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NIST Technical Note 1837

Improving Smoke Alarm 

Performance – Justification for 

New Smoldering and Flaming Test 

Performance Criteria

Thomas G. Cleary

http://dx.doi.org/10.6028/NIST.TN.1837
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NIST Technical Note 1837

Improving Smoke Alarm 

Performance – Justification for 

New Smoldering and Flaming Test 

Performance Criteria

Thomas G. Cleary

Fire Research Division

Engineering Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1837

July 2014

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Acting Director 
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Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1837

Natl. Inst. Stand. Technol. Tech. Note 1837, 27 pages (July 2014) 

CODEN: NTNOEF

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iii

ABSTRACT

Although smoke alarms provide a substantial level of safety to households, reducing the risk of 

dying in reported fires by 50 % in homes with working smoke alarms, improved smoke alarm 

performance could further reduce fire risk. Earlier alarm activation by increasing the sensitivity 

of smoke alarms to risk-significant fire scenarios is one approach to improving performance. 

Flaming and smoldering upholstered furniture fires have been identified as risk-significant 

scenarios that are not adequately addressed in current standards. New polyurethane foam 

smoldering and flaming fire tests have been suggested to address this shortcoming, but need 

scientifically sound performance criteria to be complete. 

An analysis methodology, based on the available safe egress time/required safe egress time 

(ASET/RSET) concept, is used to estimate the probability of escape given smoke alarm activation 

at specific smoke concentrations in flaming and smoldering polyurethane foam chair mock-up 

fire experiments to provide guidance in selecting new smoldering and flaming fire test 

performance criteria. The tenability limits were defined by a fractional effective dose value of 

0.3 for toxic gas and heat exposure, and two smoke optical density limits used in previous 

studies, 0.25 m-1 and 0.43 m-1, were considered. Analysis assumptions and limitations included:

 interconnected smoke alarms that alert occupants regardless of initial fire location, 

 occupant pre-movement time treated as a distribution for distinct populations,

 travel speed as a function of smoke density, 

 occupants traversing a range of equally frequent pre-determined egress routes,

 considering only one apartment-sized residential space, 

 one location for the responsive smoke alarm, and

 three flaming and three smoldering scenarios, and a total of 18 full-scale tests.

Results are presented to provide guidance in selecting performance criteria for new smoldering 

and flaming polyurethane foam fire tests proposed for ANSI/UL 217 and ANSI/UL 268. This 

study provides a rationale for how to strengthen the requirements in a manner that considers 

commensurate improvement for flaming and smoldering alarm performance. Ultimately, the 

Standards Technical panel for ANSI/UL 217 and ANSI/UL 268, other regulatory bodies and/or 

standards development organizations need to make a judgment on the addition of any new 

tests or performance requirements. 
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iv

Table of Contents

ABSTRACT ................................................................................................iii

1 INTRODUCTION...................................................................................1

2 DESCRIPTION OF EXPERIMENTS ..........................................................4

3 ASET/RSET MODEL ............................................................................11

3.1 Egress Scenarios............................................................................12

3.2 Tenability Criteria..........................................................................12

3.3 Occupant Characteristics...............................................................14

4 MODEL IMPLEMENTATION................................................................16

5 SIMULATION RESULTS.......................................................................17

6 CONCLUSIONS...................................................................................25

7 REFERENCES......................................................................................26
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1

1 INTRODUCTION

In the United States, NFPA 72, The National Fire Alarm and Signaling Code [1] requires that all 

installed smoke alarms and detectors be assessed for compliance to the minimum performance 

requirements as specified in ANSI/UL 217, Single and Multiple Station Smoke Alarms [2] and 

ANSI/UL 268, Smoke Detectors for Fire Alarm Systems [3]. 

Analysis of US fire statistics for home structure fires from 2007-2011 shows the risk of dying in 

reported fires is less than 50 % lower in homes with working smoke alarms [4]. This indicates the 

benefits of working smoke alarms. Even with this benefit, the national fire statistics show that 

between 2007 and 2011, smoke alarms were deemed to have operated in 37 % of fatal home 

structure fires with installed working smoke alarms, and in those cases, analysis of victim 

activities showed that at the time of the fatal injury, one-third of victims were attempting to 

escape or rescue others [4]. This begs the question: what can be done to improve smoke alarm 

performance?

Several factors influence the risk of dying in fires when working smoke alarms are present. 

Some examples include victims that do not respond to an operating smoke alarm while sleeping 

or incapacitated, and very rapid fire growth. Interconnected smoke alarms that activate all 

smoke alarms when any one smoke alarm senses fire conditions improves alerting, especially for 

sleeping occupants when interconnected alarms are located in all sleeping areas. Additionally, 

smoke alarms or notification devices with the low frequency 520 Hz alarm signal output provide 

improved alerting for occupants with mild to severe hearing loss. The current NFPA 72 Code 

requires interconnected alarms and alarm coverage on every level and in all sleeping areas for 

household fire alarms [1]. The number of homes protected at the level of the current Code, 

however, is uncertain. A 2010 survey found that approximately 40 % of households had smoke 

alarms in all bedrooms, but only 25 % of households had interconnected smoke alarms [4]. 

Some victims may be unable to respond due to alcohol or drug impairment, and other victims 

may be disabled and immobile, thus incapable of self-rescue. Between 2007 and 2011, in cases 

in which smoke alarms were present and operated, it was estimated that 11 % of fatal victims 

were possibly alcohol or drug impaired and 17 % were possibly physically or mentally disabled 

[4]. Finally, because of rapid fire growth and spread in contemporary furnishings containing 

flexible polyurethane foams, occupants may not have sufficient egress time even when alerted 

by a smoke alarm [5]. 

The National Institute of Standards and Technology (NIST) has conducted research on smoke 

alarm performance, including the Home Smoke Alarm Study [5] and the Smoke Alarm Sensitivity 

Study [6] that focused on fires involving modern upholstered furniture and mattress materials 

containing flexible polyurethane foams. Smoke alarm technologies examined included 

contemporary photoelectric, ionization, and dual-sensor combination photoelectric/ionization 

alarms. Both studies showed that properly installed ionization or photoelectric alarms provide 

enough time for most occupants to successfully egress for many fire scenarios. It was observed 

in both studies, however, that ionization alarms tended to react much slower to slowly growing 
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2

extended smoldering fires than photoelectric alarms, and that photoelectric alarms tended to 

react somewhat slower to quickly growing flaming fires than ionization alarms. Analysis of 

initially flaming and initially smoldering fires in full-scale experiments revealed hypothetical 

situations involving both types of photoelectric and ionization alarms for which not all occupants 

would have sufficient time to escape before experiencing thick smoke and potentially 

succumbing to toxic gases and/or heat exposure. Thus, depending on the fire scenario, a 

particular sensing technology and/or increased detection sensitivity could provide additional 

time for escape, which may be necessary for some occupants, especially slower population 

groups (elderly or mobility impaired) and those with some level of cognitive impairment. 

Analysis of the performance of smoke alarms in full-scale experiments showed that dual sensor 

photoelectric/ionization alarms with equivalent or more sensitive settings than individual 

photoelectric or ionization alarms performed better over a range of flaming and smoldering fire 

scenarios [7, 8].

In an effort to increase available escape time, consideration of the national fire statistics and 

smoke alarm research [4-8] has led to the recommendation that consumers install both 

photoelectric and ionization alarms, or combination alarms by several fire safety and research 

organizations including the US Fire Administration, the US Consumer Products Safety 

Commission, the National Institute of Standards and Technology, the National Fire Protection 

Association, Underwriters Laboratories, Inc., the International Association of Fire Chiefs, and the 

National Association of State Fire Marshals. This recommendation is also made in the 2013 

Edition of NFPA 72: ”The use of both technologies generally offers the advantage of providing a 

faster response to both flaming and smoldering fires, and is recommended for those who desire 

a higher level of protection than the minimum requirements of this Code” [1]. The concern with 

this recommendation is the identification of specific technologies as surrogates for the desired 

performance level. The desired solution is to identify a minimum set of performance 

requirements for ANSI/UL 217 and ANSI/UL 268 that are technology-independent and increase 

escape times, yet enable innovation in smoke alarm design including use of new and emerging 

technologies. 

