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EXECUTIVE SUMMARY
The National Research Council's Committee on Fire
Toxicology was formed in December 1984 in the Board on
Toxicology and Environmental Health Hazards (now the
Board on Environmental Studies and Toxicology) in the
Commission on Life Sciences. It was supported by a
consortium of federal agencies (the Consumer Product
Safety Commission, the Federal Aviation Administration,
the Department of the Navy, and the Environmental
Protection Agency) concerned with developing sound
regulatory policy. The Committee's general task was to
review the state of the art of combustion-product
toxicity testing and fire hazard assessment. In
addition, the Committee considered the relationship
between the physiologic and behavioral end points
currently used in combustion-product toxicity test
systems and the performance capabilities of humans
exposed to pyrolysis and combustion products. The
Committee was also to evaluate fire hazard models (both
available and in development), focusing on the use of
toxicity as an input, and provide guidelines for their
application.
HAZARD ASSESSMENT VS . RI SK ASSESSMENT
Risk is the product of an event's severity (i.e., its
degree of hazard) and the probability that the event will
occur. This report deals only with fire severity (i.e.,
fire hazard) and its quantification. The degree of fire
hazard is a function of a number of factors, such as fuel
load, building structure, ignition, and propagation of
flames, but also including the amount and toxicity of the
smoke, the exposure to the smoke before escape, and the
1
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exposure to heat. The assessment of fire hazard concen-
trates on combining the factors that affect the time
available for escape (TAE).
ASSESSMENT OF FIRE HAZARD
Hazard assessment, whether based on full-scale simula-
tions or on mathematical models, requires an array of
quantitative information, such as:
.
The amount of material present.
The amount of energy required to ignite the
material and spread flame over its surface.
.
The mass loss and heat release rate of the
material, both when it is burning alone and when it is
exposed to known energy fluxes from external sources.
· The toxic potency of its smoke, expressed in
terms of concentration, such as lethal concentration,
effective concentration, or lethal concentration-time
product.
· Similar information on whatever else is burning,
in addition to the material of interest.
.
Ventilation in the fire environment.
· The geometry and thermal characteristics of the
compartment that contains the fire (the fire environment)
Detection models calculate the size of a fire at the
time the detector (of smoke or heat) is activated and
therefore the extent to which smoke or heat has developed
in the compartment that encloses or is adjacent to the
detector. Fire growth and smoke transport models can be
used to predict the buildup of heat and smoke, thereby
permitting calculation of TAE. The most widely known is
the Harvard fire model, which can predict the growth of a
fire with time and the resulting buildup of smoke and
heat in up to five interconnected compartments, all on
one level. The model's outputs include the times of
ignition of second or third objects, the rate of gas
outflow from the compartment, and the concentrations of
various species in the outflowing gas. The FAST model
.
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developed at the National Bureau of Standards embodies
some of the characteristics of the Harvard code, but
emphasizes prediction of movement of fire products. It
is claimed to provide more "rugged" or ~robust" solu-
tions--e.g., those with a smaller tendency to give grossly
wrong answers or to fail to run to completion--for a wide
variety of input parameters. Such models assume that each
compartment divides into a hot upper zone and a cold lower
zone, with no mixing--hence their frequent designation as
"zone" models. In contrast, field models divide a com-
partment into hundreds or thousands of zones in a three-
dimensional array and can therefore predict fluid motion
far more realistically than zone models can. Field
models, however, require very powerful computers and are
not yet practical for routine hazard analysis.
Computer models now becoming available can, in prin-
ciple, calculate the development of a fire in a compart-
ment and the buildup of smoke at specific locations in
it. Because toxicity data are relevant to the TAE, the
fate of occupants of those locations cannot always be
predicted unless the smoke toxicity is known: TAE can be
compared with the time needed for escape or rescue (TNE)
for a selected scenario. TNE in turn also depends on the
ages and health of the occupants. Theoretically, non-
lethal exposure to toxic fire products can affect the TNE
(e.g., by impairing mental acuity and so hindering
escape), so for some scenarios data on nonlethal effects
would be more relevant than lethality data.
The details of the computations vary with the scenario
under consideration, but it should be clear that smoke
toxicity data constitute only one ingredient. Toxicity
data alone are insufficient for complete and accurate
assessment of a fire hazard.
