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Journal of Thermal Biology 29 (2004) 479–485
Extremes of human heat tolerance: life at the
precipice of thermoregulatory failure
W. Larry Kenney
, David W. DeGroot, Lacy Alexander Holowatz
102 Noll Laboratory, Department of Kinesiology, Penn State University, University Park, PA 16802-6900, USA
Abstract
1. Human life is sustainable only below an internal temperature of roughly 42–44 1C. Yet our ability to survive at
severe environmental extremes is testimony to the marvels of integrative human physiology.
2. One approach to understanding human thermoregulatory capacity is to examine the upper limits of thermal
balance between man and the air environment, i.e. the maximal environmental conditions under which humans can
maintain a steady-state core temperature. Heat acclimation expands the zone of thermal balance.
3. Human beings can and do, often willingly, tolerate extreme heat stresses well above these thermal balance limits.
Survival in all such cases is limited to abbreviated exposure times, which in turn are limited by the robustness of the
thermoregulatory response.
4. Figures are provided that relate tolerance time and the rate of change in core temperature to environmental
characteristics based on data compiled from the literature.
r2004 Elsevier Ltd. All rights reserved.
Keywords: Thermoregulation; Heat balance; Sweating; Prescriptive zone; Hyperthermia; Heat stroke
1. Introduction
The topic of ‘‘environmental extremes’’ for humans
seems relatively straightforward on the surface, yet
researching such a topic proves more difficult. ‘‘Ex-
treme’’ thermal environments have different connota-
tions to different people, even the thermal physiologists
who conduct research on the topic of human thermo-
regulation. Certainly there are extremes at either end of
the environmental temperature spectrum, and cold
tolerance is equally important to heat tolerance from a
teleological perspective. For brevity sake, the present
paper deals only with extremes of heat. One may
characterize a thermal extreme as the upper limit of
humans’ ability to maintain thermal balance, as defined
by a steady state core temperature. This approach has
been used in laboratory studies and a discussion of the
topic begins the present treatise.
While defining the upper limit of thermal balance is
instructive, human beings can and do tolerate extreme
heat stresses well above these equilibration limits, albeit
for short periods of time. A popularly retold story goes
as follows:
One morning toward the end of the eighteenth
century, the Secretary of the Royal Society of London,
one Mr. Blagden, ventured into a room heated to
1051C, taking with him some eggs, a piece of raw steak
and a dog. A quarter of an hour later, the eggs were
baked hard and the steak cooked to a crisp but
Blagden and his dog walked out unharmedy
(Ashcroft, 2000)
ARTICLE IN PRESS
www.elsevier.com/locate/jtherbio
0306-4565/$ - see front matter r2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jtherbio.2004.08.017
Corresponding author. Tel.: +1-814-863-1672; fax: +1-
425-799-1476.
E-mail address: w7k@psu.edu (W.L. Kenney).
Mr. Blagden’s experiment vividly points out how
tolerant humans truly are to dry heat—provided that
free evaporation of sweat can occur, hot surface
temperatures are avoided (Blagden’s dog was kept in a
basket to keep it from burning its feet), the circulatory
system can compensate, and the exposure time is limited.
While the above story is astonishing, situations in
which human beings tolerate such extremes form the
basis for the second part of this paper. The highest
recorded temperature on earth is apparently 58 1C in the
Sahara and many deserts routinely reach 45 1C(Ash-
croft, 2000). Sauna temperatures typically range from 80
to 100 1C with relative humidity (rh) as low as 5%, and
bathers commonly spend 15–30 min or more in these
extreme temperatures. The primary physiological com-
pensatory mechanism during these exposures is eva-
porative heat loss from sustainable sweating. This
results in progressive dehydration and cardiovascular
strain, especially in the absence of fluid and electrolyte
replacement. Where the preceding conditions rely on
evaporative heat loss, other environments rely almost
exclusively on integrated cardiovascular responses to
compensate for the heat load (such as hot water
immersion). The hottest Japanese onsen (spa baths) are
46–47 1C, yet experienced bathers routinely tolerate
these temperatures for 3 min. In summary, humans
can tolerate considerable extremes of thermal stress,
relying on different compensatory mechanisms depend-
ing on the characteristics of the environment.
