Battery-operated CO (carbon monoxide) alarm. Uses an electrochemical carbon monoxide sensor， the most accurate CO detection technology available， to detect the CO you can't see， smell， or taste in the air. Powered by (1) 9-volt replaceable battery， offering peace of mind even during power outages， and includes a permanent internal sensor lasts up to five years. Battery life varies according to use and application. An 85-decibel horn sounds if carbon monoxide levels are too high， and a pull-down action chart on each detector offers guidance on what to do if an alarm sounds. Features a test/silence button that silences the alarm and allows you to test its functionality. No installation or outlet is required， and an optional wall mount is included. UL (Underwriter's Laboratories) listed.