Ductless units operate by recirculating air through a carbon filter. These look very similar to standard ducted fume cupboard, however they are differentiated by a carbon filter and internally located fan that recirculates air back into the room to which it was removed. These units are very limited in capacity as the carbon filter is suited to the specific application. Prior to installation, the usage must be disclosed to ensure compliance with standards and that the filter is suitable. The recirculation fume cupboards are governed by AS2243.9