2. Limits to human heat balance
Over a wide range of environments, body core
temperature (T
c
) equilibrates at temperatures propor-
tional to metabolic rate, independent of ambient
conditions (Saltin and Hermansen, 1966). Thermal
environments above this designated ‘‘prescriptive zone’’
(Lind, 1963) do not allow thermal balance either because
of excessive dry heat gain or limited evaporative heat
loss, and the result is a continuous rise in T
c
. Conditions
which define the upper limit of the prescriptive zone or
‘‘psychrometric limit’’ for a given metabolic heat
production represent one type of environmental ‘‘ex-
treme’’ and such balance points have been determined
experimentally for heat-acclimated men (Belding and
ARTICLE IN PRESS
10
15
20
25
30
35
32 34 36 38 40 42 44 46 48 50 52 54
ambient dry-bulb temperature, ˚C
30% rh
50% rh
70% rh90% rh
10% rh
heat-acclimated
women
M = 152 W/m2
v = 0 m/s
unacclimated women
M = 143 W/m2
v = 0.5 m/s
heat-acclimated women
M = 152 W/m2
v = 1.0 m/s
ambient water vapor pressure, torr
Fig. 1. Psychrometric representation of upper limits of thermal balance determined from Penn State experiments. Each line represents
such an upper limit, analogous to Lind’s ‘‘upper limit of the prescriptive zone’’. The filled circles and lighter line are from unacclimated
subjects (Kenney and Zeman, 2002), while the open circles and squares are for heat-acclimated women (Kamon and Avellini, 1976).
Heat acclimation shifts the lines upward and to the right, increasing the range of environments in which heat balance is possible for a
given metabolic rate (M) and air velocity (v). Similarly, increasing vshifts the curve further to the right. Each data point represents the
mean results from a cohort of subjects.
W.L. Kenney et al. / Journal of Thermal Biology 29 (2004) 479–485480
Kamon, 1973) and women (Kamon and Avellini, 1976),
as well as for unacclimated men and women (Kenney
and Zeman, 2002).
Experiments are conducted in a programmable
environmental chamber. The experimental protocol
involves the determination of either (1) the critical water
vapor pressure (P
crit
) for the upward inflection of T
c
at
several distinct dry-bulb temperatures (T
db
), or (2) the
critical dry-bulb temperature (T
crit
) at several distinct
ambient water vapor pressures (P
a
). The methods used
to determine P
crit
and T
crit
have been previously
described in detail (Belding and Kamon, 1973;Kamon
and Avellini, 1976;Kenney, 1988). During the T
crit
experiments, P
a
is held constant and T
db
is system-
atically increased approximately 1 1C every 5 min after a
30-min equilibration period. In the P
crit
experiments,
T
db
is held constant while P
a
is increased approximately
1 Torr every 5 min after a similar equilibration period.
During each test, the subjects walked continuously on a
motor-driven treadmill for up to 3 h at a pre-determined
exercise intensity. Air velocity (v) was also varied in
some experiments. Typically, T
c
begins to plateau by
about 40 min and remains at an elevated steady state as
P
a
or T
db
is increased. The critical environment is
defined as that immediately before the upward T
c
inflection, i.e. the combination of conditions below
which thermal balance can be maintained, but above
which a steady rise in T
c
occurs.
Fig. 1 presents a portion of a psychrometric chart
showing data compiled from two sets of experiments
performed in our laboratory some 25 years apart
(Kamon and Avellini, 1976;Kenney and Zeman,
2002). Each point shown is the mean from several
subjects and each line represents the extreme critical
environments below which T
c
can achieve a biophysical
steady state, but above which T
c
rises continuously. The
effect of heat acclimation is to shift the line rightward,
encompassing more environmental conditions below the
psychrometric limit. Increased velocity of air movement
around the subject is another factor that shifts the
psychrometric limit rightward and upward.
3. Environmental extremes
3.1. Dry heat
Extreme dry heat presents a severe challenge to
homeostasis. Yet human beings have demonstrated a
remarkable capacity to withstand, and adapt to, the
extreme physiological stresses desert conditions demand.
To dissipate heat gained in a desert environment, the
free evaporation of sweat is coupled with an elevation in
skin blood flow. Tolerance time in dry heat depends on
the sustainable sweating rate and the ability to withstand
the resultant dehydration. The following is an anecdotal
account of extended desert exposure.
