The present invention relates to a vacuum demand valve (50) for use on the exhalation side of a breathing apparatus (32) for controlling the flow of exhalation gases from a patient (34) to a vacuum line (36). The demand valve comprises: a housing (1, 14) having a housing inlet (2) which can be connected to an exhalation line (40) of a breathing apparatus to allow exhalation gases to flow into the housing from a patient and a housing outlet (3) which can be connected to a vacuum line to allow gases to flow from the housing into the vacuum line； a diaphragm (4) having a flexible skirt (4b) connected to the housing and a rigid portion (4a) supported by the flexible skirt and moveable in response to gas pressure in the housing； a sealing member (10) connected to the rigid portion of the diaphragm which can engage with the housing outlet to resist the flow of gases from the housing to the vacuum line； wherein in a first condition of the diaphragm, the pressure in the vacuum line causes the sealing member to engage the housing outlet for resisting flow of gases from the housing to the vacuum line and in a second condition of the diaphragm a predetermined pressure of exhalation gases in the housing acts on the rigid portion of the diaphragm causing the sealing member to disengage at least partially from the housing outlet allowing exhalation gases to flow from the housing to the vacuum line.
