Humid Heat Stress Research Articles

Agricultural Irrigation Exacerbates Humid Heat Stress in the Mid‐Lower Reaches of the Yangtze River

AbstractMassive irrigation across eastern China (EC) could reduce extreme heat by altering energy and water budgets. However, the irrigation effect on increasing humidity has often been missed, especially in humid regions, leaving its effect and mechanism on extreme humid heat (EHH) poorly characterized. We analyzed assemblies of observations and performed regional simulations with more detailed irrigation scheme to explore the irrigation impacts and potential mechanisms on EHH. We find that irrigation in EC, despite having a cooling effect of about 0.2–0.6C, leads to an increase of about 0.4–0.8C in EHH, with a more intense impact on the Middle‐Lower Yangtze Plain (MLYP) by 0.9C. Cooling effect induced by increased latent heat fluxes through irrigation contributes to air deposition, which lowers boundary layer height, raises near‐surface moist enthalpy, and ultimately exacerbates the EHH. Results mechanistically emphasized irrigation impacts on EHH and highlighted the necessity of improving irrigation modeling reality.

Future change of humid heat extremes and population exposure in Turkey

AbstractGlobal climate projections show that humid heat extremes will expand toward the higher latitudes, making the midlatitudes hotspots for these extremes. Therefore, a thorough explanation of their regional characteristics becomes crucial, given that the changes in these extremes can potentially render a large proportion of the global population at risk. Here, we perform the first analysis of historical and projected changes in the intensity and frequency of humid heat extremes and quantify the population exposure to these extremes in Turkey, using long‐term simulations from the non‐hydrostatic mesoscale model of Consortium for Small‐scale Modeling (COSMO‐CLM) under the RCP8.5 emission scenario. We portray not only the nationwide changes in the humid heat extremes and population exposure but also their regional aspects by exploiting the K‐means clustering algorithm. Our results suggest significant future increases in the intensity and frequency of these extremes over a wide geographical area, which includes the surroundings of Adana, Antalya, Izmir, Sakarya, Ordu and Diyarbakir, most of which are coastal locations. Over most of these regions, severe humid heat stress is expected to last nearly a month every year, with almost 56% of the land area is projected to experience local historical upper tail heat stress conditions for at least an additional 10 consecutive hours. Further, we explicate a significant rise in the number of people exposed to severe humid heat stress, concentrated along most coastal regions, by as much as 1.6 million person‐days. More than 20% of Turkey's population may confront severe humid heat stress for at least 1 h, with that percentage falling to 4.15% for at least five consecutive hours, which indicates that people will not only endure more intense humid heat stress but also be exposed to these conditions consecutively over a period of many hours.

Vulnerability Assessment of a Highly Populated Megacity to Ambient Thermal Stress

The urban ambient environment is directly responsible for the health conditions of millions of people. Comfortable living space is a significant aspect that urban policymakers need to address for sustainable planning. There is still a notable lack of studies that link the spatial profile of urban climate with city-specific built-up settings while assessing the vulnerability of the city population. Geospatial approaches can be beneficial in evaluating patterns of thermal discomfort and strategizing its mitigation. This study attempts to provide a thorough remote sensing framework to analyze the summer magnitude of thermal discomfort for a city in a tropical hot and humid climate. Spatial profiles of dry bulb temperature, wet bulb temperature and relative humidity were prepared for this purpose. A simultaneous assessment of various discomfort indices indicated the presence of moderate to strong heat stress to a vast extent within the study area. The central business district (CBD) of the city indicated a ‘danger’ level of heat disorder for outdoor exposure cases. Nearly 0.69 million people were vulnerable to a moderate threat from humid heat stress, and around 0.21 million citizens faced strong heat stress. Combing city morphology in the study showed that mid-rise buildings had the maximum contribution in terms of thermal discomfort. City areas with built-up cover of more than 68%, along with building height between 5.8 m and 9.3 m, created the worst outdoor discomfort situations. Better land management prospects were also investigated through a multicriteria approach using morphological settlement zones, wind direction, pavement watering, building regulations and future landscaping plans. East–west-aligned road segments of a total 38.44 km length were delineated for water spray cooling and greener pavements. This study is likely to provide solutions for enhancing ambient urban health.

