dramatically. With only mechanical ventilation turned on, half of the droplets disappeared within 2.5 minutes, but in a room that also had a door and window open, the number of droplets was halved after 30 seconds – ten times faster than in the unventilated room.

The result is important for making better policies to slow down the spread of the coronavirus. Despite physical distancing, spaces like public transportation and nursing homes can still be centres for spreading the virus if insufficiently ventilated. When droplets remain in the air for a long time, proximity tracing via smartphone apps is also an insufficient precaution. The researchers therefore recommend healthcare authorities consider recommendations to ensure adequate ventilation wherever possible in public spaces

Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission - The Lancet Respiratory Medicine https://doi.org/10.1016/S2213-2600(20)30245-9

Indoor environments
Meanwhile, in a similar study, scientists from Surrey’s Global Centre for Clean Air Research (GCARE), with partners from Australia’s Queensland University and Technology, argue that the lack of adequate ventilation in many indoor environments – from the workplace to the home – increases the risk of airborne transmission of Covid-19.

They note that Covid-19, like many viruses, is less than 100mn in size but expiratory droplets (from people who have coughed or sneezed) contain water, salts and other organic material, along with the virus itself. However, as the water content from the droplets evaporate, the microscopic matter becomes small and light enough to stay suspended in the air and over time the concentration of the virus will build up, increasing the risk of infection – particularly if the air is stagnant like in many indoor environments.

The study highlights improving building ventilation as a possible route to tackling indoor transmission of Covid-19.

Could fighting airborne transmission be the next line of defence against COVID-19 spread? www.sciencedirect.com/science/article/pii/S2590252020300143

Modelling
Additionally, a study carried out in March this year by four Finnish research organisations modelled the transport and spread of coronavirus through the air. They note that preliminary results indicate that aerosol particles carrying the virus can remain in the air longer than was originally thought, so it is important to avoid busy public indoor spaces. This also reduces the risk of droplet infection, which remains the main path of transmission for coronavirus.

The research has been has been submitted for peer-review and published on https://arxiv.org/abs/2005.12612. The paper details how they have modelled the airborne transport of different-sized droplets. These are emitted through coughing, so the study evaluated the quantities of particles that someone could come into contact with upon entering a supermarket or any other indoor public space.

Assistant professor at Aalto University, and project coordinator, Ville Vuorinen, says that both previous related research, and a number of well-known infection spikes, indicate a substantial risk of coronavirus through inhalation of aerosol particles, as well as direct droplet transmission and transmission from surfaces. The 3D flow simulations and analyses carried out in the project also support these ideas.

The 3D simulation shows how droplets of varying size travel in an indoor airflow

https://youtu.be/f7I0O0C_eqg

credit: Aalto University / Finnish Meteorological Institute / VTT / University of Helsinki / IT Center for Science CSC. Animation: Jyrki Hokkanen, CSC - IT Center for Science Ltd.