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Questions and Answers Clarifying Findings of NIST Home Smoke Alarm Study1,2

1. In how many scenarios contained in the July 2004 NIST Technical Note 1455 on 

"Performance of Home Smoke Alarms: Analysis of the Response of Several Available 

Technologies in Residential Fire Settings" does the available time for egress exceed the 

time required for egress?

The difference between the available safe egress time (ASET) and the required safe egress time 

(RSET) is dependent on three important factors: (1) the time to activate the alarm, (2) the time to 

untenable (unsafe) conditions, and (3) the estimated egress time. The time to activate the alarm 

is a function of detector sensitivity, detector placement, and characteristics of the fire. The time 

to untenable conditions varies with the characteristics of the fire, the location within the structure 

and the selection of tenability criteria (NIST defined "untenable" as the temperature, heat flux, 

CO, or smoke conditions3at the 5 ft level outside of the room of origin that would affect 

particularly sensitive populations: the young, elderly, asthmatics, and sufferers of other lung 

conditions). The required egress times (RSET) were estimated to range between 10 s and 140 s. 

This large variability4 (See Table 26 of the report2) was meant to account for differences in the 

time of day, the capability of the occupants, the design of the residence, the character and 

location of the fire, and the location of the occupant relative to the location of the fire and smoke 

alarms. For any given test, the times to activate the alarms and to reach one of the tenability 

criteria (most often, smoke obscuration) were measured, from which the value of ASET was 

determined. Table 1 shows the number of tests (out of a total of 32 tests) when the available 

time was less than the time required for safe egress. For the 12 smoldering tests, this number 

increased from one instance to five instances for the photoelectric alarm, and from two instances 

to nine instances for the ionization alarm, over the range of values assumed for RSET. For the 

16 flaming chair and mattress fires, this number increased from one instance to eleven instances 

for the photoelectric alarm, and from zero to five instances for the ionization alarm as the time 

needed for safe egress was increased. Both the ionization and photoelectric detectors provided 

more than enough time for safe egress from the four scenarios involving kitchen fires for all 

assumed values of RSET. 

 

1 Reported in February 26, 2004 Tech Beat article "Current Smoke Alarms Pass Life-Saving Tests."

2 Bukowski, R. W., Peacock, R. D., Averill, J. D., Cleary, T. G., Bryner, N. P., Walton, W. D., Reneke, P. A., and 

Kuligowski, E. D. Performance of Home Smoke Alarms, Analysis of the Response of Several Available 

Technologies in Residential Fire Settings, Natl. Inst. Stand. Technol., Tech. Note 1455 (2004)

3 Life-threatening components of fire -- Guidelines for the estimation of time available for escape using fire data, 

ISO/TS 13571:2002." International Organization for Standardization, 2002.

4 The estimated evacuation times listed in the NIST report for nighttime fires (90 s to 140 s) are more conservative 

than the times found in actual studies of nighttime residential evacuation when occupants were sleeping, which 

reported times ranging from 36 s to 119 s, including waking time, time to call the fire department and for the family 

to escape outside the house. See Nober, E. H.; Peirce, H.; Well, A. D.; Johnson, C. C.; Clifton, C., “Waking 

Effectiveness of Household Smoke and Fire Detection Devices,” Fire Journal, Vol. 75, No. 4, 86-91,130, (1981).
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Table 1. Number of tests (out of 32) with ASET < RSET

RSET = 10 s RSET = 65 s RSET = 140 s

Fire Type Number 

of Tests

Photoelectric 

Alarms

Ionization 

Alarms

Photoelectric 

Alarms

Ionization 

Alarms

Photoelectric 

Alarms

Ionization 

Alarms

Smoldering 12 1 2 4 6 5 9

Flaming 16 1 0 4 1 11 5

Cooking 4 0 0 0 0 0 0

With alarms placed on every level plus in bedrooms or in every room, the number of tests where 

ASET was less than RSET is smaller. For example, with alarms placed in every room, ASET 

was less than RSET in two tests for photoelectric alarms and three tests for ionization alarms 

with an RSET value of 65 s. Most current building codes require smoke alarms at every level in 

existing homes and at every level plus bedrooms in newly constructed homes. No codes require 

smoke alarms in every room but this is included as (theoretically) the best performance 

achievable.

2. Do ionization detectors provide enough time to save lives? How did NIST come up with 

the "three minute warning" criteria that was cited in a February 26, 2004 Tech Beat article

"Current Smoke Alarms Pass Life-Saving Tests"?

