In order to protect against infection, interiors should be supplied with as much fresh air as possible. This is one of the most effective methods of removing potentially viral aerosols from indoor spaces.
Fresh air ventilation
The simplest form of ventilation is window ventilation. Impact ventilation, where windows are opened fully, must take place before or at the beginning of the event and then at regular intervals. A ventilation period of 3 to 10 minutes is recommended. Even more effective (if possible) is cross-ventilation via opposite windows.
The time interval for ventilation depends on the size of the room, the number of people, activities (normal speaking or e.g. choir singing) and the available window area. The SARS-CoV-2 occupational safety rule of the BMAS recommends ventilation after 60 minutes for offices and after 20 minutes for meeting rooms.
The Commission for Indoor Air Hygiene at the Federal Environment Agency has recommendations for schools that applies equally to seminar rooms at universities: For room sizes of approx. 60-75 m³ and a number of pupils of usually 20-30 children per class, intensive ventilation is to be provided in every (!) break in class with the windows wide open. In the case of teaching units of more than 45 minutes duration, i.e. also in double lessons or if only a short break (5 minutes) is planned between teaching units, this should also be done during the lessons. (...) If, during the lessons, individual students experience symptoms of illness such as repeated sneezing or coughing, the windows should be opened immediately (shock ventilation as described above). This also applies at home or in the office.
Ventilation systems (RLT systems)
HGU teaching rooms which have an air conditioning system are to be operated with a very high outside air volume flow in the winter semester.
In a statement, the indoor climate department of the German statutory accident insurance (DGUV) states: In technical ventilation, filtered fresh air is continuously fed from the outside into the interior rooms via central or decentralized ventilation systems (RLT systems). (...) In contrast to fresh air ventilation, ventilation systems, when correctly adjusted, guarantee a continuous and sufficient air exchange regardless of the external weather conditions. The risk of transmission of SARS-CoV-2 via properly maintained HVAC systems can be classified as low. For this reason, air handling systems should not be switched off, but on the contrary, the supply of fresh air via the air handling system should be increased and recirculation should be avoided or reduced as far as possible. Only a sufficient supply of fresh air contributes to a reduction of a virus-contaminated aerosol concentration.
In rooms with recirculating air coolers it is important to ventilate using the windows.
Mobile air circulation units
According to the Indoor Air Hygiene Commission (IRK) at the Federal Environment Agency, the most effective method of removing virus-containing aerosols from indoor spaces is to provide as much fresh air as possible. Mobile air filter units based on circulating air do not allow fresh air to enter the rooms. In the opinion of the IRK, the use of such air purifiers with integrated HEPA filters in rooms is not sufficient to effectively remove suspended particles (e.g. viruses) by filtering the indoor air over the entire period of use. The use of such devices cannot replace ventilation.
The Indoor Air Hygiene Commission (IRK) also describes which requirements must be checked in advance for the operation of such devices: This would require an exact recording of the airflow in the room as well as a specific placement of the mobile devices. The height of the air flow rate would also have to be adapted exactly to the local conditions and room occupancy. The use of such devices can therefore not replace ventilation measures. Treatment of air pollutants with ozone or UV light is rejected by the IDE for both health and safety reasons.
The above-mentioned documents are available at the following links (in German):