Ethnic differences in thermoregulation: Genotypic versus phenotypic heat adaptation

The effect of bi-iliac breadth on core body temperature

Thermoregulation is argued to be an important factor influencing body breadth in hominins based on the relationship of surface area to body mass first proposed by Bergmann. Selection for a narrow thorax, and thus a narrow pelvis, increases body surface area relative to body mass, which could be beneficial in hot climates if it leads to a decrease in core body temperature. However, the relationship between pelvic breadth and thermoregulation in humans has not been established. Although previous work has shown that bi-iliac breadth is significantly positively associated with latitude in humans, we lack an understanding of whether this association is due to climate-related selection, neutral evolutionary processes, or other selective pressures. A missing piece of the puzzle is whether body breadth at the iliac blades is an important factor in thermoregulation. Here, we examine this in a mixed-sex sample of 28 adult runners who ran for one hour at 3.14 m s-1 in a variety of climatic conditions while their core body temperatures were measured using internal temperature sensors. The association of maximum core temperature with anthropometric and demographic variables such as age, sex, mass, body fat percentage, and bi-iliac breadth was analyzed using a linear mixed-effect model. Due to the small sample size, the model was also bootstrapped. We found that an increase in absolute bi-iliac breadth was significantly associated with an increase in maximum core temperature. Overall, this preliminary analysis suggests a link between variation in bi-iliac breadth and maximum core body temperature during running, but further investigation is needed.

Coping with extreme heat: current exposure and implications for the future

A preview of how effective behavioral, biological and technological responses might be in the future, when outdoor conditions will be at least 2°C hotter than current levels, is available today from studies of individuals already living in extreme heat. In areas where high temperatures are common-particularly those in the hot and humid tropics-several studies report that indoor temperatures in low-income housing can be significantly hotter than those outdoors. A case study indicates that daily indoor heat indexes in almost all the 123 slum dwellings monitored in Kolkata during the summer were above 41°C (106°F) for at least an hour. Economic constraints make it unlikely that technological fixes, such as air conditioners, will remedy conditions like these-now or in the future. People without access to air conditioning will have to rely on behavioral adjustments and/or biological/physiological acclimatization. One important unknown is whether individuals who have lived their entire lives in hot environments without air conditioning possess natural levels of acclimatization greater than those indicated by controlled laboratory studies. Answering questions about the future will require more studies of heat conditions experienced by individuals, more information on indoor versus outdoor heat conditions, and a greater understanding of the behavioral and biological adjustments made by people living today in extremely hot conditions.

Myths and methodologies: Considerations for evaluating the time course of thermoregulatory adaptation during heat acclimation

Since the early 1900s, repeated heat exposure has been used as a method to induce physiological adaptations that enhance our ability to tolerate heat stress during athletic and occupational pursuits. Much of this work has been dedicated to quantifying the time course of adaptation and identifying the minimum duration of acclimation required to optimise performance or enhance safety. To achieve this, investigators have typically applied classical (constant load) heat acclimation, whereby 60-90 min exercise is performed at the same absolute or relative intensity in a hot environment for 3-24 days, with adaptations evaluated using an identical forcing function test before and after. This approach has provided a foundation from which to develop our understanding of changes in thermoregulatory function, but it has several, frequently overlooked shortcomings, which have resulted in misconceptions concerning the time course of adaptation. It is frequently suggested that most of the thermoregulatory adaptations during heat acclimation occur within a week, but this is an oversimplification and a predictable artefact of the experimental designs used. Consequently, the time course of complete human adaptation to heat remains poorly understood and appears to vary considerably due to numerous individual factors. The purpose of this communication is to highlight the key methodological considerations required when interpreting the existing literature documenting adaptation over time. We also propose potential means by which to improve the way we induce and quantify the magnitude of adaptation to expedite discovery.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 4: evolution, thermal adaptation and unsupported theories of thermoregulation

This review is the final contribution to a four-part, historical series on human exercise physiology in thermally stressful conditions. The series opened with reminders of the principles governing heat exchange and an overview of our contemporary understanding of thermoregulation (Part 1). We then reviewed the development of physiological measurements (Part 2) used to reveal the autonomic processes at work during heat and cold stresses. Next, we re-examined thermal-stress tolerance and intolerance, and critiqued the indices of thermal stress and strain (Part 3). Herein, we describe the evolutionary steps that endowed humans with a unique potential to tolerate endurance activity in the heat, and we examine how those attributes can be enhanced during thermal adaptation. The first of our ancestors to qualify as an athlete was Homo erectus, who were hairless, sweating specialists with eccrine sweat glands covering almost their entire body surface. Homo sapiens were skilful behavioural thermoregulators, which preserved their resource-wasteful, autonomic thermoeffectors (shivering and sweating) for more stressful encounters. Following emigration, they regularly experienced heat and cold stress, to which they acclimatised and developed less powerful (habituated) effector responses when those stresses were re-encountered. We critique hypotheses that linked thermoregulatory differences to ancestry. By exploring short-term heat and cold acclimation, we reveal sweat hypersecretion and powerful shivering to be protective, transitional stages en route to more complete thermal adaptation (habituation). To conclude this historical series, we examine some of the concepts and hypotheses of thermoregulation during exercise that did not withstand the tests of time.

A century of exercise physiology: concepts that ignited the study of human thermoregulation. Part 2: physiological measurements

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions.

Physiological, haematological and electroencephalographic responses to heat stress in Katjang and Boer goats

The present study aimed to determine the effect of repeated heat stress on serum levels of cortisol (CORT), acute phase proteins (APP) and heat shock protein (HSP) 70, haematological indicators, and electroencephalographic (EEG) response in the native Katjang and exotic Boer goats. Six female Katjang (15.7 kg ± 0.68) and six female Boer (16.8 kg ± 1.16) goats aged 5 to 6 months old were exposed to 38 ± 1℃ for 8 h, and the procedure was repeated at three different weeks (weeks 1, 2 and 3). Measurements of rectal temperatures and EEG activity and collection of blood samples were conducted before heat exposure (0 h), immediately after the heat exposure (8 h), and 8 h after completion of heat exposure (16 h) (recovery period). The current results revealed that the Boer animals had significantly higher rectal temperatures (RT), haemoglobin (Hb) and packed cell volume (PCV) counts than their Kajang counterparts. There were significant breed × stage of heat treatment (SHT) × week of heat treatment (WHT) interactions for neutrophil to lymphocyte ratios (NLR). In general, the Katjang animals had elevated NLR compared to those of Boer. The Boer goats had reduced capacity to express serum HSP70 compared to their Katjang counterparts following the heat challenge at week 3. Boer goats demonstrated higher delta waves than the Katjang group, which suggested the former were more stressed following the heat exposure. Breed had a negligible effect on CORT, APP, WBC counts and backfat thickness. Our findings suggested that the Katjang breed, as measured by RT, HB and PCV count, and EEG activity, could be more tolerant to heat stress than Boer. The Katjang goats showed higher HSP70 expression than their Boer counterparts, suggesting improved thermoregulation in the former.

