At what level of heat load are age-related impairments in the ability to dissipate heat evident in females?

Rating of Perceived Exertion: A Large Cross-Sectional Study Defining Intensity Levels for Individual Physical Activity Recommendations

BACKGROUND

Physical inactivity is a growing risk factor worldwide, therefore getting people into sports is necessary. When prescribing physical activity, it is essential to recommend the correct training intensities. Cardiopulmonary exercise testing (CPX) enables precise determination of individuals' training intensities but is unavailable for a broad population. Therefore, the Borg scale allows individuals to assess perceived exertion and set their intensity easily and cost-efficiently. In order to transfer CPX to rating of perceived exertion (RPE), previous studies investigated RPE on specific physiological anchors, e.g. blood lactate (bLa) concentrations, but representativeness for a broad population is questionable. Some contradictory findings regarding individual factors influencing RPE occur, whereas univariable analysis has been performed so far. Moreover, a multivariable understanding of individual factors influencing RPE is missing. This study aims to determine RPE values at the individual anaerobic threshold (LT2) and defined bLa concentrations in a large cohort and to evaluate individual factors influencing RPE with multivariable analysis.

METHODS

CPX with bicycle or treadmill ergometer of 6311 participants were analyzed in this cross-sectional study. RPE values at bLa concentrations 2 mmol/l, 3 mmol/l, 4 mmol/l, and LT2 (first rise in bLa over baseline + 1.5 mmol/l) were estimated by spline interpolation. Multivariable cumulative ordinal regression models were performed to assess the influence of sex, age, type of ergometry, VO2max, and duration of exercise testing on RPE.

RESULTS

Median values [interquartile range (IQR)] of the total population were RPE 13 [11; 14] at 2 mmol/l, RPE 15 [13; 16] at 3 mmol/l, RPE 16 [15; 17] at 4 mmol/l, and RPE 15 [14; 16] at LT2. Main influence of individual factors on RPE were seen especially at 2 mmol/l: male sex (odds ratio (OR) [95%-CI]: 0.65 [0.587; 0.719]), treadmill ergometry (OR 0.754 [0.641; 0.886]), number of stages (OR 1.345 [1.300; 1.394]), age (OR 1.015 [1.012; 1.018]), and VO2max (OR 1.023 [1.015; 1.030]). Number of stages was the only identified influencing factor on RPE at all lactate concentrations/LT2 (3 mmol/l: OR 1.290 [1.244; 1.336]; 4 mmol/l: OR 1.229 [1.187; 1.274]; LT2: OR 1.155 [1.115; 1.197]).

CONCLUSION

Our results suggest RPE ≤ 11 for light intensity, RPE 12-14 for moderate intensity, and RPE 15-17 for vigorous intensity, which slightly differs from the current American College of Sports Medicine (ACSM) recommendations. Additionally, we propose an RPE of 15 delineating heavy and severe intensity domain. Age, sex, type of ergometry, duration of exercise, and cardiopulmonary fitness should be considered when recommending individualized intensities with RPE, primarily at lower intensities. Therefore, this study can be used as a new guideline for prescribing individual RPE values in the clinical practice, predominantly for endurance type exercise.

Biophysical, thermo-physiological and perceptual determinants of cool-seeking behaviour during exercise in younger and older women

Women continue to be under-represented in thermoregulatory research despite their undergoing unique physiological changes across the lifespan. This study investigated the biophysical, thermo-physiological, and perceptual determinants of cool-seeking behaviour during exercise in younger and older women. Eleven younger (25 ± 5 years; 1.7 ± 0.1 m; 63.1 ± 5.2 kg) and 11 older women (53 ± 6 years; 1.7 ± 0.1 m; 65.4 ± 13.9 kg) performed a 40-min incremental cycling test in a thermoneutral environment (22 ± 1.7°C; 36 ± 4% relative humidity). Throughout the test, participants freely adjusted the temperature of a cooling probe applied to their wrists to offset their thermal discomfort. We continuously recorded the probe-wrist interface temperature to quantify participants' cool-seeking behaviour. We also measured changes in participants' rate of metabolic heat production, core and mean skin temperatures, and skin wetness. Finally, we body-mapped participants' skin heat, cold and wetness sensitivity. Our results indicated that: (1) older and younger women exhibited similar onset and magnitude of cool-seeking behaviour, despite older women presented reduced autonomic heat-dissipation responses (i.e., whole-body sweat losses); (2) older women's thermal behaviour was less determined by changes in core temperature (this being a key driver in younger women), and more by changes in multiple thermo-physiological and biophysical parameters (i.e., physical skin wetness, temperature and heat production); (3) older women did not present lower regional skin thermal and wetness sensitivity than younger women. We conclude that predictions of female cool-seeking behaviours based on thermo-physiological variables should consider the effects of ageing. These findings are relevant for the design of wearable cooling systems and sports garments that meet the thermal needs of women across the lifespan.

Experimental research in environmentally induced hyperthermic older persons: A systematic quantitative literature review mapping the available evidence

The heat-related health burden is expected to persist and worsen in the coming years due to an aging global population and climate change. Defining the breadth and depth of our understanding of age-related changes in thermoregulation can identify underlying causes and strategies to protect vulnerable individuals from heat. We conducted the first systematic quantitative literature review to provide context to the historical experimental research of healthy older adults - compared to younger adults or unhealthy age matched cases - during exogenous heat strain, focusing on factors that influence thermoregulatory function (e.g. co-morbidities). We identified 4,455 articles, with 147 meeting eligibility criteria. Most studies were conducted in the US (39%), Canada (29%), or Japan (12%), with 71% of the 3,411 participants being male. About 71% of the studies compared younger and older adults, while 34% compared two groups of older adults with and without factors influencing thermoregulation. Key factors included age combined with another factor (23%), underlying biological mechanisms (18%), age independently (15%), influencing health conditions (15%), adaptation potential (12%), environmental conditions (9%), and therapeutic/pharmacological interventions (7%). Our results suggest that controlled experimental research should focus on the age-related changes in thermoregulation in the very old, females, those with overlooked chronic heat-sensitive health conditions (e.g. pulmonary, renal, mental disorders), the impact of multimorbidity, prolonged and cumulative effects of extreme heat, evidence-based policy of control measures (e.g. personal cooling strategies), pharmaceutical interactions, and interventions stimulating protective physiological adaptation. These controlled studies will inform the directions and use of limited resources in ecologically valid fieldwork studies.

