Airman 1st Class Spencer Hearn, 28th Medical Operations Squadron bioenvironmental engineer technician, performs a gas mask fit test on Airman 1st Class Lauren Allen, 28th Medical Operations Squadron bioenvironmental engineer technician, on Ellsworth Air Force Base, S.D., Nov. 30, 2011. Hearn used a portacount plus fit tester to ensure the M50 gas mask had a proper seal. (U.S. Air Force photo by Airman Alystria Maurer/Released)