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Impact of protective clothing on thermal and cognitive responses

Impact of protective clothing
on thermal and cognitive
Thesis for the degree of Philosophiae Doctor
Trondheim, November 2010
Norwegian University of Science and Technology
Faculty of Natural Sciences and Technology
Department of Biology
Hilde Færevik
Norwegian University of Science and Technology
Thesis for the degree of Philosophiae Doctor
Faculty of Natural Sciences and Technology
Department of Biology
© Hilde Færevik
ISBN 978-82-471-2394-2 (printed ver.)
ISBN 978-82-471-2395-9 (electronic ver.)
ISSN 1503-8181
Doctoral theses at NTNU, 2010:206
Printed by NTNU-trykk
This thesis is submitted for the degree of Doctor of Philosophiae in the Department of
Biology, Faculty of Natural Sciences and Technology, Norwegian University of Science and
The work has been carried out at SINTEF Technology and Society, Department of Preventive
Health Research in Trondheim. Research Council of Norway grant no. 110684 financed the
main part of the thesis. The Royal Norwegian Airforce contributed financial support to parts
of the project. The final part of the work was financed by SINTEF Technology and Society.
My most grateful acknowledgements go to my supervisor Professor Randi Eidsmo Reinertsen
for introducing me to the field of thermoregulation and whom I regard as a true mentor
throughout my career to date. Her enthusiasm, constructive guidance, knowledge and
continuous encouragement during all stages of this work have given me the inspiration
necessary to finalize this thesis.
I also wish to thank my co-authors, my hard-working master's students Drude Markussen and
Gro Ellen Øglænd. In particular I would like to thank Dr. Gordon Giesbrecht at the University
of Manitoba, Canada, for fruitful project cooperation, and for bringing his knowledge,
laboratory equipment and staff all the way from Canada to Norway. Vigdis By Kampenes,
thanks for helping me out with the statistics. A special thank to the Eli Hjeltereie for technical
support in the laboratory and to the subjects who participated in the studies. I am grateful to
the late Major Oddbjørn Grande of the Royal Norwegian Airforce for his interest and
involvement in our research.
I wish to thank all my present and former colleagues and students at my department for their
collaboration, friendship, and for creating a positive work environment necessary for keeping
up the motivation. A special thank to Mariann Sandsund, my colleague and friend through all
these years, you have been a great support for me both in working and private life.
My deepest gratitude to my family; Olav you are always there for me, and I thank you for
your patience and great support. Our two children, Aksel and Vetle; you are the best that have
ever happened to me.
ACKNOWLEDGEMENTS.................................................................................................................................. 3
CONTENTS........................................................................................................................................................... 5
ABSTRACT........................................................................................................................................................... 7
LIST OF PAPERS ................................................................................................................................................ 9
INTRODUCTION............................................................................................................................................... 11
AIM OF THESIS................................................................................................................................................... 12
BACKGROUND ................................................................................................................................................. 13
THERMAL RESPONSES TO AIR AND WATER ........................................................................................................ 13
COGNITIVE PERFORMANCE DURING THERMAL STRESS ...................................................................................... 19
PROTECTIVE CLOTHING..................................................................................................................................... 20
PROBLEM ASSIGNMENT..................................................................................................................................... 23
HYPOTHESES..................................................................................................................................................... 24
SUMMARY OF INDIVIDUAL PAPERS ......................................................................................................... 25
PAPER I ............................................................................................................................................................. 25
PAPER II ............................................................................................................................................................ 26
PAPER III........................................................................................................................................................... 27
PAPER IV .......................................................................................................................................................... 28
DISCUSSION ...................................................................................................................................................... 30
THERMAL COMFORT AND PERFORMANCE IN AIR (PAPERS IAND II)................................................................... 30
IMMERSION IN COLD WATER (PAPERS III AND IV) ............................................................................................. 37
METHODOLOGICAL CONSIDERATIONS ............................................................................................................... 42
CONCLUSIONS ................................................................................................................................................. 44
PRACTICAL APPLICATIONS ................................................................................................................................45
FUTURE PERSPECTIVES...................................................................................................................................... 46
REFERENCES.................................................................................................................................................... 48
Current aircrew protective clothing is unable to the address the challenging situation that
arises when the same clothing concept needs to provide sufficient thermal protection in water
while also ensuring thermal comfort and optimal work performance during flights.
Performance, safety and health all suffer when environmental thermal stress factors exceed
the body’s ability to compensate for disturbances in heat balance. Wearing protective clothing
further increases the thermal stress, which increases the risk of human errors that can have
fatal consequences.
This thesis addresses the fundamental mechanisms of how interactions among environmental
temperature, clothing, work load, and physiological regulatory systems affect the working and
emergency responses of helicopter pilots. The first part of this thesis investigated the impact
of wearing protective clothing in a working situation on factors such as comfort, physiology
and cognitive performance. The second part focuses on immersion in cold water, and in
particular on the importance of improving heat balance during exposure to cold water.
This thesis has added to our knowledge of the ambient conditions required for thermal
comfort and optimal performance in a working situation. In the emergency situation in cold
water it also offers new knowledge about how to improve heat balance under extreme
environmental conditions when wearing an immersion suit in cold waters.
The results of the studies described in this thesis have practical implications for the
development of new types of protective clothing that will improve user safety without
reducing comfort and work performance.
List of papers
This thesis is based on the following papers, which are referred to in the text by their Roman
Paper I
Færevik H, Markussen D, Øglænd GE, Reinertsen RE (2001). The thermoneutral zone when
wearing aircrew protective clothing. Journal of Thermal Biology, 26, 419-425.
Paper II
Færevik, H, Reinertsen RE (2003). Effect of wearing aircrew protective clothing on
physiological and cognitive responses under various ambient conditions. Ergonomics. Vol.46,
no. 8, 780-799.
Paper III
Færevik H, Reinertsen RE. Initial heat stress on subsequent responses to cold water
immersion while wearing protective clothing. Aviation Space and Environmental Medicine,
under consideration, submitted May 2010, revised version sent August 2010.
Paper IV
Færevik H, Reinertsen RE, Giesbrecht GG. Leg exercise and core cooling in an insulated
immersion suit under severe environmental conditions. Aviat Space Environ Med 2010; 81:
Concern for how the thermal environment affects industrial workers has long been a public
health and safety issue (134). In the South African gold-mining industry, for example, it was
vital to acquire knowledge of how workers could acclimatize to the exceptional heat and
humidity in deep mines (139). The frequently extreme requirements of the military and space
industries have triggered further efforts to understand how thermal extremes affect human
physiology and performance (34). The large number of ships and aircraft lost at sea during the
Second World War attracted attention to survival at sea and the risks of hypothermia.
Authorities of the countries at war were forced to develop methods of protecting crews
against cold water immersion (66). Much research was done on designing protective
equipment and on determining survival time as a function of water temperature.
More recently, the rise in maritime and intercontinental air traffic and the introduction of
helicopter transport to offshore oil platforms have increased the risk of exposure to accidental
immersion in cold water, while a sharper focus on occupational health and safety has brought
this matter to more general attention. Offshore petroleum industry and fish-farm workers,
fishermen and military personnel are all at risk of falling into the water. A wide range of
protective clothing and equipment has therefore been designed to protect workers from the
potential hazards of immersion in cold water. The requirement to wear protective clothing
during work has offered new challenges to the working situation itself. By increasing external
insulation and preventing evaporative heat loss, wearing protective clothing may cause
thermal stress that can impair performance (47, 58).
Although the impact of temperature on performance and survival at sea have each been
comprehensively reviewed, few studies have paid attention to the difficulty of reconciling
thermal protection in water with requirements for thermal comfort at work. In order to ensure
the best possible performance and protection of workers who are required to wear protective
clothing, we need a better understanding of 1) the impact of thermal stress in a working
situation and 2) the physiological responses to cold water in the event of immersion. To study
this, this thesis focuses on Norwegian Sea King helicopter pilots who wear protective
clothing, in order to determine the impact of temperature exposures and protective clothing on
physiological and cognitive responses at work (in the air) and when exposed to cold water.
Aim of thesis
Norwegian Sea King helicopter pilots are required to wear immersion suits all year around,
since they mostly operate over cold ocean regions. This influences the mechanisms of heat
exchange between the body and the environment (47, 58). An immersion suit needs to protect
the wearer during all phases of an emergency situation; escaping from a ditched helicopter,
protecting against the initial cold-shock response and extending survival time in the event of
cold-water immersion under severe environmental conditions (33). The protective clothing
necessarily has a high insulation and evaporative resistance that reduces heat dissipation and
may therefore cause heat stress (58). There is growing concern that the combination of high
ambient temperature, solar radiation in the cockpit and protective clothing produce an
unacceptable level of thermal strain, a lower level of comfort and deterioration of
performance (29, 71, 90, 132). Aviator’s protective clothing concepts therefore need to
accommodate the potential conflict between thermal comfort and protection. Achieving
thermal comfort and reduced heat strain requires an understanding of physiological heat
balance and the heat exchange mechanisms between the person wearing protective clothing
and the environment.
The thesis is divided in two main parts, which focus respectively on the normal working
situation of helicopter pilots and the emergency situation in cold water after ditching. The
overarching aims of this thesis are to improve our understanding of;
- how ambient temperature influence the thermal comfort and performance of pilots
wearing aircrew protective clothing in a normal working situation.
- the impact of cold water immersion, particularly the importance of maintaining heat
balance under severe environmental conditions when wearing aircrew protective clothing.
The study of these problems requires knowledge of 1) thermal responses to air and water 2)
cognitive performance during thermal stress, and 3) the effect of protective clothing on heat-
exchange mechanisms. The following section provides a brief review of the state of the art in
these areas.
Thermal responses to air and water
When human beings are exposed to heat or cold, thermoregulatory responses that protect
them against extreme conditions are activated. The defense mechanisms against heat include
rises in peripheral blood flow and evaporation of sweat, while in the cold, vasoconstriction
and increased heat production by shivering or exercise are coupled in. The body strives to
achieve thermal balance and homeostasis (5). Helicopter pilots face extreme temperature
challenges that existing aircrew protective clothing solutions is unable to solve. In working
conditions the heat gain is larger than heat loss; in an emergency situation in cold water the
opposite situation occurs; heat loss is much larger than heat gain. The emergency situation in
cold water is one of the greatest stressors to which the human body can be exposed and has
been comprehensively studied (for reviews see 18, 25, 33, 67, 126). The many accidental
drowning- and hypothermia-related accidents at sea (1, 109, 118), further emphasize the
importance of understanding the factors that can influence survival in cold water.
To fully appreciate the extreme challenge for helicopter pilots and the influence on
performance, safety and health, it is essential to understand the characteristics of the
temperature-regulation system, the interaction between environmental temperature exposure
and the body, cognitive performance during thermal stress, and clothing physiology.
The most powerful form of human thermoregulation is behavioral; through regulating
clothing, changing posture, moving to a cooler area, seeking shelter, etc. In the case of
helicopter pilots, the working situation and the protective clothing restrict the behavioral
options and thermoregulation therefore depends to a great extent on the capacity of the body’s
own ability to thermoregulate, on cooling aids if available (12, 19, 78, 105) and on cockpit
air-conditioning (10, 19).
One particular feature of the homeothermic human is that our internal body temperature rarely
exceeds a range of ±2 °C, despite exposure to extreme variations in environmental conditions
(65). Greater deviations in deep-body temperature affect cellular structures, enzyme systems
and a wide range of temperature-dependent chemical reactions that occur in the body (4), and
thus affect health, safety and performance. Thus, throughout our lifetime we maintain a large
temperature differential between internal body temperature and the environment. Defending a
deep-body temperature within such a narrow range necessarily requires a complex system of
regulation. The temperature-regulation system consists of four main components; 1)
thermoreceptors, 2) neural pathways mediating afferent information from thermoreceptors
and the central nervous system (CNS), 3) control unit located in the hypothalamus and 4)
effector system. Thermoreceptors are nerve endings located both in the skin surface and in
deeper tissues (CNS, carotid artery, internal organs, skeletal muscles) which fire at different
temperature ranges (55, 57). The morphology of cold thermoreceptors has been described in
detail by Hensel (56). Afferent signals from peripheral and central thermoreceptors are
transmitted by neurons to the preoptic area of the hypothalamus. The anterior hypothalamus
controls heat loss, while the posterior hypothalamus participates in the regulation of
vasoconstriction and shivering (77). Although numerous studies of the neurophysiological
basis of the thermoregulatory system have been performed (6, 57, 77) it is still not fully
understood. The link between sensory input and effector output is complex and how the
signals are processed is still a matter of debate. A widely used model is the one proposed by
Bligh (6), which suggests that homeothermia depends on a system of neuronal connections
between sensors and effectors, modulation of the sensor/effectors relations by excitatory and
inhibitory signals from elsewhere in the CNS, and crossing inhibitory influences between
these pathways (6). Still under debate is the question of exactly which variable is regulated;
change in heat content (136) or change in body temperature (9). Furthermore, it is still not
certain whether temperature is regulated towards a “set point” (8, 36) or to an “interthreshold
zone” (81, 84) between the activation of the appropriate effector mechanisms. Nevertheless,
even small deviations from the preferred temperature range or set point may reduce physical
and mental performance (42, 100). Effector mechanisms such as vasomotor activity,
evaporative heat loss (sweating and respiratory) or shivering are activated to prevent
fluctuations in internal temperature and maintain heat balance. Sweat glands, skin blood
vessels and skeletal muscles serve as effector organs.
For a helicopter pilot, both the working situation in air and the emergency situation in cold
water represent ambient conditions far beyond the range within which humans can regulate.
The physiological responses (effector mechanisms) will only apply for a limited time before
the system is no longer capable of compensating. Both the working conditions and emergency
situation in cold water thus impose major stresses on the body and will affect human
performance and survival.
