U.S. Air Force Senior Airman Felicia Still, 633rd Aerospace Medical Squadron bioenvironmental engineering technician, dons her gas mask during a hazardous material exercise at Langley Air Force Base, Va., Dec. 18, 2013. The exercise aimed to enhance emergency response operations in the event of a real-world crisis. (U.S. Air Force photo by Senior Airman Brittany Paerschke-O’Brien/Released)