Astronaut???

[adam0321]

even though we do the vast majority of the actual exposures these days.

Hmmm not sure about that. One? I know sat divers decompress for weeks

[Nix]

The shuttle and station are pressed to 14.7 psi with air. Basically surface conditions. For EVA, they use a prebreathe that is approximately 4 hours using intervals of exercise and pure O2 to deplete the N2 in their bodies. Towards the end of the prebreathe they are slowly decompressed to the 4.3 psi pressure the suit maintains. The decompression is mildly staged. The bulk of their risk comes after decompression while the astronauts body is supersaturated with N2 and O2 relative to suit pressure.

[DA Aquamaster]

Yep, but decompression is a huge deal for these guys. They use N2 inside the space stations to control fire situations - but that means that they have to decompress for hours before a space walk. If you read the deco literature, any decent science is done with two groups - astronauts and military divers. We just happen to get the benefits... even though we do the vast majority of the actual exposures these days.

Right. In the Mercury, Gemini and Appollo programs NASA used a 100% O2 atmosphere and a 3.7 psi cabin pressure. This was primarily done in project Mercury due to the limited lift capability of the available boosters (mostly modified Redstone intermediate range missles and Atlas ICBMs) as the lower cabin pressures allowed thinner spacecraft walls and lighter structures to contain the lower pressures. The practice was retained in the two follow on programs.

In flight, the result was a normal PPO2 and "normal" risk for fire. This also had additional benefits in that they could quickly reduce cabin pressure further to extinguish a fire - actually adding some inflight safety. And, in Gemini and Apollo where EVAs were on the agenda, there was no pre-breathing requirement.

However, on the ground before launch it meant that the cabin was filled with 100% O2 at 14.7 psi, creating an enormous risk of a catastrophic fire. That caught up with NASA during a ground test with Apollo 1 where a fire created by an electrical short in the 100% 14.7 psi O2 environment in a spacecraft filled with highly flammable materials resulted in a catastrophic fire that killed Astronauts Grissom, White and Chaffee in a matter of seconds. Following the fire, NASA removed many of the flammable items used on board Apollo and used a "air" mix on the ground, venting the cabin on ascent and replacing the cabin atmosphere with 100% O2 at 3.7 psi in flight.

In all three programs, pre-breathing was required on the ground prior to launch given the fairly rapid drop to 3.7 psi and that procedure was (is) also used by pilots of high altitude such as the U-2/TR-1, RB-57F/WB-57F, SR-71, etc.

In the shuttle program NASA adopted a "shirt sleeve" environment using 21% O2 at normal atmospheric pressure.

The Soviets on the other hand have always used a 14.7 psi atmosphere, in large part because their nuclear scientists were not as good as ours were at reducing the size and weight of nucluear weapons. So to accommodate larger warheads, the had a much better heavy lift capability in their boosters, which gave the the luxury of more generous spacecraft weight constraints.