The thermoneutral zone (TNZ)
The idea of a neutral zone of thermoregulation was proposed as long as 50 years ago in
experimental work based on human calorimetry (45). The thermoneutral zone (TNZ) is
defined as the range of ambient temperatures (Ta) within which the metabolic rate is constant
and minimal, body core temperature (Tre) is held at steady state (81, 108) and thermal comfort
is neutral (112). Under normal circumstances, the TNZ based on ambient temperature will
necessarily be wide, since we regulate clothing and activity level to keep the
microenvironment surrounding the skin within the neutral zone (112). For sitting, resting,
nude subjects exposed to steady exposures to different ambient temperatures, the thermal
comfort and neutral temperature sensations lie within the range of physiological thermal
neutrality (28-30 °C Ta) where little physiological regulatory effort in required (30). With
aircrew protective clothing, the TNZ will obviously be shifted downwards, but this ambient
temperature range has never previously been defined. Within the TNZ, heat loss and heat
production are balanced by regulating vasomotor tone as a response to environmental changes
(112). Outside this range of Ta, the cold and warm thermoreceptors located in the skin will
activate autonomic thermoregulatory responses for heat loss through sweating/respiration, or
heat production through shivering or exercise (57). TNZ can also be affected by non-thermal
factors (81), which may alter both the Tre values at which metabolic responses are activated
and the magnitude of the metabolic response. Examples of non-thermal factors that impinge
on the thermoregulatory system include exercise/post exercise (70), state of hydration (20),
sleep (3), fever (7), motion sickness (85) and inert gas narcosis (83). Hence, non-thermal
factors such as sleep deprivation or dehydration may well shift the thermal comfort zone of
helicopter pilots during flight.
The TNZ has been widely studied and defined in animal studies (15). This is not only of
theoretical interest, but has a practical purpose such as for defining housing conditions in
which the zone of least thermoregulatory effort should be optimized (65). In human beings,
the importance of TNZ has attracted less research attention, and most studies are of theoretical
interest. Although there has been interest in defining the ambient conditions under which
humans are thermally comfortable related to work performance, health and safety, this has not
been directly related to defining the TNZ. In the example of helicopter flying, defining the
ambient TNZ when wearing protective clothing is obviously of interest to define conditions
where pilots experience minimal physiological strain and is of practical interest for the
regulation of the cockpit environment.
Heat balance
To keep the body in heat balance, heat production must be equal to heat loss, according to the
equation M - W- R - C - K - S – E = 0 (where M is metabolic heat production, W is
mechanical work, R is radiation, C is heat loss by convection, K is heat loss by conduction, S
is stored body heat and E is evaporative heat loss). Metabolic heat production is the rate of
transformation of chemical energy into heat. During rest under thermoneutral conditions, this
corresponds to a heat production rate of 80-100 Watt. Physical effort raises the metabolic rate
and, depending on the mechanical efficiency of the exercise performed (between 0-25 %),
most of the energy is converted to heat. During strenuous exercise, heat production may
exceed 1000 Watt (32). Heat production can be increased in two ways; either involuntarily by
shivering, or voluntarily by muscular exercise. Several pathways of heat loss are possible,
although they are somewhat different in air and water. In air, heat is lost by convection as air
blows over the skin, in water by replacement of the boundary layer surrounding the body.
When there is a difference between the temperature of the body and the surroundings, heat is
also lost by radiation in air, while radiation is insignificant in water. Evaporation of sweat is
the main pathway of heat loss in air but insignificant in water. Evaporation of water from the
respiratory tract is important both in water and air. Warming and moisturizing inspired cool
dry air can contribute as much as 10 % of total body heat loss at rest, increasing with exercise
and cooler air. Conduction plays a minor role in air, but becomes more important in water,
which has a thermal conductivity 25 times greater than that of air at the same temperature (18,
88). A large contact surface between water and the body in the supine position further
increases the conductive heat loss. The capacity of the body to retain or lose heat to the
environment in air is dependent on the following external factors (47): temperature (air
temperature, radiant temperature, and surface temperature), air movement and humidity
(moisture concentration, not the relative humidity that might cause dripping sweat).
Combined with metabolic heat production and clothing, these variables form the fundamental
factors that define the human thermal environment (97).
When metabolic heat production (M) increases due to work, sweat evaporation is the main
mechanism involved in maintaining body temperature within a narrow range. The
environment is usually cooler than the skin temperature if no clothing is worn, and sweat will
evaporate even at 100% relative humidity. However, clothing dramatically affects heat
exchange mechanisms and this will be described more in detail later. In working situations at
high work intensity and/or thermal stress due to ambient conditions and/or wearing protective
clothing, the thermoregulatory system of the wearer will be unable to maintain thermal
balance. As a consequence significant heat strain during work is experienced. Many
occupations have work-places close to or over water, and in the event of an accident workers
may fall into the water with a raised body temperature. Only a few studies exist on the effect
of a raised body temperature when exposed to cold water (76, 113, 137), and this has never
been investigated in subjects wearing protective clothing.
Maintenance of heat balance in cold water is much more demanding than in air, since heat
loss from the body will rise dramatically because of the large heat-removing capacity of
water. In the case of Norwegian military aircraft that operate over the North Sea and Barents
Sea, this problem is further exacerbated by the fact that an immersion accident is most likely
to occur in areas where extreme weather conditions are common (low air and water
temperatures, wind and waves). It would therefore be of interest to identify methods for
extending survival time by improving heat balance. Muscular exercise has great potential for
increasing heat production (4, 32), but is generally not recommended, since high muscle
blood perfusion during exercise increases heat loss to the water (95, 133). With adequate
insulation, enough heat is retained to improve heat balance and attenuate core cooling (107),
but this has yet to be demonstrated under the extreme environmental conditions faced by
accidentally immersed aircrew.
Thermal comfort
Thermal comfort is defined as “that state of mind which expresses satisfaction with the
thermal environment” (60, 97). The condition of thermal comfort is therefore sometimes
defined as a state in which there is no driving impulse to correct the environment by
behavioural activity (5). Thermal comfort and its determining factors have been reviewed by
Parsons (97). Thermal comfort is dependent upon both environmental and individual factors
and is influenced by the core and skin temperatures of the body (27, 31). The importance of
thermal comfort (or discomfort) in working situations has been comprehensively investigated
(77), since its relation to human health, performance and productivity is clear. A feeling of
discomfort may lower morale and even lead to a refusal to work (97). Hence, there is an
active interest in research on defining conditions of thermal comfort. Thermal sensation
indicates how a person “feels” or senses the temperature. This sensation follows the
neurological pathways of the cold and warm thermoreceptors described earlier. The actual
sensation is formed in the brain in the somatosensory cortex. A sensation of cold is
determined by skin temperature (Tsk), warmth initially on skin temperature, and then on deep
body temperature (55). However, local cooling of the hands or feet may produce a whole-
body sensation of cold that is not related to the Tsk (141). Furthermore, different skin regions
have different thermosensitivity and different degrees of importance (14, 89). Warmth
discomfort is closely related to skin wettedness (77). Rapid shifts in environmental
temperatures change thermal comfort and sensation before skin and temperature are affected
(31), indicating that thermal sensations are influenced by changes in heat loss from the body.
Under transient conditions, thermal comfort may therefore be predicted more accurately from
ambient conditions than from the skin and core temperature change (31). Fanger (27)
published in 1970 Thermal Comfort, which describes the most widely used methods and
principles for evaluating and analyzing thermal environments with respect to thermal comfort.
Fanger (27) describes four essential conditions for a person to be in thermal comfort; 1) the
body is in heat balance; 2) sweat rate is within comfort limits; 3) mean skin temperature is
within comfort limits; 4) local discomfort is absent. It appears that the preferred ambient
condition for thermal comfort is the same across geographic locations (warm/cold climates),
age and gender (97). Outside this narrow comfort zone, the sensation of cold and warmth is
affected by e.g. age, gender, and body composition and acclimation state.
Thermal discomfort represents a stimulus for behavioural activity (31). Helicopter pilots,
however, cannot change to more comfortable areas or undress to alleviate heat strain.
According to Hancock (44), a decrease in performance is more closely related to thermal
discomfort than to physiological strain. Sea King helicopter pilots experience significant
discomfort during flight (26), but how this affects their cognitive performance on certain
important tasks required for flying has not yet been investigated.
Cognitive performance during thermal stress
Cognitive performance is defined as a set of mental processes such as information processing,
learning, thinking, reasoning and remembering. Cognition thus plays a fundamental role in
orientation, safety and decision-making, especially in avoiding critical situations. A large
number of studies of accident frequency and productivity in the workplace have shown that
human error increases in hot, moderate and cold environments (21, 39, 46, 77, 79, 102). This
is highly relevant to helicopter flying, which represents a high-technology system that
requires efficient and error-free performance.
Although it is well established that hot, moderate or cold environments influence cognitive
performance, the underlying mechanisms involved are not fully understood (39, 99). It has
been difficult to draw general conclusions about the relationship between performance and
heat exposure, due to variations in experimental conditions, type of task, severity of exposure
and duration (38, 39, 40, 99, 101). Some have reported little performance loss, while others
have reported decrements in performance under identical ambient conditions (99).
Although an underlying causative model explaining the interaction between thermal
environment and cognitive performance has not been established, the physiological responses
to hot and cold environments have been well described. Work safety standards that provide
threshold values for performance in hot and cold environments are therefore largely based
upon physiological parameters. The basis of these theories is that the ambient temperature at
which an individual can perform adequately is very close to the threshold temperature at
which the body temperature can compensate physiologically for thermal strain (43). The most
obvious effects of the thermal environment are those of distraction and manual dexterity in
the cold (16, 97). Hancock and Vasmatzidis (37) challenge this basis upon which all
occupational thermal stress exposures are founded. They claim that task performance level is
the most sensitive reflection of human responses to thermal stress. Such responses are
superior indices compared with the more traditional measurement of physiological parameters
(38). More knowledge of interactions between physiological parameters and cognitive
responses is necessary for a better understanding of the critical temperature limits for safe
Pilcher et al (99) and Hancock et al (39) have performed the most comprehensive meta-
analyses to date in order to quantify the effect of thermal stress on human performance.
Pilcher (99) concluded from 22 studies that ambient temperatures above 32 °C and below
10 °C result in the greatest decrement in performance (when subjects are not wearing
protective clothing). Both metaanalyses concluded that the type of task performed (complex
vs simple), duration and intensity of exposure are key variables that influence how thermal
conditions affect performance (39, 99). The metaanalysis of Hancock was consistent with the
distraction theory that suggests that temperature stress forces the individual to allocate
attention resources to appraise and cope with the threat, and reduces his capacity to process
task-relevant information (39, 97). Other theories proposed include arousal theory (101),
which suggests that performance depends on arousal level. A “boring” task such as vigilance
will be de-arousing. A warm environment reduces arousal level and vigilance will be reduced
(97). If the task is more demanding and arousing, a cool environment (0°) may be more
arousing and may improve performance in a boring task (e.g. vigilance). There is a practical
rationale to this; a driver who is tired will tend to fall asleep in a comfortable, excessively -
warm environment (97). Furthermore, thermal sensation and comfort might also cause
dissatisfaction that could affect performance and should be avoided. This is a factor of
importance also for survival in cold water. It has been demonstrated that thermal sensation
correlates with experienced thermal strain, which in turn affects cognitive performance (129).
Both human psychological and organizational factors affect flying performance and this topic
has been comprehensively studied (87). Problems of flying performance due to environmental
stressors have not been paid the same degree of attention (87). Stressors in the physical
environment of pilots include vibration, noise, uncomfortable seats and temperature. Little
research has aimed at determining how flying performance is affected by temperature. This is
surprising since, in comparison with noise and vibration, temperature is much more severe
and exposures to extreme values of both heat and cold exposures can be fatal.
Protective clothing
The most important function of personal protective clothing (PPE) is to protect the human
body against harmful influences from the environment (e.g. physical, chemical, biological and
thermal). For several decades, the development of protective clothing therefore aimed at
improving the barrier effect of the garment which made it impermeable for water vapour
(110). As it came to be realized that the discomfort of wearing PPE lowered the acceptability
of PPE, awareness of improving thermal comfort when wearing PPE increased. The essential
problem is that protective clothing directly affects heat exchange (heat and moisture transfer)
between the skin and the environment (47, 48). This is mainly determined by the thermal
insulation and the evaporative resistance of the clothing (58). Heat transfer through clothing
takes place via dry and evaporative heat exchanges. Impaired heat exchange with the
surroundings causes accumulation of heat and water vapour within the clothing
microenvironment, and over time, skin temperatures and finally the core temperature will
increase (116). Adequate protection is therefore obtained at the expense of disturbances in the
heat balance. Wetting of the clothing by accumulation of sweat also gradually reduces the
insulation effect and causes thermal discomfort (77). Impermeable clothing increases vapour
pressure and condensation under the garment may occur (138,). This has further negative
effects on the heat balance caused by dissipation of heat in the condensation process.
Evaporative resistance is complex, but it can be directly measured on human subjects (69), by
a sweating manikin (80) or by physical skin models (e.g. sweating hotplate) (72). A number
of recent studies have developed more sophisticated methods for understanding heat and
vapour transport in clothing (49, 138).
Clothing protects the body by reducing heat loss in cold environment (air or water), and in
many cases, the insulation is the factor that can most readily be adjusted to reduce thermal
stress . Thermal insulation can be determined by measurements made on a standing thermal
manikin; values are given in Clo (1 Clo = 0.155 m2K/W). A human being at rest feels
comfortable at 21 °C Tawith a clothing insulation of 1 Clo (93, 135). In the case of Sea King
helicopter aircrew, an immersion suit is required to protect a person for a minimum of six
hours against hypothermia at sea temperatures as low as 2 °C (23). To achieve such level of
protection, a minimum of 2.0-2.3 Clo insulation (measured in air) is required (19) at the cost
of thermal comfort during flight (26). Clothing insulation is determined by clothing fit and
ensemble thickness and is influenced by the body movements of the user and by air
movement in the environment (48, 50, 51). Body movement causes a “pumping effect” that
permits air exchange in the microenvironment of the clothing through openings (collars and
cuffs etc.), reducing the thickness of the insulating air layer within the clothing. Clothing
insulation is further affected by compression by wind and water (50, 51).
The impact of protective clothing on physiological responses has been comprehensively
reviewed by several authors (47, 58, 96), all of whom emphasize the impact of clothing on the
heat exchange mechanisms between the body and the environment and how this affects
thermal comfort and heat balance. Much effort has been put into developing standards and
methods for the assessment of human response to thermal environments and defining
requirements for protective clothing (97). These can generally be divided into standards for
moderate (60), hot (61, 62) or cold environments (63), or deal with protective clothing for
immersion in cold water (59). Their principal aim is to provide guidelines for acceptable
exposure to environmental conditions (98). Although standards are useful and offer a major
contribution to describing methods to assess human responses to thermal environment,
assessment of transient thermal environments is still at an early stage and no standard method
yet exists (97). In the case of Sea King helicopter personnel a realistic accident scenario
would involve moving from a warm working cabin environment (up to 40 °C air temperature)
to sudden immersion in cold water (0-2 °C water temperature). Furthermore, the test
conditions for cold water immersion are often not very realistic (59). A general
recommendation is that standards should be used as guidelines for assessment of thermal
strain on the human, and that for each type of work, metabolic rate, environmental conditions
and the thermal properties of clothing must be individually quantified (97).
