http://smokealarm.nist.gov/

9/11/07 

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) 

1. Does the National Institute of Standards and Technology (NIST) have regulatory powers and 

does it certify smoke detectors? 

No. NIST is a non-regulatory agency of the U.S. Department of Commerce. NIST does not “certify” 

smoke detectors. 

2. Could working ionization detectors fail to detect the type of smoke most commonly produced by 

the early fire? If so, state the conditions under which this may occur. 

Both ionization and photoelectric sensors detect particles. Photoelectric technology is sensitive to the 

mass density of particles of a diameter of the same order as the wavelength of the light source in its 

sensor. Ionization technology is sensitive to the sum of all particle diameters and so it can respond to 

smaller particle diameters present in greater numbers. Because smoke from a smoldering fire 

produces relatively fewer particles and the particles are relatively larger, it is possible for an 

ionization-type detector to alarm at a time well after smoke from a smoldering fire is sensed by a 

photoelectric sensor. 

3. Did NIST ever compare the performance of a single ionization detector with a “throughout-thehome” fire detection system with a detector (heat, ionization, or photoelectric) in every room? 

If so, what did NIST find from its studies? 

No. A pioneering Canadian study published in 1962 (McGuire and Ruscoe) postulated the 

performance of a single smoke alarm outside the sleeping room(s) based on the judgment of experts 

reviewing fire reports. In 1974 this study was cited as the basis for a minimum requirement for a 

single smoke alarm outside the bedrooms in homes. In 1975, NIST (then NBS) funded tests to 

examine if one smoke alarm was sufficient. The tests indicated that it was not sufficient for multilevel homes, leading to the “every level” requirement common to most modern regulations. The 1975 

tests examined the performance of a heat detector in the room of fire origin, and in the second year 

(1976) looked at heat and smoke detectors in the room of fire origin, both equivalent to an “every 

room” alarm placement. These studies concluded that, while detectors in every room provided some 

increase in escape time, the increase was incremental and not justified by the cost as a minimum code 

requirement. The 2004 NIST tests also examined several installation strategies, including “every 

level”, “every level plus bedrooms”, and “every room.” Due mostly to the systematically lower 

escape times observed in the more recent tests, it appears that there is justification to apply the "every 

level plus bedroom" strategy (that is currently the minimum requirement for new homes) to existing 

homes. Both the Phase I and Phase II tests utilized heat, ionization, and photoelectric technologies 

and concluded that particulate sensors (either ionization or photoelectric) were adequate when placed 

appropriately and that heat sensors did not provide any additional protection. 

1/12
4. Did NIST ever conclude from any of its studies that the warning provided by a single ionization 

detector “would be so rapid that an occupant would be able to correct the pre-fire condition 

before any fire actually developed”? 

No. Over the years numerous claims were made by marketers of ionization and photoelectric 

technologies to demonstrate the superiority of their products. Some ionization manufacturers claimed 

that their detectors could sense “invisible particles of combustion that are present before any visible 

smoke or flames.” While the latter statement may be true in some flaming fire situations (such as 

burning alcohol fuels), NIST (NBS) never attempted to reproduce nor commented on these claims. 

5. Did the operating times when the fires were smoldering in Phase I of the Indiana Dunes Tests1,2

average more than one hour? If so, state the observed conditions when the fires were 

smoldering. 

The average test duration for the smoldering fires in the Phase I study was 5266 s ± 1907 s. In all of 

these tests, the test item smoldered for an extended period of time, averaging 4298 s ± 1929 s before 

transitioning to flaming. For the Phase I smoldering tests, the average time to first alarm was 

2489 s ± 1324 s for ionization alarms and 1927 s ± 1065 s for photoelectric alarms. 

6. Did the ionization detectors in the Phase I smoldering tests remain silent for over an hour? If 

so, why did they remain silent? 

In the Phase I smoldering tests, the average time to first alarm was 2489 s ± 1324 s for ionization 

alarms and 1927 s ± 1065 s for photoelectric alarms. The time to first alarm was greater than 3600 s 

in 4 of the 23 tests for the ionization alarms and in 2 of the 23 tests for photoelectric alarms. The 

reasons for the delay with both alarm technologies are associated with the slow rate at which 

smoldering fires produce smoke and the much longer transport time to the ceiling due to the low 

buoyancy of a smoldering plume. In addition, because the number of particles that eventually reach 

either detector is relatively smaller from a smoldering source, the technology that is based upon 

number density (i.e., ionization detectors) is slower to respond. 

