U.S. Navy Petty Officer 2nd Class Sean Purvis rests after going through a series of tests on a reduced oxygen breathing device at Langley Air Force Base, Va., Nov. 14, 2014. The device changes the concentration of mixtures of gases to create hypoxia, which can be experienced in high-altitude flying and parachuting. (U.S. Air Force photo by Staff Sgt. Natasha Stannard/Released)