Smoke detector

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Smoke Detector COFEM with approved EN 54-7

A smoke detector is a device that senses smoke, typically as an indicator of fire. Commercial and residential security devices issue a signal to a fire alarm control panel as part of a fire alarm system, while household detectors, known as smoke alarms, generally issue a local audible or visual alarm from the detector itself.


In the United States, the National Fire Protection Association estimates that nearly two-thirds of deaths from home fires occur in properties without working smoke alarms/detectors.[1]


The first automatic electric fire alarm was invented in 1890 by Francis Robbins Upton (U.S. patent no. 436,961).[2] Upton was an associate of Thomas Edison. George Andrew Darby patented the first electrical heat detector and smoke detector in 1902 in Birmingham, England.[3]

In the late 1930s, Swiss physicist Walter Jaeger tried to invent a sensor for poison gas.[4] He expected that gas entering the sensor would bind to ionized air molecules and thereby alter an electric current in a circuit in the instrument.[4] His device failed: small concentrations of gas had no effect on the sensor's conductivity.[4] Frustrated, Jaeger lit a cigarette and was soon surprised to notice that a meter on the instrument had registered a drop in current.[4] Smoke particles from his cigarette had apparently done what poison gas could not.[4] Jaeger's experiment was one of the advances that paved the way for the modern smoke detector.[4] In 1939 Swiss physicist Ermst Meili devised an ionisation chamber device capable of detecting combustible gases in mines.[5] He also invented a cold-cathode tube that could amplify the small electronic signal generated by the detection mechanism to a strength sufficient enough to activate an alarm.[5]

Ionisation smoke detectors were first placed on the market in the United States in 1951 and were used only in major commercial and industrial facilities in the next several years due to their high expense and large size.[5] The United States Atomic Energy Commission (USAEC) granted the first license to distribute smoke detectors using radioactive material in 1963.[4] The first truly affordable home smoke detector was invented by Duane D. Pearsall and Stanley Bennett Peterson in 1965, featuring an individual battery powered unit that could be easily installed and replaced.[6] These first units, dubbed "SmokeGuard 700," were made from strong fire resistant steel and shaped much like bee hives.[2] The idea for mass production came from Peterson, working at Pearsall’s company, Statitrol Corporation, in Lakewood, Colorado in 1975.[2] Studies in the 1960s determined that smoke detectors respond to fires much faster than heat detectors.[7] In 1969, the AEC allowed homeowners to use smoke detectors without a license.[4]

The first single-station smoke detector was invented in 1970.[7] It was an ionisation detector powered by a single 9-volt battery.[7] These detectors were made available to the public the next year.[5] They cost about $125 and sold at a rate of a few hundred thousand per year.[5] Several technological developments occurred between 1971 and 1976, reducing the cost of ionisation detectors by eighty percent and boosting sales to 8 million in 1976 and 12 million in 1977.[5] Some of these included replacement of cold-cathode tubes with solid-state circuitry, which greatly reduced the detectors' sizes and made it possible to monitor both the decrease in voltage and the build-up of internal resistance in the battery.[5] Either of these would trigger a signal to replace the battery.[5] The previous alarm horns, which required specialty batteries were replaced with horns that were more energy-efficient, enabling the use of commonly available sizes of batteries.[5] These detectors could also function with smaller amounts of radioactive source material, and the sensing chamber and smoke detector enclosure were redesigned for more effective operation.[5] The need for a quick replace battery didn't take long to show itself and the rechargeable was replaced with a pair of AA batteries along with a plastic shell encasing the detector. The small assembly line sent close to 500 units per day before Statitrol sold its invention to Emerson Electric in 1980 and Sears' retailers picked up full distribution of the 'now required in every home' smoke detector.

The Life Safety Code (NFPA 101), passed by the National Fire Protection Association in 1976, first required smoke alarms in homes.[7] By 1980, half of all Americans had smoke alarms in their homes.[7] That number soared to 75 percent by 1984.[7] As of November 2013 it is estimated that smoke detectors are installed in 93 percent of US homes and 85 percent of UK homes.[2] 30 percent of these alarms are estimated to not work, due to aging, removal of batteries, or failure of owners to replace dead batteries.[2]



Optical Smoke Detector
1: Optical chamber
2: Cover
3: Case moulding
4: Photodiode (detector)
5: Infrared LED
Optical Smoke Detector with the cover removed

An optical detector is a light sensor. The components of the light sensor are the light source (incandescent bulb or Light-emitting diode), a lens, and a photoelectric receiver (typically a photodiode). In spot-type detectors, all of these components are arranged inside a smoke chamber where smoke from a nearby fire will flow. In large open areas such as atria and auditoriums, optical beam smoke detectors are used. A wall-mounted unit emits a beam of infrared or ultraviolet light which is either received and processed by a separate device or reflected back to the transmitter/receiver by a reflector.