The overall hypothesis of hazard assessment is that
survival of any fire is likely if the TAE exceeds the
TNE. TAE depends on how quickly the environment becomes
untenable; this in turn is controlled by the material's
flammability and smoke toxicity. TNE is largely inde-
pendent of the material burning.
Besides having some importance in predicting the
outcome of a given scenario, TAE serves as a surrogate
for a material's relative fire hazard in that scenario.
A comparison of the TAES for a series of materials is
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therefore considered to give an indication of their
relative fire hazards under the selected conditions.
THE TESTING OF COMBUSTION-PRODUCT TOXICITY
At least two combustion-product test methods can be
used to provide the toxicity data required for modeling
hazard: the National Bureau of Standards (NBS) and
University of Pittsburgh methods. These methods are
relatively well documented and yield toxicity values in
substantial agreement for most materials that have been
tested with both methods. The Deutsche Industrie Norm.
. ,
.
tU1N) ~d 45b method, developed in Germany, probably would
also he adequate, but has not been thoroughly evaluated
in the United States. And it is to be expected that
these bioassays will be improved and that others will be
developed for specific uses (e.g., to measure effects on
mental acuity).
In general, the primary unit of toxicity is the LC50,
which is the concentration of a toxicant that causes death
in 50% of the exposed animals in a specified period. The
L(Ct)50, a unit that combines concentration (of fire
products) and time, where appropriate for integration
into a numerical fire hazard model, would theoretically
make more refined results possible. The few pathologic
measures that have been used (e.g., lung weights and
corneal opacity) have yielded only limited information on
the biologic effects of exposure to fire products.
No test providing data on incapacitation has yet been
developed that is demonstrably more sensitive than the
use of death as the end point, although for some fire
scenarios accurate measurement of incapacitation or
performance decrements could be important. If such a
test is developed, one would wish to demonstrate that
incorporation of an end point other than death into a
fire hazard model improved the ability to assess hazard.
In the NBS method, a quartz beaker is heated to above
or below the autoignition temperature of a sample to be
burned; the sample is then placed in the beaker. Gases
are collected in an airtight chamber, where rats are
exposed for 30 min. The test results (referred to as
LC50s) are expressed as sample weight charged per
chamber volume (mg/200 L). The animals are observed for
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5
14 days after exposure, and the ~LC50" is based on the
number of animals killed at each "concentration" (although
these are not actually exposure concentrations or dosages,
because the atmosphere in the exposure chamber is not
characterized). The method has proved reliable and
reproducible in both intralaboratory and interlaboratory
tests with Douglas fir and various other materials.
In the Pittsburgh method, the sample is placed in a
furnace that is then heated at 20°C/min. Mice are exposed
to the continuous smoke stream, which is diluted with
chilled air, for 30 min. This method also generates
reproducible results. Although it has not been subjected
to as many interlaboratory tests as the NBS method, the
data suggest good agreement among results from three
laboratories for Douglas fir and two other materials.
As is true of any small-scale test, these tests do not
model a "real fire" accurately. If only the data per-
tinent to mortality (LC50) are used in the estimation
of hazard, both methods might be equally applicable.
However, if mass loss rate and time to death were to be
used, only the Pittsburgh method could provide this
information. Although there might be exceptions to this
generality, it appears on the basis of the limited
comparative data available that the choice of one or the
other method would not alter substantially the outcome of
a fire hazard assessment.
For purposes of predicting the fire hazard of different
materials, the Committee believes that the required smoke
toxicity data are currently best obtained with animal-
exposure methods. However, chemical analysis of smoke
might be useful in the process of measuring smoke toxi-
city. The advantages of chemical tests are that many are
quicker to perform than bioassays and that they avoid the
use of test animals. The main advantage of biologic tests
is that they produce data of high validity. The major
potential danger of a chemical test is that it could
"miss" unanticipated, and perhaps unusually toxic, com-
bustion products (although unusually toxic combustion
products whose formation was not predicted by chemistry
have rarely been encountered); there is little danger of
missing biologically relevant response in a bioassay. In
addition, most current fire hazard models are designed to
accommodate toxicity data in the form of LC50 or L(Ct)50
values; the use of chemical data alone in such a model
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would require development and verification of an appro-
priate scheme to summarize and add the various measured
concentrations.
A toxicity-testing strategy that avoids the uncer-
tainties of a chemical analysis while exploiting its
advantages could have the following steps:
.
