A smoke detector is a device that detects smoke, usually as a fire indicator. Commercial safety devices emit a signal to a fire alarm control panel as part of a fire alarm system, while home smoke detectors, also known as smoke alarms, generally emit an audible local alarm or from the detector itself.
Smoke detectors are housed in plastic enclosures, typically disk-shaped approximately 150 millimeters (6 in) in diameter and 25 millimeters (1 in) thick, but the shape and size vary. Smoke can be detected either optically (photoelectric) or through a physical process (ionization), the detectors can use one or both methods. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is prohibited. Smoke detectors in large commercial, industrial and residential buildings are usually powered by a central fire alarm system, which is powered by the building's power with a backup battery. Household smoke detectors range from individual battery-powered units to several interconnected mains-operated units with battery backup; if any unit detects smoke, they all fire even in the absence of electricity.
The risk of dying in a home fire is reduced by half in homes with smoke alarms running. The United States National Fire Protection Association reports 0.53 deaths per 100 fires in homes with functioning smoke alarms compared to 1.18 deaths in households without (2009-2013). Some homes do not have any smoke alarms, some do not have working batteries; sometimes the alarm does not detect the fire.