First Alert, Battery Operated Combination Photoelectric Smoke & Carbon Monoxide Alarm, Packs Premium Detection Capabilities Into A Sleek, Compact Design, Comes Equipped With A Photoelectric Sensor & A Patented Smoke Entry System, Allowing Reliable Detection Of Smoke Produced By Smoldering Fires While Keeping False Alarms To A Minimum, Features An Advanced Electrochemical Sensor That Detects Carbon Monoxide Leaks From Multiple Sources Such As Faulty Fuel Burning Appliances & Blocked Chimneys, Sleekly Designed, Can Be Mounted Unobtrusively On A Wall Or Ceiling, A Loud Alarm Sounds When Smoke OrCarbon Monoxide Is Detected As Well As A Voice Alert Warning Of The Location & Type Of Danger.