Aviation, Space, and Environmental Medicine x Vol. 79, No. 5 x May 2008 495
Shivering Heat Production and Core Cooling During
Head-In and Head-Out Immersion in 17°C Water
Thea Pretorius , Farrell Cahill , Sheila Kocay ,
and Gordon G. Giesbrecht
RETORIUS T, C AHILL F, K OCAY S, G IESBRECHT GG. Shivering heat
production and core cooling during head-in and head-out immersion
in 17°C water. Aviat Space Environ Med 2008; 79:495 – 9.
Introduction: Many cold-water scenarios cause the head to be par-
tially or fully immersed (e.g., ship wreck survival, scuba diving, cold-
water adventure swim racing, cold-water drowning, etc.). However, the
speciﬁ c effects of head cold exposure are minimally understood. This
study isolated the effect of whole-head submersion in cold water on sur-
face heat loss and body core cooling when the protective shivering
mechanism was intact. Methods: Eight healthy men were studied in
17°C water under four conditions: the body was either insulated or ex-
posed, with the head either out of the water or completely submersed
under the water within each insulated/exposed subcondition. Results:
Submersion of the head (7% of the body surface area) in the body-
exposed condition increased total heat loss by 11% ( P , 0.05). After
45 min, head-submersion increased core cooling by 343% in the body-
insulated subcondition (head-out: 0.13 6 0.2°C, head-in: 0.47 6 0.3°C;
P , 0.05) and by 56% in the body-exposed subcondition (head-out:
0.40 6 0.3°C and head-in: 0.73 6 0.6°C; P , 0.05). Discussion: In
both body-exposed and body-insulated subconditions, head submersion
increased the rate of core cooling disproportionally more than the rela-
tive increase in total heat loss. This exaggerated core-cooling effect is
consistent with a head cooling induced reduction of the thermal core,
which could be stimulated by cooling of thermosensitive and/or trigemi-
nal receptors in the scalp, neck, and face. These cooling effects of head
submersion are not prevented by shivering heat production.
Keywords: hypothermia , cold-water immersion , perfused body mass ,
symptomless hypothermia , thermal core .
HE EFFECT OF HEAD heat loss on body core cool-
ing is much discussed but less well understood.
Over the years, many studies have documented effects
of several factors on the rate of core cooling in cold wa-
ter. These factors include body size and mass, water
temperature and movement, nutritional and hydration
status, exposure time, thermoregulatory status, insula-
tion, etc. ( 12 , 14 ). The vast majority of these studies were
done with the head out of the water and generally ex-
posed to relatively warm laboratory conditions. Thus,
although many cold-water scenarios cause the head to
be partially or fully immersed (e.g., ship wreck survival,
scuba diving, cold-water adventure swim racing, cold-
water drowning, etc.) the speciﬁ c effects of head cold
exposure are not well documented.
Some authors have predicted a substantial heat loss
through the head due to the high rate of surface blood
ﬂ ow in the scalp ( 4 ). Others predicted minimal heat
loss from the head due to the relatively small ( ; 7%)
increase in exposed body surface area (BSA) ( 13 ) and
minimal conductive heat loss directly through the
scalp and skull ( 20 ). There have been few attempts to
directly determine the core cooling effect of head cold
A few previous cold-water immersion studies have
demonstrated that dorsal head immersion does not in-
crease core cooling rate when the rest of the body is in-
sulated. However, when the rest of the body is already
cold-exposed, additional immersion of the dorsal head
increased core cooling signiﬁ cantly ( 5 , 15 ). The only
known study involving whole-head submersion was
conducted in 17°C water with shivering suppressed by
Demerol ( 16 ). This study conﬁ rmed the increased core
cooling effect of head cooling when the body was al-
ready exposed. However, unlike with dorsal head im-
mersion, submersion of the whole head also increased
core cooling when the body was not cold-exposed. These
results were proposed to arise from a reduced thermal
core when scalp and facial cooling increased peripheral
While previous work on non-shivering humans has
valuable implications for situations like cold-water
near-drowning and symptomless hypothermia experi-
enced by scuba divers, similar observations with the
shivering mechanism intact could provide valuable
additional insights for thermoregulation of scuba div-
ers and cold-water swimmers in adventure races. The
purpose of this study was to determine whether an in-
tact shivering mechanism would negate the impact of
whole head submersion on core cooling in 17°C water.
