Statement for the Record

National Institute of Standards and Technology

to the

Boston City Council Committee on Public Safety

August 6, 2007

This statement provides technical information on smoke detector technology based on research 

conducted by the U.S. Commerce Department’s National Institute of Standards and Technology 

(NIST), especially on the differences between ionization and photoelectric technology. 

NIST is a non-regulatory federal research agency that specializes in measurement and basic 

standards, and has been engaged in fire research for more than a century. Our mission is to 

promote U.S. innovation and industrial competitiveness by advancing measurement science, 

standards, and technology in ways that enhance economic security and improve our quality of 

life. 

Prior to 1988 NIST was known as the National Bureau of Standards (NBS). NIST staff has been 

involved in research and standards development related to smoke detectors and fire alarms for 

approximately four decades. We interact with the National Fire Protection Association (NFPA), 

Underwriters Laboratories (UL), the U.S. Fire Administration (USFA), the Consumer Product 

Safety Commission (CPSC), and other federal agencies on technical matters concerning fire 

sensing and fire alarm technologies. The output of NIST research on smoke alarms is freely 

available for downloading from the NIST web site (http://smokealarm.nist.gov/). 

Smoldering fires are inherently different from flaming fires, and the operating principle for a 

photoelectric detector is distinct from the operating principle of an ionization detector . 

Smoldering can occur only in a porous solid; for example, polyurethane foam, shredded paper, or 

cotton. The rate of smoldering is limited by the ability of air to penetrate the porous fuel, and 

hence the rate at which heat is released from a smoldering fire is low, as is the rate of carbon 

monoxide (CO) production. The smoke layer from a smoldering fire grows slowly and smoke 

can accumulate well below the ceiling, especially in rooms other than the room of fire origin. 

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Another important aspect of a smoldering fire is that it produces smoke particles that are 

relatively large (greater than one micrometer). 

Flaming fires can occur in all types of fuels, including porous and non-porous solids, liquids, and 

gases. Air has easy access to a flaming fire, which means that the rate at which heat is released 

is high. Smoke and fire gases, including CO, can be produced at a higher rate in a flaming fire 

than in a smoldering fire. The smoke layer is hotter and can build up near the ceiling of the room 

of fire origin as well as elsewhere in a building. Flaming fires produce a very large number of 

smoke particles that can be relatively small (less than one micrometer). 

The above discussion distinguishes between smoldering and flaming fires; the following 

describes the different characteristics of photoelectric and ionization detectors, and why they 

react the way they do in smoldering and flaming fires. The bottom line is that each type of 

detector has its advantages and disadvantages. 

An ionization detector responds to the movement through an electric field of ions produced by a 

small radioactive source in the smoke alarm chamber. When smoke particles move into the 

chamber they inhibit the motion of the ions, altering the electrical current. Fires that produce a 

large number of particles (i.e., flaming fires) are sensed more easily with an ion detector. A 

photoelectric detector operates on the principle of light scattered from the surface of particles. 

Because large particles have much more surface area than small particles, a photoelectric 

detector is more sensitive to the large particles produced in a smoldering fire. The sensitivity 

requirements of the UL standard are identical for both types of detectors, and neither detector 

responds to carbon monoxide or heat. 

The general trends from a 2004 NIST experimental study into the behavior of smoke alarms are 

consistent with several previous scientific studies showing that properly installed and 

maintained ionization and photoelectric alarms provide enough time to save lives for most of the 

population under many fire scenarios. However, ionization detectors have been shown to 

sometimes fail to alarm in a smoldering fire even when visibility in the room is significantly 

degraded by smoke. Most photoelectric detectors alarm substantially sooner in these situations. 

In the NIST experiments the photoelectric detectors sensed smoldering fires on average 30 

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minutes earlier than the ionization detectors. The same study demonstrated that ionization 

detectors responded, on average, 50 seconds earlier than photoelectric detectors during flaming 

fire experiments. The relative margins of safety associated with a 30 minute earlier warning in a 

slow growing smoldering fire compared to a 50 second earlier warning for a fast growing 

flaming fire is difficult to determine. 

