Taste 2. Sweat rates for two levels of activity at two dry-bulb temperatures
and at various humidities.

Rest Ref. Dry bulb
(80 keal/hr)
°F
37 80
37 110
37 110
37 110
Work
(350 keal/hr)
8 80
8 110
50 110
50 110

in hot environments, or at any time when sweat-
ing is maintained, is essential to insure perform-
ance standards and general well being.” Figure
9 illustrates the importance of water replacement
when men work in a hot environment. Rectal
temperature of the “no-water” group exceeds
102°F after 4 hours of work, while rectal tem-
perature of the “water ad lib” group remains
below 102°F for more than 5 hours.

EFFECT OF WATER CONSUMPTION ON MARCHING IN THE HEAT
TEMPERATURE 100° RELATIVE HUMIDITY 35 - 45%

103.0
rs ZONE OF
wy 102.5 | IMPENDING
5 EXHAUSTION
rene 7
r
© to1sh zone oF 4
Fo COMFORT van
101.0
z wey
B 100.5 — wo water
g == WATER ADL
100.0

Bb 8 EOE OE UE

00510 20 30 40 50 60
HOURS ON THE MARCH UP A GRADE OF 2.5% AT 3.5 M.P.H.

Ficure 9. The effect of water intake on rectal tem-
perature during work in the heat.”

C. Thermal Acclimatization. Chronic exposure
to hot or cold environments results in a progres-
sive change in the way an animal defends in-
ternal thermal stability. These adjustments have
been characterized® as:

1. Acclimation: systemic or cellular changes in
an individual on a daily basis in direct. re-
sponse to an identified stress,

2. Acclimatization: long-term responses or ad-
justments of an individual to an identified
stress,

3. Adaptation: phylogenetic adjustment or
compensation to any environmental condi-
tion.

10

Wet bulb Relative humidity Sweat rate

oF % hr
66 47 0.05
82 30 0.42
95 57 0.84
105 84 1.38
_ low 0.45
= low 1.05
90 46 1.60
95 37 1.90

Physiological heat acclimatization processes
for humans are well defined, ** ++ although for
cold acclimatization they remain somewhat equiv-
ocal. Heat-acclimatized individuals demonstrate
an increased ability to work in the heat and
respond to an acute heat stress with a more
rapidly elicited sudomotor reflex and the pro-
duction of a more dilute sweat. Although the
total potentials for sweat secretion is enhanced,
less sweat is produced for mildly stressing heat
exposures (at rest) compared to matching re-
sponses of non-heat-acclimatized humans. Heat
acclimatization has also been reported to be ac-
companied by a decrease in “cardiovascular
stress”" during acute heat exposures and an in-
crease in total blood volume. This latter claim
may have to be identified by the measurement
method and the experimental condition.’* * The
most striking component of human heat accli-
matization is the increased ability and ease with
which the superimposed stresses of exercise and
acute heat exposure are met during the first 5 to
7 days of chronic heat exposure. It is thought
that acclimatization to heat is not complete un-
less work is performed in the heat; exposure to
heat alone confers but little acclimatization. Re-
cent work, however, indicates that acclimatiza-
tion may be accomplished by controlled hyper-
thermia alone.**

This evidence may have practical importance
in competitive events involving teams with differ-
ent thermal histories. Men normally exercising
in warm or hot environments may well be ex-
pected to perform better in competition at high
ambient temperatures than others usually ex-
ercising at comfortable or cool temperatures. In-
dividuals from climates cooler than that in which
exercise is to be performed may improve com-
petitive potential by thermal “preconditioning”.


THE ACHIEVEMENT OF THERMAL BALANCE AND ITS MAINTENANCE
DURING ENVIRONMENTAL STRESS

I. The Achievement of Thermal Balance.

The discussion that follows cannot aspire to
be a complete analysis of the environmental or
physiological factors involved in temperature
regulation; it attempts only to be an introduc-
tion in this field and to offer a critical appraisal
of selected interests. If it can afford the be-
ginning student an understanding and apprecia-
tion of the general area, and give the advance
student and professional reader an outline of
current concepts, it will have fulfilled its respon-
sibilities. More detailed information and ex-

tended bibliographies are available in several
Teviews.® # 2% 80+ 85+ 40 69: 74

A. The Homeotherm and His Environment.
In defending its acquired thermal balance, the
homeotherm is presented with a dual thermo-
static responsibility. Whereas it must appropri-
ately adjust its own physiology to meet environ-
mental thermal demands (during heat and cold
exposures), it must also be prepared to rebalance
biothermal heat-exchange mechanisms to dis-
sipate metabolic heat loads during exercise, re-
gardless of the environmental thermal imposi-
tion. Both internal and external thermal threats
generally marshall the same physiological de-
fenses against hyperthermia, as discussed below.

Metabolic processes, like the rates of most
chemical reactions, are catalyzed by heat. Meta-
bolic independence from the influence of environ-
mental temperature, within the limits of
successfully achieved homeostasis, gives the warm-
blooded animal a considerable advantage over
the poikilotherm. Thermally affected metabolic
processes depend upon the regulated, relatively
constant internal temperature for the home-
otherm, but for the cold-blooded animal upon
the more capricious and extreme environmental
thermal changes. This freedom from other than
extreme environmental thermal exposure has
been identified as having had evolutionary im-
portance. ***° The observation that all home-
otherms thermoregulate within a relatively nar-

row temperature range (36°C for the elephant
to 41°C for the birds) supports the suggestion
that at least one other homeothermic advantage
is internal thermal stability at a metabolically
efficient level; ca. 37°C has been suggested to be
an optimum temperature for many metabolic
reactions.”°

Although a reasonably sharp distinction can
be made between homeotherms and poikilotherms
on the basis of body-temperature stability through
physiological, reflexly mediated action, it has
been demonstrated that poikilotherms can limit
internal-temperature fluctuations through be-
havioral adjustments. It is the degree of preci-
sion and the type of body temperature control
that separates the so-called cold- and warm-
blooded animals. Whereas most homeotherms
can control mean body temperature within a
fraction of a degree by reflex adjustments of
metabolism, blood flow, etc., poikilotherms are re-
stricted to providing a less discrete body tem-
perature set only by behavior and selection within
the environment.

The homeotherm’s ability to isolate meta-
bolic processes from the influence of the en-
vironment does not come without cost. Total
energy dissipation is usually greater than for
the poikilotherm even at moderate exposure tem-
peratures due to the steeper gradient between
the homeotherm’s skin-surface temperature and
environmental temperature. In winter, the warm-
blooded animal is faced not only with a greater
heat loss due to lower ambient and environ-
mental temperatures but also with reduced meta-
bolic energy sources (food supplies). Besides
improving, extending, and supplementing its
thermal protection by additional fur growth and
cold acclimatization processes, the warm-blooded
animal so stressed may augment these protections
by physically removing itself from the stressing
environment (burrowing, migrating, etc.), or
surrunder its homeothermic status and retreat
into a temporary state of modified poikilothermy



41. Harpy, J. D., and Hamer, H. T.: Temperature,
Its Measurement and Control in Science and In-
dustry. Vol. 3, Part 8, Biology and Medicine, J, D.
Hardy, ed., Chap. 54. Reinhold Publ. Corp., New
York, 1963.

