First Alert 2 pack, 10 year battery photoelectric smoke alarm with slim design and a battery carbon monoxide alarm. The CO alarm features an electrochemical CO sensor to alert you to dangerous carbon monoxide. A clear, 85-decibel alarm sounds when CO levels become hazardous. The alarm features an end-of-life warning that lets you know when it is time to replace the unit. The 10 year battery photoelectric smoke alarm detects smoke from slow-burning, smoldering fires while keeping false alarms to a minimum. The simple twist-lock mounting system installs easily into a wall or ceiling.