Evaluating Japan’s revised heat-health warning system in the face of recent escalating heat stress

In 2021, the Japanese government changed the exposure indicator of the national heat-health warning system (HHWS) from air temperature (T air) to Wet Bulb Globe Temperature (T WBG), reflecting the growing concerns about the escalating humid heat stress. However, a clear validation of the advantages of using heat stress indicators (HSIs) that combine T air and humidity in heat alerts and heat-health applications are still being explored. Here, by using the latest epidemiological data (2015–2019) before the COVID-19 pandemic, we examined the rationality of the revised HHWS for 47 prefectures in Japan. Specifically, we investigated the predictive power of different HSIs in modeling mortality and morbidity caused by different diseases (e.g. all causes, circulatory, respiratory, and heatstroke) and age groups. Our findings revealed substantial differences among the HSIs in identifying periods of intense heat stress, potentially leading to differences in the activation dates of the HHWS if various indicators were employed. While HSIs exhibited comparable performances in modeling daily mortality, our analysis demonstrates distinct advantages in using T WBG for daily morbidity predictions, and the quasi-Akaike Information Criterion of T WBG is much lower than the previously used T air. The merits of T WBG are consistent in modeling all causes, non-external, as well as heatstroke-related morbidity. Overall, this study underscores the practicality of incorporating HSIs in heat stress early warnings and provides critical insights for refining the HHWS to mitigate health impacts from heat stress under future climate change.

A comparative analysis of accelerating humid and dry heat stress in India

Extreme humid heat stress poses distinct challenges to human health and productivity that cannot be mitigated solely by heat action plans designed for dry heat stress. This study investigates the trends in extreme heat stress, differentiating between dry and moist heat, in India from 1943 to 2022 using the high-resolution hourly ERA5 reanalysis data. The analysis utilizes the Heat Index (HI) as a key parameter to identify instances of extreme humid heat stress, characterized by HI values exceeding 41 degrees Celsius and relative humidity more than 50%. The findings indicate that certain eastern coastal regions in peninsular India experience extreme humid heat stress from May to June due to persistent high humidity levels. In addition, northwest, northcentral and inland eastern coastal regions encounter extreme dry heat stress preceding the monsoon season, followed by a transition to humid heat stress immediately after the onset of the monsoon. The results also show that there has been a significant increase ofsix times (on average) in the extreme humid heat stress hours per grid over the past 80 years compared to a threefold increase in dry heat stress. Our research underscores the need to shift from exclusively addressing dry heat stress to adopting a comprehensive approach that considers the impacts of humid heat stress.

Observational Evidence Reveals Compound Humid Heat Stress‐Extreme Rainfall Hotspots in India

AbstractSequential climate hazards, such as “warm and wet” compound extremes, have direct societal implications for highly urbanized regions and agricultural production. While typically extreme temperatures and rainfall are inversely correlated during the summer, extreme humid heatwaves often lead to atmospheric instability and moisture convection, increasing the likelihood of extreme precipitation (EP). Little is known about how heatwave characteristics, such as peak intensity and duration, influence EP at a regional scale. Using high‐resolution, sub‐daily station‐based observational records over five decades (1971–2021) across India, we find a robust increase in the frequency of compound humid heat‐peak precipitation events in all seasons. Our sensitivity analysis of the impact of humid heatwave characteristics on the subsequent sub‐daily rainfall extremes reveals that, with an increase in peak heatwave intensity for a given heatwave duration, >50% of sites show an increase in the magnitude of rainfall; conversely, with an increase in heatwave duration for a given peak heatwave intensity, around 67% sites show a decline in sub‐daily rainfall extremes. An asymmetrical shift toward above‐average precipitation extremes in response to humid heat stress is mainly clustered around low‐elevation, densely populated coastal areas and the irrigation‐intensive Indo‐Gangetic Plains.

