Psychrometric limits and critical evaporative coefficients for exercising older women

Sex differences in heat stress vulnerability among middle-aged and older adults (PSU HEAT Project)

Individuals over the age of 65 yr are the most vulnerable population during severe environmental heat events, experiencing worse health outcomes than any other age cohort. The risk is greater in older women than in age-matched men; however, whether that reflects a greater susceptibility to heat in women, or simply population sex proportionality, is unclear. Seventy-two participants (29 M/43 F) aged 40-92 yr were exposed to progressive heat stress at a metabolic rate designed to reflect activities of daily living. Experiments were conducted in both hot-dry (HD; up to 53°C; ≤25% rh) and warm-humid (WH; ∼35°C; ≥50% rh) environments. After critical limits were determined for each condition, forward stepwise multiple linear regression analyses were conducted with net metabolic rate (Mnet) and age entered into the model first, followed by sex, body mass (mb), maximal oxygen consumption (V̇o2max), body surface area, and LDL cholesterol. After accounting for Mnet and age, sex further improved the regression model in the HD environment ([Formula: see text] = 0.34, P < 0.001) and the WH environment ([Formula: see text] = 0.36, P < 0.005). Sex explained ∼15% of the variance in critical environmental limits in HD conditions and 12% in WH conditions. Heat compensability curves were shifted leftward for older women, indicating age- and sex-dependent heat vulnerability compared with middle-aged women and older men in WH (P = 0.007, P = 0.03) and HD (P = 0.001, P = 0.01) environments. This reflects the heterogeneity of thermal-balance thresholds associated with aging relative to those seen in young adults and suggests that older females are more vulnerable than their age-matched male counterparts.NEW & NOTEWORTHY In contrast to young adults, there are sex differences in critical environmental limits in middle-aged and older adults. Older women exhibit lower critical environmental limits in both humid and dry extreme environments demonstrated by a leftward shift in heat compensability curves. These data confirm a true sex difference in heat vulnerability of older adults and support the epidemiological mortality data from environmental heat waves.

Critical environmental core temperature limits and heart rate thresholds across the adult age span (PSU HEAT Project)

The frequency, duration, and severity of extreme heat events have increased and are projected to continue to increase throughout the next century. As a result, there is an increased risk of excessive heat- and cardiovascular-related morbidity and mortality during these extreme heat events. Therefore, the purposes of this investigation were to establish 1) critical environmental core temperature (Tc) limits for middle-aged adults (MA), 2) environmental thresholds that cause heart rate (HR) to progressively rise in MA and older (O) adults, and 3) examine critical environmental Tc limits and HR environmental thresholds across the adult age span. Thirty-three young (Y) (15 F; 23 ± 3 yr), 28 MA (17 F; 51 ± 6 yr), and 31 O (16 F; 70 ± 3 yr) subjects were exposed to progressive heat stress in an environmental chamber in a warm-humid (WH, 34-36°C, 50-90% rh) and a hot-dry (HD, 38°C-52°C, <30% rh) environment while exercising at a low metabolic rate reflecting activities of daily living (∼1.8 METs). In both environments, there was a main effect of age on the critical environmental Tc limit and environmental HR thresholds (main effect of age all P < 0.001). Across the lifespan, critical environmental Tc and HR thresholds decline linearly with age in HD environments (R2 ≥ 0.3) and curvilinearly in WH environments (R2 ≥ 0.4). These data support an age-associated shift in critical environmental Tc limits and HR thresholds toward lower environmental conditions and can be used to develop evidence-based safety guidelines to minimize future heat-related morbidity and mortality across the adult age span.NEW & NOTEWORTHY This study is the first to identify critical environmental core temperature and heart rate thresholds across the adult age spectrum. In addition, our data demonstrate that the rate of decline in Tc and HR limits with age is environmental-dependent. These findings provide strong empirical data for the development of safety guidelines and policy decisions to mitigate excessive heat- and cardiovascular-related morbidity and mortality for impending heat events.

A Novel Conceptual Model for Human Heat Tolerance

Human "heat tolerance" has no accepted definition or physiological underpinnings; rather, it is almost always discussed in relative or comparative terms. We propose to use environmental limits to heat balance accounting for metabolic rate and clothing, that is, the environments for which heat stress becomes uncompensable for a specified metabolic rate and clothing, as a novel metric for quantifying heat tolerance.