42. Harpy, J. D.: Temperature, Its Measurement and

Control in Science and Industry. Vol. 3, Part 3,

Biology and Medicine, J. D. Hardy, ed. Reinhold

Publ. Corp., New York, 1963.

Harr, J. S.: Canad. J. Zool. 30: 90, 1952.

44. Hasama, B.: Arch. f. exper. Path. u. Pharmak, 158:
257, 1980,

45. Hesertine, E. W., and Apams, T., Jour, App. Physiol.
16 :226, 1961.

46. Henset, H.: Temperature, Its Measurement and
Control in Science and Industry. Vol. 3, Part 3,
Biology and Medicine, J. D. Hardy, ed., Chap. 19.
Reinhold Publ. Corp., New York, 1963.

47. Iampierro, P. F., VauaHan, J. A., MacLxop, A. R.,
Wercn, B. E., Marcinex, J. G, Mann, J. B.,
GRoTHEER, M, P., and FrrepeMann, T. E.: Caloric
Intake and Energy Expenditure of Eleven Men in a
Desert Environment. Tech. Rept. EP-40, QM, R &
D Center, U. S. Army, Natick, Mass., 1956.

48. Iampierno, P. F., Bass, D. E., and Buskirk, E. R.:
Metabolism. 149 ,1958.

49, Iampietro, P, F., VauaHan, J. A. Gorpman, R. F.,
Krewer, M, B., Masucct, F., and Bass, D. E.; Jour.
Appl. Physiol, 15: 632, 1960.

50, Iamprerro, P. F., and GorpMan, R. F.: Jour, Appl.
Physiol. 20: 73, 1965.

51. Irvine, L.: Scientific American, 214: 94, 1966,

52, Jounson, R. E., BELDING, H. S., Consorazio, F. O.,

and Pirrs, G C.: The requirements of water and of

sodium chloride for the best performance of men
working in hot climates. Harvard Fatigue Lab.

Rept. No. 13, 1942,

Kaurman, W. C.: Aerospace Med. 34 :889, 1968.

Kaurman, W. C., and Pirrman, J. C.: Aerospace Med.

35: 1167, 1964.

Keariner, W. R.: J. Physiol. 157: 209, 1961.

Lapett, W. 8. 8.: J. Physiol. (London), 12: 11, 1955.

Lavett, W. S. 8.: Adaptation to the Environment,

D. B. Dit, BE. F. Aporpn, and C. G, WiLBeER, eds.

American Physiological Society, Washington, D.C.,

1964.

&

ASs £8

14

58.
59.

60.

61.

63.

64.

3.

74.

5.

76.

LeBrano, J.: Jour. Appl. Physiol. 9: 895, 1956.
LeBranc, J. Hives, J., A., and Herovux, 0.: Jour
Appl. Physiol. 15: 1031, 1960.

Macoun, H. W., Harrison, F., Broseck, J. R., and
Ranson, 8. W.: J. Neurophysiol 1: 101, 1938.

Meyer, F. R., Roprnson, 8., Newron, J. L., Ts’o,
C. H., and Hoxcersen, L. 0.: Physiologist. 5: 182,
1962.

Mirier, L. K, and Irvine, L.: Jour. Appl. Physiol.
17: 449, 1962.

Nets, J. D., and Soper, D. J. G.: Jour, Appl.
Physiol. 17: 444, 1962.

Newsuren, L. H.: Physiology of Heat Regulation
and the Science of Clothing, L. H. Newburgh, ed.
W. B. Saunders Co., Phil., 1949,

. NIELSEN, B, and NiELsEN, M.: Acta Physiol. Scand.

64:314, 1965.
Orr, L: J. Nerv, Ment. Dis. 14: 152, 1887.

. Pearcy, M., Ropinson §., MItter, D. L, THoMas,

J. T., and Desrora, J.: Jour. Appl. Physiol. 8; 621,
1956.

Ricuer, C.: Pfleuger Arch ges physiol. 87: 624, 1885.
Rosrnson, 8.: Ann. Rev. Physiol. 14: 73, 1952.
Rosinson, S.: Temperature, Its Measurement and

Control in Science and Industry. Vol. 3, Part 3,
Biology and Medicine, J. D. Hardy, ed., Chap. 27.
Reinhold Publ. Corp., New York, 1963.

. SCHMIDT-NIELSEN, K.: Scientific American. 201: 140,

1959.

Scumupt-Nietsen, K.: Temperature, Its Measure-
ment and Control in Science and Industry. Vol. 3,
Part 8, Biology and Medicine, J. D, Hardy, ed.,
Chap. 14. Reinhold Publ. Corp., New York, 1963.
ScHoranper, P. F., Hock, R., and Watters, V.: Biol.
Bull. 99: 237, 1950.

Winstow, ©. E. A., and Herrineron, L, P.: Tem-
perature and Human Life. Princeton Univ. Press,
Princeton, N, J., 1949.

Woop, J. E., Bass, D. E., and Iamprerno, P. F.:
Jour. Appl. Physiol. 12: 357, 1958.

WyrnpuaM, C. H., Strypom, H. B,, Mogrtson, J. F.,
Wits, C. G., Brepett, G. A. G., Manitz, J. S.,
and Munro, ‘A.: Jour. Appl. Physiol. 20: 87, 1965.

67-3071,


(Continued from page 5)

Brample:
The heat loss (degree of thermal stress) at rest
with normal clothing at 40° F. is equal to the
thermal stress encountered at 45° F. with no cloth-
ing at rest, is equal at 20° F. with 8 clo at rest,
ete.

Beample:

‘The thermal stress at —85° F. with 3 clo at 8 met
is equal to 46° F. with no clothing at 3 met, is
equal to 0° F, with 2.1 clo at 8 met, is equal to
30° F. with normal clothing at 3 met, ete.

Example:

The heat loss at 15° F. with 4.5 clo at 1 met is
equal to the heat loss -at 10° F. with 2.7 clo at 2
met, is equal to the heat loss at 5° F. with 2 clo
at 3 met, ete.

