Answer:
Duncan Oliver
Duncan Oliver
Answered Apr 9, 2017 · Author has 430 answers and 558k answer views
Because humans are mostly dependent on the partial pressure of oxygen to be able to breathe: by filling the capsule with pure oxygen instead of 21% oxygen, 78% nitrogen, you can make the total pressure much lower without the astronauts being impaired. Lower total pressure means less force trying to rip the spacecraft apart from the inside, which means the pressure hull can be made thinner and lighter - and saving weight is HUGE on a spacecraft.
After Apollo 1, designers decided that these weight savings were not worth the massive risk that pure oxygen created if there were even the slightest fire. The Apollo spacecraft and the Skylab station were still only pressurized to 5 psi, but the Soyuz, ISS, and anything compatible with them are pressurized with gas of about the same composition and pressure of air at sea level on Earth (interesting, since airplane cabins are usually only pressurized to the equivalent of 8,000 feet of elevation).
However, spacesuits still use substantially less than atmospheric pressure: the Russian Orlan suits use 5.8 psi, NASA’s EMUs used 4.3 psi, and the Apollo suits just 3.7 psi. This is because spacesuits have to remain at least somewhat flexible for the astronauts to be able to move, and increased pressure both means the suit material itself must be thicker and stiffer, and the air pressure itself makes the suit become rigid like an inflated tire.