Sunscreen does not alter sweating responses or critical environmental limits in young adults (PSU HEAT project)

Outdoor athletes often eschew using sunscreen due to perceived performance impairments, which many attribute in part to the potential for reduced thermoregulatory heat loss. Past studies examining the impact of sunscreen on thermoregulation are equivocal. The purpose of this study was to determine the effects of mineral and chemical-based sunscreens on sweating responses and critical environmental limits in hot-dry (HD) and warm-humid (WH) environments. Nine subjects (3 M/6 F; 25 ± 2 yr) were tested with 1) no sunscreen (control), 2) chemical-, and 3) mineral-based sunscreen. Subjects were exposed to progressive heat stress with either 1) constant dry-bulb temperature (Tdb) at 34°C and increasing water vapor pressure (Pa) (WH trials) or 2) constant Pa at 12 mmHg and increasing Tdb (HD trials). Subjects walked at 4.9 ± 0.5 metabolic equivalents (METs) until an upward inflection in gastrointestinal temperature was observed (i.e., the critical environmental limit). Compared with control (39.9 ± 3.0°C), critical Tdb was not different in mineral (39.2 ± 3.5°C, P = 0.39) or chemical (39.7 ± 3.0°C, P = 0.98) sunscreen trials in HD environments. Compared with control (18.8 ± 4.0 mmHg), critical Pa was not different in mineral (18.9 ± 4.8 mmHg, P = 0.81) or chemical (19.5 ± 4.6 mmHg, P = 0.81) sunscreen trials in WH environments. Sweating rates, evaporative heat loss, skin wettedness, and sweating efficiency were not different among the three trials in the WH (all P ≥ 0.48) or HD (all P ≥ 0.87) environments. Critical environmental limits are unaffected by sunscreen application, suggesting sunscreen does not alter integrative thermoregulatory responses during exercise in the heat.NEW & NOTEWORTHY Our findings demonstrate that neither sweating nor critical environmental limits were affected by mineral-based and chemical-based sunscreen applications. The rates of change in core temperature during compensable and uncompensable heat stress were not changed by wearing sunscreen. Evaporative heat loss, efficiency of sweat evaporation, skin wettedness, and sweating rates were unaffected by sunscreen. Sunscreen did not alter integrative thermoregulatory responses during exercise in the heat.

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.

Heat stress vulnerability and critical environmental limits for older adults

The present study examined heat stress vulnerability of apparently healthy older vs. young adults and characterized critical environmental limits for older adults in an indoor setting at rest (Rest) and during minimal activity associated with activities of daily living. Critical environmental limits are combinations of ambient temperature and humidity above which heat balance cannot be maintained (i.e., becomes uncompensable) for a given metabolic heat production. Here we exposed fifty-one young (23±4 yrs) and 49 older (71±6 yrs) adults to progressive heat stress across a wide range of environments in an environmental chamber during Minimal Activity (young and older subjects) and Rest (older adults only). Heat compensability curves were shifted leftward for older adults indicating age-dependent heat vulnerablity (p < 0.01). During Minimal Activity, critical environmental limits were lower in older compared to young adults (p < 0.0001) and lower than those at Rest (p < 0.0001). These data document heat vulnerability of apparently healthy older adults and to define critical environmental limits for indoor settings in older adults at rest and during activities of daily living, and can be used to develop evidence-based recommendations to minimize the deleterious impacts of extreme heat events in this population.

Onset of cardiovascular drift during progressive heat stress in young adults (PSU HEAT project)