There are numerous combinations of fire scenarios and smoke alarm placement locations. In the 

NIST experiments, the following variables were included: smoke alarm location; room of fire 

origin, smoldering, flaming, or cooking fire; and bedroom doors open or closed. Table 2 (taken 

from Appendix A of NIST TN 1455) lists the averages and standard deviations for the measured 

times to first alarm and to untenable conditions in these tests. In all cases, the average ASET 

was positive, although the average ASET for the photoelectric alarm in the case of the flaming 

fires was less than the maximum assumed for the RSET. The average estimated time available 

for safe egress in the smoldering scenario was greater than the RSET by much more than one 

standard deviation in the case of the photoelectric alarms. For the ionization alarms in the 

smoldering situation, the average ASET was about one minute greater than the longest assumed 

RSET, but in ten of the twelve tests was less than the longest RSET assumed (See last column of 

Table 1). The general trends from the NIST experiments are consistent with previous studies 

showing that: (1) both ionization and photoelectric alarms provide enough time to save lives for 

most of the population under many fire scenarios, (2) ionization alarms may not always provide 

enough time for the most sensitive populations with mobility limitations to escape a smoldering 

fire, and (3) photoelectric alarms may not always provide enough time to escape a flaming 

furniture fire for this same population. 

Table 2. Average time to first alarm and time to untenable conditions, with standard deviations.

Time to First Alarm (s)

Time to 

Untenable 

Conditions (s)

Available Safe Egress Time (s)

Photoelectric 

Alarms

Ionization 

Alarms

Photoelectric 

Alarms

Ionization 

Alarms

Smoldering Fires 2219 ± 1061 4010 ± 1120 4316 ± 1256 2136 ± 1011 276 ± 331

Flaming Fires 94 ± 33 47 ± 36 217 ± 67 129 ± 74 177 ± 69

Cooking Fires 738 ± 103 681 ± 475 1477 ± 249 739 ± 148 796 ± 241
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For the NIST test series, the average ASET (assuming placement of detectors in all rooms) for 

ionization alarms ranged between 177 s for flaming fires and 276 s for smoldering fires; for 

photoelectric alarms the ASET averaged between 129 s and 2136 s. For the purpose of the Tech Beat 

article, 180 s (or 3 minutes) was selected as representative of the amount of time available, and 

to convey to the reader the importance of leaving their residence quickly after first hearing a smoke 

alarm of any design. 

3. Are ion and photo detectors qualitatively similar since the February 26, 2005 Tech Beat

article states that "Ionization smoke alarms respond faster to flaming fires, while 

photoelectric smoke alarms respond quicker to smoldering fires"? Why does NIST 

describe the 30-45 minute of photos in smoldering fires as "quicker" and in the same 

sentence describe the 20-30 seconds benefit of ion detectors as "faster"?

No, the ionization alarms and photo alarms are qualitatively different because they respond to 

different aspects of a fire. The Tech Beat article was correct in stating that "ionization alarms 

respond faster to flaming fires, while photoelectric smoke alarms respond quicker to smoldering 

fires." While the photo detectors sensed the smoldering fires on average 30 minutes earlier than 

the ionization detectors, the threat from a smoldering fire grows much more slowly than from a 

flaming fire. That is, the margin of safety associated with a 30 minute earlier warning in a slow 

growing smoldering fire may not be necessarily any more significant than a 30 s earlier warning 

for a fast growing flaming fire.

4. Should "necessary escape time" be replaced with "available escape time" in the 

February 26, 2004 NIST news release where it states that "The researchers determined the 

necessary escape times by considering the time that the alarms sounded in various locations 

and the development of untenable (unsurvivable) conditions"?

Yes. Necessary escape time refers to the time required for occupants to safely evacuate from a 

building. This is usually termed the required safe egress time (RSET). Available safe egress time 

(ASET) is the elapsed time between when occupants are notified of a fire and when conditions 

along the path of their egress become sufficiently untenable that escape may no longer be 

possible. The correct term in the press release should have been the available safe egress time 

rather than necessary escape time. The press release has been corrected effective March 8, 2007.

5. Why does NIST appear to install "un-modified” detectors in areas which prevent 

comparison between the unmodified and modified detectors? Where in the report does 

NIST present this comparison and associated data in a manner that justifies its analysis of 

"modified" detector response? Is the manner in which NIST models the response of 

"modified" detectors valid?

