Toxicity for the primary gases found in fires
...by Dr. Vytenis Babrauskas, Fire Science and Technology
Inc.
Background
I headed up the fire toxicity research of the National Institute of Standards and Technology
(NIST, formerly National Bureau of Standards) from 1985 until this program was completed in
1991. One of the most common questions which I was asked in that capacity was: "What are the
important gases one should study/measure/analyze in fires?" The simplest answer remains:
"Carbon monoxide." Speaking very roughly, this one gas accounts for about 1/2 of the fire
toxicity problem. Further details can be found in: Babrauskas, V., The Generation of CO in
Benchscale Fire Tests and the Prediction for Realscale Fires, Fire and Materials 19, 205-213
(1995).
In some cases, the simple answer does not suffice. Chemists, regulatory officials, etc. want to
know which additional gases may contribute a non-negligible amount to fire toxicity. No unique
answer is possible, since the circumstances of each fire are different. Certainly, different
materials produce different combustion products; for example, a polymer with no chlorine atoms
would not produce hydrogen chloride among its products. Having said this, valid reasons could
be seen for wanting a "shopping list" of gaseous agents. Thus, I created such a shopping list.
This was published in the first edition of NIST's fire model HAZARD I. While such a list will
always be subject to debate, the advantage of the NIST one was that it had to undergo the very
rigorous review to which NIST publications are subjected. Thus, instead of being just one
scientist's opinion, it had the consensus of the fire toxicity research and the fire modeling groups
and the informed consent of higher management.
Compilation of toxicity data
The table below presents the results for toxicity as the toxic potency variable LC50, and is an
inverse scale. That is, smaller numbers denote greater toxicity. For substances of undetermined
composition (e.g., burning plastics), the units used are normally g m-3. The meaning of this
variable is the amount of mass which needs to be dispersed into a volume of 1 m3 in order to
cause a 50% probability of lethality. For substances where the composition is known (e.g. pure
gases), the LC50 is usually expressed in units of ppmv. The definition here is that 1 ppmv of a gas
means the there is one part of gas per 1 million parts of air. The "v" denotes that we mean parts
by volume and not parts by weight.
In some cases, human data are available from industrial accidents and such. In most cases, it is
necessary to use animals as surrogates. These must be selected so that the biological system
which is affected by a particular toxin is one which functions relatively similarly in man. For
example, hydrogen chloride inhalation data from mice have generally been considered to be a
poorer prediction of human fatality than data from rats, due to respiratory system differences. In
all cases, of course, the numbers are only estimates and can never be actually confirmed on
humans.
The time of exposure is also important for determining the effects from toxic gases. In general, a
higher concentration allows only a shorter time, for the same biologic effect to be reached. If the
concentration is exactly hyperbolic, that is, (concentration)x(time)=constant, then this gas is said
to follow Haber's Law. Almost any gas shows deviations from this simple relationship, but for
rough estimating purposes it can be used if better data are unavailable. For fire toxicity data, the
exposure period normally used is 30 minutes.
Users often ask "Why do you compile data for lethality instead of data for incapacitation?" This
is a legitimate question, since usually we would like to know for how long a person will be all
right in fire, not just when the morgue has to be called. The reason why such a compilation was
not developed by NIST is a practical one. It is usually possible to obtain reliable measurements
of animal lethalities. By contrast, the concept of incapacitation does not even have a workable
definition. The conditions of incapacitation depend greatly on the task which is to be done.
Incapacitation can also be affected by alcohol intoxication, and by various medical conditions;
these have not been adequately studied in combination to derive usable guidance. The NIST
recommendation to users was derive a hypothetical incapacitation level (EC50) by reducing the
LC50 values by a suitable margin, say 2- to 4-fold.
As a small sidelight, while the values of EC50 will be expected normally to be several-fold lower
than the corresponding LC50 values, occasionally the opposite is seen. During the early 1980s
when the so-called USF toxicity test was being evaluated, many strongly lethal gas mixtures
were documented to have unexpected large EC50's (the test used mice and measured EC50 levels
only). These turned out not to be measurement errors, but systematic problem formulation errors.