Significant impacts of humid heat stress on city-level human mortality

Significant impacts of humid heat stress on city-level human mortality

Large humidity effects on urban heat exposure and cooling challenges under climate change

Many urban climates are characterized by increased temperature and decreased relative humidity, under climate change and compared to surrounding rural landscapes. The two trends have contrasting effects on human-perceived heat stress. However, their combined impact on urban humid heat and adaptation has remained largely unclear. Here, we use simulations from an earth system model to investigate how urbanization coupled with climate change affects urban humid heat stress, exposure, and adaptation. Our results show that urban humid heat will increase substantially across the globe by 3.1 °C by the end of the century under a high emission scenario. This projected trend is largely attributed to climate change-driven increases in specific humidity (1.8 °C), followed by air temperature (1.4 °C)—with urbanization impacts varying by location and of a smaller magnitude. Urban humid heat stress is projected to be concentrated in coastal, equatorial areas. At least 44% of the projected urban population in 2100, the equivalent of over 3 billion people worldwide, is projected to be living in an urban area with high humid heat stress. We show a critical, climate-driven dilemma between cooling efficacy and water limitation of urban greenery-based heat adaptation. Insights from our study emphasize the importance of using urban-explicit humid heat measures for more accurate assessments of urban heat exposure and invite careful evaluation of the feasibility of green infrastructure as a long-term cooling strategy.

Near-term regional climate change in East Africa

In the coming few decades, projected increases in global temperature and humidity are generally expected to exacerbate human exposure to climate extremes (e.g., humid-heat and rainfall extremes). Despite the growing risk of humid-heat stress (measured by wet-bulb temperature), it has received less attention in East Africa, where arid and semi-arid climatic conditions prevail. Moreover, no consensus has yet been reached across models regarding future changes in rainfall over this region. Here, we screen Global Climate Models (GCMs) from CMIP5 and CMIP6 and use, for boundary conditions, simulations from only those GCMs that simulate successfully recent climatic trends. Based on these GCMs and Regional Climate Model (RCM) simulations, we project that annual mean temperature is likely to rise by 2 ℃ toward midcentury (2021–2050) ​at a faster rate than the global average (about 1.5 ℃), under the RCP8.5 and SSP5-8.5 scenarios, associated with more frequent and severe climate extremes. In particular, low-lying regions in East Africa will be vulnerable to severe heat stress, with an extreme wet-bulb temperature approaching or exceeding the US National Weather Service’s extreme danger threshold of 31 ℃. On the other hand, population centers in the highlands of Ethiopia will receive significantly more precipitation during the autumn season and will see more extreme rainfall events, with implications for flooding and agriculture. The robustness of these results across all GCM and RCM simulations, and for both of CMIP5 and CMIP6 frameworks (CMIP: Coupled Model Inter-comparison Project) supports the reliability of these future projections. Our simulations of near-term climate change impacts are designed to inform the development of sound adaptation strategies for the region.

Physiological responses of Holstein calves and heifers carrying the SLICK1 allele to heat stress in California and Florida dairy farms.

Inheritance of the SLICK1 allele of the prolactin receptor gene improves thermotolerance of lactating Holstein cows under humid heat stress conditions. The aim of this study was to investigate whether pre- and postweaning Holstein heifers carrying the SLICK1 allele would show physiological responses indicative of higher tolerance to heat stress in high- and low-humidity climates. A total of 101 heifer calves of two age groups heterozygous for the SLICK1 allele and 103 wild-type half-siblings were evaluated during July 2020 in 3 dairy farms in central California and 2 in south Florida. Dry bulb temperature and relative humidity data were recorded during evaluation and used to calculate the temperature-humidity index (THI). Physiological measurements were obtained between 1600 and 1900 h in California, and 1200 and 1400 h in Florida and included rectal temperature, respiration rate, skin temperature, and sweating rate. Data were analyzed via Generalized Linear Mixed Models including the main effects of genotype, state, group, sire, farm within state, and interactions, with THI included as a covariate. The correlations between THI and dependent variables were analyzed via linear regression. The average 24-h THI was higher in Florida compared with California (90 vs. 72, respectively); the main driver of the higher THI in Florida was the high relative humidity (average 85.6% in Florida vs. 36.7% in California). In Florida, the rectal temperature of slick calves was 0.4°C lower than non-slick calves (39.5 ± 0.1 vs 39.9 ± 0.1°C); no differences were detected between slick and non-slick calves in California. Regardless of genotype, heifer calves in Florida had higher respiration rate, higher rectal and skin temperatures, and lower sweating rate than in California. This study is the first to evaluate physiological responses of calves carrying the SLICK1 allele under heat stress conditions in different climates. Our findings demonstrate that the presence of this allele is associated with lower rectal temperatures in pre- and post-weaning Holstein females. According to the physiological parameters evaluated, calves raised in Florida appeared to be under more severe heat stress; in those conditions, the SLICK1 allele was advantageous to confer thermotolerance as evidenced by lower rectal temperature in slick animals.

