The air control blades have three-dimensionally profiled contours to create an efficient swirl. Hence, the air velocity and temperature difference in the occupied zone are very low, and the level of comfort is excellent. The production of these unusually contoured blades is only possible by the use of high-quality plastics and by applying innovative production technology. The exceptionally aesthetic air control blades allow for perfect architectural integration of the circular or square swirl diffuser and therefore make for an important design element for building owners and architects. A spigot with double lip seal provides a low leakage connection of the plenum box to the ducting, and a damper for flow rate balancing simplifies commissioning.