 The high @-@ pressure oxygen atmosphere was consistent with that used in the Mercury and Gemini programs . The pressure before launch was deliberately greater than ambient in order to drive out the nitrogen @-@ containing air and replace it with pure oxygen , and also to seal the plug door hatch cover . During launch , the pressure would have been gradually reduced to the in @-@ flight level of 5 psi ( 34 kPa ) , providing sufficient oxygen for the astronauts to breathe while reducing the fire risk . The Apollo 1 crew had tested this procedure with their spacecraft in the Operations and Checkout Building altitude ( vacuum ) chamber on October 18 and 19 , 1966 , and the backup crew of Schirra , Eisele and Cunningham had repeated it on December 30 . The investigation board noted that , during these tests , the Command Module had been fully pressurized with pure oxygen four times , for a total of six hours and fifteen minutes , two and a half hours longer than it had been during the plugs @-@ out test . 