Climate Change and Occupational Heat Strain Among Women Workers: A Systematic Review

Climate change increases heat stress exposure and occupational heat strain in tropical and subtropical regions with generally hot-humid climate conditions. The present systematic review was conducted to assess the effect of climate change on occupational heat strain among women workers. In this study, three main databases (PubMed, Scopus, and Web of Science) were searched to find relevant literature on climate change and its effects using subject headings and appropriate MeSh terms. This article has been written according to the PRISMA checklist. A total of 6,176 studies were identified for screening and 13 studies were eligible for data extraction. Scientific evidence reveals that there is an imprecise but positive relationship between climate change and occupational heat strain regarding women workers. Some complications associated with occupational heat strain among women workers include fatigue, discomfort, dehydration, reduced brain function, and loss of concentration. Climate change can lead to an increase in the occurrence of heat-related illnesses and the levels of injury risk. In addition, its adverse health effects on women workers are mentioned. This systematic study identifies key priorities for action to better characterize and understand how occupational heat strain among women workers may be associated with climate change events. Strong evidence indicates that climate change will continue to cause occupational heat strain among women workers. It is essential to implement preventive measures considering multidisciplinary strategies to reduce the adverse effects of climate change on women workers health in hot weather settings. This can limit the health risks and negative effects of climate change.

Adaptation behaviors modify the effects of body fat on heat-related symptoms among Taiwanese elderly

BACKGROUND

The aging process increases body fat and susceptibility to heat-related illnesses. The relationship between body composition and symptoms associated with exposure to extreme heat among the elderly is unclear. Additionally, the influence of individual adaptive behaviors in mitigating these risks has not been adequately explored.

OBJECTIVES

This study aimed to evaluate the association between body composition and heat-related symptoms as well as the potential modifying effects of heat adaptation behavior.

METHODS

The body composition of elderly individuals was measured using bioelectrical impedance analysis. Face-to-face interviews were conducted a year later to determine the heat-related symptoms and adaptive behaviors practiced for the extremely hot days of the previous year. The association between body composition indices and the presence of more than two symptoms was assessed using logistic regression analysis, while stratified analysis and interaction term in models were used to evaluate the effect modifications of adaptive behaviors.

RESULTS

Of the 859 participants, 16% reported more than two heat-related symptoms. Increased body fat mass index (fat mass in kg/squared height in meters) was associated with an elevated risk of more heat-related symptoms (odds ratio 1.11, 95% confidence interval 1.02-1.20). Each combination of staying indoors, using an umbrella and hat, and using air conditioning at noon reduced the risk association between body fat and symptoms. For females, a combination of reducing physical activity and staying indoors provided similar protective effect. Surprisingly, bathing more frequently in hot weather with heated instead of non-heated water augmented the risk correlation. Neither fan usage nor window opening displayed protective effects.

CONCLUSIONS

Elevated body fat levels, indicative of obesity, corresponded with an increased risk of heat-related symptoms. Integrating multiple adaptive behaviors can diminish the negative health impact of body fat on heat-induced symptoms. However, certain commonly adopted practices might not confer expected benefits.

Agreement between the ventilated capsule and the KuduSmart® device for measuring sweating responses to passive heat stress and exercise

The present study assessed agreement between a wireless sweat rate monitor (KuduSmart® device) and the ventilated capsule (VC) technique for measuring: (i) minute-averaged local sweat rate (LSR), (ii) sweating onset, (iii) sudomotor thermosensitivity, and (iv) steady-state LSR, during passive heat stress and exercise. It was hypothesized that acceptable agreement with no bias would be observed between techniques for all assessed sweating characteristics. On two separate occasions for each intervention, participants were either passively heated by recirculating hot water (49 °C) through a tube-lined garment until rectal temperature increased 1 °C over baseline (n = 8), or a 60 min treadmill march at a fixed rate of heat production (∼500 W, n = 9). LSR of the forearm was concurrently measured with a VC and the KuduSmart® device secured within ∼2 cm. Using a ratio scale Bland-Altman analysis with the VC as the reference, the KuduSmart® device demonstrated systematic bias and not acceptable agreement for minute-averaged LSR (1.17 [1.09, 1.27], CV = 44.5%), systematic bias and acceptable agreement for steady-state LSR (1.16 [1.09,1.23], CV = 19.5%), no bias and acceptable agreement for thermosensitivity (1.07 [0.99, 1.16], CV = 23.2%), and no bias and good agreement for sweating onset (1.00 [1.00, 1.00], CV = 11.1%). In total, ≥73% of all minute-averaged LSR observations with the KuduSmart® device (n = 2743) were within an absolute error of <0.2 mg/cm2/min to the VC, the reference minimum detectable change in measurement error of a VC on the forearm. Collectively, the KuduSmart® device may be a satisfactory solution for assessing the sweating response to heat stress where a VC is impractical.