In 1994, Mauro Prosperi became lost in a sandstorm
during the 160-mile Marathon des Sables in the
Moroccan desert (Kamler, 2004). On day 4 of the event,
scheduled as a 50-mile leg of the race in an ambient
temperature of 46 1C (115 1F), Prosperi lost his way in a
sandstorm and wandered off course. With less that one
full canteen of water, he survived for 9 days. During his
ordeal, he drank his own urine and sought shelter in an
empty Muslin shrine, where he ate bats he was able to
catch in the shrine. He restricted his movement to the
cooler parts of the day and night. The following is a
description of Prosperi after he was rescued and brought
to an Algerian hospital:
ydoctors reported that the desert wanderer had lost
33 pounds and that 16 liters of intravenous fluids
were needed to replace his water loss. His kidneys
were barely functioning, his liver was damaged, and
he was unable to digest food. His eyes had sunk back
inside their sockets, and his skin was dry and
wrinkled. He looked like a tortoise. But he would
survive.
Assuming that this amazing tale is true, it may be the
most extreme documented case of survivable dehydra-
tion.
Physiology of dry heat exposure: The ability to
withstand extreme heat exposure depends on the aerobic
fitness, hydration status, and heat acclimation status of
the subject. In the ensuing discussions of the physiolo-
gical adaptations that make it possible for subjects to
tolerate exposure to extreme temperatures, such assump-
tions are necessary for tolerance to, and survival in, true
extremes of environment.
The primary physiological compensatory mechanism
during exposure to dry heat is evaporative cooling.
Unofficially, the highest sweating rate recorded is 5.6 L/
h, a rate based on the43 kg body weight loss of a
professional tennis player in a laboratory setting over a
50-min period of exercise (Dr. Robert Murray, Ph.D.,
personal communication). Exercise consisted of cycling
at 75–95% of maximum heart rate and voluntary fluid
consumption was 1.6 L. The environmental chamber
was 35 1C and 63% rh, with minimal air movement. In
the published literature the highest reported sweating
rate may be that of Olympic distance runner Alberto
Salazar. Based on body weight change during the 1984
Olympic Marathon, and compensating for estimated
fluid consumption, Salazar had a reported sweating rate
of 3.7 L/h. During testing in an environmental chamber
earlier that year, his measured sweating rate was 2.8 L/h
(Armstrong et al., 1986).
Hypohydration caused by high sweating rates in the
absence of adequate fluid replacement leads to eleva-
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W.L. Kenney et al. / Journal of Thermal Biology 29 (2004) 479–485 481
tions in T
c
, relative to a euhydrated state, and an
increased rate of heat storage, thereby decreasing
tolerance time. Hypohydration leads to cardiovascular
strain by reducing cardiac filling pressure, reducing
stroke volume and causing a compensatory increase in
heart rate (Sawka and Pandolf, 1990). Sawka (Sawka et
al., 1985) investigated the influence of heat stress and
graded hypohydration (0, 3, 5, 7% body weight) during
exercise at 49 1C, 20% rh. Higher levels of hypohydra-
tion resulted in a greater incidence of exhaustion
(exercise termination) at a given T
c
, such that a 7%
reduction in body weight decreased time to exhaustion
from 140 min (at 0 and 3% dehydration) to 64 min.
Additionally, hypohydration led to a predictable in-
crease in heart rate and T
c
for each exercise bout.
Interestingly, two of the subjects terminated the exercise
due to the appearance of premature ventricular con-
tractions, suggesting that this level of 7% hypohydra-
tion may be near the tolerable safe limit. We are
unaware of experimentally induced hypohydration
exceeding 8% of body weight.
3.2. Non-evaporative environments
A distinctly different hot environment in comparison
to the desert is one in which both ambient heat and
ambient water vapor pressure are high. The robustness
of the cardiovascular response and the ability to tolerate
profound elevations in T
c
determine tolerance time in
this situation. The most extreme example of a non-
evaporative environment is hot water immersion. In this
environment most avenues of heat loss are unavailable,
as skin temperature quickly equilibrates with water
temperature and evaporative cooling is limited to any
skin surface not immersed in water. Immersed skin
cannot dissipate heat through evaporation, and radia-
tion and conduction become avenues of heat gain. One
extreme example of hot water immersion is the Japanese
onsen, ancient stone communal baths where the hottest
water temperatures are reported to be 46–47 1C. Most
bathers can only withstand these extreme temperatures
for 3 min (Ashcroft, 2000). However, Japanese people
customarily bathe in 40 1C water for periods of up to
60 min. The following is an account of a Westerner’s first
experience with the Japanese onsen.
