The high-pressure oxygen atmosphere was similar to that which had been used successfully 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 the launch, the pressure would have been gradually reduced to the in-flight level of , providing sufficient oxygen for the astronauts to breathe while reducing the fire risk. The Apollo1 crew had successfully 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.