Several studies have emphasised the significant contribution made by clothing ensembles to
the development of heat stress in pilots under hot ambient conditions (10, 28, 92, 103, 119,
121). Helicopter pilots will have little benefit of ventilation cooling through garment openings
(cuffs and collars) because of their static sitting position in the seat and because openings
around the wrists, ankles and neck are sealed to protect against water ingress in the event of
cold water immersion. Vallerand et al. (132) reported that cockpit temperatures can be very
high, and environmental cooling systems do not always have sufficient capacity to handle the
heat stress associated with solar radiation, high ambient temperature and the reduced heat
dissipation ability of protective clothing ensembles. There is growing concern that the
interaction of heat stress and protective clothing may produce an unacceptable level of
thermal strain and reduced comfort, resulting in deterioration of performance. Little is known
about how ambient temperature affects physiological and psychological responses in
helicopter pilots wearing protective clothing in northern climatic zones.
Problem assignment
Protective clothing for helicopter aircrew must satisfy the end users' requirements for comfort
and mobility in a working situation and at the same time provide the best possible safety in an
emergency situation (25). A questionnaire addressed to 90 Norwegian Sea King helicopter
aircrew members demonstrated that survival in cold water is their highest priority (23). Nearly
all the aircrew (92%) stated that an immersion suit must ensure survival for 12 hours under all
weather conditions (23). In a worst-case scenario, victims may be unable to enter a dinghy,
and an immersion suit must protect from hypothermia under severe conditions, including low
ambient and water temperature and waves that continuously flush over them. Hence, visibility
(85%), prevention of water ingress (78%) and mobility in an emergency situation (68%) also
take high priority among the users. At the same time, they emphasize thermal comfort at high
and low cockpit temperatures (49%), moisture transport outwards (39%) and mobility in a
working situation (68%) as important requirements. This questionnaire makes it clear that
making protective clothing for over-water flights must involve a series of compromises
among conflicting requirements. The most difficult part is to reconcile requirements for
thermal comfort during flight and thermal protection in water.
When the end-user requirements had been identified and their importance prioritized, the
basis for further investigation of some of the thermal problems experienced by helicopter
pilots was given. The literature review further revealed some gaps in our knowledge that
remained to be filled;
First, two problem areas for the working situation for helicopter pilots were addressed.
Although heat stress during flight is a known problem, little is known about the ambient
conditions under which aircrew wearing immersion suits start to experience thermal
discomfort and heat stress (Paper I). Then, what is the impact of different ambient
temperatures on flight performance when protective clothing is being worn (Paper II)? Flying
is a task that requires sustained concentration and attention, and performance errors may well
have fatal consequences (71). Although an association between heat stress and pilot error has
been demonstrated from studies in hot climates (29), little is known about the situation for
helicopter pilots in cooler northern climate zones where pilots are required to wear well-
insulated immersion suits. Furthermore, while the physiological effects of heat stress are well
known, the mechanism underlying the relationship between physiological heat stress and
impaired performance in raised ambient temperatures is still not fully understood.
Thereafter, the emergency situation was analyzed and two problems addressed: Although
much research has been done on the effects of cold water immersion and survival at sea (25,
66, 122, 126, 128), little attention has been paid to the fact that in many emergency situations
a person may face a heat-stress problem before being exposed to cold water. Passive or active
pre-immersion warming has been shown to accelerate the onset of hypothermia in naked
subjects immersed in cold water (76, 113), but this has never been studied in subjects wearing
protective clothing (Paper III). Improving heat balance might extend survival time in cold
water. Hypothermia is the greatest long-term threat to immersed victims, and environmental
factors are significant in determining heat loss from the body (17). Voluntary leg exercise has
great potential for improving heat balance when wearing a well-insulated immersion suit
(107), but this has never been investigated under realistically severe environmental conditions
(Paper IV).
This thesis is divided in two main parts: the first part considers the impact of the thermal
environment and protective clothing on factors such as comfort, physiology and cognitive
performance. The second focuses on immersion in cold water, and in particular on the
importance of maintaining heat balance during exposure to cold water.
The principal hypothesis of this thesis is that “reconciling requirements for thermal protection
in water with requirements for thermal comfort and cognitive performance during helicopter
flights is impossible with existing aircrew protective clothing”. I further hypothesise that
wearing protective clothing produces a downward shift in the ambient temperature required
for thermoneutrality that is far beyond typical cockpit temperatures. The subsequent heat
stress will have detrimental effects on the cognitive performance of pilots. I hypothesise that
in an emergency situation, heat stress experienced during flight will affect subsequent
responses to cold-water immersion. Furthermore, under severe environmental conditions,
additional heat production is required to improve the maintenance of heat balance and
attenuate core cooling when wearing a well-insulated immersion suit.
The hypotheses were tested by pursuing the following questions:
Paper I: What is the thermoneutral zone when aircrew protective clothing is being worn?
Paper II : What impact do different ambient temperatures have on cognitive performance
when protective clothing is worn?
Paper III : How does heat stress due to wearing protective clothing during work affect
responses to subsequent immersion in cold water?
Paper IV: Can intermittent periods of leg exercise improve heat balance and attenuate core
cooling under severe environmental conditions when wearing a well-insulated immersion
Summary of individual papers
Paper I
Norwegian helicopter pilots are required to wear a dry immersion suit all year round in order
to protect themselves in the event of accidental immersion in cold water. Heat stress and
thermal discomfort can be significant problems for pilots wearing protective clothing, due to
its insulation properties and prevention of evaporative heat loss (26). Both the heat load in the
cockpit and wearing an immersion suit influence the thermal stress experienced by the user.
Regulation of ambient cockpit temperature downwards might alleviate the thermal stress
The aim of this study was to define the ambient temperature range of thermoneutrality (TNZ)
where no discomfort or heat stress is experienced when wearing protective clothing, as this
has not earlier been defined. In naked resting subjects TNZ has been determined to lie
between 28-30 °C (30). We hypothesised that wearing protective clothing will displace the
TNZ to a lower range of ambient temperatures which is far beyond typical cockpit
temperatures. Eight male volunteers participated in 12 experiments on separate days. In series
A (control), subjects wore only shorts and sat quietly on a chair in a climatic chamber for one
hour during exposure to seven different environmental temperatures (15, 20, 25, 28, 31, 35
and 40 °C). In series B, the subjects wore typical protective clothing as used by helicopter
pilots in the Royal Norwegian Airforce (2.2 Clo for the whole clothing concept) including
helmet, and were exposed to five different ambient temperatures (0, 10, 14, 18 and 25 °C).
Measures included skin (Tsk) and rectal temperature (Tre), heart rate (fc) oxygen consumption
(VO2), sweating and assessment of thermal sensation and comfort ratings. The criteria for
thermoneutrality were defined as Tsk between 33-35 °C, no change in Tre, the lowest stable
metabolic rate and the subjective sensation of temperature and thermal comfort is neutral.
This study demonstrated that the criteria for thermoneutrality were met at 28-31 °C Tain
subjects wearing shorts, and wearing a dry immersion suit caused a downward shift of the
TNZ to 10-14 °C Ta. This temperature range is far below typical cockpit temperatures in the
Sea-King helicopter (26). Subjects wearing protective clothing started to sweat and
experienced thermal discomfort even at an ambient temperature of 18 °C. The practical
implication of these findings is that efforts should be made to reduce thermal stress by
regulating cockpit temperature downwards if possible, or to consider use of personal cooling
aids at cockpit temperatures of 18 °C when aircrew are wearing protective clothing (2.2 Clo).
Paper II
Flying requires pilots to be concentrated and alert all the time, and it has been shown that
thermal stress may impair pilot performance (90, 92), which may contribute to a higher risk of
pilot error and reduced flight safety. Wearing protective clothing increases the thermal load
by increasing insulation and preventing evaporative cooling (47). Although physiological heat
stress and discomfort are experienced at typical ambient cockpit temperatures (26), little is
known of how ambient thermal conditions affect the cognitive performance of the pilots
wearing aircrew protective clothing.
The aim of this study was to investigate the effect of wearing aircrew protective clothing on
physiological and cognitive responses under low, moderate and high ambient temperatures. A
further aim was to correlate any observed performance changes with physiological
parameters. We hypothesized that typical cockpit temperatures cause heat stress, which will
have detrimental effects on the cognitive performance of aircrew wearing protective clothing.
Low (0 °C), moderate (23 °C) and high (40 °C) ambient temperatures were investigated.
Exposure to low ambient temperature (0 °C) was the control condition, and was not expected
to induce any thermoregulatory or cognitive performance changes. Eight male volunteers (six
medical students and three pilots in the Royal Norwegian Airforce) were exposed for three
hours to the three different ambient conditions on separate days. They wore typical aircrew
protective clothing (2.2 Clo for the whole clothing concept) and helmet. Physiological
variables (Tre,Tsk, heart rate, oxygen consumption, sweating), microclimate in the clothing,
subjective evaluations of thermal sensation and comfort and cognitive performance (vigilance
and multiple choice reactions) were measured during the test. Performance was measured as
correct, incorrect, missed reactions and reaction time.
The study demonstrated that there was significantly higher heat stress in the 40 °C series than
at 23 °C or 0 °C, as shown by a rise in Tre,Tsk, heart rate, increased body water loss and
subjective discomfort. Multiple choice reactions were unaffected by ambient temperature, but
a significant deterioration in vigilance performance was observed under 40 °C ambient
conditions compared to 0 and 23 °C. This performance deterioration correlated with an
increase in Tre of 1.2 °C. Although subjects started to sweat and experienced thermal
discomfort at 23 °C Ta, no negative effect on cognitive performance was observed. We
concluded from this study that cognitive performance is virtually unaffected unless the
ambient temperature is high enough to produce an increase in body core temperature. The
practical implication of this study is that moderate cockpit ambient temperature (23 °C) is
tolerable with respect to certain cognitive performance tasks required for helicopter flying,
while high cockpit temperatures (40 °C) may over time lead to deterioration of flight
performance. However, the results of laboratory studies of cognitive tasks must not be
transferred to “real life” situations without careful consideration.
Paper III
In cold-water emergency situations, helicopter aircrew will probably enter the water with a
raised body temperature due to the requirement to wear protective clothing during operations.
This study explored the potential effects of an initial raised body temperature on survival in
cold water. In the long term, maintaining body temperature during cold-water immersion is of
critical importance for avoiding the lethal effects of hypothermia. Warming by pre-immersion
exercise or passive pre-warming has been demonstrated to accelerate core cooling during
subsequent cold water immersion (CWI) (76, 113). However, wearing protective clothing
significantly alters the thermoregulatory responses to CWI by offering protection against the
effects of cold water.
The aim of this study was therefore to investigate the effect of prior warming by exercise on
the subsequent physiological response to CWI when wearing an immersion suit. We
hypothesized that wearing a dry immersion suit would eliminate long-term differences in core
cooling during CWI between normothermic and pre-warmed subjects. Two different groups
of physically similar male subjects (age; 24.7±4.2 years, ht; 183.1±6.5 cm, wt; 86.7±15.0 kg,
body fat; 16.8±3.3 %) were used to gather data under two conditions, baseline (Base-CWI)
and pre-warming by exercise (Warm-CWI) when wearing a dry immersion suit (2.97 Clo). In
Warm-CWI seven subjects rested (20 min), and then cycled on an ergometer cycle (20 min)
before immersed in water at 5° C (Tw) (140 min). In Base-CWI, six subjects were directly
immersed in 5° C Twafter resting. Physiological variables measured during the test; Tre ,Tsk,
heart rate, oxygen consumption, ventilation and respiratory frequency. Tre and Tsk was
significantly higher after Warm-CWI start of CWI, resulting in faster core cooling rate, drop
in Tre and Tsk during the first 10 min. No differences in cardiovascular or respiratory responses
were observed in the same initial period, indicating that the immersion suit protected well
against the cold-shock response in both series. In the long term (0-140 min), the overall core
cooling rate did not differ between Warm-CWI (0.34±0.11 °C • h-1) and Base-CWI
(0.31±0.05 °C • h-1). Heat production was similar between conditions.
In conclusion, when entering cold water with a raised Tre and Tsk , different thermal responses
during the first 10 min is observed, but the protection in the immersion suit eliminates long-
term differences in core cooling rate between between normothermic and pre-warmed
Paper IV
This study explored the worst-case emergency scenario in cold water, where aircrew are
exposed to severe conditions, including low ambient and water temperature, wind and waves.
Under such conditions the immersion suit must ensure survival and protect from hypothermia
for up to 12 hours (23). Additional heat production through leg exercise has great potential to
offset at least some of the heat loss to the cold water when a well-insulated immersion suit is
worn. Although exercise has been shown to increase heat loss due to increased blood
perfusion in the exercising limbs in subjects wearing swimsuits (95, 133), intermittent periods
of leg exercise reduced core cooling in subjects wearing a well-insulated immersion suit under
calm conditions (107). This has not earlier been studied under severe conditions.
The aim of this study was to evaluate the effect of intermittent periods of leg exercise on heat
balance and core cooling under severe environmental conditions when wearing a well-
insulated immersion suit. We hypothesized that compared to passive conditions, intermittent
periods of leg exercise (15 minutes per hour) will result in; 1) a greater rate of heat production
that will offset the elevated rate of heat loss and hence 2) decrease the subsequent core
cooling rate and 3) improve thermal sensation and comfort. On two separate days, seven male
subjects were immersed in 2 °C water with an air temperature of -2 C, wind 5 m · sec-1 and
waves of 30-40 cm high. Subjects wore woollen underwear, a flight suit and a 3 mm neoprene
immersion suit (2.97 Clo for the whole clothing concept). The subjects were immersed for
180 minutes while either passive (NonEx) or performing moderate leg exercise for the final
five minutes of each 20-minute period (LegEx). Heart rate, metabolism, Tre,Tsk , and skin heat
flux were measured. A subjective evaluation of thermal sensation and comfort was obtained
every 20 minutes (NonEx) or immediately before and after each exercise period (LegEx).
As predicted, intermittent periods of leg exercise resulted in a greater rate of heat production
that offset the elevated rate of heat loss resulting in a net positive heat gain (10%) compared
to lying still in the water. As a result a decreased core cooling rate and better thermal
sensation and comfort was observed in the LegEx conditions compared to NonEx. The results
suggest that when an insulated immersion suit is worn in cold water under extreme
environmental conditions, five minutes of leg exercise every twenty minute might potentially
provide a survival advantage at sea.