The unmodified smoke alarms used in the study consisted of a photoelectric model and two 

ionization types, all purchased from retail establishments by NIST for use in the test series. 

Some of the identical models of smoke alarms were modified by their manufacturers so that the 
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voltage produced by the sensor could be monitored to determine the response of the sensor to the 

changing environment in a continuous manner, rather than registering a single alarm point. This 

analog signal provided a means to monitor the environment during the build up period prior to 

the detector alarming, and had no effect on the sensitivity of the smoke alarms or the reported 

detector response times. Thus, it was appropriate to compare detector response times at different 

locations for both un-modified and modified detectors. A more appropriate term for "modified" 

would have been "continuously monitored" detectors. It was not necessary to monitor the output 

voltage of all of the smoke alarms since the monitored alarms were located judiciously. 

Chapter 2 of the July 2004 NIST report discussed the alarm calibrations in detail. Section 2.8 

discusses the response of the "modified" alarms. All of the alarms were calibrated multiple times 

throughout the test series. Any changes in the response of the alarms were incorporated into the 

analysis. NIST spot-tested "unmodified" alarms in the fire-emulator/detector-evaluator (FE/DE)5

to compare alarm point smoke levels to the equivalent alarm level for the "modified" alarms. 

The "unmodified" (off-the-shelf) ionization alarms had sensitivities near the low level sensitivity 

setting for the "modified" ionization alarms used in the analysis; otherwise, the alarm levels were 

comparable. 

Beyond simple alarm point calibration, the minimum sensitivity of smoke alarms is established 

in US and International standards by performance in fire tests. Specifically, smoldering smoke 

tests utilize cotton wicks or wood pieces on a hotplate (UL217/2686and EN54/ISO TS7240-9

7

) 

as sources, and these were used by NIST as detailed in the report. The Underwriters 

Laboratories (UL) smoldering test with wood on a hotplate was developed by UL in the late 

1970‟s to mimic the smoldering mattresses and furniture (both the smoke characteristics and 

temporal increase) in the original Indiana Dunes Tests.8Since UL uses this smoke to quantify 

smoke alarm points, NIST chose the nominal sensitivities stamped on the back of typical alarms 

to be the alarm points for this study. While NIST used the unique FE/DE apparatus as described 

in the preceding paragraph to calibrate the test alarms, the output has been correlated with the 

UL apparatus. 

Both US and International standards also include flaming sources in their fire tests. These 

include cellulosic (wood and paper – flaming wood is used by both UL and EN/ISO, paper only 

by UL), liquid hydrocarbon (heptane), and plastic (polyurethane foam – EN/ISO only). UL 

stopped conducting its flaming plastic (polystyrene foam) test in 1999. 

 

5 Cleary, T. G., “Fire Emulator/Detector Evaluator: Design, Operation, and Performance.” Proceeding of the 

International Conference on Automatic Fire Detection "AUBE '01", March 25-28, 2001, Gaithersburg, MD, Beall, 

K.; Grosshandler, W. L.; Luck, H., Editors, Natl. Inst. Stand. Technol., NIST SP 965; February 2001. 312-323 pp, 

(2001)

6 UL 217: Standard for Safety Single and Multiple Station Smoke Alarms, and UL 268: Standard for Smoke 

Detectors for Fire Protective Signaling Systems, 4th ed., Underwriters Laboratories Inc., Northbrook, IL., 1996

7 EN 54: Components of Automatic Fire Detection Systems, Part 9, Fire Sensitivity Test, European Committee for 

Standardization, Brussel, 1982.

8 Bukowski, R.W., Waterman, T.E., and Christian, W.J., “Detector Sensitivity and Siting Requirements for 

Dwellings: A Report of the NBS „Indiana Dunes Tests‟” NFPA No. SPP-43 Nat. Fire Prot. Assn., Quincy, MA, 

1975.
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6. How relevant is the calibration of detectors with smoke from flaming hydrocarbon and 

smoldering cotton for smoke from smoldering plastic? How likely is it that the ionization 

detector’s response is overestimated and the photoelectric detector’s response is 

underestimated if the detectors are not calibrated with smoke caused by smoldering 

plastic?