7. Were most Phase I smoldering tests terminated shortly after one hour? If so, why were they 

terminated? 

The average test duration for the smoldering fires in the Phase I study was 5266 s ± 1907 s with tests 

as long as 8520 s. Tests were terminated after one or more pre-established tenability limits were 

exceeded, since any devices responding after that time would not affect the escape time provided. In 

Phase I, the first 36 and the last of the 40 tests exceeded a tenability limit prior to termination. Tests 

37 (overloaded electrical motor), 38 and 39 (overloaded extension cord) did not exceed a tenability 

limit and these tests were terminated after the test items burned out on their own. 

 

1

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

Dwellings.” IIT Research Institute Report prepared for Natl. Bur. Stand. (U.S.), NBS GCR 75-51 (1975) 2

 Harpe, S.W., T.E., and Christian, W.J., “Detector Siting and Sensitivity Requirements for Dwellings – Phase 2.” 

IIT Research Institute Report prepared for Natl. Bur. Stand. (U.S.), NBS GCR 77-82 (1977) 

2
8. Was there a one-hour “average” time to warn of a smoldering fire when using ionization 

detectors in the Phase I tests? Was this based on “prematurely terminated tests”? If not, state 

why that is not the case? 

As stated above, the average time to first alarm was 2489 s ± 1324 s for ionization alarms. No tests 

were prematurely terminated since in every case (except the three electrical fires mentioned above) 

test termination did not occur until tenability had been exceeded. For the smoldering tests, the average 

time from first alarm to the onset of untenable conditions was 1926 s ± 1454 s for the ionization 

alarms. Note that many of the smoke detectors (ionization and photoelectric) were located well away 

from the origin of the fire and not in the path of egress. For those tests where not all these alarms 

activated prior to test termination (8 of 23 smoldering tests for the ionization detectors and 2 of 23 

smoldering tests for the photoelectric detectors), it is possible that the time to alarm would have 

exceeded one hour, which could increase the average time to warn of a smoldering fire. 

9. Did the Phase I report find that “photoelectric type detectors seem to respond better to the 

smoldering type fires and the ionization detectors seem to respond slightly better to the flaming 

fires”? What was the technical basis for this finding? 

Yes, at any given detector location, most photoelectric detectors responded sooner than most 

ionization detectors to smoldering fires and most ionization detectors responded sooner than most 

photoelectric detectors for flaming fires. On average, the first ionization alarm responded 147 s ± 92 s 

before the first photoelectric alarm in the flaming fires (in all cases, an ionization alarm responded 

before a photoelectric alarm). On average the first photoelectric alarm responded 562 s ± 800 s 

before the first ionization alarm in the smoldering fires (in 20 of the 23 smoldering tests, photoelectric 

alarms responded before an ionization alarm). 

The technical basis for this result lies with the differences in the character of smoke from a 

smoldering versus a flaming fire, and from the different operating principles of the detectors. Both 

ionization and photoelectric sensors detect particles. Photoelectric technology is sensitive to the mass 

density of particles of a diameter of the same order as the wavelength of the light source in its sensor. 

Ionization technology is sensitive to the number density of particles and so responds to smaller 

particle diameters present in greater numbers. Smoke from a smoldering fire produces relatively 

fewer particles and the particles are relatively larger, as compared to particles from a flaming fire 

which are much more numerous though generally smaller. 

10. Did the Phase I report conclude that “In general, all detectors responded well to fires” and “A 

residential smoke detector of either the ionization or photoelectric types with small lag time 

would provide more than adequate life saving potential under most real residential fire 

conditions when properly installed”? What was the technical basis for this conclusion? 

