A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about 150 millimeters (6 in) in diameter and 25 millimeters (1 in) thick, but shape and size vary. Smoke can be detected either optically (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.

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, do have built-in sounders.

The risk of dying in a residential fire is cut in half in houses with working smoke detectors. The 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). However, some homes do not have any smoke alarms, and some homes do not have any working batteries in their smoke alarms
he first automatic electric fire alarm was patented in 1890 by Francis Robbins Upton, an associate of Thomas Edison.[3] In 1902, George Andrew Darby patented the first European electrical heat detector in Birmingham, England. In the late 1930s, Swiss physicist Walter Jagger attempted to invent a sensor for poison gas.[6] 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.[6] However, his device did not achieve its purpose as small concentrations of gas did not affect the sensor’s conductivity.[6] Frustrated, Jagger lit a cigarette and was surprised to notice that a meter on the instrument had registered a drop in current.[7] Unlike poison gas, the smoke particles from his cigarette were able to alter the circuit’s current.[7] Jagger’s experiment was one of the developments that paved the way for the modern smoke detector.[7] In 1939, Swiss physicist Ernst Meili devised an ionization chamber device capable of detecting combustible gases in mines. 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.

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.[8] In 1955, simple “fire detectors” for homes were developed,[9] which detected high temperatures.[10] In 1963, The United States Atomic Energy Commission granted the first license to distribute smoke detectors that used radioactive material. In 1965, the first low-cost smoke detector for domestic use was developed by Duane D. Pears all and Stanley Bennett Peterson. It was an individual, replaceable, battery-powered unit that could be easily installed. The “Smoke Gard 700″[13] was beehive-shaped, fire-resistant, and made of steel.[14] The company began mass-producing these units in Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.

The first single-station smoke detector was invented in 1970 and was brought out the next year. It was an ionization detector powered by a single 9-volt battery. It cost about US$125 (equivalent to $941.95 in 2022) and sold at a rate of a few hundred thousand units per year. 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.[8] 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.[8] These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned to make operation more effective.[8] The rechargeable batteries were often replaced by a pair of AA batteries along with a plastic shell encasing the detector.

The photoelectric (optical) smoke detector was invented by Donald Steele and Robert Embark from Signal Lab and patented in

In 1995, the 10-year-lithium-battery-powered smoke alarm was introduced
A photoelectric, or optical smoke detector, contains a source of infrared, visible, or ultraviolet light—typically an incandescent light bulb or light-emitting diode (LED)—a lens, and a photoelectric receiver—typically a photo diode. 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 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 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.[16] 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 scattered and some of it reaches the sensor, triggering the alarm.[16]

According to the National Fire Protection Association “photoelectric smoke detection is generally more responsive to fires that begin with a long period of smoldering”. Studies by Texas A&M and the cited by the City of 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.”[17]

Although photoelectric alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the 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.
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.[18]

The smoke detector has two ionization chambers, one open to the air, and a reference chamber which does not allow the entry of particles. The radioactive source emits alpha particles into both chambers, which ionizes some air molecules. There is a potential difference (voltage) between pairs of electrodes in the chambers; the electrical charge on the ions 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.[19] The circuitry also monitors the battery used to supply or back up power, and 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 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 sole or backup power supply to be able to provide power for years without the need for external wiring.

Ionization smoke detectors are usually cheaper to manufacture than optical detectors. Ionization detectors may be more prone than photoelectric detectors to false alarms triggered by non-hazardous events,[20][21] and are much slower to respond to typical house fires.

Americium-241 is an alpha emitter with a half-life of 432.6 years.[22] Alpha particle radiation, as opposed to beta (electron) and 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, 241
emits gamma radiation, but it is low-energy and therefore not considered a significant contributor to human exposure.

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 37 of radioactive element americium-
), corresponding to about 0.3 µg of the isotope. 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 rather than the typical
source, in the form of reactor-grade
mixed with titanium dioxide onto a cylindrical alumina surface.

The amount of americium-241 contained in ionizing smoke detectors does not represent a significant radiological hazard.[26] If the americium is left in the ionization chamber of the alarm, the radio logical 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 natural background radiation. The radiation risk of exposure to an ionizing smoke detector operating normally is much smaller than natural background radiation.

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.[27] The EPA considers ionizing smoke detectors safe to dispose with household trash.[28] Alternatively, smoke detectors can be returned to the manufacturer.[29]
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-colored white/grey smoke.[30]

Photoelectric smoke detectors respond faster to fire that is in its early, smoldering stage. The smoke from the smoldering stage of a fire is typically made up of large combustion particles between 0.3 and 10.0 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 . Also, ionization detectors are weaker in high air flow environments.

