in a curve (dashed line) through the data of
McConnell’, Taylor? °, Webb" and some of my
own’. If we assume that various factors are
involved in tolerance, then we might reach the
following conclusion: At temperatures roughly
above 80° C. (176° F.) tolerance is determined
by a single factor since all points fall on the
curve relatively well. At temperatures below
80° C. several factors are probably involved in
tolerance since the various points show consid-
erable scatter. Blockley and Webb both indicate
that at temperatures above about 71° C. (160° F.)
and below the pain threshold, exposures are
terminated primarily because of heat storage.
Thus, Blockley’s data gives one curve based on
heat storage and the data of the others gives
another curye based on heat storage and other
limitations.

Systems Limited Tolerance.

The third type of tolerance is indicated as
being systems limited for want of a better term.
Not much attention has been paid to this category
primarily because very special environmental
conditions are required to elicit the response.
My reason for listing this type of tolerance is
that the conditions of space exploration are con-
ducive to this type of failure. As indicated
earlier, very high humidities combined with
moderately high temperatures are required.
This condition can be obtained in the space sit-
uation because we are dealing essentially with a
closed environment; either a space suit or a cap-
sule. Thus, if for any reason, there is a decrease
in the capacity to remove moisture from the
environment surrounding the voyager coupled
with a decrease in the capacity to remove heat,
the conditions will be suitable for a systems
limited tolerance. A systems limited tolerance
is manifested by (1) excessive heart rate or
(2) extreme hyperventilation or (3) nausea,
vomiting, dizziness, ete., either singly or in com-
bination. Fig. 5 shows a subject who was re-
moved from the chamber because of a systems
failure. He had hyperventilated to the point
where he exhibited intense carpopedal spasm and
had no control of his extremities. However,
excessive body temperature or heat load is not
associated with this tolerance. When a systems
limited tolerance occurs, it does so before an
excessive heat load is built up. Those individuals
who do not exhibit a systems limited tolerance

oe

will then eventually incur a limiting heat load.
Table 2 shows the tolerance times, the ambient
conditions and the percent of subjects removed
from the chamber because of systems failuret®,

TaBLe 2.—Tolerance Times for Systems Limited
Exposures. Ambient Conditions Are Expressed as
“WD" Index (0,15 Dry Bulb +0.85 Wet Bulb)

Subjects

Mean Terminated

Ambient Tolerance for Systems

Conditions ‘Time Failure

“WD” Index Minutes %
98 142 30
100 78 35
101 73 25
102 72 30
103 67 30
104 60 22
105 46 40
106 44 90
107 4 53
108 28 55
109 20 100
11 27 89
115 19 80

The tolerance times listed here include the time
for some subjects to incur a limiting heat load.
An important point to be noted is that tolerance
because of systems limitations range from about
20 minutes to about 140 minutes but by far the
greater percentage of removals occurred with
times less than 60 minutes. At the milder condi-
tions (longer tolerance times) there is a consid-
erable overlap with conditions which produce
high heat loads and thereby limit tolerance in
that way. However, when humidity is lower
than for system limited tolerances, then higher
dry bulb temperatures are required in order to
reach tolerance. For example, temperatures
around 48° C. (118° F.) with low humidity can
be tolerated indefinitely without incurring sys-
tems limitations or heat load limitations.

The tolerance time curves for the three types
of limits are summarized in Fig. 6 for quick
reference. The interesting relationship of toler-
ance times for the three types of exposure is
evident from the figure. Pain limited exposures
last from a few seconds to minutes and occur
when air temperatures are very high (above
100° C., 212° F.). Heat storage limited expo-
sures may last from minutes to hours depending
on the dry bulb temperature and humidity.


240F © BLOCKLEY & LYMAN
° A MCCONNELL
@ TAYLOR
a0 C IAMPIETRO
% WEBB
° Modified from Tay!
° 200k jodifie rom Taylor
ue
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-
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Ww
a
=
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a '60F ° °
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N
140 Ces _
|
~ —
120 ; ; ; ! 1 Arc
0 20 40 60 80 100
MINUTES HOURS

TOLERANCE TIME

Figure 4. ‘Tolerance times for heat storage limited exposures (solid curve). Dash curve represents tolerance
limits which do not conform to the heat storage curve (modified from Taylor“).


Ficure 1. The temperatures of the atmosphere for sea level to 500,000 feet.