Yes. For the “every level” and “every room” criteria, escape times provided by both smoke sensing 

technologies (ionization and photoelectric) were positive for nearly all tests. In the end, NIST (NBS) 

pointed out that the adequacy of the time provided would depend on the time needed by a specific 

group of people in a specific household. The report presents escape times in a probability plot 

(percent of the tests in which a specific escape time was provided for a specific installation location, 

sensor type, or sensitivity). An independent analysis of the Phase I data by a State Fire Protection 

3
Board considering detector legislation selected a 3 min (minimum) escape time criterion3

. On this 

basis they concluded that every level smoke detectors (either ionization or photoelectric) provided 3 

min of escape time in 89 % of the tests (average escape time +18.6 min), a single smoke detector 

outside the bedroom provided 3 min of escape time in 35 % of the tests (average escape time +4.5 

min), a rate-of-rise heat detector in every room provided 3 min of escape time in 19 % of the tests 

(average escape time -0.6 min) and a fixed temperature heat detector in every room provided 3 min of 

escape time in 11 % of the tests (average escape time -2.2 min). 

11. What criteria did the Phase I study use to determine “life saving potential”? How were the 

criteria judged to be “more than adequate”? 

“Life saving potential” was judged by the time for escape provided in advance of any untenable 

condition appearing anywhere along the “primary escape path,” which was defined as the path from 

any room to one of the doors to the exterior. While building regulations require a second means of 

egress from any residential room (normally a window) the researchers did not judge escape out a 

window as a success. The tenability criteria selected were the occurrence of a smoke optical density 

of 0.07 per foot, a temperature of 150 °F, or a CO concentration of 0.04 % averaged over one hour. 

The substantiation for these limits including literature references is provided in Appendix D of the 

reports. For CO, the report observes that the available data is all for exposure to a constant CO level 

for a given time and the observed CO levels in the tests rose almost linearly with time. This is the 

basis for using a (one hour) time averaged concentration. Smoke was always the first tenability limit 

exceeded (in two tests the CO limit was approached but not exceeded before the smoke limit was 

reached.) All three criteria were considered conservative, based on the literature cited in the report. 

This is especially true of smoke since it only relates to loss of visibility and because escape paths in 

homes are generally short and are generally familiar to the occupants. 

12. What criteria did the Phase I study use to determine “most real residential fire conditions”? 

The tests involved real detectors available for purchase installed in the locations cited in the national 

standard in real homes in which real contents were ignited in ways that are observed in data collected 

in real fires. On this basis the researchers felt that the tests represented real fire scenarios to the 

maximum extent possible. 

13. Did the report(s) from the Indiana Dunes Tests find or conclude that the smoke detector 

(ionization or photoelectric) is “the ONLY type of detector” capable of providing adequate 

warning of a flaming fire? If so, state the technical basis for this finding or conclusion. 

The report did not state that “the ONLY type of detector” capable of providing adequate warning of a 

flaming fire is the smoke detector (ionization or photoelectric). It presents escape times in a 

probability plot (percent of the tests in which a specific escape time was provided for a specific 

installation location, sensor type, or sensitivity). An independent analysis of the Phase I data by a 

State Fire Protection Board considering detector legislation selected a 3 min (minimum) escape time 

criterion. On this basis they concluded that every level smoke detectors (either ionization or 

photoelectric) provided 3 min of escape time in 89 % of the tests (average escape time +18.6 min), a 

 

3

 Wilson, R., Computer Analysis of Data on Fire Detectors Available for Purchase in Massachusetts, Data from the 

National Bureau of Standards Indiana Dunes Tests Record, Contract No. 4-36092, Massachusetts Fire Prevention 

Fire Protection Board, January 1976. 

4
single smoke detector outside the bedroom provided 3 min of escape time in 35 % of the tests 

(average escape time +4.5 min), a rate-of-rise heat detector in every room provided 3 min of escape 

time in 19 % of the tests (average escape time -0.6 min) and a fixed temperature heat detector in 

every room provided 3 min of escape time in 11 % of the tests (average escape time -2.2 min). 

14. Is the heat detector (mechanical or electrical) used as the trigger for fire sprinkler systems? 

Above what average temperature (°F) do the ideal and typical detectors trigger? 

Heat detectors are not typically used to trigger fire sprinkler systems. While there have been attempts 

at commercialization of such systems, they have not proved viable. Rather, they are most typically 

separate systems. Heat detectors are most often rated to operate at fixed temperatures from 135 °F to 

175 °F. Some fire sprinklers utilize heat sensors of a type similar to those used in the mechanical heat 

detectors used in Phase II (eutectic solder). The most common fire sprinklers have a rated activation 

temperature of 165 °F with some residential use sprinklers rated as low as 135 °F. 

