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{{short description|Device that detects smoke, typically as an indicator of fire}}
[[File:Smoke detector.JPG|thumb|Smoke detector mounted on a ceiling]]
[[File:Smoke detector alarm battery beep.webm|thumb|Knowing this sound could save your life - it means replace the battery]]

A '''smoke detector''' is a device that senses [[smoke]], typically as an indicator of [[fire]]. Smoke detectors/Alarms are usually housed in plastic enclosures, typically shaped like a disk about {{convert|125|mm|in|0}} in diameter and {{convert|25|mm|in|0}} thick, but shape and size vary. Smoke can be detected either optically ([[Photodiode|photoelectric]]) or by physical process ([[ionization]]). Detectors may use one or both sensing methods. Sensitive alarms can be used to detect and deter smoking in banned areas. Smoke detectors in large commercial and industrial buildings are usually connected to a central [[fire alarm system]].

Household smoke detectors, also known as ''smoke alarms'', generally issue an audible or visual [[alarm]] from the detector itself or several detectors if there are multiple devices interlinked. Household smoke detectors range from individual battery-powered units to several interlinked units with battery backup. With interlinked units, if any unit detects smoke, alarms will trigger at all of the units. This happens even if household power has gone out.

Residential smoke alarms are usually powered with a 9-volt battery, or by [[mains electricity]]. Some smoke alarms use a combination of the 2; usually using the 9V battery as an extra power source in the event of an outage.

Commercial smoke detectors issue a signal to a [[fire alarm control panel]] as part of a fire alarm system. Usually, an individual commercial smoke detector unit does not issue an alarm; some, however, have built-in sounders.

The risk of dying in a residential fire is cut in half in houses with working smoke detectors. The [[National Fire Protection Association|US National Fire Protection Association]] reports 0.53 deaths per 100 fires in homes with working smoke detectors compared to 1.18 deaths without (2009–2013).<ref>{{cite web |url=http://www.nfpa.org/news-and-research/fire-statistics-and-reports/fire-statistics/fire-safety-equipment/smoke-alarms-in-us-home-fires |title=Smoke Alarms in U.S. Home Fires |website=nfpa.org |date=September 2015 |access-date=2017-07-28 |url-status=live |archive-url=https://web.archive.org/web/20170729010219/http://www.nfpa.org/news-and-research/fire-statistics-and-reports/fire-statistics/fire-safety-equipment/smoke-alarms-in-us-home-fires |archive-date=2017-07-29 }}</ref>

== History ==
{{disputed section|date=September 2021}}

The first automatic electric fire alarm was patented in 1890 by [[Francis Robbins Upton]],<ref>{{Cite patent|country=US|number=436961|inventor=[[Francis Robbins Upton]]}}.</ref> an associate of [[Thomas Edison]].<ref name="party1">{{cite news |title= Birthday Party to Edison; Men Associated with Him in the Early 80s Organize the Pioneers |url= https://timesmachine.nytimes.com/timesmachine/1918/02/03/102977088.pdf |quote=Francis R. Upton of Newark, Mr. Edison's oldest associate, has been elected President of the Pioneers. |newspaper=[[The New York Times]] |date= February 3, 1918 |access-date= January 13, 2011 }}</ref> In 1902, George Andrew Darby patented the first European electrical [[heat detector]] in [[Birmingham]], [[England]].<ref>{{Cite patent|country=GB|number=190225805|title=An Electric Heat-indicator and Fire-alarm|inventor=George Andrew Darby}}.</ref><ref>{{cite book|last=Prosser|first=Richard|title=Birmingham Inventors And Inventions|year=1970|publisher=H.M. Patent Office (originally 1881) later published by S.R. Publishers 1970|isbn=0-85409-578-0}}</ref> In the late 1930s, Swiss physicist Walter Jaeger attempted to invent a sensor for poison gas.<ref name="nrc">{{cite web|url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/smoke-detectors.html|title=NRC: Fact Sheet on Smoke Detectors|date=4 September 2013|website=NRC.gov|publisher=United States Nuclear Regulation Commission|access-date=9 June 2014|url-status=live|archive-url=https://web.archive.org/web/20140727144647/http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/smoke-detectors.html|archive-date=27 July 2014}}</ref> He expected the gas entering the sensor to bind to ionized air molecules and thereby alter an electric current in a circuit of the instrument.<ref name="nrc" /> However, his device did not achieve its purpose as small concentrations of gas did not affect the sensor's conductivity.<ref name="nrc" /> Frustrated, Jaeger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current.<ref name="wallis">{{cite book |last=Wallis|first=Ian|date=1 November 2013|title=50 Best Business Ideas That Changed the World|url=https://books.google.com/books?id=WbgQAgAAQBAJ&q=duane+pearsall|publisher=Jaico Publishing House|isbn=978-81-8495-284-1|access-date=2014-11-20}}</ref> Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit's current.<ref name="wallis" /> Jaeger's experiment was one of the developments that paved the way for the modern smoke detector.<ref name="wallis" /> In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines.<ref name="madehow">{{cite web|url=http://www.madehow.com/Volume-2/Smoke-Detector.html|title=How smoke detector is made|website=MadeHow.com|publisher=Advameg|access-date=9 June 2014|url-status=live|archive-url=https://web.archive.org/web/20140607111645/http://www.madehow.com/Volume-2/Smoke-Detector.html|archive-date=7 June 2014}}</ref> He also invented a [[cold cathode]] tube that could amplify the small signal generated by the detection mechanism so that it was strong enough to activate an alarm.<ref name="madehow" />

In 1951, ionization smoke detectors were first sold in the United States. In the following years, they were used only in major commercial and industrial facilities due to their large size and high cost.<ref name="madehow" /> In 1955, simple "fire detectors" for homes were developed,<ref>{{cite journal|last=Jones|first=Hilton Ira|date=April 1955|title=Peeps at Things to Come|url=https://books.google.com/books?id=5T8EAAAAMBAJ&q=home+fire+detectors+invented+1955&pg=PA37|journal=The Rotarian|publisher=Rotary International|volume=86|issue=4|access-date=2014-11-27|url-status=live|archive-url=https://web.archive.org/web/20180508184806/https://books.google.com/books?id=5T8EAAAAMBAJ&pg=PA37&dq=home+fire+detectors+invented+1955&hl=en&sa=X&ei=8qtzVOL_B8qpNu-DgqAK&ved=0CDAQ6AEwAA#v=onepage&q=home%20fire%20detectors%20invented%201955&f=false|archive-date=2018-05-08}}</ref> which detected high temperatures.<ref name="whitepaper" /> In 1963, The [[United States Atomic Energy Commission]] (USAEC) granted the first license to distribute smoke detectors that used radioactive material.<ref name="nrc" /> In 1965, the first low-cost smoke detector for domestic use was developed by [[Duane D. Pearsall]] and Stanley Bennett Peterson. It was an individual, replaceable, battery-powered unit that could be easily installed.<ref>{{Cite web|title=History of Smoke Detectors|url=https://thedailysecure.com/history-of-smoke-detectors/|access-date=2020-12-27|website=The Daily Secure|language=en-US}}</ref><ref>{{cite magazine|last=Ha|first=Peter|date=25 October 2010|title=Smoke Detector|url=https://content.time.com/time/specials/packages/article/0,28804,2023689_2026093_2023789,00.html|magazine=[[Time (magazine)|Time]]|publisher=[[Time Inc.|Time]]|issue=ALL-TIME 100 Gadgets|page=1|access-date=9 June 2014|url-status=live|archive-url=https://web.archive.org/web/20140714194451/http://content.time.com/time/specials/packages/article/0,28804,2023689_2026093_2023789,00.html|archive-date=14 July 2014}}</ref> The "SmokeGard 700"<ref>{{cite act|title=Voluntary Standards and Accreditation Act of 1977|number=S. 825 |language=en|date=1 March 1977|url=https://books.google.com/books?id=okjQAAAAMAAJ&q=smokegard+700+pearsall|access-date=24 July 2014}}</ref> was beehive-shaped, fire-resistant, and made of steel.<ref>{{cite web |author=David Lucht |date=1 March 2013 |title=Where Theres Smoke |url=http://www.nfpa.org/newsandpublications/nfpa-journal/2013/march-april-2013/features/where-theres-smoke |url-status=live |archive-url=https://web.archive.org/web/20151220185914/http://www.nfpa.org/newsandpublications/nfpa-journal/2013/march-april-2013/features/where-theres-smoke |archive-date=20 December 2015 |access-date=7 January 2016 |newspaper=Nfpa.org}} With picture of SmokeGard.</ref> The company began mass-producing these units in 1975.<ref name="wallis" /> Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.<ref name="whitepaper">{{cite tech report|title=White Paper: Home Smoke Alarms and Other Fire Detection and Alarm Equipment|number=1|institution=Public/Private Fire Safety Council|year=2006}}</ref>

The first single-station smoke detector was invented in 1970 and was brought out the next year.<ref name="whitepaper" /> It was an ionization detector powered by a single [[Nine-volt battery|9-volt battery]].<ref name="whitepaper" /> It cost about {{US$|125|1970}} and sold at a rate of a few hundred thousand units per year.<ref name="madehow" /> Several developments in smoke detector technology occurred between 1971 and 1976, including the replacement of cold-cathode tubes with [[solid-state electronics]]. This greatly reduced the detectors' cost and size and made it possible to monitor battery life.<ref name="madehow" /> The previous alarm horns which required special batteries were replaced with horns that were more energy-efficient and allowed the use of widely available batteries.<ref name="madehow" /> These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned to make the operation more effective.<ref name="madehow" /> The rechargeable batteries were often replaced by a pair of [[AA battery|AA batteries]] along with a plastic shell encasing the detector.