It can be concluded prima facie that an improvement in smoke alarm fire test performance 

requirements necessitates earlier alarm activation and warning for both initially smoldering and 

initially flaming fires. Simply requiring smoke alarms to activate at a lower smoke concentration 

in only an initially smoldering test fire or an initially flaming test fire could bias the requirements 

to favor one stand-alone technology (photoelectric or ionization) over the other. The preferred 

situation, consistent with the recommendations of the organizations listed above, would be 

performance requirements that challenge the current generation of discrete photoelectric and 

discrete ionization alarms, leading to improved detection performance for both flaming and 

smoldering fires. 

An increase in the sensitivity of smoke alarms designed to account for risk-significant fire 

scenarios (that is, fire scenarios for which the consequences of the fire are serious and mostlikely life-threatening, and the likelihood is deemed high relative to other serious fires) not 
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3

covered in the current ANSI/UL 217 and ANSI/UL 268 fire tests would provide earlier alarm 

activation and increased time to escape for those occupants alerted by such alarms. Additional 

fire tests tailored to replicate the early stages of risk-significant fire scenarios, in conjunction 

with detection sensitivity thresholds that increase egress time and therefore, the likelihood of 

escape, would refine the ANSI/UL 217 and ANSI/UL 268 Standard’s requirements towards 

improved smoke alarm performance. 

Fire loss statistics identify fires involving upholstered furniture and mattresses and bedding as 

risk-significant scenarios. Between 2007 and 2011, upholstered furniture and mattresses and 

bedding were the two leading categories of items first ignited in home structure fires with 

fatalities, totaling 31 % of the average 2570 deaths per year [9]. Upholstered furniture

accounted for 2 % of the reported fires, but 18 % of the fire deaths, and mattresses and bedding

accounted for 3 % of the reported fires, but 13 % of the fire deaths. Between 2005 and 2009, 

smoking materials were listed as the leading cause of upholstered furniture, and mattresses and 

bedding fire deaths, accounting for 58 % and 51 % respectively, while candles, matches and 

lighters accounted for about 12 % of both upholstered furniture, and mattresses and bedding

fire deaths [10, 11]. 

A common underlying component of contemporary upholstered furniture and mattresses is 

flexible polyurethane foam. It can rapidly burn from an initial flaming ignition source or smolder 

for long periods of time, and then transition to a flaming fire. This material is not currently 

represented in the smoldering or flaming fire test protocols of ANSI/UL 217 or ANSI/UL 268. 

Requiring detection of flaming and smoldering polyurethane foam materials representing the 

earliest stages of upholstered furniture fires may result in earlier alarm activations, which should 

provide additional time for escape or rescue. This might be achieved by revising the minimum 

smoke alarm/detector performance requirements to address flaming and smoldering 

polyurethane foam in ANSI/UL 217 and ANSI/UL 268. A task group was formed to make 

recommendations to the Standards Technical Panel of ANSI/UL 217 and ANSI/UL 268, regarding 

the addition of fire tests with the goal of increasing available egress time for non-specific fires 

(both flaming and smoldering fires) by “expanding alarm responsiveness”. Previous research, 

detailed in the UL-Fire Protection Research Foundation’s Smoke Characterization Project, 

identified differences in the smoke properties of flaming and smoldering fire smokes from 

polyurethane foam as compared to flaming and smoldering fire smokes from the current UL test 

fires [12]. Based on the fact that polyurethane foam is a major fuel component in risk-significant 

fire scenarios, and that smokes from polyurethane foam combustion differ from the smokes in 

the current UL fire tests, the task group chose to focus on developing flaming and smoldering 

polyurethane foam tests in the UL fire test room to complement the existing tests. 

Suitable full-scale fire experiments can be used to guide scientifically sound selection of the 

performance criteria for new smoke sources. Such experiments include the Smoke Alarm 

Sensitivity Study, a series of 24 full-scale experiments NIST 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 
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4

scenario, and room configuration, on smoke alarm performance [6]. The experimental results 

were used in an egress analysis to examine the effects of pre-movement time, reduced travel 

speeds through smoke, and smoke optical density limits on occupant survivability given 

different smoke alarm installations and multiple egress paths [8]. The data from those 

experiments is used here in a similar available safe egress time/required safe egress time 

(ASET/RSET) analysis to identify ceiling smoke concentration ranges, for which alarm activations 

would produce a high percentage of successful escape outcomes in some very challenging 

smoldering and flaming fire egress scenarios. The objective of this analysis is to develop a 

framework to guide the performance criteria specifications for new flaming and smoldering 

polyurethane foam fire test standards. 

2 DESCRIPTION OF EXPERIMENTS

The fire experiments were conducted in a building mock-up, designed to represent a portion of 

an apartment or small home, constructed inside the National Fire Research Laboratory at NIST 

[see Reference 6 for more details]. Figure 1 is a schematic of the structure. It was wood-framed 

with interior walls and ceilings covered with gypsum wall board, which was spackled and 

painted. The structure consisted of three contiguous spaces labeled, master bedroom, kitchen, 

and living room with two hallways, and a floor to ceiling height of 2.4 m. The ceiling was 

continuous between the living room, hallways and kitchen (no headers). A door 0.9 m wide and 

2.0 m tall connected the bedroom to the adjacent hallway. The hallway to the right of the living 

room is presumed to connect to additional rooms to complete the layout. Access doors to the 

living room and the bedroom were built into the structure. 

Master 

Bedroom

(MBR)

Living Room

(LR)

Kitchen

15.8 m

4.9 m

S3

S6

S5

S4

S2 S1

TC GC TC GC

TC TC GC

HF

Chair

Mock-up

S# - Smoke alarm board

TC - Thermocouple tree

GC - Gas sampling

HF - Total Heat Flux

 - Laser extinction

Door

Chair

Mock-up

Figure 1. Schematic of the test structure. S1...S6 indicate the ceiling locations for sets of alarms. 

Gas samples were drawn at locations 1.5 m above the floor. The total heat flux gage was 

located 1.5 m above the floor and pointed toward the living room chair mock-up. The dashed 

lines represent laser beam paths located 1.5 m above the floor for extinction measurements. 

Shaded spaces were sealed off from the rest of the structure.
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5

Groups of four smoke alarms were installed on the ceiling at various fixed locations shown in 

Figure 1. Smoke alarms were pre-mounted side-by-side on a 0.3 m by 0.6 m by 6 mm thick 

paneling sheet in random order. All smoke alarms were purchased from retail establishments 

and consisted of two photoelectric alarms (P1 and P2), two ionization alarms (I1 and I2), and 

two dual alarms (D1 and D2). Smoke alarm groupings consisted of P1, I1, D1, and D2 in set 1 

and P1, P2, I1, and I2 in set 2. There were six fixed locations for smoke alarms as indicated in 

Figure 1. The alarms in Set 1 were placed at locations 1, 3, and 6, and the alarms in Set 2 alarms 

were placed at locations 2, 4, and 5. Not all locations were populated with alarms during every 

experiment.

The hallway between the master bedroom and living room contained additional light extinction, 

gas sampling, and particle sampling measurement locations near the ceiling. Figure 2 shows the 

measurement locations and the location of smoke alarm groups S3 (set 1) and S4 (set 2). In 

addition to smoke obscuration, CO and CO2 concentrations, and temperature measurements 

were made at the ceiling height (where one would most-likely place a smoke alarm) to identify 

values of these fire signatures that could provide early warning times leading to an increase in 

successful escape outcomes compared to current alarm sensitivities.

Master

Bedroom Living Room

S4 board S3 board

Door

Extinction

Beams

Aerosol 

Sampling

CO/CO2 Gas

Sampling

Light 

Fixture

Thermocouple 

Tree

3.7 m

HCN 

Sampling

2.4 m

Ceiling

Floor

Figure 2. Location of hallway measurement positions. All sampling occurred along the 

centerline of the 0.91 m wide hallway. The thermocouple tree consisted of seven 

thermocouples, spaced 0.305 m apart from each other and the ceiling, with one thermocouple 

2.5 cm below the ceiling. The measurement locations for the upper gas and aerosol sampling 

tube and the upper extinction beam was 10 cm below the ceiling, while the lower gas sampling 

tube, HCN sampling tube, and lower extinction beam was 1.5 m above the floor.

