A vacuum demand valve for controlling flow of exhalation gases from a patient to a vacuum line. The demand valve comprises: a housing having a housing inlet that allows exhalation gases to flow into the housing from a patient, and a housing outlet that allows gases to flow from the housing into the vacuum line； a diaphragm having a flexible skirt and a rigid portion moveable in response to gas pressure in the housing； a sealing member connected to the rigid portion to engage the housing outlet to resist the flow of gases from the housing to the vacuum line. In a first condition, the vacuum line pressure causes the sealing member to engage the housing outlet and in a second condition a predetermined pressure of exhalation gases in the housing causes the sealing member to disengage from the housing outlet.
