UL’s Fire Experts Research Effectiveness of Smoke Alarm Technology

 Furnishings made from synthetic materials-nylon, plastic, polyester, acrylic, and so on-burn faster and hotter than materials made from natural fibers, such as cotton, linen, silk, and wool. Source: Underwriters Laboratories.
Furnishings made from synthetic materials-nylon, plastic, polyester, acrylic, and so on-burn faster and hotter than materials made from natural fibers, such as cotton, linen, silk, and wool. Source: Underwriters Laboratories.

Home smoke alarms provide a critical first line of defense for occupants in residential settings should a fire occur. Their widespread use can be directly linked with the dramatic decline in deaths related to residential fires during the past 30 years. During this same time period, residences have also changed. Homes have increased in size, the number and amount of furnishings and possessions have grown, and petroleum-based synthetic materials have supplanted natural materials in furnishings and home construction products. The combination of these factors has changed the smoke and gas characteristics of residential fires and, in some cases, accelerated the speed of fire growth.

For the past several years, the changing nature of residential fires has been the focus of extensive research by scientists and engineers at Underwriters Laboratories (UL) and other institutions. The goal of this research is to increase the understanding of the range of expected conditions, such as smoke, temperature, and gases, in modern residential fires and to ensure that smoke alarm technologies continue to provide individuals with the greatest possible protection in the event of a fire.

A Proven History

Commercially available residential smoke detectors and smoke alarms have been largely responsible for the dramatic decline in residential fire deaths and injuries during the past 30 years. According to research conducted by the National Fire Protection Association (NFPA) on data collected from the United States Fire Administration’s (USFA) National Fire Incident Reporting System (NFIRS) and the NFPA annual fire department experience survey, home fires accounted for 5,865 deaths and more than 31,000 injuries in 1977, when only 22 percent of homes were equipped with smoke alarms.1,2

By 2009, when more than 95 percent of homes were equipped with smoke alarms, the annual death rate from home fires had dropped to 2,565-a 56 percent decline-and injuries dropped by more than 59 percent during a 32-year span.3,4 Although the entire reduction in deaths is not completely attributable to smoke alarm use adoption, it is a leading factor in reducing deaths during this period of time.

During 2003 through 2007, roughly one of every 300 households reported a fire requiring intervention by the fire service. (4) Of these 385,000 fires per year, the four percent of households that did not have smoke alarms account for 31 percent of fires and 40 percent of deaths. (4)

Furthermore, another 30 percent of deaths occurred in households with installed, viable smoke alarms that were disabled or were otherwise not working. (4) By providing occupants with advance notice of the threat of a fire and additional time to escape, working smoke alarms are often the difference between escaping a home fire without injury and succumbing to it.

In the context of these statistics, it is understandable that significant public safety efforts focus on ensuring that working smoke alarms are installed in 100 percent of homes. But, at least one recent study by the National Institute of Standards and Technology (NIST) has shown that even when working smoke alarms are present, the margin between available and safe egress times has shrunk during the past 30 years.5 This trend suggests the presence of other emerging factors that can impact smoke alarm effectiveness in home fires. Most notably, it is the changing nature of the modern residence that challenges the adequate egress time provided by smoke alarms.

Factors of Modern Residences

In a never ending effort to reduce production costs and improve product performance, home furnishings manufacturers are turning away from materials like wood and natural fibers in favor of high-performance, lower-cost synthetic materials. For example, most upholstered furniture available today uses polyurethane foam for padding and synthetic fabric covers, replacing natural padding materials like cotton; down and feathers; and cover materials made of cotton, wool, linen, or silk.

Although these material changes can lead to products that are easier to clean and more resistant to normal wear and tear, they also react differently when exposed to an ignition source. Studies by UL researchers have found that synthetic materials typically ignite faster, burn more intensely, release their fire-enabled energy faster, and create greater amounts of smoke than natural materials.6 In addition, the type and quantity of smoke particles and gases generated when synthetic materials ignite are characteristically different from those of natural materials.