Some European countries, including France,[32] and some US states and municipalities have banned the use of domestic ionization smoke alarms because of concerns that they are not reliable enough as compared to other technologies.[33] 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.”

In August 2008, the International Association of Fire Fighters 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.”

In May 2011, the Fire Protection Association of Australia’s official position on smoke alarms stated, “The Fire Prevention Association of Australia considers that all residential buildings should be fitted with photoelectric smoke alarms…”

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.

In November 2013, the Ohio Fire Chiefs’ Association published an official position paper supporting the use of photoelectric technology in Ohioan residences. The 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.”

In June 2014, tests by the Northeastern Ohio Fire Prevention Association on residential smoke alarms were broadcast on ABC’s Good Morning America program. The tests showed ionization smoke alarms were failing to activate in the early, smoldering stage of a fire.[39] 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 & Emergency Service Authorities Council and the October 2008 official position of the International Association of Fire Fighters . Both the and the recommend photoelectric smoke alarms, but not combination ionization/photoelectric smoke alarms.

According to fire tests conformant to EN 54, the CO
2 cloud from open fire can usually be detected before particulates.

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.

Obs curation is a unit of measurement that has become the standard way of specifying smoke detector sensitivity. Obs curation is the effect smoke has that reduces light intensity, expressed in percent absorption per unit length;[30] higher concentrations of smoke result in higher obs curation levels.
Carbon monoxide and carbon dioxide detection

Carbon monoxide sensors 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.

High levels of carbon dioxide (CO
2) may indicate a fire, and can be detected by a carbon dioxide sensor. Such sensors are often used to measure levels of 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 CO
2 generated by a fire. Some manufacturers say that detectors based on 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. CO
2 detectors are not susceptible to false alarms due to particles making them particularly suitable for use in dusty and dirty environments

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 units that are interconnected. 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 that triggers sounds all alarms. They are powered by mains electricity, with disposable or rechargeable battery backup. They may be interconnected by wires, or wireless. They are required in new installations in some jurisdictions.[44]

Several smoke detection methods are used and documented in industry specifications published by Underwriters Laboratories.[45] Alerting methods include:
somke 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 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 utilizing sound-based detectors.Between 2005 and 2007 research sponsored by the United States National Fire Protection Association 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 low 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.

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. It has been found that battery-powered smoke detectors in many houses have dead batteries. It has been estimated[when?] 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.[48] 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 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 recommends that home-owners replace smoke detector batteries at least once per year, when it starts chirping (a signal that the battery is low). Batteries should also be replaced when or if it fails a test, which the recommends to be carried out at least once per month by pressing the “test” button on the alarm.

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).

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

, Bill Hacker and his daughter Christine of Rotterdam, New York, died when their house caught fire and a First Alert ionization smoke detector failed to sound.[51] 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.[51] 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, Brands, was liable for millions of dollars in damages.
n the United States most state and local laws regarding the required number and placement of smoke detectors are based upon standards established in72, National Fire Alarm and Signaling Code.[52] 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 in the previous paragraph 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.[52] Many other countries have comparable requirements.

In new construction, minimum requirements are typically more stringent. All smoke detectors must be hooked directly to the electrical wiring, be interconnected and have a battery backup. In addition, 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 stairways, main hallways and garages

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 are 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 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 back up.[54]
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 systems controlled by fire alarm control panels (FACP).[55] These are the most common type of detector, and are usually significantly more expensive than single-station battery-operated residential smoke alarms.[55] They are used in most commercial and industrial facilities and other places such as ships and trains,[55] but are also part of some security alarm systems in homes.[56] 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.[55]

The word “conventional” is slang used to distinguish the method used to communicate with the control unit in newer addressable systems.[55] So-called “conventional detectors” are smoke detectors used in older interconnected systems and resemble electrical switches by their way of working.[55] 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.[55] 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.[55] In a conventional system, smoke detectors are typically wired together in each zone and a single fire alarm control panel usually monitors a number of zones which can be arranged to correspond to different areas of a building.[55] In the event of a fire, the control panel is able to 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.[55]
An addressable Simplex True Alarm smoke detector

An addressable system gives each detector an individual number, or address.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.[55] 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,[55] while in others the address and location of the detector or detectors in alarm are simply indicated.[55]

Addressable systems are usually more expensive than conventional non-addressable systems, 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.[55] 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.[58] Panels can also be interconnected to monitor a very large number of detectors in multiple buildings.[55] This is most commonly used in hospitals, universities, resorts and other large centers or institutions

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