Human temperature regulation under heat stress in health, disease, and injury

The human body constantly exchanges heat with the environment. Temperature regulation is a homeostatic feedback control system that ensures deep body temperature is maintained within narrow limits despite wide variations in environmental conditions and activity-related elevations in metabolic heat production. Extensive research has been performed to study the physiological regulation of deep body temperature. This review focuses on healthy and disordered human temperature regulation during heat stress. Central to this discussion is the notion that various morphological features, intrinsic factors, diseases, and injuries independently and interactively influence deep body temperature during exercise and/or exposure to hot ambient temperatures. The first sections review fundamental aspects of the human heat stress response, including the biophysical principles governing heat balance and the autonomic control of heat loss thermoeffectors. Next, we discuss the effects of different intrinsic factors (morphology, heat adaptation, biological sex, and age), diseases (neurological, cardiovascular, metabolic, and genetic), and injuries (spinal cord injury, deep burns, and heat stroke), with emphasis on the mechanisms by which these factors enhance or disturb the regulation of deep body temperature during heat stress. We conclude with key unanswered questions in this field of research.

Thermoregulatory responses in persons with lower-limb amputation during upper-limb endurance exercise in a hot and humid environment

BACKGROUND

Persons with an amputation may have an increased heat strain due to reduced surface area. However, there is limited evidence on the thermoregulatory responses in persons with lower-limb amputation (LLA). Although a previous study reported no difference in their rectal temperatures (Tres) in a hot environment, suggesting compensatory sweating of the intact limb, we examined the thermoregulatory responses of such persons in a hot and humid environment.

OBJECTIVE

To compare the thermoregulatory responses-through changes in Tre, sweat, and oxygen uptake (O2)-between persons with LLA and able-bodied (AB) individuals, in hot and humid environments.

STUDY DESIGN

A nonrandomized control trial.

METHODS

Nine AB men (AB group) and nine persons with LLA group performed the arm ergometer exercise at 60% peak power output intensity for 60 min in a hot and humid environment, and they were tested before and after performing. The O2, Tre and skin temperature, and total body sweating, and local sweating during exercise were measured and compared between the groups.

RESULTS

The changes in O2 and Tre after the endurance exercise did not differ between the groups (ΔTre: AB group, 1.1°C ± 0.5°C; LLA group, 1.2°C ±0.3 °C; P = 0.65), whereas the amount of local sweating of the chest (group effect, P < 0.01 by two-way analysis of variance [group × time], the group effect size was medium, η2 = 0.10) and dehydration rate (AB group, 1.5% ± 0.5%; LLA group, 2.1% ± 0.5%; P = 0.03) were higher in the LLA than in the AB group.

CONCLUSIONS

We compared the thermoregulatory responses of persons with LLA with those of AB individuals in hot and humid environments. Core body temperatures of persons with LLAs during endurance exercise were not different from those of AB men even in hot and humid environments. We found compensatory increases in the sweat rate of the chest and increased dehydration rate in persons with LLA. More sweat potentially means that athletes with LLA need to drink more fulids.

Revisiting the evaluation of central versus peripheral thermoregulatory control in humans

Human thermoregulatory control is often evaluated through the relationship between thermoeffector output and core or mean body temperature. In addition to providing a general indication of whether a variable of interest alters thermoregulatory control, this relationship is often used to determine how this alteration may occur. This latter interpretation relies upon two parameters of the thermoeffector output-body temperature relationship: the onset threshold and thermosensitivity. Traditionally, changes in the onset threshold and thermosensitivity are interpreted as "central" or "peripheral" modulation of thermoregulatory control, respectively. This mini-review revisits the origins of the thermoeffector output-body temperature relationship and its use to interpret "central" or "peripheral" modulation of thermoregulatory control. Against this background, we discuss the strengths and weaknesses of this approach and highlight that "central" thermoregulatory control reflects the neural control of body temperature whereas "peripheral" thermoregulatory control reflects properties specific to the thermoeffector organs. We highlight studies that employed more direct approaches to investigate the neural control of body temperature and peripheral properties of thermoeffector organs. We conclude by encouraging future investigations interested in studying thermoregulatory control to more directly investigate the component of the thermoeffector loop under investigation.heat; human; skin blood flow; sweat; thermoregulatory.

Challenges arising for older workers from participating in a workplace intervention addressing work ability: a qualitative study from Germany

OBJECTIVE

Studies examining what renders workplace interventions to sustain and promote work ability of older workers successful have largely neglected older workers´ perspective. This paper outlines the results of a study with regard to older workers´ experiences and expectations of a workplace intervention. Based on these findings, some reflections on how to improve the design and the implementation of workplace interventions for older workers are provided.

METHODS

Semi-structured interviews were conducted with older workers (N = 8) participating in a workplace intervention undertaken at one production site of a large manufacturing company in Baden-Wurttemberg/Germany. The interview guide included questions on participants´ experiences with and expectations of the intervention. The interviews were recorded, transcribed verbatim and analyzed using qualitative content analysis according to Mayring (2014).

RESULTS

Older workers´ reported some challenges they face due to their participation in the workplace intervention. These resulted from the work environment (physical challenges), the work process design (new long work cycle), the work organization (tight time allowances, little job rotation, change of teams, age stereotypes) and the management of the workplace intervention (bad information, feeling of occupational insecurity and lack of being valued).

CONCLUSIONS

The study shows that challenges arising for older workers from their participation in the workplace intervention may have counteracted the promotion of work ability. As findings suggest, some of these challenges might have been avoided either by considering workers´ perspective during design and implementation of an intervention or by referring to evidence on aging and work ability.

The Role of Environmental Conditions on Master Marathon Running Performance in 1,280,557 Finishers the 'New York City Marathon' From 1970 to 2019

Aim: This study investigated the influence of weather conditions on running performance in female and male age group runners in the largest marathon in the world, the “New York City Marathon.”

Methods: The analysis included data from 1,280,557 finishers the “New York City Marathon” from the years 1970 to 2019. Linear mixed models for men and women finishers with race time (min) as dependent variable and 5-year age groups, temperature, wind and relative humidity tertiles (low, medium, high) as independent factors and finisher as random intercept was performed. Additional models with an interaction between age groups and one weather variable each were performed.

