Expert: "Being in offices or classrooms is not such a good idea"

TU Berlin's Martin Kriegel compares indoor infection risks and explains why hairdressers or theatres don't pose the same threat as offices.

Exhaled aerosols can spread the virus within a room in minutes.
Exhaled aerosols can spread the virus within a room in minutes.Imago/Science Photo Library

Berlin-Hairdressers in Germany can open again from 1 March and furry manes of Berlin will be no more. But does opening hair salons make sense? Don't they pose an especially high risk of infection?

Martin Kriegel, ventilation expert and head of the Hermann Rietschel Institute at the Technical University of Berlin, used an infection risk model developed in collaboration with the Robert Koch Institute and the Charité hospital in Berlin to investigate how high the risk is in closed rooms. The result: people are exposed to a lower risk at the hairdresser's or even in the theatre than in offices and schools, for example. That is, as long as generally accepted hygiene, distance and ventilation rules are observed.

The ranking of indoor spaces is based on situational R-values – an R-value less than or equal to 1 means that in a given room with one infected person in it, theoretically at most one other person will become infected. From the calculations, this reference R-value can be determined for different premises. For hair salons, this would be 0.6, in supermarkets 1, in restaurants 2.3, and 8 in open-plan offices.

Mr. Kriegel, how does Sars-CoV-2 spread indoors?

We assume that the virus spreads via aerosols within a few minutes in a room. For example, when you light a stick of incense, you smell it very quickly throughout the room. As the smoke spreads, so do the aerosol particles. Their quantity depends on the activity. Am I breathing quietly through my nose or am I talking very loudly? The higher the activity, the more particles are emitted to which potentially reproducible viruses could adhere.

These then buzz around the room and are inhaled by everyone else who is also there. However, one does not become infected immediately. Instead, a certain dose of the virus particles is required. It is not known exactly how large this dose must be. It is very difficult to measure. But we assume that the more aerosols there are in a room, the higher the infection rate.

According to your calculations, the rate is lower in a hairdresser's than in a restaurant or in schools.

Yes, what we wanted to achieve is to be able to make a comparison between certain infection situations. And you can see that being in offices or classrooms is not such a good idea. The more people there are, the higher the infection incidence will be.

What do you base that on?

For our consideration, the amount of virus-free air supplied to the room is important. We have assumed compliance with certain rules: whether ventilation systems must be present in rooms above a certain size so that a certain air exchange takes place, for example. There are regulations about the degree to which fresh air must be supplied.

To make this a bit clearer: if 100 people are sitting in a theatre, then 30 cubic metres of air per hour should be supplied so that stale air is exchanged for fresh. Our calculations have shown that a certain amount of virus-free air is needed per person and per hour spent in the rooms in order to minimise the risk of infection. As I have already said, it's the dose that counts.

This gives us the basis for our calculation: how much air is supplied according to the applicable guidelines? And how much air would have to be supplied to the room for the situational R-value to be less than or equal to 1? Then we can compare the rooms with each other.

It is also important to note that the value always applies only per hour of stay. You might spend eight hours a day in an office. To keep the situational R-value low, you would need gigantic amounts of air. 

And since people usually spend less time at the hairdresser's than in the office, for example, the risk of infection is smaller there, according to your calculations.

Exactly. We assume that the length of stay at a hairdresser's is not that long. In detail, of course, it's more complicated: for our calculations, we assume that airing is always done correctly and regularly. But this can't be said with certainty, and not every room is the same size. There are also, from a medical point of view, many unknowns in the equation, which is why one can certainly argue about our calculated R-value. But the ratios of the rooms to each other will remain, regardless of the absolute value. And one must also remember that our consideration only illuminates aerosol transmission. So the common DHD [Distance, Handwashing, Daily mask wearing] rules are assumed.

But not every room has ventilation equipment that ensures an automatic exchange of air. How did you calculate the air volume there?

For those rooms, it's very difficult to determine. It is actually almost impossible to predict how much new air will enter the room. In our calculations, we based our findings on the recommendations for how rooms with windows should be ventilated and then derived the amount from that based on numerous existing scientific studies on window ventilation.

What are the recommendations on proper ventilation?

I refer to the UBA recommendation for schools [the Umweltbundesamt, or UBA, the German environmental agency]. It says windows should be opened for three to five minutes every 20 minutes.

Schools are to re-open in Berlin and Brandenburg from 22 February. According to your calculations, the risk of infection is relatively high. What do you think is necessary to keep the risk in the classroom as low as possible?

First of all, we deliberately did not look at elementary schools and daycare centres, but only high schools, because older students are probably just as infectious as adults. At least, that's what we assume. Medical experts can better evaluate whether that is the case.

For high schools, what I've been saying in principle for a year now applies: the length of stay must be reduced. So does the number of people in the classroom - and preferably by 50 per cent, so that the situational R-value is not so high. Say a room is designed for 100 people and we assume a 10 percent risk of infection, then about 10 people will become infected. The situational R-value is 10, but if there are only 50 people in the room, only 5 people will become infected if the risk remains the same at 10 percent. The R-value drops accordingly.

According to the results of our study, I can only ever say that staying indoors for as short a time as possible and ventilating as much as possible reduces the risk. Realistically, of course, the possibilities are limited. And of course there are other factors that play a role when it comes to schools. I cannot and do not want to evaluate those.

Infobox image
Photo: TU Berlin
Martin Kriegel ...
... is a professor at the Technical University of Berlin. He heads the Hermann Rietschel Institute, where his research includes the dispersion of aerosols.

Many are concerned about how corona mutations can affect infection. They are considered much more contagious than the original virus variant. Would the mutations change anything in your model?

Our consideration is based purely on the dose of aerosols exhaled, not on the type of virus or the number of virus particles. Therefore, mutations do not play a role in the comparative assessment of indoor environments. Although the situational R-values change, they change to the same extent in the rooms considered.

In your comparison, the situational R-value almost doubles in high schools when the people in the rooms don't wear a mask. So should a mask be worn in class?

Wearing a mask reduces the risk of infection even more. On average, we assume an overall effect of about 50 percent. This is the result of a study we did with lay people, i.e., non-medics. They were asked to put on their masks in the same way as they do in everyday life to the best of their knowledge. And we came to the conclusion that the dose of aerosol particles is reduced by about half when the mask is worn.

Does this apply to all types, both self-sewn masks and FFP2 masks?

FFP2 masks are better in terms of protection. A higher effect can be expected. However, I think - without having evaluated this statistically - in everyday life the masks are often not worn properly, because it is much harder to breathe with FFP2 masks. As soon as the mask is loosened, more air naturally flows out and more aerosol particles float around. The stated protection of 95 per cent, which is supposedly always provided by FFP2 masks, is probably never really achieved. 70 per cent would be more realistic.

Again, about schools. Could special air filters minimise the risk in classrooms?

Let me put it this way: the air filters that are recommended are roughly equivalent in effect to the ventilation rules of the UBA. But that refers to relatively powerful devices for cleaning the air. My guess is that there is less ventilation when such devices are in use. And then the situational R-value would not be minimised - the air volume would remain the same. A positive effect would only occur if one did both: switch on the air filter and still open the windows regularly in accordance with the UBA recommendations. But even then, the dose would still be too high at the end of the school day.

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