There were eight experimental configurations, each conducted three times for a total of 24 

experiments. Here, the flaming and smoldering master bedroom fire configurations with the 
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6

door to the master bedroom closed were not considered due to significant alarm activation 

delay for alarms located outside the master bedroom. 

A chair mock-up was used as the fire source for each experiment. Each mock-up was 

constructed from non-fire-retarded, flexible polyurethane foam slabs of low or high density (low 

density – 21 kg/m3{1.3 lb/ft3}, high density - 29 kg/m3{1.8 lb/ft3}) covered by a matching 

zippered seat cushion (90 cm by 70 cm by 20 cm) and seat back covers (90 cm by 50 cm by 20 

cm) of either cotton (CT) or polyester (PET) fabric. The foam cushions and fabric covers were 

obtained from retail sources. Mock-up cushion pairs weighed between 5.5 kg and 8.3 kg. The 

cushions rested on a steel frame sitting in a sheet metal pan, which in turn was supported by a 

load cell for mass loss measurements. 

One fabric type was selected that would smolder when subjected to a small ignition source like 

a cigarette. In his extensive study of cigarette ignition propensity of furniture fabrics, Hirschler 

examined smoldering ignition propensity of a set of 500 upholstery fabrics (chosen at random 

among typical upholstery fabrics) assumed to be a representative cross-section of the 

upholstery fabrics available in the early 1990s [13]. He found that of the 500 fabrics tested, only 

145 fabrics were ignitable by cigarettes, all of them predominantly (or completely) cellulosic 

(cotton, or other plant-based cellulosic fibers). Therefore, a cotton covering fabric (CT) was 

selected, reflecting the upholstered furniture market and likely to exhibit smoldering ignition 

from cigarette-like sources. 

The ignition sources in the Smoke Alarm Sensitivity Study were designed to be controlled and 

repeatable since it was important that variations in ignition strength be limited to reduce the 

uncertainty associated with the ignition process. Additionally, the sources were placed in a 

manner that could be considered representative of common ignition modes implicated in 

residential fires, which compared to the Home Smoke Alarm Study ignition sources [5] should 

produce slower growing flaming fires and longer duration smoldering before transition to 

flaming. 

For each flaming experiment, the chair seat cushion was ignited by a gas-flame ignition tube 

(similar to the flaming ignition source described in British Standard 5852 [14]) with a propane 

fuel flow of 0.75 cm3/sec. This burner tube flame is similar to a match-like small flame or candle 

flame, with a flame length of approximately 4 cm. The ignition tube flame was allowed to burn 

for two minutes before placing it next to the chair. To ignite the chair, the tube was placed near 

the front side of the seat cushion, approximately one-third down from the top of the seat and 

within 3 mm of the fabric surface. The rationale for positioning the burner tube flame near the 

upper edge of the seat cushion and away from the back cushion was to produce an initially slow 

fire growth rate. A pneumatic piston attached to a lever arm lowered the ignition tube into 

position. After 40 s (20 s for one test) of flame exposure, the arm was raised and the ignition 

flame extinguished. 
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7

A comparison of results from the two NIST studies shows that the flaming fires in the Smoke 

Alarm Sensitivity Study [15] were characterized by generally slower fire growth relative to the 

Home Smoke Alarm Study [5] experiments as indicated by the times to reach the optical density 

limit of 0.25 m-1in the room of fire origin for similar fire scenarios. For example, the average 

time to reach the optical density limit in the Home Smoke Alarm Study flaming chair 

experiments (single-story home tests 2, 10, 15, 33, and 35) was 164 s, while the average time to 

reach the optical density limit in the Smoke Alarm Sensitivity Study’s flaming chair in the living 

room experiments (polyester-covered cushion tests 4, 17, 19) was 217 s, a difference of 53 s (or 

about 30 %) on average. 

The smoldering initiation source was developed to provide a controlled low-temperature heat 

source, to induce a smolder process that looks and behaves close to a cigarette-type ignition 

event (a cigarette sized heat source inducing smoldering of a fabric capable of initiating 

smoldering and propagating the smolder front down into the polyurethane foam). For each 

experiment, smoldering was initiated by a 50 W cylindrical electric cartridge heater 50 mm long 

and 10 mm in diameter. The cartridge heater rested on a 15 cm by 15 cm square of cotton duck 

fabric that was placed on the seat cushion to ensure a sustained smoldering fire. Electrical 

power to the cartridge heater was applied in a controlled fashion to achieve an external 

temperature sufficient to produce sustained smoldering. A thermocouple was attached to the 

exterior of the cartridge heater and the temperature controller set point was fixed at 425 o C. 

After about 6 min of total contact time, the cartridge heater was removed. The cotton cushion 

covers were used during the smoldering tests. (Cushion covers made from polyester, the other 

fabric cover used in the Smoke Alarm Sensitivity Study, melts when exposed to a heat source 

like a cigarette or small power cartridge heater, instead of charring and propagating the smolder 

front into the foam (like a cotton cover fabric will allow.) Others have used cartridge heaters to 

initiate foam smoldering [16] and as a potential replacement for a standardized cigarette in 

smoldering tests [17]. Smoldering was initiated in the front corner of the seat cushion, so that 

the smolder front had to travel the length of the seat cushion to reach the back cushion and 

develop conditions favorable for transition to flaming. The intent was to maximize the in-depth 

polyurethane foam smoldering phase to the extent possible. The bulk of the smoke produced 

during smoldering was from the smoldering polyurethane foam. Smoldering fires were allowed 

to progress until they naturally transitioned to flaming fires except for one experiment that was 

stopped prior to the flaming transition. 

A comparison of the living room fire alarm activation times to the bedroom fire alarm activation 

times (with identical smoldering sources) showed that the difference between photoelectric and 

ionization alarms was much greater for the living room tests than the bedroom tests. This 

observation re-emphasizes the fact that smoke alarm activation depends on a number of factors 

and not just what is burning. For instance, the room size and the distance of the smoke alarms 

from the fire source affect smoke aerosol aging as it reaches and enters the smoke alarm. This 

is relevant because smoke aging affects smoke particle sizes which in turn influences smoke 

alarm activation. Additionally, the local air flows induced from a smoldering source are usually 
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8

low velocity, and it is well established that smoke alarms, when exposed to smoke in low 

velocity flows, can experience long activation delay times even when the smoke concentration 

in the surroundings is well above the alarm point [18]. 

The fire growth rate of the burning chair mock-up determined, to a great extent, the hazard 

development in the test enclosure [6]. Due to differences in foam density and cover fabric, 

variation in the burning rate of different chair mock-up configurations was expected. One chair 

mock-up constructed from high-density foam and polyester cushion covers was burned in the 

NIST furniture calorimeter to determine the average heat of combustion of the mock-up. The 

mock-up was ignited with the flame tube in the same manner as the full-scale tests. A peak heat 

release rate of approximately 700 kW was achieved, with a test-average heat of combustion 

(obtained by dividing the total heat released by the total mass loss) of 25 MJ/kg.1 The combined 

standard uncertainty of the average heat of combustion was estimated as ± 3 MJ/kg. 

A heat release rate estimate of most chair mock-ups was determined by differentiating the mass 

loss curve for each test to obtain the mass loss rate, and multiplying that curve by the testaverage heat of combustion. In order to characterize the heat release rate results for each 

chair mock-up, the heat release rate curve was fitted to a “t-squared” power law fire growth 

curve, following an initial linear growth phase. This functional form fits the heat release rate of 

many common materials, and is used extensively for design fires in fire hazard analysis [19]. 