Results: Temperature was positively associated with race time while wind speed and humidity were negatively associated (p < 0.001). Men were significantly greater affected wind speed and humidity than women (p < 0.001 for interaction) but not by temperature (p = 0.17 for interaction). With an average of 8 min longer race time, high temperature had the greatest effect on race time. The effect of high humidity on race time was significantly increased in 40–59 years old men and 25–65 years old women. High temperatures had an increased effect on race time in 30–64 years old men and 40–64 years old women. The inverse association between race time and high wind speed was pronounced in finishers with younger age.

Conclusion: Performance was lower on days with high temperature, low humidity and low wind speed. Men seemed to benefit more from higher humidity and wind speed than women. Aged (70 +) finishers were not greater affected by high temperatures.

The Relation between Age and Sex on Whole-Body Heat Loss during Exercise-Heat Stress

INTRODUCTION

Increasing age is associated with decrements in sweat rate that compromise whole-body total heat loss (evaporative + dry heat exchange) in both men and women during moderate-to-vigorous exercise in dry heat. Similarly, young women also display reductions in sweating (that lower evaporative heat loss) relative to young men in such conditions. Nevertheless, it remained unclear whether these effects act synergistically to exacerbate the age-related decline in whole-body total heat loss in women relative to men. We therefore assessed the interrelation between age and sex on whole-body total heat loss during light, moderate, and vigorous exercise in dry heat.

METHODS

To achieve this, we used direct and indirect calorimetry to assess whole-body total heat loss and metabolic heat production (respectively) in 46 men and 34 women age between 18 and 70 yr. Participants performed three, 30-min bouts of cycling at metabolic heat productions of 150 (light), 200 (moderate), and 250 (vigorous) W·m, each separated by 15-min recovery in dry heat (40°C, ~15% relative humidity).

RESULTS

Whole-body total heat loss was ~5% lower in women relative to men during moderate and vigorous exercise (both, P < 0.01), irrespective of age. Total heat loss declined with age during moderate and vigorous exercise in both men and women (all, P < 0.050), although the rate of that decline (~4% per decade) was similar between men and women across all exercise bouts (all, P > 0.050).

CONCLUSIONS

We show that, when assessed in dry heat, whole-body total heat loss is lower in women relative to men, irrespective of age. Furthermore, total heat loss declines with increasing age in both men and women during moderate-to-vigorous exercise, albeit the rate of that decline is not appreciably modified by sex.

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.

Temperature regulation during exercise in the heat: Insights for the aging athlete

OBJECTIVE

The purpose of this review is to evaluate the currently-available literature regarding the impact of both primary aging and age-related fitness on thermoregulatory function during exercise in the heat. In so doing, we aim to (1) characterize the influence of fitness in mitigating age-related declines in thermoregulation, (2) address the limitations of prior experimental approaches for investigating age-related thermoregulatory impairments, (3) examine to what extent aerobic fitness can be maintained in the aging athlete, and (4) begin to address the specific environmental conditions in which age-related impairments in thermoregulatory function may place highly active older adults at increased risk for heat-related illness and injury and/or limited performance.

DESIGN

Mini-review.

METHODS

Review and synthesis of available information.

RESULTS

The earth's climate is warming, accompanied by a consequently greater frequency and severity of extreme heat events. At the same time, lifespan is increasing and people of all ages are staying increasingly active. Age-related impairments in thermoregulatory function are well-documented, leading to increased heat-related health risks and reduced exercise/athletic performance for older adults in hot environmental conditions. High aerobic fitness improves body temperature regulation during exercise via augmented sweating and improved cardiovascular function, including cardiac output and skin blood flow, in humans of all ages.

CONCLUSIONS

The masters athlete is better suited for exercise/heat-stress compared to his or her less fit peers. However, while age and thermoregulation in general has been studied extensively, research on the most fit older adults, including highly competitive athletes, is generally lacking.

Age-Related Changes in the Thermoregulatory Properties in Bank Voles From a Selection Experiment

As with many physiological performance traits, the capacity of endotherms to thermoregulate declines with age. Aging compromises both the capacity to conserve or dissipate heat and the thermogenesis, which is fueled by aerobic metabolism. The rate of metabolism, however, not only determines thermogenic capacity but can also affect the process of aging. Therefore, we hypothesized that selection for an increased aerobic exercise metabolism, which has presumably been a crucial factor in the evolution of endothermic physiology in the mammalian and avian lineages, affects not only the thermoregulatory traits but also the age-related changes of these traits. Here, we test this hypothesis on bank voles (Myodes glareolus) from an experimental evolution model system: four lines selected for high swim-induced aerobic metabolism (A lines), which have also increased the basal, average daily, and maximum cold-induced metabolic rates, and four unselected control (C) lines. We measured the resting metabolic rate (RMR), evaporative water loss (EWL), and body temperature in 72 young adult (4 months) and 65 old (22 months) voles at seven ambient temperatures (13-32°C). The RMR was 6% higher in the A than in the C lines, but, regardless of the selection group or temperature, it did not change with age. However, EWL was 12% higher in the old voles. An increased EWL/RMR ratio implies either a compromised efficiency of oxygen extraction in the lungs or increased skin permeability. This effect was more profound in the A lines, which may indicate their increased vulnerability to aging. Body temperature did not differ between the selection and age groups below 32°C, but at 32°C it was markedly higher in the old A-line voles than in those from other groups. As expected, the thermogenic capacity, measured as the maximum cold-induced oxygen consumption, was decreased by about 13% in the old voles from both selection groups, but the performance of old A-line voles was the same as that of the young C-line ones. Thus, the selection for high aerobic exercise metabolism attenuated the adverse effects of aging on cold tolerance, but this advantage has been traded off by a compromised coping with hot conditions by aged voles.