Internal body temperature can be protected
during a cold exposure not only by appropriate
and effective physiological reflexes that reduce
heat loss at the body surface and increase meta-
bolic heat production, but also by critically re-
distributing stored calories. Analogous to the
conservation of body fluids by renal counter-
current mechanisms, decreased heat loss has been
described to take place in the extremities of many
animals (including humans) by a circulatory,
counter-current heat-exchange system.” Figure
6 shows how this effect occurs. During a cold
exposure, venous blood normally returning
through cutaneous channels is shunted by vaso-
motor adjustments to veins deeper in the axis
of the extremity. Through the anatomical jux-
taposition of these arteries and veins (vena
comitees) warm arterial blood coursing periph-
erally can loose heat by conduction (C,) to the
corresponding vein carrying cooler blood cen-
trally. The temperature difference between the
warm arterial blood and the cooler venous blood
is due, of course, to some heat loss to the environ-
ment at the skin surface. The end effect is to
establish an additional site for this exchange of
heat within the body (by heat transfer to the
returning venous blood), which keeps the thermal
energy within the body and reduces heat loss to
the environment (extremity temperature is re-
duced since the arterial blood entering the pe-
ripheral portions of the limb is cooler). This
type of thermal response, in conjunction with
skin vasoconstriction reflexes, established a tem-
perature gradient longitudinally as well as trans-
versely in the extremity.”

Figure 7 illustrates how the alternate super-
ficial venous-return pathways can also have ther-

ain (79)

PROXIMAL,
"1

ain O76)
(SOLD exposure :
1 Veascaattetion 0b (TT

= Seatsttgt tyros

rl grote
ala gradient Wm exireminy (4 Hee! Won
tlt gradient ae)

Ficure 6, Section through an extremity showing the
cireulatory basis for counter-current heat exchange.

moregulatory importance. During exposure to
air warmer than skin temperature, venous blood
shunted to the skin surface decreases the gradient
along which (and the rate at which) heat is
gained from the environment (skin temperature
approaches environmental temperature). Simi-
larly, during exercise at ambient temperatures
lower than skin temperature, the increased heat
loss required to maintain thermal balance in the
face of increased metabolic heat production is
facilitated by increasing (even more) the skin-
ambient temperature gradient.

Both in the case of heat exposure and of ex-
ercise at lower air temperatures, required heat
loss from the skin surface by evaporation is in-
creased due to the increase in water-vapor pres-
sure accompanying the rise in skin temperature;
sweat-gland activity consequently becomes a more

AIR (Te)

PROXIMAL
7%)

AIR (Tq)

1, eat Exponue-ovonetoion-9h( Te nro rata t ar beet gla
2 6 -vosedil tT aTaah bat *redtal took <f heat toss
(len neat exposure, Te, > Ty + with exercaa, Ty 7)

Fieure 7. Section through an extremity showing the
circulatory adjustment for environmental heat ex-
change at the skin surface.


Heat exchange by convection between the body
surface and the surrounding air (or water) can
be reduced by the interposal of any material
that would inhibit free fluid flow. Trapping air
within or beneath clothing or within fur pro-
vides a still fluid layer in contact with the skin,
which reduces heat exchange by convection and
establishes an important protective microenviron-
ment immediately surrounding the animal. Any
factor that damages the integrity of this still
fluid layer facilitates increased convective heat
exchange. For example, exercise in the cold air
can increase heat loss markedly from the skin
surface due to limb and body movements. *
Less heat would be lost were the same amount
of exercise performed isometrically.

Heat transfer by the exchange of electro-
magnetic energies between facing surfaces is
called radiation. This heat-exchange route is
unique in that it does not depend upon contact
between heat-exchanging masses, but can take
place through low-density environments (for ex-
ample, as solar radiation does through “space”).
All objects above a temperature of absolute zero
(278°C) exchange radiative energy with every
other facing object (or with its own facing sur-
faces), with the net gain or loss of heat de-
pending upon differences in temperature and
other surface characteristics (e.g., emissivity :
whether an object is a good “reflector” or “emit-
ter” of radiant energy) 2% *

Heat loss by evaporation depends upon ther-
mal transfer in the conversion of a material from
a liquid to a gas phase. Evaporation is always
a heat-dissipating phenomenon; the energy stored
by gas molecules can be released during the
process of condensation. The dissipation of heat
by evaporation can occur from any ‘wetted body
surface exposed to a less-than-saturated gaseous
environment (e.g., from the respiratory-tract sur-
faces, the skin with or without sweat-gland
activity, or from the wetted fur). Homeotherms
exposed to environmental temperatures higher
than skin temperature depend solely upon heat
loss by evaporation to retain a thermal steady
state (equation 3). If exposed under such a con-
dition in air that is saturated with water vapor
(ie., 100% relative humidity; no vapor-pressure
gradient can be established), progressive hyper-
thermia develops at the accumulated rates of
metabolic heat production and heat gains from
the environment through, conduction, convection,

and radiation. Even though sweat-gland activ-
ity (or panting) were maximal under these con-
ditions, no effective cooling could occur since
evaporative processes would be precluded.

An analogy expressing the interaction of these
factors is presented in Figure 2.

HYPERTHERMIA

il NORMAL RANGE
i } (Variation due to!

individual and
E species differences,
E diurnal effect, etc.)

“= HYPOTHERMIA

4 HEAT LOSS (-Cy,-Cy,-Rg.-Ey)
Ficure 2. Body temperature is maintained within a

normal range as the amount of stored heat is
balanced by rates of net heat gain and loss.

In conceptualizing a specific thermal exposure,
it is important to recognize that seldom do
any of the four avenues of heat exchange operate
alone in effective biothermal responses; the ther-
mal threat to the homeotherm and the counter-
ing physiological defense is effected rather in
terms of the algebraic sum of these interacting
forces.

C. The Physiological. Basis for Biothermal
Control. Figure 3 shows how the avenues of heat:
exchange operate together to transfer thermal
energy (during an air exposure) from the clothed
or bare skin surface. This body cross-section
shows the three major temperatures to be con-
sidered, air (T,), skin (T,), and “core” (T;)
or deep body temperature, usually evaluated as
rectal temperature. Modifying heat-exchange
rates along this axis are the summing insulations
of the outer body area (“shell”) [I,J], the
clothing [In], and the air [I,]. Tissue insula-
tion [I,] can vary due to rates of local blood
flow [Cy] (ie., internal body convection) to
make the “shell” (that portion of the body mass
in which there are temperature gradients and
that will change its temperature being ther-
mal stress) occupy approximately one-half
of the total body mass (during massive pe-
ripheral vasoconstriction and extended cold ex-
posure), or bring the “core” (that portion of



(hibernation or pseudo-hibernation). In either
event, the obligated high-energy flux is avoided.
Humans adapt further by using the “psycho-
sociological’®? tools of shelter, clothing, heating
and air-conditioning systems, etc., and thereby
establish for themselves greater thermal free-
dom than any other warm-blooded animal.

B. The Physical Basis for Homeostasis. A
constant temperature for any object is achieved
(equation 1, Figure 1) when sources of heat gain
are equally matched by avenues of heat loss,
with such a balance represented by an unchang-
ing heat content (AH=0). For animals (equa-
tion 2), the thermal steady state is reached
when metabolism (always a source of heat pro-
duction) is coupled with, or balanced against,
effective avenues of heat exchange, and no net
heat gain or loss takes place. A constant in-
ternal temperature is the direct result of the
algebraic sum of these influences, and nothing
else. Such a thermal situation exists for the
homeotherm after enough heat has been stored
to elevate its temperature above that of the en-
vironment and within the thermal-control zone
(37° to 39° C for most mammals).