The photoelectric (optical) smoke detector was invented by Donald Steele and Robert Emmark from Electro Signal Lab and patented in 1972.<ref>{{Cite patent|country=US|number=3863076|status=patent|title=Optical Smoke Detector|pridate=1973-07-24|gdate=1975-01-28|invent1=Donald F. Steele|invent2=Robert B. Enemark}}</ref>

In 1995, the 10-year-lithium-battery-powered smoke alarm was introduced.<ref name="whitepaper" />

== Design ==

Smoke can be detected using a [[#Photoelectric|photoelectric]] sensor or an [[#Ionization|ionization]] process. Fire without smoke can be detected by [[#Carbon monoxide and carbon dioxide detection|sensing carbon dioxide]]. Incomplete burning can be detected by [[#Carbon monoxide and carbon dioxide detection|sensing carbon monoxide]].

=== Photoelectric === <!-- [[Photoelectric smoke detector]] and [[Optical smoke detector]] redirect here. -->
{{more citations needed section|date=August 2016}}
[[File:OpticalSD.jpg|thumb|Optical smoke detector with the cover removed; the angled plastic in an arc across the top is a light baffle]]
[[File:OpticalSmokeDetector.png|thumb|Optical smoke detector{{ordered list|Optical chamber|Cover|Case moulding|Photodiode (transducer)|Infrared LED}}]]

A '''photoelectric''', or '''optical smoke detector''', contains a source of [[infrared]], [[visible light|visible]], or [[ultraviolet]] light—typically an [[incandescent light bulb]] or [[light-emitting diode]] (LED)—a [[lens (optics)|lens]], and a [[photoelectric receiver]]—typically a [[photodiode]]. In spot-type detectors, all of these components are arranged inside a chamber where air, which may contain smoke from a nearby fire, flows. In large open areas such as atria and auditoriums, [[optical beam smoke detector|optical beam]] or projected-beam smoke detectors are used instead of a chamber within the unit: a wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device or reflected to the receiver by a reflector. In some types, particularly optical beam types, the light emitted by the light source passes through the air being tested and reaches the photosensor. The received [[Radiative transfer|light intensity will be reduced]] due to [[scattering]] from particulates of smoke, air-borne dust, or other substances; the circuitry detects the light intensity and generates the alarm if it is below a specified threshold, potentially due to smoke.<ref name="AirflowPatterns">
  {{cite web
    | url = http://www.afcom8-21.afcom-miami-admin.com/AFCOM%20%20-%20Effects%20of%20High%20Airflow%20and%20Complex%20Airflow%20Patt.pdf
    | title = The Effects of High Air Velocity and Complex Airflow Patterns on Smoke Detector Performance
    | archive-url = https://web.archive.org/web/20120320024300/http://www.afcom8-21.afcom-miami-admin.com/AFCOM%20%20-%20Effects%20of%20High%20Airflow%20and%20Complex%20Airflow%20Patt.pdf
    | last = Brazzell
    | first = D.
    | website = AFCOM8-21.AFCOM-Miami-Admin.com
    | archive-date = 2012-03-20
    | access-date = 2009-05-13
    }}</ref>

In other types, typically chamber types, the light is not directed at the sensor, which is not illuminated in the absence of particles. If the air in the chamber contains particles (smoke or dust), the light is [[scattering|scattered]] and some of it reaches the sensor, triggering the alarm.<ref name="AirflowPatterns" />

According to the [[National Fire Protection Association]] (NFPA), "photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering". Studies by Texas A&M and the NFPA cited by the City of Palo Alto, California state, "Photoelectric alarms react slower to rapidly growing fires than ionization alarms, but laboratory and field tests have shown that photoelectric smoke alarms provide adequate warning for all types of fires and have been shown to be far less likely to be deactivated by occupants."<ref>{{Cite web |title=Risk Analysis of Residential Fire Detector Performance |url=https://propertyevaluation.net/Texas%20A&M%2095-0%20Risk%20Analysis%20of%20Residential%20Fire%20Detector%20Performance.pdf |archive-url=https://web.archive.org/web/20160411233139/http://propertyevaluation.net/Texas%20A&M%2095-0%20Risk%20Analysis%20of%20Residential%20Fire%20Detector%20Performance.pdf |archive-date=2016-04-11 |access-date=2022-07-19 |website=Walker Property Evaluation Services}}</ref>

Although photoelectric alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the NFPA recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke or use both the ionization and photoelectric smoke sensing methods. Some combination alarms may also include a carbon monoxide detection capability.

The type and sensitivity of light source and photoelectric sensor and type of smoke chamber differ between manufacturers.

=== Ionization === <!-- [[Ionization smoke detector]] redirects here. -->
[[File:Smoke-engineerguy.ogv|thumb|A video overview of how an ionization smoke detector works]]
[[File:smokealarm.JPG|thumb|Inside a basic ionization smoke detector. The black, round structure at the right is the ionization chamber. The white, round structure at the upper left is the [[piezoelectricity|piezoelectric]] horn that produces the alarm sound.]]
[[File:Americium-241.jpg|thumb|An americium container from a smoke detector]]

An '''ionization smoke detector''' uses a [[radioisotope]], typically [[americium-241]], to ionize air; a difference due to smoke is detected and an alarm is generated. Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage.<ref name="ionizaton">Fleming, Jay. [https://www.scribd.com/doc/14390291/Smoke-Detector-Technology-Research-Chief-Jay-Fleming "Smoke Detector Technology Research"] {{webarchive|url=https://web.archive.org/web/20160420071735/https://www.scribd.com/doc/14390291/Smoke-Detector-Technology-Research-Chief-Jay-Fleming|date=2016-04-20}}, retrieved 2011-11-07.</ref>

The smoke detector has two [[ionization chamber]]s, one open to the air, and a reference chamber which does not allow the entry of particles. The radioactive source emits [[alpha particle]]s into both chambers, which [[ionizes]] some air [[molecule]]s. There is a [[potential difference]] (voltage) between pairs of [[electrode]]s in the chambers; the [[electrical charge]] on the [[ion]]s allows an [[electric current]] to flow. The currents in both chambers should be the same as they are equally affected by air pressure, temperature, and the ageing of the source. If any smoke particles enter the open chamber, some of the ions will attach to the particles and not be available to carry the current in that chamber. An electronic circuit detects that a current difference has developed between the open and sealed chambers, and sounds the alarm.<ref>{{cite book |last1=Cote |first1=Arthur |title=Principles of fire protection |last2=Bugbee |first2=Percy |publisher=National Fire Protection Association |year=1988 |isbn=0-87765-345-3 |location=Quincy, Massachusetts |page=249 |chapter=Ionization smoke detectors}}</ref> The circuitry also monitors the battery used to supply or back up power. It sounds an intermittent warning when it nears exhaustion. A user-operated test button simulates an imbalance between the ionization chambers and sounds the alarm if and only if the power supply, electronics, and alarm device are functional. The current drawn by an ionization smoke detector is low enough for a small battery used as a sole or backup power supply to be able to provide power for years without the need for external wiring.

