Pressure regulator assembly

A pressure demand regulator assembly for use with a breathing apparatus having a valve assembly that includes an inlet for connection to a source of breathing gas, an outlet for connection to a facepiece to provide breathing gas to the user and an actuator for controlling the flow of breathing gas between the inlet and the outlet in response to the user's respiration. The regulator assembly further includes a flexible elastomeric diaphragm in operative connection with the actuator. The diaphragm is exposed to ambient pressure on a first side thereof and exposed to a positive pressure within the facepiece on a second side thereof. The regulator assembly also includes an impermeable and flexible shield that seals the first side of the diaphragm from certain toxic substances in the ambient atmosphere while allowing the first side of the diaphragm to experience ambient pressure, such that the flexible shield moves along with the diaphragm during respiration without dampening the movement of the diaphragm during respiration of the user.