The invention relates to an apparatus, in particular for processing toxic and/or medical media, including a closed-off work chamber, in which the medium can be manipulated. A ventilating device delivers a particle-free or laminar or turbulent air stream to the work chamber. A return air conduit communicates with the work chamber via an outlet opening and by way of which air is carried away from the work chamber. A cleaning element is disposed in the work chamber, to enable cleaning of the work chamber and of the return air conduit. At least one blocking device blocks off an additional connecting portion between the work chamber and the return air conduit. The blocking device is able to be opened for cleaning of the return air conduit.