Differences in characteristics of the smoke from a smoldering plastic (which is not used for 

calibration) and from smoldering cotton and for wood on a hot plate (which are used for 

calibration) is an active area of research and does have implications for the design and standard 

test methods for smoke alarms. Given smoke from, for example, smoldering upholstered 

furniture, an ionization alarm of a given design will respond based primarily on the smoke 

concentration and size distribution. A photoelectric alarm‟s response will also depend on its 

design, the smoke concentration, size distribution and optical properties. It is possible, and 

likely, that a photoelectric alarm would respond before an ionization alarm (exactly what was 

observed multiple times in the NIST Home Smoke Alarm study) to a rising concentration of such 

a smoke. This does not imply any underestimate or overestimate of the various alarm type 

calibrations. 

The flaming hydrocarbon smoke calibrations were performed to allow further study of these fire 

tests, specifically modeling of the flaming fire tests, and the response of the smoke alarms. 

These calibrations were never used in any of the analysis in NIST TN 1455. 

7. How was a detection time of 1830 seconds determined for both types of detectors 

(ionization and photoelectric) in the smoldering bedroom fire involving the 2-story house, 

especially since many of the detectors did not respond? Does this raise questions about the 

adequacy of the methodology used to determine detection times?

The method NIST used to determine the detection time in the smoldering fire involving the 2-

story house was in error. The results of the smoldering mattress test (test SDC 21) should not 

have been included in the analysis since the fire development did not allow enough alarms to 

respond by the time the test concluded. In addition, test SDC 13 was inadvertently excluded 

from the original analysis. The report will be revised to reflect these changes, although this is 

unlikely to change any key conclusions or recommendations.

8. Was the “end of test” assumed as the time to untenability, since the smoking bedroom 

test did not reach untenable conditions?

For most tests, one or more criteria for untenable conditions were reached prior to the end of test. 

For six of the tests, untenable conditions were not reached outside the room of fire origin. For 

these tests, the time to untenable conditions was assumed to be as long as the time to the end of 

test. This provides a conservative estimate of the time to untenable conditions. 
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9. Has NIST concluded that adding smoke alarms to the bedrooms provides no benefit in 

smoldering mattress fires with detectors placed in the room of fire origin, since the test 

results show that for an installation with detectors in every room and in every bedroom, the 

ASET is the same as when they are only on every level? It appears the same conclusion can 

be drawn about the benefit of placing detectors in the room of fire origin from the flaming 

living room fire test.

No. The results from the single-story manufactured home show the value of adding smoke 

alarms in the bedroom. NIST did not place any detectors in the room of origin (bedroom or 

living room) for the 2-story tests. The two-story home was an open floor plan. Alarms were 

placed in the foyer and den, with no doors blocking the smoke path. In the analysis, this was 

considered the same space (room) as the living room. There were also smoke alarms in the 

entranceway, comparable to the den mid-level detector locations. Scheduling constraints and the 

number of available unmodified alarms precluded inclusion of alarms in all locations for some of 

the field tests.

10. Are both types of detectors (i.e., ionization and photoelectric) equally susceptible to 

nuisance alarms?

In the 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.

11. Why did NIST run half the nuisance tests with a large fan only on one side of the 

kitchen to blow smoke away from the detectors? Would this not favor the ionization 

detectors in the tests?

The fan was added to provide an additional variable to the data set. About half of the 

manufactured home tests were conducted with all exterior doors and windows closed and no 

ventilation of any kind, which is unrepresentative of situations including open windows, HVAC 

flows, cooking range fans, movement of occupants, etc. Flow velocities were monitored just 

below the ceiling at three locations. Air speeds at the ceiling were typically below 0.1 m/s 

without the fan and up to 0.5 m/s with the fan. The fan had a tendency to break up the plumes 

generated from the cooking activities, and to dilute the aerosols. To the extent that the fan 
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diluted the nuisance aerosols and distributed them throughout the connected rooms and 

subsequently impacted the nuisance alarms produced, it was precisely the result intended. 

12. Have ionization detectors been de-sensitized over time (since the late 1980s) and are 

they relatively poor at detecting the kind of smoke given off by today’s synthetic 

furnishings? 

NIST is not aware of any definitive data on the actual sensitivities of the detectors over the past 

20 or 30 years. All of the smoke alarms used in the current study met the sensitivity 

requirements specified in the 2002 version of UL 217, and those in the 1975 study met the 

requirements of the applicable UL standard at that time.9 The average ionization alarm 

sensitivity based on smoke obscuration was found to be 4.2 %/m in the current study and 

6.8 %/m in the 1975 study. In other words, the ionization detectors were, on average, slightly 

more sensitive in the current study. Photoelectric alarms were, on average, slightly less sensitive 

in the current study (6.8 %/m versus 4.8 %/m in 1975).

