| Smoke Alarm Research |
The Fire Research Division is conducting research to provide the technical basis for modifications and/or additions to standard test methods used to certify residential smoke alarms by developing performance metrics related to the hazards posed by fires, and to the susceptibility to nuisance alarms.
Smoke alarms are now installed in 96% of all U.S. homes. While 4% of homes do not have smoke alarms, approximately 20% of homes with smoke alarms have non-operational smoke alarms. It is estimated that if every home had working smoke alarms, U.S. residential fire deaths could drop by 36% (~1100 lives saved per year.) U.S. fire statistics for home structure fires from Years 2000-2004 reveal that 34% of civilian deaths occurred in homes with operating smoke alarms, 22% of civilian deaths occurred in homes with smoke alarms present, but that failed to operate, and 43% of civilian deaths occurred in homes with no smoke alarms. The statistics point to three major problems or obstacles to further reducing residential fire deaths through smoke alarm usage, leading to the following questions: First, how can 100 % of homes have smoke alarms? Second, how can the percentage of working smoke alarms be significantly increased? Third, how can smoke alarms, when they are functioning, be more effective in warning occupants? Research points to two main reasons for non-working smoke alarms: missing or dead batteries and intentional power source interruption. A reduction in nuisance alarms would tend to reduce intentional disabling of alarms. There is a need for research to improve the performance measurement of smoke alarms including performance for a range of fire scenarios, and the susceptibility to nuisance alarms.
This page provides a summary of recent NIST research related to smoke alarms.
Cleary T.G., and Chernovsky, A. "Smoke Alarm Performance in Kitchen Fires and Nuisance Alarm Scenarios," NIST Technical Note 1784 (January, 2013)
Cleary, T.G., "An Analysis of the Performance of Smoke Alarms," presented at 10th International Symposium on Fire Safety Science, University of Maryland, USA, June 19-24 (2011).
Test results from the NIST 2008 Smoke Alarm Sensitivity Study were used in a smoke alarm performance analysis to examine the effects of pre-movement time, reduced travel speeds through smoke, and smoke optical density limit on occupant survivability given different smoke alarm installations. Smoke alarm installations that meet the requirements in the current National Fire Alarm Code NFPA 72 were considered. Alarm times from commercially-available photoelectric, ionization, and dual photoelectric/ionization alarms were used in the analysis to examine the effects of smoke alarm type on the predicted survivability for a range of fire and egress scenarios. Fire scenarios included both flaming and initially smoldering upholstered chair mock-ups. Egress scenarios considered occupants located in, or remote from the room of fire origin. Reduced travel speed through smoke was included in the analysis. Prior to occupant movement and as an occupant travels to the exit, the fractional effective dose from toxic gas and heat exposure were computed to determine survivability. The concept of relative effectiveness as performance metric for smoke alarms is introduced. The relative effectiveness is the fraction of occupants that successfully escape a given fire and egress scenario. It is computed by considering a frequency distribution for the pre-movement time and determining the cumulative fraction of occupants that successfully escape. Thus, the relative effectiveness of a smoke alarm type or installation requirement can be averaged over a large number of fire and egress scenarios. The pre-movement frequency distribution was modeled as a log-normal function. Experimental studies suggest that the median value of the distribution relates to characteristics of the population and a geometric standard deviation of 1.6 characterizes the width the distributions. The distribution median was varied to examine relative effectiveness skewed to more vulnerable populations (those slower to react). Travel speed was modeled as a function of smoke optical density which predicts reduced travel speed as thicker smoke is encountered. Model results showed photoelectric alarms had the lowest relative effectiveness values for flaming fires, while ionization alarms had the lowest relative effectiveness values for smoldering fires. These trends were expected based the results of previous studies. It was observed that there can be a steep increase in relative effectiveness, depending on the smoke alarm type and fire scenario, as the smoke optical density limit was increased from 0.25 m-1 to 0.50 m-1. However, the ranking of smoke alarms tend to remain the same. Given the magnitude of statistically significant mean values of relative effectiveness for all flaming and smoldering fires considered, the model results suggest that there is a benefit from a combination of alarm technologies, and that vulnerable populations who may require significantly more time to escape, regardless of the fire scenario, would benefit the most from dual alarms or side-by-side photoelectric and ionization alarms.
