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Finnish study shows coronavirus aerosols remain airborne for minutes

Findings suggest that aerosols remain airborne but no new recommendations will be issued on the basis of the study.

The study looked at the behaviour of aerosols in a typical supermarket setting with regular ventilation systems. Image: Famveld / Alamy / AOP. Kuvituskuva

Coronavirus droplets ejected when a person coughs, sneezes or even speaks can remain suspended in the air for minutes rather than immediately sinking to ground, according to new research data.

The finding comes from studies conducted by researchers from Aalto University, the Finnish Meteorological Institute (FMI), the state-owned technical and innovation centre VTT and Helsinki University.

The organisations investigated how extremely small aerosol particles are released and travel on air currents when people cough, sneeze or even speak. The research teams included dozens of scientists from flow physicists to virologists, as well as experts in medical technology and infectious diseases.

The study modelled how novel coronavirus might spread using a trip to the supermarket as an example. For example if a person who has been infected with the virus sneezes between the aisles at the supermarket, they release a cloud of droplets that may also contain the virus. The model suggests that the cloud spreads some distance from the infected person.

Aerosols take minutes to dissipate

All of the research groups arrived at the same initial conclusion: the aerosol cloud spreads farther than the immediate vicinity of the infected person. While the cloud will dissipate over time, that process could take minutes.

"If someone infected with coronavirus coughs and walks away and some one else nearby comes over, aerosol particles containing the virus could end up in the other person's respiratory tract," Aalto university associate professor Ville Vuorinen explained.

In the model used by the researchers, the test subject coughs in the aisle between the shelves in a typical supermarket, which has a ventilation system that is typical for such outlets.

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A visualisation of the aerosol cloud emitted by a person coughing. Image: Petteri Peltonen / Aalto-yliopisto

The model looked at the movement in air of aerosol particles smaller than 20 micrometres. In a dry cough, one of the common symptoms of Covid-19, the disease caused by novel coronavirus, the particle size is normally under 15 micrometres.

Such minute particles do not sink to the ground, but float about on air currents or remain suspended in the air. In previous studies involving Influenza A, researchers found that the virus is most prevalent in smaller particles under five micrometres in size.

Follow-up studies with Swedish colleagues

The movement of the droplets and their ability to remain suspended in the air were modelled using a super-computer and a 3D model was created. The CSC – IT Center for Science super-computer's huge computing capacity as well as multi-disciplinary cooperation produced the first results within a week.

The modelled findings as such were already well known. The goal of the research consortium was to use visualisation to provide a better understanding of the behaviour of aerosol particles.

"Visualisation of the invisible particles containing the virus is extremely important, so that we can better understand the spread of infectious diseases and the phenomena behind it, now and in the future," Vuorinen noted.

No new recommendations can yet be issued on the basis of the initial results, but the findings highlight the importance of the guidelines already provided, said THL senior specialist Jussi Sane.

The Finnish Institute for Health and Welfare (THL) has recommended maintaining physical distance in public places, coughing into a sleeve or tissue and maintaining good hand hygiene. People who are sick should stay at home.

Meanwhile work on the model will continue. Infectious diseases specialists and virologists will examine the models in light of new information about coronavirus infections and the virus itself. The consortium will also be further strengthened when two Swedish universities come on board the project.

The idea for the research project was first proposed by South Savo social and healthcare municipal association primary care and emergency services medical chief of staff Janne Kuusela.