Chemically analyze the test material's smoke for
expected major toxicants, such as carbon monoxide,
hydrogen cyanide, and hydrogen chloride.
· Calculate an "expected" LC50 for the smoke, on
the basis of the response of test animals to the
toxicants identified in the chemical analysis.
.
Perform a bioassay of the material's smoke at,
slightly above, and slightly below the expected LC50.
If all the important toxicants have been identified in
the chemical analysis, this test should be sufficient to
confirm that identification and to yield an approximate
LC50. If the observed LC50 is very different from
the expected LC50, the difference will be apparent, and
more extensive bioassays must be carried out.
Beyond LC50 data, the routine measurement of carbon
monoxide in smoke or of carboxyhemoglobin in the blood of
exposed animals, however useful such measures are for
research purposes, provides no information of utility to
hazard assessment efforts that is not provided with more
certainty by the LC50 itself.
CONCLUSIONS AND RECOMMENDATIONS
There is a strong need for additional research in
combustion-product toxicity testing and fire hazard
assessment. Indeed, knowledge in these fields is still
quite incomplete. Thus, the approaches embodied in the
following conclusions and recommendations should be
viewed as being of an interim nature. The issues should
be reviewed again in 5 years and the recommendations
revised in the light of new knowledge.
No model or test method comes close to reproducing the
peril in which fire places human life. The results of
mathematical models are only as good as the data used in
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them, and those data, whether they are abstractions of
smoke movement and ventilation or toxicity values deter-
mined in rodents, are only representations of the reality
that takes the lives of some 5,000 people a year in this
country. Although efforts to reduce the likelihood of
fire must continue, we must assume that there will always
be fires. We must therefore work to improve our ability
to cope with fire by improving the fire performance of
the materials that furnish the places in which we live
and work, while continuing to improve building designs,
fire codes, and fire detection and suppression techniques.
· Although mung the highest in the world, the
number of fire fatalities in the United States has been
declining for the last 30 years.
The concern that the introduction of synthetic mater-
ials into general use in homes has increased the risk
associated with fire is not supported by data on recent
fire-death trends. New materials are being used in homes
and commercial buildings, and these materials have dif-
ferent combustion characteristics, but the number or rare
fatalities per 100,000 people has declined. This decline
cannot be fully explained by improvements in firefighting
equipment, sprinkler systems, or detectors, although they
_ _ ~
are relevant.
· The best-characterized threats associated with
fire are the acute results of exposure to heat, the toxic
agents and irritants that make up smoke, and perhaps
oxygen depletion. Long-term effects of repeated exposure,
such as would be encountered by firefighters, have not
been conclusively characterized.
Smoke inhalation is the cause of death in the majority
of fire fatalities. The toxic components of smoke are
largely carbon monoxide and other gases, such as hydrogen
cyanide. Carbon monoxide is well accepted as a factor in
50-80% of all fire fatalities; the role of hydrogen
cyanide is still under investigation. Other components,
such as respiratory and sensory irritants, might con-
tribute to the inability of people to escape from fire,
as well as to long-term pulmonary complications in
survivors. The cause of death of many fire victims is
not fully understood. Even less is understood about the
potential long-term health consequences in survivors of
single exposures to fire. The influence of the type of
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materials involved in a fire on mortality has not been
established. To determine cause of death, postmortem
study of fire victims would include both autopsy and
blood-gas investigations (for carbon monoxide, hydrogen
cyanide, etc.). Prospective studies of pulmonary and
necrologic function in both single-exposure fire victims
and occupationally exposed firefighters are needed to
evaluate long-term health consequences. There seems to
be no conclusive evidence that the long-term effects of
repeated exposure to fire include an increased risk of
developing cancer, although some groups of firefighters
have been found to have excesses of some cancers.
Finally, research on chemical and cellular biologic
markers of combustion-product toxicity (e.g., with
bronchoalveolar ravage) should continue, inasmuch as such
markers might provide early indicators of pathologic
effects of smoke toxicity.
· The dynamics of a fire in generating heat and
toxic products will determine the ability of people to
escape; the use of fire hazard assessment to estia te
ability to escape a given fire is currently the best
approach to measuring the hazard associated with
materials.