A series of 24 full-scale fire experiments was conducted in a multi-room structure to examine the effects of alarm type (photoelectric, ionization, and dual sensor), alarm location, fabric type (cotton and polyester), polyurethane foam density, ignition scenario (smoldering or flaming), and room configuration on smoke alarm performance. The fire source was a chair mock-up consisting of a seat and back cushion of a specific fabric and foam density, resting on a metal frame. Each fire progressed for a time sufficient to produce multiple hazards (smoke, heat, toxic gases) throughout the compartment. Photoelectric, ionization, and dual photoelectric/ionization alarms were co-located at multiple locations to facilitate comparisons of each type of alarm. In the room of fire origin, a smoke optical density of 0.25 m-1 was reached before a fractional effective dose of 0.3 for either toxic gases or heat exposure. The available safe egress time (ASET) for both flaming and smoldering fires was sensitive to the imposed optical density limit. Further study is needed to deduce the impact of visibility-limiting smoke levels on the time needed to egress residential fires to justify any particular optical density limit value.
Cleary, T.G., "Results from a Full-Scale Smoke Alarm Sensitivity Study," Presented at the Fire Protection Research Foundation's 13th annual Suppression and Detection Research & Applications Symposium (SUPDET 2009), February 24-27, 2009, Orlando, FL, and published in Fire Technology (2010)
Cleary, T.G., "Performance of Dual Photoelectric/Ionization Smoke Alarms in Full-Scale Fire Tests," Presented at the Fire Protection Research Foundation's 13th annual Suppression and Detection Research & Applications Symposium (SUPDET 2009), February 24-27, 2009, Orlando, FL, and published in Fire Technology (2010)
Data from two full-scale residential smoke alarm fire test series were analyzed to estimate the performance of dual sensor photoelectric/ionization alarms as compared to co-located individual photoelectric and ionization alarms. Dual alarms and aggregated photoelectric and ionization alarm responses were used to estimate dual alarm performance. It was observed that dual alarms with equivalent or higher sensitivity settings performed better than individual photoelectric or ionization alarms over a range of flaming and smoldering fire scenarios. In one test series, dual alarms activated 539 s faster than ionization alarms and 79 s faster than photoelectric alarms on average. In another test series, individual alarm sensor outputs were calibrated against a reference smoke source in terms of light obscuration over a path length (percent smoke obscuration per unit length) so that alarm thresholds could be defined by the sensor outputs. In that test series, dual alarms, with individual sensor sensitivities equal to their counterpart alarm sensitivities, activated 261 s faster on average than ionization alarms (with sensitivity settings of 4.3%/m smoke obscuration for the ionization sensors) and 35 s faster on average than the photoelectric alarms (with sensitivity settings of 6.6%/m, for the photoelectric sensors.) In cases where an ionization sensor was the first to reach the alarm threshold, the dual alarm activated 67 s faster on average than the photoelectric alarm. While in cases were a photoelectric sensor was the first to reach the alarm threshold, the dual alarm activated 523 s faster on average than the ionization alarm. Over a range of ionization sensor settings examined, dual alarm response was insensitive to the ionization sensor setting for initially smoldering fires and fires with the bedroom door closed, while dual alarm response to the kitchen fires was very sensitive to the ionization sensor setting. Tests conducted in the National Institute of Standards and Technology (NIST) fire emulator/detector evaluator showed that the ionization sensors in off-the-shelf ionization alarms and dual alarms span a range of sensitivity settings. While there appears to be no consensus on sensitivity setting for ionization sensors, it may be desirable to tailor sensor sensitivities in dual alarms for specific applications, such as near kitchens where reducing nuisance alarms may be a goal, or in bedrooms where higher smoke sensitivity may be a goal.
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 1455-1 (February 2008 Revision)
Other Related InformationQuestions and Answers Clarifying Findings of NIST Home Smoke Alarm Study (2/25/2008)
Supplementary Questions and Answers Clarifying "Detector Sensitivity and Siting Requirements for Dwellings," Phase I (NBS GCR 75-51) and Phase II (NBS GCR 77-82) (9/11/07)
Statement for the Record, National institute of Standards and Technology to the Boston City Council Committee on Public Safety, August 2007 (8/6/07)