Proceedings: Indoor Air 2002
A MODEL FOR ESTIMATING SUBLETHAL EFFECTS OF IRRITANT
GASES ON EGRESS TIME IN HOME FIRE SCENARIOS
T Thomas*, S White, S Inkster, M Neily, A Lee and L Saltzman
U.S. Consumer Product Safety Commission
ABSTRACT
CPSC staff, in partnership with the National Institute of Standards and Technology (NIST)
and other organizations is completing a study into the efficacy of various smoke alarm
technologies in different home environments. A part of the study involves quantifying the
concentrations of irritant and asphyxiant gases in specific fire scenarios. CPSC staff has
developed a model to estimate the severity of the physiological effect of each gas and
approximate the delay in egress time resulting from the gas exposure.
INDEX TERMS
Irritant gases, Asphyxiant gases, Egress, Fire, Smoke
INTRODUCTION
CPSC staff, in partnership with the National Institute of Standards and Technology (NIST)
and other organizations is completing a study on the efficacy of various smoke alarm
technologies in different home environments. Data from this testing will be used to assess the
effects of asphyxiant and irritant gases on egress times. The majority of the available research
concerning the effects of irritants focuses on tenability, where an escaping occupant is
incapacitated by the presence of irritant gases. The tenability limits are usually within the
range of 1000 ppm or higher (ISO 1999). There is a dearth of data quantifying the effects of
irritant gases at lower concentrations (i.e. 10 ppm). It is assumed that the asphyxiant gases,
primarily CO, are the underlying cause of victim incapacitation. However, depending on how
rapidly asphyxiant concentrations increase during development of the fire, types and
concentrations of irritant gases might also contribute an additional effect on egress time. The
remainder of the paper discusses an approach to estimate irritant gas effects on egress time
and how a delay in escape due to irritants may provide sufficient time for the onset of
symptoms related to asphyxiant gas exposure.
METHODOLOGY
In each test site, fires were set using three fuel sources: mattresses, upholstered furniture and
kitchen grease. An array of detection devices including various smoke alarm technologies;
CO sensors, and heat, temperature, and smoke obscuration detectors, were placed in various
locations throughout each testing site. Nondispersive infrared spectroscopy (NDIR) was used
to determine the concentrations of the following gases associated with asphyxiation: oxygen
(O2), carbon monoxide (CO), and carbon dioxide (CO2). The concentrations of irritant gases,
hydrogen bromide, chloride (HCl), hydrogen fluoride (HF), oxides of nitrogen (NOx) and one
asphyxiant gas, hydrogen cyanide (HCN), were quantified by Fourier Transform infrared
spectroscopy (FTIR).
*
Contact author email: [email protected]
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Proceedings: Indoor Air 2002
The testing of fire alarms and the quantification of combustion by-products resulting from
fires have been completed in a manufactured home and a two-story brick home. Additional
tests will be conducted in a ranch house and the manufactured home. The concentration
profiles of CO, HCN, and the irritant gases will be compared to determine: 1) when CO
effects are expected to impact egress, and 2) if prior to this time, irritants reach concentrations
that can sufficiently slow egress, and 3) if the delay in egress from irritants is sufficient to
allow time for asphyxiant effects.
DISCUSSION
Asphyxiant Gases – Potential Effects
Asphyxiant gases exert their toxic effects by interfering with oxygen uptake, transport and/or
utilization, each of which ultimately causes oxygen deprivation or cellular hypoxia. Smoke
contains two major asphyxiant gases, CO and HCN. CPSC staff has extensively reviewed CO
exposures of consumers in non-fire scenarios. These scenarios are the result of consumer
products that generally produce CO at concentrations lower than those found in most fire
scenarios.
Carbon monoxide is the primary asphyxiant of concern, since it is present in all fires. In
unintentional non-fire related CO poisoning scenarios, CO elevations are typically in the 1001000 ppm range (0.01- 0.10 % CO) and are frequently maintained at a plateau level for
extended times. In contrast, fire scenarios ultimately produce much higher CO levels in the
range of several thousand ppm (>0.1%). These levels can evolve within a few minutes or
over several hours for flaming and smoldering fires respectively. CO levels closest to the fire
origin and in other areas of the building can differ dramatically.