The following discussion of the papers is divided into two main parts; thermal comfort and
performance in air when wearing aircrew protective clothing (papers I and II) and the
emergency situation in cold water (papers III and IV).
Thermal comfort and performance in air (papers I and II)
Ambient conditions required for thermoneutrality when wearing aircrew protective clothing
Paper I demonstrated that the existing solution for aircrew protective clothing did not fulfil
requirements for thermal comfort at typical cockpit temperatures in Sea King helicopters
operating in cool northern climatic zones. 18 °C Tarepresented the threshold for when pilots
start to experience sweating and discomfort when wearing protective clothing (2.2 Clo). The
study confirmed that for sitting resting nude subjects the criteria for thermoneutrality (for
definition see paper I) lie in the range of 28-30 °C Ta(30), and that wearing aircrew protective
clothing affected heat exchange between the body and the surroundings and produced a
downward displacement of the TNZ to 10-14 °C. These findings correlate well with data from
our field study of Sea King rescue helicopter pilots (26), in whom increased skin temperatures
and sweating caused significant discomfort during flights when wearing protective clothing
(2.2 Clo) at an ambient cockpit temperature of 18.6±1.3 °C. A more recent study of 26 flights
of Canadian Search and Rescue helicopters over a period of eight months (winter, spring and
summer season), confirmed that an cockpit temperature of 18 °C represents the cockpit
temperature at which aircrew wearing immersion suits (2.2 Clo) start to experience thermal
discomfort, and 25 °C Tarepresented a condition of thermal discomfort and perceived heat
stress (19).
Paper I further demonstrated that the criteria for thermoneutrality (as defined in Paper I) are
not met at one single ambient temperature, but rather at a range of temperatures. This is
supported by several studies suggesting that there is a range of ambient temperatures within
which heat loss and heat production are regulated by adjusting blood flow (81, 112). Small
oscillations in finger and foot temperature were observed at Taof 10-14 °C when protective
clothing was worn, indicating regulation of heat balance through vasomotor control to keep
the body within the thermal neutral zone (112, paper I). Metabolic and thermal responses
stabilised within this range, but outside these Tametabolic and thermal responses changed.
Once the capacity of the vasomotor control response is exceeded, the autonomic responses of
sweating (above the upper critical limit of the TNZ) or shivering (below the lower critical
limit of the TNZ) are coupled in. Paper I further demonstrated that the insulation in the
clothing was not sufficient to prevent shivering at a Taof 0 °C (Tlc).
Thermal comfort and work rate
Thermal comfort is important for the acceptance of wearing PPE by the users, and this is
closely related to achieving body heat balance (27). However, the body may well be in heat
balance but still uncomfortable because of sweating at high temperatures or vasoconstriction
at low Ta(97). The main factors contributing to disturbances in heat balance during flights are
metabolic heat production due to work, ambient temperature in the cockpit and protective
clothing. For comfort, both sweat rate and mean skin temperature should be within a certain
range determined by the work rate; outside this range complaints about thermal discomfort
will be made (96). Metabolic rate during flying varies, depending on different phases of flight
and type of aircraft (120). Oxygen consumption within the TNZ (10-14 °C) when wearing
aircrew protective clothing was 0.3 l · min-1 (Paper I) as compared to 0.5 l · min-1 during
helicopter flights (26) . This corresponds to a metabolic heat production of 88 W· m-2 during
flight, and for Sea-King helicopter pilots, the metabolic rate is relatively stable over time. A
higher work rate during flight will significantly affect the sweat rate and shift the comfort
zone to a lower ambient temperature (96). Ducharme (19) simulated aircrew backender
activities that involve a workrate above resting level. Interestingly, the level of physiological
strain (increased Tsk,Tre and dehydration) and saturation of the microenvironment in the
clothing after 60 minutes of work in 25 °C (19), was equivalent to ours when subjects were
exposed to an environmental temperature of 40 °C at rest wearing the same clothing (Paper
II). This emphasises that the combined effect of wearing protective clothing at sufficiently
high Tahas a significant impact on the level of heat stress even at a low metabolic rate (Paper
II). A relatively low metabolic rate, a stable heart rate and no increase in Tre during flight,
demonstrates that Sea King Helicopter pilots are in heat balance at ambient cockpit
temperature of at 18 °C, but are still uncomfortable due to raised skin temperature and sweat
rate (26, Paper I).
Thermal comfort and sweating
Warmth discomfort is highly related to skin wettedness due to sweating (77). In papers I and
II, the onset of sweating was determined by the increase in humidity in the microclimate of
the clothing and the subjective sensation of sweating. Impaired heat exchange with the
surroundings due to wearing protective clothing (47) caused a downward shift of the threshold
for sweating compared to nude subjects (paper I). Both papers I and II demonstrated
accumulation of water vapour due to sweating inside the immersion suit at Taabove 18 °C. At
an ambient temperature 40 °C, relative humidity inside the clothing continuously rose until
100% saturation was reached (paper II). Ducharme’s more recent study of Canadian
helicopter search and rescue aircrew demonstrated similar findings (19). At 40 °C Ta, the
accumulation of heat and water vapour within the clothing microenvironment over time
caused cardiac output, skin temperatures and finally Tre to increase, demonstrating a non-
compensable physiological strain (paper II). This thesis supports the conclusions of previous
studies, that wearing immersion suits prevents evaporative cooling, resulting in increased heat
storage in the body (116). With the restrictions on the body heat exchange, physical work in
personal protective clothing becomes even more stressful (58).
Insulation and evaporative resistance of protective clothing
The evaporative resistance of protective clothing is influenced by the fabric construction and
design of the immersion suit (110). The suit worn in papers I and II was the British Mark 10
survival suit (2.2 Clo for whole clothing concept), consisting of a double layer of cotton
ventyle that permits transmission of water vapour in air, while in water the fibres expand, so
that the interfibre spaces no longer transfuse liquid. The same immersion suit was used in a
study by Sullivan and Mekjavic (116), who investigated the effect of the clothing
microenvironment of four different types of protective clothing worn by helicopter personnel
operating in Canadian coastal waters (Gore-tex, cotton ventyle, Nomex insulate and Nomex
neoprene). This study demonstrated that for all concepts the increase in environmental heat
load (when Tawas gradually increasing from 20-40 °C), was accompanied by increases in
temperature in the clothing microenvironment. The vapour pressure within the clothing
microenvironment increased in spite of little increase in ambient vapour pressure, and was
dependent on differences in the evaporative resistance in the fabric of the suit. The dry suits
with a water-permeable fabric (made of cotton ventyle and Goretex) resulted in less of an
increase in Tre (0.2-0.3 °C) than the neoprene suit (1.2 °C). In comparison, our study (Paper II)
demonstrated an increase in Tre of 1.2 °C in the cotton ventyle suit when exposed to a Taof 40
°C. This difference can be explained by differences between the test protocols (test duration
and gradually increasing Ta) and higher insulation values in the clothing concept in our study.
We used two layers of woollen underwear (as normally used during Sea King helicopter
flights in Norwegian coastal waters), while Sullivan and Mekjavic only used one layer of
cotton underwear. The higher total insulation value due to more layers of underwear in the
clothing concept has been shown to eliminate the benefit of higher evaporative efficiency in
the fabric of the outer garment, resulting in similar thermal strain during helicopter flights
(24). This underlines the importance of taking the whole clothing concept and the heat
exchange mechanisms through the clothing system into account in the design of protective
clothing. The choice of fabrics in the outer layer must allow for evaporative cooling during
flights, while careful selection of the type, thickness and number of layers of underwear is
necessary to alleviate thermal stress during flights. Recent studies in our laboratory (105)
have demonstrated that phase-change materials (PCM) integrated in clothing can be used to
reduce thermal stress and improve thermal comfort at low work rates when protective clothing
is worn. However, this is only possible if such adaptive materials are carefully positioned and
evaluated as a part of the total heat exchange mechanism through the clothing system,
together with the capacity of the body to maintain thermal neutrality and comfort (105). The
challenge is to achieve thermal comfort during flights without decrements in the protection in
the event of immersion in cold water. One way to achieve this is by improving insulation in
those areas of the body that are particularly exposed to heat loss in cold water (106), while
allowing for evaporative cooling in zones of the body with a high potential for heat exchange
through sweating (13, 117).
Aircrew protective clothing and performance in warm climatic zones
Paper II hypothesized that typical cockpit temperatures cause heat stress, which will have
detrimental effects on cognitive performance when wearing protective clothing. The finding
in Paper II that a 1.2 °C increase in Tre in 40 °C causes decrements in performance is most
relevant to flights in warmer climatic zones. Although ambient cockpit temperatures in the
Sea King helicopter occasionally rise to 40 °C in flight, cockpit temperatures seldom remain
so high for very long. In warm climatic zones, heat stress is a concern for aviators regardless
of whether or not they are wearing protective clothing (29, 90). Protective clothing further
increases the heat stress and has been shown to cause severe decrements in operational
tolerance limits and performance in pilots (10, 92, 104, 121). Ambient temperatures up to
40 °C are commonly reported when flying in hot, humid conditions (10), and the situation
inside the cabin is even worse due to solar radiation. As a result, the temperature inside the
cockpit is often reported to be 2-4 °C higher than the exterior ambient temperature (119).
Such ambient conditions are far above the TNZ defined in Paper I, and as reported in Paper II,
aTaof 40 °C causes Tre to increase to 38.4 °C, which resulted in impaired performance. In
agreement with the findings in Paper II, Caldwell (10) reported that US army helicopter pilots
could not fly safely when wearing chemical biological protective clothing (CPC) in 40 °C
without some kind of cockpit cooling equipment (10). Reardon et al (104) reported increases
in heart rate, body core temperature, dehydration, poorer performance and other symptoms
such as nausea, dizziness, headache and thirst, during simulated 2*2 hours helicopter flights at
38 °C when protective clothing was being worn. This is particularly noteworthy considering
that many US military conflicts take place in part of the world where high ambient
temperatures are common and the threat of chemical warfare is high (10). Norwegian military
pilots too are currently engaged in military operations in geographical areas where ambient
heat stress is a risk. Personal air or liquid water cooling systems has been shown to alleviate
heat stress during flights under hot conditions (12, 86). However, not all helicopters have
access to personal cooling systems or are able to regulate cockpit thermal conditions. Liquid
and other cooling garments have also been tried out by Norwegian Sea King helicopter pilots;
however, these were not widely accepted by the users.
Aircrew protective clothing and performance in cooler climatic zones
The results of Paper I suggest that heat stress caused by wearing protective clothing might be
a problem even under winter conditions in northern countries, by shifting the TNZ to a much
lower range of ambient temperatures. In northerly climatic areas, 23 °C represents a more
realistic cockpit temperature (19). Although wearing protective clothing during flights causes
thermal discomfort and increased skin temperatures at an ambient temperature of 18 °C (19,
Paper I), Paper II found no decrements in performance in 23 °C and explained this by the lack
of an increase in deep-body temperature. As far as cognitive performance is concerned,
therefore, 23 °C is a tolerable ambient temperature in spite of the fact that cockpit temperature
lies outside the thermoneutral zone when PPE is worn. Grether (35) and Hancock (42)
similarly concluded that higher ambient temperatures result in more severe decrements in
performance than moderate temperatures.
Possible mechanisms explaining degradations in performance under thermal stress
Uncompensable physiological strain
The mechanisms explaining the results are thoroughly discussed in Paper II, but some further
discussion is provided in the following paragraphs. Paper II correlated the physiological
findings with performance parameters, and the results support the theory that decrements in
performance are related to the actual physiological thermal state of the body. We did not
observe any decrements in performance before Tre passively rose to above 38 °C. This finding
supports earlier studies that demonstrated that changes in performance are linked to dynamic
changes in deep body temperature (2, 21, 38, 40, 44). When the total thermal load causes the
deep body temperature to increase out of the comfort level, heat storage in the body will
accumulate over time and performance breakdown will soon be observed (40). Hancock and
Warm (43) suggest that performance is relatively stable over a wide range of ambient
temperatures until a specific threshold limit is reached, at which point compensatory
mechanisms begin to fail. When an individual can use physiological mechanisms (such as
evaporation of sweat) to partially neutralize the impact of the increased ambient thermal load,
this does not represent an uncompensable change (2). In our case, the combination of the
evaporative resistance in the immersion suit combined with sufficiently high ambient
temperature caused a situation in which compensatory effector mechanisms were not
sufficient to keep the body in heat balance, with the result that Tre increased and performance
was affected (Paper II).
Thermal sensation and comfort
The results of Paper II further demonstrated that performance decrements are not so closely
linked to thermal sensation and comfort. If the latter correlation was found, we would expect a
deterioration of performance at an ambient temperature of 23 °C Ta. This is in contrast to the
theory of Hancock and Vastmatzidis (37), which claims that work performance begins to fail
before current physiological heat stress limits are reached. The threshold at which comfort
fails is much lower in terms of stress level than physiological threshold values (43). The
results of Paper II suggest that subjects compensate for the thermal discomfort, possibly
through increased arousal or motivation. However, we should not overlook the importance of
the subjective reports of the pilots during flight (26), and further investigations are needed to
determine whether experienced thermal discomfort in itself might result in performance
Acclimatization and duration of exposure
Other factors influencing performance include level of acclimatization, personal arousal and
level of training (39). Exposure to heat has been demonstrated to be made somewhat more
tolerable by increased duration, possibly due to heat acclimatization (39). Pilcher (99)
suggested that experimental sessions of less than two hours in hot conditions had a stronger
negative impact on performance than longer durations. This suggests that working in
environments with high ambient temperatures would be expected to produce an initial
deterioration in performance, which could be explained by the notion that people do adapt to
some extent to extreme ambient conditions. The results of Paper II do not support this theory;
on the contrary, better performance was found at the beginning of the test in some of the more
complex tasks, which might be explained by a higher level of arousal. This is in accordance
with Hancock and Vastmatzidis (37) who explain the arousal theory as follows: “when
environmental temperature (or body core temperature) rises, the arousal level of the
performer increases, which in turn causes performance to improve. At some critical point of
ambient (or core) temperature, no further improvement is possible and performance
decreases with increasing heat (and arousal)”.
Nature of the tasks
Although it has been demonstrated that heat stress can eventually lead to impaired pilot
performance and operational endurance (10, 90, 91, 119), this is still a controversial issue in
practice, since the nature of the task and skill in performing it can be important variables in
performance degradation during thermal stress (44). To define threshold limits for
performance, therefore, the nature of the activity must be described (38, 39, 97). Attention,
vigilance and fast decision-making are important cognitive tasks for a pilot, and paper II
simulated these tasks under different thermal ambient conditions in a controlled laboratory
setting. Like earlier studies, Paper II demonstrated that Vigilance was particularly vulnerable
to heat stress (Paper II, 39, 99).