Annex B, Engineering Design Guide for Automatic Fire Detector Spacing, in the 13 ed. of NFPA 72 

specifies a design fire range as slow being a time to reach 1055 kW in a period greater than or 

equal to 400 s, medium for growth times between 150 s to 400 s, and fast for growth times less 

than 150 s [1]. Figure 3 shows two fitted heat release rate curves for an initially flaming and an 

initially smoldering chair mock-up fire. The linear portion spanned from ignition to the time to 

reach a fixed value (typically 25 kW for initially flaming fires, and 10 kW for initially smoldering 

fires). Table 1 shows the linear time span for each fit, the fitting parameter, the expected fire 

growth time to reach 1055 kW (after the linear growth phase), and the fire growth category. All 

but one of the chair mock-up fires fall within the medium “t-squared” fire category after the 

linear growth phase, and the linear growth phase was estimated to be at least 150 s for initially 

flaming fires and 4840 s or greater for initially smoldering fires. The combination of the linear 

growth phase and the “t-squared” fire growth rate suggests that these fire sources were not so 

fast developing as to render smoke detection inconsequential, but that alarm activation during 

the linear growth phase could provide substantial available safe egress times. 

 

1 The heat of combustion for the smoldering phase is expected to be lower than the specified 25 MJ/kg, 

but the transition to flaming occurred before significant mass loss rate was measured. 
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9

Figure 3. Experimental heat release rate curves for an initially flaming fire (Exp. 10, left figure) 

and initially smoldering fire (Exp. 5, right figure). Dashed lines are fitted curves consisting of a 

linear segment and a “t-squared” segment. Fitting parameters are listed in Table 1.

Data on hydrogen cyanide (HCN) was not included in previous analysis of the Smoke Alarm 

Sensitivity Study [6, 8 and 15], but is included here with conservative HCN estimates used in the 

following tenability calculations. The HCN sampling in the hallway was accomplished by drawing 

a fixed flow of hallway air through soda lime HCN sorbent tubes for a fixed time. The sorbent 

tubes were located inside the hallway at the centerline. A rigid metal pipe terminating at the 

center of the hallway supported the sorbent tube, and the flow tube attached to it. The flow 

tube led from the structure connecting to a mass flow controller and then to a vacuum pump. 

Multiple sorbent tubes could be switched out during individual tests. The samples were 

analyzed for adsorbed HCN mass using the NIOSH 6010 method [20]. The HCN mass was 

converted to a mean volume fraction (at room conditions) over the sampling time (sampling 

times restricted to smoldering phase or flaming phase) and scaled by the volume fraction of 

carbon monoxide. HCN has been shown to correlate with CO in combustion gas yields of 

furnishing materials [21]. Figure 4 shows the HCN yield values for samples taken during flaming 

and smoldering phases of combustion. The combined relative uncertainty is estimated as ± 

15 %. Also shown are best fit lines through all of the flaming mode and smoldering mode data 

points. In order to compute toxic gas tenability limits using HCN along with CO in a conservative 

manner, fixed values of HCN yields of 0.075 and 0.025 (approximately double the best fit lines) 

were specified for flaming and smoldering phases of combustion. 

0

100

200

300

400

500

600

700

0 100 200 300 400

Experiment

Fitted Curve

Heat Release Rate (kW)

Time (s)

0

100

200

300

400

5900 6000 6100 6200 6300

Experiment

Fitted Curve

Heat Release Rate (kW)

Time (s)
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Table 1. Fire growth rate fitting parameters2for chair mock-up fires based on mass loss rate 

and an average effective heat of combustion.

Material*, Exp. No. Ignition Mode,

Location#

Linear 

Phase

tL(s), 

Q̇

L

(kW)

a

(kW/s2) R

%

Time to

1055 

kW (s)

Category+

PET fabric, LD foam, 4 Flaming, LR 150, 10 0.0142 0.995 271 Medium

PET fabric, LD foam, 17 Flaming, LR 200, 25 0.0229 0.983 212 Medium

PET fabric, LD foam, 19 Flaming, LR 175, 25 0.0256 0.970 201 Medium

PET fabric, HD foam, 8 Flaming, BR 260, 30 0.0207 0.977 223 Medium

PET fabric, HD foam, 10 Flaming, BR 280, 30 0.0397 0.986 161 Medium

PET fabric, HD foam, 24 Flaming, BR 240, 25 0.0241 0.972 207 Medium

PET fabric, LD foam, 7 Flaming, BR 180, 25 0.0246 0.990 205 Medium

PET fabric, LD foam, 11 Flaming, BR 160, 10 0.0315 0.973 182 Medium

CT fabric, LD foam, 1 Flaming, LR 1200, 25 0.0106 0.987 312 Medium

CT fabric, LD foam, 6 Flaming, LR 220, 25 0.0127 0.975 285 Medium

CT fabric, LD foam, 18 Flaming, LR 800, 25 0.0284 0.954 190 Medium

CT fabric, LD foam, 5 Smoldering, LR 6180, 10 0.0285 0.991 191 Medium

CT fabric, LD foam, 22 Smoldering, LR 5790, 10 0.0333 0.980 177 Medium

CT fabric, HD foam, 16 Smoldering, LR 5240, 10 0.0334 0.976 177 Medium

CT fabric, HD foam, 21 Smoldering, LR 6210, 10 0.0401 0.995 160 Medium

CT fabric, HD foam, 23 Smoldering, LR 5180, 10 0.0395 0.980 163 Medium

CT fabric, LD foam, 12 Smoldering, BR 4840, 10 0.0344 0.983 174 Medium

CT fabric, LD foam, 15 Smoldering, BR 10910, 20 0.0579 0.986 131 Fast

CT fabric, LD foam, 2 Smoldering, BR 6125, 10 0.0118 0.963 298 Medium

CT fabric, LD foam, 9 Smoldering, BR 6790, 10 0.0201 0.986 228 Medium

*PET – Polyester, CT – Cotton, LD – low density, HD – high density 

#

LR – living room, BR – bedroom

% Correlation coefficient R

+NFPA 72 Annex B definition

 

2

Fitting equations given below, where Q̇(t) is the heat release rate at time t, Q̇

L

is the heat 

release rate maximum during the linear growth phase, tlis the linear growth phase time and a

is the pre-exponential. 

Q̇(t) =

Q̇

L

tL

t t < tl

Q̇(t) = a (t − tL

)

2 + Q̇

L

t ≥ tL
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11

0.001

0.01

0.1

1

10

100

1 10 100 1000

Smoldering Phase 

Flaming Phase

Smoldering Bound

Flaming Bound

HCN Concentration (Vol. frac. x10

6

)

CO Concentration (Vol. frac. x106)

Figure 4. Mean HCN concentration as a function of mean CO concentration for samples taken at 

a height of 1.5 m above the floor in the center of the hallway leading to the master bedroom. 

Symbols are experimental time-averaged values, and thin lines are linear fits through the data

(combined relative uncertainty is estimated as ± 15 %.) The thick lines define upper bounds of 

HCN yields for flaming and smoldering phases used in the fractional effective dose (FED) 

calculations.

Toxic gas and thermal exposure tenability criteria were computed using the fractional effective 

dose (FED) concept and equations ISO 13571 [22]. Figure 5 shows two examples of toxic gas FED 

and thermal exposure FED in initially flaming and initially smoldering fires. The location of the 

fire in each case was the living room, and the FEDs were calculated using the average of the 

hallway toxic gas concentrations and living room temperature and heat flux values. Two FED 

toxic gas calculations were performed using either, CO and CO2, or CO, CO2 and HCN. The FED 

calculations that included HCN rose faster than the calculations that excluded HCN. Only in the 

initially smoldering fire does the more rapid HCN rise significantly impact the calculated hazard. 

When HCN was considered, the toxic gas FED reached a value of 0.3 approximately 400 s sooner 

compared to the toxic gas FED computed without HCN. Additionally, the toxic gas FED 

(including HCN) reached a limit of 0.3 more than 400 s sooner than the thermal FED. 

3 ASET/RSET MODEL

An ASET/RSET analysis was performed using the model presented in Reference 7 where 

scenario-averaged outcomes were computed for specific alarm activation times to provide a 

comparison between the performance of photoelectric, ionization, and dual sensor alarms. 
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12

Figure 5. Comparison of FED calculations in living room fires with and without HCN. Left figure 

flaming fire, right figure initially smoldering fire that transitioned to flaming.