The seemingly insignificant change from natural to synthetic materials in home furnishings has led to the faster development of residence fires and to the more rapid onset of untenable conditions. As such, the amount of time available for safe egress from a home fire is much shorter than in the past, placing a greater burden on smoke alarms to respond at the earliest possible stages of a fire.

Smoldering fires extend the time before lethal conditions are reached but also provide more time for smoke detection and warning to occupants. These fires are slow-growing and may or may not transition to rapidly growing flaming fires. In a recent NIST study, initial smoldering phases lasted anywhere from 30 to 120 minutes before fire conditions became untenable. (5)

Although NFPA studies have determined that more than 25 percent of home fire deaths involve an extended initial smoldering phase, estimates are that roughly three percent of the deaths involve fires that did not transition from smoldering to flaming.7

Smoke Detection Technologies

Today’s residential smoke alarms are largely based on one of two prevailing detection technologies: photoelectric or ionization. Ionization-based smoke alarms operate by monitoring a small current created by ionized air between electrically charged plates; smoke particles will reduce the current. A photoelectric-based smoke alarm detects the scattering or obscuration of light caused by smoke particulates. In both cases, the units trigger when the signal crosses a set threshold value.

Research has shown that each smoke alarm technology has unique advantages under certain fire conditions. In controlled experiments, smoke alarms based on ionization technology tend to activate more quickly than those based on photoelectric technology in flaming fires, while photoelectric alarms tend to activate earlier than ionization alarms in smoldering fires.8

Additional research by UL on individual materials and items further clarifies these trends, even for the same material. For example, when polyurethane foam (used in mattresses and upholstered furniture) was ignited with a cigarette lighter to flame, the ionization alarms activated earlier; when the same polyurethane foam was smoldered, such as from a cigarette, the photoelectric alarm activated earlier. (6)

The key challenge in selecting the appropriate smoke alarm technology is the inability to predict the type of home fire likely to occur. For that reason, nationally recognized fire safety organizations, including the NFPA, the USFA, the International Association of Fire Chiefs (IAFC), NIST, the National Association of State Fire Marshals (NASFM), and UL, all currently recommend using both photoelectric and ionization smoke alarms in residential settings or smoke alarms incorporating both types of these sensing technologies in a single device to provide the earliest possible warning and the longest possible escape time, regardless of the type of fire encountered.

At the same time, ongoing studies provide researchers with a more advanced understanding of the characteristics of various types of fires along with their smoke and gas byproducts, leading to development of more complex and detailed fire profiles that can be integrated into current fire detection technologies.

Smoke Alarm Innovations

In addition to ionization and photoelectric smoke detection technologies, industry is developing a new generation of smoke detection technologies. The goal of these efforts is to produce a smoke alarm that reacts more effectively to fires in the modern home.

To promote new smoke detector technology innovation, the UL 217 smoke alarm standard does not restrict the types of smoke detection technologies that can be employed in smoke alarms, provided they can meet the performance tests specified in the standard. Similarly, the NFPA 72 National Fire Alarm Code does not place restrictions on what smoke detection technologies can be used.

Researchers at UL have been actively engaged in ongoing investigations regarding the changing nature of modern fires and current smoke detection technologies’ effectiveness. This research has led to some important findings that will guide future UL standards development activities involving smoke alarms. The following sections summarize some of UL’s research regarding smoke alarms and modern residence fires, detail the key recommendations produced by the studies that have been completed, and outline future steps for those studies still in progress.

Smoke Characterization Project

In 2006, in conjunction with the Fire Protection Research Foundation (FPRF) of the NFPA, and as a follow up to a 2004 NIST study, UL initiated a smoke characterization project. (6) In the 2004 NIST study, researchers observed reduced available safe egress times that they attributed to significantly faster fire growth caused by the types of materials used in modern furnishings.

