“To prevent the further spread of the coronavirus SARS-CoV-2, it is necessary to jointly implement important infection control measures. The focus is on the AHA formula – that means: keep your distance, observe hygiene and wear an everyday mask (mouth-nose cover),” writes the Federal Center for Health Education (BZgA) on its portal “infektionsschutz.de”. But according to researchers, these measures are not enough.
Experts repeatedly point out that the so-called AHA rules must be observed in order to contain the spread of the novel coronavirus SARS-CoV-2: in other words: keep your distance, observe hygiene rules and wear an everyday mask. But researchers now report that these protective rules are not sufficient.
The common recommendations to contain the COVID-19 epidemic are to wear a mask, keep your distance and avoid crowds, explains the Vienna University of Technology (TU Vienna) in a recent communication. However, the scientific basis on which these recommendations are based is decades old and no longer reflects the current state of knowledge. In order to change this, several research groups from the field of fluid dynamics have now joined forces and developed a new, improved model of the spread of infectious droplets.
No longer the current state of knowledge
It turns out that wearing masks and keeping distances is useful, but you should not be lulled into a false sense of security. Even with a mask, infectious droplets can be transmitted over several meters and remain in the air longer than previously assumed. The TU Vienna, the University of Florida, the Sorbonne in Paris, Clarkson University (USA) and the Massachusetts Institute of Technology (MIT) in Boston were involved in the research project. The new fluid dynamics model for infectious droplets was published in the “International Journal of Multiphase Flow”.
“The picture of droplet propagation that has been accepted worldwide so far is based on measurements from the 1930s and 1940s”, explains Prof. Alfredo Soldati from the Institute of Fluid Mechanics and Heat Transfer at the Vienna University of Technology. “At that time, the measuring methods were not as good as today, we suspect that especially small droplets could not be measured reliably at that time”. According to the information provided, previous models made a strict distinction between large and small droplets: The large droplets are pulled downwards by gravity, the small ones move forward almost in a straight line, but evaporate very quickly. “This picture is a bit too simple,” says Soldati. “It was therefore high time to adapt the models to the latest research in order to better understand the propagation of COVID-19”.
According to the researchers, the situation is complicated from a fluid mechanics point of view – after all, we are dealing with a so-called multiphase flow: The particles themselves are liquid, but they move in a gas. It is precisely such multiphase phenomena that are Soldati’s specialty: “Small droplets have been considered harmless until now, but this is clearly wrong,” the scientist explains. “Even when the water droplet has evaporated, an aerosol particle remains, which can contain the virus. This allows viruses to spread over distances of several meters and remain in the air for a long time”.
A particle with a diameter of ten micrometers (the average size of the ejected saliva droplets) takes almost 15 minutes to fall to the ground in typical everyday situations. This means that one can also come into contact with viruses if one observes distance rules – for example in an elevator that was used by infected persons shortly before. Particularly problematic are environments with high relative humidity, such as poorly ventilated conference rooms. Special care is needed in winter, when the relative humidity is higher than in summer.
“Masks are useful because they stop large droplets. And keeping a distance is just as useful. But our results show that neither of these can provide guaranteed protection,” says Soldati. With the mathematical model that has now been presented, and with the simulations currently running at the VSC, it is possible to calculate the concentration of virus-carrying droplets at different distances at different times.
COVID-19: Protection rules are not enough
“Until now, political decisions on corona protection measures have mainly been based on studies from the fields of virology and epidemiology,” explains the scientist. “We hope that in the future the findings from fluid mechanics will also be included.” (ad)
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