13. Given the clear evidence that ionization alarms should not be used near kitchens since 

they often are intentionally disabled to avoid nuisance alarms, why does HUD allow them 

to be installed in manufactured homes?

NIST is a non-regulatory agency and does not set the rules for smoke alarm placement. HUD 

released the final rule "Manufactured Construction and Safety Standards: Smoke Alarms Rules 

and Regulations." 10 It was based on the requirements of NFPA 501. Concerning kitchens the 

final rule requires: 

“At least one smoke alarm must be installed in each of the following locations: 

(i) To protect both the living area and kitchen space. Manufacturers are 

encouraged to locate the alarm in the living area remote from the kitchen and 

cooking appliances. A smoke alarm located within 20 feet horizontally of a 

cooking appliance must incorporate a temporary silencing feature or be of a 

photoelectric type.” 11

This requirement should reduce nuisance alarms, which should reduce the negative effects 

(disabling of alarm) of repeated nuisance alarms. This requirement is consistent with the 

requirements of the National Fire Alarm Code, NFPA 72.12

 

9 The UL standard requires that alarm point based upon smoke obscuration be within the range of 1.6 %/m to 12.5 

%/m. This has not changed. In the 1980s, manufacturers were allowed to shift production windows (based on a 

sample of 24 alarms originally submitted by a manufacturer for testing) by 1.6 %/m to reduce susceptibility to 

nuisance alarms. The production window is the sensitivity range the manufacturer must meet during production 

after Listing with UL. A shift in the production window to a less-sensitive range is allowed to ensure that alarms 

produced represent the least sensitive range. 

10 67 FR 12811 (March 19, 2002)

11 Manufactured Home Construction and Safety Standards, 24 CFR 3280.208(b)(1)(i) (2002).

12 “NFPA 72, National Fire Alarm Code, 1999 Edition,” Volume 5 of the National Fire Codes, Natl. Fire Protection 

Association, Quincy MA 2002.
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14. Did NIST consider (1) the different ignition methodologies and (2) unlikelihood of 

change in materials as being factors that changed/decreased the time to untenable 

conditions in the recent NIST tests compared with the Indiana Dunes8tests of the mid1970s?

NIST considered a number of factors for the increased fire growth rates, including ignition 

methodology and materials of construction. 

Although the ignition methodologies were not identical between the NIST tests2and the original 

Indiana Dunes tests,8 NIST does not feel they were sufficiently different to fully account for the 

difference in time to untenable conditions. The smoldering ignition technique for both test series 

utilized a heated wire loop in contact with the item ignited for a similar time period (typically 2 

minutes). For flaming ignition, a larger wastebasket ignition source was used in the original 

Indiana Dunes tests compared to the single matchbook ignition source used in the current NIST 

study. By design, the amount of paper ignition source in the wastebasket in the original Indiana 

Dunes tests was controlled so that the ignition source was fully consumed within 2 min of 

ignition. For particularly difficult to ignite items of furniture, additional newspaper was added 

until ignition was achieved. NIST did not attempt to adjust the ignition times reported in the 

original Indiana Dunes tests to account for these difficult ignitions.

While the 1975 study also used actual upholstered furniture and mattresses, these were purchased 

from a (charity) resale shop from donated articles. Such items purchased in 1975 would have 

been new in the early 1960s or even late 1950s and represented materials and constructions of 

that period with typically natural materials. The chairs used in the present study were purchased 

from a furniture rental store and, while used, were only a year or two old and of synthetic 

materials. The mattresses were purchased new. Thus, the materials were certainly significantly

different. 

While the internationally 

accepted ISO tenability criteria 

used for the NIST study2 were 

not identical to those used in 

the 1975 study,8the criteria 

were equivalent or slightly 

higher in the current study, 

which, if anything, would have 

a tendency to increase the time 

to untenable conditions for the 

current study. Table 3 shows a 

comparison of tenability criteria used in the two studies. For the temperature and smoke 

obscuration criteria, values used in the current study were similar to or slightly higher than those 

used in the 1975 study. For CO concentration, the range for the FED-based model used in the 

current study includes the value used in the 1975 study, but is also quite a wide range, depending 

Table 3. Comparison of Tenability Criteria Used in the 1975 

and Current Studies

1975 Study8 Current Study2

Temperature T ≥ 66 °C T ≥ 88 °Ca

Gas 

Concentration

CO ≥ 0.04 % 

volume fraction

CO ≥ 0.02 % – 0.3 % 

volume fractionb

Smoke 

Obscuration

O.D. ≥ 0.23 m-1 O.D. ≥ 0.25 m-1

a – value for flaming fires calculated from ISO TS 13571 

equation for convected heat

b – range of average values calculated from ISO TS 

13571 equation for asphyxiant gases with tenability times 

for flaming fires and smoldering fires
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on duration of the fire. For nearly all tests, the smoke or temperature criterion was met prior to 

the CO criterion.

