This study examined the effects of sauna air temperature and relative humidity (RH) on acute cardiovascular and thermoregulatory responses under typical Finnish sauna conditions, where participants were free to throw water onto the heated stones (löyly). Fifty recreationally active healthy adults (32 males, 18 females, 35.4 ± 10.4 years, 174 ± 9.9 cm, 77 ± 17 kg) completed four 10-minute sauna sessions, each separated by 30-minute recovery. Core temperature (Tcore), skin temperature, heart rate (HR), body mass, and perceptual responses were measured, alongside detailed multi-point environmental monitoring of temperature and relative humidity (RH) monitoring. Generalized estimating equations assessed associations between environmental factors and physiological responses. HR increased markedly during sauna exposure (mean +41.6 bpm), while Tcore rose moderately (+0.4°C). Temperature and RH were independently associated with increases in HR (β = 1.40 bpm/°C; β = 0.37 bpm/%RH) and Tcore (β = 0.032°C/°C; β = 0.0079°C/%RH). These associations remained significant after adjustment for sex, physical activity, body fat percentage, and skin surface area. The difference between dew point and skin temperature, used as an index of condensation potential, showed weak associations with HR and no significant relationship with Tcore. Repeated sauna sessions did not meaningfully influence HR responses, and only minor differences were observed in Tcore between sessions. Perceptual responses were primarily driven by temperature, with smaller contributions from humidity. These findings demonstrate that humidity, beyond temperature, appears to independently contribute to physiological strain during sauna exposure. The results highlight the importance of characterizing environmental conditions and suggest that humidity should be considered when optimizing sauna use for potential health benefits.
Exertional heat stress (ExHS) has a significant impact on human health and performance in various occupational and athletic sectors. While exercise is known to affect cytokine levels, there is limited information quantifying the circulating inflammatory response to acute exercise in the heat. This meta-analysis aims to quantify acute changes in circulating pro- and anti-inflammatory markers in response to ExHS among nonacclimatized humans and to determine if these responses are moderated by peak core temperature, heat exposure duration, or exercise intensity. Following PRISMA guidelines, four databases were searched for studies measuring cytokines IL-1β, IL-6, TNF-α, IL-8, IL-10, IL-1ra, and IFN-γ after exercise in temperatures ≥26°C (universal thermal climate index). Random-effects meta-analyses and meta-regressions were performed. Thirty‑six studies (N = 498, 86% male) were included. Acute ExHS induced significant immediate increase in IL-6, TNF-α, IL-8, IL-10, and IL-1ra (all p < 0.05). Both IL-6 and IL-10 remained significantly elevated 1-h post‑ExHS with IL-6 significant at 2 h post. Peak core temperature was a significant moderator for TNF-α and IL-10, and trended for IL-6 at immediate post-ExHS. No significant effects were observed for IL-1β or IFN-γ. Thermal strain in ExHS causes an abrupt disruption of immune homeostasis. The response shifts toward an anti-inflammatory state in early recovery, mirroring the cytokine profile of exertional heat stroke but at a lower magnitude. However, substantial heterogeneity makes future predictions from the model challenging and possibly unreliable.
This study aimed to analyze the running performance of highly trained young male football players during official matches played under different heat stress risk conditions. Young highly trained male football players (n = 37) from one club in Brazil were monitored during 50 official matches. Relative and absolute metrics were monitored, including total distance, low-to-moderate running, high-speed running and activities, sprinting distance, number of sprints, accelerations, and decelerations. Heat stress risk conditions were calculated using the wet-bulb globe temperature and were categorized as low, moderate, high. Linear mixed models were used to compare performance metrics under different heat stress risk conditions. Overall, relative and absolute performance metrics were reduced under high-risk conditions compared to low-risk: high-speed running distance and number of sprints and decelerations were reduced by 24%, 25%, and 19%, respectively (p < 0.05); in particular, high-speed running distance was markedly reduced in central defenders (-43%), fullbacks (-25%) and forwards (-19%) under high-risk conditions. Under moderate-risk conditions, compared to low-risk, high-speed running distance and sprinting were reduced (-16% and -26%, respectively; p < 0.01), with major differences in central defenders (-32%) and forwards (-16%). When high-risk conditions were compared with moderate-risk conditions, differences were observed in absolute metrics (p < 0.05): central defenders had reduced total distance and low-to-moderate running by 4-5%, whereas high-speed running distance was reduced in fullbacks (9%). These findings indicate that young highly trained football players reduce high-intensity running performance under heat stress, likely as a pacing strategy to cope with increased physiological strain during matches.