Thermoregulation of Tuvan pastoralists and Western Europeans during cold exposure

OBJECTIVES

This study compared the metabolic and vascular responses, to whole-body and finger cold exposure, of a traditional population lifelong exposed to extreme cold winters with Western Europeans.

METHODS

Thirteen cold acclimatized Tuvan pastoralist adults (45 ± 9 years; 24.1 ± 3.2 kg/m2 ) and 13 matched Western European controls (43 ± 15 years; 22.6 ± 1.4 kg/m2 ) completed a whole-body cold (10°C) air exposure test and a cold-induced vasodilation (CIVD) test, which involved the immersion of the middle finger into ice-water for 30 min.

RESULTS

During the whole-body cold exposure, the durations until the onset of shivering for three monitored skeletal muscles were similar for both groups. Cold exposure increased the Tuvans' energy expenditure by (mean ± SD) 0.9 ± 0.7 kJ min-1 and the Europeans' by 1.3 ± 1.54 kJ min-1 ; these changes were not significantly different. The forearm-fingertip skin temperature gradient of the Tuvans was lower, indicating less vasoconstriction, than the Europeans during the cold exposure (0 ± 4.5°C vs. 8.8 ± 2.7°C). A CIVD response occurred in 92% of the Tuvans and 36% of the Europeans. In line, finger temperature during the CIVD test was higher in the Tuvans than the Europeans (13.4 ± 3.4°C vs. 3.9 ± 2.3°C).

CONCLUSION

Cold-induced thermogenesis and the onset of shivering were similar in both populations. However, vasoconstriction at the extremities was reduced in the Tuvans compared to the Europeans. The enhanced blood flow to the extremities could be beneficial for living in an extreme cold environment by improving dexterity, comfort, and reducing the risk of cold-injuries.

The relative value of sociocultural and infrastructural adaptations to heat in a very hot climate in northern Australia: a case time series of heat-associated mortality

BACKGROUND

Climate change is increasing heat-associated mortality particularly in hotter parts of the world. The Northern Territory is a large and sparsely populated peri-equatorial state in Australia. The Northern Territory has the highest proportion of Aboriginal and Torres Strait Islander people in Australia (31%), most of whom live in remote communities of over 65 Aboriginal Nations defined by ancient social, cultural, and linguistic heritage. The remainder non-Indigenous population lives mostly within the two urban centres (Darwin in the Top End region and Alice Springs in the Centre region of the Northern Territory). Here we aim to compare non-Indigenous (eg, high income) and Indigenous societies in a tropical environment and explore the relative importance of physiological, sociocultural, and technological and infrastructural adaptations to heat.

METHODS

In this case time series, we matched temperature at the time of death using a modified distributed lag non-linear model for all deaths in the Northern Territory, Australia, from Jan 1, 1980, to Dec 31, 2019. Data on deaths came from the national registry of Births, Deaths and Marriages. Cases were excluded if location or date of death were not recorded or if the person was a non-resident. Daily maximum and minimum temperature were measured and recorded by the Bureau of Meteorology. Hot weather was defined as mean temperature greater than 35°C over a 3-day lag. Socioeconomic status as indicated by Index of Relative Socioeconomic Disadvantage was mapped from location at death.

FINDINGS

During the study period, 34 782 deaths were recorded; after exclusions 31 800 deaths were included in statistical analysis (15 801 Aboriginal and 15 999 non-Indigenous). There was no apparent reduction in heat susceptibility despite infrastructural and technological improvements for the majority non-Indigenous population over the study period with no heat-associated mortality in the first two decades (1980-99; relative risk 1·00 [95% CI 0·87-1·15]) compared with the second two decades (2000-19; 1·14 [1·01-1·29]). Despite marked socioeconomic inequity, Aboriginal people are not more susceptible to heat mortality (1·05, [0·95-1·18]) than non-Indigenous people (1·18 [1·06-1·29]).

INTERPRETATION

It is widely believed that technological and infrastructural adaptations are crucial in preparing for hotter climates; however, this study suggests that social and cultural adaptations to increasing hot weather are potentially powerful mechanisms for protecting human health. Although cool shelters are essential during extreme heat, research is required to determine whether excessive exposure to air-conditioned spaces might impair physiological acclimatisation to the prevailing environment. Understanding sociocultural practices from past and ancient societies provides insight into non-technological adaptation opportunities that are protective of health.

FUNDING

None.

How humans adapt to hot climates learned from the recent research on tropical indigenes

This review mainly aimed to introduce the findings of research projects comparing the responses of tropical and temperate indigenes to heat. From a questionnaire survey on thermal sensation and comfort of Indonesians and Japanese, we found that the thermal descriptor “cool” in tropical indigenes connotes a thermally comfortable feeling, suggesting that linguistic heat acclimatization exists on a cognitive level. Ten male students born and raised in Malaysia were invited to Fukuoka, Japan, and compared their responses with 10 Japanese male students with matched physical fitness and morphological characteristics. Cutaneous thermal sensitivity: The sensitivities were measured at 28 °C. The forehead warm sensitivity was significantly blunted in Malaysians. The less sensitivity to the warmth of tropical indigenes is advantageous in respect to withstanding heat stress with less discomfort and a greater ability to work in hot climates. Passive heat stress: Thermoregulatory responses, especially sweating, were investigated, during the lower leg hot bathing (42 °C for 60 min). The rectal temperature at rest was higher in Malaysians and increased smaller during immersion. There was no significant difference in the total amount of sweating between the two groups, while the local sweating on the forehead and thighs was lesser in Malaysians, suggesting distribution of sweating was different from Japanese. Exercise: Malaysian showed a significantly smaller increase in their rectal temperature during 55% maximal exercise for 60 min in heat (32 °C 70% relative humidity), even with a similar sweating and skin blood flow response in Japanese. The better heat tolerance in Malaysians could be explained by the greater convective heat transfer from the body core to the skin due to the greater core-to-skin temperature gradient. In addition, when they were hydrated, Malaysian participants showed better body fluid regulation with smaller reduction in plasma volume at the end of the exercise compared to the non-hydrated condition, whereas Japanese showed no difference between hydration conditions. We further investigated the de-acclimatization of heat adaptation by longitudinal observation on the heat tolerance of international students who had moved from tropical areas to Fukuoka for several years.