Ionization smoke detectors are usually less expensive to manufacture than optical detectors. Ionization detectors may be more prone than photoelectric detectors to false alarms triggered by non-hazardous events,<ref>Residential Smoke Alarm Performance, Thomas Cleary, Building and Fire Research Laboratory, National Institute of Standards and Technology, UL Smoke and Fire Dynamics Seminar. November, 2007.</ref><ref name="NIST">{{cite web|title=Performance of Home Smoke Alarms Analysis of the Response of Several Available Technologies in Residential Fire Settings|url=http://www.fire.nist.gov/bfrlpubs/fire07/art063.html|work=Bukowski, Cleary et al|url-status=live|archive-url=https://web.archive.org/web/20100822192559/http://www.fire.nist.gov/bfrlpubs/fire07/art063.html|archive-date=2010-08-22}}</ref> and are much slower to respond to typical house fires.{{Citation needed|reason=At least one source not mentioned in the article indicates that most house fires are fast burning "flaming" kind, and the only thing saying something like this claim that this is the typical (rather than some or many types of) house fire that I found in the citations (many of which are dead links) was only one quote in a magazine article.|date=July 2024}}

==== Radiation ====
[[File:Americium button hd.jpg|thumb|A 141 [[nanogram|ng]] speck of [[Americium dioxide|americium-241 dioxide]] on a coin-sized aluminum button.<ref>{{cite magazine |last1=Bettenhausen |first1=Craig |title=Chemistry in Pictures: Americium the beautiful |magazine=[[Chemical & Engineering News]] |date=July 7, 2021 |url=https://cen.acs.org/materials/Chemistry-Pictures-Americium-beautiful/99/web/2021/07 |access-date=11 June 2023 |publisher=[[American Chemical Society]] |issn=0009-2347}}</ref>]]
[[Americium-241]] is an [[Alpha decay|alpha emitter]] with a [[half-life]] of 432.6 years.<ref>{{Cite web |title=NuDat 3.0 database |url=https://www.nndc.bnl.gov/nudat3/ |website=NNDC.BNL.gov |publisher=[[Brookhaven National Laboratory]] |access-date=24 September 2022 }}</ref> Alpha particle radiation, as opposed to [[Beta decay|beta]] (electron) and [[Gamma ray|gamma]] (electromagnetic) radiation, is used for two reasons: the alpha particles can ionize enough air to make a detectable current; and they have low penetrative power, meaning they will be stopped, safely, by the air or the plastic shell of the smoke detector. During the alpha decay, {{SimpleNuclide|Americium|241}} emits [[gamma radiation]], but it is low-energy and therefore not considered a significant contributor to human exposure.<ref group=Note name=Note01/><ref group=Note name=Note02/><ref group=Note name=Note03/>

The amount of elemental americium-241 in ionization smoke detectors is small enough to be exempt from the regulations applied to larger deployments. A smoke detector contains about {{convert|37|kBq|abbr=on|lk=on}} of radioactive element americium-241 ({{SimpleNuclide|Americium|241}}), corresponding to about 0.3&nbsp;μg of the isotope.<ref>{{cite web | url = http://media.cns-snc.ca/pdf_doc/ecc/smoke_am241.pdf | title = Smoke detectors and americium-241 fact sheet | publisher = Canadian Nuclear Society | access-date = 2009-08-31 | url-status = live | archive-url = https://web.archive.org/web/20110706173242/http://media.cns-snc.ca/pdf_doc/ecc/smoke_am241.pdf | archive-date = 2011-07-06 }}</ref><ref>{{cite web|url=http://www.atsdr.cdc.gov/toxprofiles/tp156.pdf|format=PDF; 2.1MiB|title=Toxicological Profile for Americium|first=Julie Louise|last=Gerberding|publisher=[[United States Department of Health and Human Services]]/[[Agency for Toxic Substances and Disease Registry]]|access-date=2009-08-29|date=April 2004|url-status=live|archive-url=https://web.archive.org/web/20090906112953/http://www.atsdr.cdc.gov/toxprofiles/tp156.pdf|archive-date=2009-09-06}}</ref> This provides sufficient ion current to detect smoke while producing a very low level of radiation outside the device. Some Russian-made smoke detectors, most notably the RID-6m and IDF-1m models, contain a small amount of plutonium (18&nbsp;MBq), rather than the typical {{SimpleNuclide|Americium|241}} source, in the form of reactor-grade {{SimpleNuclide|Plutonium|239}} mixed with titanium dioxide onto a cylindrical alumina surface.<ref>{{Cite web |date=2017-02-07|title=Analysis of Soviet smoke detector plutonium|url=https://carlwillis.wordpress.com/2017/02/07/analysis-of-soviet-smoke-detector-plutonium/|access-date=2021-12-23|website=Special Nuclear Material|language=en}}</ref>

The amount of americium-241 contained in ionizing smoke detectors does not represent a significant radiological hazard.<ref>{{cite web |title=Backgrounder on Smoke Detectors |url=https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/smoke-detectors.html |website=NRC Library |publisher=U.S. [[Nuclear Regulatory Commission]] |access-date=24 September 2022}}</ref> If the americium is left in the ionization chamber of the alarm, the radiological risk is insignificant because the chamber acts as a shield to the alpha radiation. A person would have to open the sealed chamber and ingest or inhale the americium for the dose to be comparable to [[Background radiation#Natural background radiation|natural background radiation]]. The radiation risk of exposure to an ionizing smoke detector operating normally is much smaller than natural background radiation.

==== Disposal ====
Disposal regulations and recommendations for ionization smoke detectors vary from region to region. The government of New South Wales, Australia considers it safe to discard up to 10 ionization smoke detectors in a batch with domestic rubbish.<ref name="FireRescueNSWDisposal">{{cite web |date=26 November 2012 |title=Safe disposal of smoke alarms – Fire and Rescue NSW |url=http://www.fire.nsw.gov.au/page.php?id=704 |url-status=live |archive-url=https://web.archive.org/web/20130420103030/https://fire.nsw.gov.au/page.php?id=704 |archive-date=20 April 2013 |access-date=2013-06-26 |publisher=New South Wales Government |language=en |publication-place=Australia}}</ref> The U.S. [[United States Environmental Protection Agency|EPA]] considers ionizing smoke detectors safe to dispose with household trash.<ref>{{cite web |title=Americium in Ionization Smoke Detectors |url=https://www.epa.gov/radtown/americium-ionization-smoke-detectors |website=RadTown |date=27 November 2018 |publisher=[[United States Environmental Protection Agency|EPA]] |access-date=24 September 2022}}</ref> Alternatively, smoke detectors can be returned to the manufacturer.<ref name="EPADisposal">{{cite web |url=http://www.epa.gov/radiation/sources/smoke_dispose.html |title=Disposing of Smoke Detectors &#124; Radiation Protection &#124; US EPA |date=27 June 2012 |publisher=[[United States Environmental Protection Agency|EPA]] |access-date=2013-06-26 |url-status=live |archive-url=https://web.archive.org/web/20130602015411/http://www.epa.gov/radiation/sources/smoke_dispose.html |archive-date=2 June 2013 }}</ref>

=== Performance differences ===
Photoelectric detectors and ionization detectors differ in their performance depending on the type of smoke generated by a fire.

A presentation by [[Siemens]] and the [[Canadian Fire Alarm Association]] reports that the ionization detector is the best at detecting incipient-stage fires with invisibly small particles, fast-flaming fires with smaller 0.01–0.4 [[micron]] particles, and dark or black smoke, while more modern photoelectric detectors are best at detecting slow-smouldering fires with larger 0.4–10.0 micron particles, and light-coloured white/grey smoke.<ref name=siemens/>

Photoelectric smoke detectors respond faster to fire that is in its early, smoldering stage.<ref name="criticalapps">{{cite web |title=Fire and Life Safety in Mission-Critical Applications |url=http://www.systemsensor.com/lifesafety/2011/05/sophisticated-strategic-fire-and-life-safety-in-mission-critical-applications/ |archive-url=https://web.archive.org/web/20120416013553/http://www.systemsensor.com/lifesafety/2011/05/sophisticated-strategic-fire-and-life-safety-in-mission-critical-applications/ |archive-date=April 16, 2012 |access-date=2011-07-01 |publisher=Life Safety Magazine}}</ref> The smoke from the smoldering stage of a fire is typically made up of large combustion particles between 0.3 and 10.0&nbsp;[[μm]]. Ionization smoke detectors respond faster (typically 30–60 seconds) to the flaming stage of a fire. The smoke from the flaming stage of a fire is typically made up of microscopic combustion particles between 0.01 and 0.3&nbsp;μm. Also, ionization detectors are weaker in high airflow environments.<ref name="criticalapps" />

Some European countries, including France,<ref>{{Cite web |title=Lycée Blaise Pascal Rouen – Smoke alarms |url=http://pascal-lyc.spip.ac-rouen.fr/spip.php?article1399&debut_article_actuel=5 |access-date=2015-12-28 |website=pascal-lyc.spip.ac-rouen.fr}}</ref> and some US states and [[Municipality|municipalities]] have banned the use of domestic ionization smoke alarms because of concerns that they are not reliable enough as compared to other technologies.<ref>{{cite web | url = http://cfpa-e.eu/wp-content/uploads/files/guidelines/CFPA_E_Guideline_No_10_2008.pdf | title = Smoke Alarms in the Home | website = CFPA-E.eu | publisher = Confederation of Fire Protection Associations in Europe | page = 5 | date = 2008 | access-date = 2015-05-11 | url-status = live | archive-url = http://archive.wikiwix.com/cache/20150511004024/http://cfpa-e.eu/wp-content/uploads/files/guidelines/CFPA_E_Guideline_No_10_2008.pdf | archive-date = 2015-05-11 }}</ref> Where an ionizing smoke detector has been the only detector, fires in the early stages have not always been effectively detected.

