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Interview
How did the super-spreader singing event in Schwyz come about? An aerosol expert explains
The high rate of infection with the coronavirus in a multipurpose room was not a surprise, says a ventilation expert. And suggests improvements. Not only ventilation plays an important role.
At the end of September, almost 600 spectators watched two performances of a yodel musical in the multipurpose hall in Schwyz. The happy occasion turned into a super spreader event because some singers were infected with the coronavirus. Most of the 391 new cases that the canton registered in a week are attributed to this location.
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Shouldn’t one go to more demonstrations of any kind? We asked this question to aerosol expert Michael Riediker, who works at the Swiss Center for Occupational and Environmental Health SCOEH in Winterthur and has developed a model for estimating the concentration of viruses indoors. According to Riediker, in many events you don’t take a great risk in terms of the crown. In the multipurpose hall in Schwyz, however, the situation was unfavorable. And it would have been predictable, says Riediker.
What went wrong with the demo?
Michael Riediker: At first glance, the room with an estimated 2500 cubic meters has a large volume of air and therefore good condition. But a group of people singing and dancing at the same time is the most extreme thing you can do on stage. This means that more aerosols are emitted. For a super emitter yodel and air change per hour, my calculations result in the number of 5500 viruses that a viewer inhales in an hour. That’s more than ten times the dose, which still seems to be a small risk and clearly in the range of other super spreader events. Even if the ventilation changes the air three times, 3500 viruses are still inhaled per hour.
It seems that the room does not have displacement ventilation from the floor like in the KKL or the Zurich Schauspielhaus, what does that matter?
In this multi-purpose room, as in many other places, it is clear that fresh air enters the room from the ceiling. It sinks into the audience, warms up, and sucks in again at another point above.
What is the downside?
The exhaled air is first distributed around the room before it is extracted. Comparable to a well in which colored water flows. With floor displacement ventilation, air is slowly drawn straight up. It’s more like a slow flowing stream. Exhaled air mixes less.
What happened on stage?
I suspect spray mist, i.e. larger droplets, was involved here. If you dance and sing together, it’s hard to avoid it.
Obviously, the air of the stage and the room has also been mixed.
This happens quickly with mixed ventilation. With displacement ventilation, on the other hand, the air from the stage only mixes overhead with the exhaust air from the audience area. If the air that had risen from the stage actually flew down on Schwyz it would have to be cleared in situ.
What can the organizers do?
For example, you could try to introduce fresh air not from the ceiling, but from the side of the floor. That should be clear in terms of ventilation and the tested effect.
More on the subject of the coronavirus in Switzerland:
What helps on stage if the actors can’t wear masks?
You must first keep your distance. For extreme activities, you could try hiding the air purifiers in stage decorations, such as trees or walls, or blowing clean air from the floor through a grill and feeding it through a filter on top. But the question quickly arises of what is technically and financially feasible.
Do not recommend this for theatrical performances.
The number of people speaking or singing and how active people are on stage plays an important role. This is much less intense in the scenarios examined at the Schauspielhaus Zurich. Not only is ventilation better there, but we also expect much lower emissions of aerosols that can contain viruses.