Furthermore, temperature stress is not always a bad thing, e.g. cognitive responses have been
shown to actually benefit from mild cold exposure but are negatively affected by heat (39).
Thermal ambient conditions seem to have various influences on different performance tasks.
For example, cold exposure has a negative effect on performance tasks such as reasoning,
learning and memory (below 18 °C WBGT or below), while hot exposure (26 °C WBGT or
above) results in small improvement in these tasks (39). In contrast, attention and perceptual
tasks are more negatively affected by hot exposure than by cold (99). Paper II thus
demonstrated that attention tasks (vigilance) were more affected by heat stress than more
complex cognitive tasks. This thesis supports current theories that heat stress affects cognitive
performance differentially, depending on the type of cognitive task, and that it appears that a
relationship can be established between the effects of heat stress and deep body temperature
The magnitude of the stress level experienced by a pilot will necessarily involve the
interaction of several stressors, and in that context, occupational stress exposure limits should
be based on evaluation of degradation of the task itself under realistic exposures (39).
Although it is difficult to distinguish between different stressors (e.g., noise, vibration,
temperature, etc.), our results emphasize the necessity of always including the thermal
environment in occupational stress analysis when considering performance degradation.
In summary, having analyzed the thermal working environment for helicopter pilots wearing
immersion suits, this thesis concludes that pilots experience thermal discomfort at ambient
cockpit temperatures above 18 °C (Paper I). However, cognitive performance is virtually
unaffected unless the combination of wearing an immersion suit and sufficiently high ambient
temperature results in an uncompensable physiological strain that produces a dynamic
increase in Tre (Paper II).
Immersion in cold water (papers III and IV)
While the above discussions concentrated on the work situation and the potential hazards of
wearing protective clothing in flight, the next two papers focused on the emergency situation
in cold water (Papers III and IV).
Short-term response to cold water
Exposure to cold water causes a rapid fall in Tsk that induces a cold-shock response that
include an inspiratory gasp reflex, hyperventilation, reduction in breath hold time and
cardiovascular responses (52, 82, 127). This response is extremely critical if the immersion
occurs in choppy water or includes submersion from a ditched helicopter, and increases the
chances of aspirating water and drowning. As discussed in paper III, the immersion suit
attenuated the cold shock response by reducing the area of skin directly exposed to cold
water, in both prewarmed and normothermic subjects. This is in agreement with previous
studies, which have demonstrated that wearing protective clothing significantly alters the
thermoregulatory response to cold water immersion by offering protection against the
immediate cold shock response (53, 68, 73, 124, 125). Although convective heat loss results
in different thermal responses during the first ten minutes after entering cold water with a
raised body temperature, the insulation and air layer within the immersion suit allows for a
higher Tsk, reducing the temperature gradient from the skin to the water (Paper III). As a
result, Tsk were kept above the maximal firing rate of cold receptors (17-20 °C; 5, 123). This
was also demonstrated in paper IV. This is very important, because it reduces the powerful
drive to increase respiration and hence the risk of aspirating water and drowning (127). A
much larger heat loss and rapid fall in Tsk has been demonstrated in studies where subjects
were only wearing swimsuits (76, 113, 137) than in the studies in this thesis (Papers III and
IV). This due to a much steeper body to water temperature gradient and a much larger surface
area of skin exposed to cold water. In the studies of subjects wearing swimsuits, they were
exposed in a sitting position with water up to their neck exposing a large area of the body to
cold water (76, 113, 137). In Papers III and IV, subjects were kept in a supine floating
position in the immersion suit, which meant that the upper parts of the body were exposed to
air, which has much lower heat conductivity than water. The skin surface area exposed to cold
water is therefore much less when an immersion suit is worn.
Long-term responses to cold water
In cold water, the innate protection mechanism against heat loss is shivering, but this is not
sufficient to balance the heat loss in the long term, and deep body temperature will eventually
drop. An unprotected individual exposed to cold water loses heat rapidly, and shortly after
exposure to cold water will reach a critical core temperature below 35 °C, which is defined as
hypothermia. The time taken to reach critical lower body temperatures depends on the rates of
heat loss and heat production. This will depend on a range of individual factors; insulation in
body fat and muscles, clothing, fitness level, nutritional status, gender, age and health status
(11, 18, 74, 75). Jacob et al (64) suggest that aerobic fitness level can significantly influence
heat balance and the core cooling rate during water immersion. Furthermore, insulation and
the surface area-mass ratio is of particular importance in water (25, 54, 66). Hence, children
are more vulnerable to heat loss than adults (114). Active or passive warming before
immersion in cold water has been shown to increase heat loss from the body and accelerate
the onset of hypothermia in subjects wearing swimsuits (76, 113,137). Paper III demonstrated
that the immersion suit eliminates these long-term differences in core cooling between pre
exercised and resting individuals. Paper IV further showed that increased heat production
through leg exercise during CWI reduces core cooling, improves thermal comfort and reduces
the sensation of cold.
Toner et al (129) showed that a more intense shivering response is associated with perceived
thermal stress. Paper IV demonstrated that subjects felt uncomfortable due to increased
shivering response when not performing leg-exercise. Five minutes of intermittent leg
exercise every 20 minute was sufficient to reduce the discomfort associated with the more
vigorous shivering (Paper IV). Thus, activity improves both physiological and psychological
factors that favor survival in cold water.
Exercise in cold water has great potential for increasing heat production and thus improving
heat balance. Physical activity increase metabolic heat production tenfold or more, while
shivering has less capacity (4, 22, 32, 33, Paper IV). However, the benefit of exercising in
water is controversial, since several studies have demonstrated increased convective heat loss
due to high muscle perfusion (52, 68, 95, 133). Exercise produces a tenfold increase in muscle
blood flow which has significant consequences for total body insulation (95, 133). The
amount of heat delivered to the exercising limbs increases and overall tissue conductance
increase largely through removal of the 70-90% of total body insulation which has been
thought to be provided by poorly perfused muscle in resting individuals (133). As a result, a
higher rate of heat loss from the skin is observed in subjects wearing only swimsuits (76, 113,
137). In contrast, Papers III and IV demonstrated that with adequate insulation provided by an
immersion suit, enough of the body heat content from exercise before or during immersion is
retained, in spite of the increased muscle blood flow. Paper IV further demonstrated that
wearing an insulated dry immersion suit attenuated the convective heat loss observed when
physically active. Although heat loss increased due to exercise (especially in the legs), there
was a net heat gain due to the much greater heat production from exercise compared to
shivering. Furthermore, leg movements alone disturb both the water and the boundary layer
on the outside of the body and the potential air layer that is inside the suit (115). This
contributes to a change in the thermal resistance of the suit system and explains the increase
in convective heat loss during periods of exercise periods. During exercise, both increased
vasodilatation and decreased fixed insulation in the suit due to water splashing over the front
of the suit (which was normally exposed to air), may have contributed to the increased heat
loss during the periods of exercise (Paper IV).
Severe environmental conditions affect heat balance during CWI
Paper IV is the first study to demonstrate reduced core cooling when wearing a well-insulated
immersion suit in severe environmental conditions. The impacts of environmental factors
(wind, waves, low sea and air temperatures), are significant in determining the rate of heat
loss from the body during cold water immersion and are associated with significantly shorter
survival times (17, 115). According to Steinman’s field observations, increased activity levels
is required to maintain stable body posture and airway freeboard, thus increasing peripheral
circulation and decreasing effective tissue insulation in rough environmental conditions (115).
However, the results of this thesis imply that when insulation in clothing is sufficiently high,
activity in rough water has a significant positive effect by decreasing core cooling rate (Paper
IV). This is in agreement with earlier studies that emphasised the importance of the thickness
of insulation in the dry suit for the core cooling rate (107, 135). This is of great importance for
Norwegian Sea King helicopter aircrew operating in the North and Barents Sea regions where
severe environmental conditions are normal. If the aircraft suffers engine failure when flying
over sea, they must perform a controlled emergency landing and will then be exposed to cold
water (~2 °C), low air temperatures, waves and wind. According to Sea King rescue crew,
search and rescue operations in these remote areas may well last for up to 12 hours (23).
Under these severe conditions the immersed victim will continuously lose heat even when
wearing a well-insulated immersion suit. Paper IV showed that a 5-15 minutes work –rest
schedule of leg-exercise at a moderate intensity results in a net positive heat gain (10 %)
compared to lying still. This procedure probably provides a practically significant survival
advantage at sea for victims awaiting rescue, by extending the time to severe hypothermia
(Paper IV).
The importance of wearing a well insulated dry suit
The importance of wearing a dry immersion suit in preventing heat loss from the body when
lying still in the water has been demonstrated by Hayward (53), who found a 60-fold greater
core cooling rate when wearing light clothing (52) than an insulated dry immersion suit (53).
Other studies of subjects wearing wet suits have also demonstrated higher rates of heat loss
and core cooling, and reduced tissue insulation when exercising than when at rest (94, 140).
These studies indicate that, even in a wet suit-protected individual, exercise increases heat
loss as much as heat production in cold water, emphasizing the importance of the degree of
insulation in a immersion suit (94, 140). A dry immersion suit prevents direct contact between
the water and the skin, thus keeping skin temperatures higher than in a wet suit, for example.
Reinertsen et al (107) studied the importance of insulation value of dry immersion suits when
performing leg exercises in cold water. This study demonstrated that intermittent periods of
leg exercise did not alter the core cooling rate when wearing uninsulated dry immersion suits,
while such behavior delayed core cooling in subjects wearing well-insulated dry suits.
Without any confirming measurement of heat loss, they assumed that the overall heat loss is
not enhanced by periods of leg exercise when wearing a well insulated immersion suit (107).
In paper IV we introduced eight heat flux sensors to provide measurements of heat loss under
more severe environmental conditions that included wind, waves, cold ambient temperature
and near-freezing water. Although heat loss is enhanced during the leg exercise periods, it is
counteracted by the high heat production, resulting in a more positive thermal situation
(decreased core cooling) for the subjects compared to lying still. This thesis extends previous
work by demonstrating the importance of insulation in the dry immersion suit in improving
heat balance under more severe conditions.
To summarize, heat loss in cold water is highly dependent on the severity of the conditions
and whether or not one is wearing an immersion suit. A well-insulated immersion suit
provides excellent protection against critical short-term responses when exposed to CWI, in
both prewarmed and normothermic subjects (Paper III). The immersion suit eliminates long-
term differences in core cooling between pre-warmed and resting individuals (Paper III).
Increasing heat production by intermittent periods of leg exercise is likely beneficial for better
maintenance of heat balance under severe environmental conditions when wearing a well
insulated immersion suit (Paper IV).
Methodological considerations
The methods used for evaluation of thermo physiology, heat exchange and thermal comfort in
this thesis are widely used in research laboratories worldwide. They are based on years of
experience in the Work Physiology Laboratory, and have already been described in a number
of journal papers and textbooks.
Ambient condition
One weakness of the study described in Paper I is that we lack information about temperatures
between the selected temperatures in the study. We do not know what would have happened
at 9 °C or 15 °C, for example, and can therefore not exclude a broader zone of
thermoneutrality. Nor did we include radiation in our laboratory setting in Papers I and II.
This was included in a more recent study by Ducharme (19), and our results are still
comparable to this study, which concluded that ambient temperatures of 23-25 °C do not
cause any significant detrimental change in the physiology of aircrew when workload is low,
as it is for helicopter pilots.
Protective clothing
The immersion suit worn in Papers I and II was the Mark 10 (Beaufort, UK) consisting of
cotton ventyle (2.2 Clo for the whole clothing concept). In Papers III and IV we used a 3 mm
neoprene suit (Helly Hansen, Norway) with higher insulation (2.97 Clo for the whole clothing
concept). The neoprene suit is also frequently used during Sea King helicopter flights in
Norway. A study comparing these two suits using the same test protocol as in paper III
recorded higher heat stress and discomfort ratings during work, but better protection in cold
water when the neoprene suit was worn (24). For this reason the neoprene suit was selected in
the cold water immersion studies.
Choice of subjects
In Paper III, we were unable to use the same subjects in the two groups, so that a pair-wise
statistical analysis could not be carried out. In order to reduce the effect of individual variance
we matched the anthropometric data of the subjects, but the results should be interpreted with
some caution. In paper IV the test subjects were a mixture of pilots and medical student’s i.e.
a selected cohort of highly motivated subjects who were more capable of withstanding the
detrimental effects of heat stress. A different population with lower motivation might not have
the same ability to withstand the heat stress, and decrements in performance might occur
earlier. In paper IV we selected young fit subjects with little variation in body fat. However,
we did not measure the subjects' VO2max, and can therefore not exclude the possibility that
differences in aerobic fitness level influenced the cooling rate and the ability to sustain a
certain level of leg-exercise intensity over time. However, they were their own controls, so
comparison between series still persists.
Work intensity in cold water
In Paper IV we deliberately did not control work intensity precisely because we wanted to
simulate a very realistic test in which the subject lies in a supine position as he would in a real
accident. For the same reason we did not introduce ergometer cycles or similar equipment to
control intensity as in previous studies (131, 133). Earlier studies have demonstrated that the
intensity and duration of exercise and whether it involved legs, arms or both (131) are factors
influencing its effects on core cooling. Leg exercise has been demonstrated to be many more
times thermally efficient than arm or a combination of arm and leg exercise, because the
higher surface-to-volume ratio of the arms results in greater heat loss than from the legs (131).
Most studies on exercise in cold water involved continuous exercise. A work intensity of 4-5
MET is required to maintain thermal equilibrium for one hour below the thermoneutral water
temperature (defined to be 34 °C at rest) (111). In practice such a high intensity cannot be
continued consistently for a long period of time in water (111), so an interval-based regime of
leg exercise is more adequate. In the present study work periods of five minutes every 20
minutes were based on the experience gained in previous experiments, where this work/rest
schedule could be maintained for a long period of time (up to six hours) (107, 135). The study
demonstrates that the length of the bouts of exercise (5 minutes) and the self-controlled
exercise intensity (between 600-900 W) in this study was sufficiently high to slow the core
cooling rate compared to lying still. It is important to note that even within this
anthropometric group there were differences between subjects in the leg exercise intensity
chosen. One subject chose an exercise intensity that was so high that he was able to increase
his core temperature during three hours of immersion.