Very high temperatures (around 100° C., 212° F.)
can be tolerated for about 30 minutes when
humidity is low (<20mm Hg), while tempera-
tures below about 55° C. (131° F.) and low
humidity can be tolerated for several hours.
Systems limited exposures are of relatively short
duration and are terminated because of extreme
hyperventilation, excessive heart rate, vomiting,
dizziness, ete. High humidity with moderately
high temperatures induces this tolerance.

Additional work is required to determine more
fully the factors involved in tolerances to hot
environments. The three types of limits pre-
sented here are arbitrary and certainly improve-
ments can be made on them. The important
point to be made is that a single criterion for
tolerance to hot environments is not. justified.
The tolerance time and type of limitation will
be determined by the type of hot environment
encountered.

REFERENCES

. Blockley, W. V.: Heat Storage Rate as a Determi-
nant of Tolerance Time and Duration of Unimpaired
Performance above 150° F., FED. PROC., 22 :887-890,
1963.

2. Blockley, W. V. and J. Lyman: Studies of Human
Tolerance for Heat III: Mental Performance under
Heat Stress as Indicated by Addition and Number
Cheeking Tests, AF Tech. Rep. No. 6022 U.S. Air
Materiel Command, Wright-Patterson AFB, Dayton,
Ohio, 1950.

. Buettner, K.: Effects of Extreme Heat on Man,
JAMA, 144 :732-738, 1950.

4. Goldman, R. F., E. B. Green and P. F. Iampietro:
Tolerance of Hot, Wet Environments by Resting
Men, J. APPL. PHYSIOL., 20:271-277, 1965.

. Hardy, J. D.: Physiological Problems in Space Ex-
ploration, (J. D, Hardy ed.) Springfield, Ill.
Charles C. Thomas, 1964, Ch. 1.

6. Iampietro, P. F.: Heat-Induced Tetany, FED. PROC.,

22 884-886, 1963.

7. Iampietro, P. W. D. Chiles, E. A, Higgins and

H. L. Gibbons: Complex Performance During Ex-

posure to High Temperatures, AEROSPACE MED.,

40 :1331-1335, 1969.

2

a

8. McConnell, N. J., F. C. Houghten and ©. P. Yaglou:
Air Motion—High Temperatures and Humidities—
Reaction on Human Beings, ASHVE TRANS.,
302167, 1924. ‘

9. Taylor, C, L.:; Human Tolerance for Short Exposures
to Heat and Humidity, USAF Memo Report TSEAL
695-56B, Air Materiel Command, Wright-Patterson
AFB, Dayton, Ohio, 1946.

10. Taylor, C. L.: Physics and Medicine of the Upper
Atmosphere, (C. 8. White and ©. 0. Benson, Jr.,
eds.) Albuquerque, New Mexico, University of New
Mexico Press, 1952, Ch. 21.

11, Webb, P.: Aerospace Medicine, (H. G. Armstrong,
ed.) Baltimore, Williams and Wilkins Co., 1961,
Ch. 19.

12. Webb, P.: Temperature, Its Measurement and Con-
trol in Science and Industry, (J. D, Hardy, ed.)
New York, Reinhold Publishing Corp., 1963, Ch. 3.

13. Wing, J. F.: A Review of the Effects of High Am-
bient Temperature on Mental Performance, AMRL-—
TR-65-102, Wright-Patterson AFB, Dayton, Ohio,
1965.


oF

WALL TEMPERATURE IN

Fievre 3.

500
400
300
200+ PAIN
100 N I l ! I \ !
fe) 2 4 6 8 10 12 14 16
500 }
400
300 F-
L l
Pp “AY
200 T
PAIN STORAGE
100 F-
! ! 1 1 ! ! 1
te) 10 20 30 40 50 60 70 80

TIME IN MINUTES

Tolerance times for pain limited heat exposures (top panel) and curves showing transition areas be-

tween pain limited and heat storage limited exposures (bottom panel) (from Webb”).


AM 70-22

TOLERANCES TO THERMAL EXTREMES IN
AEROSPACE ACTIVITIES

P. F. Iampietro, Ph. D.
FAA Civil Aeromedical Institute
P.O. Box 25082
Oklahoma City, Oklahoma 73125

Approved by Released by
°
Stemteg Q Neh [UV sege€ wo
Srantey R. Monier, M.D. P. V. Sreczn, M.D.
Curer, ArroMEpIcaL APPLICATIONS Feprrat Arr Surgeon
Division

Availability is unlimited. Document may be released to the National Technical Infor-
mation Service, Springfield, Virginia 22151, for sale to the public. The contents of this
report reflect the views of the FAA Civil Aeromedical Institute which is responsible for
the facts and the accuracy of the data presented herein. The contents do not necessarily
reflect the official views or policy of the Department of Transportation. This report does
not constitute a standard, specification or regulation.