Human total fertility rate affected by ambient temperatures in both the present and previous generations

Elevated temperatures negatively affect human reproduction through several processes that regulate nutrient uptake and resource allocation in pregnant women. These can interfere with foetal development, resulting in low birth weight neonates with altered development trajectories. Temperatures that affect the current generation could, therefore, also have an impact on the following generation. We asked whether heat stress affected offspring fertility by asking if current and past ambient temperatures influenced total fertility rates (TFR) in human populations distributed across the world. We analysed time series data in 65 countries using simple regression analyses based on maximum temperatures and temperature amplitudes over 55 years. Supplemental longer time series (up to 100 years) provided information on response patterns in Northern Europe and Greenland's colder climates. There were clear and strong effects of temperatures on the TFR in the concurrent and the previous generation. Our temperature-based models account for 71-95% of the variation in TRF in European countries and Greenland, and 56-99% of the variation in 65 countries worldwide. Our findings are consistent with studies of seasonal variation in fertility and suggest that increased temperatures will negatively influence populations subjected to monthly maximum temperatures above 15-20 °C, while fertility in colder climates benefits from elevated temperatures. Our results provide strong evidence that ambient temperatures have important effects on human fertility, and that these effects persist into the following generation.

Computer modelling of heat strain responses of exercising personnel in tropical climate

Physiologically based thermoregulatory models are useful for deriving predictions of heat strain for pragmatic applications such as planning of continuous exercise/work-rest protocols. The SCENARIO model is an example of a thermoregulatory model that predicts heat strain including body core temperature (Tc) from individual characteristics, physical activity, clothing properties and environmental conditions. This paper presents work to optimize and enhance the SCENARIO model for prediction of Tc during high intensity load carriage tasks under predominantly tropical climate conditions. Data for model optimization (in-sample analysis) and model external validation were derived from four and two load carriage studies respectively. A total of four parameters characterizing metabolic heat production, sweat evaporation and ice ingestion for hydration were identified for model optimization based on physiological reasoning. The accuracy of Tc estimates was evaluated based on bias, root mean square deviation (RMSD), RMSD based on mean values (RMSD-Mean), and standard deviation fall-in percentage (SDP). Under in-sample analysis, the optimized model achieved bias, RMSD, RMSD-Mean and SDP of 0.01°C, 0.39°C, 0.14°C and 99%, respectively. When externally validated against two sets of unseen data, the model achieved comparable bias, RMSD, RMSD-Mean and SDP values of 0.06°C, 0.32°C, 0.13°C, 92% and 0.08°C, 0.39°C, 0.19°C, 92%, respectively. Overall, the results indicate the robustness of the optimized SCENARIO model for predicting the Tc responses during prolonged, high-intensity physical tasks under hot and humid environments. Future work to further validate the model against data beyond the range of the present study's experimental data and enhancing it for more accurate simulations of other heat strain markers including heart rate is recommended.

Physiological interactions with personal-protective clothing, physically demanding work and global warming: An Asia-Pacific perspective

The Asia-Pacific contains over half of the world's population, 21 countries have a Gross Domestic Product <25% of the world's largest economy, many countries have tropical climates and all suffer the impact of global warming. That 'perfect storm' exacerbates the risk of occupational heat illness, yet first responders must perform physically demanding work wearing personal-protective clothing and equipment. Unfortunately, the Eurocentric emphasis of past research has sometimes reduced its applicability to other ethnic groups. To redress that imbalance, relevant contemporary research has been reviewed, to which has been added information applicable to people of Asian, Melanesian and Polynesian ancestry. An epidemiological triad is used to identify the causal agents and host factors of work intolerance within hot-humid climates, commencing with the size dependency of resting metabolism and heat production accompanying load carriage, followed by a progression from the impact of single-layered clothing through to encapsulating ensembles. A morphological hypothesis is presented to account for inter-individual differences in heat production and heat loss, which seems to explain apparent ethnic- and gender-related differences in thermoregulation, at least within thermally compensable states. The mechanisms underlying work intolerance, cardiovascular insufficiency and heat illness are reviewed, along with epidemiological data from the Asia-Pacific. Finally, evidence-based preventative and treatment strategies are presented and updated concerning moisture-management fabrics and barriers, dehydration, pre- and post-exercise cooling, and heat adaptation. An extensive reference list is provided, with >25 recommendations enabling physiologists, occupational health specialists, policy makers, purchasing officers and manufacturers to rapidly extract interpretative outcomes pertinent to the Asia-Pacific.

A comparison of the effect of weather and climate on emergency department visitation in Roanoke and Charlottesville, Virginia

Compared with mortality, the impact of weather and climate on human morbidity is less well understood, especially in the cold season. We examined the relationships between weather and emergency department (ED) visitation at hospitals in Roanoke and Charlottesville, Virginia, two locations with similar climates and population demographic profiles. Using patient-level data obtained from electronic medical records, each patient who visited the ED was linked to that day's weather from one of 8 weather stations in the region based on each patient's ZIP code of residence. The resulting 2010-2017 daily ED visit time series were examined using a distributed lag non-linear model to account for the concurrent and lagged effects of weather. Total ED visits were modeled separately for each location along with subsets based on gender, race, and age. The relationship between the relative risk of ED visitation and temperature or apparent temperature over lags of one week was positive and approximately linear at both locations. The relative risk increased about 5% on warm, humid days in both cities (lag 0 or lag 1). Cold conditions had a protective effect, with up to a 15% decline on cold days, but ED visits increased by 4% from 2 to 5 days after the cold event. The effect of thermal extremes tended to be larger for non-whites and the elderly, and there was some evidence of a greater lagged response for non-whites in Roanoke. Females in Roanoke were more impacted by winter cold conditions than males, who were more likely to show a lagged response at high temperatures. In Charlottesville, males sought ED attention at lower temperatures than did females. The similarities in the ED response patterns between these two hospitals suggest that certain aspects of the response may be generalizable to other locations that have similar climates and demographic profiles.

