Senior Airman Jerod Hopkins, 99th Comptroller Squadron, pulls his straps tight on his gas mask to ensure a proper seal during a chemical, biological, radiological and nuclear defense survival skills training course at Nellis Air Force Base, Nev., April 30, 2015. CBRN training is intended to prepare Airmen for the possibility of a chemical, biological, radiological or nuclear attack. (U.S. Air Force photo by Airman 1st Class Jake Carter)