By contrast, the United States used a pure oxygen atmosphere for its 1961 Mercury, 1965 Gemini, and 1967 Apollo spacecraft, mainly in order to avoid decompression sickness. Mercury used a cabin altitude of (); Gemini used an altitude of (); and Apollo used () in space. This allowed for a lighter space vehicle design. This is possible because at 100% oxygen, enough oxygen gets to the bloodstream to allow astronauts to operate normally. Before launch, the pressure was kept at slightly higher than sea level at a constant above ambient for Gemini, and above sea level at launch for Apollo), and transitioned to the space cabin altitude during ascent. However, the high pressure pure oxygen atmosphere proved to be a fatal fire hazard in Apollo, contributing to the deaths of the entire crew of Apollo 1 during a 1967 ground test. After this, NASA revised its procedure to use a nitrogen/oxygen mix at zero cabin altitude at launch, but kept the low-pressure pure oxygen atmosphere at in space.