 When designing the Mercury spacecraft , NASA had considered using a nitrogen / oxygen mixture to reduce the fire risk near launch , but rejected it based on two considerations . First , nitrogen used with the in @-@ flight pressure reduction carried the clear risk of decompression sickness ( known as " the bends " ) . But the decision to eliminate the use of any gas but oxygen was <unk> when a serious accident occurred on April 21 , 1960 , in which McDonnell Aircraft test pilot G.B. North passed out and was seriously injured when testing a Mercury cabin / spacesuit atmosphere system in a vacuum chamber . The problem was found to be nitrogen @-@ rich ( oxygen @-@ poor ) air leaking from the cabin into his spacesuit feed . North American Aviation had suggested using an oxygen / nitrogen mixture for Apollo , but NASA overruled this . The pure oxygen design was judged to be safer , less complicated , and lighter in weight . 