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 (called smoldering fires)." Also, 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."

Although optical alarms are highly effective at detecting smoldering fires and do provide adequate protection from flaming fires, fire safety experts and the National Fire Protection Agency recommend installing what are called combination alarms, which are alarms that either detect both heat and smoke, or use both the ionization and photoelectric / optical processes. Also some combination alarms may include a carbon monoxide detection capability.

Not all optical detection methods are the same. The type and sensitivity of light source and photoelectric sensor, and type of smoke chamber differ between manufacturers.


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 piezoelectric buzzer that produces the alarm sound.
A video overview of how an ionization smoke detector works.

An ionization smoke detector uses a radioisotope such as americium-241 to produce ionization in 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.[8]

An Americium container from a smoke detector

The radioactive isotope americium-241 in the smoke detector emits ionizing radiation in the form of alpha particles into an ionization chamber (which is open to the air) and a sealed reference chamber. The air molecules in the chamber become ionized and these ions allow the passage of a small electric current between charged electrodes placed in the chamber. If any smoke particles pass into the chamber the ions will attach to the particles and so will be less able to carry the current. An electronic circuit detects the current drop, and sounds the alarm. The reference chamber cancels effects due to air pressure, temperature, or the aging of the source.[9] Other parts of the circuitry monitor the battery (where used) and sound an intermittent warning when the battery nears exhaustion. A self-test circuit simulates an imbalance in the ionization chamber and verifies the function of power supply, electronics, and alarm device. The standby power draw of an ionization smoke detector is so low that a small battery can provide power for months or years, making the unit independent of AC power supply or external wiring; however, batteries require regular test and replacement.

An ionization type smoke detector is generally cheaper to manufacture than an optical smoke detector; however, it is sometimes rejected because it is more prone to false (nuisance) alarms than photoelectric smoke detectors.[10][11] It can detect particles of smoke that are too small to be visible.

Americium-241, an alpha emitter, has a half-life of 432 years. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation. The amount of elemental americium-241 is small enough to be exempt from the regulations applied to larger sources. It includes about 37 kBq or 1 µCi of radioactive element americium-241 (241Am), corresponding to about 0.3 µg of the isotope.[12][13] This provides sufficient ion current to detect smoke, while producing a very low level of radiation outside the device.

The americium-241 in ionizing smoke detectors poses a potential environmental hazard. Disposal regulations and recommendations for smoke detectors vary from region to region.[14][15] Some European countries[which?] have banned the use of domestic ionic smoke alarms.[16]

Radiation data[17]
Type Energy Percentage
Alpha 5485 keV 84.5%
Alpha 5443 keV 13.0%
Gamma 59.5 keV 35.9%
Gamma 26.3 keV 2.4%
Gamma 13.9 keV 42%

Specific activity is 3.5 Ci/g.[17]


An air-sampling smoke detector is capable of detecting microscopic particles of smoke. Most air-sampling detectors are aspirating smoke detectors, which work by actively drawing air through a network of small-bore pipes laid out above or below a ceiling in parallel runs covering a protected area. Small holes drilled into each pipe form a matrix of holes (sampling points), providing an even distribution across the pipe network. Air samples are drawn past a sensitive optical device, often a solid-state laser, tuned to detect the extremely small particles of combustion. Air-sampling detectors may be used to trigger an automatic fire response, such as a gaseous fire suppression system, in high-value or mission-critical areas, such as archives or computer server rooms.

Most air-sampling smoke detection systems are capable of a higher sensitivity than spot type smoke detectors and provide multiple levels of alarm threshold, such as Alert, Action, Fire 1 and Fire 2. Thresholds may be set at levels across a wide range of smoke levels. This provides earlier notification of a developing fire than spot type smoke detection, allowing manual intervention or activation of automatic suppression systems before a fire has developed beyond the smoldering stage, thereby increasing the time available for evacuation and minimizing fire damage.

Carbon monoxide and carbon dioxide detection[edit]

Some smoke alarms use a carbon dioxide sensor or Carbon monoxide sensor to detect characteristic products of combustion.[18][19] However, some gas sensors react on levels that are dangerous for humans but not typical for a fire; these are therefore not generally sensitive or fast enough to be used as fire detectors. Other gas sensors are even able to warn about particulate-free fires (e. g. certain alcohol fires).