Circulating sex hormones play important roles in reproductive and non-reproductive physiological homeostasis. We evaluated the relationships of sex hormones measured in urine and serum with ambient temperature (maximum and average daily temperatures) in male and female soldiers undergoing initial military training (n = 237, 125F; data are mean ± SD; age: 21 ± 4 years, BMI: 24 ± 3, body fat: 28 ± 7%). Participants provided intermittent blood samples for assessment of 17β-estradiol, total and free-testosterone, luteinizing hormone (LH), and sex hormone binding globulin. Females not using hormonal contraceptives (non-HC users) additionally provided daily first morning urine samples which were analyzed for metabolites of estrogen normalized to creatinine (estrone conjugates, E1C) and progesterone (pregnanediol glucuronide, PDG), LH, and follicle stimulating hormone (FSH). Linear mixed effects models were used to evaluate the relationship between maximum and average daily temperatures and circulating hormonal and gonadotropin concentrations. In urine collected from female non-HC users, maximum daily temperature was inversely associated with E1C and FSH (p = 0.0019, p < 0.001, respectively). Total testosterone was inversely related to maximum daily temperature in males (p < 0.001) and positively associated in both groups of females (non-HC users: p = 0.004, HC users: p = 0.018). Estradiol was inversely related to maximum daily temperature in female non-HC users only (p = 0.001). There was no relationship between temperature (maximum or average) and serum or urine LH, or urine PDG. These findings suggest that ambient temperature is associated with modest sex- and hormone-specific variation in circulating sex hormone concentrations during prolonged physical training.
Sleep is recognized as a critical factor in athletic recovery and performance. However, elite athletes often experience inadequate sleep, and research suggests that disturbances in thermoregulatory processes may be a contributing factor. This study aimed to investigate the effects of a temperature-controlled mattress cover on thermal comfort and sleep outcomes in professional athletes. Thirty-two elite male Australian Rules Football athletes (mean ± SD age: 23 ± 4 years) participated in a 14-night intervention using a temperature-controlled mattress cover (Eight Sleep Pod 3) (POD, n = 16) or their own mattress (CON, n = 16). The Brief-Pittsburgh Sleep Quality Index (B-PSQI) was used to analyze changes in sleep quality. Post-intervention, the POD group rated perceived thermal comfort, sleep impact, satisfaction, and recovery on a 1-10 Likert scale. Linear mixed models showed significant group ×time interaction favoring POD over CON for total sleep time (TST; +29 min, g = 1.41, p = 0.008), improvements in sleep quality (-1.0, g = -3.06, p < 0.001), and global score (-2.4, g = -2.43, p < 0.001) all taken from the B-PSQI. Perceived thermal comfort in the POD group did not change significantly from pre- to post-intervention (median [Q1-Q3]l 3.0 [2.0-4.0] and 4.0 [2.0-4.0], p > 0.05). The use of a temperature-controlled mattress cover was associated with improvements in subjective sleep measures in our cohort of athletes. These preliminary findings suggest that the device may be a promising noninvasive strategy for improving perceived sleep quality, pending confirmation in randomized studies with validated objective sleep measures.
Exertional heat illness (EHI) is an under-recognized cause of emergency department admissions in otherwise healthy young people in athletic and occupational settings. Specifically, the frequency and severity of end-organ damage in EHI, which include exertional heat stroke (EHS), heat injury (HI), and heat exhaustion (HE) require better characterization to optimize triage and patient care. In the present study, we characterized end-organ damage following emergency department admission for EHI in young, otherwise healthy military service members who collapsed during training exercises. Standard clinical measures of creatinine (Cr), alanine transaminase (ALT), aspartate aminotransferase (AST), creatine kinase (CK), and troponin were used. Active-duty service members who were seen in the Martin Army Community Hospital emergency department and diagnosed with an EHI were eligible. Relevant laboratory data was retrieved from the electronic medical record after informed consent was obtained. Acute kidney injury was the most common form of end organ damage observed (73-86%) followed closely by liver damage (59-73%). EHS cases had higher peak damage levels for Cr, CK, AST, and ALT compared to either HI or HE. Muscle damage only occurred concurrently with liver or kidney damage. There was no disseminated intravascular coagulopathy, cardiovascular complications, or organ failure observed in this study. The frequency and severity of end-organ damage was greatest in EHS, followed by HI and HE. No patients developed organ failure or required transport to a higher level of care. Data suggest that cooling modality did not impact end-organ damage.
