A safety interface for connection between an anesthesia patient circuit and a disposal system, and a combined positive and negative pressure relief valve for the safety interface.The safety interface comprises a housing defining a passageway, means for connecting an upstream end of the passageway to the waste anesthetic gas outlet of an anesthesia patient circuit, means for connecting the downstream end of the passageway to a disposal system, a negative pressure relief valve and a positive pressure relief valve in communication with the passageway. The positive pressure release valve and negative pressure release valve are preferably provided in the form of a combined positive and negative pressure release valve, and a second negative pressure release valve is preferably also provided. Preferably, a restrictor partially obstructs the passageway at a point on its upstream to downstream extent, and the positive pressure relief valve communicates with the passageway at a point downstream of the restrictor. Means for connecting a reservoir in communication with the passageway upstream of the restrictor are preferably provided.The double acting pressure relief valve comprises a spindle, a first disc mounted on the spindle, a spindle receptacle for supporting said spindle, a coil spring seated in said spindle receptacle and surrounding said spindle, and a second disc having at least one aperture therein which may be closed by contact with said first disc, said second disc slidingly engaging said spindle and situated between said first disc and said spindle receptacle. The coil spring biases the second disc towards the housing so that the second disc may engage a raised ridge surrounding an opening in the housing.