The importance of improving indoor air quality is becoming more apparent with the current corona pandemic. Increasing the ventilation rates with outside air is the general recommendation. Recirculation should be avoided. To remove or destroy contamination from recirculated air, try to maximize the outdoor air share and use mechanical air filters (HEPA or ULPA filtration) or use germicidal ultraviolet irradiation (UVGI). If possible, it is preferable to install filters of a higher class.