CPSC staff considers that for fire scenarios, the CO exposures at which the victim’s ability to
egress the building will start to be compromised will be equivalent to attainment of blood
carboxyhemoglobin (COHb) levels in the range of 20 to 30%. These levels can be produced
by an infinite number of CO exposure profiles. The minimum CO level that can elevate blood
levels to approximately 20% COHb is 150 ppm within several minutes of exposure. Since the
buildup of COHb is dependent on the time course profile of CO, the best way to estimate the
appropriate CO limits for fire scenarios may be to model COHb levels based on fire test data
for CO buildup using the Coburn Forster Kane (CFK) equation.
Hydrogen cyanide, another extremely potent chemical asphyxiant, can be present in smoke
only if the burning material contains nitrogen (e.g. natural and synthetic nitrogen-containing
polymers in wood, silk and some plastics). Respiratory absorption of HCN occurs almost
immediately and severe exposures can cause extreme life-threatening cellular hypoxia within
seconds.
Irritant Gases – Potential Effects on Egress Time
The effects of irritant gases include sensory and respiratory irritation, and changes in the
upper respiratory tract that may occur during short exposure episodes (e.g. fire scenarios), and
at relatively low concentration, such as a smoldering fire.
There is a dearth of data on the impact of irritant gases on egress time at lower concentrations.
Models such as the N-gas, Purser tenability criteria, and the ISO 13571 focus on tenability
limits (Purser 1988, Levin 1997, ISO 2001). At the irritant gas levels expected in these
models, typically around 1000 ppm, incapacitation is possible due solely to irritant gases. At
lower irritant gas concentrations that are expected in the smoke alarm study (between 10 and
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100 ppm), incapacitation is not expected. The effects of irritant gases on egress time at lower
concentrations is not well established.
Exposure to irritant gases is likely to delay evacuation. At low concentrations, irritants
produce mild effects which may impair the speed of an individual’s movement through a
building (as would simple visual obscuration from smoke). The combination of irritant
health effects and smoke obscuration may be more serious and may reduce one’s ability to
respond and leave the building. At moderate concentrations, the speed of movement may
decrease due to worsening conditions. When irritants are present at high levels, the
physiological effects become more severe, and the gases are likely to have severe effects on
an occupant’s escape capability.
Smoke Toxicity – Irritant Gas Effects in Emergency Scenarios
The CPSC staff approach to estimating irritant gas effects on a person’s ability to escape is to
utilize existing occupational and environmental irritant exposure guidelines and use them as
an index in a model to predict the effects of irritant and asphyxiant gases on egress time. A
scaling factor, or egress coefficient, will be used to predict the relative reduction in egress
time due to irritant effects. It will be based on a comparison of the sum of the irritant gases
found in the smoke alarm study fires against the immediately dangerous to life and health
(IDLH), acute exposure guideline levels (AEGL’s) and other existing exposure values
(NIOSH 1994, OECD 1999). The IDLH and AEGL values may be more relevant to fire
scenarios because these recommended exposure levels are designed for emergency situations.
The threshold limit value-time weighted average (TLV-TWA) for 8-hour workplace
exposures will serve as a No/Minimal effect concentration, while the tenability limits derived
from the ISO 13571 document will be used as the most conservative concentration limit for
comparison purposes. The ISO 13571 values are similar to the tenability limits in other
models that quantify the effects of irritant gases such as the N-gas model (Levin, 1997).
Health Effects Model To Estimate Irritant Gas Effects on Egress
In the initial model, CPSC staff will apply the egress coefficient to a basic escape scenario
that involves residential fires and assumes the following conditions: a lone healthy adult with
no pets, children, or other persons who will need assistance; the decision to escape is
instantaneous; and the individual will continue to attempt to escape despite the potential
health effects, and will not attempt to save any material nor attempt to fight the fire.
In addition to the human performance effects of irritant gases on egress time, there are other
potential factors that influence evacuation. These include the effects of smoke on the
occupant’s ability to navigate and make decisions; psychological factors as fear, panic,
perception of the fire threat; and type of smoke alarm or other active intervention device (i.e.
sprinklers). These factors are not addressed in the proposed initial model. The irritant gas
concentrations will be compared to the exposure limit values to estimate the potential
physiological effects. These effects have been categorized by the degree of severity of the
expected physical effects. Each of the irritant gases may have effects at different
concentrations. It appears that most of the acid gases behave similarly, and at identical
concentrations, should roughly fall within the same effect strata.
The values listed in the Tables 2-5 are to serve as an example only to demonstrate the
methodological approach. The concentrations of each irritant gas will be converted into a
model to estimate the impact of irritant gases on escape time. The values of the gases will be
compared to the various exposure guidelines to determine the estimated degree of severity.
