Soldiers of Headquarters and Headquarters Company, 1st Stryker Brigade Combat Team, 4th Infantry Division, clear their M40-series protective mask of contaminants while inside the range 72 the gas chamber during annual chemical, biological, radiological and nuclear training, Jan. 8, 2015. The face-mounted canister (gas and aerosol filter) equipped on the mask can be worn on either the left or the right cheek.