In Phase I the response of fixed temperature heat detectors was estimated from thermocouples at 

temperatures of 135 °F and 150 °F. Actual rate-of-rise heat detectors were present beginning with 

test 14. In Phase II actual (fixed temperature) mechanical heat detectors with activation temperatures 

of 135 °F were installed in the fire room through test 63 at which time all devices supplied by the 

manufacturers were expended. 

15. How does the heat detector (either type) in a fire sprinkler system perform in the two major 

types of fires (smoldering fires and flaming fires)? Is the heat detector reliable for this 

application? 

Heat detectors are not typically used to trigger fire sprinkler systems. Rather, they are most typically 

separate systems. Heat detectors are most often rated to operate at fixed temperatures from 135 °F to 

175 °F. Some fire sprinklers utilize heat sensors of a type similar to those used in the mechanical heat 

detectors used in Phase II (eutectic solder). A heat detector or a fire sprinkler cannot detect a 

smoldering fire while it is smoldering since it does not raise the room temperature appreciably. The 

purpose of residential fire sprinklers is distinctly different from residential smoke alarms however. 

From the National Fire Protection Association standard, NFPA 13D4

, these sprinklers are “expected 

to prevent flashover (total involvement) in the room of fire origin,” thus suppressing the fire and 

limiting the spread of fire and toxic conditions in rooms remote from the fire. 

Activation of heat detectors or fire sprinklers is often significantly later in the fire than smoke alarms. 

In the Phase I tests, heat detector activation was well after the time to untenable conditions in all fires 

and after the recorded transition to flaming in all but one of the smoldering tests. Neither the Phase I 

nor Phase II tests included residential fire sprinklers since these were not developed until several 

years later. There were rare residential sprinkler systems that utilized commercial sprinkler heads, but 

these did not represent a significant force in the market. In the NIST 2004 study5

 so-called “tell tale” 

sprinklers (commercially available fire sprinklers typical of those installed in residential systems but 

not connected to a water supply) with a temperature rating of 155 °F were installed in the fire room 

 

4

 “NFPA 13D, Installation of Sprinkler Systems in One- and Two-Family Dwellings and Manufactured Homes, 

2007 Edition,” Volume 2 of the National Fire Codes, Natl. Fire Protection Association, Quincy MA 2007. 

5

 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) 

5
for all tests. Activation times recorded for flaming fires ranged between 126 s and 247 s, and for 

smoldering fires they did not activate until after transition to flaming. Specific times are documented 

in the report. 

16. Did the Dunes study find that the heat detector (mechanical or electrical) is not a reliable alarm 

device to warn of a flaming fire? What was the technical basis for this finding? 

The tests showed that heat detectors in every room were far less effective than smoke alarms on every 

level in the majority of tests (see answer to question 10). 

17. Did the Phase I study find that the smoldering fires involving heat detectors did not produce 

“any significant heat”? If so, why was that the case? Is this representative of typical fires in a 

residential dwelling to evaluate detector performance? 

Smoldering fires by their nature do not produce any significant heat as smoldering is a very slow 

combustion process. At the time, NFPA data showed smoldering fires might make up 2/3 of all 

(fatal) residential fire starts as estimated from fire reports where the ignition source was smoking 

materials and the first item ignited was upholstered furniture or mattresses. For this reason the test 

scenarios in the 1975/76 tests were approximately 2/3 smoldering ignitions and 1/3 flaming ignitions. 

18. Did the Phase I study find that the flaming fires involving heat detectors did not “raise the 

ceiling temperatures in the fire rooms to the modest 135 °F which was the set temperature[s] for 

the heat detectors”? If so, why was that the case? Is this representative of typical fires in a 

residential dwelling to evaluate detector performance? 

An independent analysis (Patton 1992) listed 27 tests (12 in Phase I and 15 in Phase II) that reached 

or exceeded 200 °F. Some of these began as flaming and others were smoldering and transitioned to 

flaming. Several conditions must occur to raise the temperature of the air below the ceiling. First 

there must be sufficient heat release from the fuel, second the plume entrains cooler ambient air that 

reduces the temperature by mixing, and third is heat lost to the ceiling by convection (initially as 

much as 90 % of the energy in the plume is lost to the ceiling). These processes are typical of fires in 

compartments. 