EQUATION

| AH=(HEAT GAIN)-(HEAT LOSS)

2 AH «METABOLISM s HEAT EXCHANGE

3 AH =METABOLISM + CqsCysRy-Ey

WHERE :
AH =NET HEAT FLUX: IF EQUAL TO ZERO,
A THERMAL STEADY STATE IS ACHIEVED.

Cg = CONDUCTION
Cy = CONVECTION
Rg * RADIATION
Ey = EVAPORATION

Ficure 1, Basic heat-exchange equations.

Specifically, the avenues of heat exchange are
(equation 8, Figure 1) conduction (Cd), con-
vection (Cy), radiation (Rd), and evaporation
(Ev), each of which (except evaporation) can be
a source of heat gain or loss, depending on envir-
onmental and physiological circumstances. Heat
flow along any of these routes requires an energy
potential gradient. For conduction, convection,
and radiation, a thermal gradient is required;
heat loss by evaporation requires a difference in
water vapor pressures. Detailed examination of
the physical variables associated with each of

these energy exchange avenues is available else-
where* ** 2 #74; the present discussion will be
limited to evaluating each of these forces as they
supply the physical framework for homeostasis.

Heat transfer by conduction is defined as ther-
mal energy exchange through a medium, or be-
tween objects in physical contact, by the transfer
of kinetic intramolecular energy, without involv-
ing the physical transfer of material. With an
existing temperature difference, heat can be ex-
changed (either gained or lost) between the bot-
toms of the feet and the floor, between the skin
and contact clothing, between and through ad-
jacent body tissues, etc. Since homeotherms
normally have only a small body area in con-
tact with other objects in the environment (ex-
cept for clothing), conduction is usually not a
major avenue of environmental heat exchange
for the warm-blooded animal.

Convection, a special case of conduction, is
most often considered as a separate avenue of
heat exchange. It is distinguished as a route
of thermal energy flow depending upon the
movement of a fluid over a surface that is at a
different temperature. Two types of convection
are usually identified. In natural (or passive)
convection, the fluid (liquid or gas) flow is a
function of differences in density within the
fluid produced by differences in temperature (for
example, warm air rising from a ftame or over
a warm surface). Forced convection, which in-
creases the rate of heat exchange, requires an
additional energy source external to the heat-ex-
changing system itself to move the fluid (for
example, an electric fan or a stirring motor).
In addition to the physical considerations for
conduction, heat exchange by convection also
varies with the fluid characteristics (viscosity,
density, etc.), and the surface features of the ob-
ject that affect surrounding fluid movement
(surface texture, irregularities, shape, etc.). Most
important and effective convective heat ex-
change within the body occurs by tissue blood
flow. Since the movement of blood through
the circulatory system depends ultimately on
the expenditure of myocardial metabolic energy,
tissue perfusion is an example of forced con-
vection. The free movement of blood through
the vasodilated body serves not only to minimize
intracorporeal temperature differences, but also
is an important mechanism for exchange of body
heat with the environment.




be enjoyed, as shown in Figure 4. Although the
ambient temperature at which “chemical tem-
perature regulation” begins (so-called “critical
temperature”)’* is only a degree or so below
normal room temperature for the human, some
furred animals are able to maintain a constant
internal temperature without augmenting physi-
cal defenses by shivering in air temperatures as
low as —40° C."> Increases in heat production
by shivering are effective in maintaining thermal
balance only in acute cold stress; protection dur-
ing chronic cold exposure must rest upon mech-
anisms for retaining rather than generating
heat.*

Tare 1, Different levels of physical activity with cor-
responding value of heat production’

Activity Metabolic rate* “Met”**
Sleeping 36 07
Basal 40 0.8
Sitting (resting) 50 10
Standing 60 12
Walking (level)

1.5 mph 90 18

8.0 mph 130 2.6

4.0 mph 180 3.6
Running (level)

10 mph 500 100
Heavy work 860 17.2

* keal/ hr . m2.
#* 1 “met” 50 Keal/hr . m?.

i
"CHEMICAL 4_CLOTHED "PHYSICAL
TEMPERATURE  CLOTHED WUDE ‘TEMPERATURE
REGULATION * | REGULATION

A NUDE.

INCREASED EVAPORATION
METABOLISM REQUIRED TO
REQUIRED TO PREVENT
PREVENT cLoTHED nung | HYPERTHERMIA
HYPOTHERMIA

1@ 20 22 24 2 28 30 32 34
AMBIENT TEMPERATURE (°C)

Ficure 4. The temperature ranges of effective vaso-
motor regulation are shown for the human nude and
with light clothing. Exposure to temperatures above
each upper limit requires sweat-gland activity to
maintain normothermia; normal temperature stability
during an exposure to temperatures below each lower
limit requires metabolic heat production.

A very general estimate of the flexibility of the
interacting influences of clothing and physical
activity during exposure to cold is shown in Fig-
ure 5. More accurate relations for “comfort”

and behavioral integrity are available else-
where.2* 5%" Predictions of homeothermal sta-
tus are being made available for extreme ex-
posures through the use of computor analoges of
operating thermoregulatory systems * ** as well
as through human testing.

70: 20
65 33°C ~Tg
60 Tat la" Sowany) ry
35
so 10
a4“ 5
Ei 28
w 35 Y tl ow
°
5 30 mn 1° 3
ga &
5
rd 20 \3 fi]
1s % 0 §
Fi 10 4 Fd
ot, 15
bo ‘
Z = 20%
wos Eg w
a a
2° rn oS ae
= std, > 3 a Ly <
20420 3 4 fo 30
2st 38 % ZG
20138] | ss
3st3e
el 113
40 \ 40
itf 2 3 afs5s 6 7
ormat ve ca MAX. insulation with
clothing CLO" arctic clothing
Hmet = 50 keat /tr- m2
[met = 0.18 *C/ keal»m2-hr
= the amount of insulative clothing required to maintain @
resting-sitting man with a metab. of 1 met indefinitely
‘comfortable at on envir, temp. of 21°C( 70°F)
Reference: T. Environmental Factors Infivencing Therma,

Exchange. Tech. Rept. No. 59-22, Lodd AFB, Alesha, 1960.

Ficurr 5. This chart shows the approximate relation-
ship between ambient temperature and the units of
insulation (expressed as “‘clo”) required to maintain
thermal comfort. It will in addition indicate the
varying degrees of heat loss (or gain) and levels of
thermal equilibrium under varying degrees of heat
production and exercise. No estimates can be made
with this diagram to include the effects of wind
velocity greater than 20 ft/min.

Ewample:
72° KF. with normal clothing at rest (1 met) is
‘thermally equal to 54° F, with 3 clo, is equal to
41° F, with 4 clo, is equal to 27° F. with 5 clo, ete.

