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The purge port on the exterior of the hatch.

After the Apollo 1 fire, the atmosphere inside the command module was changed from 100% oxygen to a sea-level pressure mixture of 60% oxygen and 40% nitrogen, which reduced the fire hazard. The pressure and composition of the cabin gas during the prelaunch period was controlled by ground support equipment through the purge port. During launch, a pressure relief valve vents the excess gas overboard. During normal space operations, cabin atmosphere was pure oxygen and the pressure was maintained at 5 psi.