19. Did the Phase II study find that in the six flaming fires involving heat detectors “the ceiling 

temperatures went above 200 °F but never above 500 °F”? If so, why was that the case? Is this 

representative of typical fires in a residential dwelling to evaluate detector performance? 

The analysis cited above (Patton 1992) indicated that all 27 tests that reached or exceeded 200 °F did 

not exceed 500 °F. This should be expected since the tenability criterion for temperature was 150 °F 

(at the 5 foot level) that was never exceeded prior to test termination based on exceeding the smoke 

criterion. Since the activation temperature of all of the heat detectors was 135 °F (150 °F for the 

thermocouple reading to simulate thermal inertia) this would have provided an opportunity for the 

heat detectors to respond had the temperature exceeded their activation temperature. 

6
20. Were any of the heat detectors (electrical or mechanical) removed from the fire rooms before 

lighting the fires in the six Phase II flaming fire tests? If so, why were they removed? 

None of the heat detectors were removed. In the Phase II tests actual mechanical heat detectors were 

used. Since these are not sold in retail stores they were obtained from the manufacturers. Each of 

two manufacturers (that utilized a horn or bell as the sounding device) provided a case of 24 units. 

One of each was included in each Phase II test starting with test 41 and ending with test 63 at which 

time all 24 samples had been used. Both devices used eutectic solder sensors which have a 

characteristic that the activation temperature can decrease if they are exposed to an elevated 

temperature without activating. Thus each unit could be used in only one test even if it did not 

activate. 

21. In how many of the six Phase II flaming fire tests were there no electrical heat detectors? 

Mechanical detectors? If so, what was the technical basis for this condition(s)? 

Officially there were 15 flaming tests in Phase II (44, 45, 49, 50, 58, 59, 63, 64, 66, 68, 69, 70, 71, 74, 

and 76). The mechanical heat detectors were present in 7 (44, 45, 49, 50, 58, 59, and 63). Electrical 

heat detectors were not used in Phase II since the mechanical devices were present. Estimated 

response of heat detectors from thermocouple readings was not made although a thermocouple was 

present in the room of fire origin and the data were published in the report. 

22. How many smoldering fire tests were conducted in the Phase I test series? 

23 of 37 tests (not counting the three electrical fires) in Phase I were smoldering. 

23. Were nine ionization detectors installed in each of the Phase I smoldering fire tests? If not, 

state how many were installed in each of the tests. 

The smoke alarms used in the Phase I tests were arranged on one large board and two “half” boards. 

The large board had 4 ionization, 4 photoelectric, and a dual gate (an older technology that used an 

early ionization detector paired with an electrical resistance circuit) detector. One half board had 3 

ionization and 2 photoelectric detectors, and the other half board had 2 ionization, 2 photoelectric, 

and a dual gate detector. The alarm boards were located in the first and second floor hallways 

(outside sleeping rooms) and one of the half boards was moved to the top of the basement stairs for 

fires in the basement. The half boards were constructed to assess any differences between wall 

mounting and ceiling mounting (one half board on the wall and one on the ceiling at the same 

location). For some first floor fires the half boards were in the first floor hallway and for some on the 

second floor hallway to examine wall vs. ceiling at that location. The large board was used at the 

other hall location. In some tests the half boards were switched (wall for ceiling) to eliminate 

systematic errors. These detectors; 9 ionization, 8 photoelectric, and two dual gate (plus heat 

detectors in the fire room), were present in all tests. 

Not all detectors were operational in all tests. For tests 28 through 40, one of the detector locations 

(clock No. 25, a photoelectric detector) was taken out of service due to physical breakage of the 

detector. 

7
24. Did the Phase I study find that: 

(a) not a single ionization detector sounded within five minutes of the initiation of the Phase I 

smoldering fire tests? 

(b) Only one time did an ionization detector sound within 10 minutes of the initiation of the 

Phase I smoldering fire tests? 

(c) Only 26 times (or about 16 percent of possible times) did the ionization detector sound 

within the first 30 minutes of the initiation of the Phase I smoldering fire tests? 

(d) The ionization detector never sounded 44 times at the time of the termination of the Phase I 

smoldering fire tests? 

State the technical basis for each of the above with the responses. 

(a) Neither ionization detectors nor photoelectric detectors alarmed within the first five minutes in 

any test. The shortest alarm times in the smoldering tests were 473 s in test 6 for an ionization 

and 468 s in test 6 for a photoelectric alarm 

(b) Only tests 5 and 6 had alarm times of less than 10 min for both ionization detectors and 

photoelectric detectors. 

