In the air conditioning systems of a building, especially in case of hospitals, there are several sensitive places where bacteria colonize and thrive, such as air ducts, humidifying, spaces and evaporative coolers. Due to the urge of energy saving, hermetically sealed windows are more and more common. As a consequence, moisture must be removed from the building without using fresh air. To reduce the ventilation heat demand, air treatment devices are required in the building to prevent mold growth and pathogen reproduction. Especially in entrance areas and next to critical emission sources such as toilets, impeccable hygiene is mandatory.