U.S. Service members train to recognize and mitigate symptoms of hypoxia in the hypobaric chamber at Langley Air Force Base, Va., Dec. 11, 2014. The hypobaric chamber changes air pressure and chemistry to simulate different heights in altitude to train pilots and aircrew members to handle the physiological stresses of fights. (U.S. Air Force photo by Airman 1st Class Devin Scott Michaels/Released)