Cabin Atmosphere Control Systems (1) Oxygen used in aircraft systems is at least 99.5 percent pure and is practically water free. (2) Oxygen used in aircraft systems is 99.5 percent pure and is hospital quality. Regarding the above statements, a. only No. 1 is true. b. both No. 1 and No. 2 are true. c. neither No. 1 nor No. 2 is true.