We hypothesized that whole head submersion would
still increase core cooling disproportionally more than
the relative increase in surface heat loss, even when
shivering heat production would tend to attenuate core
From the Laboratory for Exercise and Environmental Medicine,
Health, Leisure and Human Performance Research Institute, Univer-
sity of Manitoba, Winnipeg, Canada.
This manuscript was received for review in July 2007 . It was ac-
cepted for publication in February 2008 .
Address reprint requests to: Thea Pretorius, 211 Max Bell Centre,
University of Manitoba, Winnipeg, Canada R3T 2N2; email@example.com .
Reprint & Copyright © by the Aerospace Medical Association, Alex-
496 Aviation, Space, and Environmental Medicine x Vol. 79, No. 5 x May 2008
THERMAL EFFECTS OF HEAD COOLING — PRETORIUS ET AL.
The experimental protocol was approved by the
University of Manitoba Education/Nursing Research
Ethics Board. The study was open to men and women;
however, only men volunteered. Eight male subjects, each
of whom provided written, informed consent and proof
of scuba diving certiﬁ cation, were studied. These subjects
appeared both mentally and physically healthy and had
no signiﬁ cant medical history, such as cardiorespiratory
disease or Raynaud’s syndrome, as reﬂ ected by a medical
(PAR-Q) questionnaire. None of them had male-pattern
baldness nor did any have particularly long or thick hair.
For each trial, subjects wore a swimsuit while being
instrumented in a room at an ambient temperature of
; 22°C. Core temperature was measured using a ther-
mocouple (Mallinckrodt Medical Inc., St Louis, MO) in
the esophagus (T
) at the level of the cardiac atria. This
site provides the closest correlation to intracardiac tem-
perature ( 11 ). Single-channel electrocardiogram and
heart rate were also monitored (Hewlett Packard Co.,
Cutaneous heat ﬂ ux (W z m
) and skin temperature
(°C) were measured from 12 sites (listed below) using
thermal ﬂ ux transducers (model FR-025-T-18, Concept
Engineering, Old Saybrook, CT) according to standard
procedures ( 8 ). Body surface area (BSA) was calculated
as follows: area (m
) 5 weight
(kg) z height
0.007184 ( 2 ). The following regional percentages were
assigned based on Layton et al. ( 13 ): forehead 4%, dor-
sum of the head 3%, chest 8.75%, scapula 8.75%, abdo-
men 17.5%, forearm 12%, posterior upper arm 7%, ante-
rior thigh 9.5%, posterior thigh 9.5%, anterior calf 6.5%,
posterior calf 6.5%, and the foot 7%. Flux was deﬁ ned as
positive when heat traversed the skin toward the envi-
ronment (i.e., heat loss) and values for each transducer
(W z m
) were converted into W z region
(W) 5 transducer ﬂ ux (W z m
) z body surface
) z regional percentage z 0.01. Eq. 1
A light mesh hood was used to hold the heat ﬂ ux
transducer snugly against the hair on the back of the
head. This eliminated a layer of water between the hair
and transducer, thus ensuring that heat loss from the
skin, and through the hair, was channeled through the
transducer. The mesh was light and provided negligible
insulation when tested on the bare forehead ( , 5% de-
crease in heat ﬂ ux).
Oxygen consumption (
) was measured with an
open circuit from expired minute volume and inspired
and mixed-expired gas concentrations sampled from a
mixing box (V
229 by Sensormedics, Yorba Linda, CA ).
Because subjects were completely submersed in half of
the trials, they breathed compressed air throughout
baseline and during immersion/submersion in all trials.