Tomato (Solanum lycopersicum L.) Genotypes Respond Differently to Long-Term Dry and Humid Heat Stress

Tomato production in coastal areas in West Africa is constrained by heat stress. There is currently limited empirical evidence on the extent of the effect of heat stress on tomato yield in the sub-region. In this study, we assessed the effects of heat stress on yield and yield components among 16 tomato genotypes with varying heat tolerance status and explored the potential of stress tolerance indices to identify heat tolerant genotypes. The experiments were conducted under three temperature and humidity regimes, namely optimal season (28.37/23.71 °C and 71.0/90.4% day/night), long-term mild and humid (greenhouse, 30.0/26.2 °C and 77.6/97.2%), and long-term mild and dry (open field, 31.50/28.88 °C and 66.72/77.82%) heat stress (HS). All genotypes exhibited significantly higher fruit set percentage, fruit number per plant, fruit weight, and fruit weight per plant in the optimal season compared to both heat stress conditions. In general, the genotypes demonstrated higher performance under dry HS (i.e., HS in open field HSO) than humid HS (i.e., HS in greenhouse HSG). Fruit set decreased by 71.5% and 68.3% under HSG and HSO, respectively, while a reduction of 75.1% and 50.5% occurred in fruit weight per plant under HSG and HSO, respectively. The average sum of ranks values from nine stress tolerance indices and fruit weight per plant (used as proxy trait of yield) identified CLN2498D, CLN3212C, CLN1621L, and BJ01 as heat tolerant under HSG and BJ01, BJ02, Fla.7171, and P005 as heat tolerant under HSO. Fruit weight per plant under long-term heat stress (Ys) and optimal growing conditions (Yp) were suitable to select high performing genotypes under HSO, HSG, and optimal conditions while relative stress index, yield stability index, yield index, stress susceptibility index, and harmonic mean were suitable to select heat tolerant genotypes under either HSG or HSO. Our findings shed light on the extent of the effect of HS on tomato production in the off-season in coastal areas in West Africa and provide new insight concerning the heat tolerance status of the evaluated tomato genotypes.

Near-term regional climate change over Bangladesh

Bangladesh stands out as a climate change hot spot due to its unique geography, climate, high population density, and limited adaptation capacity. Mounting evidence suggests that the country is already suffering from the effects of climate change which may get worse without aggressive action. Here, we use an ensemble of high-resolution (10 km) regional climate model simulations to project near-term change in climate extremes, mainly heat waves and intense rainfall, for the period (2021–2050). Near-term climate projections represent a valuable input for designing sound adaptation policies. Our climate projections suggest that heatwaves will become more frequent and severe in Bangladesh under the business-as-usual scenario (RCP8.5). In particular, extremes of wet-bulb temperature (a temperature and humidity metric important in evaluating humid heat stress) in the western part of Bangladesh including Bogra, Ishurdi, and Jessore are likely to exceed the extreme danger threshold (according to U.S. National Weather Service criterion), which has rarely been observed in the current climate. The return periods of extreme heat waves are also significantly shortened across the country. In addition, country-averaged rainfall is projected to increase by about 6% during the summer months, with the largest increases (above 10%) in the eastern mountainous areas, such as Sylhet and Chittagong. Meanwhile, insignificant changes in extreme rainfall are simulated. Our results suggest that Bangladesh is particularly susceptible to climate extremes in the near future, in the form of extreme heat waves over the western part of the country.

