At the monthly Apollo spacecraft design review meeting with NAA, MSC officials directed NAA to design the spacecraft atmospheric system for 5 psia pure oxygen. From an engineering standpoint, the single-gas atmosphere offered advantages in minimizing weight and leakage, in system simplicity and reliability, and in the extravehicular suit interface.

From the standpoint of physiological considerations, the mixed-gas atmosphere (3.5 psia oxygen, 3.5 psia nitrogen) had the advantages of offering protection against dysbarism and atelectasis, whereas the single-gas atmosphere afforded greater decompression protection. The atmosphere validation program demonstrated the known fire hazard of a pure oxygen atmosphere. Two fires occurred, one at the Air Force School of Aerospace Medicine, Brooks Air Force Base, Tex., on September 10 and the other at the U.S. Naval Air Engineering Center, Philadelphia, Penna., on November 17. The answer to this problem appeared to be one of diligent effort on the part of spacecraft designers to be aware of the fire hazard and to exercise strict control of potential ignition sources and material selection. The official authorization was issued to NAA by NASA on August 28.