According to engineer Martin Kriegel, he would like to recommend again for this winter what he and his colleagues have always propagated in the almost two years of the pandemic: more ventilation and put on a mask.

But in view of the high number of infections one has to honestly say: “We cannot prevent an infection with it, at most we can reduce the risk.

But now it is important to prevent infections. "

Karin Truscheit

Editor in the section “Germany and the World”.

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For Kriegel, who heads the Hermann Rietschel Institute of the TU Berlin, Department of Energy, Comfort and Health in Buildings, it's a simple calculation: We have known for a long time that around 20 percent of infected people, the so-called super spreaders, account for 80 percent of the Are responsible for the occurrence of infection.

“And with these exploding incidences, the number of superemitters is also increasing.” In addition, the health authorities could hardly follow up the contacts quickly enough.

"A hurricane would have to sweep through it"

There are therefore too many of these people who carry such a high viral load that the risk for non-infected people is drastically increased indoors. After all, people who have been vaccinated can also become infected - and pass on viruses. So if you sit in a room with a super spreader, there is no escape from the above-average number of virus-contaminated aerosol particles. “You can ventilate or filter as much as you want. The amounts of air that dilute these such high concentrations cannot get into the rooms: neither with window ventilation, air conditioning or mobile air filters. A hurricane would have to sweep through it. ”Masks also reduce the risk, but they too would reach their limits. In the case of low incidences, on the other hand, and thus relatively few superemitters, one could use the common,Non-medical preventive measures, with AHA rules and with ventilation, "let go": In this way, the infection process can be controlled. "But the times are over."

The inhaled dose is decisive for infection indoors: It is made up of the length of stay and the number of virus-laden aerosol particles in the air.

The higher the viral load in the infected person, the more viruses get into the air, and the faster you have inhaled the critical dose - and the weaker the masks and ventilation.

Outbreaks are currently unavoidable

Kriegel recently presented the effectiveness of the measures in relation to the increasing viral load at a meeting of the German government's crisis team. A mean viral load of 108 virus copies per milliliter of secretion is assumed for the delta variant, which humans "aerosolize" through breathing alone, i.e. atomize them into tiny aerosol particles and release them into the air. According to Kriegels Institut, FFP2 masks or increased ventilation are "just enough" to keep the risk of infection sufficiently low.

But if the viral load in the infected person's body rises to 109 virus copies per milliliter, the measures have to be bundled: FFP2 mask, ventilation - and above all a reduction in the length of stay by, for example, 50 percent. This is the only way to reduce the risk so that there is no "outbreak": It is defined as an infection of more than two people. But this bundling of measures can hardly be achieved in many areas, for example in school and working life. “And from 1010 virus copies per milliliter, so much and more are spread by super emitters, hops and malt are lost. You can no longer prevent an outbreak. "It is therefore irritating to Kriegel when the idea is often spread that" with an air filter,Ventilation and masks “, even in the current situation, the interior can still be protected from an outbreak.

Testing is no longer enough

In order to be aware of the risks, in Kriegel's opinion it is “extremely important” to precisely name the rooms in which, statistically speaking, the highest doses of virus-contaminated aerosols are inhaled.

Kriegel's Institute has shown the relative risk of different rooms: When visiting a supermarket with a mask, a value of 1 is assumed for the dose, based on the average length of stay, the size of the shop and a typical ventilation rate.

One can deduce from this that, given the virus concentration, a person can become infected there.

In comparison, one of the highest is the inhaled dose during a working day in the office with several people without a mask - the risk is estimated at 32, i.e. 32 times as high as in the supermarket. Even a day at school without a mask is associated with a twenty times higher risk than in the supermarket, since the dose is twenty times as high. When visiting a restaurant for 1.5 hours, however, an average dose of 6 is achieved. “In the current situation that is far too high. Because it would mean that an average of six people there would be infected by a superemitter. ”Bars and clubs are about as high a risk as offices - but loud conversation and dancing increase aerosol exposure many times over.

The primary goal of protection concepts for airborne pathogens is, as Kriegels put it, still: "to eliminate" the source of the emissions.

"The infected person is not even allowed to enter the room." Therefore, a strict test must be passed before entering the interior.

But even that is no longer enough, in Kriegel's assessment, to significantly reduce the number of infections.

"This wave can only be contained with contact restrictions."

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