Mask with anti-asphyxia valve

A full face mask for a patient comprising:

• a frame defining a breathing chamber;
• an elbow coupled to the frame, said elbow including a housing having an interior defining a blower side and a patient side, at least one port to provide fluid communication between the housing interior and atmosphere;
• and an anti-asphyxia valve including at least one flap having a first portion adapted for mounting to the housing and a second portion adapted to flex between a first open position allowing gas to pass from the patient side to the housing interior through the at least one port, and from the blower side through the at least one port, to atmosphere when a difference in gas pressure in the housing interior and atmosphere is below a predetermined operating threshold, and a second forcibly closed position substantially occluding the at least one port when the difference in gas pressure between the housing interior and atmosphere is substantially equal to or above the operating threshold.