Aging Impairs Whole-Body Heat Loss in Women under Both Dry and Humid Heat Stress.

This study was designed to determine whether age-related impairments in whole-body heat loss, which are known to exist in dry heat, also occur in humid heat in women. To evaluate this possibility, 10 young (25 ± 4 yr) and 10 older (51 ± 7 yr) women matched for body surface area (young, 1.69 ± 0.11; older, 1.76 ± 0.14 m, P = 0.21) and peak oxygen consumption (V˙O2peak) (young, 38.6 ± 4.6; older, 34.8 ± 6.6 mL·kg·min, P = 0.15) performed four 15-min bouts of cycling at a fixed metabolic heat production rate (300 W; equivalent to ~45% V˙O2peak), each separated by a 15-min recovery, in dry (35°C, 20% relative humidity) and humid heat (35°C, 60% relative humidity). Total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat storage was measured as the temporal summation of heat production and loss. Total heat loss was lower in humid conditions compared with dry conditions during all exercise bouts in both groups (all P < 0.05), resulting in 49% and 39% greater body heat storage in young and older women, respectively (both P < 0.01). Total heat loss was also lower in older women compared with young women during exercise bouts 1, 2 and 3 in dry heat (all P < 0.05) and bouts 1 and 2 in humid heat (both P < 0.05). Consequently, body heat storage was 29% and 16% greater in older women compared with young women in dry and humid conditions, respectively (both P < 0.05). Increasing ambient humidity reduces heat loss capacity in young and older women. However, older women display impaired heat loss relative to young women in both dry and humid heat, and may therefore be at greater risk of heat-related injury during light-to-moderate activity.

Influence of menstrual phase and arid vs. humid heat stress on autonomic and behavioural thermoregulation during exercise in trained but unacclimated women.

Despite an attenuated fluctuation in ovarian hormone concentrations in well-trained women, one in two of such women believe their menstrual cycle negatively impacts training and performance. Forthcoming large international events will expose female athletes to hot environments, and studies evaluating aerobic exercise performance in such environments across the menstrual cycle are sparse, with mixed findings. We have identified that autonomic heat loss responses at rest and during fixed-intensity exercise in well-trained women are not affected by menstrual cycle phase, but differ between dry and humid heat. Furthermore, exercise performance is not different across the menstrual cycle, yet is lower in humid heat, in conjunction with reduced evaporative cooling. Menstrual cycle phase does not appear to affect exercise performance in the heat in well-trained women, but humidity impairs performance, probably due to reduced evaporative power. We studied thermoregulatory responses of ten well-trained [V̇O2 max , 57 (7) ml min-1 kg-1 ] eumenorrheic women exercising in dry and humid heat, across their menstrual cycle. They completed four trials, each of resting and cycling at fixed intensities (125 and 150W), to assess autonomic regulation, then self-paced intensity (30min work trial), to assess behavioural regulation. Trials were in early-follicular (EF) and mid-luteal (ML) phases in dry (DRY) and humid (HUM) heat matched for wet bulb globe temperature (WBGT, 27°C). During rest and fixed-intensity exercise, rectal temperature was ∼0.2°C higher in ML than EF (P<0.01) independent of environment (P = 0.66). Mean skin temperature did not differ between menstrual phases (P ≥ 0.13) but was higher in DRY than HUM (P<0.01). Local sweat rate and/or forearm blood flow differed as a function of menstrual phase and environment (interaction: P≤0.01). Exercise performance did not differ between phases [EF: 257 (37), ML: 255 (43)kJ, P = 0.62], but was 7 (9)% higher in DRY than HUM [263 (39), 248 (40)kJ; P<0.01] in conjunction with equivalent autonomic regulation and thermal strain but higher evaporative cooling [16 (6)Wm2 ; P<0.01]. In well-trained women exercising in the heat: (1) menstrual phase did not affect performance, (2) humidity impaired performance due to reduced evaporative cooling despite matched WBGT and (3) behavioural responses nullified thermodynamic and autonomic differences associated with menstrual phase and dry vs. humid heat.