Contribution of nitric oxide synthase to cutaneous vasodilatation and sweating in men of black-African and Caucasian descent during exercise in the heat

NEW FINDINGS

What is the central question of this study? Nitric oxide modulates cutaneous vasodilatation and sweating during exercise-induced heat stress in young men. However, it remains uncertain whether these effects are reduced in black-African descendants, who commonly demonstrate reduced nitric oxide bioavailability. Therefore, we assessed whether black-African descendants display reduced nitric oxide-dependent cutaneous vasodilatation and sweating compared with Caucasians in these conditions. What is the main finding and its importance? Nitric oxide-dependent cutaneous vasodilatation and sweating were similar between groups, indicating that reduced nitric oxide bioavailability in black-African descendants does not attenuate these heat-loss responses during an exercise-induced heat stress.

ABSTRACT

Men of black-African descent are at an increased risk of heat-related illness relative to their Caucasian counterparts. This might be attributable, in part, to reduced cutaneous nitric oxide (NO) bioavailability in this population, which might alter local cutaneous vasodilatation and sweating. To evaluate this, we compared these heat-loss responses in young men (18-30 years of age) of black-African (n = 10) and Caucasian (n = 10) descent during rest, exercise and recovery in the heat. Participants were matched for physical characteristics and fitness, and they were all born and raised in the same temperate environment (i.e. Canada; second generation and higher). Both groups rested for 10 min and then performed 50 min of moderate-intensity exercise at 200 W m^-2 , followed by 30 min of recovery in hot, dry heat (35°C, 20% relative humidity). Local cutaneous vascular conductance (CVC_%max ) and sweat rate (SR) were measured at two forearm skin sites treated with either lactated Ringer solution (control) or 10 mm N^G -nitro-l-arginine methyl ester (l-NAME, a nitric oxide (NO) synthase inhibitor). l-NAME significantly reduced CVC_%max throughout rest, exercise and recovery in both groups (both P < 0.001). However, there were no significant main effects for the contribution of NO to CVC_%max between groups (all P > 0.500). l-NAME significantly reduced local SR in both groups (both P < 0.050). The contribution of NO to SR was similar between groups such that l-NAME reduced SR relative to control at 40 and 50 min into exercise (both P < 0.05). We demonstrate that ethnicity per se does not influence NO-dependent cutaneous vasodilatation and sweating in healthy young men of black-African and Caucasian descent during exercise in dry heat.

Diversity and evolution of human eccrine sweat gland density

The human eccrine sweat gland is central to the evolution of the human genus, permitting an enormous thermoregulatory sweating capacity that was essential to the human niche of high physical activity in open, hot, semi-arid environments. Despite a century of research inventorying the structure and function of eccrine glands and the physiological responses of human heat acclimation, we do not have a clear understanding of how intraspecific differences in eccrine density affect thermoregulation. Similarly, existing data does not comprehensively catalogue modern human diversity in this trait, nor do we understand the relative influences of evolutionary forces and phenotypic plasticity in shaping this diversity.

Drivers of diversity in human thermal perception - A review for holistic comfort models

Understanding the drivers leading to individual differences in human thermal perception has become increasingly important, amongst other things due to challenges such as climate change and an ageing society. This review summarizes existing knowledge related to physiological, psychological, and context-related drivers of diversity in thermal perception. Furthermore, the current state of knowledge is discussed in terms of its applicability in thermal comfort models, by combining modelling approaches of the thermoneutral zone (TNZ) and adaptive thermal heat balance model (ATHB). In conclusion, the results of this review show the clear contribution of some physiological and psychological factors, such as body composition, metabolic rate, adaptation to certain thermal environments and perceived control, to differences in thermal perception. However, the role of other potential diversity-causing parameters, such as age and sex, remain uncertain. Further research is suggested, especially regarding the interaction of different diversity-driving factors with each other, both physiological and psychological, to help establishing a holistic picture.

Heat acclimatization blunts copeptin responses to hypertonicity from dehydrating exercise in humans

Acclimatization favors greater extracellular tonicity from lower sweat sodium, yet hyperosmolality may impair thermoregulation during heat stress. Enhanced secretion or action of vasopressin could mitigate this through increased free water retention. Aims were to determine responses of the vasopressin surrogate copeptin to dehydrating exercise and investigate its relationships with tonicity during short and long-term acclimatization. Twenty-three participants completed a structured exercise programme following arrival from a temperate to a hot climate. A Heat Tolerance Test (HTT) was conducted on Day-2, 6, 9 and 23, consisting of 60-min block-stepping at 50% VO2 peak, with no fluid intake. Resting sweat [Na+ ] was measured by iontophoresis. Changes in body mass (sweat loss), core temperature, heart rate, osmolality (serum and urine) and copeptin and aldosterone (plasma) were measured with each Test. From Day 2 to Day 23, sweat [Na+ ] decreased significantly (adjusted P < 0.05) and core temperature and heart rate fell. Over the same interval, HTT-associated excursions were increased for serum osmolality (5 [-1, 9] vs. 9 [5, 12] mosm·kg-1 ), did not differ for copeptin (9.6 [6.0, 15.0] vs. 7.9 [4.3, 14.7] pmol·L-1 ) and were reduced for aldosterone (602 [415, 946] vs. 347 [263, 537] pmol·L-1 ). Urine osmolality was unchanging and related consistently to copeptin at end-exercise, whereas the association between copeptin and serum osmolality was right-shifted (P = 0.0109) with acclimatization. Unchanging urine:serum osmolality argued against increased renal action of vasopressin. In conclusion, where exercise in the heat is performed without fluid replacement, heat acclimatization does not appear to enhance AVP-mediated free water retention in humans.