Impact of uncomplicated controlled hypertension on thermoregulation during exercise-heat stress

Exercise is promoted for management of hypertension and as a general healthy behavior, but environmental conditions are seldom considered in these recommendations. Hypertension may affect skin blood flow and sweating, two of the primary mechanisms which prevent continued elevations in core temperature by facilitating whole-body heat loss during exercise-heat stress. We show that during incremental exercise-heat stress (in hot-dry conditions), controlled and uncomplicated hypertension is unlikely to exert a meaningful effect on whole-body heat loss in individuals who are already physically active.

Individual Responses to Heat Stress: Implications for Hyperthermia and Physical Work Capacity

Background

Extreme heat events are increasing in frequency, severity, and duration. It is well known that heat stress can have a negative impact on occupational health and productivity, particularly during physical work. However, there are no up-to-date reviews on how vulnerability to heat changes as a function of individual characteristics in relation to the risk of hyperthermia and work capacity loss. The objective of this narrative review is to examine the role of individual characteristics on the human heat stress response, specifically in relation to hyperthermia risk and productivity loss in hot workplaces. Finally, we aim to generate practical guidance for industrial hygienists considering our findings. Factors included in the analysis were body mass, body surface area to mass ratio, body fat, aerobic fitness and training, heat adaptation, aging, sex, and chronic health conditions.

Findings

We found the relevance of any factor to be dynamic, based on the work-type (fixed pace or relative to fitness level), work intensity (low, moderate, or heavy work), climate type (humidity, clothing vapor resistance), and variable of interest (risk of hyperthermia or likelihood of productivity loss). Heat adaptation, high aerobic fitness, and having a large body mass are the most protective factors during heat exposure. Primary detrimental factors include low fitness, low body mass, and lack of heat adaptation. Aging beyond 50 years, being female, and diabetes are less impactful negative factors, since their independent effect is quite small in well matched participants. Skin surface area to mass ratio, body composition, hypertension, and cardiovascular disease are not strong independent predictors of the heat stress response.

Conclusion

Understanding how individual factors impact responses to heat stress is necessary for the prediction of heat wave impacts on occupational health and work capacity. The recommendations provided in this report could be utilized to help curtail hyperthermia risk and productivity losses induced by heat.

Psychrometric limits and critical evaporative coefficients for exercising older women

Critical environmental limits are those above which human heat balance cannot be maintained for a given metabolic heat production. These limits, and associated critical evaporative coefficients (K_e') that can be used to model responses in hot environments, have not been determined for older subjects. The present paper graphically characterizes psychrometric limits and environmental isotherms and derives K_e' values for a group of unacclimated older (n = 10; age 62 - 80 yr) women exercising at 30% V̇o_2max. Uniquely, we compare and contrast these data with published data from young, unacclimated and young, heat-acclimated women tested across a four-decade span using the same protocol in the same environmental chamber. These loci are presented graphically on a psychrometric chart (with confidence intervals). Isotherms constructed from biophysical modeling and sweating capacity closely fit the data but underestimated empirically derived data points in hotter, drier environments. Compared with the young (age 19-26 yr) women previously tested, the older women had significantly constrained (lower) critical environmental limits, in part due to lower sweating rates. Age-specific values of the critical evaporative coefficient, K_e', derived by partial calorimetry in the more humid environments (in which skin wettedness approached 1), were likewise lower for the older women (overall mean = 9.1 W·m^-2·mmHg^-1; P < 0.05) vs. unacclimated (15.4 W·m^-2·mmHg^-1) and acclimated (17.0 W·m^-2·mmHg^-1) young women. Constrained psychrometric limits and lower critical evaporative coefficients lend biophysical clarity to decreased abilities of older women for prolonged exercise in the heat.NEW & NOTEWORTHY This study is the first to describe, graphically and quantitatively, critical environmental limits for women between the ages of 62 and 80 yr based on the biophysics of heat exchange. These psychrometric limit lines define combinations of ambient temperature and humidity above which human heat balance cannot be maintained for a given metabolic heat production. These limits, and associated critical evaporative coefficients (K_e'), can be used to model low- to moderate-intensity exercise responses in hot environments and have directly translatable data that can be used for evidence-based policy decisions, to prepare for impending heat events, and for implementation of other safety interventions.

Impairments to Thermoregulation in the Elderly During Heat Exposure Events

Heat waves represent a public health risk to elderly people, and typically result in an increased rate of hospital admissions and deaths. Studies of thermoregulation in this cohort have generally focused on single elements such as sweating capacity. Sweating capacity and skin blood flow reduce with age, reducing ability to dissipate heat. Perception of effort during heat exposure is emerging as an area that needs further investigation as the elderly appear to lack the ability to adequately perceive increased physiological strain during heat exposure. The role of the gut and endotoxemia in heat stress has received attention in young adults, while the elderly population has been neglected. This shortcoming offers another potential avenue for identifying effective integrated health interventions to reduce heat illnesses. Increasing numbers of elderly individuals in populations worldwide are likely to increase the incidence of heat wave-induced deaths if adequate interventions are not developed, evaluated, and implemented. In this narrative-style review we identify and discuss health-related interventions for reducing the impact of heat illnesses in the elderly.