The following principal conclusions can be drawn from the findings of this thesis:
Existing aircrew protective clothing is unable to reconcile the requirements for thermal
protection in water with those for thermal comfort during helicopter flights. Wearing aircrew
protective clothing (2.2 Clo) causes a downward shift of the ambient temperature range
required for thermoneutrality to 10-14 °C. These ambient conditions are below typical Sea
King Helicopter cockpit temperatures operating in Northern cool climatic zones. Thermal
discomfort is experienced at cockpit temperatures of 18-23 °C when wearing protective
clothing, but this is tolerable as far as cognitive performance is concerned.
The thesis has further demonstrated that at an ambient condition of 40 °C, the evaporative
resistance of the protective clothing causes accumulation of heat and water vapor within the
clothing microenvironment. The resulting increase in sweat rate, cardiac output, skin
temperatures and finally Tre is evidence of an uncompensable physiological strain. This thesis
thus concludes that cognitive performance is virtually unaffected unless wearing an
immersion suit combined with sufficiently high ambient temperature results in an
uncompensable physiological strain resulting in increase core temperature (Paper II).
Heat stress due to working in aircrew protective clothing will not affect long-term core
cooling rate during subsequent cold water immersion (Paper III). Aircrew protective clothing
protects well against the initial effects of CWI, described as the “cold shock response” (Papers
III, IV). The thesis has also shown that under severe environmental conditions five minutes of
leg exercise every twenty minutes might improve heat balance resulting in a net positive heat
gain (10%) compared to lying still in the water (Paper IV). This procedure reduces the core
cooling rate and has a positive effect on subjective perception of thermal comfort and reduced
cold sensation (Paper IV).
Practical applications
Paper I
The practical implications of the findings of this paper are that efforts should be made to
increase thermal comfort by regulating cockpit temperature downwards (if possible) or
considering the introduction of personal cooling aids at cockpit temperatures of as low as
18 °C when aircrew protective clothing (2.2 Clo) is worn.
Paper II
The practical implications of this study are that moderate cockpit ambient temperatures
(23 °C) are quite tolerable as far as certain cognitive performance tasks required for helicopter
flying are concerned, while high cockpit temperatures (40 °C) may over time lead to
deterioration of flight performance when aircrew protective clothing is worn.
Paper III
The results of Paper III are relevant to occupations in which people are required to wear
protective clothing due to the risk of being exposed to cold water when their body temperature
is elevated. The practical implication of this study is that heat stress after working in
protective clothing will not increase the risk incurred by falling in cold water, because 1) of
aspirating water and drowning, since the protective clothing attenuates the cold shock
response and 2) the immersion suit protects well against the long-term core cooling.
Paper IV
Five minutes of leg exercise every twenty minutes might improve heat balance when a well-
insulated immersion suit is worn under severe environmental conditions. This procedure
potentially provides a significant survival advantage at sea for victims awaiting rescue by
extending the time to severe hypothermia. This is of significant importance for Norwegian
Sea King helicopter aircrew operating in the North and Barents Sea regions where severe
environmental conditions are common.
Future perspectives
This thesis addresses the difficulties of reconciling requirements for thermal protection and
thermal comfort for helicopter pilots when wearing aircrew protective clothing. However,
protective clothing for many sectors of the workforce has the same problem of resolving
trade-offs between protection and comfort (firefighters' clothing, cold-weather clothing,
chemical-biological clothing, etc.). Even people working in extreme cold may face the
problem of heat stress at work when wearing protective clothing if the intensity of their work
is high. In situations with rapid changes in work intensity and/or variations in ambient
conditions, the limitations of the thermoregulatory system of the wearer will not be able to
handle this alone to maintain thermal balance; for example, the rise in petroleum activity in
the high north, willl face workers with conditions of extreme cold
( Thermal protection and thermal comfort are both related to
heat and moisture transfer in clothing. This has traditionally been regarded as two separate
processes; however this issue needs to be addressed as a set of related phenomena (138).
More recently, we have started to look at the influence of liquid sweat, which is often a
problem for impermeable clothing evaporation and condensation. Complex models have been
developed to understand these mechanisms (49, 138).
Recent years have seen a sharpening of focus on developing “smart textiles” that might help
to solve the difficulties of reconciling contradictory requirements of personal protective
clothing. An example is the development of the Helly Hansen SeaAir helicopter transportation
suit (
helikopterdrakt/). Today, developments in materials have led to a revolutionary way of
thinking of protection. Performance and comfort can be improved by new textiles that can
adapt their thermal and moisture-transmission properties to changing environment conditions
and exercise levels. The safety and efficiency of operations can be improved by incorporating
instrumentation in clothing for monitoring vital physiological parameters of the wearer. To do
this, future research will need to develop a fundamental understanding of how comfort and
performance can be improved by the use of stimulus-responsive textiles that adapt their
properties to environmental changes. Research should be done to develop advanced materials
that will provide a significant increase in performance because they are utilized in accordance
with the body’s own regulatory mechanisms to provide optimal function. The main goal for
the development of new materials for protective clothing must be to improve ergonomics and
thermal comfort while maintaining an adequate level of protection. Clearly, making clothing
that supports the thermoregulation of the human body is the most effective way of improving
thermal comfort. By improving the mechanisms that affect thermal and moisture transport
through several layers of fabric, thermal comfort can be improved. Material experts, product
designers and clothing physiologist therefore need to collaborate in the development of more
sophisticated protective clothing.
Many of the processes that occur in textiles are still not completely understood, and we can
expect that intensive research will be done on this topic in the future (110). There is a trend
towards realistic test methods that more closely resemble “real-life conditions”. Test
conditions should be set according to a thorough assessment of how and under what
conditions protective clothing will be used. Future research will need to pay more attention to
understanding the situation of the worker and how specific tasks are performed. Development
in the field of protective clothing must be take place on the basis of user needs; only then will
appropriate solutions to some very complex problems be found.
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Paper I
Journal of Thermal Biology 26 (2001) 419–425
The thermoneutral zone when wearing aircrew
protective clothing
H. Færevik
*, D. Markussen
, G.E. Øglænd
, R.E. Reinertsen
Department of Health and Work Physiology, SINTEF Unimed, N-7465 Trondheim, Norway
Norwegian University of Science and Technology, N-7491 Trondheim, Norway
The aim of this study was to determine the thermoneutral zone (TNZ) in subjects wearing aircrew protective clothing.
TNZ were first defined in naked subjects to a temperature range of 28–311C. Wearing aircrew protective clothing caused
a displacement of the TNZ to 10–141C ambient temperature (Ta). Discomfort increased at ambient temperatures above
this range, as a result of increases in metabolic rate, mean skin temperature (MST) and sweating. The practical
implication of this study is that cockpit temperature in Sea King helicopters should be regulated to lie between 101C and
141C(Ta) in order to prevent heat stress in pilots when wearing aircrew protective clothing. r2001 Elsevier Science
Ltd. All rights reserved.
Keywords: Temperature regulation; Basal metabolism; Protective clothing; Mental performance; Mean skin temperature; Thermal
comfort; Heat stress; Helicopter pilot
1. Introduction
Humans are not very well adapted to cold and can
tolerate only small falls in body temperature. In the
cold, man uses behavioural adaptation (clothing, shelter,
moving to a warmer environment) or physiological
mechanisms (vasoconstriction) to decrease heat loss, or
increases heat production through exercise or shivering.
Humans cope better with heat and are capable of
increasing heat loss either behaviourally (by moving to a
colder environment) or physiologically (by increased
peripheral blood flow or evaporation of sweat). The
range of ambient temperature (Ta) within which basal
metabolic rate is minimal and constant is called the
thermoneutral zone (TNZ) (Risbourg et al., 1991).
Despite the obvious importance of the TNZ, this zone
of thermoregulation has attracted little research in
humans since experiments revealed its existence more
than 50 years ago.
Within the range of TNZ, the control of thermal
balance is accomplished by the regulation of skin blood
flow (Hardy, 1961). Metabolic rate rises at a Tabelow
the lower critical temperature (Tlc) and above the upper
critical temperature (Tuc) of TNZ (Withers, 1992). The
relationship between metabolic rate and Tabelow Tlc
generally conforms to a physical model for endothermic
heat balance, where M¼CðTbTaÞ(Mis metabolic
rate, Tbis body temperature, Tais ambient temperature
and Cis the thermal conductance). This is not a physical
constant, but can be varied by behavioural and
physiological thermoregulation. The conductance varies
in naked subjects, and is dependent on the thickness of,
and bloodflow to, subcutaneous tissues. A large blood-
flow to the skin increases heat loss and conductance is
high. When Tadecreases below Tlc vasoconstriction
occurs and the insulation value of the superficial shell of
the body is higher while conductance is lower.
Under normal circumstances evaporation of sweat is
sufficient to keep the body in heat balance. However,
some working situations require protective clothing
capable of limiting heat exchange with the surroundings
by increasing insulation and preventing evaporative heat
loss, but which may cause significant heat stress for the
*Corresponding author. Tel.: +47-7359-2355; fax: +47-
E-mail address:
(H. Færevik).
0306-4565/01/$ - see front matter r2001 Elsevier Science Ltd. All rights reserved.
PII: S 0306-4565(01)00054-7
user. This is known to be a severe problem for military
pilots, who are required to wear protective clothing
during flights, possibly resulting in fatigue and impaired
pilot performance (Nunneley, 1989; Hancock, 1981).
Field studies of Sea King helicopter pilots, for example,
have demonstrated that they experience significant heat
stress during flight (Færevik and Reinertsen, 1998). Sea
King pilots and crew members in Norway are required
to wear a survival suit all year around when operating in
the North Sea and Barents Sea region with low sea
temperatures. The heat stress is caused by a combination
of the survival suit, the large cockpit canopy that
increases radiation, and limited possibilities to regulate
ambient temperature inside the cockpit. Nor do pilots
have the possibility to behaviourally move to a cooler
environment. It is therefore interesting to define the
ambient conditions within which pilots are thermally
comfortable and do not experience any physiological
heat stress.
The aim of this study was to determine the thermo-
neutral zone (TNZ) in subjects wearing aircrew protec-
tive clothing, as this has not earlier been defined. The
study also aimed to define how thermal conductance
under ambient conditions below Tlc changes in naked
subjects and subjects wearing aircrew protective cloth-
ing. In naked, resting subjects the TNZ has been defined
as lying between 28 and 301C ambient temperature
(Gagge et al., 1967), and we hypothesised that wearing
protective clothing will cause displacement of the TNZ
to a lower range of ambient temperatures.
2. Material and methods
The following criteria were used to define the TNZ;
(1) The range of ambient temperature within which the
basal metabolic rate is minimal and constant (Risbourg
et al., 1991), predicting that the metabolic rate will
increase below Tlc and above Tuc:(2) Mean skin
temperature (MST) between 33–351C (Savage and
Brengelmann, 1996), predicting that the MST will
decrease below Tlc and increase above Tuc:(3) The
subjective sensation temperature and thermal comfort is
neutral, predicting that subjects will be thermally
uncomfortable below Tlc and above Tuc (
AAstrand and
Rodahl, 1986).
Eight male volunteers participated in the study. The
mean (7SD) age, weight, height and percent body fat of
the subjects was 23.671.3 years, 83.7711.0 kg,
184.175.3 cm, 12.571.7%. The subjects were asked to
retire to bed at their usual time on the night before each
exposure. They abstained from exercise and taking
caffeine, alcohol or snuff for 24 h before exposure.
Subjects were not permitted to eat 3 h before the test in
order to avoid a rise in metabolic rate after feeding.
None of the subjects were smokers. All subjects were in
good health and had undergone an electrocardiogram
test. The Ethical Review Committee of the Faculty of
Medicine at the Norwegian University of Science and
Technology approved the experimental procedure. The
subjects were free to withdraw from the chamber
environment at any time.
2.1. Experimental protocol
Subjects reported to the preparation room at least 1 h
before the test. Registration of weight, height and body
fat were made. Percentage body fat was calculated using
the Durnin and Wommersley 4-site skinfold thickness
measure (Durnin and Womersley, 1974). Body surface
in square metres (ADu) was calculated using the
following formula (DuBois, 1919): ADu ¼0:202
b;where Wbis the body weight in kg, and
Hbis the body height in meters. The subjects were then
fitted with thermistors and heart rate recorder. To
provide baseline measurements of oxygen consumption
), skin and rectal temperature and subjective
evaluations, the subjects sat quietly outside the climatic
chamber for 20 min. The subjects were then moved to
the climatic chamber where they sat at rest for a further
60 min.
Each subject was first exposed to a range of ambient
temperatures wearing only shorts (series A). They were
then exposed to a lower range of temperatures, wearing
aircrew protective clothing (series B) in order to
determine the displacement of TNZ. In series B they
dressed as pilots normally do for helicopter flights, with
long legged/long sleeved underwear (200 g Ullfrotte), a
woollen whole-body overall, leather gloves, helmet and
uninsulated survival suit (Mark 10, Beaufort, England).
The insulation value of the whole clothing ensemble was
2.20 Clo, measured on a thermal manikin. In series A
they were exposed to seven different ambient conditions;
151C, 201C, 251C, 281C, 311C, 351C, and 401C,
respectively. In series B they were exposed to five
different conditions; 01C, 101C, 141C, 181C, and 251C,
respectively. The subjects were exposed randomly to the
different environmental condition to avoid any order
effects. The tests were carried out at the same time of the
day on separate days with at least a one-day interval
between the tests. Subjects were not permitted to drink
or eat during the experiment.
During the experiments, rectal temperature (Tre) was
measured with a thermistor probe (YSI-700, Yellow
Springs Instrument, USA, accuracy 70.151C) inserted
10 cm beyond the anal sphincter. Skin temperatures were
measured using thermistors (YSI-400 Yellow Springs
Instrument, USA, accuracy 70.151C) at 13 locations
(forehead, chest, lower arm, abdomen, underarm,
middle finger, neck, scapula, front of thigh, back of
thigh, shin, calf and surface of foot). The weighted
average formula of Teichner (1958) was used to define
H. F
revik et al. / Journal of Thermal Biology 26 (2001) 419–425420
mean skin temperatures (MST). Heart rate (fc) was
recorded using a Polar Sports Tester (Polar Electro,
Finland). All logged data were transferred to a computer
for graphically and numerically displaying the results
every minute and processed using TempLog 3.1. Oxygen
consumption (VO
) was logged for a period of 10 min
after 40 and 60 min in the climatic chamber using a
Cortex MetaMax Portable Metabolic Testsystem (Cor-
tex Biophysic GmbH, Germany). To ensure stabilised
measures of VO
the mean of the last 2 min (of the
10 min measure period) were used for statistical analysis.
