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 one way damper in it to prevent air from being drawn in through it. If for some reason the air pressure in the airlock exceeded the limits, the flap would open and air would exhaust 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. Any potential locations of air leaks needed to be sealed with gaskets or with flaps/valves that would only allow air out, not in.

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