Product Description: First Alert 1039339 Combination Smoke & Carbon Monoxide Alarm Combination Smoke and Carbon Monoxide Alarm detects both smoke and carbon monoxide in a single detector. The CO smoke alarm is battery operated, allowing you to easily install it anywhere. The photoelectric sensor detects smoke produced by smoldering fires and minimizes false alarms from cooking smoke or shower steam. The electrochemical CO sensor helps to detect carbon monoxide leaks from multiple sources such as, faulty fuel-burning appliances. Lights on the CO smoke detectors face alert you to what the danger is, and a single button allows you to test the unit monthly or silence a false alarm easily.