With climate change, humans are at a greater risk for heat-related morbidity and mortality, often secondary to increased cardiovascular strain associated with an elevated core temperature (Tc). Critical environmental limits (i.e., the upper limits of compensable heat stress) have been established based on Tc responses for healthy, young individuals. However, specific environmental limits for the maintenance of cardiovascular homeostasis have not been investigated in the context of thermal strain during light activity. Therefore, the purposes of this study were to 1) identify the specific environmental conditions (combinations of ambient temperature and water vapor pressure) at which cardiovascular drift [i.e., a continuous rise in heart rate (HR)] began to occur and 2) compare those environments to the environmental limits for the maintenance of heat balance. Fifty-one subjects (27 F; 23 ± 4 yr) were exposed to progressive heat stress across a wide range of environmental conditions in an environmental chamber at two low metabolic rates reflecting minimal activity (MinAct; 159 ± 34 W) or light ambulation (LightAmb; 260 ± 55 W). Whether systematically increasing ambient temperature or humidity, the onset of cardiovascular drift occurred at lower environmental conditions compared with Tc inflection points at both intensities (P < 0.05). Furthermore, the time at which cardiovascular drift began preceded the time of Tc inflection (MinAct P = 0.01; LightAmb P = 0.0002), and the difference in time between HR and Tc inflection points did not differ (MinAct P = 0.08; LightAmb P = 0.06) across environmental conditions for either exercise intensity. These data suggest that even in young adults, increases in cardiovascular strain precede the point at which heat stress becomes uncompensable during light activity.NEW & NOTEWORTHY To our knowledge, this study is the first to 1) identify the specific combinations of temperature and humidity at which an increase in cardiovascular strain (cardiovascular drift) occurs and 2) compare those environments to the critical environmental limits for the maintenance of heat balance. We additionally examined the difference in time between the onset of increased cardiovascular strain and uncompensable heat stress. We show that an increase in cardiovascular strain systematically precedes sustained heat storage in young adults.

Relatively minor influence of individual characteristics on critical wet-bulb globe temperature (WBGT) limits during light activity in young adults (PSU HEAT Project)

Critical environmental limits are temperature-humidity thresholds above which heat balance cannot be maintained for a given metabolic heat production. This study examined the association between individual characteristics [sex, body surface area (AD), aerobic capacity (V̇o2max), and body mass (mb)] and critical environmental limits in young adults at low metabolic rates. Forty-four (20 M/24 F; 23 ± 4 yr) subjects were exposed to progressive heat stress in an environmental chamber at two low net metabolic rates (Mnet); minimal activity (MinAct; Mnet = ∼160 W) and light ambulation (LightAmb; Mnet = ∼260 W). In two hot-dry (HD; ≤25% rh) environments, ambient water vapor pressure (Pa = 12 or 16 mmHg) was held constant and dry-bulb temperature (Tdb) was systematically increased. In two warm-humid (WH; ≥50% rh) environments, Tdb was held constant at 34°C or 36°C, and Pa was systematically increased. The critical wet-bulb globe temperature (WBGTcrit) was determined for each condition. During MinAct, after entry of Mnet into the forward stepwise linear regression model, no individual characteristics were entered into the model for WH (R2adj = 0.01, P = 0.27) or HD environments (R2adj = -0.01, P = 0.44). During LightAmb, only mb was entered into the model for WH environments (R2adj = 0.44, P < 0.001), whereas only V̇o2max was entered for HD environments (R2adj = 0.22; P = 0.002). These data demonstrate negligible importance of individual characteristics on WBGTcrit during low-intensity nonweight-bearing (MinAct) activity with a modest impact of mb and V̇o2max during weight-bearing (LightAmb) activity in extreme thermal environments.NEW & NOTEWORTHY Our laboratory has recently published a series of papers establishing the upper ambient temperature-humidity thresholds for maintaining heat balance, termed critical environmental limits, in young adults. However, no studies have investigated the relative influence of individual characteristics, such as sex, body size, and aerobic fitness, on those environmental limits. Here, we demonstrate the contributions of sex, body mass, body surface area, and maximal aerobic capacity on critical wet-bulb globe temperature (WBGT) limits in young adults.

Critical Environmental Limits for Human Thermoregulation in the Context of a Changing Climate

Human-caused climate change has increased the average temperature of the Earth by over 1°C since the 19th century with larger increases expected by 2100 due to continued human influence. This change in mean ambient temperature has had nonlinear effects, resulting in more high temperature extremes, i.e., heat waves, that have increased in frequency, duration, and magnitude. Additional occurrences of humid heatwaves have significantly affected human health due to the physiological strain associated with a relative inability for evaporative cooling. Inability to efficaciously cool the body, whether during passive heat exposure or physical activity, not only leads to elevated core temperatures but also places strain on the cardiovascular system, often exacerbating age-related co-morbidities. As part of the PSU HEAT (Pennsylvania State University - Human Environmental Age Thresholds) Project, a progressive environmental strain protocol has been developed to determine critical environmental limits - combinations of ambient temperature and humidity -- associated with uncompensable heat stress and intractable rises in core temperature (Tc). These human heat balance thresholds, well below those originally theorized by climatologists, have been surpassed in recent heatwaves and be exceeded on a more regular basis in the future, providing additional impetus to the urgency of adaptative measures and climate change mitigation.