Extreme Terrestrial Environments: Life in Thermal Stress and Hypoxia. A Narrative Review

Living, working and exercising in extreme terrestrial environments are challenging tasks even for healthy humans of the modern new age. The issue is not just survival in remote environments but rather the achievement of optimal performance in everyday life, occupation, and sports. Various adaptive biological processes can take place to cope with the specific stressors of extreme terrestrial environments like cold, heat, and hypoxia (high altitude). This review provides an overview of the physiological and morphological aspects of adaptive responses in these environmental stressors at the level of organs, tissues, and cells. Furthermore, adjustments existing in native people living in such extreme conditions on the earth as well as acute adaptive responses in newcomers are discussed. These insights into general adaptability of humans are complemented by outcomes of specific acclimatization/acclimation studies adding important information how to cope appropriately with extreme environmental temperatures and hypoxia.

Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability

Athletes lose water and electrolytes as a consequence of thermoregulatory sweating during exercise and it is well known that the rate and composition of sweat loss can vary considerably within and among individuals. Many scientists and practitioners conduct sweat tests to determine sweat water and electrolyte losses of athletes during practice and competition. The information gleaned from sweat testing is often used to guide personalized fluid and electrolyte replacement recommendations for athletes; however, unstandardized methodological practices and challenging field conditions can produce inconsistent/inaccurate results. The primary objective of this paper is to provide a review of the literature regarding the effect of laboratory and field sweat-testing methodological variations on sweating rate (SR) and sweat composition (primarily sodium concentration [Na⁺]). The simplest and most accurate method to assess whole-body SR is via changes in body mass during exercise; however, potential confounding factors to consider are non-sweat sources of mass change and trapped sweat in clothing. In addition, variability in sweat [Na⁺] can result from differences in the type of collection system used (whole body or localized), the timing/duration of sweat collection, skin cleaning procedure, sample storage/handling, and analytical technique. Another aim of this paper is to briefly review factors that may impact intra/interindividual variability in SR and sweat [Na⁺] during exercise, including exercise intensity, environmental conditions, heat acclimation, aerobic capacity, body size/composition, wearing of protective equipment, sex, maturation, aging, diet, and/or hydration status. In summary, sweat testing can be a useful tool to estimate athletes’ SR and sweat Na⁺ loss to help guide fluid/electrolyte replacement strategies, provided that data are collected, analyzed, and interpreted appropriately.

The evolution of eccrine sweat glands in human and nonhuman primates

Sweating is an unusual thermoregulatory strategy for most mammals, yet is critical for humans. This trait is commonly hypothesized to result from human ancestors moving from a forest to a warmer and drier open environment. As soft tissue traits do not typically fossilize, this idea has been difficult to test. Therefore, we used a comparative approach to examine 15 eccrine gland traits from 35 primate species. For each trait we measured phylogenetic signal, tested three evolutionary models to explain trait variation, and used phylogenetic models to examine how traits varied in response to climate variables. Phylogenetic signal in traits varied substantially, with the two traits exhibiting the highest values being gland distribution on the body and percent eccrine vs. apocrine glands on the body. Variation in most traits was best explained by an Ornstein-Uhlenbeck model suggesting the importance of natural selection. Two traits were strongly predicted by climate. First, species with high eccrine gland glycogen content were associated with habitats exhibiting warm temperatures and low rainfall. Second, species with increased capillarization were associated with high temperature. Glycogen is a primary energy substrate powering sweat production and sodium reabsorption in the eccrine gland, and increased capillarization permits greater oxygen, glucose and electrolyte delivery. Thus, our results are evidence of natural selection for increased sweating capacity in primate species with body surface eccrine glands living in hot and dry climates. We suggest that selection for increased glycogen content and capillarization may have been part of initial increases in hominin thermoregulatory sweating capacity.

Heart rate variability responses to acute and repeated postexercise sauna in trained cyclists

Short- to medium-term (i.e., 4-14 days) heating protocols induce physiological adaptations including improved cardiac autonomic modulations, as assessed using heart rate variability, which may contribute to greater exercise performance. Whether similar cardiac autonomic changes occur during an intense heating protocol (sauna) reported to increase plasma volume in athletes remains to be confirmed. This study examined changes in heart rate and its variability during a single extreme heat (sauna) exposure and repeated exposures in athletes. Six well-trained male cyclists undertook sauna bathing (30 min, 87 °C, 11% relative humidity) immediately after normal training over 10 consecutive days. Heart rate recordings were obtained during each sauna bout. Heart rate and its variability (natural logarithm of root mean square of successive differences, lnRMSSD) were analysed during 10-min periods within the first bout, and changes in heart rate and lnRMSSD were analysed during each bout via magnitude-based inferences. During the first sauna bout, heart rate was almost certainly increased (∼32%, effect size 1.68) and lnRMSSD was almost certainly reduced (∼62%, effect size -5.21) from the first to the last 10-min period, indicating reduced parasympathetic and (or) enhanced sympathetic modulations. Acute exposure to extreme heat stress via sauna produced alterations in heart rate and cardiac autonomic modulations with successive postexercise heat exposures producing unclear changes over a 10-day period. The physiological benefits of intense heating via sauna on cardiac control in athletes remain to be elucidated.

Effect of portable non-invasive ventilation & environmental conditions on everyday activities

The current study examined the effect of non-invasive ventilation (NIV) within environments of differing temperature and humidity on several physiological and perceptual responses while performing six activities of daily living (i.e. putting on shirt/shoes/trousers, vacuuming, hanging towels, and walking on a treadmill). Sixteen healthy participants completed the activities of varying difficulty within four experimental conditions: with and without NIV; and in temperate (22°C, 40% relative humidity) and hot-humid environments (32°C, 70% relative humidity). Comparisons of physiological responses between conditions were examined via repeated measures ANOVAs. Overall, NIV resulted in similar physiological and perceptual responses within all environmental conditions for healthy participants. Further, NIV use increased heart rate during the most strenuous task (29.5±12.7 vs. 22.8±12.0bpm, p=0.008) indicating NIV use may stress cardiovascular functioning during moderate-high intensity activities. Tropical conditions did not alter physiological or perceptual responses during everyday tasks with NIV use by healthy adults. Future investigations examining the independent and combined impacts of task intensity, extreme environments and NIV use will clarify the benefits of NIV for healthy and clinical populations.

