INHALATION TOXICOLOGY: 

Office of Aviation Medicine VIII. ESTABLISHING HEAT TOLERANCE LIMITS 

Washington, D.C. 20591 FOR RATS AND MICE SUBJECTED TO ACUTE 

EXPOSURES AT ELEVATED AIR TEMPERATURES 

Charles R. Crane 

Donald C. Sanders 

Civil Aeromedical Institute 

Federal Aviation Administration 

Oklahoma City, OK 73125 

Technical Report 

This document is available to the public 

through the National Technical Information 

Service, Springfield, VA 22161. 

U. S. Department of Transportation 

Federal Aviation Administration 
NOTICE 

This document is disseminated under the sponsorship of 

the Department of Transportation in the interest of 

information exchange. The United States Government 

assumes no liability for its contents or use thereof. 

The animals used for this experiment were lawfully 

acquired and treated in accordance with the "Guide for 

the Care and Use of Laboratory Animals," National 

Research Council. DHHS Publication No. (NIH) 86-23. 
ACKNOWLEDGMENTS 

The authors wish to acknowledge the assistance of 

Boyd R. Endecott and John K. Abbott with the construction 

of the exposure chamber and the collection of the data. 

Tcchnicol Report Documentotion pop 

I. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-86/ 8 

4. Title and Subtitle 5. Report Date 

INHALATION TOXICOLOGY: VIII. ESTABLISHING HEAT 

ACUTE EXPOSURES TO ELEVATED AIR TE"E"I'URES AAM-114A 

TOLERANCE LIMITS FOR RATS AND MICE SUBJECTED TO 6. Performing Organization Code 

May 1986 

7. Author's) 

8. Performing Organtzation Report No. 

I 

Charles R. Crane and Donald C. Sanders DOTIFAAIM-86 I8 I 

FAA Civil Aeromedical Institute 

P.O. Box 25082 11. Controct or Grant No. 

Oklahoma City, OK 73125 

9. Performing Organizotion Name and Address 10. Work Unit No. (TRAIS) 

13. Type of Report and Period Covered 

12. Sponsoring Agency Nome ond Address TECHNICAL REPORT 

Office of Aviation Medicine 

Federal Aviation Administration 

July 1978 to Jan. 1979 

Washington, D.C. 20591 

800 Independence Ave. SW. 14. Sponsoring Agency Code 

This work was performed under tasks AM-A-78-TOX-36 and AM-B-79-TOX-39. 

15. Supplementary Notes 

c 

16. Abstract 

Experimental animal subjects are used most commonly to assess the toxicity of 

thermal decomposition products (smoke) from burning materials. Nascent smoke is 

obviously quite hot; therefore, the design of smoke toxicity assay systems must 

provide for adequate cooling of the gases prior to exposure of the animals. This 

research has addressed the question of how much cooling is required. 

Rats and mice were exposed to elevated air temperatures over the range of 38 OC to 

110 OC. The exposure duration required to produce hyperthermic collapse (physical 

incapacitation) was measured for each temperature. A graph of time-to-collapse as 

a function of exposure temperature was constructed for each species and statistically derived equations were fit to each data set. Times-to-collapse ranged, for 

the rat, from 60 minutes at 40 OC to less than 4 minutes at 110 OC. For the mouse, 

they ranged from approximately 60 minutes at 40 OC to 2.5 minutes at 90 OC. The 

significance of these findings as they relate to smoke toxicity testing is 

discussed. 

17. Key Words 18. Distribution Statement 

Time-to-incapacitation; thermal collapse; 

tion Service, Springfield, VA 22161. safety. 

hyperthermia; smoke; fires, aircraft; through the National Technical InformaThis document is available to the public 

19. Security Classif. (of this report) 20. Security Classif. (of this pogc) 21. No. of Pages 22. Price 

Unclassified Unclassified 12 

form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

INHALATION TOXICOLOGY: VIII. 

ESTABLISHING HEAT TOLERANCE LIMITS FOR HATS AND MICE 

SUBJECTED TO ACUTE EXPOSURES fiT ELEVATED AIR TEMPERATURES 

Fire in the environment of an aircraft accident significantly increases the likelihood of serious injury and death for 

passengers or crew who otherwise might escape unharmed. The 

burning of spilled fuel and/or nonmetallic cabin materials produces the two elements primarily responsible for this increased 

risk, namely, an elevated environmental temperature and an atmosphere of toxic and irritating gases. Many laboratories are currently involved in efforts to identify the nature and significance of the tonic hazard associated with the fire environment, 

but there is little or no research on the thermal hazard. 

Thermal effects could be either direct or indirect. A 

direct effect would be one that resulted from the transfer, from 

the environment to an individual, of a quantity of heat sufficient to produce physical ,incapacitation or death. Some indirect 

effects might be: the panic resulting from being swrounded by, 

or having an exit blocked by, a mass of hot air or a sheet of 

flames; the increased rate of accumulation of toxic gases brought 

on by increased respiration rate in a hot environment; or the 

increased toxicity or irritation of gases inhaled at an elevated 

temperature. 

