If you want to counteract the thick air of a full classroom, you basically have two options. He exchanges it for fresh air or filters it. The simplest technology for air exchange is the window. Systems that automatically regulate the ventilation of individual rooms or entire buildings are more complex. To prevent corona infections, the responsible Federal Environment Agency (UBA) recommends air exchange as the first choice method.
The office only recommends resorting to the filter method and the use of additional mobile air cleaners if rooms cannot be adequately ventilated. In this case, the air remains in the room, it is instead drawn through a filter in which small particles such as pathogens, dust or pollen get stuck. Moisture and CO₂, which above a certain concentration can cause tiredness and headaches, are not eliminated by the filters, which is why rooms with mobile air purifiers still have to be ventilated regularly.
How well the filters clean the air depends on two factors. One factor is the frequency at which the devices pull the room’s air through the filter. The Association of German Engineers (VDI) recommends in its Air Purifiers in Schools Policythat the room air should be filtered at least four times an hour.
If the fan is too loud, concentrated work is only possible with difficulty
The second factor is the filter’s ability to retain particles. In theory, many of the filters, mostly made of fleece, achieve impressive values. The so-called HEPA filters (high efficiency particulate air filters) theoretically manage to bind at least 99.95 percent of the particles. The VDI guidelines for schools also recommend this percentage.
In fact, this value only applies if all particles reach the filter. In practice, however, far from all the smallest particles from every corner of the room reach the fleece. If a room is too large for the device, if it is angled or if there are obstacles in the airflow of the cleaner, areas of the room can remain unreachable. In reality, around 80 percent of the viruses are held back by the air purifiers, says Hans-Jörn Moriske, director at the UBA until a few days ago. The devices cannot prevent virus transmissions at close range – for example, if a student coughs on his neighbor – if they are several meters away from the infectious child.
The society for aerosol research points in a opinion points out that the rate at which air is drawn through the filter is more critical to the overall performance of the device than the fineness of the filter. A somewhat stronger air circulation can compensate for the disadvantage of simpler filters and thus significantly reduce the costs of the device.
However, if the blowers suck in more air, they can become louder. The VDI guideline stipulates that the devices should not exceed the value of 35 decibels. This roughly corresponds to the volume of a table fan. This takes into account the fact that the fans generate a continuous noise that is added to other sources of noise and can therefore make it difficult to work in a very concentrated manner or to understand precisely, for example when learning a foreign language.
Some filter devices also have technologies for inactivating pathogens, for example through UV radiation. It must be ensured that the radiation cannot escape from the device. All equipment should only be serviced by properly trained personnel.