Source RKI, as of 24.11.2020:
In general, the risk of transmission of SARS-CoV-2 is highest in the near field and during prolonged and close contact. In case of prolonged stay in e.g. small, poorly or not ventilated rooms, where many people are present, a transmission by infectious, small airborne particles (aerosols) can also occur over a distance of more than 1.5 m. Such aerosols can best be reduced or removed by regular ventilation or, in the case of ventilation systems, by exchanging the room air with a supply of fresh air (or by appropriate filtration) indoors (see also “What role do aerosols play in the transmission of SARS-CoV-2?”).
At present, a wide variety of (mobile) devices are offered as possible measures to clean or disinfect indoor air. The use of these devices should prevent indoor transmission of SARS-CoV-2. Further information on the various techniques and concepts, e.g. the use of air filters or fogging or radiation techniques, as well as on the efficiency of air exchange can be found, among others, in the statement “The risk of transmission of SARS-CoV-2 indoors can be reduced by appropriate ventilation measures” of the Federal Environment Agency (UBA). The Commission for Indoor Air Hygiene at the UBA commented on air purifiers in schools on 17.11.2020.
In this context, however, it is important to emphasize that even efficient depletion (reduction) of aerosols in indoor air cannot effectively reduce the risk of transmission in the near field, e.g., in face-to-face contact at a distance of < 1.5 m. In addition, some important questions are still unresolved, such as the actual effectiveness in practical use, the health safety of the substances or processes used, or the sufficient distribution of a disinfecting agent or the filtered/disinfected air throughout the room. Also, the risk of indirect transmission via (droplet) contaminated surfaces cannot be reduced by the use of such devices (see also “Guidance on cleaning and disinfection of surfaces outside healthcare facilities in the context of the COVID 19 pandemic”).
The false assumption that when a certain device is used within a room, further measures, e.g. compliance with distance rules or the wearing of a mouth-nose covering, can be dispensed with, should be avoided at all costs. Therefore, it is important to ensure that the use of such devices does not lead to a sense of “false security” and that the recommended infection prevention measures (AHA+L rule) continue to be followed.