I stepped boldly into the pool—and leapt straight out
again. It was scalding hot. At least 45 1C. I thought I
must have first-degree burns. yWhen I emerged
from my pool five minutes later I was a bright cherry
red, like a boiled lobster. All of my blood had been
directed to my skin as my body tried desperately to
cool down — to no avail, for not only could I not get
rid of the heat I generated myself, but I was rapidly
accumulating that of the bath. I sat on the edge of the
pool, my skin pouring sweat. But I felt marvellous
yfor although a short dip is marvellously invigorat-
ing, to remain there too long would, quite literally, be
fatal.
Warm water SCUBA diving is a unique example of
thermoregulatory stress that adds metabolic heat
production in addition to hot water immersion. Rescue
divers working in warm water wear vulcanized rubber
suits and carry equipment that can weigh up to 45 kg.
These divers can experience increases in T
c
as high as
31C during a 1-h rescue mission (White et al., 1998).
Rescue operations are carried out with dive pair teams
with one diver remaining on the surface, where
evaporative cooling is still limited by the wet suit. White
(White et al., 1998) measured T
c
changes during both
warm and cold water rescue training sessions; the
ambient temperature for the warm water training rescue
situation was 23 1C and the water temperature was
21 1C. These investigators found that the mean increase
in T
c
in all divers was 1.2 1C, however 2 divers had an
increase in T
c
greater than 2 1C in 30–50 min. T
c
continued to climb even after exiting the water at the
conclusion of the training session. This situation
represents another example of extreme heat stress
because evaporative cooling is severely limited while
metabolic heat is being generated causing core tempera-
ture to climb rapidly.
Physiological responses to extreme non-evaporative
heat: In the laboratory, a standard method to test
subjects’ upper limits of thermal tolerance is the water-
perfused suit. In numerous experiments (Minson et al.,
1998;Rowell et al., 1970;Rowell, 1974, 1983, 1986)
investigators raised skin temperature to 40 1C by the use
of these tube-filled suits to examine the cardiovascular
adjustments to severe hyperthermia. This experimental
paradigm maximizes skin blood flow and allows for
investigation of integrated cardiovascular adjustments
when the T
c
–T
sk
thermal gradient is reversed (as occurs
in hot water immersion as well).
When subjects are heated to thermal tolerance using
water-perfused suits, an integrated cardiovascular re-
sponse ensues. During severe hyperthermia, skin blood
flow can increase to as much as 7–8 L/min. To support
this increase in skin blood flow, cardiac output is raised
by 3L/min/ 1C rise in right atrial blood temperature
(Rowell, 1986). This increase is primarily mediated
through an increase in heart rate although small
increases in stroke volume do occur, despite the falling
right atrial filling pressure. Additionally, approximately
0.6 L/min of blood is redistributed to the skin from the
splanchnic circulation, and 0.4 L/min of blood from the
renal circulation. Muscle blood flow may also be
reduced during passive heat stress and redirected toward
the cutaneous circulation. Due to the volume of blood in
the cutaneous circulation and the lack of muscle pump
activity, right atrial pressure declines.