Heat acclimation: Gold mines and genes

The underground gold mines of South Africa offer a unique historical setting to study heat acclimation. The early heat stress research was conducted and described by a young medical officer, Dr. Aldo Dreosti. He developed practical and specific protocols to first assess the heat tolerance of thousands of new mining recruits, and then used the screening results as the basis for assigning a heat acclimation protocol. The mines provide an interesting paradigm where the prevention of heat stroke evolved from genetic selection, where only Black natives were recruited due to a false assumption of their intrinsic tolerance to heat, to our current appreciation of the epigenetic and other molecular adaptations that occur with exposure to heat.

Absolute humidity and the human nose: A reanalysis of climate zones and their influence on nasal form and function

OBJECTIVES

Investigations into the selective role of climate on human nasal variation commonly divide climates into four broad adaptive zones (hot-dry, hot-wet, cold-dry, and cold-wet) based on temperature and relative humidity. Yet, absolute humidity-not relative humidity-is physiologically more important during respiration. Here, we investigate the global distribution of absolute humidity to better clarify ecogeographic demands on nasal physiology.

METHODS

We use monthly observations from the Climatic Research Unit Timeseries 3 (CRU TS3) database to construct global maps of average annual temperature, relative humidity and absolute humidity. Further, using data collected by Thomson and Buxton (1923) for over 15,000 globally-distributed individuals, we calculate the actual amount of heat and water that must be transferred to inspired air in different climatic regimes to maintain homeostasis, and investigate the influence of these factors on the nasal index.

RESULTS

Our results show that absolute humidity, like temperature, generally decreases with latitude. Furthermore, our results demonstrate that environments typically characterized as "cold-wet" actually exhibit low absolute humidities, with values virtually identical to cold-dry environments and significantly lower than hot-wet and even hot-dry environments. Our results also indicate that strong associations between the nasal index and absolute humidity are, potentially erroneously, predicated on individuals from hot-dry environments possessing intermediate (mesorrhine) nasal indices.

DISCUSSION

We suggest that differentially allocating populations to cold-dry or cold-wet climates is unlikely to reflect different selective pressures on respiratory physiology and nasal morphology-it is cold-dry, and to a lesser degree hot-dry environments, that stress respiratory function. Our study also supports assertions that demands for inspiratory modification are reduced in hot-wet environments, and that expiratory heat elimination for thermoregulation is a greater selective pressure in such environments.

Load carriage, human performance, and employment standards

The focus of this review is on the physiological considerations necessary for developing employment standards within occupations that have a heavy reliance on load carriage. Employees within military, fire fighting, law enforcement, and search and rescue occupations regularly work with heavy loads. For example, soldiers often carry loads >50 kg, whilst structural firefighters wear 20–25 kg of protective clothing and equipment, in addition to carrying external loads. It has long been known that heavy loads modify gait, mobility, metabolic rate, and efficiency, while concurrently elevating the risk of muscle fatigue and injury. In addition, load carriage often occurs within environmentally stressful conditions, with protective ensembles adding to the thermal burden of the workplace. Indeed, physiological strain relates not just to the mass and dimensions of carried objects, but to how those loads are positioned on and around the body. Yet heavy loads must be borne by men and women of varying body size, and with the expectation that operational capability will not be impinged. This presents a recruitment conundrum. How do employers identify capable and injury-resistant individuals while simultaneously avoiding discriminatory selection practices? In this communication, the relevant metabolic, cardiopulmonary, and thermoregulatory consequences of loaded work are reviewed, along with concomitant impediments to physical endurance and mobility. Also emphasised is the importance of including occupation-specific clothing, protective equipment, and loads during work-performance testing. Finally, recommendations are presented for how to address these issues when evaluating readiness for duty.

The impact of acclimatization on thermophysiological strain for contrasting regional climates

During acclimatization to heat and cold, the body experiences additional thermally induced physiological strain. The first signs show up in the respiratory organs because respiration is a continuous heat exchange process in which the body is in closest contact with the ambient air. There are no behavioral or other adjustments to prevent the ambient air from entering into the body's core area through the respiratory tract. The Acclimatization Thermal Strain Index (ATSI) describes the acclimatization thermal loading (ATL) on respiratory organs until full adaptation is achieved. The aim here is to further assess the ATSI scheme using a range of actual but contrasting bioclimatic conditions. To simulate ATL, scenarios of the consequences of acclimatization due to movement to or from five contrasting climates are used. The results show that adjusting to cold comes with greater physiological strain than adjusting to heat, the biggest impact occurring for a change of location from hot-humid to cold-dry climatic conditions. The approach can be used to assess risks due to increases in short-term thermal variability in weather conditions such as encountered during heat waves and cold snaps. The information could also be useful for assessing the need for public health services and measures that might be used to help mitigate impacts.

Racial and socioeconomic disparities in heat-related health effects and their mechanisms: a review

Adaptation to increasing extreme heat in a changing climate requires a precise understanding of who is most vulnerable to the health effects of extreme heat. The evidence for race, ethnicity, income, education and occupation, at the individual and area levels, as indicators of vulnerability is reviewed. The evidence for the social, behavioral and technological mechanisms by which racial and socioeconomic disparities in vulnerability exist is also reviewed. These characteristics include cardiorespiratory, renal and endocrine comorbidities; cognitive, mental or physical disabilities; medication use; housing characteristics; neighborhood characteristics such as urban heat islands, crime and safety; social isolation; and individual behaviors such as air conditioning use, opening windows and using fans and use of cooler public spaces. Pre-existing and future research identifying these more proximal indicators of vulnerability will provide information that is more generalizable across locations and time to aid in identifying who to target for prevention of heat-associated morbidity and mortality.

Time course of physiological and psychological responses in humans during a 20-day severe-cold-acclimation programme

The time course of physiological and psychological markers during cold acclimation (CA) was explored. The experiment included 17 controlled (i.e., until the rectal temperature reached 35.5°C or 170 min had elapsed; for the CA-17 session, the subjects (n = 14) were immersed in water for the same amount of time as that used in the CA-1 session) head-out water immersions at a temperature of 14°C over 20 days. The data obtained in this study suggest that the subjects exhibited a thermoregulatory shift from peripheral-to-central to solely central input thermoregulation, as well as from shivering to non-shivering thermogenesis throughout the CA. In the first six CA sessions, a hypothermic type of acclimation was found; further CA (CA-7 to CA-16) led to a transitional shift to a hypothermic-insulative type of acclimation. Interestingly, when the subjects were immersed in water for the same time as that used in the CA-1 session (CA-17), the CA led to a hypothermic type of acclimation. The presence of a metabolic type of thermogenesis was evident only under thermoneutral conditions. Cold-water immersion decreased the concentration of cold-stress markers, reduced the activity of the innate immune system, suppressed specific immunity to a lesser degree and yielded less discomfort and cold sensation. We found a negative correlation between body mass index and Δ metabolic heat production before and after CA.

