U.S. Air Force Senior Airman Eric Purnell, 823d Base Defense Squadron fireteam member, tightens a gas mask, April 13, 2016, Avon Park Air Force Range, Fla. Part of the obstacle course challenge involved dressing in full mission-oriented-protective-posture gear and being cleared by cadre before moving on to the next obstacle. (U.S. Air Force photo by Airman 1st Class Janiqua P. Robinson/Released)