Department of Transportation

FEDERAL AVIATION ADMINISTRATION
Office of Aviation Medicine


PAIN LIMITED HEAT STORAGE SYSTEMS LIMITED
LIMITED DB 104-130 °F
WB 97-114 °F
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Vapor Pressure - Low Vapor Pressure- Low Vapor Pressure - High
{<20 mmHg) (<20 mm Hg) (45-70 mmHg)
1 L 1 1 1 1 L 1 1 1 i rT 1
o 5 10 18 20 ° 80 100 ° 50 100 150
TOLERANCE TIME, MINUTES
Figure 6. Summary graph of tolerance times for three types of physiological limits and the conditions which elicit

each tolerance.

10


Fieure 5. Subject exhibiting one form of systems limited tolerance (carpopedal spasm) (from Iampietro‘).


EFFECTIVE TEMPERATURE

T T T T T T T T T
°C oF
48.97120
acitis | \
ae Tevlor
43.4 4110 Marginal Physiological Limit
(Imminent Collapse )
40.6 7105
\
37.8 +100
Lovelace and Gagge
35.0 7 95 ~~ Recommended Physiological Limit
32.2 4 THIS STUDY
. Unimpaired Complex Performance
29.5 +85 C.
Unimpaired Mental Performance
n 1 1 1 1 i 1 1 1
° 30 60 90 120 150 180 210 240
EXPOSURE TIME (minutes)
Vierre 2. Physiological and performance limits in hot environments (modified fron Wing").



characteristics which separates it from the others.
Also, each of these tolerances could be important
in space activities depending on the exposure
conditions of the astronaut. Table 1 lists the
three types of tolerance and the environmental
conditions which might produce each. There
does not: seem to be much question that there is a
differentiation of the physiological events promi-
nent in the deterioration of tolerance for these
three categories, as well as some differentiation
of the tolerance times. However, it is to be
expected that, because of individual variation
and other variables, there will be some overlap
of tolerance times, as well as overlap of the
physiological determinants of tolerance. For
purposes of simplification, only seminude or
lightly clothed, resting or sedentary men will
be considered in this analysis.

There may be some question concerning these
three categories of physiological tolerance, es-
pecially because of the 2nd and 3rd categories.
Those workers who are firmly committed to the
idea that rate of heat storage is the ultimate
determinant of tolerance may not agree that
other factors can determine tolerance under es-
sentially identical exposure conditions.

Tapie 1.—Tolerance Times for Three Types of Physio-
logical Limitations During Exposure to Hot Environ-
ments

Ambient Conditions Tolerance
Type of Dry Bulb, °C (°F) Time
Thermal Tolerance Vapor Pressure, mm Hg Minutes
Pain Limited 100(212)—260(500) <1-15
Low ( <20)
Heat Load Limited 55(130)—110(230) 25—> 180
Low (<20)
Systems Limited 37(98)—55(130) 20—140

High (45—70)

Pain Limited Tolerance.

Pain limited tolerance can occur by exposure
of skin areas to direct radiation of sun, by skin
coming into contact with hot objects or by ex-
posure to radiation other than solar, for example,
hot walls of the capsule. Webb’s"? presentation
of information on pain limited heat exposures
in 1963 illustrated very well not only the range
of tolerance times for pain limited exposures,
but also that there is a transition region where
pain limited exposures may overlap with heat
storage limited exposures (Fig. 3). Exposures

which are pain limited occur only when the ex-
posure temperature is very high or when the
heat flux is high. Webb’s temperatures were on
the order of 115 (238) to 260° C. (500° F.).
However, pain may be incurred by contact with
objects at a much lower temperature than is
necessary to cause pain by radiation. In our
experience contact with metal objects at tem-
peratures around 71° C. (160° F.) causes dis-
comfort? and temperatures probably not much
higher would cause pain. The statement is made
that subjects in the UCLA?? studies exposed to
temperature above 100° C. (212° F.) spent con-
siderable time protecting hot spots, blowing on
knuckles, etc. It could be construed that toler-
ance was reached when the subjects attempted
to protect. themselves from the environment.
However, the point to be made here is that at
very high temperatures there is no question that
pain is the determiner of tolerance. Hardy* has
estimated that in space near the earth and moon,
radiation from the sun (about 2.0 gm cal/cm?/
sec) is enough to cause pain on exposed skin
surfaces. Hardy*, Buettner’, Webb’? and others
have found that skin temperatures around 45° C.
(113° F.) are always associated with pain. Re-
covery from pain limited exposures is rapid if
burns have not been incurred.