Eeample:
0° F. with 7.2 clo at rest is equal to 3.7 clo at 0° F.
at 2 met, is equal to 2.5 clo at 0° F. at 8.met, ete.

Eeample:
—40° F. with 2.5 clo at 4 met is an isothermal con-
dition equal to 57° F. with normal clothing at 2
met, is equal to 54° F. with 3 clo at rest, ete.

(Continued on page 6)



AM 66-23

THE ACHIEVEMENT OF THERMAL BALANCE AND ITS
MAINTENANCE DURING ENVIRONMENTAL STRESS

Approved by

J. Rosert Duties, M.D.
Cuter, Crvit AEROMEDICAL
InstrrurTe

P. F. Iampietro
Thomas Adams

June 1966

FEDERAL AVIATION AGENCY
Office of Aviation Agency

Released by

(USege€ wd
P. V. Sreeeu, M.D.
Feprrar Air Surceon



integrational area; the terms “thermal sensibil-
ity” (to denote the action of temperature directly
on central nervous system cells) and “thermal
sensitivity” (to identify peripheral (skin) tem-
perature receptor function) seem appropriate.

Local anterior hypothalamic heating and cool-
ing in unanesthetized animals ® * ** have empha-
sized the earlier suggestion that local central
temperature as well as skin temperature must be
considered as functional temperature-regulation
control-system inputs. Local diencephalic heat-
ing produces many of the characteristic heat
responses as does (to a more limited extent)
local hypothalamic cooling initiate predictable
cold defenses. Questions related to the prefer-
ential influence of either of these factors in cold
and heat exposure and acclimatization largely re-
main open. The identification of the neural
characteristics of anterior hypothalamic neurons,
in contrast to others within the central nervous
system, is not complete? The basic features of
control-system regulation that are most likely
involved in body-temperature control have been
presented.*?

It should not be forgotten that thermal-control
effectors serve other functions for the organism
and may operate effectively outside of hypothe-
sized biothermal control. For example, it has been
shown recently that sweat-gland activity is trig-
gered almost immediately upon initiation of
voluntary activity,'* ° before there are any meas-
ureable changes in skin or central temperature;
the total response, however, can (under some cir-
cumstances) be greatly influenced by skin temper-
ature. Although it can be argued teleologi-
cally that this response is involved in the prepar-
atory phases of thermal control and is effected
in anticipation of the increased heat production
associated with exercise, details of its action are
missing. The control of sweating during exercise
is discussed in greater detail in the following
section.

Sweating, panting, shivering, peripheral vaso-
motor responses, and other thermoregulatory ef-
fector actions serve the same biothermal ends, to
offset net heat fluxes impinging through environ-
mental and exercise induced heat stress and to
retain a constant internal temperature. Matching
of appropriate responses to the thermal situation
is the role of central neural reflexes subserving
temperature regulation. Research leading to the

detailed elucidation of these controls has only

begun.

II. Thermal Balance During Exercise and En-
vironmental Stress.

When man exercises, he must dissipate the heat
produced by the muscles or increase body tem-
perature. If heat generated during exercise can
be lost to the environment, a new thermal steady
state can be achieved; voluntary activity is then
limited by factors other than elevated body tem-
perature. If humidity is high, or if clothing in-
hibits heat loss by radiation, convection, conduc-
tion, and the evaporation of sweat, body
temperature increases as a function of the exer-
cise rate with voluntary activity terminating as
hyperthermia ensues.

A. Ewercise in Heat and Humidity. Semi-
nude man resting at 82° F. ( a “neutral” temper-
ature) produces heat at the rate of 80 kcal/hr.
Approximately 20 kcal/hr are dissipated as
water evaporates from respiratory and skin sur-
faces . (without sweating, so-called “insensible
water loss”); 60 kcal/hr are lost by radiation,
convection, and conduction. During heavy exer-
cise, heat production may rise as high as 960
keal/hr (12 times resting). The major portion
of this heat is brought to the body surface by in-
creased skin blood flow (400 cc/min at rest com-
pared to 1,200 cc/min during exercise)*”; skin-
surface heat loss is further increased by the
evaporation of sweat. Since each cubic centi-
meter of water evaporated requires 0.58 kcal, a
sweating rate of 3,000 cc/hr can dissipate heat
at a rate of 1,740 keal/hr.

In addition to attempting heat dissipation
during exercise, an advantage may be taken of
the thermal capacitance of the “shell” with some
heat being stored (ingreasing body temperature;
see page 00). This Sect is valuable in heavy
exercise for short periods (e.g., track events of
a mile or less). No satisfactory explanation is
available to indicate why body heat storage is
tolerated better during exercise than by resting
man during heat exposure.

The neurophysiological control system through
which heat is dissipated by increased skin-surface
evaporative water loss is particularly important
in extending the upper limit of “physical temper-
ature regulation” (see Figure 4) during whole-
body heat exposure, and especially during
prolonged exercise. There appears to be a ther-


efficient’ mechanism for loosing heat as long as the
environment is not saturated with water vapor.

In addition to satisfying skin metabolic re-
quirements, the cutaneous circulation can vary
considerably in performing its role in thermo-
regulation. Skin blood flow in a nude resting
man at 82° F. ambient temperature is estimated
to be 400 cc/min.*® This flow can be increased
by a factor of 3 with work at 70° F.," or during
rest at 100° F; working at 100° F. will raise
cutaneous circulation to approximately 9,000 cc/-
min.”

Suceinctly, an acute human response to cold
js characterized by: (1) peripheral cutaneous
vasoconstriction (which results in an increase in
tissue insulation); expansion of the “shell;”
withdrawal of the “core;” (2) increase heat pro-
duction by shivering; and (8) shunting of
venous blood in the extremities to deep vessels
for counter-current heat exchange.* An acute
human response to heat stress would entail :
(1) peripheral cutaneous vasodilation (decrease
in peripheral tissue insulation) ; expansion of the
“core”; reduction of the “shell,” (2) increased
sweat-gland activity, and (8) the superficial re-
turn of venous blood.**

In spite of the cursory presentation here, it
should be clearly recognized that the physiologi-
cal responses to heat and cold stress and the
consequent readjustment of heat within the body
are far from simple and their details are not
completely understood. More detailed descrip-
tions of the characteristics of heat, cold, hiber-
nation, etc., are available elsewhere.” “

D. The Neurophysiological Control of Body
Temperature. As might be expected for any
unit that remains as stable as homeothermic deep
body temperature (although not necessarily total
body-heat content due to thermal transients with-
in the “shell”) under the buffeting of internal
and external thermal loads, the subserving con-
trol system is complex. As with any control
system, however, it retains the basic components
of input (internal and skin temperature) and
output (metabolism, shivering, sweating, vaso-
motor control, ete.) operating through an in-

*In furred animals, this series of responses would be sup-
plemented by pilocrection, which would trap air near the skin
‘and reduce the rate of skin-surface convection heat loss.