The purpose of the UL-FPRF Smoke Characterization Project was to more fully characterize the products of both flaming and nonflaming combustion on a variety of products and materials typically found in residential settings. This study used smoke particle and gas effluent characterization technology not previously available for commercial testing purposes.

Testing scenarios included the standard UL 217 smoke alarm fire test protocols, including a burning coffee maker, a toaster with a bypassed shutoff, and flaming and smoldering upholstered furniture components.

Key findings of the Smoke Characterization Project study include the following:

  • Synthetic materials ignite faster, burn more intensely, and create greater amounts of smoke and other types of gases than natural materials.
  • The response time of photoelectric and ionization smoke alarms was influenced by different smoke particle sizes and counts because of changes in the combustion mode (flaming vs. nonflaming).
  • Commercially available ionization smoke alarms triggered earlier than commercially available photoelectric smoke alarms for flaming and high-energy nonflaming (toaster) fires.
  • Photoelectric alarms triggered earlier for lower energy nonflaming fires.
  • Smoke from low-energy nonflaming fires was found to stratify with time.

The full report is available at http://www.nfpa.org/assets/files//PDF/Research/SmokeCharacterization.pdf.


Smoke Alarm Fire Tests

In the UL-FPRF Smoke Characterization Project, researchers determined that flaming and nonflaming polyurethane foam produces smoke with characteristics that are different from those used to evaluate smoke alarms under UL 217.

Accordingly, in 2008 UL formed a task group under the UL 217 standards technical panel (STP) to develop appropriate tests for flaming and nonflaming polyurethane foam. The objective of the task group is to expand the number of smoke signatures to which smoke alarms are evaluated under the standard.

To date, the task group has established target performance criteria for the new fire tests that will not inadvertently cause an increase in nuisance alarm frequency. UL has also investigated the smoke produced by samples of commercially available foams used in mattresses and upholstered furniture covering a range of densities. The task group has also investigated how sample size, geometry, density, mode of combustion, and mode of heating impact smoke particle size, count distribution, and smoke concentration buildup rates.

In the final stages of its work, the task group is using the results to select the test foam material and the flaming and smoldering test protocols to be proposed to the UL 217 STP. The task group is formulating test material specifications and test consistency limits for the selected test protocols it generated.

One unanticipated issue in developing material specifications and test consistency limits has been the discovery that the cell size of polyurethane foam, independent of the foam density, significantly impacts the smoke buildup rate, particularly for the slower smoldering fire test protocol. To address this issue, the task group is pursuing two approaches: first, develop test material specifications and test consistency limits for a range of commercially available foams meeting the test material property targets; and second, develop a standard reference for polyurethane foam. Once the material properties (chemistry, density, indentation load density, cell size, and so on) have been established, the proposed test protocols will be repeated 30 times to establish the test consistency limits. The task group will submit the developed test protocols (including test sample specifications) to the UL 217 STP for review and consideration.

Home Furnishings Comparison10

Funded through an Assistance to Firefighters Grant from the United States Department of Homeland Security (DHS), UL researchers recently completed an investigation covering the impact of fire service ventilation practices on fire growth in modern and legacy residences.

As part of this study, researchers examined fire growth behavior in modern and legacy furnished living rooms in side-by-side fires. Each living room measured 12 by 12 feet, with an eight-foot ceiling and an eight-foot-wide by seven-foot-tall opening on the front wall.

The modern room was lined with a layer of ½-inch painted gypsum board, and the floor was covered with carpet and padding. The furnishings included a microfiber-covered polyurethane-foam-filled sectional sofa, engineered-wood coffee table, end table, television stand, and bookcase. The sofa had a polyester throw placed on its right side. The end table had a lamp with a polyester shade on top of it and a wicker basket inside it. The coffee table had six color magazines, a television remote, and a synthetic plant on it. The television stand had a color magazine and a 37-inch flat-panel television. The bookcase had two small plastic bins, two picture frames, and two glass vases on it. The right rear corner of the room had a plastic toy bin, a plastic toy tub, and four stuffed toys. The rear wall had polyester curtains hanging from a metal rod, and the side walls had wood framed pictures hung on them.