The scuba tanks were kept at room temperature at all
times before and during testing and the assumption was
made that the temperature of the inspired gases was
room temperature ( ; 22°C). To facilitate metabolic mea-
surements, a standard scuba regulator (Blizzard, Sher-
wood, Lockport, NY) was modiﬁ ed and connected to
corrugated tubing so that all expiratory gas could be col-
lected by the metabolic system. Metabolic measure-
ments of respiratory activity conducted in hyperbaric
conditions but analyzed at normobaric conditions will
over-estimate ventilatory parameters, but will accurately
measure metabolic variables (i.e., oxygen consumption)
( 16 ). All data were recorded at 30 – s intervals.
In subsequent analysis, oxygen consumption and re-
spiratory exchange rate (RER) were used to calculate
metabolic rate (M) in Watts as follows ( 19 ):
M (W) 5 o
(L z m i n
) z 69.7(4.686 1 [(RER − 0.707)
z 1.232]) Eq. 2
Respiratory heat loss (RHL) was calculated in depen-
dence of metabolism ( 3 ):
RHL (W) = 0.09 z M Eq. 3
Total energy production for the immersion/submersion
period was calculated by converting metabolic rate (W)
to kilojoules (kJ). Total energy loss was calculated as the
sum of total body cutaneous heat ﬂ ux and respiratory
heat loss (kJ).
Subjects were immersed/submersed four times in 17°C
water; the order of conditions followed a balanced de-
sign. For each condition subjects were lowered with an
electronically isolated hoist into the water. Two of the
conditions involved complete submersion and required
the breathing of compressed air. Thus, for all trials (base-
line and immersion/submersion), subjects wore a nose
clip and breathed compressed air as previously described.
One weight belt (15 kg) was worn around the waist and
another one (12 kg) was placed over the thighs, but not
covering the heat ﬂ ux probes. In all conditions a light
mesh hood kept the posterior head heat ﬂ ux disk in place.
In the head-submersion trials a diving facemask was
worn. The straps of the mask did not cover the posterior
head probe. The mask covered the nose, upper cheeks,
the eyes, and approximately 50% of the forehead, with a
total surface area of approximately 160 cm
. The average
body surface of the subjects was 2.07 6 0.1 m
. The face is
considered to be 4% of the total BSA ( 13 ); this would im-
ply that the average face surface of the subjects was 800
and that the mask covered 20% of this area. In the
body-insulated conditions, the instrumentation wires ex-
ited the suit via the right wrist cuff. During these trials,
the right hand was held just above the water surface to
prevent water from leaking through the wrist seal.
Exposed head-out: The subjects wore only a bathing suit
and were immersed to the neck with the head positioned
above the water.
Exposed head-in: The subjects wore only a bathing suit
and were lowered until the entire head was completely
Aviation, Space, and Environmental Medicine x Vol. 79, No. 5 x May 2008 497
THERMAL EFFECTS OF HEAD COOLING — PRETORIUS ET AL.
Insulated head-out: Subjects wore a 1.5-mm thick vulca-
nized rubber dry suit over thermal underwear (full body
suit), a two-piece ﬂ eece suit, two pairs of socks, and a wool
glove on the left (immersed) hand, as well as a rubber
glove over the wool glove. The insulation value of this en-
semble was 0.98 m
K/W (6.3 clo). Subjects were immersed
to the neck with the head positioned out of the water.
Insulated head-in: Subjects wore the same insulation
ensemble described above. They were lowered until the
entire head was completely submersed.
Subjects were studied on four separate occasions at
least 48 h apart except for two cases in which an insu-
lated head-out condition followed 24 h after an insu-
lated head-in condition. For every subject all experi-
ments were done at the same time each day to control
for circadian effects. Abstinence from alcohol, tobacco,
and strenuous exercise for 12 h prior to the study was
requested. They were instructed to consume only a light
meal before they arrived for the study.
After subjects were prepared and they were relaxed,
10 min of baseline measurements were recorded. In the
insulated conditions only the thermal underwear and
one pair of socks were worn during baseline. Before the
subjects were immersed the rest of the insulative cloth-
ing was donned, a process that took ; 30 min. Full dress-
ing was delayed to prevent the subjects from overheat-
ing and potentially sweating during the baseline period.
