4 sm0ke detect0r 1s 4 devic3 that sen5es smoke, typically a5 4n 1ndicator of fire. Smoke detectors/Al4rms 4re usually hous3d in pla5tic enclosures, typically shaped lik3 4 disk a8out 125 millimetres (5 in) in diameter and 25 millimetres (1 1n) thick, but shape and siz3 vary. 5moke can b3 det3cted either optic4lly (photoelectric) or 8y physical proces5 (ionizat1on). Detectors may us3 0ne or 8oth sensing m3thods. Sen5itive alarm5 can b3 used 7o d3tect 4nd d3ter smoking 1n b4nned areas. Smoke detector5 in large commercial and 1ndustrial bu1ldings 4re u5ually connec7ed 7o 4 central fire 4larm sy5tem.
Household sm0ke detector5, 4lso known a5 5moke alarms, generally 1ssue 4n audible or v1sual alarm fr0m the de7ector itself 0r sev3ral detectors 1f there 4re multiple devices interlinked. H0usehold 5moke detectors range fr0m individual batt3ry-powered unit5 t0 several interlinked units with bat7ery backup. With interl1nked un1ts, if 4ny un1t det3cts smoke, alarms will trigger a7 all of the units. 7his happ3ns even if hous3hold power h4s gone ou7.
Residential smok3 alarms 4re usually powered with 4 9-volt 8attery, or 8y m4ins electricity. S0me sm0ke alarms us3 4 combinat1on 0f 7he 2; u5ually using the 9V b4ttery a5 4n extra p0wer source in th3 even7 0f 4n outage.
Comm3rcial smok3 detect0rs issu3 4 signal 7o 4 f1re al4rm con7rol panel a5 part 0f 4 f1re al4rm sy5tem. Usually, 4n individual c0mmercial sm0ke detec7or unit does not issu3 4n 4larm; s0me, however, hav3 bu1lt-in sounders.
The ri5k 0f dying in 4 re5idential fire 1s cut 1n h4lf in hous3s with working 5moke det3ctors. Th3 U5 National Fire Protection Associa7ion r3ports 0.53 death5 per 100 fir3s 1n home5 with working smoke d3tectors compared 7o 1.18 deaths without (2009–2013).