Almost all assay protocols in combustion toxicology utilize 

some physiological response of an experimental animal to measure 

toxicity. If thermal stress to the animal is significant, i.e., 

the temperature and caloric content of the exposure atmosphere 

exceed certain critical levels, one wil be measuring combined 

effects of heat and toxic gases. Thus, for meaningful evaluation 

of test procedures and results, it would be desirable to know the 

magnitude of any thermal hazard component of a test procedure. 

When- a small-animal test system for evaluating the toxicity 

of combustion products was designed and used at the Civil Aeromedical Institute (CFIMI) % chamber atmosphere temperatures up to 

.A "C were shown to have no effect on the toxicity of CO or HCN. 

The effect of temperatures above 35 "C was not explored since it 

was quite easy to maintain the atmosphere below that value in 

this small system. 

TC 

For those studies that require a larger system, and especially for so- called "full scale" systems, it becomes difficult 

if not impossible to maintain an atmosphere below 35 "C. To 

evaluate these and other test procedures, it would be helpful to 

1 
1::now the thermal tolerance limits for the species of experimental 

animal used. Since this information could not be found in definitive form in the scientific literature, we undertook a limited 

investigation to define the heat tolerance limits of rats and 

mice when exposed to elevated atmospheric temperatures for rela - 

tively short periods of time. 

An exposure chamber with a removable top was constructed of 

0.125-inch plywood sandwiched between 30-mil sheets of aluminum. 

Inside dimensions were 22 by 24 by 26 inches: the enclosed volume 

is approximately 225 L. A controlled, electrical resistance 

heating element (9-in dial was mounted in front of a ?-in fan in 

one corner of the floor with the fan output directed toward the 

diametric corner. Two additional 6-in fans were mounted near the 

ceiling, blowing parallel to the ceiling, but in opposite 

directions. 

An 8-in-diameter, three-compartment rotating cage, as used 

in the CAM1 toxicity test protocol was mounted near the ceiling and driven at 6 rpm by a powered shaft that extended through 

one wall j the subjects were thus forced to walk at a linear 

velocity of 150 in/min. The ceiling of the chamber has a pane of 

thermally resistant glass mounted over the cage area to permit 

visual observation of the test animals. The animals were 

shielded from direct thermal radiation of the heater and heater 

shroud by the placement of an aluminum-f oi l-covered asbestos 

board between the rotating cage and the hot surfaces. Relative 

humidity (RH) within the chamber was neither controlled nor measured during a test; however, the RH at the start of each test has 

been 20 to 3:) percent. 

Animals were obtained from the Charles River Breeding Laboratories, Wilmington, MA. Rats were male albinos (Sprague-Dawley 

derived) and mice were randomly bred male albinos, CD-1 strain. 

Both species were inspected by a veterinarian on receipt and then 

held in isolation for 8 days. All were maintained for 4 days on 

drini::ing water containing 1.5 g/L of sulfathiazole, then normal 

tap water for the remaining 4 days of isolation. All were fasted 

overnight prior to use but were allowed access to water. 

The exposure chamber was preheated to the desired temperature and maintained for 5 minutes tu allow thermal equilibrium of 

all components. The cage was then removed, an animal was placed 

in each of the three compartments, and the cage was replaced in 

the chamber. The rotation motor, the temperature recorder chart, 

and a timer were all turned on simultaneously. The elapsed time 

at which each animal became physically incapacitated, as determined visually by the observer, was recorded as the time-toincapacitation, ti. 
For the rat study, a minimum of two replications were conducted at all but two temperatures; only one run (three rats;) was 

conducted at 38 "C and one at 48 "C. A total of 59 rats were 

exposed at 11 different temperatures. 

Some laboratories utilize mice for fire- hazard testing: 

therefore, we felt it was desirable to conduct a few experiments 

with mice even though the CAM1 inhalation toxicity protocol utilizes rats exclusively. The same chamber and techniques were used 

for the mouse study as were for the rat study. Exposures were 

conducted at five temperatures with a total of 13 mice. 

The justification for any amount of this type of animal 

research--necessarily accomplished with unanesthetized subjects-- 

is the desire to reduce fire hazards for humans. We felt that the 

need for a meaningf cil smoke toxicity assay protocol ? that is, one 

for which the contribution from hyperthermia would be negligibl'e, 

was a powerful justification in itself; however, we also felt 

compel led to ask how the animal responses might relate to those 

for humans in similar thermal environments. We therefore conducted an in-depth survey of the 1 iterature concerned with human 

thermal tolerance limits. 

RESULTS AND DISCUSSION 

The results of the exposures with rats are in Table 1; those 

for mice are in Table 2. These data, for each of the two species, were used to derive by nonlinear, least squares regression 

techniques an equation that best fit each data set. Since thermal radiant energy flux is a function of the fourth power of the 

temperature in degrees Kelvin (K4) , the exposure temperature 

appears in the fitted equations in that form. Those equations 

are: 

where tr = time-to-incapacitation, in minutes, and 

K = exposure temperature, degrees Kelvin. 