In June 2006, the Australian Fire & Emergency Service Authorities Council, the peak representative body for all Australian and New Zealand fire departments, published an official report, 'Position on Smoke Alarms in Residential Accommodation'. Clause 3.0 states, "Ionization smoke alarms may not operate in time to alert occupants to escape from a smoldering fire."<ref>{{cite web | url = http://knowledgeweb.afac.com.au/positions/documents/AFACSmokeAlarmposition1June2006.pdf | title = Position on Smoke Alarms in Residential Accommodation | publisher = Australasian Fire & Emergency Service Authorities Council | access-date = 2006-06-01 | archive-url = https://web.archive.org/web/20121224033049/http://knowledgeweb.afac.com.au/positions/documents/AFACSmokeAlarmposition1June2006.pdf | archive-date = 2012-12-24 }}</ref>

In August 2008, the [[International Association of Fire Fighters]] (IAFF) passed a resolution recommending the use of photoelectric smoke alarms, saying that changing to photoelectric alarms "Will drastically reduce the loss of life among citizens and firefighters."<ref>{{cite web | url = https://www.scribd.com/doc/76855071/IAFF-Resolution-Photoelectric-Smoke-Alarms-August-2008 | title = International Association of Fire Fighters Resolution 15 | publisher = The International Association of Fire Fighters, California, USA | access-date = 2013-06-27 | url-status = live | archive-url = https://web.archive.org/web/20130828021903/http://www.scribd.com/doc/76855071/IAFF-Resolution-Photoelectric-Smoke-Alarms-August-2008 | archive-date = 2013-08-28 }}</ref>

In May 2011, the Fire Protection Association of Australia's (FPAA) official position on smoke alarms stated, "The Fire Prevention Association of Australia considers that all residential buildings should be fitted with photoelectric smoke alarms..."<ref>{{cite web |date=May 2011 |title=Position Statement – Selection of Residential Smoke Alarms – clause 5.0 |url=http://www.fpaa.com.au/media/38231/ps_01_v1_selection_of_residential_smoke_alarms.pdf |url-status=live |archive-url=https://web.archive.org/web/20130510011502/http://www.fpaa.com.au/media/38231/ps_01_v1_selection_of_residential_smoke_alarms.pdf |archive-date=2013-05-10 |access-date=2013-06-27 |publisher=Fire Protection Association Australia |page=7}}</ref>

In December 2011, the Volunteer Firefighter's Association of Australia published a World Fire Safety Foundation report, "Ionization Smoke Alarms are DEADLY", citing research outlining substantial performance differences between ionization and photoelectric technology.<ref>{{cite web | url = https://www.scribd.com/doc/76543158/The-Volunteer-Fire-Fighter-Magazine-December-2011 | title = Ionization Smoke Alarms Are DEADLY | publisher = The World Fire Safety Foundation | access-date = 2001-06-27 | url-status = live | archive-url = https://web.archive.org/web/20140416175526/http://www.scribd.com/doc/76543158/The-Volunteer-Fire-Fighter-Magazine-December-2011 | archive-date = 2014-04-16 }}</ref>

In November 2013, the Ohio Fire Chiefs' Association (OFCA) published a position paper supporting the use of photoelectric technology in Ohioan residences. The OFCA's position states, "In the interest of public safety and to protect the public from the deadly effects of smoke and fire, the Ohio Fire Chiefs' Association endorses the use of photoelectric smoke alarms in both new construction and when replacing old smoke alarms or purchasing new alarms."<ref>{{cite web | url = http://www.photoelectricsaves.com/wp-content/uploads/2013/12/Ohio-Fire-Chiefs-Association-Position-Paper.pdf | title = OFCA Position Statement on Smoke Alarms | publisher = Ohio Fire Chief's Association | access-date = 2014-10-03 | url-status = live | archive-url = https://web.archive.org/web/20141006094232/http://www.photoelectricsaves.com/wp-content/uploads/2013/12/Ohio-Fire-Chiefs-Association-Position-Paper.pdf | archive-date = 2014-10-06 }}</ref>

In June 2014, tests by the Northeastern Ohio Fire Prevention Association (NEOFPA) on residential smoke alarms were broadcast on [[American Broadcasting Company|ABC's]] ''[[Good Morning America]]'' program. The NEOFPA tests showed ionization smoke alarms were failing to activate in the early, smoldering stage of a fire.<ref>{{cite web | url = http://neofpa.org/announcements/neofpa-abcs-good-morning-america-conduct-smoke-alarm-tests/ | title = 'GMA' Investigates: Will Your Smoke Detector Respond Fast Enough? | publisher = Good Morning America | access-date = 2014-05-29 | url-status = live | archive-url = https://web.archive.org/web/20140903105443/http://neofpa.org/announcements/neofpa-abcs-good-morning-america-conduct-smoke-alarm-tests/ | archive-date = 2014-09-03 }}</ref> The combination ionization/photoelectric alarms failed to activate for an average of over 20 minutes after the stand-alone photoelectric smoke alarms. This vindicated the June 2006 official position of the [[Australasian Fire and Emergency Service Authorities Council|Australasian Fire & Emergency Service Authorities Council]] (AFAC) and the October 2008 official position of the [[International Association of Fire Fighters]] (IAFF). Both the AFAC and the IAFF recommend photoelectric smoke alarms, but not combination ionization/photoelectric smoke alarms.<ref>{{cite web | url = https://www.scribd.com/doc/35526418/Smoke-Alarm-Myths-Explained | title = Smoke Alarm Myths Explained | publisher = The World Fire Safety Foundation | access-date = 2014-09-03 | url-status = live | archive-url = https://web.archive.org/web/20141006092145/https://www.scribd.com/doc/35526418/Smoke-Alarm-Myths-Explained | archive-date = 2014-10-06 }}</ref>

According to fire tests conformant to [[EN 54]], the {{chem|CO|2}} cloud from open fire can usually be detected before particulates.<ref name=senseair />

Due to the varying levels of detection capabilities between detector types, manufacturers have designed multi-criteria devices which cross-reference the separate signals to both rule out false alarms and improve response times to real fires.<ref name=criticalapps />

[[Wikt:obscuration|Obscuration]] is a unit of measurement that has become the standard way of specifying smoke detectors' [[Sensitivity (electronics)|sensitivity]]. Obscuration is the effect smoke has in reducing light intensity, expressed in percent absorption per unit length;<ref name="siemens">[http://www.cfaa.ca/Files/flash/EDUC/FIRE%20ALARM%20ARTICLES%20AND%20RESEARCH/SMOKE_DETECTOR_SENSITIVITY.pdf Smoke Detector Sensitivity testing: Siemens and Canadian Fire Alarm Association], {{webarchive|url=https://web.archive.org/web/20160222054527/http://www.cfaa.ca/Files/flash/EDUC/FIRE%20ALARM%20ARTICLES%20AND%20RESEARCH/SMOKE_DETECTOR_SENSITIVITY.pdf|date=2016-02-22}}.</ref> higher concentrations of smoke result in higher obscuration levels.