Skin Temperature in Master Long-Distance Runners-Results From a Field Study at the 2018 World Master Athletics Championships

Older people and athletes show impairments in thermoregulation, but this has not yet been studied during a running competition. The aim of the study was to assess (1) whether there are age-related differences in skin temperature during the last stage of a race in well-trained master athletes and (2) to what extent such differences are related to running speed and sex. To investigate this, we used thermography to measure maximum skin temperatures of the head, legs and hands of participants of the 2018 World Master Athletics (WMA) Championships when they were approximately 9,600 m into a 10,000-m road race. Of the 813 runners, 404 were analyzed (142 women, 262 men) including athletes of age groups 35 to 85. All ≥70-year-old athletes completed the race; all 16 non-finishers were younger. The hand temperature was lower than that of the head and legs (p < 0.001). Stepwise regression revealed that head ( R adj 2 = 0.143; p < 0.001) and hand temperature decreased with increasing speed ( R adj 2 = 0.092; p < 0.001). Sex was the most important determinant of leg skin temperature ( R adj 2 = 0.054; p < 0.001), men having higher leg temperatures than women, with a small negative contribution of speed ( R adj 2 increased to 0.069). In conclusion, higher running speed is associated with lower skin temperatures, and leg skin temperature is lower in women than men. The absence of an age effect on skin temperature suggests that there is no impairment in heat dissipation in well-trained older athletes.

Age differences in cardiac autonomic regulation during intermittent exercise in the heat

PURPOSE

This study aimed to detect potential differences in heart-rate variability (HRV) during a moderate-intensity intermittent exercise in the heat among physically active young (25.8 ± 1.9 years), middle-aged (43.5 ± 2.8 years), and older (62.9 ± 3.7 years) men.

METHODS

Thirty-three participants (11/group) performed four successive bouts of 15-min cycling at a moderate fixed rate of metabolic heat production of ~ 400 W; each separated by a 15-min recovery with 1 h of final recovery in a hot and dry environment (35 °C, 20% relative humidity). Twelve HRV indices were computed that have been commonly described in the literature, and characterized various domains of the variability and complexity of heart rate.

RESULTS

Cardiac autonomic regulation during intermittent exercise in the heat, as well as during pre-exercise rest and recovery was significantly affected by age, as changes were observed among the three different aged groups in five indices (p ≤ 0.05). Similarly, time influenced cardiac autonomic regulation as three indices showed changes across time (p ≤ 0.05) during intermittent exercise, whilst five indices displayed significant changes (p ≤ 0.05) during rest and recovery in the heat.

CONCLUSIONS

This study supports that moderate-intensity intermittent exercise in the heat is associated with significant cardiac autonomic dysregulation in older men, as compared to young and middle-aged men, yet it highlights the importance of developing preventative health strategies for heat-related illness in aged individuals.

Occupational heat stress management: Does one size fit all?

Heat stress is a deadly occupational hazard that is projected to increase in severity with global warming. While upper limits for heat stress designed to protect all workers have been recommended by occupational safety institutes for some time, heat stress continues to compromise health and productivity. In our view, this is largely explained by the inability of existing guidelines to consider the inter-individual (age, sex, disease, others) and intra-individual (medication use, fitness, hydration, others) factors that cause extensive variability in physiological tolerance to a given heat stress. In conditions that do not exceed the recommended limits, this 'one size fits all' approach to heat stress management can lead to reductions in productivity in more heat-tolerant workers, while compromising safety in less heat-tolerant workers who may develop heat-related illness, even in temperate conditions. Herein, we discuss future directions in occupational heat stress management that consider this individual variability.

Is There a Need to Integrate Human Thermal Models with Weather Forecasts to Predict Thermal Stress?

More and more people will experience thermal stress in the future as the global temperature is increasing at an alarming rate and the risk for extreme weather events is growing. The increased exposure to extreme weather events poses a challenge for societies around the world. This literature review investigates the feasibility of making advanced human thermal models in connection with meteorological data publicly available for more versatile practices and a wider population. By providing society and individuals with personalized heat and cold stress warnings, coping advice and educational purposes, the risks of thermal stress can effectively be reduced. One interesting approach is to use weather station data as input for the wet bulb globe temperature heat stress index, human heat balance models, and wind chill index to assess heat and cold stress. This review explores the advantages and challenges of this approach for the ongoing EU project ClimApp where more advanced models may provide society with warnings on an individual basis for different thermal environments such as tropical heat or polar cold. The biggest challenges identified are properly assessing mean radiant temperature, microclimate weather data availability, integration and continuity of different thermal models, and further model validation for vulnerable groups.

Interactive effects of age and hydration state on human thermoregulatory function during exercise in hot-dry conditions

AIM

Ageing and hypohydration independently attenuate heat dissipation during exercise; however, the interactive effects of these factors remain unclear. We assessed the hypothesis that ageing suppresses hypohydration-induced reductions in whole-body heat loss during exercise in the heat.

METHODS

On two occasions, eight young (mean [SD]: 24 [4] years) and eight middle-aged (59 [5] years) men performed 30-minute bouts of light (heat production of 175 W m^-2 ) and moderate (275 W m^-2 ) cycling (separated by 15-minute rest) in the heat (40°C, 15% relative humidity) when euhydrated and hypohydrated (~4% reduction in body mass). Heat production and whole-body net heat exchange (evaporative heat loss + dry heat gain) were measured via indirect and direct calorimetry (respectively) and heat storage was calculated via their temporal summation.

RESULTS

Net heat exchange was reduced, while heat storage was elevated, in the middle-aged men during moderate exercise when euhydrated (both P ≤ 0.01). In the young, evaporative heat loss was attenuated in the hypohydrated vs euhydrated condition during light (199 ± 6 vs 211 ± 10 W m^-2 ; P ≤ 0.01) and moderate (287 ± 15 vs 307 ± 13 W m^-2 ; P ≤ 0.01) exercise, but was similar in the middle-aged men, averaging 223 ± 6 and 299 ± 15 W m^-2 , respectively, across conditions (both P ≥ 0.32). Heat storage was thereby exacerbated by hypohydration in the young (both P < 0.01) but not the middle-aged (both P ≥ 0.32) during both exercise bouts and, as a result, was similar between groups when hypohydrated (both P ≥ 0.50).

