First Alert, 2 pack, Battery Operated, Carbon Monoxide Alarm, Uses An Electrochemical Carbon Monoxide Sensor To Detect The Carbon Monoxide You Can't See, Smell, Or Taste In The Air, Each Unit Is Powered By One 9V Replaceable Battery, Offering Peace Of Mind Even During Power Outages, Includes A Permanent Internal Sensor That Lasts Up To 5 Years, An 85 Decibel Horn Sounds If Carbon Monoxide Levels Are Too High, & A Pull Down Action Chart On The Detector Offers Guidance On What To Do If An Alarm Goes Off, Features A Test/Silence Button That Silences The Alarm & Allows You To Test Its Functionality, No Installation Or Outlet Is Required, & An Optional Wall Mount Is Included.