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

Formula Gas Assumed LC50 for man
(ppmv)

5 min
Assumed LC50 for man
(ppmv)

30 min
Reference data (species, minutes)
h=man; r=rat; m=mouse; p=primate;
EC=conc. for effect;
LC0=conc. at which first lethal effects are observed
CO2 carbon dioxide >150,000 >150,000 [1] LC(r,30) =470,000
C2H4O acetaldehyde -- 20,000 [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
NH3 ammonia 20,000 9,000 [4] EC(m,5)=20,000 EC(m,30)=4400 [5] EC(r,5)=10,000 EC(r,30)=4000
HCl hydrogen chloride 16,000 3,700 [6] r,p [7] LC(r,5)=40,989
CO carbon monoxide -- 3,000 [1] LC(r,30)=4600 [8] LC(h,30) 3000
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 [2] LC(m,60)=673 LC0(h,30)=600 LC0(mam,5)=800 [8] LC(h,30) 2000
HF hydrogen fluoride 10,000 2,000 [2] LC(gpg,15)=4327 LC(p,60)=1774 LC0(h,30)=50 LC(m,60)=456 LC(r,60)=1276 [7] LC(r,5)=18,200 [8] LC(gpg,2)=300 LC(m,5)=6247 LC(r,5)=18,200
C3H4N acrylonitrile -- 2,000 [2] LC(gpg,240)=576 LC(r,240)=500
COF2 carbonyl fluoride   750 [3] LC(r,60)=360
NO2 nitrogen dioxide 5000 500 [4] EC(m,5)=2500 EC(m,30)=700 [5] EC(r,5)=5000 EC(r,30)=300 [7] LC(m,5)=831 LC(r,5)=1880
C3H5O acrolein 750 300 [2] LC(m,360)=66 LC0(p,10)=153 [9] LC(p,5) 505 to 1025
CH2O formaldehyde -- 250 [2] LC0(r,240)=250 [8] LC(r,30)=250 LC(r,240)=830(??) [3] LC(cat,480)=700 LC(m,120)=700
SO2 sulfur dioxide 500 -- [2] rodents poor; LC0(m,300)=6000 [8] LC(var.,5) 600 to 800
HCN hydrogen cyanide 280 135 [10] LC(r,5)=570 LC(r,30)=110 [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
COCl2 phosgene 50 90 [2] rec. 50 ppm short exp. [11] LC(h,30) 90
C4F8 perfluoroisobutylene 28 6 [2] LC(r,10)=17 LC(r,5)=28


References

[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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