The legacy room was lined with a layer of ½-inch painted cement board and the floor was covered with unfinished hardwood flooring. The furnishings included a cotton-covered/cotton-batting filled sectional sofa, a solid wood coffee table, two end tables, and a traditional television stand. A cotton throw was placed on the right side of the sofa. Both end tables had a lamp with a polyester shade on top. Two paperback books were placed near the lamp on the left side of the sofa and a wicker basket was located on the floor to the right of the sofa at floor level. The coffee table had three hard-covered books, a television remote, and a plant made of synthetic materials. The television stand had a 27-inch tube television. The right front corner of the room had a wood toy bin and multiple wood toys. The rear wall had cotton curtains hanging from a metal rod and the side walls had wood framed pictures hung on them.

A lit stick candle placed at the right side of each sofa served as the ignition source for both rooms. Researchers allowed the fires to grow until flashover. The legacy room transitioned to flashover in 29 minutes and 30 seconds, whereas the modern room transitioned in just 3 minutes and 30 seconds.

Smoke Alarm Response Project

This study is intended to characterize smoke and gas conditions in various locations throughout a modern, two-story open floor plan residence to evaluate the response rate of different smoke detection technologies and assess the benefits of having alarms in multiple locations. Scenarios include cooking in the kitchen-e.g., making toast, frying bacon, a cooking oil fire; smoldering and flaming upholstered furniture fires in the two-story family room; smoldering and flaming upholstered furniture fires in a den; and mattress fires in the bedrooms. The standard test protocols found in UL 217 to evaluate smoke alarms were also conducted in the living room.

UL is currently analyzing the recorded data, including smoke particle size and count distribution, effluent gas composition, reference obscuration and ionization signals, analog photoelectric and ionization detector signals, and temperature. A final report summarizing the research from this project is expected to be released in the near future.


National and local building codes and regulations have been responsible for the almost universal installation of smoke alarms in residential structures during the past decade. These codes are continually revised to reflect the knowledge gained through ongoing research and product development. Because of these efforts, smoke alarms installed in today’s residences are more effective and reliable than ever.

As the results of this research are made available, expect further changes to the UL 217 smoke alarm standard and model codes. Although the transition of enhanced safety requirements from product safety standards to codes and regulations often proceeds in a seemingly nonlinear fashion, such enhancements are also critical to ensure that codes and regulations provide the highest possible level of safety.

The ultimate goal of UL’s smoke alarm research is to provide the technological data that can help eliminate fire deaths in residential dwelling units. This can lead to advancements in product safety standards, model codes, and regulations. Achieving that goal also depends on having working smoke alarms installed in every home, on continuing programs that effectively educate consumers about the dangers of residence fires, and on the actions they can take to ensure their safety. Taken together, these steps will lead to safer homes and fewer injuries and lives lost to fire. Visit www.ul.com/fireservice for more information on these studies and their accompanying videos.


