The IRK recommends - given the current state of knowledge about viruses - that the highest possible supply of fresh air should be provided indoors. This is necessary regardless of other protective measures such as keeping minimum distances or wearing a mouth-nose cover.
Cross-ventilation is the best option with window ventilation because it quickly exchanges room air for fresh air by means of a draught through widely opened windows that are opposite each other if possible. Care should be taken to ensure that the ventilation does not spread infectious aerosols to other rooms. In addition, several minutes of intensive ventilation with widely opened windows (preferably several windows in one room at the same time) is considered effective. Should individuals be coughing and sneezing, whether at home, in the office or at school, intensive airing should be carried out immediately. In high-occupancy rooms, mere tilting of the windows is considered rather ineffective, even if it is constant.
When operating air handling units (AHUs), the proportion of circulating air during the SARS-CoV-2 pandemic should be reduced to as close to zero as possible, unless high-efficiency (virus-removing) filters are installed in the ventilation system. If ventilation and air conditioning systems are available in schools, which is rarely the case, they should be run as continuously as possible during the current pandemic.
For indoor sports, sufficient ventilation must also be provided. Even at low levels of exertion, breathing rate is significantly higher than when resting. The amount of emitted particles therefore increases further with physical activity. Rooms in which sports activities take place should therefore be ventilated much more frequently, exchanging the air at least five times per hour, according to the IRK.
The IRK does not consider the use of mobile air purifiers in classrooms or at home to be suitable, as they cannot replace active ventilation. At best, they can act as a supplementary measure in individual cases. For health reasons the IRK rejects chemical additives such as ozone to the supply air or recirculated air that is fed back into the room. This also applies to UV-C lamps in non-commercial use.
For more information about airing in schools and other indoor spaces and on the operation of ventilation systems, see the detailed statement by the IRK (available in German).