Away and masks are not enough

Drops with a viral load. Sincerely: Vienna University of Technology

Wear a mask, keep your distance, avoid overcrowding – these are common recommendations to prevent COVID-19 epidemics. However, the scientific foundations to which these recommendations are based are decades old and no longer reflect the current state of knowledge. 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. It has been shown that it makes sense to wear a mask and maintain a distance, but it should not have you in a false sense of security. Even with a mask, infectious droplets can be transmitted up to several meters and stay in the air for longer than previously thought.

TU Wien (Vienna), University of Florida, Sorbonne in Paris, Clarkson University (US) and MIT in Boston were involved in the research project. New fluid dynamics model for infectious droplets was published in International Journal of Multiphase Flow.

A new look on old data

“Our understanding of small droplets accepted worldwide is based on measurements from the 1930s and 1940s,” says Alfredo Soltati, professor at the Institute of Fluid Mechanics and Heat Transfer at TU Vein. “At the time, the methods of measuring were not as good as today, we suspect that especially small droplets cannot be measured reliably at that time.”

In the previous model, a strict distinction was made between large and small droplets: the larger droplets are drawn downward by gravity, the smaller ones move almost in a straight line, but evaporate very quickly. “This picture is overseen,” says Alfredo Soltati. “Therefore, it is time to adapt the model for the latest research to better understand the spread of COVID-19.”

From a fluid mechanics point of view, the situation is complex – after all, we are working with a so-called multiphase flow: the particles themselves are liquid, but they move in a gas. Solti states that these are the most common phenomena that are characteristic of Solati: “Small droplets were previously thought to be harmless, but this is clearly erroneous.” “Even when the water droplets evaporate, an aerosol particle remains, which may contain the virus. This allows the virus to spread over several meters and remain in the air for a long time.”

In normal everyday conditions, it takes about 15 minutes for a particle with 10 micrometers (the average size of emitted salivary drops) to fall to the ground. So when possible regulations are observed, it is still possible to come into contact with the virus – for example in an elevator that was used some time ago by infected people. Particularly problematic are environments with high relative humidity, such as poorly ventilated living rooms. Special care is required in winter as the relative humidity is higher in summer.

Protection rules: useful, but not sufficient

“Masks are useful because they prevent large droplets. And it is useful to maintain distance. But our results show that none of these measures can provide guaranteed protection,” Solati says. With the mathematical model that is now presented, and under the current simulation it is possible to calculate the concentration of droplets carrying the virus at different distances at different times. Sollati says, “So far, political decisions on COVID-safeguards have been based primarily on studies in the fields of virology and epidemiology. We hope that in the future, findings from fluid mechanics will also be included.”

Humid air can extend the lifetime of virus-filled aerosol droplets

more information:
S. Balachander et al. Host-to-host airborne transmission as a multiphase flow problem for science-based social distance guidelines, International Journal of Multiphase Flow (2020). DOI: 10.1016 / j.ijmultiphaseflow.20202020.103439

Provided by Vienna University of Technology

Quotes: COVID-19: Distancing and Masks Not Enough (2020, 20 October) Retrieved 20 October 2020 from

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