The ULA.4 is part of the new generation of operating theatre clean air canopies. It is installed as a full system in the suspended ceiling of operating theatres and consists of an air outlet element (single-layered, depending on the requirement double-layered or differential flow polyester fabric), the H14 high-efficiency particulate air filter (alternatively H13) as per DIN EN 1822 at the ends, a plenum made of anodised aluminium or stainless steel with sound absorbers and also air circulating modules. The air circulating modules comprise a circulating air intake (stainless steel microfabric or polyester fabric) in the direction of the airflow, a F9 filter in accordance with DIN EN 779, a sound absorber, fans with non-return flap valves and air supply connection. Mode of operation: The room air is sucked into the air circulating module, is mixed in the module with the fresh air conditioned by the air conditioner and discharged to the plenum located above the filter canopy. Then, the filtered, clean air flows into the operating theatre and forms the protection zone. The ideal clean air outlet for the low-turbulence ULA.4 canopy has a uniform air discharge area without dead zones and a peripheral skirting of laminated safety glass or acrylic glass that goes down to height of the door lintel.