Here, the results are presented in terms of activation time. The time to reach a specific smoke 

obscuration value provided by the ceiling extinction meter was compared to the fraction of 

successful escape outcomes assuming that an alarm activates at that smoke obscuration level. 

The basis for the analysis assumed that smoke alarms were interconnected and alerted 

occupants, regardless of their initial location, and occupants started to move after a predetermined, pre-movement delay time. 

3.1 Egress Scenarios

The space considered was the residential test structure with a fictitious attached space 

representing a second bedroom and an entrance/exit door off the living room, as shown in 

Figure 6. Again, it was sized to represent a small, single-floor apartment space. The shaded 

spaces were closed-off from the rest of the structure. The second bedroom is assumed to have 

an interconnected smoke alarm. 

Table 2 shows 13 specified occupant travel paths. These routes were selected to include direct 

egress paths, investigational and rescue activity paths, and include initially locating the occupant 

in the room of fire origin in some cases. All 13 scenarios were included in the analysis of every 

experiment, and equally weighted to provide a basis for calculation of ASET and RSET. 

3.2 Tenability Criteria

The tenability criteria for toxic gas and heat exposure followed the criteria used in other studies 

[5, 8, 15 and 23], specifically, an FED limit of 0.3 for both toxic gases and heat exposure. These 

limits are conservative in that only the most vulnerable population would potentially become

0

0.05

0.1

0.15

0.2

0.25

0.3

0 100 200 300 400 500

FED CO, CO2

FED CO, CO2, HCN

FED Thermal

Fractional Effective Dose

Toxic Gas and Thermal Exposure 

Time from Ignition (s)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1000 2000 3000 4000 5000 6000 7000

FED CO, CO2

FED CO, CO2, HCN

FED Thermal

Fractional Effective Dose

Toxic Gas and Thermal Exposure 

Time from Ignition (s)
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13 Master 

Bedroom

(MBR)

Living Room

(LR)

Kitchen

Bedroom

(BR)

Door

(closed)

Door

(open or closed)

Exit 

3.7 m

8.9 m

3.0 m

4.0 m

Chair

Mock-up

Chair

Mock-up

4.0 m

Figure 6. Layout of the residential structure as defined for egress analysis. Dashed lines 

represent travel path segments with their respective distances indicated. 

Table 2. Travel paths indicating order of rooms visited, number of segments and total distance 

traveled for each egress scenario considered (Figure 6).

Scenario Travel Path Travel 

Segments

Travel Distance (m)

1 MBR – Exit 2 7.7

2 LR – Exit 1 8.9

3 BR – Exit 3 15.9

4 MBR – BR – Exit 7 35.5

5 BR – MBR – Exit 6 27.3

6 LR – BR – Exit 5 27.8

7 LR – MBR – Exit 4 20.3

8 LR – BR – MBR – Exit 8 39.2

9 LR – MBR – BR – Exit 9 48.1

10 MBR – LR – MBR – Exit 6 28.0

11 BR – LR – BR – Exit 7 34.8

12 MBR – BR – MBR – Exit 10 46.9

13 BR – MBR – BR – Exit 11 55.1

MBR – master bedroom 

LR – living room

BR – Bedroom

incapacitated at these levels. The smoke obscuration limit, while not a tenability criteria, was 

nevertheless used as a limiting smoke value, as an occupant’s escape can be significantly 

affected in a negative manner. Studies have shown that increasing smoke obscuration 

decreases visibility distance and reduces travel speed. A limiting value of smoke optical density 

of 0.25 m-1 was used in the NIST Home Smoke Alarm Study, whereas a value of 0.43 m-1 was 

used in an NFPA task group report that performed a re-analysis of the NIST data [23]. Neither 

analysis considered the effect of smoke on occupant travel speed. Analysis of the NIST Smoke 

Alarm Sensitivity Study results, that did consider the effect of smoke on travel speed, showed 

that the assumed smoke obscuration limit had a significant effect on the escape outcomes for 
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14

flaming and smoldering fire scenarios over a range of 0.25 m-1to 1.0 m-1[8]. In the analysis 

below, the results are presented for four smoke limits (0.20 m-1, 0.25 m-1, 0.30 m-1, and 0.43 m-1) 

and when the smoke limit is not enforced (“no smoke” limit). The 0.20 m-1 and 0.30 m-1 limits 

represent ± 20 % differences from the 0.25 m-1value and are included to show the sensitivity of 

the results about the nominal uncertainty of the optical density measurement at 0.25 m-1. The 

“no smoke” limit results force either the heat or toxic gas tenability limits to become the limiting 

condition. The “no smoke” limit conveys a relative urgency for a given scenario, whereas a small 

time difference between a smoke limit and “no smoke” limit implies little time for alternate 

outcomes like being rescued. 

3.3 Occupant Characteristics

In each egress simulation, the occupant is alerted at some time from an interconnected smoke 

alarm. Since the ceiling smoke obscuration was only measured in the south hallway (leading 

from the master bedroom, and next to the exit door), it was defined as the location of the 

initiating alarm; the actual recorded alarm times throughout the structure were ignored. After 

receiving the alarm, the occupant remained at their initial location for a specified time before 

moving, designated as the pre-movement time. Pre-movement times are represented as a 

probability distribution in the model. In previous work, three probability distributions were 

proposed to characterize a fast reacting, “primed” group, a more measured normal-reacting 

group, and a slower-to-react group characteristic of elderly or mobility-impaired occupants [8]. 

Here, these distributions are postulated from discrete pre-movement activities tied to similar 

activities considered in the NIST Home Smoke Alarm Report [5]. 

The Home Smoke Alarm Report considered the following pre-movement activities: calling the 

fire department, getting dressed, gathering belongings, and waking/rescuing two individuals. 

Waking to the alarm was not included. This omission is contradicted by research that showed 

waking to an alarm is not instantaneous, but may take 30 s or longer in some cases [24]. The 

action of calling the fire department was included in human response studies conducted in the 

1980’s. However, here, it is speculated that with the prevalence of cell phones presently, an 

individual would not make a call to the fire department from inside the home. Therefore, in this 

analysis, calling fire department was dropped and waking/arising was added as pre-movement 

activities. The Home Smoke Alarm Study used estimated mean values for the pre-movement 

times obtained from previous research [25]. Here, a distribution was associated with each premovement activity. The “awaken and arise” pre-movement distribution was postulated given 

the experimental results of Duncan [26] and Ashley [27]. The fast-responding “primed” 

occupants are described as responding sooner and with less spread in their responses. This 

group represents the response of test subjects who were expecting an alarm, i.e. “primed”, as 

part of a study. This group does not alert others as defined to represent the quickest expected 

individual occupant response and this group is not included in the following analysis. Both the 

normal-responding mobile occupants and the slower-to-react occupants take somewhat longer 

to arise, awaken, and accomplish pre-movement activities on average. 
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15

As described in [8], the total pre-movement time was characterized by a median time and a 

geometric standard deviation (g) with a log-normal distribution. The Fenton-Wilkinson method 

was used to estimate a single log-normal distribution that approximates the sum of the 

individual pre-movement activity distributions [28]. Table 3 details the individual contributions 

and the final distribution for three separate populations which, as it turns out, are the same as 

those considered previously [8]. Response times for 84.1 % of a group to complete individual 

tasks are presented to show how long it would take a large portion of the population to 

complete each task (for a log-normal distribution, the median time multiplied by the geometric 

standard deviation defines the 84.1 % response time, thus, it is a convenient way to compare 

different distributions ). For the normal-responding occupants group (median = 35 s, g = 1.6),

84.1 % accomplished all pre-movement tasks in 56 s. Whereas for slower-to-react occupants 

group (median = 55 s, g = 1.6), 84.1 % accomplished all pre-movement tasks in 88 s. This 

compares with the cumulative worst-case pre-movement times of 55 s for the young family at 

night and 80 s for the elderly family at night estimated in the NIST Home Smoke Alarm Study [5]. 

Table 3. Pre-movement activities and the corresponding log-normal distribution that 

approximates the sum of the individual pre-movement activities for three populations.