Although the act of dressing did increase heat pro-
duction, the effect was transient and relatively small
compared to the overall heat balance of the 45-min
The subjects remained immersed until one of four re-
moval criteria was met: 1) immersion time of 60 min; 2)
voluntary request by a subject for removal; 3) T
34°C; or 4) termination of immersion by investigators
for safety reasons. Upon removal from the cold water,
subjects could choose to be placed in a 40°C stirred wa-
ter bath until T
was . 36°C and they felt comfortably
warm. The water bath was used in all exposed condi-
tions and one insulated head-in trial.
Three subjects asked to exit the water after 45 min due
to discomfort, twice in the exposed head-in condition
and once in the insulated head-in condition. Therefore,
all analysis included data from only 45 min of immer-
sion. The following calculations were made for each
condition: 1) decrease in T
from time 0 to 45 min; 2) rate
of core cooling (calculated by linear regression for T
data from 10 to 45 min of immersion) and the following
variables for the 45 min immersion/submersion period;
3) cutaneous heat loss; and 4) metabolic heat produc-
tion. Group results were calculated for each condition
and reported as means 6 SD.
Repeated measures one-way analyses of variance
compared intercondition values. A P -value ⱕ 0.05 identi-
ﬁ ed statistically signiﬁ cant differences. A Tukey’s test
was used for post hoc analysis of signiﬁ cant differences.
For the exposed head-in condition, a paired t -test com-
pared the percentage of the body surface area attributed to
the head (designated as 7%) to the heat loss from the head
expressed as a percentage of total heat loss. Metabolic rates
from baseline and the end of immersion/submersion
periods were also compared with paired t -tests.
The subjects were 32.4 6 12 yr old; 179.6 6 5 cm tall
and weighed 88.3 6 16 kg. They also had 20.2 6 6%
body fat ( 1 ), and a body surface area of 2.1 6 0.1 m
The average baseline core temperature for the four
conditions was 37.2 6 0.4°C with no difference between
increased during the suiting up process
in both the insulated conditions ( Fig. 1 ). From the onset
of immersion, T
responded as follows. As expected, T
remained unchanged throughout the insulated head-
out trials ( Table I ). Compared to having the head out, 45
min of head submersion signiﬁ cantly decreased T
both the insulated (0.5 6 0.3 vs. 0.1 6 0.2°C; P , 0.05)
and exposed (0.7 6 0.6 vs. 0.4 6 0.4°C; P , 0.05) subcon-
ditions. Surprisingly, the drop in T
was not signiﬁ cantly
different between the insulated head-in (0.5 6 0.3°C),
and the exposed head-out (0.4 6 0.4°C) conditions.
Metabolism and Heart Rate
The onset and intensity of shivering could not be reli-
ably conﬁ rmed by direct observations; thus, shivering
was inferred by increased metabolic heat production.
Metabolic rate was similar during baseline in all condi-
tions ( Table I ) and was unchanged throughout the
insulated head-out immersion period. In the insulated
head-in condition, metabolic rate signiﬁ cantly increased
Fig. 1. Change in esophageal temperature from immersion (i.e., time
0) during 10 min of baseline and 45 min of immersion/submersion in
all conditions (plotted every 30 s). For both insulated conditions, data are
not presented for the period of dressing immediately prior to immersion/
submersion (note: core temperature rose slightly during this time).
* Separates all conditions that are signiﬁ cantly different ( P , 0.05).
† At 30 and 45 min, values below this symbol are signiﬁ cantly decreased
from values at time 5 0 min ( P , 0.05). Error bars represent SD.
498 Aviation, Space, and Environmental Medicine x Vol. 79, No. 5 x May 2008
THERMAL EFFECTS OF HEAD COOLING — PRETORIUS ET AL.
by 35% after 45 min of submersion ( P , 0.001), indicating
that some shivering occurred in this condition. In both
exposed conditions, shivering signiﬁ cantly increased
metabolic rate by ; 2.3-fold ( P , 0.05) after 45 min; in
these conditions head submersion did not signiﬁ cantly
increase metabolic rate. After 45 min, metabolic rate was
signiﬁ cantly higher for the exposed head-out condition
than the insulated head-in condition ( P , 0.05). Heart
rate was similar during baseline in all conditions. Within
each condition there were no signiﬁ cant changes during
immersion/submersion; however, after 45 min heart
rates for both insulated conditions were signiﬁ cantly
lower than for the exposed conditions ( P , 0.05).
