read up on rapid decompression...like if an airliner hatch/door blows out...the higher atmospheric pressure inside the pressurized cabin rushes out of the cabin quickly. Airliners tend to operate around 7 psi over the outside pressure. The Apollo atmosphere, being 100% O2 meant the same amount/concentration of O2 at a much lower pressure, around 3.5 psi. Normal sea level atmospheric pressure is 14.7 psi.