The animals which showed the surprisingly high EC50 values were, in fact, the "walking dead."
They were able to perform the tasks required of them in the EC50 testing procedure while they
were already mortally injured.
By inspecting the list, one can readily see that CO is not greatly toxic and that there are many
more gases found in fires which are notably more toxic. The reason why CO is considered the
primary agent is simply due to its copious generation by all fires. The importance of any toxic
gas species to a particular fire must reflect both its toxicity and its actual concentration in that
particular fire. This is the fractional effective dose concept and algorithms are available for using
it. Such data cannot be tabulated in a handbook fashion, however, since the concentrations are
completely specific to the fire in question.
It may be noted from the references that most of the data are from the 1980s and earlier. This can
largely be explained by the fact that major fire toxicity research groups at NIST, the Southwest
Research Institute, Hungtingdon, and other institutes were closed or dispersed around the start of
the 1990s. This was largely due to 'a job well-done,' but budgetary considerations were also an
issue.
Miscellaneous caveats
The data assembled here refer only to acute exposures. Thus, they are pertinent to civilians
encountering an unwanted fire. A fire fighter, by contrast, may be exposed to fire gases on a
recurring basis. A number of research studies have been conducted in this area, but they have not
yielded any similar table to this one for chronic exposure effects.
Burn and thermal exposure injuries have been extensively studied, but there is little data on
combinations of thermal and toxic gas injuries. Some arithmetic scaling laws have been
proposed, but there is no satisfactory validation of them. At the moment, the situation is viewed
non-interactive. In fire modeling, it is common to compute thermal and toxic exposures and to
declare as governing the first one where limit values are reached.
Table of primary fire gas toxicities
Assumed Assumed
LC50 for
LC50 for Reference data (species, minutes)
man
man
Formula
h=man; r=rat; m=mouse; p=primate
Gas
CO2
carbon dioxide
C2H4O
acetaldehyde
(ppmv)
(ppmv)
5 min
30 min
>150,000
>150,000
--
20,000
EC=conc. for effect; LC0=conc. at which first lethal
effects are observed
[1] LC(r,30) =470,000
[2] LC(m,240)=1500 LC0(r,240)=4000 LC(ham,240)=17,000
[3] LC(r,30)=20,000 LC(r,240)=16,000
C2H4O2
acetic acid
--
11,000
[2] LC(m,60)=5620
[4] EC(m,5)=20,000 EC(m,30)=4400
NH3
ammonia
20,000
9,000
[5] EC(r,5)=10,000 EC(r,30)=4000
[6] r,p
hydrogen
chloride
16,000
carbon
monoxide
--
HBr
hydrogen
bromide
--
3,000
[2] LC(m,60)=814 LC(r,60)=2858
NO
nitric oxide
10,000
2,500
[3] 1/5 as toxic as NO2 LC(h,1) 15,000
COS
carbonyl
sulfide
--
2,000
[2] LC0(var.,3590)=10001400
H2S
hydrogen
sulfide
--
2,000
HCl
CO
3,700
[7] LC(r,5)=40,989
[1] LC(r,30)=4600
3,000
[8] LC(h,30) 3000
[2] LC(m,60)=673 LC0(h,30)=600 LC0(mam,5)=800
[8] LC(h,30) 2000
[2] LC(gpg,15)=4327 LC(p,60)=1774 LC0(h,30)=50
LC(m,60)=456 LC(r,60)=1276
HF
hydrogen
fluoride
10,000
2,000
[7] LC(r,5)=18,200
[8] LC(gpg,2)=300 LC(m,5)=6247 LC(r,5)=18,200
C3H4N
acrylonitrile
COF2
carbonyl
fluoride
--
2,000
750
[2] LC(gpg,240)=576 LC(r,240)=500
[3] LC(r,60)=360
[4] EC(m,5)=2500 EC(m,30)=700
NO2
nitrogen
dioxide
5000
500
[5] EC(r,5)=5000 EC(r,30)=300
[7] LC(m,5)=831 LC(r,5)=1880
[2] LC(m,360)=66 LC0(p,10)=153
C3H5O
acrolein
750
300
[9] LC(p,5) 505 to 1025
[2] LC0(r,240)=250
CH2O
formaldehyde
--
250
[8] LC(r,30)=250 LC(r,240)=830(??)