A comparison of hydration effect on body fluid and temperature regulation between Malaysian and Japanese males exercising at mild dehydration in humid heat

BACKGROUND

This study investigated the effect of hydration differences on body fluid and temperature regulation between tropical and temperate indigenes exercising in the heat.

METHODS

Ten Japanese and ten Malaysian males with matched physical characteristics (height, body weight, and peak oxygen consumption) participated in this study. Participants performed exercise for 60 min at 55% peak oxygen uptake followed by a 30-min recovery at 32°C and 70% relative air humidity with hydration (4 times each, 3 mL per kg body weight, 37°C) or without hydration. Rectal temperature, skin temperature, heart rate, skin blood flow, and blood pressure were measured continuously. The percentage of body weight loss and total sweat loss were calculated from body weight measurements. The percentage change in plasma volume was estimated from hemoglobin concentration and hematocrit.

RESULTS

Malaysian participants had a significantly lower rectal temperature, a smaller reduction in plasma volume, and a lower heart rate in the hydrated condition than in the non-hydrated condition at the end of exercise (P <0.05), whereas Japanese participants showed no difference between the two hydration conditions. Hydration induced a greater total sweat loss in both groups (P <0.05), and the percentage of body weight loss in hydrated Malaysians was significantly less than in hydrated Japanese (P <0.05). A significant interaction between groups and hydration conditions was observed for the percentage of mean cutaneous vascular conductance during exercise relative to baseline (P <0.05).

CONCLUSIONS

The smaller reduction in plasma volume and percentage body weight loss in hydrated Malaysians indicated an advantage in body fluid regulation. This may enable Malaysians to reserve more blood for circulation and heat dissipation and thereby maintain lower rectal temperatures in a hydrated condition.

Regional variations in transepidermal water loss, eccrine sweat gland density, sweat secretion rates and electrolyte composition in resting and exercising humans

Literature from the past 168 years has been filtered to provide a unified summary of the regional distribution of cutaneous water and electrolyte losses. The former occurs via transepidermal water vapour diffusion and secretion from the eccrine sweat glands. Daily insensible water losses for a standardised individual (surface area 1.8 m2) will be 0.6-2.3 L, with the hands (80-160 g.h-1) and feet (50-150 g.h-1) losing the most, the head and neck losing intermediate amounts (40-75 g.h-1) and all remaining sites losing 15-60 g.h-1. Whilst sweat gland densities vary widely across the skin surface, this same individual would possess some 2.03 million functional glands, with the highest density on the volar surfaces of the fingers (530 glands.cm-2) and the lowest on the upper lip (16 glands.cm-2). During passive heating that results in a resting whole-body sweat rate of approximately 0.4 L.min-1, the forehead (0.99 mg.cm-2.min-1), dorsal fingers (0.62 mg.cm-2.min-1) and upper back (0.59 mg.cm-2.min-1) would display the highest sweat flows, whilst the medial thighs and anterior legs will secrete the least (both 0.12 mg.cm-2.min-1). Since sweat glands selectively reabsorb electrolytes, the sodium and chloride composition of discharged sweat varies with secretion rate. Across whole-body sweat rates from 0.72 to 3.65 mg.cm-2.min-1, sodium losses of 26.5-49.7 mmol.L-1 could be expected, with the corresponding chloride loss being 26.8-36.7 mmol.L-1. Nevertheless, there can be threefold differences in electrolyte losses across skin regions. When exercising in the heat, local sweat rates increase dramatically, with regional glandular flows becoming more homogeneous. However, intra-regional evaporative potential remains proportional to each local surface area. Thus, there is little evidence that regional sudomotor variations reflect an hierarchical distribution of sweating either at rest or during exercise.

The use of language to express thermal sensation suggests heat acclimatization by Indonesian people

The purpose of this study was to explore whether there is evidence of heat acclimatization in the words used to express thermal sensation. A total of 458 urban Japanese and 601 Indonesians participated in a questionnaire. In addition, in a preliminary survey, 39 native English speakers in the UK participated. Our results showed that (1) for Indonesians, the closest thermal descriptor of a feeling of thermal comfort was 'cool' (75%) followed by 'slightly cool' (7%), 'slightly cold' (5%) and 'cold' (5%), while Japanese responses were distributed uniformly among descriptors 'cool', 'slightly cool', 'neither', 'slightly warm', and 'warm'; (2) the closest thermal descriptors of a feeling of discomfort for Indonesians were less affected by individual thermal susceptibility (vulnerability) than those for Japanese; (3) in the cases where 'cool' and 'slightly cold' were imagined in the mind, the descriptors were cognized as a thermal comfortable feeling by 97% and 57% of Indonesians, respectively; (4) the most frequently voted choice endorsing hot weather was 'higher than 32°C' for Indonesians and 'higher than 29°C' for Japanese respondents; for cold weather, 'lower than 15°C' for Japanese and 'lower than 20°C' for Indonesians. In summary, the descriptor 'cool' in Indonesians connotes a thermally comfortable feeling, but the inter-zone between hot and cold weather that was judged in the mind showed a upward shift when compared to that of Japanese. It is suggested that linguistic heat acclimatization exists on a cognitive level for Indonesians and is preserved in the words of thermal descriptors.

Effects of duration of stay in temperate area on thermoregulatory responses to passive heat exposure in tropical south-east Asian males residing in Japan

BACKGROUND

In this study, we investigated the effects of duration of stay in a temperate area on the thermoregulatory responses to passive heat exposure of residents from tropical areas, particularly to clarify whether they would lose their heat tolerance during passive heat exposure through residence in a temperate country, Japan.

METHODS

We enrolled 12 males (mean ± SE age 25.7 ± 1.3 years) from south-east Asian countries who had resided in Japan for a mean of 24.5 ± 5.04 months, and 12 Japanese males (age 24.1 ± 0.9 years). Passive heat exposure was induced through leg immersion in hot water (42°C) for 60 minutes under conditions of 28°C air temperature and 50% relative humidity.