(c) A total of 21 ionization detectors of a possible 207 alarmed within the first 30 min of test 

initiation. A total of 30 photoelectric detectors of a possible 166 alarmed within the first 30 

minutes of test initiation. 

(d) A total of 21 ionization detectors out of a possible 207 did not alarm prior to test termination. A 

total of 5 photoelectric alarms out of a possible 166 did not alarm prior to test termination in all 

the tests. 

It is important to note that the number of detectors that alarm within a particular time may not be that 

valuable. More important is a comparison of alarm time to the time to untenable conditions. In all 

tests, multiple detectors of both types provide alarms well before untenable conditions. For the 

smoldering tests, the average time from first alarm to the onset of untenable conditions was 1926 s ±

1454 s for the ionization alarms and 2511 s ± 1667 s for the photoelectric alarms. 

25. Did the Phase I study find that the ionization detector did not respond to the smoldering fires in 

the Phase I tests so long as they remained smoldering fires? If so, why was that the case? 

No, the Phase I study did not find this. In all tests, at least one ionization alarm responded well before 

the transition to flaming. 

26. Did the Phase I study find that “long before an hour has passed (average time to operate) 

potentially deadly levels of carbon monoxide and other toxic gases were created” in the 

smoldering fire tests? Is this statement a reasonable representation of knowledge among 

experts at the time the tests were conducted? Today? 

The tenability limit used for the Phase I study for CO was a time averaged (one hour) concentration 

of 400 ppm (based on a review of literature available at the time). This limit is roughly consistent 

with limits in today’s literature6

. In only 2 of the 40 experiments carbon monoxide concentrations 

 

6

 Most typically, tenability is judged using the ISO Technical Specification 13571. For CO, this is a fractional 

effective dose calculation, FED, of FED = ([CO] 35000 Δt t1

t 2

∑ ) where the CO concentration, [CO] is in ppm and the 

time, t, is in min. Incapacitation is considered to have occurred at FED values between 0.3 and 0.5; lethality at a 

value of 1. The 400 ppm one hour average limit is equivalent to an FED of 0.68. 

8
approached but did not exceed the time averaged (one hour) concentration of 400 ppm before the 

optical density reached 0.07 per ft. Nevertheless, the occurrence of the critical optical density 

preceded the occurrence of critical carbon monoxide levels in all of the experiments. 

Today, the guidance regarding CO toxicity is more conservative, with an accepted tenability limit of 

175 ppm for one hour for the most sensitive population. We are reexamining the data from Phase I 

and the more recent NIST home smoke alarm study5

 to determine how often this limit may have been 

exceeded inside the room of origin and outside the room of origin along the egress path prior to 

alarm. 

27. Did the Phase I study find that the ionization detector sounded only after the smoldering fire 

was heating up and converting, or had already converted, to a flaming (hot) fire? If not, why 

was this not the case? 

No, in all tests, at least one alarm of each type, ionization and photoelectric, responded well before 

the transition to flaming. On average, the time from first alarm to the transition to flaming was 1962 s 

± 1678 s for ionization alarms and 2445 s ± 1967 s for photoelectric alarms. In all cases, the time to 

first alarm preceded the transition to flaming in the smoldering tests. The minimum time from first 

alarm to the transition to flaming was 40 s for the ionization alarms and 41 s for the photoelectric 

alarms (both in test JR-22). 

28. Is the type of smoke created in the standard smoldering fire test developed by Underwriters 

Laboratories (i.e., where Ponderosa Pine sticks on a hot plate are raised closed to their autoignition temperature of about 700 °F) similar to the type of smoke created by a burning 

cigarette on upholstered furniture or bedding? If not, state the major differences. 

At the time of publication of the study (1975/76) there was no smoldering test in the UL standard 

(UL217). The Dunes tests clearly showed the importance of response to such fires. UL conducted a 

study to develop a suitable test. They smoldered cotton-stuffed mattresses similar to those used in the 

Dunes tests in their fire test room to develop smoke vs. time profiles. They then experimented with 

different techniques and materials to generate the observed conditions in a reproducible manner. 