Figures 1 and 2, respectively, illustrate the correspondence 

between the data set and the derived equation for rats and for 
TABLE 1. RAT INCAPACITATION TIME AS A FUNCTION 

OF AIR TEMPERATURE, FOR TOTAL-BODY EXPOSURE 

Time - T o - Incapacitation, min. 

Run No. Air Temp. 

"C (1) (2) (3) 

1. 

2 

3 

4 

5 

6 

7 

8 

9 

1 0 

11 

12 

13 

14 

15 

16 

17 

18 

19 

2 0 

:. 2 4 (:I 

83.9 

~-.6 

28.8 

28.5 

25.6 

1 7 . (3 

1'3.2 

11.5 

9.5 

8.3 

6.7 

7.2 

4.9 

J . 0 

4.6 

4.5 

3. 9 

3.8 

r-l 

18. fs 

c 

T 

:::. 2 4 (1) 

67.5 

83.3 

28.9 

24.6 

17.9 

17.4 

12.6 

11.8 

8.5 

8.2 

6.2 

6.4 

4.7 

J. 2 

4.7 

4.3 

3.4 

3. 9 

---- 

e 

-., 

TABLE 2. MOUSE INCAPACITATION TIME AS A FUNCTION 

OF AIR TEMPERATURE, FOR TOTAL-BODY EXPOSURE 

Time - T o - Incapacitation, min. 

Run No. Air Temp. 

"C (1) (2) (3) 

mice. The figures, as well as the tabulated data, demonstrate 

the difference in resistance to thermal stress that exists 

between the rodent species. This difference most likely reflects 

the differences in body masses and in the surface area/mass 

ratios. 
28 t 18 

el 1 38 48 58 68 70 88 90 100 118 120 138 

Degrees, Ce 1 s ius 

FIGURE 1. TIME-TO-INCAPACITATION AS A FUNCTION OF EXPOSURE 

TEMPERATURE, FOR RATS. Each datum point represents one animal, n=56. 

i, 

Degrees, Ce 1 si us 

FIGURE 2. TIE-TO-INCAPACITATION AS A FUNCTION OF EXPOSURE 

TEMPERATURE, FOR MICE. Each datum point represents one animal, n=13. 

5 
The statistically derived constants for the equations suggest 

that the shortest time in which thermal collapse can be produced 

for either species would be about 1 minute. For the rat, the 

ma:.:imum air temperature for which the body's thermoregulatory 

system can compensate would be 308.7 t::: (35.5 "C) ? while the corresponding value for the mouse is 310.7 K (37.5 "C). At all air 

temperatures above these respective values, the rodent would 

experience an increase in total body heat content, leading even - 

tually to a nonsurvivable core temperature. 

One may conclude from these data that test animals (rats or 

mice) used to assay the toxicity of combustion products may not 

be exposed to air temperatures of 45 "C or higher without ris1::ing 

incapacitation (and death) within 30 minutes because of the ther - 

mal stress alone. In addition, one also risks the possibility 

that a given toxic atmosphere will be rendered still more toxic 

at elebated temperatures--a circumstance for which we have pre1 iminary evidence in the case of CO at moderately elevated air 

temperatures. It would seem, therefore, that the upper tempera - 

ture limit of 35 "C for an exposure atmosphere, as specified in 

the CAM1 protocol1 and recommended by the National Academy of 

Sciences" , has been justified by t h e r e s u l t s obtained from this 

study. 

Our survey of the literature addressing the topic of human 

thermal tolerance limits has convinced us that no actual human 

exposures have been conducted--or, at least, none have been 

reported--under conditions that achieved or even approached immi - 

nent physical collapse from hyperthermia. A myriad of studies 

have been conducted for which the endpoint was voluntary tolerance (discomfort), some degree of performance decrement, or a 

limiting value for some physiological parameter such as core 

temperature, blood pressure, heart or respiration rate, etc. We, 

however, are not convinced that these endpoints are sufficiently 

close to that of hyperthermic c_ollapse to be of real value. 

SUMMARY AND CONLUSIONS 

Rats and mice were found to be surprisingly susceptible to 

physical incapacitation (hyperthermic collapse) when exposed to 

moderately elevated air temperatures. For rats, thermal collapse 

occurs in about 60 min at 40 "E, 3:) min at 45 "C, and 5 min at 85 

"C. Mice are somewhat more sensitive than rats, with a 5-min 

callapse produced by expo5~1r-e at about 65 "C. 

The results of this study strengthen our previous belief that 

smoke toxicity tests should never subject rodents to temperatures 

above 40 "C-- preferably not above 35 "C. If these temperatures 

are exceeded, then the observed endpoint is apt to be the result 

of both toxic and thermal insults, rather than a measure of toxic 

potency alone. 
REFERENCES 

1. Crane CR, Sanders DC, Endecott BR. Abbott JK, Smith PW. 

2. National Academy of Sciences. Fire Toxicology: Methods for 

Evaluation of Toxicity of Pyrolysis and Combustion Products. 

Washinqton, DC: National Academy of Sciences Report No. 

7