{| class="wikitable"
|+ Typical smoke detector obscuration ratings
|-
! Detector type
! Obscuration
|-
| Photoelectric
| 0.70–13.0% obs/m (0.2–4.0% obs/ft)<ref name="AirflowPatterns" />
|-
| Ionization
| 2.6–5.0% obs/m (0.8–1.5% obs/ft)<ref name="AirflowPatterns" />
|-
| [[Aspirating smoke detector|Aspirating]]
| 0.005–20.5% obs/m (0.0015–6.25% obs/ft)<ref name="AirflowPatterns" />
|-
| Laser
| 0.06–6.41% obs/m (0.02–2.0% obs/ft)<ref>{{cite web | url = http://www.systemsensor.com/en-us/Documents/7251_DataSheet_A05-0314.pdf | title = Low-Profile Plug-in Intelligent Laser Smoke Detector | website = SystemSensor.com | access-date = 2014-05-01 | url-status = live | archive-url = https://web.archive.org/web/20140502002646/http://www.systemsensor.com/en-us/Documents/7251_DataSheet_A05-0314.pdf | archive-date = 2014-05-02 }}</ref>
|}

=== Carbon monoxide and carbon dioxide detection ===
[[Carbon monoxide sensor]]s detect potentially fatal concentrations of [[carbon monoxide]], which may build up due to faulty ventilation where there are combustion appliances such as gas heaters and cookers, although there is no uncontrolled fire outside the appliance.<ref>{{cite web | last = New York City Fire Department | title = Carbon monoxide alarms | url = http://www.nyc.gov/html/fdny/pdf/safety/fire_safety_education/2010_02/08_smoke_and_carbon_monoxide_alarms_english.pdf | access-date = 2012-05-28 | url-status = live | archive-url = https://web.archive.org/web/20120131024146/http://www.nyc.gov/html/fdny/pdf/safety/fire_safety_education/2010_02/08_smoke_and_carbon_monoxide_alarms_english.pdf | archive-date = 2012-01-31 }}</ref>

High levels of [[carbon dioxide]] ({{chem|CO|2}}) may indicate a fire, and can be detected by a [[carbon dioxide sensor]]. Such sensors are often used to measure levels of {{chem|CO|2}} which may be undesirable and harmful, but not indicative of a fire. This type of sensor can also be used to detect and warn of the much higher levels of {{chem|CO|2}} generated by a fire. Some manufacturers say that detectors based on {{chem|CO|2}} levels are the fastest fire indicators. Unlike ionization and optical detectors, they can also detect fires that do not generate smoke, such as those fueled by alcohol or gasoline. {{chem|CO|2}} detectors are not susceptible to false alarms due to particles making them particularly suitable for use in dusty and dirty environments.<ref name="senseair">{{cite web |title=Carbon Dioxide – Life and Death |url=http://senseair.senseair.com/wp-content/uploads/2011/12/carbon_dioxide.pdf |access-date=2018-12-21 |publisher=senseair.se |page=4}}<!--archiveurl=https://web.archive.org/web/20130522011323/http://senseair.se/wp-content/uploads/2011/12/carbon_dioxide.pdf|archive-date=2013-05-22}}--></ref>

==Residential==
Smoke alarm systems used in a home or residential environment are typically smaller and less expensive than commercial units. The system may include one or more individual standalone units, or multiple, interconnected units. They typically generate a loud acoustic warning signal as their only action. Several detectors (whether standalone or interconnected) are normally used in the rooms of a dwelling. There are inexpensive smoke alarms that may be interconnected so that any detector triggers all alarms. They are powered by mains electricity, with disposable or rechargeable battery backup. They may be interconnected by wires, or wirelessly. They are required in new installations in some jurisdictions.<ref>{{cite web |url=https://nest.com/support/article/Why-are-interconnected-smoke-alarms-better-than-standalone-smoke-alarms |title=Why are interconnected smoke alarms better than standalone smoke alarms? |newspaper=Nest Web site |date=17 June 2015 |author=Nest Labs |access-date=7 January 2016 |url-status=live |archive-url=https://web.archive.org/web/20160304110902/https://nest.com/support/article/Why-are-interconnected-smoke-alarms-better-than-standalone-smoke-alarms |archive-date=4 March 2016 }}</ref>

Several smoke detection methods are used and documented in industry specifications published by [[Underwriters Laboratories]].<ref>{{Cite document |publisher=Underwriters Laboratories 217 | title = Single and Multiple Station Smoke Alarms, UL 1971: Signaling Devices for the Hearing Impaired, UL 268: Smoke Detectors for Fire Alarm Signaling Systems }}</ref> Alerting methods include:

* Audible tones
** Varies between 2,900 and 3,500 [[Hertz|Hz]] depending on brand and model name
** 95 [[decibel|dB]] loudness at 3 ft, can vary between brands and models.
* Spoken voice alert
* Visual [[strobe light]]s
** 177 [[candela]] output
* [[Emergency light]] for illumination
* Tactile stimulation (e.g. bed or pillow shaker), although no standards existed as of 2008 for tactile stimulation alarm devices

Some models have a hush or temporary silence feature that allows silencing, typically by pressing a button on the housing, without removing the battery. This is especially useful in locations where false alarms can be relatively common (e.g. near a kitchen), or situations where users might remove the battery permanently to avoid the annoyance of false alarms, preventing the alarm from detecting a fire should one break out.

While current technology is very effective at detecting smoke and fire conditions, the deaf and hard-of-hearing community has raised concerns about the effectiveness of the alerting function in awakening sleeping individuals in certain high-risk groups. People part of groups like the elderly, those with hearing loss, and those who are intoxicated, may have a more difficult time using sound-based detectors.<ref>{{cite web|url=http://www.hearingloss.org/advocacy/pressrelease080207.asp|title=Alarm concern from the Hearing Loss Association of America|website=hearingloss.org|access-date=8 May 2018|archive-url=https://web.archive.org/web/20070927051359/http://www.hearingloss.org/advocacy/pressrelease080207.asp|archive-date=27 September 2007}}</ref> Between 2005 and 2007, research sponsored by the United States [[National Fire Protection Association]] (NFPA) focused on understanding the cause of the higher number of deaths in such high-risk groups. Initial research into the effectiveness of the various alerting methods is sparse. Research findings suggest that a mid-frequency (520 Hz) square wave output is significantly more effective at awakening high-risk individuals. Wireless smoke and carbon monoxide detectors linked to alert mechanisms such as vibrating pillow pads for the hearing impaired, strobes, and remote warning handsets are more effective at waking people with serious hearing loss than other alarms.<ref>{{cite web|url=http://www.hearinglossweb.com/tech/alrt/smoke/low.htm|title=All about Hearing Loss|website=www.hearinglossweb.com|access-date=8 May 2018|archive-url=https://web.archive.org/web/20100618073004/http://www.hearinglossweb.com/tech/alrt/smoke/low.htm|archive-date=18 June 2010}}</ref>

=== Batteries ===
Batteries are used either as sole or as backup power for residential smoke detectors. Mains-operated detectors have disposable or rechargeable batteries; others run only on 9-volt disposable batteries. When the battery is exhausted, a battery-only smoke detector becomes inactive; most smoke detectors chirp repeatedly if the battery is low in power. It has been found that battery-powered smoke detectors in many houses have dead batteries. It has been estimated{{When|date=January 2016}} that in the UK, over 30% of smoke alarms have dead or removed batteries. In response public information campaigns have been created to remind people to change smoke detector batteries regularly. In Australia, for example, a public information campaign suggests that smoke alarm batteries should be replaced on [[April Fools' Day]] every year.<ref>{{cite web|last=Beacham|first=Janine|title=Don't be a fool: change alarm batteries|url=http://www.margaretrivermail.com.au/news/local/news/general/dont-be-a-fool-shange-alarm-batteries/2119255.aspx|publisher=Augusta Margaret River Mail|access-date=19 April 2011|archive-url=https://web.archive.org/web/20110403140053/http://www.margaretrivermail.com.au/news/local/news/general/dont-be-a-fool-shange-alarm-batteries/2119255.aspx|archive-date=3 April 2011}}</ref> In regions using [[daylight saving time]], campaigns may suggest that people change their batteries when they change their clocks or on a birthday.

Some mains-powered detectors are fitted with a non-rechargeable [[lithium battery]] for backup with a life of typically ten years. After this, it is recommended that the detector be replaced. User-replaceable disposable [[Nine-volt battery#Lithium 9V/PP3|9-volt lithium batteries]], which last at least twice as long as alkaline batteries, are also available for smoke detectors.

