How long is the room

Why the corona risk is growing indoors


CoronaCorona is in the fresh air
almost no problem - most researchers are convinced of that. But it becomes dangerous in closed rooms, whereby it makes a difference whether you are in the office or in the gym, in a restaurant or on an airplane.

The majority of the research community is now convinced that in addition to smear infections - for example when using the same handle - droplets and the even smaller aerosol aerosol
Aerosols are mixtures of solid or liquid particles in a gas mixture such as air. The tiny particles can float in it for a long time. -Partiekln play a crucial role in the transmission of Sars-CoV-2. "I believe that just breathing is enough," says the former President of the International Society for Aerosols in Medicine, Gerhard Scheuch, with regard to symptom-free (symptom-free) infected people who have been shown to have transmitted the virus. Only recently, US researchers have confirmed in tests that aerosols emitted by corona infected people are aerosols
Aerosols are mixtures of solid or liquid particles in a gas mixture such as air. The tiny particles can float in it for a long time. may contain intact virus particles.

And this is exactly where the problem lies: In a closed room, an infected person breathes, coughs and sneezes again and again in bursts of virus clouds. If there is no wind, the viruses spread throughout the room and the corona concentration increases. The Robert Koch Institute (RKI) therefore warns that if you stay in small, poorly or non-ventilated rooms for a long time, the probability of transmission by aerosols over a distance greater than two meters can increase. Of course, other factors also play a role here - such as how many virus-containing particles the infected person emits and how long others stay in the same room and breathe the air.

How much higher is the danger indoors than outdoors? There are no concrete statements on this. That cannot be quantified so precisely, explains an RKI spokeswoman. Scheuch refers to a study from China, according to which of the 318 outbreaks examined with three or more cases of infection, only one took place outdoors. The evaluation refers to data from January and February - i.e. if the weather is better to stay in.

Scheuch makes an example calculation. He assumes that there are 50 viruses per liter of air in a room. If a person were to inhale around 150 liters of air in ten minutes, it would contain around 7500 viruses. “According to my American colleagues from Harvard University, 300 to 1000 viruses are probably enough to trigger an infection,” Scheuch makes clear. "That means: This person received at least seven times the limit dose."

But interior space is not always interior space, as Scheuch explains: "In fitness studios, of course, physical exertion can significantly increase the production of aerosols through breathing." In a classroom with many screaming, confused children, the risk is also greater than in an office few (well-behaved) adults. In the tavern, on the other hand, loud speaking, noise and singing could increase the spread.

Here, too, the solution is: wind. And the air should ideally be as fresh as possible. The head of the Hermann Rietschel Institute, the Institute for Energy Technology at the TU Berlin, Martin Kriegel, and his team investigated how the particles are distributed in space.

He comes to the conclusion: “In principle, it can be said that with typical air exchange rates in residential and office buildings, the pathogens remain in the room for hours. The rate of descent and the air renewal take a very long time. Any increase in the outside air supply is therefore generally sensible. "

Dieter Scholz from the Department of Vehicle Technology and Aircraft Construction at the Hamburg University of Applied Sciences argues similarly. Cross ventilation with open windows on opposite sides of an apartment, for example, is best. Tilted windows also bring more than a built-in ventilation system, says Scholz. The problem with this, especially with a view to autumn: The heat disappears just as quickly as possible viruses are blown out.

Scholz also took a closer look at the situation in airplanes. The air in the cabin is permanently flushed with virus-free air from the outside and filtered air in a mixing process, he explains. As a result, the corona concentration - in the case of an infected person on board - remains at a constant value, but it is not zero. Part of the cabin air flows through so-called Hepa filters. These particulate filters can stop significantly smaller particles than FFP or even self-sewn community masks and are considered the safest variant in the fight against corona aerosols. But the cabin is not yet completely virus-free, emphasizes Scholz. The source doesn't just stop emitting viruses: “A sick person coughs, sneezes or continues to breathe,” explains Scholz. "So there are always new corona viruses." In addition, the air in an aircraft cabin circulates in such a way that viruses are verifiably spread to the left and right, but also several rows to the front and back.

The investigation of an infection chain in a Chinese restaurant fits in with this. She suggests, among other things, that the simple air flow of an air conditioning system was more likely to contribute to the aerosols being spread in the room - and precisely those guests who were sitting in the direction of this air flow were infected.

The cold seasons of autumn, winter and probably also spring bring one more problem with them this year. What to do? A team from the Institute for Fluid Mechanics and Aerodynamics at the University of the Federal Armed Forces in Munich has examined a room air purifier, with its filter combination even very small aerosol aerosols
Aerosols are generally understood to be a mixture (or a multiphase system) of gases (especially air) with irregularly distributed solid particles (e.g. dust, fiber dust, smoke) or with liquid components. Dusts are mixtures of irregularly distributed solid particles in gases that result from mechanical work or whirling up. Fiber dusts are mixtures of irregularly distributed fibers (of organic or inorganic origin - for example asbestos) in gases. Fumes are the finest mixtures of solid particles in gases that arise from thermal processes (e.g. welding smoke or metal oxide smoke) or from combustion processes (e.g. soot or fly ash).
- 99.995 percent of the particles are separated from the room air. In an 80 square meter room, the aerosol concentration can be halved in six minutes. Because the aerosols are filtered out, the devices would not turn into viruses, says the team around Christian J. Kähler. They recommend room air purifiers for schools, offices, shops, waiting rooms, club houses, lounges and dining rooms.

But such a device costs several thousand euros. In addition, there is usually the aforementioned constant source of virus - such as an infected colleague. The scientists at the Bundeswehr University therefore recommend mouth and nose protection: room air purifiers could not reduce the risk of infection through direct coughing or when talking for a long time over a short distance. Therefore, despite room air filters, sufficiently large distances to others and mouth-nose covers or particle-filtering respiratory protection masks are important.

Aerosol expert Scheuch also considers CO2 measuring devices to be helpful in closed rooms. “The CO2 content is a measure of the air quality in a room with several people. Then they would help as a warning system, ”he explains. But if you use room air purifiers at the same time, they no longer help. “Because then the CO2 content in the room goes up, but the air still remains fairly virus-aerosol-free.” An additional particle measuring device, which determines the aerosol concentration, could help here. The prices for these devices are usually in the three-digit range.

Source: dpa from August 18, 2020