CONCLUSION

Hypohydration attenuates heat loss via sweating in young but not middle-aged men, indicating that ageing impairs one's ability to mitigate further sweat-induced fluid loss during hypohydration.

Physiological and perceptual responses in the elderly to simulated daily living activities in UK summer climatic conditions

OBJECTIVES

The elderly population is at an increasingly significant health risk to heat-related illnesses and mortality when compared with younger people in the same conditions. This is due to an increased frequency and severity of heatwaves, attributed to climate change, and reduced ability of elderly individuals to dissipate excess heat. Consequently, most excess deaths and emergency visits during heatwaves occur in people aged more than 65 years. The aim of this investigation was to assess the physiological and perceptual responses of elderly people during exercise sessions equating to activities of daily living in UK summer climatic conditions.

STUDY DESIGN

Mixed-method, randomised research design.

METHODS

Twenty-eight participants (17 males, 10 females and 1 transgender female) were randomly assigned into three experimental groups; 15°C, 25°C or 35°C, with 50% relative humidity. Participants completed one preliminary and three experimental trials within their assigned environment. The data from the preliminary incremental recumbent cycling test was used to calculate participant's individual exercise intensities equating to 2, 4 and 6 metabolic equivalents (METs) for the subsequent trials. During experimental trials, participants completed 30-min seated rest and 30-min cycling.

RESULTS

No change was observed in thermal comfort ([TC] just uncomfortable in both trials), and only modest changes in ratings of perceived exertion (14 ± 2 vs 15 ± 2) at 6 METs in 25°C compared with those in 35°C were observed. In contrast, thermal strain markers did significantly increase (P < 0.05) across the same conditions, including change in rectal temperature (ΔT_re) during exercise (0.27 ± 0.17°C vs 0.64 ± 0.18°C) and peak skin temperature ([T_skin] 32.94 ± 1.15°C vs 36.11 ± 0.44°C).

CONCLUSION

When completing exercise that equates to activities of daily living, elderly people could have a decreased perceptual awareness of the environment even though physiological markers of thermal strain are elevated. Consequently, the elderly could be less likely to implement behavioural thermoregulation interventions (i.e. seek shade and/or remove excess layers) due to a decreased awareness of an increasingly thermally challenging environment.

Towards establishing evidence-based guidelines on maximum indoor temperatures during hot weather in temperate continental climates

Rising environmental temperatures represent a major threat to human health. The activation of heat advisories using evidence-based thresholds for high-risk outdoor ambient temperatures have been shown to be an effective strategy to save lives during hot weather. However, although the relationship between weather and human health has been widely defined by outdoor temperature, corresponding increases in indoor temperature during heat events can also be harmful to health especially in vulnerable populations. In this review, we discuss our current understanding of the relationship between outdoor temperature and human health and examine how human health can also be adversely influenced by high indoor temperatures during heat events. Our assessment of the existing literature revealed a high degree of variability in what can be considered an acceptable indoor temperature because there are differences in how different groups of people may respond physiologically and behaviorally to the same living environment. Finally, we demonstrate that both non-physiological (e.g., geographical location, urban density, building design) and physiological (e.g., sex, age, fitness, state of health) factors must be considered when defining an indoor temperature threshold for preserving human health in a warming global climate.

Age alters cardiac autonomic modulations during and following exercise-induced heat stress in females

The aim of this study was to examine the effect of natural ageing on heart rate variability during and following exercise-induced heat stress in females. Eleven young (∼24 years) and 13 older (∼51 years), habitually active females completed an experimental session consisting of baseline rest, moderate intensity intermittent exercise (four 15-min bouts separated by 15-min recovery) and 1-hour of final recovery in a hot and dry (35°C, 20% relative humidity) environment. Respiratory and heart rate recordings were continuously logged with 10-min periods analysed at the end of: baseline rest; each of the exercise and recovery bouts; and during the 1-hour final recovery period. Comparisons over time during exercise and recovery, and between groups were conducted via two-way repeated-measures ANCOVAs with rest values as the covariate. During baseline rest, older females exhibited lower heart rate variability compared to young females with similar levels of respiration and most (∼71-79%) heart rate variability measures during repeated exercise and recovery. However, older females exhibited heart rate variability metrics suggestive of greater parasympathetic modulation (greater long axis of Poincare plot, cardiac vagal index; lower low-high frequency ratio) during repeated exercise with lower indices during the latter stage of prolonged recovery (less very low frequency component, Largest Lyapunov Exponent; greater cardiac sympathetic index). The current study documented several unique, age-dependent differences in heart rate variability, independent of respiration, during and following exercise-induced heat stress for females that may assist in the detection of normal heat-induced adaptations as well as individuals vulnerable to heat stress.

Screening criteria for increased susceptibility to heat stress during work or leisure in hot environments in healthy individuals aged 31-70 years

Population aging and global warming generate important public health risks, as older adults have increased susceptibility to heat stress (SHS). We defined and validated sex-specific screening criteria for SHS during work and leisure activities in hot environments in individuals aged 31-70 years using age, anthropometry, and cardiorespiratory fitness. A total of 123 males and 44 females [44 ± 14 years; 22.9 ± 7.4% body fat; 40.3 ± 8.6 peak oxygen uptake (mlO2/kg/min)] participated, separated into the Analysis (n = 111) and Validation (n = 56) groups. Within these groups, participants were categorized into YOUNG (19-30 years; n = 47) and OLDER (31-70 years; n = 120). All participants performed exercise in the heat inside a direct calorimeter. Screening criteria for OLDER participants were defined from the Analysis group and were cross-validated in the Validation group. Results showed that 30% of OLDER individuals in the Analysis group were screened as SHS positive. A total of 274 statistically valid (p < 0.05) criteria were identified suggesting that OLDER participants were at risk for SHS when demonstrating two or more of the following (males/females): age ≥ 53.0/55.8 years; body mass index ≥29.5/25.7 kg/m2; body fat percentage ≥ 28.8/34.9; body surface area ≤2.0/1.7 m2; peak oxygen uptake ≤48.3/41.4 mlO2/kg fat free mass/min. In the Validation group, McNemar χ2 comparisons confirmed acceptable validity for the developed criteria. We conclude that the developed criteria can effectively screen individuals 31-70 years who are at risk for SHS during work and leisure activities in hot environments and can provide simple and effective means to mitigate the public health risks caused by heat exposure.