Adverse heat-health outcomes and critical environmental limits (Pennsylvania State University Human Environmental Age Thresholds project)

BACKGROUND

The earth's climate is warming and the frequency, duration, and severity of heat waves are increasing. Meanwhile, the world's population is rapidly aging. Epidemiological data demonstrate exponentially greater increases in morbidity and mortality during heat waves in adults ≥65 years. Laboratory data substantiate the mechanistic underpinnings of age-associated differences in thermoregulatory function. However, the specific combinations of environmental conditions (i.e., ambient temperature and absolute/relative humidity) above which older adults are at increased risk of heat-related morbidity and mortality are less clear.

METHODS

This review was conducted to (1) examine the recent (past 3 years) literature regarding heat-related morbidity and mortality in the elderly and discuss projections of future heat-related morbidity and mortality based on climate model data, and (2) detail the background and unique methodology of our ongoing laboratory-based projects aimed toward identifying the specific environmental conditions that result in elevated risk of heat illness in older adults, and the implications of using the data toward the development of evidence-based safety interventions in a continually-warming climate (PSU HEAT; Human Environmental Age Thresholds).

RESULTS

The recent literature demonstrates that extreme heat continues to be increasingly detrimental to the health of the elderly and that this is apparent across the world, although the specific environmental conditions above which older adults are at increased risk of heat-related morbidity and mortality remain unclear.

CONCLUSION

Characterizing the environmental conditions above which risk of heat-related illnesses increase remains critical to enact policy decisions and mitigation efforts to protect vulnerable people during extreme heat events.

Core temperature responses to compensable versus uncompensable heat stress in young adults (PSU HEAT Project)

With global warming, much attention has been paid to the upper limits of human adaptability. However, the time to reach a generally accepted core temperature criterion (40.2°C) associated with heat-related illness above (uncompensable heat stress) and just below (compensable heat stress) the upper limits for heat balance remains unclear. Forty-eight (22 men/26 women; 23 ± 4 yr) subjects were exposed to progressive heat stress in an environmental chamber during minimal activity (MinAct, 159 ± 34 W) and light ambulation (LightAmb, 260 ± 55 W) in warm-humid (WH; ∼35°C, >60% RH) and hot-dry (HD; 43°C-48°C, <25% RH) environments until heat stress became uncompensable. For each condition, we compared heat storage (S) and the change in gastrointestinal temperature (ΔTgi) over time during compensable and uncompensable heat stress. In addition, we examined whether individual characteristics or seasonality were associated with the rate of increase in Tgi. During compensable heat stress, S was higher in HD than in WH environments (P < 0.05) resulting in a greater but more variable ΔTgi (P ≥ 0.06) for both metabolic rates. There were no differences among conditions during uncompensable heat stress (all P > 0.05). There was no influence of sex, aerobic fitness, or seasonality, but a larger body size was associated with a greater ΔTgi during LightAmb in WH (P = 0.003). The slopes of the Tgi response during compensable (WH: MinAct, 0.06, LightAmb, 0.09; HD: MinAct, 0.12, LightAmb, 0.15°C/h) and uncompensable (WH: MinAct, 0.74, LightAmb, 0.87; HD: MinAct, 0.71, LightAmb, 0.93°C/h) heat stress can be used to estimate the time to reach a target core temperature from any given starting value.NEW & NOTEWORTHY This study is the first to examine heat storage and the rate of change in core temperature above (uncompensable heat stress) and just below (compensable heat stress) critical environmental limits to human heat balance. Furthermore, we examine the influence of individual subject characteristics and seasonality on the change in core temperature in warm-humid versus hot-dry environments. We provide the rate of change in core temperature, enabling projections to be made to and from any hypothetical core temperature.

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.