A questionnaire developed by Nielsen et al. (1989) was
used to obtain information about local and overall
thermal sensation, shivering/sweating and thermal com-
fort. Thermal sensation of the body, feet and hands was
evaluated according to the following rating: 1 very cold,
2 cold, 3 cool, 4 slightly cool, 5 neutral, 6 slightly warm,
7 warm, 8 hot, 9 very hot. Thermal comfort was
evaluated according to the following rating: 1 comfor-
table, 2 slightly uncomfortable, 3 uncomfortable, 4 very
uncomfortable. Shivering/sweating was also evaluated
on a graded rating; 1 heavily shivering, 2 moderately
shivering, 3 slight shivering, 4 not at all shivering/
sweating, 5 slightly sweating, 6 moderately sweating, 7
heavily sweating. Environmental conditions (air tem-
perature, radiation and relative humidity) were mea-
sured continuously with an Indoor Climate Analyser
T1213 (Br.
uuel & Kjær A/S, Denmark). Thermal con-
ductivity (C) was calculated from the following formula
(Withers, 1992): M¼CðTbTaÞ;C¼M=ðTbTaÞ;
where Mis metabolic rate, Tbis body temperature
and Tais ambient temperature.
2.2. Statistical analysis
Time-dependent changes in rectal temperature, mean
skin temperatures and heart rate were assessed by two-
way analysis of variance for repeated measures (ANO-
VA). A within-group study design was used. All data
were tested for effects of time, ambient conditions and
interactions between the two measures. When ANOVA
revealed a significant main effect, a contrast test was
used as a post hoc test to identify significant differences
between temperatures. Kawashima (1993) has shown
that it takes 30 min for rectal and skin temperature,
heart rate and metabolism to stabilise at a higher
ambient temperature, so the mean values of the last
30 min were analysed further using Student’s t-test for
paired samples. Differences in oxygen consumption and
the various ratings on thermal comfort, thermal sensa-
tion, degree of shivering or sweating were also assessed
by Student’s t-test for paired samples. The measures
after 60 min were used in the statistical analysis in order
to be sure that the readings had stabilised. The Shapiro
Wilks test was used to test for normal distribution.
Results are presented as means 7SD for eight subjects.
All differences reported are significant at the pp0:05
level. SPSS 10.0 (SPSS inc. Chicago, USA) was the
software used for processing the statistical material.
3. Results
3.1. Oxygen consumption
In series A (naked) VO
was lowest at 281C
(3.970.7 ml kg
) and highest at 151C
(5.171.1ml kg
). VO
at 281Cwassignicantly
lower than at 151C, 351Cand401C(Ta) (Fig. 1). In series
B (wearing aircrew protective clothing), VO
was lowest at
an ambient condition of 101C(4.170.7 ml kg
and 141C(3.970.4 ml kg
at both
101Cand141C was significantly lower than at 01C, 181C,
and 251C. There were no significant differences between
3.2. Rectal temperature
For both series A and B, statistical analysis by
ANOVA demonstrated that the time-dependent change
in rectal temperature (Tre) was highest during the first
30 min under all ambient conditions, and that the Tre
then stabilised during the last 30 min. One exception was
the 401C (series A) where Tre continued to increase
throughout the experiment. When the means for the last
30 min for all subjects in series A were compared, it was
Fig. 1. Changes in metabolic rate (VO
) in subjects wearing
only shorts (series A) after 60 min exposure to seven different
ambient temperatures (Ta), or wearing aircrew protective
clothing (series B) at five different ambient temperatures
(n¼8). In series A (nude), (*) indicates significantly lower
at 281C(Ta) as compared to 151C, 351C and 401C. In series
B (wearing aircrew protective clothing), (*) indicates signifi-
cantly lower VO
at 101C as compared to 01C, 181C and 251C,
and (E) indicates significantly lower VO
at 141C as compared
to 01C, 181C and 251C.
H. F
revik et al. / Journal of Thermal Biology 26 (2001) 419–425 421
found that Tre was unchanged at temperatures below
281C, but was significantly higher at ambient condition
311C, 351C and 401C compared to 281C (Fig. 2). In
series B, Tre was significantly lower at 101C
(36.970.21C) compared to 251C (37.170.21C) (Fig. 3).
No other differences in Tre between ambient conditions
were observed.
3.3. Mean skin temperature
Most skin temperatures stabilised after 30 min at the
different ambient temperatures in series A. However, at
the two lowest ambient conditions (151C and 201C)
MST continued to fall throughout the experiment. MST
was very responsive to changes in Ta:When MSTs
during the last 30 min in series A were compared,
significant differences were found between all ambient
temperatures (Fig. 2). Mean skin temperature rose at Ta
higher than 281C and fell at Tabelow 281C. The criteria
for thermoneutrality (MST of 33–351C) were fulfiled at
both Taof 281C (33.470.61C) and 311C (34.570.41C).
In series B, MST stabilised after 30 min at all ambient
temperatures, except at 01C, where it continued to fall
throughout the experiment. At 101C and 141C(Ta) there
were no significant time-dependent changes in MST
throughout the whole experiment. When MSTs during
the last 30 min in series B were compared, they were
found to rise with increasing ambient temperatures and
fall at lower ambient temperatures (Fig. 3). MST
differed significantly between all ambient conditions.
An Taof 101C (33.670.61C), 141C (34.270.41C) and
181C (34.870.31C) fulfiled the criteria for thermoneu-
trality (MST of 33–351C). Finger and foot temperatures
stabilised and demonstrated the least variations at 251C,
281C and 311C in series A. Finger and foot temperature
showed a slight increase and thereafter stabilised when
Tawas 351Cor401C. At Taof 151C and 201C finger and
foot temperatures did not stabilise and continued to fall
throughout the experiment. In series B, finger and foot
temperatures stabilised at 101C, 141C, 181C and 251C,
but at 01C(Ta) continued to fall throughout the
3.4. Subjectiveevaluations
Taking all subjective evaluations together, the feeling
of thermal neutrality was not limited to a single
temperature, but was closest to the ambient tempera-
tures of 251C, 281Cor311C in series A. All subjects
stated that they felt most comfortable at 251C(Ta). At
lower ambient temperatures (151C and 201C) and higher
ambient temperatures (351C and 401C) subjects were
significantly more uncomfortable. Similarly, the overall
thermal sensation of the body, hands and feets was
closest to neutral at ambient temperatures of 251C, 281C
and 311C. Subjects stated that they were slightly warmer
at Taof 311C than 251C, but there were no significant
differences between 251C and 281C. At ambient tem-
peratures of 351C and 401C, all subjects stated they were
sweating significantly more than at 311C. At lower
temperatures, subjects started shivering at 201C, and
shivered significantly more compared to temperatures at
and above 251C.
In series B, there were no significant differences in
subjective sensations of thermal comfort between
ambient temperatures of 101C, 141C and 181C. The
subjects were significantly more uncomfortable at 01C
and 251C than at either 101Cor141C. Most subjects
stated that they were not shivering/sweating at all at
ambient temperatures of 01C and 101C. Some subjects
Fig. 2. Mean7SD of mean skin (MST) and rectal temperature
(Tre) during the last 30 of 60 min exposure to seven different
ambient temperatures (Ta) in subjects only wearing shorts
(series A) (n¼8). (E) indicates significantly lower Tre at 281C
(Ta) as compared to 311C, 351C and 401C. (*) indicates
significantly differences in MST between all Tain relation to
Fig. 3. Mean7SD of mean skin (MST) and rectal temperature
(Tre) during the last 30 of 60 min exposure to seven different
ambient temperatures (Ta) in subjects wearing aircrew protec-
tive clothing (series B) (n¼8). (E) indicates significantly
higher Tre at 251C(Ta) as compared to 101C. (*) indicates
significantly differences in MST between all Tain relation to
H. F
revik et al. / Journal of Thermal Biology 26 (2001) 419–425422
voted that they started sweating slightly at 141C and
181C. At 251C all subjects were sweating significantly.
The thermal sensation of the body was closest to neutral
at 101C (4.970.6, 5=neutral) and 141C (5.470.5).
Thermal sensation of hands and feet’s were closest to
neutral at 101C (5.070.5 and 4.870.7, respectively).
The subjects were significantly cooler at 01C and
significantly warmer at 181C and 251C compared to
101C and 141C.
3.5. Heart rate
In series A, the heart rate (fc) fell at low temperatures
(151C and 201C) and increased at higher temperatures
(351C and 401C). There were wide inter-subject varia-
tions in heart rate. When the results for the last 30 min
were compared, there were no differences in heart rate
between 251C (61710 beats min
), 281C (6474
beats min
) and 311C (6377 beats min
). Heart rate
was lowest at Taof 151C (5677 beats min
), and
highest at 401C (7078 beats min
). The only Taat
which fcstabilised were 281C and 311C. Heart rate at
281C and 311C(Ta) was significantly higher than fcat
151C and 201C and significantly lower than fcat 351C
and 401C. In series B, fcincreased significantly with time
when Tawas 251C, and the mean of the last 30 min at
this temperature was also highest (6877 beats min
The heart rate at 251C was significantly higher than fcat
01C (5879 beats min
)101C (5675 beats min
) and
141C (5877 beats min
4. Discussion
The maintenance of a constant body temperature is
important for pilots, since raised body temperature is
correlated with impaired pilot performance (Nunneley,
1989). The main purpose of this study was to determine
the thermoneutral zone (TNZ) in subjects wearing
aircrew protective clothing. The hypothesis that wearing
protective clothing will affect heat exchange with the
surroundings and cause displacement of TNZ was
clearly supported.
In naked resting subjects, VO
was lowest at 281C
(0.3370.08 l min
) and this is close to the value that
Nielsen et al. (1989) demonstrated under thermoneutral
condition (VO
at 0.32 l min
). Weight-specific VO
was used when comparing the different exposures in
order to compensate for weight differences. TNZ has
earlier been defined as lying between 28 (Tlc) and 301C
(Tuc) ambient temperatures (Gagge et al., 1967). Resting
subjects will strive to combat the rise in body
temperature by sweating above Tuc and to prevent a
fall in body temperature by shivering below Tlc:This
energy demanding thermoregulatory processes will
cause an increase in metabolism below Tlc and above
Tuc in order to keep the body in heat balance (Davson
and Sagal, 1975). Even though some subjects reported
that they started shivering at an ambient temperature of
201C, we did not observe any significant increase in
metabolic rate at 201Cor251C. Most subjects did not
report shivering until Tawas as low as 151C, this was
also demonstrated by an increase in VO
. The results of
the VO
measurements suggest that Tlc in naked resting
subjects is lower than the earlier suggested 281C. This
suggestion is in accordance with Kawashima (1993),
who demonstrated a rise in metabolic rate at 221C but
no significant increase at 251C(Ta). A significant
increase in metabolic rate was found at higher ambient
temperatures (351C and 401C). This suggests that Tuc is
close to 311C in naked, resting subjects. This suggestion
is supported by the observation that subjects also started
sweating at 351C(Ta). Protective clothing (2.20 Clo)
clearly affected metabolism in these sitting, resting
subjects. The observation of the lowest values of
metabolic rate shifted from 281C(Ta)to101C
(0.3370.06 l min
) and 141C (0.3270.03 l min
(Ta) when with protective clothing. At both lower
(01C) and higher Ta(181C and 251C) the metabolic rate
increased significantly. The insulation of the clothing
was not sufficient to prevent shivering at 01C, as was
demonstrated by an increase in VO
. Shivering caused
subjects to feel more uncomfortable and they also stated
that they felt slightly cooler at 01C compared to higher
Ta:At ambient temperatures closer to Tuc vasoactivity is
no longer sufficient to regulate heat balance and active
sweating occurs. To prevent an increase in body
temperature, the produced sweat must evaporate. Even
though the survival suit used in the present study has
some evaporative properties, it was not sufficient to
ensure evaporative cooling at 181C(Ta). As a result,
there was a significant increase in metabolism at 181C
(Ta). Subjects stated that they were sweating and that
their clothing felt damp and they were warm and
significantly uncomfortable at ambient temperatures
above 181C.
The significant differences in MST between all
different ambient temperatures in naked subjects de-
monstrates that skin temperatures are very sensitive to
changes in ambient conditions, due to the rapid
implementation of vasoconstrictive responses. Calori-
metric studies of naked male subjects have shown that
MST values of 331C–351C cover the TNZ (Hardy and
BuBois, 1937; Savage and Brengelman, 1996). Ambient
temperatures of 281C–311C fulfilled the criteria for
thermoneutratility in naked subjects. MST did not
stabilise after 30 min when naked subjects were exposed
to the lowest temperatures (151C and 201C), and both
the low mean values (28.570.71C, 30.370.8) and the
continuous fall in MST suggests that 151C and 201C(Ta)
is below Tlc:This is in agreement with the findings of
Kawashima (1993), who demonstrated that MST does
H. F
revik et al. / Journal of Thermal Biology 26 (2001) 419–425 423
not stabilise at lower temperatures. At higher tempera-
tures (351C and 401C) skin temperatures stabilised close
to Tre temperature and were outside the TNZ. The high
values of MST (36.01C and 36.91C) at 351C and 401C
indicate that vasodilatation is at its maximum while Tre
is also increasing, and this helps to maintain the core-
skin gradient and thereby ensure dissipation of heat. Tre
was significantly higher at 311C, 351C and 401C than at
281C. Rectal temperature is not very sensitive to small
changes in Ta;and is therefore not a very good indicator
of the TNZ. The observed increase in metabolic rate
below Tlc at 151C prevented Tre from falling. Wearing
aircrew protective clothing clearly affected MST and
caused a displacement of thermoneutral temperatures
from 281C and 311C to a lower range of 101C, 141C and
181C(Ta). The increased metabolism at 251C caused a
significant rise in MST. Within the TNZ, control of
thermal balance is achieved by regulation of the skin
blood flow (Hardy, 1961). This is accomplished by
vasomotoractivity in the extremities (hands and feet),
characterised by small oscillations within the TNZ. This
was observed at Taof 101C and 141C, indicating that
these temperatures produce the TNZ. This is supported
by the finding that subjective thermal sensation of hands
and feet was close to neutral at these Ta:At the lowest
ambient temperature (01C) the skin temperatures in
fingers and feet were falling throughout the experiment,
indicating that vasoconstriction of peripheral vessels was
occurring. Finger and foot temperatures were not
oscillating and continuously increasing at 181C and
251C(Ta), indicating that the vessels were dilated in
order to increase heat loss. The result from the skin
temperatures of the extremities suggests that TNZ while
wearing protective clothing lie within the range of 141C
to 181C.