A Lower Heat Activated Sweat Gland Density in Larger Individuals Does Not Impair Local Sweat Rates During Hot and Humid Heat Stress

While the absolute number of sweat glands is primarily determined in utero, a larger body surface area (BSA) will naturally reduce the density of sweat glands per surface area which may compromise sweat rate per unit BSA, and consequently the maximal skin surface sweat coverage (i.e. skin wettedness: ωmax) during exercise in a hot/humid environment. The purpose of the present study was to compare the heat activated sweat gland density (HASGD), sweat gland output (SGO), local sweat rate (LSR) and ωmax between groups differing greatly in BSA during hot and humid active heat stress. Twelve participants separated into two groups based on body size (Small (SM): n=6, 65±6 kg, 1.8±0.1 m2; Large (LG): n=6, 102±13 kg, 2.3±0.1 m2) exercised on an upright cycle ergometer at an evaporative heat balance requirement (Ereq) of 240 W/m2 and 290 W/m2 for up to 75 min at 36°C, 70% RH. LSR of the upper back was measured throughout and expressed relative to surface area (mg/cm2/min). HASGD in number of sweat glands per cm2 (glands/cm2) was measured medially to LSR using the starch iodine technique every 15 minutes and averaged throughout the trial. Mean SGO was derived from the division of LSR and mean HASGD, and expressed as μg/gland/min. On a separate occasion ωmax was measured using a humidity‐ramp protocol (increase of 0.3 kPa from 2.3 kPa every 7.5‐min) during exercise at a fixed external workload of 100W at 36°C until an inflection of esophageal temperature was observed. LSR was similar at 240 W/m2 (SM: 1.24±0.24; LG: 1.40±0.47 mg/cm2/min, P=0.25) and 290 W/m2 (SM: 1.36±0.18; LG: 1.36±0.36 mg/cm2/min, P=0.48). HASGD was greater in SM compared to LG group at 240 W/m2 (SM: 67±12; LG: 55±6 glands/cm2, P=0.03) and 290 W/m2 (SM: 73±9; LG: 57±11 glands/cm2, P&lt;0.01). SGO was trending to be greater in the LG group at 240 W/m2 (SM: 18.6±5.0; LG: 25.2±10.2 μg/gland/min, P=0.10) while significantly greater in the LG group at 290 W/m2 (SM: 17.3±1.6; LG: 21.2±5.7 μg/gland/min, P=0.03). On the other hand, ωmax was not different between groups (SM: 0.82±0.16; LG: 0.72±0.07, P=0.13). In conclusion, a lower HASGD in people with a greater BSA is compensated by a greater SGO to yield a similar LSR up to an Ereq of 290 W/m2 and a similar ωmax. Future research should attempt to assess how even greater reductions in HASGD due to disproportionally larger differences in BSA (ie morbid obesity) compromise LSR, ωmax, and therefore the level of heat stress that can be physiologically compensated.Support or Funding InformationSupported by a NSERC Discovery Grant (P. Imbeault &amp; O. Jay)

Humid heat exposure induced oxidative stress and apoptosis in cardiomyocytes through the angiotensin II signaling pathway.

Exposure to humid heat stress leads to the initiation of serious physiological dysfunction that may result in heat-related diseases, including heat stroke, heat cramp, heat exhaustion, and even death. Increasing evidences have shown that the humid heat stress-induced dysfunction of the cardiovascular system was accompanied with severe cardiomyocyte injury; however, the precise mechanism of heat stress-induced injury of cardiomyocyte remains unknown. In the present study, we hypothesized that humid heat stress promoted oxidative stress through the activation of angiotensin II (Ang II) in cardiomyocytes. To test our hypothesis, we established mouse models of humid heat stress. Using the animal models, we found that Ang II levels in serum were significantly up-regulated and that the Ang II receptor AT1 was increased in cardiomyocytes. The antioxidant ability in plasma and heart tissues which was detected by the ferric reducing/antioxidant power assay was also decreased with the increased ROS production under humid heat stress, as was the expression of antioxidant genes (SOD2, HO-1, GPx). Furthermore, we demonstrated that the Ang II receptor antagonist, valsartan, effectively relieved oxidative stress, blocked Ang II signaling pathway and suppressed cardiomyocyte apoptosis induced by humid heat stress. In addition, overexpression of antioxidant genes reversed cardiomyocyte apoptosis induced by Ang II. Overall, these results implied that humid heat stress increased oxidative stress and caused apoptosis of cardiomyocytes through the Ang II signaling pathway. Thus, targeting the Ang II signaling pathway may provide a promising approach for the prevention and treatment of cardiovascular diseases caused by humid heat stress.