The US National Fire Protection Association (NFPA) recommends that homeowners replace smoke detector batteries at least once per year when they start chirping (a signal that the battery is low on power output). Batteries should also be replaced when or if they fail a test, which the NFPA recommends to be carried out at least once per month by pressing the "test" button on the alarm.<ref>{{cite web
 |url         = http://www.nfpa.org/itemDetail.asp?categoryID=1647&itemID=39905&URL=Research%20&%20Reports/Fact%20sheets/Smoke%20alarms/Smoke%20alarm%20safety%20tips&cookie_test=1
 |title       = SMOKE ALARM SAFETY TIPS
 |publisher   = National Fire Protection Association
 |work        = Safety Information.
 |access-date  = 2009-05-17
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20090821222032/http://www.nfpa.org/itemDetail.asp?categoryID=1647&itemID=39905&URL=Research%20&%20Reports%2FFact%20sheets%2FSmoke%20alarms%2FSmoke%20alarm%20safety%20tips&cookie_test=1
 |archive-date = 2009-08-21
}}</ref>

=== Reliability ===
A 2004 [[NIST]] report concluded that "Smoke alarms of either the ionization type or the photoelectric type consistently provided time for occupants to escape from most residential fires," and, "Consistent with prior findings, ionization type alarms provided somewhat better response to flaming fires than photoelectric alarms (57 to 62 seconds faster response), and photoelectric alarms provided (often) considerably faster response to smoldering fires than ionization type alarms (47 to 53 minutes faster response)."<ref name="NIST" />

Regular cleaning can prevent false alarms caused by the build-up of dust and insects, particularly on optical-type alarms as they are more susceptible to these factors. A vacuum cleaner can be used to clean domestic smoke detectors to remove detrimental dust. Optical detectors are less susceptible to false alarms in locations such as near a kitchen producing cooking fumes.<ref>{{Cite web|title=Cleaning Smoke and Heat Alarms|url=http://www.sdfirealarms.co.uk/cleaning-smoke-and-heat-alarms|website=SDFireAlarms.co.uk|publisher=Hav Direct|date=2011|access-date=2015-07-31|url-status=live|archive-url=https://web.archive.org/web/20150925103532/http://www.sdfirealarms.co.uk/cleaning-smoke-and-heat-alarms|archive-date=2015-09-25}}</ref>

On the night of May 31, 2001, Bill Hackert and his daughter Christine of [[Rotterdam (town), New York|Rotterdam, New York]], died when their house caught fire and a [[First Alert]] brand ionization smoke detector failed to sound.<ref name="hackert">{{cite web | title = Federal appeals court upholds $2.8M award for faulty smoke alarm | url = http://www.wthr.com/global/story.asp?s=8109077 | date = April 2, 2008 | first = Bob | last = Segall | website = WTHR.com | publisher = [[WTHR]] | access-date = 2008-10-28 | archive-url = https://web.archive.org/web/20081207064422/http://www.wthr.com/global/story.asp?s=8109077 | archive-date = December 7, 2008 }}</ref> The cause of the fire was a frayed electrical cord behind a couch that smoldered for hours before engulfing the house with flames and smoke.<ref name="hackert" /> The ionization smoke detector was found to be defectively designed, and in 2006 a jury in the United States District Court for the Northern District of New York decided that First Alert, and its then parent company, [[BRK Brands]], was liable for millions of dollars in damages.<ref name="hackert" />

=== Installation and placement ===
[[File:SmokeAlarmPlacement.svg|thumb|A 2007 U.S. guide to placing smoke detectors, suggesting that one be placed on every floor of a building, and in each bedroom]]

In the United States most [[U.S. state|state]] and [[Local government|local]] [[laws]] regarding the required number and placement of smoke detectors are based upon standards established in [[National Fire Protection Association|NFPA]] 72, National Fire Alarm and Signaling Code.<ref name=nfpa72>{{cite act|title=NFPA 72|number=72-2013|language=en|year=2013|url=http://www.nfpa.org/Assets/files/AboutTheCodes/72/72-13ROPDraft.pdf|access-date=6 August 2014|url-status=live|archive-url=https://web.archive.org/web/20140922234529/http://www.nfpa.org/Assets/files/AboutTheCodes/72/72-13ROPDraft.pdf|archive-date=22 September 2014}}</ref> Laws governing the installation of smoke detectors vary depending on the locality. However, some rules and guidelines for existing homes are relatively consistent throughout the developed world. For example, Canada and Australia require a building to have a working smoke detector on every level. The United States NFPA code, cited earlier, requires smoke detectors on every habitable level and within the vicinity of all bedrooms. Habitable levels include attics that are tall enough to allow access.<ref name=nfpa72/> Many other countries have comparable requirements.

In new construction, minimum requirements are typically more stringent. For example, all smoke detectors must be hooked directly to the [[electrical wiring]], be interconnected and have a [[battery backup]]. In addition, typically, smoke detectors are required either inside or outside every [[bedroom]], depending on local codes. Smoke detectors on the outside will detect fires more quickly, assuming the fire does not begin in the bedroom, but the sound of the alarm will be reduced and may not wake some people. Some areas also require smoke detectors in [[stairway]]s, main [[hallway]]s and [[garage (house)|garages]].<ref name="rcahw">{{cite book|last=Fletcher|first=Gregory|date=18 May 2011|title=Residential Construction Academy: House Wiring|url=https://books.google.com/books?id=ugOkSeJlfisC&q=placing+smoke+detectors|publisher=Technology & Engineering|isbn=978-1-111-30621-2|access-date=24 November 2014|url-status=live|archive-url=https://web.archive.org/web/20180508184806/https://books.google.com/books?id=ugOkSeJlfisC&dq=placing+smoke+detectors&source=gbs_navlinks_s|archive-date=8 May 2018}}</ref>

A dozen or more detectors may be connected via wiring or wirelessly such that if one detects smoke, the alarms will sound on all the detectors in the network, improving the likelihood that occupants will be alerted even if smoke is detected far from their location. Wired interconnection is more practical in new construction than for existing buildings.

In the UK, the installation of smoke alarms in new builds must comply with British Standard BS5839 pt6. BS 5839: Pt.6: 2004, which recommends that a new-build property consisting of no more than 3 floors (less than 200 square metres per floor) should be fitted with a Grade D, LD2 system. Building Regulations in England, Wales and Scotland recommend that BS 5839: Pt.6 should be followed, but as a minimum a Grade D, LD3 system should be installed. Building Regulations in Northern Ireland require a Grade D, LD2 system to be installed, with smoke alarms fitted in the escape routes and the main living room and a heat alarm in the kitchen; this standard also requires all detectors to have a mains supply and a battery backup.<ref>{{cite act|title=BS 5839-6:2013 Fire detection and fire alarm systems for buildings - Code of practice for the design, installation, commissioning and maintenance of fire detection and fire alarm systems in domestic premises|number=5839-6|language=en-GB|year=2013|url=http://www.safelincs.co.uk/pages/bs5839-6.html|access-date=24 November 2014|url-status=live|archive-url=https://web.archive.org/web/20141202161054/http://www.safelincs.co.uk/pages/bs5839-6.html|archive-date=2 December 2014}}</ref>

== Commercial ==
[[File:Integrierte-feststellanlage.JPG|thumb|An integrated locking mechanism for commercial building doors. Inside an enclosure are a locking device, smoke detector and power supply.]]

Commercial smoke detectors are either conventional or addressable, and are connected to [[security alarm]] or [[fire alarm system]]s controlled by [[fire alarm control panel]]s (FACP).<ref name="ssspl">{{cite web|url=http://www.ssspl.org/uploads/Products/Pdf/firealarmsystem.pdf|title=Fire Alarm System|website=ssspl.org|access-date=8 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170829094729/http://www.ssspl.org/uploads/Products/Pdf/firealarmsystem.pdf|archive-date=29 August 2017}}</ref> These are the most common type of detector and are usually significantly more expensive than single-station battery-operated residential smoke alarms.<ref name="ssspl" /> They are used in most commercial and industrial facilities and other places such as ships and trains,<ref name="ssspl" /> but are also part of some security alarm systems in homes.<ref>{{cite web|url=http://wiki.hometech.com/tiki-index.php?page=Adding+Smoke+Detectors+to+a+Security+System|title=Adding Smoke Detectors to a Security System|last1=Fisher|first1=Jeff|website=Wiki.hometech.com|publisher=TechWiki|access-date=6 June 2014|url-status=live|archive-url=https://web.archive.org/web/20140714145756/http://wiki.hometech.com/tiki-index.php?page=Adding+Smoke+Detectors+to+a+Security+System|archive-date=14 July 2014}}</ref> These detectors do not need to have built-in alarms, as alarm systems can be controlled by the connected FACP, which will set off relevant alarms, and can also implement complex functions such as a staged evacuation.<ref name="ssspl" />

=== Conventional ===
The word "conventional" is slang used to distinguish the method used to communicate with the control unit in newer addressable systems.<ref name="ssspl" /> So-called "conventional detectors" are smoke detectors used in older interconnected systems and resemble electrical switches by their way of working.<ref name="ssspl" /> These detectors are connected in parallel to the signaling path so that the current flow is monitored to indicate a closure of the circuit path by any connected detector when smoke or other similar environmental stimuli sufficiently influences any detector.<ref name="ssspl" /> The resulting increase in current flow (or a dead short) is interpreted and processed by the control unit as a confirmation of the presence of smoke and a fire alarm signal is generated.<ref name="ssspl" /> In a conventional system, smoke detectors are typically wired together in each zone and a single fire alarm control panel usually monitors several zones which can be arranged to correspond to different areas of a building.<ref name="ssspl" /> In the event of a fire, the control panel can identify which zone or zones contain the detector or detectors in alarm. However, they cannot identify which individual detector or detectors are in a state of alarm.<ref name="ssspl" />