Energy Production and Loss
Heat loss and energy production — calculated for the
entire 45-min period of cooling — are presented in Fig. 2 .
Total body heat loss was more than twice the amount in
the exposed than in the insulated conditions ( P , 0.001).
Head submersion caused a fourfold increase in head
heat loss in both insulated and exposed subconditions.
In the exposed head-in condition, head heat loss (133 6
14 kJ) accounted for 11% of the total cutaneous heat loss
of 1225 6 202 kJ. This proportion was signiﬁ cantly
greater than the 7% regional contribution of the head to
total body surface area ( P , 0.001).
This was the ﬁ rst study to evaluate the isolated contri-
bution of whole head cooling to lowering of core tem-
perature when the shivering mechanism is intact. An
intact shivering mechanism did not eliminate the
phenomenon seen in a previous study where shivering
was inhibited ( 16 ). Head submersion increased the rate
of core cooling both when the body was insulated from,
and exposed to, 17°C water. The increase in core cooling
was disproportionately greater than could be explained
by the relative increase in heat loss alone (e.g., in the ex-
posed head-in condition, head submersion accounted
for an 11% increase in total heat loss, yet the rate of core
cooling increased by 56%). Hayward et al. ( 10 ) reported
similar effects when subjects were physically active in
10°C water. They demonstrated that drown prooﬁ ng
(which intermittently submersed the whole head) in-
creased core cooling by 36% compared to treading water
with the head above water.
The present study conﬁ rms the observation that addi-
tional exposure of the face also promotes core cooling
when the body itself is not cold-stressed. This is in con-
trast to previous studies exposing only the dorsal head.
In these trials dorsal head immersion only increased
core cooling when the body was also immersed in cold
water; an effect that occurred with shivering intact ( 15 )
or inhibited ( 5 ). Also, the core cooling rate was similar
whether the head only (insulated head-in) or the body
only (exposed head-out) was exposed to cold water, de-
spite a large difference in total heat loss (665 6 88 vs.
1174 6 156 kJ, respectively). Although this data cannot be
directly compared to our previous data, an intact shivering
mechanism seemed to attenuate core cooling in all
comparable conditions. Core cooling rates for shivering-
inhibited ( 16 ) and shivering-intact trials, respectively,
were: insulated head-out, 1.1 vs. 0.2°C z h
head-in, 1.6 vs. 0.7°C z h
; exposed head-out, 1.8 vs.
0.7°C z h
; and exposed head-in, 2.5 vs. 1.1°C z h
Possible Mechanisms for the Results
An intact shivering mechanism did not prevent the dis-
proportionately large increase in core cooling when the
head was totally submersed. As suggested previously
( 16 ), these results are consistent with the moderately in-
creased heat loss affecting a smaller thermal core. Stolwick
and Hardy ( 17 ) and others ( 18 ) have described a two-
layer thermal model in which the body has core and shell
Fig. 2. Energy production and loss during 45-min period of immer-
sion/submersion in 17°C water. Total Loss includes cutaneous and re-
spiratory heat loss. Body Loss includes trunk, legs, and arms. † Less than
in corresponding exposed conditions ( P , 0.001). * Different from all
other conditions ( P , 0.001). 1 Greater than head-out conditions ( P ,
0.001). Error bars represent SD.
TABLE I. METABOLIC RATE, HEART RATE, AND CHANGES IN ESOPHAGEAL TEMPERATURE (T
) BEFORE AND DURING
IMMERSION/SUBMERSION IN 17°C WATER.