Critical environmental limits for young, healthy adults (PSU HEAT Project)

Critical environmental limits are those combinations of ambient temperature and humidity above which heat balance cannot be maintained for a given metabolic heat production, limiting exposure time, and placing individuals at increased risk of heat-related illness. The aim of this study was to establish those limits in young (18-34 yr) healthy adults during low-intensity activity approximating the metabolic demand of activities of daily living. Twenty-five (12 men/13 women) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress was performed with either 1) constant dry-bulb temperature (T_db) and increasing ambient water vapor pressure (P_a) (P_crit trials; 36°C, 38°C, or 40°C) or 2) constant P_a and increasing T_db (T_crit trials; 12, 16, or 20 mmHg). Each subject was tested during MinAct and LightAmb in two to three experimental conditions in random order, for a total of four to six trials per participant. Higher metabolic heat production (P < 0.001) during LightAmb compared with MinAct trials resulted in significantly lower critical environmental limits across all P_crit and T_crit conditions (all P < 0.001). These data, presented graphically herein on a psychrometric chart, are the first to define critical environmental limits for young adults during activity resembling those of light household tasks or other activities of daily living and can be used to develop guidelines, policy decisions, and evidence-based alert communications to minimize the deleterious impacts of extreme heat events.NEW & NOTEWORTHY Critical environmental limits are those combinations of temperature and humidity above which heat balance cannot be maintained, placing individuals at increased risk of heat-related illness. Those limits have been investigated in young adults during exercise at 30% V̇o_2max, but not during metabolic rates that approximate those of light activities of daily living. Herein, we establish critical environmental limits for young adults at two metabolic rates that reflect activities of daily living and leisurely walking.

Validity and reliability of a protocol to establish human critical environmental limits (PSU HEAT Project)

The PSU HEAT protocol has been used to determine critical environmental limits, i.e., those combinations of ambient temperature and humidity above which heat stress becomes uncompensable and core temperature rises continuously. However, no studies have rigorously investigated the reliability and validity of this experimental protocol. Here, we assessed the 1) between-visit reliability and 2) validity of the paradigm. Twelve subjects (5 M/7W; 25 ± 4 yr) completed a progressive heat stress protocol during which they walked on a treadmill (2.2 mph, 3% gradient) in a controllable environmental chamber. After an equilibration period, either dry-bulb temperature (T_db) was increased every 5 min while ambient water vapor pressure (P_a) was held constant (T_crit experiments) or P_a was increased every 5 min while T_db was held constant (P_crit experiments) until an upward inflection in gastrointestinal temperature (T_gi) was observed. For reliability experiments, 11 subjects repeated the same protocol on a different day. For validity experiments, 10 subjects performed a T_crit experiment at their previously determined P_crit or vice versa. The between-visit reliability (intraclass correlation coefficient, ICC) for critical environmental limits was 0.98. Similarly, there was excellent agreement between original and validity trials for T_crit (ICC = 0.95) and P_crit (ICC = 0.96). Furthermore, the wet-bulb temperature at the T_gi inflection point was not different during reliability (P = 0.78) or validity (P = 0.32) trials compared with original trials. These findings support the reliability and validity of this experimental paradigm for the determination of critical environmental limits for maintenance of human heat balance.NEW & NOTEWORTHY The PSU HEAT progressive heat stress protocol has been used to identify critical environmental limits for various populations, clothing ensembles, and metabolic intensities. However, no studies have rigorously investigated the reliability and validity of this experimental model. Here, we demonstrate excellent reliability and validity of the PSU HEAT protocol.

ClimApp-Integrating Personal Factors with Weather Forecasts for Individualised Warning and Guidance on Thermal Stress

This paper describes the functional development of the ClimApp tool (available for free on iOS and Android devices), which combines current and 24 h weather forecasting with individual information to offer personalised guidance related to thermal exposure. Heat and cold stress assessments are based on ISO standards and thermal models where environmental settings and personal factors are integrated into the ClimApp index ranging from -4 (extremely cold) to +4 (extremely hot), while a range of -1 and +1 signifies low thermal stress. Advice for individuals or for groups is available, and the user can customise the model input according to their personal situation, including activity level, clothing, body characteristics, heat acclimatisation, indoor or outdoor situation, and geographical location. ClimApp output consists of a weather summary, a brief assessment of the thermal situation, and a thermal stress warning. Advice is provided via infographics and text depending on the user profile. ClimApp is available in 10 languages: English, Danish, Dutch, Swedish, Norwegian, Hellenic (Greek), Italian, German, Spanish and French. The tool also includes a research functionality providing a platform for worker and citizen science projects to collect individual data on physical thermal strain and the experienced thermal strain. The application may therefore improve the translation of heat and cold risk assessments and guidance for subpopulations. ClimApp provides the framework for personalising and downscaling weather reports, alerts and advice at the personal level, based on GPS location and adjustable input of individual factors.

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.