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W.L. Kenney et al. / Journal of Thermal Biology 29 (2004) 479–485482
3.3. Heat tolerance: duration of exposure
While anecdotal accounts of extreme heat exposure
provide some qualitative insight into the limits of human
heat tolerance, laboratory experiments over the decades
provided ample data relating tolerance time to a given
set of environmental characteristics. Numerous investi-
gators have measured the ‘‘maximal’’ tolerance time in a
given environment, and have used that information to
determine ‘‘safe’’ exposure limits. Figs. 2 and 3 are
drawn using data compiled from the literature in an
attempt to examine the nature of the relation between
environment, rate of rise in T
c
, and tolerance time. To be
included in this analysis, experiments had to satisfy
several critical criteria. These studies needed to report
the time to exhaustion for the group, the change in T
c
(or provide enough data to calculate this variable), and a
description of the subject’s activity level. Additionally,
chamber T
db
and either T
wb
or rh were necessary. As few
laboratory studies over the years have included globe
temperature data (a measure of radiant heat load),
calculation of wet-bulb globe temperature (WBGT)
across the studies was not possible. Alternatively, the
modified discomfort index (MDI) proposed by Moran
and Pandolf (1999) was calculated. Briefly, the MDI is
calculated as (0.75T
wb
)+(0.30T
db
) and correlates well
with WBGT (r40.95). Fig. 2 presents data for the rate
of change in T
c
/h as a function of MDI, compiled from
several studies meeting the aforementioned criteria
(Avellini et al., 1980;Goldman et al., 1965;Iampietro
and Goldman, 1965;Nag et al., 1999, 1997;Pandolf et
al., 1988;Randle and Legg, 1985;Sawka et al., 1983,
1992;Shapiro et al., 1980;Shvartz and Benor, 1972).
Compared to resting exposure, exercise is associated
with a larger rate of change in T
c
/h in a given
environment, in part because of the additional cardio-
vascular strain superimposed on the thermoregulatory
strain. The highest absolute change in T
c
recorded was
2.6 1C (several studies only reported the per hour value,
and the absolute T
c
change was not reported). Fig. 3
shows the relationship between tolerance time (time to
exhaustion or test termination) and MDI. During either
resting or exercising conditions, an increase in MDI
reduced tolerance time in a relatively predictable
fashion. However, exercise data are again shifted to
the left.
4. Thermoregulatory failure
Heat stroke is traditionally defined as a rectal
temperature greater than 40.6 1C, with untoward neu-
rological involvement. This condition is the consequence
of severe cardiovascular strain accompanied by thermo-
regulatory failure. In heat stroke, the internal heat load
overwhelms the capacity of the system, and the
ARTICLE IN PRESS
0
1
2
3
4
5
6
7
8
9
25 30 35 40 45 50
MDI, ˚
rate of rise in core temperature, ˚C/h
C
Fig. 2. Rate of rise in core temperature plotted against the mean discomfort index (MDI, a measure of the combined non-radiant
thermal environment similar to WBGT). These data points were derived from a series of published studies meeting finite criteria (see
text). Open circles are from resting exposures while filled circles come from exercise trials. Above an MDI of about 32 1C, both
responses are linear (resting: y=7.4x2.8, R
2
=0.92; exercise: y=4.3x1.4, R
2
=0.84).
W.L. Kenney et al. / Journal of Thermal Biology 29 (2004) 479–485 483
maintenance of core temperature is sacrificed so that
arterial pressure and blood flow to vital organs may be
better maintained (Rowell, 1986). The ensuing cardio-
vascular strain due to heat and progressive dehydration
causes a reduction in skin blood flow and sweating and a
rapid increase in core temperature. Heat stroke is often
associated with multiple organ system damage including
cognitive impairment, acute renal failure, rhabdomyo-
lysis, liver dysfunction with hepatic enzyme elevation,
disseminated intravascular coagulation, and acid base
disturbances (Sutton, 1990).
While a T
c
in excess of 42 1C is usually considered
fatal, T
c
correlates poorly with the severity of organ
dysfunction (Gardner and Kark, 2001). The highest T
c
survived without lasting complications was 46.5 1C
(Slovis et al., 1982). On that day, the ambient high T
db
was 38 1C, with 44% rh. The 52-year-old male victim
reported that he had been cooking indoors all day in a
poorly ventilated room. Emergency medical services
were summoned when the victim failed to answer his
door. Medical personnel recorded a rectal temperature
at the scene of 42 1C, which represented the upper limit
of their thermometer. Upon arriving at the hospital, and
25 min after aggressive cooling had been initiated,
doctors recorded a rectal temperature of 46.5 1C.
Calibration of the thermometer was validated later that
day. In spite of the severe hyperthermia, liver and renal
complications, and disseminated intravascular coagula-
tion, the patient survived and was discharged after 24
days of hospitalization.
5. Summary
This paper has attempted to present anecdotes,
clinical cases, and results of laboratory experiments
related to extremes of heat stress. The purpose has been
two fold: to examine environmental ‘‘extremes’’ in their
many facets, and to highlight the exquisite precision and
adaptability of human thermoregulation.
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