Heating ponderosa pine wood strips on a temperature-programmed hotplate obtained the desired 

result7

. Recently, UL has conducted tests following the NIST Home Smoke Alarm Project and 

concluded that smoldering and flaming polyurethane foam smoke has characteristics different from 

the smoke in their standard test fires8

. UL is now investigating the possibility of adding additional 

standard tests using polyurethane foam. 

 

7

 Harpe, S.W., and Christian, W.J., “Development of a Smoldering-Fire Test for Household Smoke Detectors,” Fire 

Journal, Vol. 73, No. 3, 1979 

8

 Smoke Characterization Project Final Report, Underwriters Laboratories Inc. 333 Pfingsten Road, Northbrook, IL 

60062, April 24, 2007) 

9
29. Does the industry standard used to certify fire alarms in flaming fires allow the smoke 

obscuration to go as high as 37 percent? If so, why is that the case when the International 

Association of Fire Chiefs set 4 percent smoke obscuration as the maximum for travel along an 

exit path on the basis of the live fire tests conducted by the Los Angeles Fire Department in the 

aftermath of the December 1958 school fire? 

The UL fire tests are not tests of sensitivity but rather reproduce a range of conditions observed in 

real fires. The paper fire produces a high smoke density (about 30 % per foot at the ceiling and 37 % 

at the wall mounting location) for a short duration (about 10 s). Detecting this fire assures that the 

device will not ignore a high initial pulse of smoke due to internal time delays. The basic sensitivity 

(alarm threshold) is measured in the sensitivity test in a test compartment where the alarm threshold 

must be within the range of sensitivities that passed the fire tests and that which passed the false 

alarm tests, but not outside the range on 0.5 to 4.0 percent per foot of grey smoke (black smoke 

sensitivity can go to 10% but the mass density ratio of black to grey smoke is about 5:1, meaning that 

the mass density of 10 % black smoke is equal to that of 2 % grey smoke). 

30. Why were detectors with a sensitivity of 1 %/ft used in Phase I and Phase II testing when 

typical sensitivities of smoke alarms used by consumers are 2 %/ft? 

The studies used detectors that covered a range of sensitivities, nominally 1 %/ft or 2 %/ft in order to 

evaluate the life safety impact of detector sensitivity. Actual sensitivities were measured and reported 

in the appendices of each report. 

31. Has any study conducted by NIST (or the predecessor National Bureau of Standards) 

concluded that a combination of both the ionization and the photoelectric smoke detector 

provides more than adequate life saving potential under most real residential fire conditions 

when properly installed? If so, what is the technical basis for this conclusion? 

The subject of combination (ion and photo) detectors was never raised in the 1975/76 test reports. 

However in the 1979 report (NBSIR 79-1915) on the results of similar tests conducted in mobile 

(manufactured) homes, conclusion 6 stated, 

“Based on the series of fire tests conducted in this program and the evaluation criteria discussed in 

section 2.3, a properly functioning smoke detector of either the ionization or photoelectric type 

should provide an alarm in sufficient time to permit an alert and mobile occupant to escape from the 

mobile home. While either type detector provided enough time for escape, the use of a detector which 

combines both ionization and photoelectric sensors in the same unit (or one of each) could provide 

significant improvement in alerting the occupants of a mobile home to either a flaming or smoldering 

fire.” 

32. Has any publication or presentation by a NIST researcher suggested that a “combination of 

both the ionization device and the photoelectric smoke detector” may provide more than 

adequate life saving potential under most real residential fire conditions when properly 

installed? If so, provide complete citations for all publications that make this suggestion. 

Yes. See: Bukowski, R.W., Investigation of the Effects of Heating and Air Conditioning on the 

Performance of Smoke Detectors in Mobile Homes, NBSIR 79-1915, Nat Bur Stand, Gaithersburg, 

MD, 1979. 

10
Also, there is a fact sheet jointly produced by the USFA, HUD, and NIST which states: 

“There are two kinds of smoke alarms -- ionization and photoelectric. The ionization 

activate quicker to fast, flaming fires and the photoelectric are quicker for slow, 

smoldering fires. Either one will provide enough time to get out, but having a mix of the 

two types is a good idea. Models with both sensors are better than single sensor units, but 

of course they cost more.” 