In the 2004 NIST study, average times to untenable conditions for flaming and smoldering 

furniture fires were found to be shorter by 17% and 47%, respectively, than those found in the 

1975 NBS study (also known as the Indiana Dunes study). On the other hand, the average time 

for the cooking fires to reach untenable conditions was 120% longer in the current study. Since 

the cooking materials were similar in the two studies, NIST concluded—on the basis of the 

shorter time to untenable conditions in the furniture fires—that a major factor in the increase in 

fire growth rate is due to differences in modern furniture materials and construction compared to 

furniture manufactured four decades ago. The reduction in time to untenable conditions from a 

fire (either flaming or smoldering) in modern, synthetic materials indicates the need to determine 

the ability of standard test methods to ensure safe performance of modern (and legacy) 

residential fire alarms. Both NIST and UL are involved in research that will assess whether or 

not modifications are required in the standard to accommodate the changing threat. 

An important conclusion from the 2004 NIST study was that the available safe egress time 

provided by a smoke alarm would be sufficient, in many cases, only if households follow the 

requirements in NFPA’s National Fire Alarm Code (NFPA 72) for new construction, which 

requires the installation of fire alarms at more locations in order to improve audibility in 

bedrooms where occupants sleep with the door closed, and to provide warning to the occupants 

of bedrooms with closed doors when the fire starts in that bedroom. NFPA 72 also requires two 

ways out of a sleeping room, one of which is generally a window. With the bedroom door closed 

there is more time in which to use the window exit should the primary exit be blocked. 

Audibility of smoke alarms remains an issue, particularly for sleeping children and adults 

impaired with alcohol or other drugs. For existing residences that do not fall under the "new 

construction" requirements of NFPA 72, or for new residences where the state or local building 

regulatory agency has not adopted the requirements, the following approaches are suggested to 

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reduce problems associated with inaudibility: placing smoke alarms in bedrooms, 

interconnecting smoke alarms, changing alarm tones, and providing better home fire escape 

planning. Interconnection of the smoke alarms ensures all alarms respond to a fire event. 

Nationally-collected data on fire incidents do not specifically classify fire sources as smoldering 

and/or flaming; however, NFPA estimates that more than one-fourth of home fire deaths involve 

fires with an extended period of initial smoldering. 

In NIST's smoke alarm research, and in applications in the field, it is documented that most 

common ionization detectors have a propensity to produce nuisance alarms during cooking 

activities. NIST examined a broad range of activities (including cooking) that yield nuisance 

alarms. The published field observations guided the nuisance alarm scenarios studied. 

Specifically, the sensitivity to alarm threshold, distance from the source, background air flows, 

and alarm sensor (photoelectric or ionization) were examined. Additional measurements were 

made with aerosol instrumentation to provide a more fundamental understanding of nuisance 

alarm sources than has been previously published. Given the scenarios examined, both 

photoelectric and ionization alarms produced nuisance alarms, but NIST does not mean to imply 

that they are equally susceptible to such nuisance alarms. Most field data suggest that ionization 

alarms have a greater propensity to nuisance alarm than photoelectric alarms, possibly indicating 

that certain activities such as cooking dominate reported nuisance alarms in the field. 

To reduce the impact of nuisance alarms, NFPA 72 requires that smoke alarms not be located 

directly in cooking areas, and that any alarm located within 20 feet of a cooking appliance be 

photoelectric. Studies have shown that this should be reasonably effective except where some 

cooking techniques (blackening, deep frying, and broiling especially in a broiler that is not clean) 

are employed. However, photoelectric detectors may also alarm to these more egregious 

cooking styles. The second biggest nuisance alarm culprit is steam from showers where both 

detector types are equally susceptible. 

In summary, the research conducted by NIST staff leads to the conclusion that both 

ionization and photoelectric alarms provide enough time to save lives for most of the 

population under many fire scenarios; however, ionization alarms may not always alarm 

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even when a room is filled with smoke from a smoldering fire, exposing the most sensitive 

populations with mobility limitations to an undetermined risk. Photoelectric detectors can 

provide a lot more warning time than ionization detectors in a smoldering fire; at the same 

time a smoldering fires can take a longer period to become dangerous. Ionization detectors 

can provide a little more time than photoelectric detectors in a flaming fire; in this case 

there can be little time to spare. Changes in furnishing materials and construction over the past 

decades have reduced the time available for safe egress in any fire. NIST is currently conducting 

research to assess whether or not modifications may be needed in the standard test method for 

certifying residential smoke alarms to accommodate the changing threat. 

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