The perception of dental pain and thermal stimuli is governed by the specialized structural and physiological organization of the dentin - pulp complex. Teeth are subjected to rapid thermal changes during normal oral functions, while the dental pulp - an intensely vascularized and innervated tissue - is confined within rigid mineralized structures. Under these anatomical conditions, even mild thermal stimuli - especially cold - can induce rapid and intense pain sensations. The mechanisms underlying thermal sensitivity of dentin have been explained by several theories. The direct neural theory proposes that external stimuli directly activate trigeminal nerve endings, whereas the hydrodynamic theory suggests that temperature changes induce dentinal fluid movement within tubules, generating mechanical forces that stimulate pulpal afferent fibers. Increasing experimental evidence supports an integrated model in which odontoblasts (ODs) function as mechanosensory transducers. In this concept, thermal stimuli generate dentinal fluid movement and mechanical stress that activate ion channels and enzymes in ODs, leading to the release of signaling molecules such as ATP, glutamate and possible other mediators including nitric oxide. This thermomechanical coupling mechanism, linking dentinal fluid dynamics, odontoblast mechanotransduction, and neuronal activation, is the most widely accepted explanation for thermal pain and dentin hypersensitivity. Understanding these mechanisms provides important insights into dental nociception and may guide the development of more accurate clinical diagnosis and improved strategies for managing thermal dentin hypersensitivity.
Despite numerous studies on children's thermoregulation, there is limited collective evidence on thermoregulatory function and its outcomes in children. We aimed to systematically review the existing evidence on the thermoregulatory function during heat stress in children, specifically as compared with adults. Electronic databases were searched for studies assessing core temperature (Tc), sweating rate (SR), and skin blood flow (SkBF) in children and adults. Of the 4683 studies initially screened, 22 met the eligibility criteria, which included 507 research participants (20% female). Of 19 studies that reported Tc, 5 studies reported significantly higher resting and final Tc in children, but the change in Tc was not different between groups in any of the studies. In 19 of 22 studies that examined SR, 16 studies reported significantly lower (13-74%) SR per surface area (or per gland, or absolute) in children compared to adults, while 3 studies reported no significant difference between groups. Of 10 studies that examined SkBF, 7 studies reported higher SkBF in children compared to adults in the chest and/or forearm (16-83%), 2 studies reported lower SkBF in children (in the forearm and finger), and 3 studies reported no group differences in SkBF. In summary, data show similar body temperature regulation in children and adults, despite different use of means of heat dissipation. Inconsistencies among studies reporting differences in heat dissipation between children and adults may be related to differing states of training or acclimatization, as well as to the lack of systematic examination of thermoregulation in girls and women.
Regular exposure to Finnish sauna bathing (FSB) has been associated with reduced morbidity and mortality. This study aimed to examine the acute effects of FSB on immune cell mobilization, circulating cytokines, and their associations with changes in body temperature. A total of 51 adults - 27 women (mean age 50 ± 9 years, body mass index (BMI) 27 ± 5 kg/m2) and 24 men (mean age 50 ± 10 years, BMI 27 ± 3 kg/m2) - were exposed to a 30-minute session of acute FSB at a temperature of + 73°C. Venous blood samples were collected at baseline, immediately after and 30 minutes after the FSB and analyzed for immune cells and 37 cytokines. Subjects were allowed to drink water throughout. FSB increased body temperature from 36.4 ± 0.5°C to 38.4 ± 0.7°C, without altering plasma volume. Total white blood cell (WBC) count rose significantly and remained slightly elevated 30 minutes post-sauna in women. Neutrophil and lymphocyte counts increased immediately after the FSB but returned to baseline after 30 minutes, whereas MXD cells (monocytes, eosinophils, basophils) remained elevated. The levels of only two cytokines changed significantly. Although only a few correlations were observed between changes in immune cells and cytokines, 18 significant associations were identified between changes in body temperature and circulating cytokines - particularly immediately post-sauna - but not with WBC changes. Thus, a 30-minute session of acute FSB induces immune cell mobilization. The observed associations between changes in body temperature and circulating cytokines suggest that sauna-induced heat stress, along with immune activation, may partly mediate the health benefits of FSB.