1. Karter, M.J., “Fire Loss in the United States During 2003,” NFPA Fire Analysis and Research Division, Quincy, MA, October 2004.
2. Ahrens, M., “U.S. Experience with Smoke Alarms and other Fire Detection/Alarm Equipment,” NFPA Fire Analysis and Research Division, Quincy, MA, April 2007.
3. Karter, M.J., “Fire Loss in the United States During 2009,” NFPA Fire Analysis and Research Division, Quincy, MA, August 2010.
4. Ahrens, M., “Home Structure Fires,” NFPA Fire Analysis and Research Division, Quincy, MA, March 2010.
5. Bukowski, R.W. et al., “Performance of Home Smoke Alarms-Analysis of the response of several available technologies in residential fire settings,” NIST Technical Note 1455, National Institute of Standards and Technology, Gaithersburg, MA, July 2004, Reissued 2008.
6. Fabian, T.Z, and Gandhi, P.D., “Smoke Characterization Project: Technical Report,” UL, Northbrook, IL, April 2007. (Available at http://www.nfpa.org/assets/files//PDF/Research/SmokeCharacterization.pdf.)
7. Hall, J.R. Unpublished NFIRS national estimates statistics cited in A brief history of home smoke alarms, presentation to National Fire Protection Association conference, Denver, CO, May 2000.
8. Position Paper, “Smoke Alarms: Ionization and Photoelectric Technology,” International Association of Fire Chiefs, Fairfax, VA, April 2008.
9. Fabian, T.Z., Borgerson, J., Kerber, S.I., Baxter, C.S., Ross, C.S., Lockey, J.E., and Dalton, J.M., “Firefighter Exposure to Smoke Particulates: Final Report,” UL, Northbrook, IL, April 2010. (Available at http://www.ul.com/global/documents/offerings/industries/buildingmaterials/fireservice/WEBDOCUMENTS/EMW-2007-FP-02093.pdf.)
10. Kerber, S., “Impact of Ventilation on Fire Behavior in Legacy and Contemporary Residential Construction,” UL, Northbrook, IL, December 2010.

THOMAS FABIAN, Ph.D., is the manager, fire hazards research team, for Underwriters Laboratories Inc. He has a Ph.D. in polymer science from the University of Connecticut and a bachelor of science in chemical engineering from Carnegie Mellon University. He is an Underwriters Laboratories Mark of Excellence recipient for research on smoke characterization and a Fire Protection Research Foundation Ronald Mengel Award recipient for research on smoke detection.

PRAVIN GANDHI, Ph.D., is the business development director for Underwriters Laboratories Inc. A fire researcher who has translated the product of fire research into economically useful applications in testing and certification of materials, products, and systems, Gandhi is an accomplished fire researcher across a wide array of technologies. Through his work, UL is able to study a range of variables through analytical modeling techniques, permitting a wide range of features and conditions to be evaluated through fewer tests.

Firefighter Exposure to Smoke Particulates9

One of the key observations from the UL-FPRF Smoke Characterization Project was the predominance of submicron-sized smoke particles generated by combustion. Other studies have shown that such particles penetrate the human cardiovascular system and can be subsequently absorbed into the body. Throughout their professional careers, firefighters are exposed to intense heat, smoke particulate, and fire gas effluents. Firefighters also have a history of greater cardiovascular risks and certain types of cancers than the general population.

In 2007, to further investigate the causal relationship between submicron smoke particles and the risk of cardiovascular problems, UL partnered with the Chicago (IL) Fire Department and the University of Cincinnati College of Medicine to collect data on the smoke and gas effluents to which firefighters are exposed during routine firefighting operations, as well as contact exposure from contaminated personal protective equipment. This research was funded by a grant from the U.S. Department of Homeland Security (DHS).

As a component of this study, the combustibility, smoke, and gas characteristics of 42 different residential construction and furnishing materials were characterized using the methodology developed in the UL-FPRF Smoke Characterization Project. This increased the number of measured smoke signatures from the 18 materials originally completed in the UL-FPRF Smoke Characterization Project to the 60 smoke signatures now identified.

The Firefighter Exposure to Smoke Project study produced the following key findings:

  • Concentrations of combustion products vary tremendously from fire to fire depending on the size, the chemistry of materials involved, and the ventilation conditions of the fire.
  • The type and quantity of smoke particles and gases generated depended on the chemistry and physical form of the materials being burned. However, synthetic materials produced more smoke than natural materials.
  • Combustion of the materials generated asphyxiants, irritants, and airborne carcinogenic byproducts that could be potentially debilitating.
  • Multiple asphyxiants, irritants, and carcinogenic materials were found in smoke during both the suppression and overhaul phases. Carcinogenic chemicals may act topically, following inhalation, or following dermal absorption, including from contaminated equipment.
  • Long-term repeated exposure may accelerate cardiovascular mortality and the initiation and/or progression of atherosclerosis.

The full report of this research is available at www.ul.com/fireservice.


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