Hyperthermia and cardiovascular strain during an extreme heat exposure in young versus older adults

We examined whether older individuals experience greater levels of hyperthermia and cardiovascular strain during an extreme heat exposure compared to young adults. During a 3-hour extreme heat exposure (44°C, 30% relative humidity), we compared body heat storage, core temperature (rectal, visceral) and cardiovascular (heart rate, cardiac output, mean arterial pressure, limb blood flow) responses of young adults (n = 30, 19-28 years) against those of older adults (n = 30, 55-73 years). Direct calorimetry measured whole-body evaporative and dry heat exchange. Body heat storage was calculated as the temporal summation of heat production (indirect calorimetry) and whole-body heat loss (direct calorimetry) over the exposure period. While both groups gained a similar amount of heat in the first hour, the older adults showed an attenuated increase in evaporative heat loss (p < 0.033) in the first 30-min. Thereafter, the older adults were unable to compensate for a greater rate of heat gain (11 ± 1 ; p < 0.05) with a corresponding increase in evaporative heat loss. Older adults stored more heat (358 ± 173 kJ) relative to their younger (202 ± 92 kJ; p < 0.001) counterparts at the end of the exposure leading to greater elevations in rectal (p = 0.043) and visceral (p = 0.05) temperatures, albeit not clinically significant (rise < 0.5°C). Older adults experienced a reduction in calf blood flow (p < 0.01) with heat stress, yet no differences in cardiac output, blood pressure or heart rate. We conclude, in healthy habitually active individuals, despite no clinically observable cardiovascular or temperature changes, older adults experience greater heat gain and decreased limb perfusion in response to 3-hour heat exposure.

Restoration of thermoregulation after exercise

Performing exercise, especially in hot conditions, can heat the body, causing significant increases in internal body temperature. To offset this increase, powerful and highly developed autonomic thermoregulatory responses (i.e., skin blood flow and sweating) are activated to enhance whole body heat loss; a response mediated by temperature-sensitive receptors in both the skin and the internal core regions of the body. Independent of thermal control of heat loss, nonthermal factors can have profound consequences on the body's ability to dissipate heat during exercise. These include the activation of the body's sensory receptors (i.e., baroreceptors, metaboreceptors, mechanoreceptors, etc.) as well as phenotypic factors such as age, sex, acclimation, fitness, and chronic diseases (e.g., diabetes). The influence of these factors extends into recovery such that marked impairments in thermoregulatory function occur, leading to prolonged and sustained elevations in body core temperature. Irrespective of the level of hyperthermia, there is a time-dependent suppression of the body's physiological ability to dissipate heat. This delay in the restoration of postexercise thermoregulation has been associated with disturbances in cardiovascular function which manifest most commonly as postexercise hypotension. This review examines the current knowledge regarding the restoration of thermoregulation postexercise. In addition, the factors that are thought to accelerate or delay the return of body core temperature to resting levels are highlighted with a particular emphasis on strategies to manage heat stress in athletic and/or occupational settings.

A shirt containing multistage phase change material and active cooling components was associated with increased exercise capacity in a hot, humid environment

Recent advances in clothing design include the incorporation of phase change materials (PCM) and other active cooling components (ACC) to provide better body heat dissipation. The purpose of this study was to determine the effect of wearing a shirt containing multistage PCM/ACC on exercise capacity at low (5.0), moderate-high (7.5) and extreme (9.0) levels of the physiological strain index (PSI). Fourteen individuals tested two shirts (control vs. cooling) during 45-min of interval running in a hot, humid (35 ± 1 °C; 55 ± 6% RH) environment. The cooling shirt resulted in an 8% improvement in exercise capacity at a PSI of 7.5 (p < 0.05). The observed increase in exercise capacity would likely translate to a significant improvement in exercise performance. More research is needed to determine a best practice approach for the use of cooling clothing as a counter to exercise-induced heat exposure. Practitioner Summary: In this report, we demonstrate that when forced to exercise in a hot, humid environment, an individual's exercise capacity may increase by as much as 8% when wearing a shirt composed of multistage phase change material and active cooling components.

Age, human performance, and physical employment standards

The proportion of older workers has increased substantially in recent years, with over 25% of the Canadian labour force aged ≥55 years. Along with chronological age comes age-related declines in functional capacity associated with impairments to the cardiorespiratory and muscular systems. As a result, older workers are reported to exhibit reductions in work output and in the ability to perform and/or sustain the required effort when performing work tasks. However, research has presented some conflicting views on the consequences of aging in the workforce, as physically demanding occupations can be associated with improved or maintained physical function. Furthermore, the current methods for evaluating physical function in older workers often lack specificity and relevance to the actual work tasks, leading to an underestimation of physical capacity in the older worker. Nevertheless, industry often lacks the appropriate information and/or tools to accommodate the aging workforce, particularly in the context of physical employment standards. Ultimately, if appropriate workplace strategies and work performance standards are adopted to optimize the strengths and protect against the vulnerability of the aging workers, they can perform as effectively as their younger counterparts. Our aim in this review is to evaluate the impact of different individual (including physiological decline, chronic disease, lifestyle, and physical activity) and occupational (including shift work, sleep deprivation, and cold/heat exposure) factors on the physical decline of older workers, and therefore the risk of work-related injuries or illness.