The conclusion of faster fire growth rates for flaming fires in the current test series was based on 

a comparison of gas temperature histories near ceiling level in the room of fire origin. With the 

exception of one room in each of the two test homes in the original Indiana Dunes tests, ceiling 

heights were similar in both tests, so comparisons of this gas temperature provides a relative 

indication of fire growth rate. From Table 32 of TN 1455,2flaming fires reached a near-ceiling 

gas temperature of 65 °C more than 7 times faster in many of the current tests than in the original 

Indiana Dunes tests (130 s versus 970 s). Including an adjustment for delayed ignition for some 

of the flaming fire in the original Indiana Dunes tests brings this ratio down to a factor of four. 

Fire development in the current tests was generally similar to the growth of heat release rate 

determined in other recent studies for upholstered furniture13 and mattresses14. The same trend 

was not evident for smoldering fires.

15. Why does NIST in its February 26, 2004, Tech Beat article highlight the increase in 

growth rate of fires and fail to point out that this finding applies only to flaming fires? 

Why does NIST fail to mention that smoldering fires are not growing faster?

The Tech Beat article was designed to highlight trends that were different between the two 

studies, especially one which was so dramatic and critical to life safety. The executive summary 

of the NIST report provides a more detailed description of the findings, including the statement:

“...the smoldering fire scenarios are very difficult to reproduce experimentally and 

tenability times in the present study have an uncertainty (based upon one standard 

deviation) which overlaps the uncertainty from the 1975 study. Therefore, 

caution should be exhibited in drawing conclusions based upon comparisons of 

smoldering tenability times between the two studies.”

16. Do the data indicate that the increase in fire growth rate, which may be far less than 

NIST estimates due to the different ignition methodologies for the flaming fire tests, cannot 

be the reason for the increase in fatalities when the detector operates?

NIST did not study fatalities with working smoke alarms in the study. Average times to 

untenable conditions in the current NIST study for flaming and smoldering furniture fires were 

found to be 17 % and 49 %, respectively, of those found in the 1975 Indiana Dunes tests, 4as can 

be seen in Table 4 based on a revised analysis of the NIST data (as noted in Question 7, test SDC 

13 was added and test SDC 21 was removed from the analysis). On the other hand, the average 

time for the cooking fires to reach untenable conditions was 23 % longer in the current study. 

Since the cooking materials were similar in the two studies, the shorter time to untenable 

 

13 Cleary, T. G., Ohlemiller, T. J, and Villa, K. M., “The Influence of Ignition Source on the Flaming Fire Hazard of 

Upholstered Furniture.” Natl. Inst. Stand. Technol., NISTIR 4847, 1992.

14 Ohlemiller, T. J, and Gann, R. G.., “Estimating Reduced Fire Risk Resulting From an Improved Mattress 

Flammability Standard.” Natl. Inst. Stand. Technol., Tech. Note 1446, 2002.
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conditions in the 

furniture fires 

supports the NIST 

statement 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. 

17. Why is NIST highlighting the difference in tenability criteria, when the obscuration 

criterion that matters for the smoldering case is essentially the same as in the Indian Dunes 

study?

Tenability criteria are important since it is the difference between the alarm time and the time to 

untenable conditions that determines the available safe egress time. As noted in the response to 

Question #15, above, these times were quite different for the current study and the original 

Indiana Dunes tests, even with similar tenability criteria in the two studies (see Question #13).

18. NIST states in its February 26, 2004 Tech Beat article that "The tests also showed how 

closed bedroom doors and proper placement of smoke alarms affect one's chance of 

survival. In both cases, the time to escape untenable conditions was increased, providing 

that the individual was not in the room where the fire originated." This finding would help 

a very small group of homeowners who (1) can be rescued from the bedroom window and 

(2) have interconnected detectors including the bedroom. What is the basis for this finding 

since the approach is harmful to the larger group of homeowners that has battery-powered 

smoke alarms and needs to hear the detector to exit the home safely? 