Population Pre-movement 

Activity

Mean 

Time 

(s)

Median 

Time 

(s)

Geometric

Standard

Deviation (g)

Time for 84.1 % 

of Population to 

complete task(s)

Fast-responding, 

primed occupants

Awaken and arise 8.0 6.1 2.1 12.8

Dress or don robe 5.0 3.8 2.1 8.0

Gather belongings 5.0 3.8 2.1 8.0

Distribution 

representing the sum 18 16 1.6 26

Normalresponding, mobile 

occupants

Awaken and arise 12 8.2 2.4 17.2

Dress or don robe 8 5.5 2.4 11.5

Gather belongings 8 5.5 2.4 11.5

Awaken occupant 1 6 4.1 2.4 8.6

Awaken occupant 2 6 4.1 2.4 8.6

Distribution 

representing the sum 40 35 1.6 56

Elderly, mobility 

impaired, slowerto-react occupants

Awaken and arise 15 10.2 2.4 21.5

Dress or don robe 15 10.2 2.4 21.5

Gather belongings 12 8.2 2.4 17.2

Awaken occupant 1 10 6.8 2.4 14.3

Awaken occupant 2 10 6.8 2.4 14.3

Distribution 

representing the sum 62 55 1.6 88

Travel speed was treated as a function of the smoke optical density along the travel path in the 

model. The NIST Home Smoke Alarm Study analysis used fixed floor travel speeds of 1.2 m/s for 

young occupants and 0.75 m/s for elderly occupants [5]. Here, the initial travel speed for all 

occupants was an intermediate value of 1.05 m/s and, as the smoke optical density increased, 

the occupant travel speed was assumed to decrease from 1.05 m/s to a limiting speed of 0.1 

m/s [8]. Figure 7 shows the travel speed along with an estimated visibility range for various 
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16

reflective surfaces (walls, doors, furniture objects, etc. under normal lighting conditions) as a 

function of optical density. The visibility was estimated by a dimensionless constant divided by 

the optical density. Here, the constants were 0.9 and 1.7 for the lower and upper range of 

visibility for reflective surfaces [29]. For an optical density of 0.5 m-1 in Figure 7, the travel speed 

drops by about 85 %, whereas the visibility is estimated to be between 3.5 m and 7.0 m. 

0.1

1

10

1

10

0 0.25 0.5 0.75 1 Visibility (m) and Walking Speed (m/s)

Visibility (ft) and Walking Speed (ft/s)

Optical Density (m-1)

Walking Speed

Lower Range Visibility 

Upper Range Visibility 

Figure 7. The walking speed as a function of the optical density correlation used in the model 

[8], and the estimated upper and low reflective surface visibility range as a function of optical 

density [29]. 

4 MODEL IMPLEMENTATION 

The start of each experiment (application of heat source or flame to the seat cushion) was 

considered time zero. At some time well before non-tenable conditions, a fictitious hallway 

alarm was assumed to activate. From this alarm activation time, egress calculations were 

conducted for 41 pre-movement times from 0 s to 200 s in 5 s intervals, which covered more 

than 99 % and 97 % of the normal-responding occupants and the slower-to-react occupants 

distributions, respectively. If egress was deemed unsuccessful, because a tenability or smoke 

limit was reached for a given pre-movement time, then the fraction of the population deemed 

to successfully escape was estimated by the integral of the pre-movement frequency 

distribution from zero to the last successful pre-movement time step (and normalized by 0.99 or 

0.97, depending on the occupancy group.) The fraction of successful escapes was calculated for 

each of the 13 egress scenarios, and those values were averaged to represent a single fraction of 

successful escapes for the presumed alarm time of a given fire experiment. The initial alarm 
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17

time was then increased by 10 s in the case of initially flaming fires and 30 s in the case of 

initially smoldering fires and the fraction of successful escapes re-calculated. The alarm time 

was increased and the calculations repeated until essentially all egress attempts were 

unsuccessful. This process was conducted for the two characteristic occupant groups 

considered here, the normal-responding occupants and the slower-to-react occupants.

5 SIMULATION RESULTS

There were a total of six fire scenarios considered: two flaming fires located in the living room 

(with different cover fabrics, either cotton or polyester) and one located in the master bedroom, 

and two initially smoldering fires located in the living room (with different foam densities) and 

one located in the master bedroom. Each fire scenario experiment was conducted three times, 

to quantify the variability in the ignition, fire growth, and time to alarm, for a total of 18 

individual experimental fires used for the simulations considered in this analysis. 

Figures 8-11 are sample plots showing the results for flaming and smoldering fire experiments in 

the living room and master bedroom, respectively. For each experiment, the fraction of 

successful escapes is plotted versus the presumed alarm time. In addition, the smoke 

obscuration at the ceiling is plotted. In the analysis presented, results are given for four 

hypothetical smoke limit criteria (0.20 m-1, 0.25 m-1, 0.30 m-1, and 0.43 m-1) and when “no 

smoke” limit is enforced. Figures 8 and 9 show the results of a flaming fire in the living room 

with occupant pre-movement distributions representing the normal-responding occupant group 

and the slower-to-react occupant group, respectively. Figures 10 and 11 show the results for a 

smoldering fire in the master bedroom with occupant pre-movement distributions representing 

the normal-responding occupant group, and the slower-to-react occupant group, respectively. 

There are significant differences in egress outcomes for the various smoke limit criteria that 

define smoke alarm performance. For the smoldering cases, there were long time differences 

for different smoke limits in the slowly developing smoldering fire, whereas in the flaming fire 

case, the egress outcomes for the different smoke limits were relatively close together, 

indicating rapid smoke development. The flat portion of some curves at approximately 31 % in 

Figures 10 and 11 was due to the fact that 4 of the 13 egress scenarios that did not include 

traversing the south hallway nor the master bedroom, and combined with the relatively long 

time for the smolder smoke to fill the living room as it traveled from the master bedroom, led to 

extended safe egress time for those scenarios. 

The surrogate for smoke alarm performance is the ceiling smoke obscuration measurement at 

the desired smoke alarm activation time which would be considered as the target alarm 

criterion for new polyurethane foam smoldering and flaming fire sensitivity tests in ANSI/UL 217 

and ANSI/UL 268. A performance level of 100 % successful escapes may be a target that is too 

challenging to achieve given that the continuous pre-movement frequency distributions for 
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18

which 99 % or 97 % of a population’s pre-movement time is longer than 200 s, and that three 

egress scenario travel distances were greater than 45 m, requiring significant travel time. 

Likewise, a performance level of 50 % successful escapes or less may be too lax. Therefore, three 

performance levels were examined, with 95 %, 85 % and 75 % success rates. 

The results for the three performance levels, with values of 95 %, 85 % and 75 % egress success 

for two target populations (normal-responding occupants, and slower-to-react occupants) are 

given in Tables 4 - 6 to provide a comparison between the levels of performance and the smoke 

obscuration values at which alarms would need to respond to attain different levels of 

performance. Results are also tabulated for two different optical density limits, namely a more 

conservative 0.25 m-1 and a less conservative value, 0.43 m-1. The tabulated values include the 

average smoke obscuration3 and standard deviation (std dev) for the nine smoldering and nine 

flaming experiments, along with the singular values that would produce the desired egress 

success rate (95 %, 85 % and 75 %) across all 9 flaming or smoldering experiments. Obviously, 

the smoke alarm sensitivity, as indicated by the smoke obscuration value, needed to achieve a 

given performance level decreased (indicated by an increase in smoke obscuration) as the 

desired egress success rate decreased. The sensitivity required for the most conservative 

assumptions versus the least conservative assumptions (optical density limit of 0.25 m-1for the 

slower-to-react occupant group versus optical density limit of 0.43 m-1for the normalresponding occupant group) varied by 4 %/ft -6 %/ft obscuration for flaming fire scenarios to 6 

%/ft -8 %/ft obscuration for smoldering fire scenarios for each of the three performance levels.