RESULTS

Compared with the Japanese group, the tropical group displayed a higher pre-exposure rectal temperature (P < 0.01) and a smaller increase in rectal temperature during 60 minutes of leg immersion (P = 0.03). Additionally, the tropical group showed a tendency towards a lower total sweat rate (P = 0.06) and lower local sweat rate on the forehead (P = 0.07). The tropical group also had a significantly longer sweating onset time on the upper back (P = 0.04) compared with the Japanese groups. The tropical group who stayed in Japan for > 23 months sweated earlier on the forehead and upper back than those who stayed in Japan < 11 months (P < 0.01 and P = 0.03 for the forehead and upper back, respectively). There was a positive correlation between duration of stay in Japan and total sweat rate (r = 0.58, P <0.05), and negative correlations between duration of stay and sweating onset time on the forehead (r = -0.73, P = 0.01) and on the upper back (r = -0.66, P = 0.02). Other physiological indices measured in this study did not show any difference between the subjects in the tropical group who had lived in Japan for a shorter time and those who had lived there for a longer time.

CONCLUSIONS

We conclude that the nature of heat acclimatization of the sweating responses to passive heat exposure that are acquired from long-term heat acclimatization is decayed by a stay in a temperate area, as shown by the subjects in our tropical group. We did not find any evidence of a decay in the other physiological indices, indicating that heat tolerance acquired from long-term heat acclimatization is not completely diminished through residence in a temperate area for less than 4 years, although some aspects of this heat tolerance may be decayed.

Comparison of heat dissipation response between Malaysian and Japanese males during exercise in humid heat stress

This study investigated the differences in heat dissipation response to intense heat stress during exercise in hot and humid environments between tropical and temperate indigenes with matched physical characteristics. Ten Japanese (JP) and ten Malaysian (MY) males participated in this study. Subjects performed exercise for 60 min at 55% peak oxygen uptake in 32°C air with 70% relative humidity, followed by 30 min recovery. The increase in rectal temperature (T(re)) was smaller in MY during exercise compared to JP. The local sweat rate and total body mass loss were similar in both groups. Both skin blood flow and mean skin temperature was lower in MY compared to JP. A significantly greater increase in hand skin temperature was observed in MY during exercise, which is attributable to heat loss due to the greater surface area to mass ratio and large number of arteriovenous anastomoses. Also, the smaller increase in T(re) in MY may be explained by the presence of a significantly greater core-skin temperature gradient in MY than JP. The thermal gradient is also a major factor in increasing the convective heat transfer from core to skin as well as skin blood flow. It is concluded that the greater core-skin temperature gradient observed in MY is responsible for the smaller increase in T(re).

Comparison of thermoregulatory responses to heat between Malaysian and Japanese males during leg immersion

The objective of this study was to investigate thermoregulatory responses to heat in tropical (Malaysian) and temperate (Japanese) natives, during 60 min of passive heating. Ten Japanese (mean ages: 20.8 ± 0.9 years) and ten Malaysian males (mean ages: 22.3 ± 1.6 years) with matched morphological characteristics and physical fitness participated in this study. Passive heating was induced through leg immersion in hot water (42°C) for 60 min under conditions of 28°C air temperature and 50% RH. Local sweat rate on the forehead and thigh were significantly lower in Malaysians during leg immersion, but no significant differences in total sweat rate were observed between Malaysians (86.3 ± 11.8 g m(-2) h(-1)) and Japanese (83.2 ± 6.4 g m(-2) h(-1)) after leg immersion. In addition, Malaysians displayed a smaller rise in rectal temperature (0.3 ± 0.1°C) than Japanese (0.7 ± 0.1°C) during leg immersion, with a greater increase in hand skin temperature. Skin blood flow was significantly lower on the forehead and forearm in Malaysians during leg immersion. No significant different in mean skin temperature during leg immersion was observed between the two groups. These findings indicated that regional differences in body sweating distribution might exist between Malaysians and Japanese during heat exposure, with more uniform distribution of local sweat rate over the whole body among tropical Malaysians. Altogether, Malaysians appear to display enhanced efficiency of thermal sweating and thermoregulatory responses in dissipating heat loss during heat loading. Thermoregulatory differences between tropical and temperate natives in this study can be interpreted as a result of heat adaptations to physiological function.

Human cold exposure, adaptation, and performance in high latitude environments

Cold exposure is present to significant amounts in the everyday occupational and leisure time activities of circumpolar residents. A cross-sectional population study demonstrated that Finns reported being exposed to cold on average 4% of their total time. Factors modifying cold exposure are: age, gender, employment, education, health, and amount of physical exercise. Several symptoms and complaints are associated with wintertime cold exposure and start to appear more commonly when temperatures decrease below -10 degrees C. Urban circumpolar people do not evidently demonstrate cold acclimatization responses in terms of changes in thermoregulation, probably due to behavioral factors (adequate protective clothing, short cold exposures, and high housing temperatures). With regard to performance, we observed that moderate cold exposure, which may occur in everyday life, affects cognition negatively through the mechanisms of distraction and both positively and negatively through the mechanism of arousal (increased vigilance). It seems that especially simple cognitive tasks are adversely affected by cold, while in more complex tasks performance may even improve in mild or moderate cold. Repeated, short cold exposures in the laboratory, causing cold habituation responses, do not markedly improve neuromuscular or cognitive performance. The article discusses the functional significance of cold exposure, adaptation, and the specific environmental conditions and physiological mechanisms that affect behavior and performance in high latitude environments.

Challenges to temperature regulation when working in hot environments

The focus of this review is upon acute exposure to hot environments and the accompanying physiological changes. The target audience includes physiologists, physicians and occupational health and safety practitioners. Using the principles of thermodynamics, the avenues for human heat exchange are explored, leading to an evaluation of some methods used to assess thermally-stressful environments. In particular, there is a critique of the wet-bulb globe temperature (WBGT) index, and an overview of an alternative means by which such assessments may be undertaken (the heat stress index). These principles and methods are combined to illustrate how one may evaluate the risk of heat illness. Three general areas of research are briefly reviewed: the physiological impact of wearing thermal protective clothing, heat adaptation (acclimation) and whole-body pre-cooling. These topics are considered as potential pre-exposure techniques that may be used to reduce the threat of hyperthermia, or to enhance work performance in the heat.