Abstract: An air processing plant for a building transports air from rooms through first air ducts to a central and from there back to the rooms through second air ducts. Several consecutive ozone generators or ultraviolet light sources are arranged in the second air ducts. Ozone and nitrogen oxide catalyzers are arranged at the exits of the second air ducts. This arrangement allows to keep a large part of the duct system under ozone. In contrast to conventional plants, the maximum ozone concentration can therefore be lower while the formation of new germs is prevented.