The 2-pack First Alert carbon monoxide alarm utilizes an electrochemical carbon monoxide sensor to detect carbon monoxide. The battery-powered design of this personal carbon monoxide detector means your space can be monitored for dangerous carbon monoxide gas. Simply insert the included 9-volt battery and the carbon monoxide detector is ready to go. When CO levels become elevated, a loud 85-decibel alarm alerts you to the threat. Designed to be simple to 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.