Performance differences[edit]


In June 2006, the Australasian 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 early enough to escape from smouldering fires." [21]

In August 2008, the International Association of Fire Fighters (IAFF-300,000+ members throughout the USA and Canada) passed a Resolution recommending the use of photoelectric smoke alarms. The IAFF states that changing to photoelectric alarms, "Will drastically reduce the loss of life among citizens and fire fighters."[22]

In June 2010, the City of Albany, California enacted photoelectric-only legislation after a unanimous decision by the Albany City Council. This was a catalyst for several other Californian and Ohioan cities to enact legislation requiring photoelectric smoke detectors.[23]

In May 2011, the Fire Protection Association of Australia's (FPAA) official position on smoke alarms states, "Fire Prevention Association Australia considers that all residential buildings should be fitted with photoelectric smoke alarms...".[24]


In December 2011, the Volunteer Fire Fighter'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.[26]



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.[20] Examples include Photo/heat, photo/CO, and even CO/photo/heat/IR.

Obscuration is a unit of measurement that has become the standard definition of smoke detector sensitivity. Obscuration is the effect that smoke has on reducing sensor visibility; higher concentrations of smoke result in higher obscuration levels.

Typical smoke detector obscuration ratings[28]
Type of Detector Obscuration Level
Photoelectric 6.5–13.0% obs/m (2–4% obs/ft)
Beam 3% obs/m (0.9% obs/ft)[citation needed]
Laser 0.06–6.41% obs/m (0.02–2.0% obs/ft)[29]

Interconnected smoke detectors[edit]

An integrated locking mechanism for commercial building doors. Inside an enclosure are a locking device, smoke detector and power supply.

Interconnected smoke detectors are either conventional or analog addressable, and are wired up to security alarm systems or fire alarm systems controlled by fire alarm control panels (FACP).[30] These are the most common type of detector, and usually cost a lot more than single-station battery-operated smoke alarms.[30] They exist in most commercial and industrial facilities and other places such as ships and trains,[30] but are also part of some security alarm systems in homes.[31] These detectors don't 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.[30]


The word "conventional" is slang used to distinguish the method used to communicate with the control unit in newer addressable systems.[30] So called “conventional detectors” are smoke detectors used in older interconnected systems and resemble electrical switches in their information capacity.[30] These detectors are connected in parallel to the signaling path or (initiating device circuit) 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 stimulus sufficiently influences any detector.[30] The resulting increase in current flow is interpreted and processed by the control unit as a confirmation of the presence of smoke and a fire alarm signal is generated.[30] In a conventional system, 32 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.[30] In the event of a fire, the control panel is able to identify which zone or zones contain the detector or detectors in alarm, but can not identify which individual detector or detectors are in a state of alarm.[30]


An addressable Simplex smoke detector

An analog addressable system gives each detector an individual number, or address.[30] Addressable systems allow the exact location of an alarm to be plotted on the FACP.[30] In certain systems, a graphical representation of the building is provided on the screeen of the FACP which shows the locations of all of the detectors in the building,[30] while in others the address and location of the detector or detectors in alarm are simply indicated.[30]

Analog addressable systems are usually more expensive than conventional non-addressable systems,[32] 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.[30] 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.[33] Panels can also be interconnected to control a very large number of detectors in multiple buildings.[30] This is most commonly used in hospitals, universities, resorts and other large centres or institutions.[30]

Single station smoke detectors[edit]

Photoelectric smoke detector equipped with strobe light for the hearing impaired
A Kidde single-station ionisation smoke detector, showing "test" and "hush" buttons.

The main function of a single station or "standalone" smoke detector is to alert persons at risk. Several methods are used and documented in industry specifications published by Underwriters Laboratories[34] Alerting methods include:

  • Audible tones
    • Usually around 3200 Hz due to component constraints (Audio advancements for persons with hearing impairments have been made; see External links)
    • 85 dBA at 10 feet
  • Spoken voice alert
  • Visual strobe lights
  • Tactile stimulation, e.g., bed or pillow shaker (No standards exist as of 2008 for tactile stimulation alarm devices.)

Some models have a hush or temporary silence feature that allows silencing without removing the battery. This is especially useful in locations where false alarms can be relatively common (e.g. due to "toast burning") or users could remove the battery permanently to avoid the annoyance of false alarms, but removing the battery permanently is strongly discouraged.