**In furred animals, in the absence of effective sweat-gland
activity, increased evaporative cooling would occur through
panting, and in extreme body heating, by copious salivation and
licking of the fur.

Figure 8 A general outline of the reflex pathways in-
volved in homeothermie protections against hypoth-
ermia,

tegrational level (hypothalamic central-nervous-
system areas). Figure 8 presents an outline of
these operational levels for responses to cold.

The general response pattern for cold-elicited
reflexes shown in Figure 8 is that temperature
receptors within the skin ** provide information
(via the lateral spinothalamic tract) afferently
into neurointegrational levels within the dien-
cephalon (hypothalamus), which reflectly relays
appropriate effector responses through the sym-
pathetic and somatic motor pathways and into
metabolic channels. A similar, grossly over-
simplified circuit can be described for defenses
against hyperthermia.

Just before the turn of this century, it be-
came apparent that internal body temperature,
in addition to the more obvious influence of local
skin temperature, was influential in triggering
thermal responses.* ** It became clear later
that the neurons and synaptic activity in regions
of the anterior hypothalamus were especially sus-
ceptible to local temperature effects,** and that
the temperature of this area serves functionally
as an additional thermal input into the bio-
thermal control system. Since there are no ana-
tomically identifiable temperature receptors in
the anterior hypothalamus, as there are in the
skin, it should be emphasized that the effect of
temperature within the central nervous system
is a direct, thermal modification of activity
rather than one in which information (in the
form of action potentials) is fed into a central



the body that is at a near constant temperature) *
close to the skin surface (during cutaneous vaso-
dilation and heat exposure). Heat exchange
(double-headed arrows) can occur by conduction
(Ca) both within the body between adjacent
tissues at different temperatures or to objects
touching the skin surface (including clothing).
Heat loss (single-headed arrows) by evaporation.
can occur directly from deep body masses (with-
in the “core”) by evaporation from nasopharyn-
geal, buccal, lingual, palatine, and respiratory
tract surfaces (E,,) or from the skin and clothing
surfaces (Ey). Heat exchange by radiation
(Ra) and convection (C,) to the surrounding
environment can take place from any exposed
surface. The terms “core” and “shell” are, of
course, functionally defined in terms of the heat
content and thermal gradients within different
body masses; they do not have fixed anatomical
counterparts.

Ew

Ficure 3. Internal and environmental physical heat-
exchange routes are indicated for both clothed (up-
per) and nude (lower-air exposures).

Partitional calorimetric testing has shown that
during a whole-body exposure (human) with air
‘and wall temperatures at 24° C., with low rela-
tive humidity and little forced-air movement,
evaporation accounts for approximately 21%,
radiation 37%, and conduction and convection
together 42% of the total dissipated heat.”* These
proportions can be drastically different for ex-

* Small temperature differences exist between organs and
structures deep in the body and even in the ventricular cavities
during different phases of the cardiac cycle (due to heat re-
leased during myocardial contraction). Although these differ-
ences in temperature should be considered in evaluating the
rationale for selecting a single temperature measurement to
reflect accurate internal mean temperature, it should not be con-
fusing in the general but conceptually valuable identification of
the “core.”

posures in environments with other character-
istics.

A situation analogous to that presented in Fig-
ure 3 can be envisioned for a water rather than
an air exposure. Additional appreciation of the
physical factors involved in the avenues of heat
exchange may also be gained by speculating how
the equations in Figure 1 would have to be re-
balanced, or the avenues of heat exchange shown
in Figure 3 reevaluated for a water exposure or
during exposure to environments of low density
(“space”).

At least three separate thermal functions can
be attributed to that portion of the body near the
skin surface, the “shell,” which is not (as de-
fined) at deep body temperature. In addition
to serving as a site for convective heat exchange
by perfusing blood, decreased heat loss during
cold exposure results as the uniformly warm
body mass (the “core”) withdraws from the skin
surface to deeper body areas, interposing a vaso-
constricted tissue mass between itself and the en-
vironment.’*> This “shell,” so established, offers
some degree of thermal insulation ([I.], Fig-
ure 3) that can be calculated. 2° These cir-
culatory adjustments can bring about a change
in tissue insulation from approximately 25 to
10 keal/hr . m? . ° C with a change in ambient
temperature from 35 to 10° C.° Deposition of
subcutaneous fat layers can increase tissue in-
sulation over that achieved by vasoconstriction
alone in the human,” ** as it can in arctic ani-
mals. Since the “shell” can transiently change
its heat content, it can serve an additional role
of thermal capacitance that acts as a buffer to
internal-temperature changes by means of its
ability to affect either positive or negative heat
storage. This effect has been shown to be of
functional importance at least to desert mam-
mals."

Besides the physiological adjustment of the
avenues of heat exchange (“physical temperature
regulation”), most homeotherms can provide large
amounts of thermal energy during an acute cold
exposure by involuntary muscular activity,
shivering *° (“chemical temperature regulation”) ,
or by voluntary activity (Table 1). For the
human, both metabolic heat production and skin-
surface evaporation of secreted sweat are criti-
cally important in extending the environmental
thermal limits provided only by circulatory ad-
justments in which a thermal steady state can




Moderate exercise (about 350 kcal/hr) for 2 to
3 hours daily at an ambient temperature a few
degrees warmer than that anticipated for an
impending event, 10 to 14 days before competi-
tion, could conceivably extend thermal-exercise
limits, improve performance, and reduce physio-
logical strain.

The experimental identification of human,
whole-body cold acclimatization has stirred a
controversy for many years, which at present is
far from resolved. Although regional skin cold
acclimatization for the human is well docu-
mented,* ** 5% 6 63 the precise character of whole-
body changes with chronic cold stress remains
elusive. Experimental programs reporting
physiological changes in “indoor” men who
(for the purposes of the experiment) live in
a cold environment must face the question of
possible artifacts introduced by physical fitness
changes.* “55 These programs must also iden-
tify how thermal stress occurs when, in many
studies, the subjects are provided with thermally
insulating clothing; it would seem that exposure
to the microclimate established by such clothing
would preélude the required experimental condi-
tion of cold exposure (except perhaps for the
hands, feet, and face). Attempts to demon-
strate cold acclimatization in human populations
indigenous to arctic or subarctic regions leaves
unresolved problems of (at least) racial, ethnic,
and dietary involvements! Although many re-
ports have been published in this area, equivoca-
tion on the basis of subject selection, use, train-
ing and maintenance, the nature of the chronic
and acute cold exposures used in the tests, ade-
quacy, appropriateness and precision of employed
physiological tests, and, of course, individual
data interpretation in view of these variables,
serve to cloud the issue.