=== Addressable ===
[[File:Truealarmdetector.JPG|thumb|right|An addressable Simplex TrueAlarm smoke detector]]
An addressable system gives each detector an individual number or address.<ref name="ssspl" /> Addressable systems allow the exact location of an alarm to be plotted on the FACP while allowing several detectors to be connected to the same zone.<ref name="ssspl" /> In certain systems, a graphical representation of the building is provided on the screen of the FACP which shows the locations of all of the detectors in the building,<ref name="ssspl" /> while in others the address and location of the detector or detectors in alarm are simply indicated.<ref name="ssspl" />

Addressable systems are usually more expensive than conventional non-addressable systems,<ref>{{cite web |title=Addressable Equipment |url=http://www.wi-ltd.com/fire/Fire_Detection_Systems/Addressable_Systems/Addressable_Equipment |url-status=live |archive-url=https://web.archive.org/web/20091124071304/http://www.wi-ltd.com/fire/Fire_Detection_Systems/Addressable_Systems/Addressable_Equipment |archive-date=2009-11-24 |access-date=2010-06-09 |publisher=Westminster International Limited}}</ref> and offer extra options, including a custom level of sensitivity (sometimes called Day/Night mode) which can determine the amount of smoke in a given area and contamination detection from the FACP that allows determination of a wide range of faults in detection capabilities of smoke detectors.<ref name="ssspl" /> Detectors become contaminated usually as a result of the build-up of atmospheric particulates in the detectors being circulated by the heating and air-conditioning systems in buildings. Other causes include carpentry, sanding, painting, and smoke in the event of a fire.<ref>{{cite web|url=http://firewize.com/contaminated-smoke-detectors|title=Contaminated (dirty) Smoke Detectors|date=2012|website=Firewize.com|publisher=Firewize Holdings Pty|access-date=6 June 2014|url-status=live|archive-url=https://web.archive.org/web/20140729162623/http://firewize.com/contaminated-smoke-detectors|archive-date=29 July 2014}}</ref> Panels can also be interconnected to monitor a large number of detectors in multiple buildings.<ref name="ssspl" /> This is most commonly used in hospitals, universities, resorts and other large centres or institutions.<ref name="ssspl" />

== Standards ==

=== EN54 European standards ===
Fire detection products have the European Standard ''[[EN 54]] Fire Detection and Fire Alarm Systems'' that is a mandatory standard for every product that is going to be delivered and installed in any country in the European Union (EU). ''EN 54 part 7'' is the standard for smoke detectors. European standards are developed to allow free movement of goods in the EU countries. EN 54 is widely recognized around the world. The EN 54 certification of each device must be issued annually.<ref>{{cite web |date=2012-11-11 |title=CEN – Technical Bodies |url=http://www.cen.eu/cen/Sectors/TechnicalCommitteesWorkshops/CENTechnicalCommittees/Pages/Standards.aspx?param=6055&title=Fire%20detection%20and%20fire%20alarm%20systems |archive-url=https://web.archive.org/web/20130520093208/http://www.cen.eu/cen/Sectors/TechnicalCommitteesWorkshops/CENTechnicalCommittees/Pages/Standards.aspx?param=6055&title=Fire%20detection%20and%20fire%20alarm%20systems |archive-date=2013-05-20 |access-date=2014-08-22 |publisher=Cen.eu}}</ref><ref>{{cite web |url=http://shop.bsigroup.com/ProductDetail/?pid=000000000030151892 |title=BS EN 54-11:2001 - Fire detection and fire alarm systems. Manual call points – BSI British Standards |publisher=Shop.bsigroup.com |access-date=2014-08-22 |url-status=live |archive-url=https://web.archive.org/web/20141018011717/http://shop.bsigroup.com/ProductDetail/?pid=000000000030151892 |archive-date=2014-10-18 }}</ref>

====Coverage of smoke and temperature detectors with European standard EN54====
{| class="wikitable"
|-
! rowspan="2" | Surface area (square meters)
! rowspan="2" | Type of detector
! rowspan="2" | Height (m)
! colspan="2" | Ceiling slope ≤20°
! colspan="2" | Ceiling slope >20°
|-
! Smax (square meters) !! Rmax (m) !! Smax (square meters) !! Rmax(m) 
|-
| SA ≤80 || EN54-7 || ≤12 || 80 || 6,6 || 80 || 8,2
|-
| rowspan="2" | SA >80 || EN54-7 || ≤6 || 60 || 5,7 || 90 || 8,7
|-
|  || 6 < h ≤ 12 || 80 || 6,6 || 110 || 9,6
|-
| rowspan="2" | SA ≤30 || EN54-5 Clase A1 || ≤7,5 || 30 || 4,4 || 30 || 5,7
|-
| EN54-5 Clase A2,B,C,D,F,G || ≤ 6 || 30 || 4,4 || 30 || 5,7
|-
| rowspan="2" | SA >30 || EN54-5 Clase A1 || ≤7,5 || 20 || 3,5 || 40 || 6,5
|-
| EN54-5 Clase A2,B,C,D,E,F,G || ≤6 || 20 || 3,5 || 40 || 6,5
|}

* EN54-7: Smoke detector
* EN54-5: Temperature detector
* SA: Surface area
* Smax (square meters): Maximum surface coverage
* Rmax (m): Maximum radio

Information that is in bold is the standard coverage of the detector. Smoke detector coverage is '''60 square meters''' and temperature smoke detector coverage is '''20 square meters'''. The height from the ground is an important issue for correct protection.<ref>{{cite web |url=http://www.aenor.es/DOCUMENTOS/NORMALIZACION/NORMASNACIONALES/EXTRACTOS/(EX)UNE_23007-14=2009.pdf |title=AENOR Noticias |access-date=2013-08-26 |url-status=live |archive-url=https://web.archive.org/web/20131103070833/http://www.aenor.es/DOCUMENTOS/NORMALIZACION/NORMASNACIONALES/EXTRACTOS/(EX)UNE_23007-14=2009.pdf |archive-date=2013-11-03 }}</ref>

An additional (harmonised) EN14604 also exists, which tends to be the standard usually cited at the domestic point of sale. This standard expands on the EN54 recommendations for domestic smoke alarms and specifies requirements, test methods, performance criteria, and manufacturer's instructions. It also includes additional requirements for smoke alarms, which are suitable for use in leisure accommodation vehicles.<ref>{{Cite web |title=CE marking to Smoke Alarm Device–EN 14604_Firefighting Device_Fire test center_FireTC.net |url=http://www.firetc.net/firetesting/show.php?itemid=706 |website=firetc.net}}</ref> However, much of EN14604 is voluntary. A study published in 2014 assessed six areas of compliance and found that 33% of devices claiming to meet this standard did not do so in one or more of the specifics. The study also found 19% of the products to have a problem with actual fire detection.<ref>{{cite web |url=https://cfpa-e.eu/wp-content/uploads/2017/docs/ARTICLE%20final%20aube%202014%20bst.pdf |title=Compliance of Smoke Alarm Devices to EN 14604 - Statistical Analysis of Market Surveillance Results |publisher=ANPI |date=October 2014 |archive-url=https://web.archive.org/web/20181222162026/https://cfpa-e.eu/wp-content/uploads/2017/docs/ARTICLE%20final%20aube%202014%20bst.pdf |archive-date=December 22, 2018 |access-date=October 22, 2022}}</ref>

=== Australia and United States ===
In the United States, the first standard for home smoke alarms was established in 1967.<ref name="whitepaper" /> In 1969, the USAEC allowed homeowners to use smoke detectors without a license.<ref name="nrc" /> The [[Life Safety Code]] (NFPA 101), passed by the US National Fire Protection Association (NFPA) in 1976, first required smoke alarms in homes.<ref name="whitepaper" /> Smoke alarm sensitivity requirements in [[UL 217]] were modified in 1985 to reduce susceptibility to nuisance alarms.<ref name="whitepaper" /> In 1988 [[Building Officials Code Administrators International|BOCA]], [[International Conference of Building Officials|ICBO]], and [[Southern Building Code Congress International|SBCCI]] model building codes begin requiring smoke alarms to be interconnected and located in all sleeping rooms.<ref name="whitepaper" /> In 1989 NFPA 74 first required smoke alarms to be interconnected in every new home construction, and 1993 NFPA 72 first required that smoke alarms be installed in all bedrooms.<ref name="whitepaper" /> The NFPA began requiring the replacement of smoke detectors after ten years in 1999.<ref name="whitepaper" /> In 1999, the [[Underwriters Laboratory]] (UL) changed smoke alarm labeling requirements so that all smoke alarms must have a manufactured date written in plain English.

