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As a new and improved model of infectious droplet spread shows, they can spread out for several meters and remain in the air for a long time, reports an international research team with the participation of TU Vienna in the “International Journal of Multiphase Flow”.
According to a broadcast from the Technical University (TU) on Tuesday, the globally common picture of small droplet propagation would be based on measurements from the 1930s and 1940s and no longer reflect the current state of knowledge.
“To calculate the extent, you must know the size of the droplets expelled. You determined it by having people sneeze in front of a sheet of paper hanging on the wall and deducting the diameter of the droplets from the size of the spots on the paper.” says Alfredo Soldati from TU Wien’s Institute for Fluid Mechanics and Heat Transfer. Today one has completely different and more reliable measurement methods.
Therefore, together with colleagues from the US and France, Soldati has developed a new and improved model of infectious droplet spread, “which is based on a theoretical analysis of the relevant physical processes,” as the researchers write in his work. Current knowledge is that heavy droplets fall rapidly due to gravity, while small droplets evaporate rapidly. “But this image is too simple,” says Soldati. Because the tiny droplets would not only consist of water, but would also contain other things like proteins or the virus itself.
15 minutes in the air
When water evaporates, they become aerosol particles that can be carried by air currents over distances of several meters and remain in the air for a long time. A particle with a diameter of ten micrometers, which corresponds to the average size of the expelled droplets of saliva, normally takes almost 15 minutes before falling to the ground. “If you use an elevator only from which an infected person has just left, you can come into contact with viruses through inhaled aerosols,” the scientist emphasized. The situation is exacerbated in environments with high relative humidity, such as poorly ventilated meeting rooms.
Soldati does not want to make recommendations based on the new work, he is not a virologist. The decisive question is, for example, how many viruses must be inhaled to become infected. In any case, “masks are useful because they stop large drops. And keeping your distance is just as useful. But our results show that neither can offer guaranteed protection,” Soldati emphasized. The new model can now be used in conjunction with experiments and simulations to investigate a variety of scenarios involving breathing, speaking, coughing and sneezing and under a variety of environmental conditions such as a humid or dry atmosphere, a closed or open environment, the scientists write. .