Nonetheless, some experiments in which sub-
jects have been chronically exposed to cold
(unprotected by microenvironments generated
by thermally insulating clothing) strongly sug-
gest that increased heat production without
shivering (so called “nonshivering  thermo-
genesis”) can be developed in humans ** * as it
can in other species.” * Although many more

11

data must be available before these questions are
answered, the general pattern of human cold ac-
climatization is hinted to be related to increased
heat production through shivering** and en-
docrine or intracellular adjustments, a greater
heat flux with increased “shell” and decreased
“core” partitioning of the main body mass (i.e.,
increased tissue insulation),?* but reduced pe-
ripheral vasoconstriction,’ leading to warmer
hands and feet. Although obligating a source
of increased heat loss, the warmer extremities
may play the protective role of reducing pe-
ripheral cold injury and maintaining manual
dexterity. There is little evidence to support
the suggestion that a reduced thermoregulatory
set-point” accompanies these changes.* Paral-
leling tests of heat acclimatization and exercise,
some effort has been directed toward defining in
man cold acclimatization in terms of metabolic
responses to exercise in the cold.?

In spite of difficulty in identifying the whole-
body cold acclimatization indexes, local cold
acclimatization for the human has been well
documented both by “field” and laboratory test-
Humans who chronically expose
their extremities to cold air or water show an
alteration in local circulation during acute cold
stress (extremity immersion in well-stirred ice
water) that can be interpreted as a protective
adjustment. Their extremities do not cool as
deeply and remain generally warmer than those
of persons not having experienced long-term
local cold exposure. Increased protection against.
local cold injury and maintained manual dex-
terity may well be achieved prior to de facto
cold stress by repeated, local, cold exposures.

The physiology of exercise cannot be separated
from the homeothermic demands of the environ-
ment. Performance capability can be easily sac-
rificed by a lack of understanding or respect
for thermoregulatory requirements. The cost is,
of course, the termination of voluntary exercise
by hyperthermia before cardiovascular, muscular,
and pulmonary limits are met. The recognition
of only a few thermoregulatory concepts can pay
a multifold return in improved individual and
group performances.

ing.® 94 9% 68,



L

2.

3.

10.

11.

12,

13.

14.

15.
16.

17.

18.

19.

20.

21.

REFERENCES

ApaMs T. and Covino, B. G.: Jour, Appl. Physiol.
12: 9, 1958.

ApaMs, T., and HEBERLING, E, J.:; Jour. Appl. Physiol.
13: 226, 1958.

Apas, T.: The Control of Body Temperature.
TR 59-21, AAO, AAL, Alaska, 1960.

. ApaMs, 'T.: Environmental Factors Influencing Ther-

mal Exchange. TR 59-22, AAC, AAL, Alaska, 1960.

. AvaM, T., and SmirH, R. E.: Jour, Appl. Physiol.

17: 812, 1962.
Avams, T.: Jour. Appl. Physiol. 18: 772, 1968.
Apass, T.: Jour. Appl. Physiol. 18: 778, 1963.

. ADOLPH, E, F., and Coworkers : Physiology of Man in

the Desert.
1947.

Interscience Publishers, Inc., New York,

). ANDERSON, K, L., StromMis, S., and Exsner, R. W.:

Metabolic and Thermal Responses to Muscular Exer-
cise in the Cold. TD Rpt. No, AAC-TDR-61-52,
Fort Wainwright, Alaska, 1952.

Bass, D. B., Kiweman, C. R., QuINN, M., HENSCHEL,
A, Heonaver, A, H.: Medicine. 34: 323, 1955.
Bass, D. E.: Temperature, Its Measurement and
Control in Science and Industry. Vol. 3, Part 3,
Biology and Medicine, J. D. Hardy, ed., Chap. 28.
Reinhold Publ. Corp., New York, 1963.

Bazert, H. ©., Sunperman, F. W., Dovrs, J. and
Scorr, J. C.; Am. J. Physiol. 129: 69, 1940.
Beaumont, W. VAN, and Burxarp, R. W.: Science.
141; 643, 1963.

Benzincer, T. H., Krrzincer, C., and Pratt, A.W.:
‘Temperature, Its Measurement and Control in Science
and Industry. Vol. 3, Part 3, Biology and Medicine,
J. D. Hardy, ed., Chap. 56, Reinhold Publ. Corp.,
New York, 1963.

Boarrr, C. M.: Scientific American. 200:105, 1959.
BRENGELMANN, G., and Brown, A. C.: In Physiology
and Biophysics. T. C. Ruch and H. D. Patton, eds.,
Chap. 54. W. B. Saunders Co., Phil, 1965.

Brovua, W. L., and Raprorp, EB. P., Jr.: Science
and Medicine of Exercise and Sports, W. R. Johnson,
ed., Chap. 10. Harper and Brothers Publishers, New
York, 1960,

Brown, A. ©.: Analogy Computer Simulation of Tem-
perature Regulation in Man. TD Rpt. No, AMRI-
'TDR-63-116. Wright-Patterson AFB, Ohio, 1963.
Bruck, K. and Hensen, H.: Pflug. Arch. ges physiol.
257: 70, 1953.

Burron, A. ©. and Epxors, O. G.: Man in Cold
Environment. Edward Arnold Ltd., Lond., 1955.
Buskirk, B. R., Iamprerro, P. F., and Bass, D.B.:
Jour, Appl. Physiol. 12: 189, 1958.

Buskirk, E. R., THompson, R. H., and WHEDON,
G. D.: Temperature, Its Measurement and Control

13

24,

25.

26.

27.

28.

29.

31.

32.

34,

35.
36.

3T.

38.

40.

. Day, R.: The Effect of Cold on Man.

Vol. 3, Part 8, Biology and
Reinhold Publ.

in Science and Industry.
Medicine, J. D. Hardy, ed., Chap. 37.
Corp., New York, 1963.

. CaRLsoN, L. D., Youne, A. C., Burns, HL, and

Quinton, W. F.: Acclimatization to Cold Environ-
ment. USAF Tech. Rept. No. 6247, 1951.

Carson, L. D., Peart, D., and ScHryer, W.: Am. J.
Physiol. 625, 1954,

Cartson, L. D., Hsten, A. 0. L., Futteston, F., and
Exsner, R. W.: J. Aviat. Med, 29:145, 1958.
Cartsoy, L. D., and Tuursn, H. L.: Human Ac-
climatization to Cold. Contract No. AF 41 (657)-212,
1959.

Cormx, W. H., and Cartson, L. D.: Proc, Soc. Eeper.
Biol. Med. 92: 845, 1956.

Crossir, R. J., Harpy, J. D. and Fessenven, E.:
‘Temperature, Its Measurement and Control in Science
and Industry, Vol. 8, Part 3, Biology and Medicine,
J. D. Hardy, ed., Chap. 55. Reinhold Publ. Corp.,
New York, 1963.

Davis, T. R. A.: Temperature, Its Measurement and
Control in Science and Industry. Vol. 8, Part 3,
Biology and Medicine, J. D.. Hardy, ed., Chap. 38.
Reinhold Publ. Corp., New York, 1963.

am

Josiah Macy,
Jr. Foundation Review Series, 1(2)L1-64, 1948.
Exsner, R. W., Nevas, J. D., and Irvine, L.: Jour.
Appl. Physiol. 15: 662, 1960.