To provide some context to the three performance levels chosen, the performance of actual 

smoke alarms used in the experiments were assessed. Table 7 shows the corresponding egress 

success rate for the experimentally determined hallway-mounted ionization or photoelectric 

alarm times used as model inputs. For fires located in the living room or bedroom, alarm 

activations at the S3 or S4 locations were used, respectively. The alarm locations and the ceiling 

smoke measurements were not at the same locations in the hallway, with the alarms are 

located closer to the fire source, so a direct correlation is not expected. However, the alarm 

time results are suggestive of the level of relative performance current photoelectric and 

ionization alarms achieve for the tenability criteria and occupant characteristics investigated. 

The lowest average egress success rates for the most conservative assumptions (optical density 

limit of 0.25 m-1for the slower-to-react occupant group) were 55 % for ionization alarms in 

smoldering fire scenarios and 66 % for photoelectric alarms in flaming fire scenarios. These 

values are below the lowest performance level of 75 % success rate specified above. This 

contrasts with the lowest average egress success rates for the least conservative assumptions 

(optical density limit of 0.43 m-1for the normal-responding mobile occupant group) of 95 % for 

ionization alarms in smoldering fire scenarios and 94 % for photoelectric alarms in flaming fire 

scenarios.

 

3

Smoke obscuration is reported throughout this paper in terms of percent obscuration per foot (%/ft 

obscuration) as specified in ANSI/UL 217 and ANSI/UL 268 and per US fire detection industry norms. 
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19

0

0.2

0.4

0.6

0.8

1

0

5

10

15

20

0 50 100 150 200 250 300

0.20

0.25

0.30

0.43

None

Obsc.

Fraction of Successful Escapes

Smoke Obscuration at the Ceiling

(%/ft obscuration)

Time to Alert from Start of Fire (s)

OD Limit 

(m-1)

Figure 8. Calculated fraction of successful escapes as a function of the time to alarm for a 

flaming fire originating in the living room for the normal-responding mobile occupant group. 

Also shown is the measured smoke obscuration at the ceiling.

0

0.2

0.4

0.6

0.8

1

0

5

10

15

20

0 50 100 150 200 250 300

0.20

0.25

0.30

0.43

None

Obsc.

Fraction of Successful Escapes

Smoke Obscuration at the Ceiling

(%/ft obscruation.)

Time to Alert from Start of Fire (s)

OD Limit 

(m-1)

Figure 9. Calculated fraction of successful escapes as a function of the time to alarm for a 

flaming fire originating in the living room for the slower-to-react occupant group. Also shown is 

the measured smoke obscuration at the ceiling.
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20

0

0.2

0.4

0.6

0.8

1

0

5

10

15

20

25

30

35

40

2000 2500 3000 3500 4000 4500 5000

0.20

0.25

0.30

0.43

None

Obsc.

Fraction of Successful Escapes

Smoke Obscuration at the Ceiling

(%/ft Obsc.)

Time to Alert from Start of Fire (s)

OD Limit 

(m-1)

Figure 10. Calculated fraction of successful escapes as a function of the time to alarm for a 

smoldering fire originating in the master bedroom for the normal-responding mobile occupant 

group. Also shown is the measured smoke obscuration at the ceiling.

0

0.2

0.4

0.6

0.8

1

0

5

10

15

20

25

30

35

40

2000 2500 3000 3500 4000 4500 5000

0.20

025

0.30

0.43

None

Obsc.

Fraction of Successful Escapes

Smoke Obscuration at the Ceiling

(%/ft Obsc.)

Time to Alert from Start of Fire (s)

OD Limit 

(m-1)

Figure 11. Calculated fraction of successful escapes as a function of the time to alarm for a 

smoldering fire originating in the master bedroom for the slower-to-react occupant group. Also 

shown is the measured smoke obscuration at the ceiling.
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21

Table 4. Ceiling smoke obscuration required to achieve a calculated 95 % egress success rate 

for the target populations and optical density limits.

Optical 

Density 

Limit (m-1)

Pre-movement 

Distribution

(median (s) /g)

Initially smoldering fire scenario 

ceiling smoke obscuration to 

achieve 95 % egress success rate

(%/ft obsc.)

Flaming fire scenario 

ceiling smoke obscuration to 

achieve 95 % egress success rate

(%/ft obsc.)

Average of 9 

experiments 

± std dev

Value to 

achieve 95 % 

success rate 

across all 9

experiments

Average of 9 

experiments

± std dev

Value to 

achieve 95 % 

success rate 

across all 9 

experiments

0.25 35/1.6 17.2 ± 5.2 11.3 3.1 ± 1.7 2.5

55/1.6 16.7 ± 5.1 10.7 0.9 ± 0.8 0.4

0.43 35/1.6 23.3 ± 7.6 18.3 5.6 ± 2.2 4.7

55/1.6 22.3 ± 7.2 13.3 2.5 ± 1.7 2.0

Table 5. Ceiling smoke obscuration required to achieve a calculated 85 % egress success rate 

for the target populations and optical density limits.

Optical 

Density 

Limit (m-1)

Pre-movement 

Distribution

(median (s) /g)

Initially smoldering fire scenario 

ceiling smoke obscuration to 

achieve 85 % egress success rate

(%/ft obsc.)

Flaming fire scenario 

ceiling smoke obscuration to 

achieve 85 % egress success rate

(%/ft obsc.)

Average of 9 

experiments 

± std dev

Value to 

achieve 85 % 

success rate 

across all 9

experiments

Average of 9 

experiments 

± std dev

Value to 

achieve 85 % 

success rate 

across all 9 

experiments

0.25 35/1.6 17.5 ± 5.3 12.7 5.0 ± 1.6 4.5

55/1.6 17.1 ± 5.2 12.5 2.3 ± 1.5 2.1

0.43 35/1.6 24.6 ± 8.9 19.9 8.6 ± 3.2 7.5

55/1.6 23.3 ± 7.8 18.9 5.5 ± 2.1 4.5

Table 6. Ceiling smoke obscuration required to achieve a calculated 75 % egress success rate 

for the target populations and optical density limits.

Optical 

Density 

Limit (m-1)

Pre-movement 

Distribution

(median (s) /g)

Initially smoldering fire scenario 

ceiling smoke obscuration to 

achieve 75 % egress success rate

(%/ft obsc.)

Flaming fire scenario 

ceiling smoke obscuration to 

(%/ft obsc.)achieve 75 % egress 

success rate

Average of 9 

experiments 

± std dev

Value to 

achieve 75 % 

success rate 

across all 9

experiments

Average of 9 

experiments 

± std dev

Value to 

achieve 75 % 

success rate 

across all 9 

experiments

0.25 35/1.6 17.7 ± 5.3 14.0 5.9 ± 1.7 5.5

55/1.6 17.4 ± 5.3 13.9 3.7 ± 1.6 3.4

0.43 35/1.6 25.3 ± 9.0 22.4 9.9 ± 3.2 9.7

55/1.6 24.3 ± 8.6 21.5 7.1 ± 2.5 6.2
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22

Table 7. Calculated relative egress performance using experimental activation times for 

photoelectric and ionization alarms in the hallway where the ceiling smoke obscuration was 

measured.

Optical 

Density 

Limit (m-1)

Pre-movement 

Distribution

(median (s) /g)

Initially smoldering fire scenario 

egress success rate percentage 

using local (hallway) alarm times

Flaming fire scenario 

egress success rate percentage 

using local (hallway) alarm times

Ionization 

Alarms 

(average of 9 

experiments

± std dev)

Photoelectric 

Alarms 

(average of 9 

experiments

± std dev)

Ionization 

Alarms 

(average of 9 

experiments

± std dev)

Photoelectric 

Alarms 

(average of 9 

experiments

± std dev)

0.25 35/1.6 59 ± 40 100 ± 0 95 ± 13 83 ± 18

55/1.6 55 ± 44 100 ± 0 89 ± 18 66 ± 23 

0.43 35/1.6 95 ± 16 100 ± 0 100 ± 1 94 ± 13

55/1.6 94 ± 21 100 ± 0 97 ± 4 85 ± 20

Another way to examine the results is to tabulate the egress success rate as a function of alarm 

activation at a given smoke obscuration. Table 8 shows the corresponding egress success rate 

for flaming fire scenarios with smoke obscuration ranging from 2 %/ft obscuration to 10 %/ft 

obscuration. Table 9 shows the corresponding success rate for smoldering fire scenarios with 

smoke obscuration ranging from 8 %/ft obscuration to 24 %/ft obscuration. Additional columns 

are shown for the egress success rate for a given smoke obscuration plus a 30 s delay (dwell 

time, which has been suggested by some manufacturers as part of the fire test performance 

criteria). 

