SMOKE DETECTORS – A Matter of Life and Death

A smoke detector or smoke alarm is a device that detects smoke and issues an alarm to alert nearby people that there is a potential fire. A household smoke detector will typically be mounted in a disk shaped plastic enclosure about 150mm in diameter and 25mm thick, but the shape can vary by manufacturer.

Because smoke rises, most detectors are mounted on the ceiling or on a wall near the ceiling. To avoid the nuisance of false alarms, most smoke detectors are mounted away from kitchens. To increase the chances of waking sleeping occupants, most homes have at least one smoke detector near any bedrooms; ideally in a hallway as well as in the bedroom itself.

Smoke detectors are usually powered by one or more batteries but some can be connected directly to household wiring. Often smoke detectors that are directly connected to household wiring also have a battery as a power supply backup in case the household wiring goes out. It is usually necessary to replace the batteries once a year to ensure appropriate protection.

Most smoke detectors work either by optical detection or by ionization, but some of them use both detection methods to increase sensitivity to smoke. Smoke detectors may operate alone, be interconnected to cause all detectors in an area to sound an alarm if one is triggered, or be integrated into a fire alarm or security system. Smoke detectors with flashing lights are available for the deaf or hearing impaired, although recent research suggests that their waking effectiveness is poor (see below).

A smoke detector cannot detect carbon monoxide to prevent carbon monoxide poisoning unless it has an integrated carbon monoxide detector.




In 1902 George Andrew Darby, an electrical engineer of 211 Bloomsbury Street, Birmingham, England, patented the electrical Heat-Indicator and Fire Alarm. The device was a heat detector rather than a smoke detector and indicated an increase temperature in the apartment where it was fixed. The device operated by closing an electrical circuit to sound an alarm if the temperature rose above the safe limit. The contact was made by bridging a gap with a conductor, or allowing one plate to fall on another. The connection of the two plates was caused simply by a block of butter which melted as the temperature rose. This early device subsequently gave way to more modern fire and eventually smoke alarms.

The first home smoke detectors were invented by Duane D. Pearsall in 1967.


Optical Detector


Optical Smoke Detector 1: optical chamber 2: cover 3: case moulding 4: photodiode (detector) 5: infra-red LED

An optical detector is a light sensor. When used as a smoke detector it includes a light source (infra-red LED), a lens to collimate the light into a beam, and a photodiode or other photoelectric sensor at right-angles to the beam as a light detector. In the absence of smoke, the light passes in front of the detector in a straight line. When smoke enters the optical chamber into the path of the light beam, some light is scattered by the smoke particles, and some of the scattered light is detected by the sensor. An increased input of light into the sensor sets off the alarm.

Another type of optical detector works by using a straight line infra-red beam from the sender to the receiver. When smoke enters the beam, some light is scattered which results in less light detected by the receiver. A decreased input of light into the receiver sets off the alarm.

Also seen in large rooms, such as a gymnasium or an auditorium, are projected beam detectors. A unit on the wall sends out a beam, which is either received by a receiver, or reflected back via a mirror. When the beam is less visible to the “eye” of the sensor, it sends an alarm signal to the Fire alarm control panel.

Optical smoke detectors are quick in detecting slow burning, smoky fires. They are less sensitive to false alarming from cooking and bathroom steam than ionization smoke alarms.


Ionization Detector

Inside a basic ionization smoke detector.

This type of detector is cheaper than the optical detector, however it is sometimes rejected for environmental reasons. It can detect particles of smoke that are too small to be visible. It includes a tiny mass of radioactive americium-241, which is a source of alpha radiation. The radiation passes through an ionization chamber, which is an air-filled space between two electrodes, and permits a small, constant current to flow between the electrodes. Any smoke that enters the chamber absorbs the alpha particles, which reduces the ionization and interrupts this flow of current, setting off the alarm.

Approximately 0.2 micrograms of Am-241 is used, which has a half life of 432.2 years. This means that it does not have to be replaced very often, and also makes it safer for people at home (as it is less radioactive). Alpha radiation (as opposed to beta and gamma) is used for two further reasons: alpha particles have high ionization (so sufficient air particles will be ionized for the current to flow), and they have low penetrative power – meaning they will be stopped by the plastic of the smoke detector and/or the air, reducing the risk of harm to people.


Alarms and Alerts

The second function of the detector is to alert persons at risk. Several methods are used and documented in industry specifications published by UL (see References section below).

Alerting methods include:

  • Audible tones
    • usually around 3200 Hz due to component constraints (Audio advancements for hearing impaired have been made, see External Links)
    • 85 dBA at 10 feet
    • A spoken voice alert
  • Visual strobe lights
    • 110 candela output
  • Tactile stimulation
    • bed shaker
    • pillow shaker
    • no specifications and currently no UL approved devices

While current technology is very effective at detecting smoke and fire conditions, there have been concerns about the effectiveness of the alerting function in awakening sleeping individuals in certain high risk groups.

Initial research into the effectiveness of the various alerting methods is sparse. Recently (2005-2007), research sponsored by the NFPA has focused on understanding the cause of a higher number of deaths seen in high risk groups such as the elderly, those with hearing loss, and those who are intoxicated.

Research findings suggest that a low frequency (520 Hz) square wave output is significantly more effective in the high risk individuals at awakening. It is expected that products using the new alerting technology will be available in the future. (See bottom for current availability.) Special concern has been raised regarding waking effectiveness by the deaf and hard of hearing community following research released in June of 2007. A press release from August of that year is in the external links section below.