Fox, R. H.: The Physiology of Human Survival,
0. G. Edholm and A. L. Bacharach, eds. Chap. 3.
Academic Press, New York, 1965.

. Freeman, W. J., and Davis, D. D.: Am. J. Physiol.

197: 145, 1959.

Freoty, M. J., and Iamprerao, P. F.: Metabolism
7: 624, 1958.

Fry; F. E. J.: Ann, Rev. Physiol. 20: 207, 1958.
Fusco, M.: Temperature, Its Measurement and Con-
trol in Science and Industry. Vol. 8, Part 3, Biology
and Medicine, J. D. Hardy, ed., Chap. 51. Reinhold
Publ. Corp., New York, 1963.

GowwMan, R. F., Green, B. B., and Iamprerno, P. F.:
Jour, Appl. Physiol. 20: 271, 1965.

Greenrietp, A. D. M.: Circulation, W. F. Hamilton
and P. Dow, eds., Chap. 39. American Physiological
Society, Washington, D.C., 1963.

. Hannon, J. P.: Temperature, Its Measurement and

Control in Science and Industry. Vol. 8, Part 3,
Biology and Medicine, J. D. Hardy, ed. Chap. 41.
Reinhold Publ. Corp., New York, 1963.

Harpy, J. D.: The Physiology of Temperature Regu-
lation Rept. No. 22, NADC-MA-G015. U. S. Naval
Air Development Center, Johnsville, Pa., 1960.



TABLE OF CONTENTS

y
S
%

I. Achievement of Thermal Balance
A. The Homeotherm and His Environment

B. The Physical Basis for Homeostasis ----

C. The Physiological Basis for Biothermal Control

D. The Neurophysiological Control of Body Temperature ------

IL. Thermal Balance During Exercise and Environmental Stress -
A. Exercise in Heat and Humidity -

B. Water and Work -----------

©. Thermal Acclimatization -
References ~-------------

waorwwore

Be
woo



FOREWORD

Aviation activities are conducted in a wide variety of environments,
and knowledge concerning the effects of these environments on man is neces-
sary in order to insure adequate performance. Environmental temperature
is one factor that may profoundly alter performance or personal well-being
or both. This paper describes in some detail the means, both physical and
physiological, available to man for maintenance of thermal balance. It is
also concerned with some aspects of thermal balance and performance when
work is performed in hot or cold environments and the enhancement of
performance that thermal acclimatization imparts.

Qualified requestors may obtain Aviation Medical Reports from Defense Documentation
Center. The general public may purchase from Clearinghouse for Federal Scientific and
Technical Information, U.S. Dept. of Commerce, Springfield, Va, 22151


moregulatory response to exercise in which in-
ternal body temperature is controlled at a new,
higher level. The elevation over resting internal
temperatures depends upon the rate of work °
and is generally uninfluenced by ambient tem-
perature (within the limits of biothermal abil-
ity). Further, during different steady state
workloads at a uniform ambient temperature,
sweat rates and internal temperatures are linearly
related and skin temperature remains constant.
Conversely, when the same level of work is per-
formed at different ambient temperatures, inter-
nal body temperature remains constant and sweat
rates rise as a linear function of average skin,
temperature.** These data provide a valuable
extention of the relationship between sweat rate
and internal temperature rported earlier."

Resting man (non-heat-acclimatized) exposed
to a hot environment (as high as 120° F.) can
maintain a normal body temperature by increas-
ing cardiac output and skin blood flow and
evaporating sweat. Superimposition of require-
ments to dissipate metabolic heat during exercise
increases circulatory strain( the requirement to
provide blood flow to the exercising muscles and
to the skin for heat dissipation), leading to in-
creased internal-body and skin-surface tempera-
tures, nausea, dizziness, and syncope.” Heat
production approximately § times resting at
100°F increases skin blood flow about 4 times over
resting levels at the same temperature.* Heat
dissipation at ambient temperatures greater than
skin temperature depends solely on the evapora-
tion of secreted sweat. The less clothing that
may form a vapor barrier and the drier the sur-
rounding air, the greater ease with which ex-
ercise can be performed in the heat. Human heat
acclimatization involves changes in blood volume
and peripheral circulation, which reduces cardio-
vascular strain during exercise in the heat and
improves sweating efficiency. *

The comfort and ease of physiological adjust-
ment to a heat stress, either at rest or during
exercise, is a combined function of ambient tem-
perature and humidity.” °° Those geographical
areas that typically have high ambient tempera-
tures in the summer (100°F or more), usually
have low relative humdity. This allows for the
rapid and complete evaporation of sweat on the
skin and clothing surfaces and a_ relatively
effortless thermal adjustment. Those areas, how-
ever, with high water-vapor pressures (although

lower ambient temperatures) have a much less
tolerable summer climate. Since the ready eva-
poration of sweat is not possible with high rela-
tive humidity, exercise thermal tolerances are
thereby limited.”* The same, or greater, amount
of water and salt may be lost in secreted sweat
in the wet-hot climate, in contrast to the dry-hot
environment; the thermal balance question is
more related to its evaporation than simply its
production: Table 2 illustrates how sweat secre-
tion increases when humidity is increased. Rest-
ing man at 110°F ambient temperature more
than triples sweat secretion when relative hu-
midity increases from 30% to 84%. Working
man at the same dry-bulb temperature almost
doubles sweat secretion when relative humidity
increases from “low” (about 30%) to 57%. The
excellent chapter by Ladell®’ should be consulted
for a comprehensive treatment of the physio-
logical effects of humid heat in man.

B. Water and Work. Sweating removes from
the body both water and salts. The loss of
water leads to dehydration and, eventually to de-
creased performance. As blood volume de-
creases with the progressive loss of body water,
metabolic as well as heat-exchanging circulatory
functions become impaired. Further dehydra-
tion (3.0% of body weight) reduces sweat-gland
activity.” Sweat-gland function during exercise
in either hot or cold environments brings about
the same general effects. After exercise, rehydra-
tion by voluntary water intake does not replace
the total amount of water lost until several
days* 7; the physiological mechanism for this
latency is not clear.

Inattention to body-hydration levels during
competitive sports can result directly in reduced
performance or, more seriously, set the stage for
heat exhaustion. A decrease in body water by
5% reduces maximum oxygen consumption (the
highest rate at which aerobic work can be per-
formed)? and performance (the skill required
for a particular task).’ Whereas particular at-
tention should be paid to drinking adequate
amounts of water during and after exercise,*
the requirement to replace lost Na, K, and Cl
can be normally met during meals. ** Since
both water and inorganic salt are lost in the
sweat, excessive water intake alone can result,
in below normal levels of tonicity and consequent
discomfort or heat cramps or both. The periodic
replacement of body water and salts during work