Editing Smoke detector (section)

== 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
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    | 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? | work = NEOFPA | 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>