This fact sheet can be found at: http://www.fire.nist.gov/factsheets/Smoke&CO.pdf and 

http://www.usfa.dhs.gov/downloads/pdf/smokeco-mh.pdf 

33. Does the ionization detector provide more than adequate life saving potential under most real 

residential fire conditions when properly installed? If not, cite specific limitations of the 

ionization detector for flaming fires and for smoldering fires. 

In the 1975/76 tests the answer is clearly yes. In the 2000 tests the performance of both types is often 

close to the margin, possibly due to the much more rapid development of fires in modern furniture as 

opposed to the detectors. If the furniture produces conditions that exceed tenability limits in a much 

shorter time, then detectors that meet the current UL test standards may be inadequate to provide 

sufficient warning to occupants intimate with the fire or who need 3 minutes or more to escape. 

For example, victims located in the room of fire origin are considered intimate with the fire and may 

not have adequate warning based on the established tenability criteria for smoldering or flaming fires, 

especially if the room of fire origin does not contain a smoke alarm. Impaired occupants, small 

children, and mobility-limited occupants incapable of self-rescue may be unable to evacuate safely. 

In summary, the NIST studies conclude that both ionization and photoelectric alarms provide enough 

time to save lives for most of the population under many fire scenarios; however, ionization alarms 

may not always alarm even when a room is filled with smoke from a smoldering fire, exposing the 

most sensitive populations with mobility limitations to an undetermined risk. Photoelectric detectors 

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

smoldering fire can take a longer period to become dangerous. Ionization detectors can provide a little 

more time than photoelectric detectors in a flaming fire; in this case there can be little time to spare. 

Changes in furnishing materials and construction over the past decades have reduced the time 

available for safe egress in any fire. NIST is currently conducting research to assess whether or not 

modifications may be needed in the standard test method for certifying residential smoke alarms to 

accommodate the changing threat. 

34. Does the photoelectric detector provide more than adequate life saving potential under most 

real residential fire conditions when properly installed? If not, cite specific limitations of the 

photoelectric detector for flaming fires and for smoldering fires. 

If the room of fire origin is to be explicitly addressed in the adequacy of life saving potential then, 

photoelectric detectors would most likely be deemed adequate for smoldering fires. For flaming fires, 

the photoelectric detector would tend to alarm after an ionization alarm, so the rate of fire growth 

would determine the outcome. 

11
In summary, the NIST studies conclude that both ionization and photoelectric alarms provide enough 

time to save lives for most of the population under many fire scenarios; however, ionization alarms 

may not always alarm even when a room is filled with smoke from a smoldering fire, exposing the 

most sensitive populations with mobility limitations to an undetermined risk. Photoelectric detectors 

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

smoldering fire can take a longer period to become dangerous. Ionization detectors can provide a little 

more time than photoelectric detectors in a flaming fire; in this case there can be little time to spare. 

Changes in furnishing materials and construction over the past decades have reduced the time 

available for safe egress in any fire. NIST is currently conducting research to assess whether or not 

modifications may be needed in the standard test method for certifying residential smoke alarms to 

accommodate the changing threat. 

35. Why is the ionization detector, which senses the products of combustion and not the large 

smoke particles caused by a fire, called a “smoke detector”? If the ionization detector does not 

sense smoke, is it still technically appropriate to call it a smoke detector? Why? 

All US Codes and Standards consider both ionization and photoelectric detectors as smoke sensing 

fire detectors responding to particles produced by combustion. The official NFPA definition of smoke 

(found in NFPA 90A) is: 

Smoke. The airborne solid and liquid particles and gases evolved when a material 

undergoes pyrolysis or combustion, together with the quantity of air that is entrained or 

otherwise mixed into the mass. 

In particular, the ionization alarm senses smoke particles of all sizes that enter its sensing chamber. 

Its sensitivity is linear with the particle diameter. The photoelectric, light-scattering, detector’s 

sensitivity is proportional to the smoke particle diameter raised an exponent that decreases from a 

power of 6 to a power of two as the diameter increases from smallest to largest sizes9

.

36. Did the results from one or more studies conducted by NIST indicate that “more than 50 

percent of the time” a new ionization detector with a fresh battery will fail to warn in a timely 

manner? 

No. 

 

9

 Mulholland, G. W., and Liu, B. Y. H. “Response of Smoke Detctors to Monodisperse Aerosols,” Journal of 

Research of the National Bureau of Standards, Vol. 85, No. 3, 223-238, May/June 1980. 

12