In the July 2004 Technical Note 1455, NIST concluded that the available safe egress time was 

often quite short. In many cases, available escape time would be sufficient only if households 

follow the advice of fire safety educators, including sleeping with doors closed while using 

interconnected smoke alarms to provide audible alarm in each bedroom, and pre-planning and 

practicing escape so as to reduce pre-movement and movement times. It is this interconnection 

that insures all alarms respond to a fire event. Further, building codes require 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.

The latest version of NFPA 72 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. 

However, audibility of smoke alarms remains an issue, particularly for sleeping children and 

Table 4. Comparison of alarm times and times to untenable conditions for 

1975 and current studies

1975 Tests (s) Current Tests (s)

Alarm Times

Flaming 146 ± 93 47 ± 35

Smoldering 1931 ± 1103 2042 ± 876

Tenability Times

Flaming 1036 ± 374 177 ± 69

Smoldering 4419 ± 1790 2148 ± 1023
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adults impaired with alcohol or other drugs.15 For existing residences that do not fall under the 

"new construction" requirements of NFPA 72, the following approaches are suggested to reduce 

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

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

19. Why does NIST conclude that increased usage of smoke alarms between 1975 and 2000 

can be credited with decrease in home fire deaths by nearly a half when data indicate that 

(1) the rate of reduction of fire deaths was relatively independent of the rate of increase in 

smoke alarm usage, and (2) the rate of death and injury per 100 cigarette fires increased as 

smoke alarm usage tripled?

This is not a NIST finding. NIST is citing data based on a 2004 study by the National Fire 

Protection Association16. A newer white paper from a broad coalition of public and private 

organizations, including NFPA, also supports this conclusion17.

20. What fraction of fatal residential fires is initially smoldering in nature and occurs when 

people are asleep? 

While nationally-collected data on fire incidents do not specifically classify fire sources as 

smoldering and/or flaming, one can get an indication from the reported cause of death (reported 

for example as smoke inhalation, burns, or a combination), the extent of damage, and victim 

location. An analysis of U.S. data from 1986 to 1990 shows that two-thirds to three-quarters of 

fire deaths in the United States were due to smoke inhalation18. It is estimated that more than 

one-fourth of home fire deaths involve an extended period of initial smoldering19. According to 

an analysis of home structure fires from 1999 to 2002, 29 % of civilian fire deaths are due to 

fires from smoking materials20. Intentionally set fires and fires originating from heating 

equipment were responsible for 19 % and 11 % of civilian fire deaths, respectively.

Although kitchens were seen as the leading area of origin for home structure fires (34 %) and for 

civilian home fire injuries, 21 % of reported home fires and 51 % of home fire deaths occur 

between 11 pm and 7 am when people are unlikely to be cooking in the kitchen and are likely to 

asleep in their bedrooms.

 

15 A Review of the Sound Effectiveness of Residential Smoke Alarms, CPSC-ES-0502, U.S. Consumer Product 

Safety Commission, Washington, DC (2004).

16 Ahrens, M., “U.S. Experience with Smoke Alarm and Other Fire Detection/Alarm Equipment.” National Fire 

Protection Association, Quincy, MA (2004).

17 Home Smoke Alarms and other Fire Detection and Alarm Equipment, Public/Private Fire Safety Council, 

available from National Fire Protection Association, Quincy, MA (2006)

18 Gann, R. G., Babrauskas, V., Peacock, R. D., and Hall, J. R., “Fire Condition for Smoke Toxicity Measurement,” 

Fire and Materials, Vol. 18, 193-199 (1994).

19 Home Smoke Alarms and other Fire Detection and Alarm Equipment, Public/Private Fire Safety Council, 

available from National Fire Protection Association, Quincy, MA (2006)

20 Ahrens, M. “U.S. Fires in Selected Occupancies, National Fire Protection Association, Quincy, MA (2006)
2/25/08 http://smokealarm.nist.gov/

12

21. A true understanding of the historical tests shows the ionization alarm to be deficient 

whenever synthetic material is smoldered for 30 minutes or more. The recent NIST study is 

consistent with this finding. Why has NIST ignored the results of research conducted in 

Norway (1991), Australia (1986) and England (1978) which concluded that ionization 

detectors provide inadequate warning and escape times for smoldering fires while 

photoelectric detectors provide a more effective alternative for such fires?