Age-related differences in heat loss capacity occur under both dry and humid heat stress conditions.

This study examined the progression of impairments in heat dissipation as a function of age and environmental conditions. Sixty men (n = 12 per group; 20-30, 40-44, 45-49, 50-54, and 55-70 yr) performed four intermittent exercise/recovery cycles for a duration of 2 h in dry (35°C, 20% relative humidity) and humid (35°C, 60% relative humidity) conditions. Evaporative heat loss and metabolic heat production were measured by direct and indirect calorimetry, respectively. Body heat storage was measured as the temporal summation of heat production and heat loss during the sessions. Evaporative heat loss was reduced during exercise in the humid vs. dry condition in age groups 20-30 (-17%), 40-44 (-18%), 45-49 (-21%), 50-54 (-25%), and 55-70 yr (-20%). HE fell short of being significantly different between groups in the dry condition, but was greater in age group 20-30 yr (279 ± 10 W) compared with age groups 45-49 (248 ± 8 W), 50-54 (242 ± 6 W), and 55-70 yr (240 ± 7 W) in the humid condition. As a result of a reduced rate of heat dissipation predominantly during exercise, age groups 40-70 yr stored between 60-85 and 13-38% more heat than age group 20-30 yr in the dry and humid conditions, respectively. These age-related differences in heat dissipation and heat storage were not paralleled by significant differences in local sweating and skin blood flow, or by differences in core temperature between groups. From a whole body perspective, combined heat and humidity impeded heat dissipation to a similar extent across age groups, but, more importantly, intermittent exercise in dry and humid heat stress conditions created a greater thermoregulatory challenge for middle-aged and older adults.

Age-related Differences In Heat Loss Capacity Occur Under Both Dry And Humid Heat Stress Conditions

Age-related Differences In Heat Loss Capacity Occur Under Both Dry And Humid Heat Stress Conditions

Induction of Heat-Shock Protein 70 Expression by Geranylgeranylacetone Shows Cytoprotective Effects in Cardiomyocytes of Mice under Humid Heat Stress

BackgroundIncreasing evidence has revealed that humid heat stress (HHS) causes considerable damage to human health. The cardiovascular system has been suggested to be the primary target of heat stress, which results in serious cardiovascular diseases. However, there is still a lack of effective approaches for the prevention and treatment of cardiovascular diseases induced by HHS.ObjectiveHeat-shock proteins (Hsps), especially Hsp70, are reported to provide effective cytoprotection under various stress stimuli. In the present study, we evaluated the cytoprotective effect of geranylgeranylacetone (GGA), which was previously been reported to induce Hsp70 expression in cardiomyocytes under HHS.Methods and Principal FindingsUsing a mouse model of HHS, we showed that the pretreatment of GGA enhanced Hsp70 expression under HHS, as examined by quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot. We then examined the effect of GGA pretreatment on the cardiomyocyte apoptosis induced by HHS using terminal-deoxynucleoitidyl transferase mediated nick end labeling (TUNEL) staining, and found that GGA pretreatment inhibited mitochondria-mediated apoptosis. GGA pretreatment could reverse the effect of HHS on cell apoptosis by increasing expression of Bcl-2, decreasing cytochrome c in cytosol, and increasing cytochrome c in mitochondria. However, GGA pretreatment had no effect on the oxidative stress induced by HHS as determined by levels of superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione (GSH).ConclusionWe have demonstrated that GGA pretreatment suppressed HHS-induced apoptosis of cardiomyocytes through the induction of Hsp70 overexpression.

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).