Determination of the likelihood of escape from a
burning building requires evaluation of the time avail-
able for escape and the time needed for escape. TAE can
be calculated from the time at which the fire is detected,
the temperature and the quantity of smoke, and the growth
curve of the fire or smoke. TAE is not to be used at
face value as a measurement of real time, but as a tool
for comparing materials under some set of conditions.
a rapidly growing fire, temperature can increase so
rapidly that the toxicity of smoke is irrelevant. In a
fire that is growing slowly or if the potential victim is
not in the same room as the origin of the fire, the
toxicity of smoke might be the prime source of danger.
In
TNE is calculated from factors associated with the
potential victim's ability to find a safe escape route,
the nature of the building, the victim's age and related
characteristics, etc.
· If a combustion-product toxicity test is to be
useful in a hazard model, it should have good inter-
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laboratory reproducibility, should differentiate between
materials on the basis of relative toxicities, should be
able to determine dose-response relationships for a given
material, and should yield data in units that are co"
patible with hazard models.
Just as there is no single set of fire conditions,
there is no "correct" set of toxicity-test parameters.
Different tests can be expected to yield different
rankings of the same group of materials. This failure of
agreement is unlikely to be resolved by additional
research and test development; and total agreement might
not even be desirable, in that use of a single "standard"
test could lead to unnecessarily restricted sampling of
combustion conditions.
· Laboratory metbDds for measuring the toxicity of
combustion products have been developed to the point
where relative toxicities of materials can be reliably
measured.
Both the NBS and the Pittsburgh test methods provide a
comparison of relative toxicities; when used by different
laboratories, each has reasonable reproducibility.
Neither method provides a complete model of a real fire,
but each provides data on some aspect of combustion.
Data from these methods can be used in hazard models.
Currently used toxicity test methods use
lethality as the end point; other end points remain to be
remelted.
The use of death as the end point provides a reliable
index of smoke toxicity, but fails to provide information
on the inability of people to escape fires. This inabil-
ity could be due to sublethal exposure to toxic gases,
such as carbon monoxide, or to the effects of irritants
in impeding escape. No animal model of sublethal effects
has been found more useful than measures of lethality in
providing the desired information. A sensitive measure
of sublethal effects would theoretically improve the
ability to assess hazard; at the least, it would allow
for a more logically consistent representation of ability
to escape.
In order to understand the effects of combustion
products on mental acuity, specific tests of impairment
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of judgment or performance in animals and in humans should
be developed. The results of such tests could be used in
a fire hazard evaluation in various ways, e.g., with
respect to toxicity or human performance.
.
As a basis for judging or regulating materials'
performance in a fire, co~bustionrproduct toxicity data
must be used only within the context of fire hazard
assessment.
In determining overall hazard to people, toxicity data
obtained from animal experiments should be incorporated
into a fire hazard model. Given all the other factors
that are relevant to fire hazard, such as rate of burning
and heat generation, toxicity cannot be the sole criterion
for defining the hazard. If products for some intended
use have been shown to be very similar in composition and
other fire properties, a pass/fail decision that depends
on a toxicity test could be justified. Although this
appears to be a screening test, it is in fact simply the
final point of discrimination in a less formal hazard
analysis. For uses with no regulatory component te.g.,
manufacturer's surveillance of products under develop-
ment), any chosen test can be used for screening, with
specific performance criteria set by the user.
.
Because of the possibility that new toxic
chemicals will not be detected in chemical tests of
combustion products, biologic tests Bust remain the
ultimate toxicity assays.
Chemical tests can be extremely useful in measuring
concentrations of known chemicals in combustion products
and thus might become a first screen for testing toxicity.
An animal biologic model acts as the ultimate integrator
of the combined toxicities of combustion products, whereas
a chemical assay is a selective measure of specific
chemicals and might or might not detect all toxic agents.
Therefore, animal tests must remain as the final deter-
minants of human hazard. This necessity could become
even more evident as nonlethal measures of toxicity
become available.
.
Although techniques will continue to improve,
fire hazard assessment can already be used to answer
fundamental questions about the suitability of materials.
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Computational models require a knowledge of the
burning rate of materials and critical concentrations of
toxic combustion products, if TAE is to be calculated.
TAE is used as a surrogate of fire hazard and makes it
possible to compare relative fire performance of
materials in a given application.
Powerful computers and increasingly detailed analysis
of more complex spaces will lead to better understanding
of the dynamics of fires and to more realistic
approximation of TAE. In the meantime, however, many
regulatory questions can already be answered.
Representative terms from entire chapter:
hazard assessment