Personal cooling strategies are used to lower body core temperature. This controlled cross-over trial evaluates the efficacy of cooling strategies following exercise-induced heat exposure resulting in increased body temperature. Ten healthy active males (25 ± 3 years, 84.5 kg with interquartile range 81.4-89.4) were randomized to a mixed cooling protocol every week within a five-week period. After completing a 60-minute intervention to increase body core temperature, they participated in one of four cooling interventions in thermoneutral (~24°C, ~45% relative humidity) conditions: (1) cool vest (18°C), (2) damp neck towel (24°C) in combination with forearm water immersion (15°C), 3) a combination of all methods, or 4) no intervention. In addition, 500 mL ice-cold water (0°C) was consumed during each cooling intervention (except for no intervention). Time to reach pre-exercise baseline core temperature and physiological responses (gastrointestinal temp-, skin temp- and heart rate) were measured. No significant differences were found between cooling conditions for heart rate (p ≥ 0.15) and body core temperature (p ≥ 0.18). Median cooling rates ranged between 0.02°C/min and 0.04°C/min across the groups with no significant difference (p = 0.51). Additionally, cooling duration (time to reach pre-exercise core body temperature) was not different between the groups (p = 0.38). Skin temperature decrement between the groups was significantly different (p < 0.001), specifically, control vs. combination (p = 0.029). In conclusion, no significant differences in cooling rate were found between mixed cooling interventions, but in all conditions with an average body core temperature ≥38.3°C gastrointestinal temperature decreased to safe levels within 60 minutes after moving to a thermoneutral room.
Lowering body temperature with cooling vests has been shown to increase energy intake (EI), but it is unclear whether a heating strategy would have the opposite effect and modify EI. We examined the acute effects of combined torso and foot heat exposure via a hot vest and a warm foot bath on EI. Eight young females and eight young males performed two randomized and crossover trials, 28 d apart. Trials on female were conducted during the early follicular phases. In the control trial (CON), participants remained seated from 09:30 to 11:30. In the hot trial (HOT), they remained seated from 09:30 to 10:30, after which they wore a hot vest (~45°C) and took a foot bath (~43°C) from 10:30 to 11:30. From 11:40, participants were given up to an hour to consume a buffet meal. EI was calculated from the buffet meal. Tympanic and skin temperatures, appetite, and thermal sensation were assessed. Tympanic temperature, skin temperature and thermal sensations were higher in HOT than in CON during 10:30-11:30 (all p < 0.05). EI, food amount, hot foods, and hot beverage consumption were lower in HOT compared with CON (all p < 0.05). Subjective appetite was similar between the two trials. Linear mixed model revealed that lower EI trended to be linked to higher tympanic temperature (β = -6.060, p = 0.069). Combined torso and foot heat exposure decreased EI in young healthy adults, with tympanic temperature predicting this reduction and no changes in subjective appetite.