Table 8. Calculated relative egress performance for an alarm activated at a prescribed ceiling 

smoke obscuration for flaming fires.

Smoke 

Obscuration

(%/ft obsc.)

Success Rate (%)

Normal-responding occupants

(Pre-movement distribution – 35 s, g 1.6)

Success Rate (%)

Slower-to-react occupants

(Pre-movement distribution – 55 s, g 1.6)

Optical density limit

0.25 m-1

Optical density limit

0.43 m-1

Optical density limit

0.25 m-1

Optical density limit

0.43 m-1

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

2 97 85 99 97 86 65 95 86

3 94 73 98 91 78 55 91 78

4 89 63 97 87 70 43 88 71

5 80 50 94 80 60 33 82 62

6 70 40 91 71 50 26 76 52

7 62 32 87 63 42 20 70 44

8 55 28 83 56 37 17 64 38

9 49 23 79 49 32 13 59 33

10 44 19 73 42 27 11 52 27

 
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Table 9. Calculated relative egress performance for an alarm activated at a prescribed ceiling 

smoke obscuration for smoldering fires.

Smoke 

Obscuration

(%/ft obsc.)

Success Rate (%)

Normal-responding occupants

(Pre-movement distribution – 35 s, g 1.6)

Success Rate (%)

Slower-to-react occupants

(Pre-movement distribution – 55 s, g 1.6)

Optical density limit

0.25 m-1

Optical density limit

0.43 m-1

Optical density limit

0.25 m-1

Optical density limit

0.43 m-1

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

Delay 0 

s

Delay 

30s 

8 100 100 100 100 100 100 100 100

10 100 100 100 100 100 99 100 100

12 91 88 98 94 88 85 95 92

14 77 74 97 94 75 72 95 92

16 67 66 96 93 66 66 94 92

18 67 65 96 92 66 64 93 88

20 59 52 84 77 54 47 79 74

22 43 39 78 73 40 37 74 70

24 27 25 66 60 26 24 62 56

For the flaming fire scenarios and smoke obscuration values from 2 %/ft to 10 %/ft, the success 

rate ranged from 99 % to 27 %, and 87 % to 11 % for the no delay and 30 s delays cases, 

respectively. The 30 s delay is equivalent to about a 3 %/ft to 6 %/ft obscuration increase to 

achieve the same egress success rate. For the smoldering fire scenarios and smoke obscuration 

values from 8 %/ft to 24 %/ft, the success rate ranged from 100 % to 26 %, and 100 % to 24 % 

for the no delay and 30 s delays cases, respectively. In the smoldering scenarios, the 30 s delay 

has very little impact on the smoke obscuration increase needed to achieve the same egress 

success rate. This is due to the relatively slow smoke concentration build-up in the smoldering 

fires

Since the simulated egress success rates are a function of egress scenarios (Table 2), occupant 

characteristics (Table 3), optical density limits (Tables 4 - 6) and the experimental results, a 

success rate averaged over the four occupant characteristic/optical density limit results was 

used to characterize an averaged relative success rate for a given smoke obscuration 

performance criterion. Averaging the normal-responding occupant group results and the 

slower-to-react occupant group results yields an equal weighting of both groups in lieu of a 

specific combined population distribution. An average of the results from the two optical 

density limits (0.25 m-1 and 0.43 m-1) is an approximation of the mean smoke concentration limit 

that starts to inhibit egress of more and less susceptible occupants, given their psychophysical 

response to smoke.

It is asserted here that the performance criterion for flaming and smoldering fire test scenarios 

should lead to nominally equal relative egress success rates to provide commensurate 

improvement in flaming and smoldering fire detection performance. Table 10 gives matched 

pairs of flaming and smoldering fire performance criteria based on the averaged simulated 

egress success rates. The first tabulated performance criteria representing an average egress 
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24

success rate of about 94 % yielded a flaming fire performance criterion (2 %/ft obscuration) 

that would significantly challenge photoelectric alarms, whereas the smoldering fire 

performance criterion (12 %/ft obscuration) would significantly challenge ionization alarms. 

Most-likely, a combination of sensors would be needed to meet these performance criteria. The 

last tabulated performance criteria representing an average egress success rate of about 47 % 

yielded flaming and smoldering fire performance criteria (10 %/ft obscuration and 24 %/ft 

obscuration, respectively) that most-likely would not challenge current existing standalone 

photoelectric or ionization alarms and detectors. Intermediate levels of performance are also 

presented in the table.

The selection of performance criteria for new fire tests ought to consider factors other than just 

the relative alarm performance in a series of full-scale fire experiments. The smoke production 

rate and the smoke concentration build up in the fire test room should be similar to the fullscale fire experiments to assure alarms in the fire test room respond in a similar fashion. In fact, 

tests should be conducted in the fire test room using the flaming and smoldering smoke sources

and current smoke alarm technologies to assess baseline performance. This would provide a 

reality check on the performance criteria developed from the analysis here, similar to the 

relative performance of smoke alarms placed in the hallway of the NIST Smoke Alarm Sensitivity 

Study (Table 7). In addition, nuisance alarm resistance needs to be considered in lieu of specific 

nuisance resistance tests. 

Table 10. Matched pairs of flaming and smoldering fire performance criteria where the 

average success rate is nominally equal for smoke obscuration target values on the same row.

Flaming fire test alarm criterion Smoldering fire test alarm criterion

Smoke Obscuration

(%/ft obsc.)

Averaged success rate 

and standard deviation 

(%/%)

Smoke Obscuration

(%/ft obsc.)

Averaged success rate 

and standard deviation 

(%/%)

2* 94.3/5.7 12* 93.0/4.4

4 86.0/11.4 14 86.0/11.6

5 79.0/14.1 16 80.8/16.5

6 71.8/17.0 20 69.0/19.7

8 59.8/19.1 22 58.8/20.0

10** 49.0/19.1 24** 45.3/21.7

* Matched performance achievable with combination photoelectric/Ionization alarm 

**Current standalone photoelectric and ionization alarms would most likely pass with these 

criteria [4]. 
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6 CONCLUSIONS

An analysis methodology based on the ASET/RSET concept was used to estimate the relative 

performance of smoke alarms designed to alarm at specific smoke concentrations in flaming and 

smoldering polyurethane foam chair mock-up fires. The tenability limits were defined by a 

fractional effective dose value of 0.3 for toxic gas and heat exposure, and two smoke optical 

density limits used in previous studies, 0.25 m-1 and 0.43 m-1, were considered. Analysis 

limitations and modeling assumptions included:

 interconnected smoke alarms that alert occupants regardless of initial fire location, 

 occupant pre-movement time treated as a distribution for distinct populations,

 travel speed as a function of smoke density, 

 occupants traversing a range of equally frequent pre-determined egress routes,

 considering only one apartment-sized residential space, 

 one location for the responsive smoke alarm, and

 three flaming and three smoldering scenarios, and a total of 18 full-scale tests.

Results were presented to provide guidance in selecting performance criteria for new 

polyurethane foam fire tests proposed for ANSI/UL 217 and ANSI/UL 268. Averaged 

performance criteria for flaming and smoldering fire tests were specified in matched pairs of 

smoke concentrations representing commensurate relative performance improvements. The 

matched pairs ranged from an egress success rate of about 47 % with little to no expected 

improvement achieved by adding tests with these criteria to an egress success rate of about 94 

%, representing the potential performance of combination photoelectric/ionization sensor 

alarms. Other considerations including the smoke production rate of the smoldering and flaming 

test samples and smoke transport in the fire test room may also likely affect the final 

performance criteria for the new proposed tests. In addition, some level of resistance to 

sources of nuisance alarms needs to be factored into the fire performance criteria.

Ultimately, the ANSI/UL 217 Standards Technical Panel or other regulatory bodies or standards

development organizations needs to make a judgment on the performance it would like to 

achieve with the addition of the new tests.
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