[3] LC(cat,480)=700 LC(m,120)=700
[2] rodents poor; LC0(m,300)=6000
SO2
sulfur dioxide
500
-[8] LC(var.,5) 600 to 800
[10] LC(r,5)=570 LC(r,30)=110
HCN
hydrogen
cyanide
280
135
[7] LC(r,5)=503 LC(m,5)=323
[8] LC(h,30)=135 LC(h,5) 280
C9H6O2N2
toluene
diisocyanate
--
100
[2] LC(gpg,240)=13 LC(rbt,180)=1500 LC(r,360)=600
LC(m,240)=10
[8] LC(m,r,rbt,gpg,240)=9.7 to 13.9
[2] rec. 50 ppm short exp.
COCl2
phosgene
C4F8
perfluoroisobu
tylene
50
90
[11] LC(h,30) 90
28
6
[2] LC(r,10)=17 LC(r,5)=28
References cited
[1] Levin, B. C., Paabo, M., Gurman, J. L., Harris, S. E., and Braun, E., Toxicological
Interactions Between Carbon Monoxide and Carbon Dioxide, Toxicology 47, 135-164 (1987).
[2] Sax, N.I., Dangerous Properties of Industrial Materials, Van Nostrand Reinhold, New
York (1984).
[3] American Conference of Governmental Industrial Hygienists, Inc., Documentation of the
Threshold Limit Values, ACGIH, Cincinnati (1980).
[4] Sakurai, T., Toxic Gas Test by the Several Pure and the Mixture Gas, in the Third Expert
Meeting of the Canada-Japan-United States Trilateral Cooperative Study on Fire Gas Toxicity,
Ottawa (1984). Published as NBS report NBSIR 88-3753 (1988).
[5] Nishimaru, Y., Study on Evaluation of Toxicity of Gas Using Pure Gas, pp. 613-628 in Proc.
of the 7th Panel Meeting of the U.J.N.R. Panel on Fire Research and Safety (NBSIR 85-3118), N.
H. Jason and K. Davis, eds., [U.S.] Nat. Bur. Stand. (1985).
[6] Hartzell, G. E., et al., Intoxication of Rats by Carbon Monoxide in the Presence of an Irritant,
J. Fire Sciences 3, 263-279 (1985).
[7] Higgins, E.A., Fiorica, V., Thomas, A.A., and Davis, H.V., Acute Toxicity of Brief Exposure
to HF, HC , NO2, and HCN with and without CO, Fire Technology 8, 120-130 (1972).
[8] Kimmerle, G., Aspects and Methodology for the Evaluation of Toxicological Parameters
during Fire Exposure, JFF/Combustion Toxicology 1, 4-51 (1974).
[9] Kaplan, H. L., et al., Combustion Gases in Postcrash Aircraft Fires (DOT/FAA/CT-84/16),
Federal Aviation Administration, Atlantic City (1984).
[10] Levin, B. C., et al., Effects of Exposure to Single or Multiple Combinations of the
Predominant Toxic Gases and Low Oxygen Atmospheres Produced in Fires, Fundamental and
Applied Toxicology 9, 236-250 (1987).
[11] Clayton, G.D., and Clayton, F.E., eds., Patty's Industrial Hygiene and Toxicology, vols.
2a-2c, Wiley-Interscience, New York (1982).
This article © Copyright 1996, 1997 by Vytenis Babrauskas.
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