 The cabin atmosphere at launch was adjusted to 60 % oxygen and 40 % nitrogen at sea @-@ level pressure : 14 @.@ 7 psi ( 101 kPa ) . During ascent the cabin rapidly vented down to 5 psi ( 34 kPa ) , releasing approximately 2 / 3 of the gas originally present at launch . The vent then closed and the environmental control system maintained a nominal cabin pressure of 5 psi ( 34 kPa ) as the spacecraft continued into vacuum . The cabin was then very slowly purged ( vented to space and simultaneously replaced with 100 % oxygen ) , so the nitrogen concentration fell asymptotically to zero over the next day . Although the new cabin launch atmosphere was significantly safer than 100 % oxygen , it still contained almost three times the amount of oxygen present in ordinary sea level air ( 20 @.@ 9 % oxygen ) . This was necessary to ensure a sufficient partial pressure of oxygen when the astronauts removed their helmets after reaching orbit . ( 60 % of 5 psi is 3 psi , compared to 20 @.@ 9 % of 14 @.@ 7 psi ( 101 kPa ) , or 3 @.@ 07 psi ( 21 @.@ 2 kPa ) in sea @-@ level air . ) 
