From inside the GPMTCP with the hall door mostly closed so that you can see the inside of the vent through the wall. Based on what's still in the HECP this would have had a damper in it to prevent air from being drawn in through it, air exhausts out into the stairwell. An essential part of gas proofing is to create positive pressure inside the structure so that air doesn't infiltrate on it's own, it's all run through the filters. That means that any potential sources of air coming into the structure need to be blocked by gaskets (on the doors) or flaps/valves on the vents.

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