Smoke Alarm Research
Understanding the value of smoke alarms and the real world role they play in saving lives and property is an ongoing issue of concern for both metro and rural communities. There have been many studies conducted, and nearly all come to one core conclusion: maintaining working smoke alarms greatly reduces the chances of dying in a house (structure) fire.
There are some key facts that The National Institute of Standards and Technology (NIST) has determined during its focused research into smoke alarm performance, including:
- Smoke alarms are now installed in 96% of all homes in the United States
- Approximately 20% of homes with installed alarms are unable to use them – they are disabled
- If every home had working smoke alarms, U.S. residential fire deaths could drip by 36% (or approx. 1,100 lives saved annually)
- There are some significant obstacles to overcome relative to the “smoke alarm question.” These include:
How can 100 % of homes have smoke alarms?
How can the percentage of working smoke alarms be significantly increased? 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 series of ongoing research projects underway to help establish quality data regarding smoke alarms.
Some of the more recent NIST research into smoke alarms and their operation includes:
— Cleary T.G., and Chernovsky, A. “Smoke Alarm Performance in Kitchen Fires and Nuisance Alarm Scenarios,” NIST Technical Note 1784 (January, 2013)
Experiments were conducted to assess the performance of various residential smoke alarms to kitchen fires and nuisance alarm cooking scenarios. A structure representing a kitchen, living room and hallway was constructed to conduct the experiments. Eight different residential smoke alarms types, two photoelectric models (P1 and P2), two ionization models (I1 and I2), two dual sensor photoelectric/ionization models(D1 and D2), and two multi-sensor, intelligent models (M1 and M2) were used in this study. The data gathered provided insight into the susceptibility of alarm activation from exposures to typical cooking events and alarm times for actual kitchen fires. The effects of alarm technology and installation location on the propensity of an alarm to activate were examined. In the kitchen fire experiments, all smoke alarms responded before hazardous conditions developed. An ionization alarm (I1) tended to respond first compared to other co-located alarms. Results show smoke alarms placed greater than 6 m from the kitchen range may provide less than 120 s of available safe egress time, which suggests the importance of a more central alarm location closer to the kitchen for this configuration. Experiments were conducted to determine an alarm’s propensity to activate when exposed to particulates generated from eight typical cooking activities including toasting, frying, baking and broiling. In most cases, the propensity to nuisance alarm decreased as the distance from the cooking source increased. Two alarms, I1 and D2, experienced more nuisance alarm activations across the eight cooking activities than the other alarms. The remaining alarms experienced about the same combined nuisance alarm frequency by averaging all cooking events for installation locations outside the kitchen. Experiments showed combustible materials typically found on a counter top can spread flames to overhead cabinets, and a single empty 0.6 m wide 1.0 m tall wood-framed, pressboard cabinet can produce a peak heat release rate nearly sufficient to flashover a small room. Alternatively, protective metal barrier on the bottom and side facing the range tended to limit the spread of flames to the cabinet and reduce the heat release rate.
— 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.
— Cleary, T.G., “Full-scale Residential Smoke Alarm Performance,” 14th International Conference on Automatic Fire Detection, University of Duisburg-Essen, Duisburg, Germany (2009).
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)
A series of 24 full-scale experiments was conducted during the summer of 2008 to examine the effects of alarm type (photoelectric, ionization, and dual sensor), alarm location, fabric type (100 % cotton and 100 % polyester), polyurethane foam density, ignition scenario, and room configuration, on smoke alarm performance. A two-level, fractional factorial design of eight experimental configurations was developed around the five factors: fabric type, foam density, fire location, ventilation, and ignition scenario. A structure, designed to represent a single-story home or apartment, was constructed inside the Large Fire Laboratory at the National Institute for Standards and Technology for the experiments. The fire source was a chair mockup consisting of a seat and back cushion of a specific cover fabric and foam density, weighing between 5.5 kg and 8.3 kg. It rested on a metal frame and was subjected to a small propane gas flame, or an electric cartridge heater to initiate smoldering. Each experimental configuration was conducted three times. Smoldering fires were allowed to progress until they naturally transitioned to flaming fires except for one test that was terminated early due to time constraints. The smoldering to flaming transition times ranged from (81 to 182) min. Each fire progressed for a time sufficient to produce multiple hazards (smoke, heat, and toxic gases). All alarms tested were purchased from retail outlets and activated at their preset levels. Photoelectric, ionization, and dual photoelectric/ionization alarms were co-located at multiple locations to facilitate comparisons of each alarm type, and different designs of the same type of alarm. For smoke alarms in the room of fire origin, it was observed that each of the five factors had an effect on the measured alarm times that was primarily a result of fire growth rate (fabric type, foam density, and ignition scenario), or smoke dilution (fire location and ventilation). The photoelectric alarm responded quicker on average than ionization alarm in two of four smoldering fire configurations, responding before the ionization alarm in all 6 trials, while the ionization alarm responded before the photoelectric alarm in two of three trials for the other two configurations. The ionization alarm responded quicker on average than photoelectric alarm in all four flaming fire configurations, and responded before the photoelectric alarm in all 12 flaming fire trials. One dual alarm had the fastest average alarm time for all four smoldering fire configurations, and responded first in 11 of the 12 trials. It also yielded faster average alarm times than the other dual alarm in seven of eight configurations, and was the first dual alarm to respond in 22 out of 23 trials where dual alarms were present.
— 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)
This report presents the results of the project and provides details of the response of a range of residential smoke alarm technologies in a controlled laboratory test and in a series of real-scale tests conducted in two different residential structures. The data developed in this study include measurement of temperature and smoke obscuration in addition to gas concentrations for a range of fire scenarios and residences. The results are intended to provide both insight into siting and response characteristics of residential smoke alarms and a set of reference data for future enhancements to alarm technology based on fires from current materials and constructions. Smoke alarms of either the ionization type or the photoelectric type consistently provide time for occupants to escape from most residential fires, although in some cases the escape time provided can be short. Consistent with prior findings, ionization type alarms provide somewhat better response to flaming fires than photoelectric alarms, and photoelectric alarms provide (often) considerably faster response to smoldering fires than ionization type alarms. Escape times in this study were systematically shorter than those found in a similar study conducted in the 1970’s. This is related to some combination of faster fire development times for today’s products that provide the main fuel sources for fires, such as upholstered furniture and mattresses, different criteria for time to untenable conditions, and improved understanding of the speed and range of threats to tenability.
These research suites are only a partial overview of the ongoing research into the operational benefits and issues related to smoke alarms. Ongoing research by the NFPA, USFA, Vision 20/20, and other entities will continue as solutions for reduction in fire fatalities are sought.