Heart rate rose at the highest Ta;and fell at lower Ta:
The increase in heart rate and vasodilatation causes an
increase in peripheral bloodflow and thus heat dissipa-
tion at higher Ta(Hardy and BuBois, 1937). Heart rate
decreases in colder environments due to vasoconstric-
tion. Although wide individual variations made it
difficult to determine the TNZ, heart rate was signifi-
cantly higher at 351C and 401C in naked subjects, and
lower at 151C and 201C. This indicates that the TNZ is
between 251C and 311C. When subjects were wearing
protective clothing, the only significant difference in
heart rate was observed in an increased heart rate at
251C compared with 01C, 101C and 141C, while the
TNZ shifted downwards. It may be concluded that heart
rate was not a good indicator of thermoneutrality.
Fanger (1970) suggested that heat balance was
necessary for a subjective sensation of thermal comfort.
Discomfort increased when MST was outside the TNZ.
This is in agreement with Gagge et al. (1967), who
demonstrated that the subjective sensation of discomfort
correlates well with changes in MST. The subjective
evaluations of thermal sensation and comfort in this
study indicate that TNZ is in the range of 251C and 311C
in naked subjects, and shifted to a lower Tarange of
101Cto181C when subjects were wearing aircrew
protective clothing. The subjective evaluations indicated
a wider TNZ than suggested by the physiological
measurements, and it is possible that the scale for
subjective evaluation is not sensitive enough to locate
significant changes with small variations in Ta:
The results from this study suggests that TNZ wearing
protective clothing is between 101C and 141C. During
helicopter flights VO
has been measured to
0.5370.02 l min
(n¼6) using the same clothing as in
the current experiment (Færevik and Reinertsen, 1998).
The ambient condition in cockpit during these 2 h flights
were 18.671.31C. This Tais somewhat higher than the
suggested thermoneutral condition determined when
subjects were sitting still in the laboratory. Even this
cool cockpit condition (181C) caused MST above 351C,
subjective sensation of thermal discomfort and increased
sweat production, indicating that the subjects was
outside TNZ.
Thermal conductance at 281C was calculated to be
0.04 W m
in naked subjects and 0.01 W m
at 101C(Ta) in those wearing protective clothing. The
fall in heat conductance with protective clothing reflects
a reduction in heat flow to the surroundings from the
body when wearing protective clothing. In naked
subjects the insulation is low and conductance is
correspondingly high. Protective clothing is a barrier
to heat dissipation, and heat has to be transported to all
the clothing layers, resulting in a fall in thermal
conductance. The results of this study are summarised
in Fig. 4. This figure illustrates the change in thermal
conductance calculated from the Tlc values in both series
A and B.
Fig. 4. The displacement of the thermoneutral zone (TNZ)
when wearing aircrew protective clothing. The figure illustrates
the thermal conductance based on the estimated lower critical
temperature (Tlc) for both series. In series A (nude) thermal
conductance was calculated at Tlc of 281C to 0.04 W m
and in series B (wearing aircrew protective clothing) it was
calculated at Tlc of 101C to 0.01 W m
.Tbis body
H. F
revik et al. / Journal of Thermal Biology 26 (2001) 419–425424
5. Summary
This study has shown that the criteria for thermo-
neutrality were met at 281C and 311C(Ta) in naked
subjects, and shifted downward to 101C and 141C(Ta)
when subject wore aircrew protective clothing. It also
suggests that 101C mark the Tlc when wearing protective
clothing, given that we lack information about tempera-
tures between 01C and 101C, and that 281C marks the
Tlc in naked subjects.
The practical implication of this study is that cockpit
temperature in Sea King helicopters should be regulated
to lie between 101C and 141C(Ta) in order to prevent
heat stress in pilots when wearing aircrew protective
clothing. Pilot discomfort will increase at cockpit
temperatures above 181C, as a result of increases in
metabolic rate, MST and sweating.
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Paper II
Is not included due to copyright
Paper III
Is not included due to copyright
Paper IV
Is not included due to copyright
Doctor al theses in Biology
Norwegian Univer sity of Science an d Technology
Depar tment of Biology
1974 Tor-Henning Iversen Dr. philos
The roles of statholiths, auxin transport, and auxin
metabolism in root gravitropism
1978 Tore Slagsvold Dr. philos.
Breeding events of birds in relation to spring temperature
and environmental phenology.
1978 Egil Sakshaug Dr.philos
"The influence of environmental factors on the chemical
composition of cultivated and natural populations of
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1980 Arnfinn Langeland Dr. philos.
Interaction between fish and zooplankton populations and
their effects on the material utilization in a freshwater
1980 Helge Reinertsen Dr. philos
The effect of lake fertilization on the dynamics and
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the phytoplankton
1982 Gunn Mari Olsen Dr. scient
Gravitropism in roots of
Pisum sativum
thalia na
1982 Dag Dolmen Dr. philos.
Life aspects of two sympartic species of newts (
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ecological niche segregation.
1984 Eivin Røskaft Dr. philos.
Sociobiological studies of the rook
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1984 Anne Margrethe
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Effects of alcohol inhalation on levels of circulating
testosterone, follicle stimulating hormone and luteinzing
hormone in male mature rats
1984 Asbjørn Magne Nilsen Dr. scient
Alveolar macrophages from expectorates – Biological
monitoring of workers exosed to occupational air
pollution. An evaluation of the AM-test
1985 Jarle Mork Dr. philos.
Biochemical genetic studies in fish.
1985 John Solem Dr. philos.
Taxonomy, distribution and ecology of caddisflies
) in the Dovrefjell mountains.
1985 Randi E. Reinertsen Dr. philos.
Energy strategies in the cold: Metabolic and
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1986 Bernt-Erik Sæther Dr. philos.
Ecological and evolutionary basis for variation in
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1986 Torleif Holthe Dr. philos.
Evolution, systematics, nomenclature, and zoogeography
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1987 Helene Lampe Dr. scient.
The function of bird song in mate attraction and
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1987 Olav Hogstad Dr. philos.
Winter survival strategies of the Willow tit
1987 Jarle Inge Holten Dr. philos
Autecological investigations along a coust-inland transect
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1987 Rita Kumar Dr. scient
Somaclonal variation in plants regenerated from cell
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1987 Bjørn Åge Tømmerås Dr. scient.
Olfaction in bark beetle communities: Interspecific
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1988 Hans Christian Pedersen Dr. philos.
Reproductive behaviour in willow ptarmigan with special
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1988 Tor G. Heggberget Dr. philos.
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1988 Marianne V. Nielsen Dr. scient.
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1989 Helga J. Vivås Dr. scient.
Theoretical models of activity pattern and optimal
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1989 Reidar Andersen Dr. scient.
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1989 Kurt Ingar Draget Dr. scient
Alginate gel media for plant tissue culture,
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1990 Hege Johannesen Dr. scient.
Respiration and temperature regulation in birds with
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1990 Åse Krøkje Dr. scient
The mutagenic load from air pollution at two work-places
with PAH-exposure measured with Ames
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1990 Arne Johan Jensen Dr. philos.
Effects of water temperature on early life history,
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1990 Tor Jørgen Almaas Dr. scient.
Pheromone reception in moths: Response characteristics
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1990 Magne Husby Dr. scient.
Breeding strategies in birds: Experiments with the
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1991 Tor Kvam Dr. scient.
Population biology of the European lynx (
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1991 Jan Henning L'Abêe
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Reproductive biology in freshwater fish, brown trout
Salmo trutta
and roach
Rutilus rutilus
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1991 Asbjørn Moen Dr. philos
The plant cover of the boreal uplands of Central Norway.
I. Vegetation ecology of Sølendet nature reserve;
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1991 Else Marie Løbersli Dr. scient
Soil acidification and metal uptake in plants
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Reflctometric studies of photomechanical adaptation in
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1991 Thyra Solem Dr. scient
Age, origin and development of blanket mires in Central
1991 Odd Terje Sandlund Dr. philos.
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1991 Nina Jonsson Dr. philos. Aspects of migration and spawning in salmonids.
1991 Atle Bones Dr. scient
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Food supply as a determinant of reproduction and
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1992 Arne Vollan Aarset Dr. philos.
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1993 Geir Slupphaug Dr. scient
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Habitat shifts in coregonids.
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Cortisol dynamics in Atlantic salmon,
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Theoretical studies of life history evolution in modular
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Molecular studies of myrosinase in Brassicaceae
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Reproductive strategy and feeding ecology of the
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1993 Kjetil Bevanger Dr. scient.
Avian interactions with utility structures, a biological
1993 Kåre Haugan Dr. scient
Mutations in the replication control gene trfA of the
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Sexual selection in the lekking great snipe (
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1994 Nils Røv Dr. scient.
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Light harvesting and utilization in marine phytoplankton:
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1994 Morten Bakken Dr. scient.
Infanticidal behaviour and reproductive performance in
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1994 Arne Moksnes Dr. philos.
Host adaptations towards brood parasitism by the
1994 Solveig Bakken Dr. scient
Growth and nitrogen status in the moss
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Sm. as influenced by nitrogen supply
1994 Torbjørn Forseth Dr. scient.
Bioenergetics in ecological and life history studies of
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1995 Hanne Christensen Dr. scient.
Determinants of Otter
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Mustela vision
1995 Svein Håkon Lorentsen Dr. scient.
Reproductive effort in the Antarctic Petrel
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1995 Chris Jørgen Jensen Dr. scient.
The surface electromyographic (EMG) amplitude as an
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1995 Martha Kold Bakkevig Dr. scient.
The impact of clothing textiles and construction in a
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1995 Vidar Moen Dr. scient.
Distribution patterns and adaptations to light in newly
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and constraints
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1995 Hans Haavardsholm
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A revision of the
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1996 Jorun Skjærmo Dr. scient
Microbial ecology of early stages of cultivated marine
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1996 Ola Ugedal Dr. scient.
Radiocesium turnover in freshwater fishes
1996 Ingibjørg Einarsdottir Dr. scient.
Production of Atlantic salmon (
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1996 Christina M. S. Pereira Dr. scient.
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1996 Jan Fredrik Børseth Dr. scient.
The sodium energy gradients in muscle cells of
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1996 Gunnar Henriksen Dr. scient.
Status of Grey seal
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1997 Gunvor Øie Dr. scient
Eevalution of rotifer
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1997 Håkon Holien Dr. scient
Studies of lichens in spurce forest of Central Norway.
Diversity, old growth species and the relationship to site
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1997 Ole Reitan Dr. scient.
Responses of birds to habitat disturbance due to
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Physiological effects of reduced water quality on fish in
1997 Per Gustav Thingstad Dr. scient.
Birds as indicators for studying natural and human-
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1997 Torgeir Nygård Dr. scient.
Temporal and spatial trends of pollutants in birds in
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1997 Signe Nybø Dr. scient.
Impacts of long-range transported air pollution on birds
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1997 Atle Wibe Dr. scient.
Identification of conifer volatiles detected by receptor
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1997 Rolv Lundheim Dr. scient.
Adaptive and incidental biological ice nucleators.
1997 Arild Magne Landa Dr. scient.
Wolverines in Scandinavia: ecology, sheep depredation
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1997 Kåre Magne Nielsen Dr. scient
An evolution of possible horizontal gene transfer from
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1997 Jarle Tufto Dr. scient.
Gene flow and genetic drift in geographically structured
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1997 Trygve Hesthagen Dr. philos.
Population responces of Arctic charr (
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1997 Trygve Sigholt Dr. philos.
Control of Parr-smolt transformation and seawater
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Cold sensation in adult and neonate birds
1998 Seethaledsumy
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Influence of environmental factors on myrosinases and
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1998 Thor Harald Ringsby Dr. scient.
Variation in space and time: The biology of a House
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1998 Erling Johan Solberg Dr. scient.
Variation in population dynamics and life history in a
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consequences of harvesting in a variable environment
1998 Sigurd Mjøen Saastad Dr. scient
Species delimitation and phylogenetic relationships
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1998 Bjarte Mortensen Dr. scient
Metabolism of volatile organic chemicals (VOCs) in a
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1998 Gunnar Austrheim Dr. scient
Plant biodiversity and land use in subalpine grasslands. –
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1998 Bente Gunnveig Berg Dr. scient.
Encoding of pheromone information in two related moth
1999 Kristian Overskaug Dr. scient.
Behavioural and morphological characteristics in
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1999 Hans Kristen Stenøien Dr. scient
Genetic studies of evolutionary processes in various
populations of nonvascular plants (mosses, liverworts and
1999 Trond Arnesen Dr. scient
Vegetation dynamics following trampling and burning in
the outlying haylands at Sølendet, Central Norway.
1999 Ingvar Stenberg Dr. scient.
Habitat selection, reproduction and survival in the White-
backed Woodpecker
Dendrocopos leucotos
1999 Stein Olle Johansen Dr. scient
A study of driftwood dispersal to the Nordic Seas by
dendrochronology and wood anatomical analysis.
1999 Trina Falck Galloway Dr. scient.
Muscle development and growth in early life stages of
the Atlantic cod (
Gadus morhua
L.) and Halibut
Hippoglossus hippoglossus
1999 Marianne Giæver Dr. scient.
Population genetic studies in three gadoid species: blue
whiting (
Micromisistius poutassou
), haddock
Melanogrammus aeglefinus
) and cod (
Gradus morhua
in the North-East Atlantic
1999 Hans Martin Hanslin Dr. scient
The impact of environmental conditions of density
dependent performance in the boreal forest bryophytes
Dicranum majus
Hylocomium splendens
Ptilium crista-castrensis
Rhytidiadelphus lokeus
1999 Ingrid Bysveen
Dr. scient.
Aspects of population genetics, behaviour and
performance of wild and farmed Atlantic salmon (
) revealed by molecular genetic techniques
1999 Else Berit Skagen Dr. scient
The early regeneration process in protoplasts from
Brassica na pus
hypocotyls cultivated under various g-
1999 Stein-Are Sæther Dr. philos.
Mate choice, competition for mates, and conflicts of
interest in the Lekking Great Snipe
1999 Katrine Wangen Rustad Dr. scient.
Modulation of glutamatergic neurotransmission related to
cognitive dysfunctions and Alzheimer’s disease
1999 Per Terje Smiseth Dr. scient.
Social evolution in monogamous families:
mate choice and conflicts over parental care in the
Bluethroat (
Luscinia s. svecica
1999 Gunnbjørn Bremset Dr. scient.
Young Atlantic salmon (