Battery-Operated Combination Smoke & Carbon Monoxide Alarm. 2-In-1 Smoke & CO Protection. Protects against the threat of smoke and carbon monoxide. Smoke particles of varying number and size are produced in all fires. For maximum protection, use both ionization and photoelectric sensing technologies. Ionization alarms are generally more sensitive to fast flaming fires and photoelectric alarms are generally more sensitive to smoldering fires.