Valve assembly for breathing systems

Control valve for a user-donnable breathing device selectively switches from a normal breathing oxygen source to a pressurized emergency bottle. A valve body has a normal oxygen source inlet and also an emergency source inlet for oxygen flow to the user of the device when donned. A poppet permits normal source oxygen flow into the valve body, normally keeping a piston in a spring-biased cocked position spaced from a seal of the emergency bottle. A user-operated release permits the poppet to move to a position for blocking the normal source inlet but allowing the piston to become uncocked and so be driven by spring force to pierce the emergency bottle seal. Only emergency breathing oxygen from the bottle then flows to the user for emergency breathing. The device can be an escape hood for an aircraft passenger.