NIST had no intention of overlooking research conducted elsewhere. The shortening in time to 

untenable conditions from a fire (either flaming or smoldering) in modern, synthetic materials 

indicates the need to determine the ability of the 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 changes are required in the standard to accommodate the 

changing threat. 

22. Why do dual ion/photo alarms appear to perform worse than individual photo or ion 

alarms in tables 23, 24, 27 and 28 of the report?

The dual ion/photo alarms appear to perform worse than individual photo or ion alarms in the 

tables because the dual-alarms were often located further from the fire source than the individual 

photo or ion alarms listed in tables 23, 24, 27 and 28. The affected tables and figures 206-208 

have now been revised by removing the instances where a particular alarm type was not colocated. It is stressed that the individual alarm times reported in the appendix of the report have 

not been changed and remain available for direct comparison of the individual alarms and the 

dual ion/photo alarm in every case where these alarms were co-located. 

Table 5 shows examples of the comparison of alarm time for the two ionization alarms, the

single photoelectric alarm and the dual ion/photo alarm for when all the alarms were co-located 

and functioning. The specific test number is indicated in parentheses. The alarm times come 

directly from Appendix A of NIST TN 1455-1 (2008). Examples from the manufactured home 

(single-story) and the two-story home are presented. The shaded block indicates the alarm 

(either one of the ionization alarms, the photoelectric alarm or the dual photo/ion alarm) that 

reached the alarm threshold first at the particular location of the alarms. Thus, the values 

presented in the dual ion/photo column refer to either the photo or ionization sensor, whichever 

one sounded first in the dual sensor device. 

NIST TN 1455-1 was not meant to provide a detailed comparison of individual alarms to dual 

technology alarms. Rather, it was intended to provide an overall assessment of the performance 

of smoke alarms placed according to the broad placement categories specified in building and 

fire codes.
2/25/08 http://smokealarm.nist.gov/

13

Table 5. Sample comparison of individual ionization and photoelectric alarm times with 

collocated dual photo/ion alarms

Fire Scenario Location of 

Alarms

Alarm Time (seconds)

Ion #1 Ion #3 Photo #1 Dual 

Photo/Ion

Flaming Chair in Living 

room (SDC02)

Util. Hall 70 78 122 66

Master Bedroom 106 124 154 102

Smoldering Chair in Living 

Room (SDC01)

Util. Hall 6087 6112 5262 5282

Master Bedroom 6262 6287 5447 5512

Smoldering Mattress in 

bedroom (SDC04)

Util. Hall 3393 3413 3418 3398

Master Bedroom 3523 3518 3523 3503

Flaming Mattress in 

bedroom (SDC05)

Util. Hall 117 117 127 107

Master Bedroom 147 147 167 147

Kitchen Fire (SDC12) 2nd bedroom 977 797 839 797

Bedroom hall 737 795 847 657

Flaming Chair in Living 

room (SDC25)* Den 122 124 152 118

Smoldering Chair in Living 

Room (SDC23)* Upstairs Hallway 4824 4886 1542 1508

Kitchen Fire (SDC24)* Den 1290 1500 1194 876

Upstairs Hallway 1554 1554 880 898

*Two-story home test

23. Did the modifications to the dual-sensor smoke alarms that were used in the test affect 

their ability to react to smoke conditions, and did the modified dual-sensor smoke alarms 

meet the UL listing criteria? 

The photoelectric and ionization sensing chambers in the dual ion/photo alarms were calibrated 

in the same manner as the individual ionization and photoelectric alarms (see answer to question 

5 of this FAQ). Additionally, in the computation of alarm times the same alarm levels were 

specified for either the individual photoelectric alarm and dual alarm photoelectric sensor, or the 

individual ionization alarms and the dual alarm ionization sensor as specified in table 5 of the 

report. Given that the alarm levels specified in table 5 fall within the range allowed by UL 217, 

the modified dual ion/photo alarms would meet the UL requirements just like the individual 

photoelectric and ionization alarms. In dual ion/photo alarms that meet UL 217 requirements, 

manufacturers are allowed to set the sensitivity of the photoelectric and ionization sensing 

chambers to any allowable values so long as the dual alarm passes all applicable sensitivity tests. 

Thus manufacturers are free to set alarm sensitivities in dual photoelectric/ionization alarms less 

sensitive than in individual sensor alarms in order to reduce nuisance alarms.