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Proceedings: Indoor Air 2002
The exposure guidelines for the gases quantified in the smoke alarm study are very similar,
which facilitates combining their concentrations into a single effect category. The three toxic
endpoints that are expected to result from exposure to irritant gases are coughing, respiratory
irritation, and eye irritation. The effects for each symptom will be assigned a numerical
severity score. The severity scores for each symptom will then summed to determine the
overall severity of the physiological effects. The summed severity is then converted into an
egress coefficient that estimates the relative impact on escape time.
Table 2. Irritant Gas Concentrations versus Degree of Effect
Gas Concentration (ppm)* Degree of Effect
0-50
Mild – Moderate
50-100
Moderate-Severe
100 +
Severe - Incapacitating
*Values in table are for descriptive purposes only
Table 3. Lung/Breathing Effects
Effect Severity
Physiologic Effect
0-2
Infrequent coughing, Breath holding, Shallow breathing
Mild
Frequent, deep lung, heavy coughing
Moderate 2-4
5
Body contortions, movement slowed or stopped, extremely
Severe
labored breathing
*Values in table are for descriptive purposes only
Table 4. Eye Irritation
Effect Severity* Physiologic Effect
0-2
Slow tearing, semi-regular eye wiping
Mild
Steady flow of tears, individual wipes eyes, eyes are
Moderate 3-5
mostly open
5-10 Eyes close frequently, vigorously rubbing/wiping eyes,
Severe
significant vision impairment
*Values in table are for descriptive purposes only
Table 5. Respiratory Irritation/Pain
Effect Severity*
Physiologic Effect
0-2
Irritation effects minimal
Mild
Chest clutching, movement slightly
Moderate 2-3
impacted
3-5
Airway constriction, impaired breathing
Severe
*Values in table are for descriptive purposes only
Based on the combination of expected physiological reactions to the gases, the endpointspecific effects will be combined into an overall effect. A scale will be created to quantify the
degree of impact on the escape time. The level of impact can be converted to a numerical
value in Table 6. This value will give a multiplying factor for escape time.
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Table 6. Determination of the Egress Coefficient
Egress Coefficient*
ΓEffect Severity*
0-5
Negligible 1-1.3
5-10
Minimal
1.3 – 1.5
10-15
Moderate
1.5-2
15-18
Significant 2-3
18-20
Extreme
∃3
*Values in table are for descriptive purposes only
The egress coefficient will be multiplied by the “drill” escape time to determine the delay in
egress time due solely to the irritant gases. For example, assume that in a drill, a lone, healthy
adult can escape from an upstairs room in a two-story home in two minutes. In this scenario,
the irritant gases from the fire scenario are placed in the model and result in an egress
coefficient of 1.5. The final escape time increases to three minutes (2 minute drill time x 1.5
egress coefficient) which is the product of the “drill” escape time and the egress coefficient.
CONCLUSIONS
The smoke alarm study is expected to provide data with extensive scientific and practical
application. The data derived from the study are expected to demonstrate the capabilities of
different types of alarms to respond to different kinds of severe residential fires and to provide
a more precise definition of the capabilities of today's fire alarms. If the detection of
compounds such as irritant or asphyxiant gases is determined to increase the likelihood of
escape, one result of this study may be the development of new alarms that can detect these
gases.
The model presented is an approach developed by CPSC staff for estimating the potential
effects of irritant gases on egress time. The values for the egress coefficient are a range,
which is reflective of the uncertainty involved in quantifying the effects of these compounds
on human behavioral reaction in emergency scenarios. The model will be expanded to
include other factors that affect egress. The egress coefficient is a useful tool to give an
approximate range for the effects that can be utilized to determine whether sensor
technologies should be modified to detect irritant gases.
AKNOWLEDGEMENTS:
The authors would like to express their gratitude to Richard Bukowski, Richard Gann, and
Jason Averill and their colleagues at the National Institute of Standards and Technology
(NIST). We would also like to thank the US Fire Administration, Dept. of Housing and
Urban Development, Center for Disease Control, Underwriters Laboratories, National
Research Council of Canada, National Fire Protection Association and other organizations
who have supported this research.
DISCLAIMER:
The opinions are those expressed by the authors, and do not necessarily represent the views of
the Commission. Because this material was prepared by the authors in their official capacity,
it is in the public domain and may be freely copied or reprinted.
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