Heat Load Limited Tolerance.

The second category of tolerance limit is heat
storage and is probably the most commonly de-
scribed and used. Most exposures to heat are in
reality heat storage limited and therefore there
is some justification for so much preoccupation
with this category. This type of limit could be
achieved in the space situation in several ways.
Failure of the cooling system would be a prime
avenue, or heat loading by excessive exposure to
thermal radiation could be another.

Many workers have espoused the idea that
limitation on exposure to heat can always be
defined in terms of heat storage. Blockley*?
and his co-workers have been serious advocates
of this concept. Fig. 4 shows the data of Block-
ley (solid line) and others on tolerance times
modified from Taylor’. Taylor states that at
lesser temperatures factors of judgment enter
into the interpretation of tolerance. He also
states that nausea, air hunger, tingling of ex-
tremities should be considered in evaluation of
tolerance. I have taken the liberty of drawing



TOLERANCES TO THERMAL EXTREMES IN AEROSPACE ACTIVITIES

It is presumed that all participants in aero-
space activities are protected from the hazards
of the space environment. The protection takes
the form of a self-contained space suit, or a
system in which the individual lives in a cabin
or capsule without a protective suit, or some
combination of the two. The primary and most
immediate hazards to the space voyager are asso-
ciated with extremes of pressure and temperature.
Also, in both civil and military aviaion there are
potential high temperature problems. High per-
formance aircraft (Air Force fighters, SST) can
experience surface temperatures in excess of
260° C. (500° F.). In the event of a failure of
the cooling system, crewmembers and passengers
could be exposed to very high temperatures.
This is not meant to minimize other potential
physical hazards of aerospace activities such as
radiation and meteor strikes. It is possible, al-
though unlikely, that a thermal threat could
occur in conjunction with a partial loss of suit
or cabin pressure. However, attempts to sepa-
rate and ascribe the physiological events of such
a dual exposure to the individual stresses would
be difficult at best. This paper will concern it-
self only with some aspects of physiological
tolerance to the thermal threats of aerospace
activity and in addition will consider only prob-
lems of excessive heat since these are more
prevalent than problems associated with cold ex-
posure. This is attested to by the fact that space
suits are designed for heat elimination rather
than for conservation or production of heat.
Hardy® in an excellent paper stated that “shad-
ing or cooling will have to be provided for the
astronaut in the space environment near the
earth or moon and that this problem will be
greater as one goes near the sun.” However,
situations may occur in which moderately low
temperatures may be a problem. The flight of
Apollo 13 indicated that under certain unique
emergency situations moderate discomfort could
occur because of temperatures in the range
4-10 C. (40-50° F.).

The temperatures of the atmosphere are indi-
cated in Fig. 1 for reference. It is seen that at
altitudes above 800,000 ft. the temperature rises
very rapidly. However, temperature at extreme
altitude or in space does not have the same mean-
ing as temperature in our atmosphere. Ambient
temperature in our atmosphere is the net result
of convective and radiative heat exchanges be-
tween the temperature sensing device and its
surroundings, while in space the temperature is
the net result of radiative exchange only. The
heat sources in space near earth are the earth,
sun and moon and the heat sink is outer space.
The point to be made here is that heat exchanges
between man’s suit or cabin and his environment
in space must be accomplished by radiation alone.
Thus, special problems of heat exchange are im-
posed on the astronaut. Man in space may be
faced with several types of thermal stress de-
pending on the kind of failure of his protective
equipment or the kind of environmental situation
with which he is confronted. Before categoriz-
ing and describing the types of physiological
tolerances the space traveler may be faced with,
it should be emphasized that physiological toler-
ance is a rather extreme circumstance and the
individual approaching physiological tolerance
is probably well beyond any ability to perform
complex duties which might help him to improve
his condition. Fig. 2 illustrates this. This figure
shows the marked differences in tolerance times
for impaired performance and for physiological
tolerance. Performance deteriorates, for com-
parable exposure conditions at much earlier
times than does physiological function. In ad-
dition, the more complex the performance, the
earlier the deterioration. The physiological
tolerance curves shown here are based primarily
on data relating to body temperature and to heat
storage. It is apparent now that there is no one
clear cut criterion which can be used to determine
physiological tolerance to all thermal stresses.

There are at least three kinds of physiological
tolerance to thermal stress and each of these has