In June 2013, a World Fire Safety Foundation report titled, 'Can Australian and U.S. Smoke Alarm Standards be Trusted?' was published in the official magazine of the Australian Volunteer Firefighter Association. The report brings into question the validity of testing criteria used by American and Australian government agencies when undergoing scientific testing of ionization smoke alarms.<ref>{{cite web|url=https://www.scribd.com/doc/139524431/Can-Australian-and-US-Smoke-Alarm-Standards-be-Trusted
|title=Can Australian and U.S. Smoke Alarm Standards be Trusted?|publisher=The World Fire Safety Foundation|access-date=2013-06-27}}</ref>

== Legislation ==
In June 2010 the City of [[Albany, California]], enacted a photoelectric-only legislation after a unanimous decision by the Albany City Council; several other Californian and Ohioan cities enacted similar legislation shortly afterwards.<ref>{{cite web | url = http://www.albanyca.org/index.aspx?page=926 | title = Albany, California Ordinance 2010-06 Photoelectric Specific Requirements | publisher = The Albany City Council, Albany, California, USA | access-date = 2013-06-27 | url-status = live | archive-url = https://web.archive.org/web/20120203192234/http://www.albanyca.org/index.aspx?page=926 | archive-date = 2012-02-03 }}</ref>

In November 2011, the [[Northern Territory]] enacted Australia's first residential photoelectric legislation mandating the use of photoelectric smoke alarms in all new Northern Territory homes.<ref>{{cite web |title=Northern Territory Photoelectric smoke alarm legislation |url=http://pfes.nt.gov.au/Fire-and-Rescue/Territory-laws-on-smoke-alarms.aspx |url-status=live |archive-url=https://web.archive.org/web/20111001081206/http://www.pfes.nt.gov.au/Fire-and-Rescue/Territory-laws-on-smoke-alarms.aspx |archive-date=2011-10-01 |access-date=2001-06-27 |publisher=Northern Territory Fire & Rescue Service |language=en |publication-place=Australia}}</ref>

From January 1, 2017, the Australian state of [[Queensland]] mandated all smoke alarms in new dwellings (or where a dwelling is substantially renovated) must be photoelectric, and not also contain an ionization sensor. They also were to be hardwired to the mains power supply with a secondary power source (i.e. battery) and be interconnected with every other smoke alarm in the dwelling. This is so all would be activated together. From that date, all replacement smoke alarms must be photoelectric; from January 1, 2022, all dwellings sold, leased, or where a lease is renewed must comply as for new dwellings; and from January 1, 2027, all dwellings must comply as for new dwellings.<ref>{{cite web |url=https://www.qfes.qld.gov.au/community-safety/smokealarms/Documents/New-Smoke-Alarm-Legislation.pdf |title=Archived copy |access-date=2017-02-06 |url-status=live |archive-url=https://web.archive.org/web/20170218233453/https://www.qfes.qld.gov.au/community-safety/smokealarms/documents/New-Smoke-Alarm-Legislation.pdf |archive-date=2017-02-18 }}</ref>

In June 2013, in an Australian Parliamentary speech, the question was asked, "Are ionization smoke alarms defective?" This was further to the Australian Government's scientific testing agency (the Commonwealth Scientific and Industrial Research Organisation – [[CSIRO]]) data revealing serious performance problems with ionization technology in the early, smoldering stage of a fire, a rise in litigation involving ionization smoke alarms, and increasing legislation mandating the installation of photoelectric smoke alarms. The speech cited in May 2013, World Fire Safety Foundation report published in the Australian Volunteer Firefighter Association's magazine titled, 'Can Australian and U.S. Smoke Alarm Standards be Trusted?' The speech concluded with a request for one of the world's largest ionization smoke alarm manufacturers and the CSIRO to disclose the level of visible smoke required to trigger the manufacturers' ionization smoke alarms under CSIRO scientific testing.<ref>{{cite web |date=20 June 2013 |title=Smoke Alarms |url=http://www.parliament.nsw.gov.au/Prod/parlment/hansart.nsf/V3Key/LA20130620041?open&refNavID=HA8_1 |url-status=live |archive-url=https://web.archive.org/web/20131029203221/http://www.parliament.nsw.gov.au/Prod/parlment/hansart.nsf/V3Key/LA20130620041?open&refNavID=HA8_1 |archive-date=29 October 2013 |access-date=2013-06-26 |publisher=Hansard – Mr Christopher Gulaptis MP, Private Member's Statements, New South Wales Parliamentary Debates, Legislative Assembly, New South Wales |page=22218 |publication-place=Australia}}</ref> The US state of California banned the sale of smoke detectors with replaceable batteries.<ref>{{cite web|url=http://abc7news.com/news/law-bans-sale-of-smoke-detectors-with-replaceable-batteries/891576/|title=Law bans sale of smoke detectors with replaceable batteries|first=Michael|last=Finney|date=29 July 2015|website=abc7news.com|access-date=8 May 2018|url-status=live|archive-url=https://web.archive.org/web/20170703172923/http://abc7news.com/news/law-bans-sale-of-smoke-detectors-with-replaceable-batteries/891576/|archive-date=3 July 2017}}</ref>

== Safety concerns regarding smart detectors ==
Smart smoke detectors, like other [[Internet of things]] devices, can collect and transmit a significant amount of data. This can include data about when and where the device is used, the frequency of alarms, and even audio and video data if the device includes a microphone or camera. This data can potentially infer sensitive information about a user's habits, routines, and lifestyle. Since smart smoke detectors are connected to the internet, they are vulnerable to hacking. An unauthorized person could potentially access the device and the data it collects. In extreme cases, if the device includes a camera or microphone, a hacker could use it to spy on the home's inhabitants.<ref name=":0">{{Cite web |title=How to secure IoT devices and protect them from cyber attacks {{!}} TechTarget |url=https://www.techtarget.com/iotagenda/post/How-to-secure-IoT-devices-and-protect-them-from-cyber-attacks |access-date=2023-05-12 |website=IoT Agenda |language=en}}</ref>

Many smart device manufacturers share user data with third parties, often for advertising or data analysis purposes. This can be a significant privacy concern if the data includes sensitive or personally identifiable information. Some manufacturers may also cooperate with law enforcement agencies, potentially providing them with access to users' data without their knowledge or consent.<ref name=":1">{{Cite web |title=IoT security: How we are keeping consumers safe from cyber threats |url=https://www.weforum.org/impact/iot-security-keeping-consumers-safe/ |access-date=2023-05-12 |website=World Economic Forum |language=en}}</ref>

Many users have taken steps to protect their privacy when using smart smoke detectors. This can include using strong, unique passwords for their devices, disabling unnecessary features, and regularly updating device software to protect against security vulnerabilities. Some users may also choose to use traditional smoke detectors that do not connect to the internet, to completely avoid these privacy concerns.<ref>{{Cite web |title=IoT Security: Risks, Examples, and Solutions {{!}} IoT Glossary |url=https://www.emnify.com/iot-glossary/iot-security |access-date=2023-05-12 |website=www.emnify.com |language=en}}</ref><ref name=":1" />

==Notes== <!-- format for Notes from https://en.wikipedia.org/wiki/Help:Explanatory_notes -->
{{reflist|group=Note|refs=
<ref name=Note01>{{cite book |title=Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials, NUREG-1717 |date=June 2001 |publisher=U.S. [[Nuclear Regulatory Commission]] |page=2-3 |url=https://www.nrc.gov/docs/ML0119/ML011980333.pdf |access-date=24 September 2022}}</ref>
<ref name=Note02>
A study published by [[Argonne National Laboratory]] evaluates the exposure risk as low but cites 33 keV as the primary component of gamma radiation.
{{cite web |title=Radiological and Chemical Fact Sheets to Support Health Risk Analyses for Contaminated Areas |url=https://remm.hhs.gov/ANL_ContaminantFactSheets_All_070418.pdf |website=[[United States Department of Health and Human Services]] Radiation Emergency Medical Management |publisher=[[Argonne National Laboratory]] |access-date=25 September 2022 |page=5 |date=August 2005 }}</ref>
<ref name=Note03>
According to the physical study cited at [[Americium-241#Decay]], the primary component of the gamma radiation is 60 keV. 
</ref>
}}

== References ==
{{reflist}}

== External links ==
{{Commons|Smoke detector}}
* [https://www.nist.gov/el/fire_protection/buildings/home-smoke-alarm-tests.cfm Smoke alarm research from the National Institute of Standards and Technology]

{{Fire protection}}
{{Firefighting}}

{{DEFAULTSORT:Smoke Detector}}
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