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 such as the elderly, those with hearing loss and those who are intoxicated.[35] Between 2005 and 2007, research sponsored by the United States' National Fire Protection Association (NFPA) has focused on understanding the cause of a higher number of deaths seen 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.[36]


Most residential smoke detectors run on 9-volt alkaline or carbon-zinc batteries. When these batteries run down, the smoke detector becomes inactive. Most smoke detectors will signal a low-battery condition. The alarm may chirp at intervals if the battery is low, though if there is more than one unit within earshot, it can be hard to locate. It is common, however, for houses to have smoke detectors with dead batteries. It is estimated, in the UK, that over 30% of smoke alarms may have dead or removed batteries. As a result, 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.[37] In regions using daylight saving time, campaigns may suggest that people change their batteries when they change their clocks or on a birthday.


Common NiMH and NiCd rechargeable batteries have a high self-discharge rate, making them unsuitable for use in smoke detectors. This is true even though they may provide much more power than alkaline batteries if used soon after charging, such as in a portable stereo. Also, a problem with rechargeable batteries is a rapid voltage drop at the end of their useful charge. This is of concern in devices such as smoke detectors, since the battery may transition from "charged" to "dead" so quickly that the low-battery warning period from the detector is either so brief as to go unnoticed, or may not occur at all.

The NFPA, recommends that home-owners replace smoke detector batteries with a new battery at least once per year, when it starts chirping (a signal that its charge is low), or when it fails a test, which the NFPA recommends to be carried out at least once per month by pressing the "test" button on the alarm.[38]


In 2004, NIST issued a comprehensive report[11] that concludes, among other things, 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)".

The NFPA strongly recommends the replacement of home smoke alarms every 10 years. Smoke alarms become less reliable with time, primarily due to aging of their electronic components, making them susceptible to nuisance false alarms. In ionization type alarms, decay of the 241Am radioactive source is a negligible factor, as its half-life is far greater than the expected useful life of the alarm unit.

Regular cleaning can prevent false alarms caused by the buildup of dust or other objects such as flies, particularly on optical type alarms as they are more susceptible to these factors. A vacuum cleaner can be used to clean ionization and optical detectors externally and internally. However, on commercial ionization detectors it is not recommended for a lay person to clean internally. To reduce false alarms caused by cooking fumes, use an optical or 'toast proof' alarm near the kitchen. [39]

On the night of May 31, 2001, Bill Hackert and his daughter Christine of Rotterdam, New York died when their house caught fire and a First Alert ionisation smoke detector failed to sound.[40] 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.[40] The ionisation 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 parent company, BRK Brands, was liable for millions of dollars in damages.[40]

Installation and placement[edit]

A 2007 U.S. guide to placing smoke detectors, suggesting that one be placed on every floor of a building, and in each bedroom.


Laws governing the installation of smoke detectors vary depending on the locality. Homeowners with questions or concerns regarding smoke detector placement may contact their local fire marshal or building inspector for assistance. 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.


Wired units with a third "interconnect" wire allow a dozen or more detectors to be connected, so that if one detects smoke, the alarms will sound on all the detectors in the network, improving the chances that occupants will be alerted, even if they are behind closed doors or if the alarm is triggered one or two floors from their location. Wired interconnection may only be practical for use in new construction, especially if the wire needs to be routed in areas that are inaccessible without cutting open walls and ceilings. As of the mid-2000s, development has begun on wirelessly networking smoke alarms, using technologies such as ZigBee, which will allow interconnected alarms to be easily retrofitted in a building without costly wire installations. Some wireless systems using Wi-Safe technology will also detect smoke or carbon monoxide through the detectors, which simultaneously alarm themselves with vibrating pads, strobes and remote warning handsets. As these systems are wireless they can easily be transferred from one property to another.

In the UK, the placement of detectors is similar however the installation of smoke alarms in new builds need to comply to the British Standards BS5839 pt6. BS 5839: Pt.6: 2004 recommends that a new-build property consisting of no more than 3 floors (less than 200sqm 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 main supply and a battery back up.

Standards for smoke alarms[edit]

EN54 European standards[edit]


Coverage of smoke detector and temperature detectors with European standard EN54 (sqm)[edit]

Superfice area (sqm) Type of Detector Heigh (m) Roof Slope ≤20º Roof Slope >20º
Smax(sqm) Rmax (m) Smax (sqm) Rmax(m)
SA ≤80 EN54-7 ≤12 80 6,6 80 8,2
SA >80 EN54-7 ≤6 60 5,7 90 8,7
6< h ≤ 12 80 6,6 110 9,6
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
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 = Superfice Area.
  • Smax(sqm) = Maximum Superfice coverage.
  • Rmax (m) = Maximum Radio.

Information in "bold" is the standard coverage of the detector. Smoke detector coverage is 60sqm and temperature smoke detector is 20sqm. Height from ground is an important issue for a correct protection. You can see "High" in the table for this information.[43]

Australia and United States[edit]



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External links[edit]