Nearly a century ago, cold-induced vasodilation (CIVD) was first described as repeated episodes of warm blood flow to the fingers during cold-water immersion. Since then, hundreds of studies have examined this phenomenon, yet no comprehensive synthesis exists. To address this gap, we conducted a meta-analysis of studies in which the hand, or parts thereof, were immersed for 30 minutes in water below 20°C. A total of 80 studies met the inclusion criteria. Across studies, the weighted onset time of CIVD averaged 7.9 minutes [7.4-8.3], and the mean finger temperature averaged 10.0°C [9.5-10.6]. Onset time was weakly related to finger temperature during immersion (r = -0.21 to -0.27), supporting the theory that the onset of CIVD is triggered by low local tissue temperatures, while the magnitude is dependent on sympathetic activity. Onset time was longer for hand versus finger-only immersion, for individuals with a larger surface area, and for males compared to females. Onset time was shorter with higher ambient temperatures, in cold-indigenous populations, and with increasing age. To enrich the meta-analysis, we conducted a narrative literature review of the individual factors and previously proposed mechanisms of CIVD. Current evidence suggests that CIVD is mediated by 1) impaired transfer of noradrenaline from sympathetic nerves to the smooth muscle of the arterio-venous anastomoses or 2) nitric oxide release from these nerves, however, further research is needed to confirm these mechanisms. Future investigations should prioritize including more females and older adults, as these populations remain underrepresented in the literature.
Cold-induced metabolic responses across human organs and tissues vary markedly and do not regulate metabolism uniformly. The magnitude and nature of these responses differ depending on the type of cold exposure, ranging from mild surface cooling and beta-adrenergic stimulation to deep tissue cooling impacting intracellular biophysical and metabolic properties. Upregulating brown adipose tissue (BAT) activity has been proposed to improve whole-body metabolism. Despite its high metabolic activity, BAT mass is typically only 50-100 g and may contribute less than 1% of total heat production during thermogenesis. In contrast, skeletal muscles and white adipocytes may play greater roles in thermogenic and metabolic regulation. Cold exposure triggers a cascade of metabolic responses across tissues, extending beyond fuel partitioning and the regulation of uncoupling proteins. It also alters gene expression, protein synthesis, and metabolic pathways. In response to cold, the body increases sympathetic nervous system activity, leading to peripheral vasoconstriction and energy substrate mobilization. Brown adipocytes increase mitochondrial uncoupling to produce heat, while skeletal muscle contributes through shivering and non-shivering thermogenesis. The liver adjusts glucose production and lipid metabolism, the heart and circulatory system adapt to altered hemodynamic demands, and the kidneys modify fluid balance. Endocrine systems, including the thyroid, amplify thermogenic capacity, and the brain integrates thermal sensing with behavioral responses. Cold exposure also modulates immune function, cytokine profiles and inflammatory pathways across tissues, and shifts in gut microbiome composition influence nutrient absorption, bile acid metabolism and energy homeostasis. These coordinated tissue-specific adaptations enable the maintenance of core temperature during cold stress.
The 2025 FIFA Club World Cup was held primarily during the summer season in the Northern Hemisphere, with reports of athletes exposed to significant environmental heat stress. We investigated whether environmental conditions, along with other factors (e.g., time of day, players' age and field position, and club geographic origin), influenced physical performance in this tournament. Information about the performance during 57 matches (n = 1070 observations) was extracted from FIFA technical reports, whereas environmental conditions were obtained through mathematical modeling (ERA5 reanalysis). Linear mixed models were used to identify factors that explained variance in total distance covered and in distances covered at high, moderate, and low speeds. Mean wet-bulb globe temperature (WBGT) exceeded 28°C in 31 of the 57 matches analyzed, confirming that players were exposed to conditions of extreme heat illness risk. WBGT and air temperature explained total distance and distances at different speeds, while relative humidity explained distance only at high speeds (p < 0.001). More specifically, high WBGT, air temperature, and relative humidity values reduced the distances covered. Other factors also influenced players' performance, including their position and age, time of day, and club geographic origin: longer distances were observed in midfielders/forwards, younger players, in the evening, and in clubs from cold climates (p < 0.05). In conclusion, the findings from this tournament, which featured many matches under extreme heat, highlight the multifaceted regulation of physical performance in soccer and emphasize the prominent role of environmental conditions in determining the distance players cover at different speeds.