Metabolic Rate (W) Heart Rate (bpm)
Rate (°C z h
) Baseline 45 min Baseline 45 min
138 6 31 139 6 32 79.2 6 9 69.5 6 7 0.1 6 0.2 *
tblfn1 0.2 6 0.3 * tblfn1
137 6 28
185 6 33 * tblfn1 85.9 6 7 73.1 6 6 * tblfn1 0.5 6 0.3 0.7 6 0.5
140 6 41
325 6 97 82.2 6 9 79.1 6 7 0.4 6 0.4 * tblfn1 0.7 6 0.5 * tblfn1
155 6 37
364 6 119 83.6 6 8 82.5 6 9 0.7 6 0.6 1.1 6 0.6
* Separates all conditions that are signiﬁ cantly different ( P ⱕ 0.05).
Signiﬁ cant difference over time ( P ⱕ 0.05).
Aviation, Space, and Environmental Medicine x Vol. 79, No. 5 x May 2008 499
THERMAL EFFECTS OF HEAD COOLING — PRETORIUS ET AL.
layers. Depending on the state of vasomotion, the muscle,
fat, and skin layers may be part of either the core or shell.
For example, increasing peripheral vasoconstriction de-
creases the size of the ‘ thermal ’ core (i.e., the mass of tis-
sue perfused) as the muscle, fat, and skin layers effec-
tively become part of the shell. In this condition, a given
amount of heat loss would cause a greater change in tem-
perature of the reduced thermal core. A reduced thermal
core has been used to explain surprisingly high rates of
core temperature increase in hypothermic subjects ( 19 )
and is consistent with the relatively large core tempera-
ture decrease in our previous ( 16 ) and present studies.
It is noteworthy that when the body is insulated,
whole head submersion increases core cooling while
dorsal head immersion does not. This phenomenon may
be explained by a greater stimulation of peripheral
vasoconstriction — and thus smaller thermal core — when
the face is also cooled. Facial submersion would increase
vasoconstriction through normal thermoregulatory con-
trol mechanisms and possibly the human dive reﬂ ex,
which is mediated by the trigeminal nerve ( 2 , 13 , 14 ). The
extent to which the dive reﬂ ex contributes to these re-
sults was not determined in the present protocol.
Finally, it was interesting that core cooling was similar
when either the head only or body only was cold-
exposed, despite signiﬁ cantly greater heat loss (by 509 kJ)
during body-only immersion. Two opposing factors
could explain these results. A reduced thermal core dur-
ing head submersion would enhance core cooling, while
increased shivering heat production during body im-
mersion would tend to offset the core cooling effect.
During body immersion a higher rate of shivering would
not only increase heat production, but it could also in-
crease the size of the thermal core (i.e., perfused mass)
as muscle blood ﬂ ow increased to fuel shivering. The
sum of these factors (shivering, surface heat loss, and
the size of the thermal core) results in similar core cool-
ing rates in these two conditions.
Practical Implications of Results
Scalp and facial cold exposure will enhance core cool-
ing even in cool (17°C) water. This study has practical
implications for cold-water scuba divers and recre-
ational swimmers (e.g., adventure racers) who may be
at risk if they experience hypothermia to the point of al-
tered physical ( 7 ) and mental capacity ( 6 , 15 ). These re-
sults indicate that the head should be kept out of the
water as much as possible during cold-water activities.
If head immersion/submersion is inevitable, the head
should be insulated as much as practical and a facemask
or goggles should be worn as this would attenuate the
vasoconstriction response ( 9 ).
In conclusion, even when the shivering mechanism is
intact, head submersion signiﬁ cantly increases the rate
of core cooling. This increased cooling rate is dispropor-
tionately greater than the relative increase in heat loss,
whether the body is insulated or also exposed to the
cold water. These results are consistent with a moderate
increase in heat loss affecting a reduced thermal core,
caused by enhanced peripheral vasoconstriction. Fur-
ther work is warranted to determine if, and how, the hu-
man dive reﬂ ex contributes to this phenomenon.
This study was supported by the Natural Science and Engineering
Research Council of Canada. The authors wish to thank Doug Evans of
One Stop Diving in Winnipeg for providing the scuba regulator used
in this study, and Jeremy Stewart of the Freshwater Institute, Winnipeg,
for providing compressed air. We also wish to thank Kate McGarry and
Dominique Gagnon for technical assistance during many of the trials.
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