First Alert, 2 pack, carbon monoxide plug-In alarm features an electrochemical CO sensor to alert you to dangerous carbon monoxide. This personal carbon monoxide detector plugs into any standard electrical socket and monitors your space for elevated carbon monoxide levels. A clear, 85-decibel alarm sounds when levels become hazardous. This CO detector also features a battery backup that allows for continuous monitoring, even in the event of a power failure. The simple, user friendly design makes this CO detector easy to setup and operate, and it features an end-of-life warning that lets you know when it is time to replace the unit.