Turbulent airflow is a fundamental characteristic of real-world outdoor and mechanically ventilated environments, yet most thermoregulation models rely on heat transfer coefficients derived from steady indoor airflows with low turbulence intensity. Using an updated Stolwijk thermoregulation model and a turbulence-informed heat transfer correlation, we evaluated the impact of turbulence intensity and integral length scale on human thermophysiological responses. Simulations were conducted across three environmental conditions (hot-dry, hot-humid, temperate), two clothing levels (0 and 0.6 clo), two activity levels (1.2 and 4.0 MET), and air speeds ranging from 0.4 to 5 m/s. Results show that turbulence significantly enhances convective and evaporative heat loss in temperate and hot-dry environments when unclothed. Compared to baseline simulations that neglect turbulence characteristics at equivalent air speed, core temperature differed by up to 0.3°C, and skin temperature by up to 1.8°C, highlighting the potential physiological relevance of turbulence. In contrast, the influence of turbulence is minimal in hot-humid environments and when clothed. These findings demonstrate that turbulence should not be viewed as inherently beneficial or detrimental, but rather as a mechanistic modifier of heat and mass transfer whose physiological impact depends on context, including ambient temperature, metabolic rate, clothing, and the skin-air temperature difference. This work advances the field by introducing a turbulence-resolved approach to support the improved assessment of heat exposure across vulnerable populations, including outdoor workers and athletes, and to guide the design of more effective cooling strategies and ventilation systems, such as fans, based on different climate and personal contexts.
Thermoregulatory processes are closely linked to sleep initiation and maintenance throughout the circadian cycle, and may contribute to the increased tendency to nap in older adults. This cross-sectional study examined whether habitual napping in healthy older individuals is associated with altered skin temperature-derived heat-loss dynamics and their relationship with sleep onset. Thirty self-reported habitual nappers and 28 non-nappers (59-82 y) completed a 40-hour multiple-nap protocol under controlled laboratory conditions, with continuous polysomnography and distal-proximal skin temperature gradients (DPG) recordings. DPG was analyzed across scheduled wake episodes and at lights-off preceding each nap opportunity. Habitual nappers exhibited distinct changes in thermoregulatory dynamics compared to non-nappers. Overall, they had a lower DPG during scheduled wakefulness, particularly during the afternoon nap window (14:45-17:30). Their circadian organization of the DPG also differed markedly: they had a higher 24-hour DPG amplitude, a more pronounced 12-hour component, and an earlier DPG phase than the non-nappers. During nap opportunities, shorter sleep onset latency (SOL) was associated with a faster increase in DPG after lights-off in both groups. However, in habitual nappers, sleep onset occurred more rapidly despite a reduced dependence on pre-sleep DPG increase. Together, these findings indicate that habitual napping in older adults is accompanied by altered heat-loss rhythms across the circadian cycle and a reduced coupling between pre-sleep thermoregulatory dynamics and sleep initiation. Circadian-driven thermoregulatory changes may underlie the greater propensity to nap in older adults and differentiate habitual nappers from non-nappers. However, the causal direction of this relationship requires further investigation.
This study assessed the effects of hydration and hydration/cooling on various psycho-physiological and cognitive responses in staff during a simulated burns surgery. Twelve participants completed three 2.5-h trials in the heat (33.6°C, 36.4% RH) whilst walking on a treadmill at a rating of perceived exertion of 12 on the Borg scale. Trials consisted of: i) ingestion of 37°C water (HYD); ii) ingestion of 5°C water (COLD); and iii) a no cooling/hydration control (CON). Water ingestion (0.9% of body-mass) was based on fluid loss calculated during a previous 2.5-h burn surgery. Results demonstrated that while treadmill distance was similar between trials (p > 0.05), cold water ingestion resulted in improved manual dexterity (p = 0.03), better thermal comfort (p < 0.01) and lower core and skin temperatures (p < 0.01), compared to CON. Skin temperature was also lower in COLD vs HYD (p < 0.01). Moderate to large effect sizes (ES, g = 0.38-0.77) were observed in favor of COLD versus CON and/or HYD for manual dexterity, counting span, grammatical reasoning and several perceived workload subsets at various time points, however associated 95% confidence intervals were wide and crossed zero, suggesting statistical uncertainty. Similarly, moderate to large ES (g = 0.45-0.77) favored HYD over CON for counting span (120 min) and various perceived workload outcomes, though again confidence intervals suggest that these effects were not statistically conclusive. No differences were observed between trials for sweat loss, thermal sensation, or heart-rate (p > 0.05). Overall, cold water ingestion resulted in benefit to numerous variables assessed here. Small boluses of cold water ingestion are recommended during hot burn surgeries.