Residential ceiling-mounted smoke detector

In 2004, NIST issued a comprehensive report entitled Performance of Home Smoke Alarms – Analysis of the Response of Several Available Technologies in Residential Fire Settings. The report 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, and photoelectric alarms provided (often) considerably faster response to smoldering fires than ionization type alarms”.

The National Fire Protection Agency has issued a fact sheet urging the replacement of home smoke alarms every 10 years.



Most residential smoke detectors run on 9 volt alkaline batteries. If these batteries run out, the smoke detector will become inactive. Most smoke detectors should signal a low battery condition, but it is common for houses to have smoke detectors with dead batteries. As a result, public information campaigns have been created to remind people to change their smoke detector batteries regularly. In regions using daylight saving time, these campaigns usually suggest that people change their batteries when they change their clocks. Another option is to change batteries on a birthday.

Some detectors are also being sold with a lithium battery that can run for about 7 to 10 years, though this might actually make it less likely for people to change batteries since their replacement is needed so infrequently. By that time, the whole detector should be replaced. Though relatively expensive, user-replaceable 9 volt lithium batteries (in the same configuration as the common alkaline ones) are also available. They should only be used in a fairly new detector.

Smoke detectors with missing batteries are also a concern. As a result, many detectors sold today are designed to provide a visual indication of a missing battery. One popular brand of smoke detector will not allow the user to close the battery door until a battery has been placed in the alarm; another contains a spring-loaded protrusion obstructing the attachment holes when the battery is missing, preventing reattachment to the wall or ceiling and making a missing battery situation immediately obvious. Some local governments do not permit the installation of smoke detectors with removable batteries.

In new construction, most building codes today require smoke detectors to be wired to the main electricity flow of buildings. Many of these units also include a battery backup to ensure operation during a power outage.

Rechargeable batteries should never be used in smoke detectors, since common NiMH and NiCd rechargeable batteries have a short life in between charges—in other words, they self-discharge relatively quickly. 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 particularly prevalent in older technology rechargeables is a rapid voltage drop at the end of their useful charge. This is concerning in devices like smoke detectors since the battery may transition from “charged” to “dead” so quickly that the low battery warning from the detector is either very brief, or may not occur at all.

A quality alkaline battery should be installed and replaced every six months or so. The used battery will still probably have the majority of its charge, and can be reused in less critical applications such as a backup for a digital alarm clock. For those living in areas that observe daylight saving time, one handy way to remember this important maintenance event is to replace your smoke detector batteries the same day you adjust your clocks for the new season.

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.



Virtually all modern smoke alarm units come equipped with a “test” button. Alternatively, artificial smoke can be purchased, which has the advantage of also testing the detection mechanism itself. Many people simply wave a lit match underneath the detector to test it; this is dangerous, however, as it can set the smoke alarm and the rest of the house on fire. Another way is to blow cigarette smoke into the detector, but this is also dangerous.

The National Fire Protection Association, through its fire protection program, urges homeowners to replace smoke detector batteries every six months when changing your clock for Daylight saving time, and to replace the entire smoke detector after ten years of use.


Installation and Placement


Place smoke detectors where they will do the most good.

In the United States, most state and local laws regarding the required number and placement of smoke detectors are based upon standards established in Article 72 of NFPA fire code.

Laws governing the installation of smoke detectors vary depending on the locality. Homeowners with questions or concerns regarding smoke detector placement are encouraged to contact their local fire marshal or building inspector for assistance. However, there are some rules and guidelines that are relatively consistent throughout the country. In older existing homes, smoke detectors are generally required on every habitable level and within the vicinity of all bedrooms. Habitable levels include attics that are tall enough to allow access. In new construction, the minimum requirements are typically much greater. 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.

Detectors on the ceiling should be placed several inches away from any wall. If the ceiling is not flat, the detector should be placed at or near the highest point. If the highest point is a small recess, then the detector should be placed at the next highest level. Detectors placed on the wall should be several inches, but no more than a foot, from the top. Detectors should not be placed on a wall if the ceiling has a deep recess or if the ceiling slopes steeply or for a long distance. Detectors should be several horizontal feet away from a heating or cooling register, window, corner, the edge of a ceiling fan’s sweep and doors to a kitchen or bathroom. They should be placed as far as possible away from combustion sources, like oil and gas-fired furnaces, space heaters, clothes dryers and water heaters, without compromising coverage or safety. Smoke detectors in a basement should be placed at the bottom of the stairs and an additional detector should be placed in or near sleeping areas in the basement.

It is recommended, and sometimes required, that smoke detectors not be placed in kitchens because the small amounts of smoke and particulates generated while cooking can set them off. Detectors designed for use near a kitchen may have a silence button to cancel accidental triggering.

Detectors should not be placed in a bathroom or near a bathroom door because moisture may cause false alarms or damage the detector. False alarms reduce the effectiveness of smoke detectors in preventing harm and property damage because people soon begin to assume that the alarm is false. Heat detectors, which sound an alarm when the temperature reaches a certain point and/or when it climbs more rapidly than a certain rate, can be used in kitchens, garages and areas with combustion sources that would otherwise generate nuisance alarms.

Canada and Australia requires a building to have a working smoke detector on every level.


Alarm Manufacturers (this should not be considered an endorsement or recommendation of any manufaturer and is provided for informational purposes only)



  • Journal of Applied Fire Science, Volume 6, Number 2, June 1997, article Risk Analysis of Residential Fire Detector Performance
  • National Fire Protection Code, Article 72
  • Underwriters Laboratories UL 217: Single and Multiple Station Smoke Alarms
  • Underwriters Laboratories UL 1971: Signaling Devices for the Hearing Impaired


External Links