First Alert #1039734 plug-in CO (carbon monoxide) alarm with battery back-up utilizes an electrochemical carbon monoxide sensor to accurately and effectively warn you of elevated CO levels. The plug-in design and included battery back-up mean your space can be continuously monitored for dangerous carbon monoxide gas, even during power outages. Battery life varies according to use and application. Simply plug the alarm into a 120-volt outlet, and the personal carbon monoxide detector is ready to go. When CO levels rise above a safe threshold, a loud 85-decibel alarm alerts you to the threat. Designed for simple use, the CO detector has just one button that silences or tests the unit, as well as an end-of-life timer that lets you know when it is time to replace the device.