Review on fabric thermal comfort in wet conditions

Synthetic progestins in oral contraceptives are thought to blunt heat dissipation by reducing skin blood flow and sweating. However, whether progestin-releasing intrauterine devices (IUDs) modulate heat loss during exercise-heat stress is unknown. We used direct calorimetry to measure whole-body total (dry + evaporative) heat loss in young, physically active women (mean (SD); aged 24(4)years, 39.3(5.3) ml/kg/min) with (IUD; n=19) and without (Control; n=17) IUDs in the follicular and luteal phases of the menstrual cycle during light- and moderate-intensity exercise at fixed rates of heat production (∼175 and ∼275W/m2 ) in 30°C, ∼21% relative humidity. Between-group and -phase differences were evaluated using traditional hypothesis testing and statistical equivalence testing within pre-determined bounds (±11W/m2 ; difference required to elicit a ±0.3°C difference in core temperature over 1h) in each exercise bout. Whole-body total heat loss was statistically equivalent between groups within ±11Wm-2 (IUD-Control [90% CIs]; Light: -2 [-8, 5] W/m2 , P=0.007; Moderate: 0 [-6, 6] W/m2 , P=0.002), as were dry and evaporative heat loss (P ≤ 0.023), except for evaporative heat loss during moderate-intensity exercise (equivalence: P=0.063, difference: P=0.647). Whole-body total and evaporative heat loss were not different between phases (P≥0.267), but dry heat loss was 3 [95% CIs: 1, 5] W/m2 greater in the luteal phase (P≤0.022). Despite this, all whole-body heat loss outcomes were equivalent between phases (P≤0.003). These findings expand our understanding of the factors that modulate heat exchange in women and provide valuable mechanistic insight of the role of endogenous and exogenous female sex hormones in thermoregulation. KEY POINTS: Progestin released by hormonal intrauterine devices (IUDs) may negatively impact heat dissipation during exercise by blunting skin blood flow and sweating. However, the influence of IUDs on thermoregulation has not previously been assessed. We used direct calorimetry to show that IUD users and non-users display statistically equivalent whole-body dry and evaporative heat loss, body heat storage and oesophageal temperature during moderate- and high-intensity exercise in a warm, dry environment, indicating that IUDs do not appear to compromise exercise thermoregulation. However, within IUD users and non-users, dry heat loss was increased and body heat storage and oesophageal temperature were reduced in the luteal compared to the follicular phase of the menstrual cycle, though these effects were small and unlikely to be practically meaningful. Together, these findings expand our understanding of the factors that modulate heat exchange in women and have important practical implications for the design of future studies of exercise thermoregulation.

Background: Acute mirabegron (MIRA) ingestion, a pharmaceutical that activates β3-adrenergic receptors on brown adipose tissue (BAT), enhances energy expenditure during thermoneutral and cool air exposure (i.e., 20°C). Cold-water immersion (CWI) accelerates heat loss, thus challenging thermoregulation during immersion. It is not known whether the acute pharmacological activation of BAT can aid thermoregulation during a progressive CWI challenge. We tested the hypothesis that acute MIRA ingestion would increase markers of BAT activation, mitigate the reduction in body temperature, and extend CWI duration during a progressive CWI challenge vs. placebo (PLA). Methods: 12 healthy adults (age: 25±6 y; BMI: 26±5 kg/m 2 , body fat: 27±5%, 6 women) completed two, double-blind, randomized study visits: ingestion of 100 mg of MIRA or PLA. 30 min after ingestion, participants rested in an empty water immersion tank. Following baseline measures, 35°C water rapidly filled the tank up to the sternum. The temperature of the water was progressively cooled by 2.5°C every 15 min until reaching 10°C, where it was held constant for up to 240 min of total immersion time. Participants were asked to remain in the water for as long as possible or complete 240 min of immersion. O 2 consumption (VO 2 ; ml/kg/min), CO 2 production (VCO 2 ; ml/kg/min), and respiratory exchange ratio (RER) were measured using indirect calorimetry. BAT activity was estimated via supraclavicular skin temperature (T sc ; infrared thermography) and expressed relative to shoulder skin temperature. Mean skin temperature (T sk ; 12 sites), and rectal temperature (T c ) were measured and used to calculate mean body temperature (T b ). Shivering was assessed via mechanomyography (MMG; 3 tri-axial accelerometers) and the bedside shivering scale (BSS; 0 = no shivering, 3=severe shivering). Perceptual measures of thermal discomfort (1=comfortable, 4=very uncomfortable) and sensation (1=cold, 7=hot) were also measured. Data were captured at 15 min intervals up until the last common timepoint for all CWIs (i.e., 60 min). Values are reported as mean ± SD. Results: Total CWI duration was greater following MIRA ingestion vs PLA (127.1±55.0 vs 142.2±44.3 min; p =0.050). VO 2 (PLA: 7.2±3.1 vs MIRA: 7.6±3.3 ml/kg/min; condition effect p =0.579), VCO 2 (PLA: 6.2±2.8 vs MIRA: 6.5±2.9 ml/kg/min; condition effect p =0.559), and RER (PLA: 0.85±0.07 vs MIRA: 0.86±0.07; condition effect p =0.825) did not differ between conditions. MIRA elicited greaterT sc (PLA: 36.0 ± 0.1 vs MIRA: 36.3±0.4°C; condition effect p =0.014) and relative T sc (PLA: 1.5± 0.4 vs MIRA: 1.9±0.4°C; condition effect p =0.003) vs PLA. MIRA caused higher T sk (PLA: 29.9± 2.3 vs MIRA: 31.2±1.6°C; condition effect p =0.013) and T b (PLA: 34.5±0.8 vs MIRA: 34.9±0.5°C; condition effect p =0.015) vs PLA, but T c did not differ between conditions (PLA: 37.0±0.2 vs MIRA: 37.0±0.2°C; condition effect p =0.812). MMG (PLA: 0.7±0.1 vs MIRA: 0.7±0.1 cm 2 ; condition effect p =0.762), BSS (PLA: 0.1±0.3 vs MIRA: 0.1±03; condition effect p =0.777), thermal discomfort (PLA: 1.6±0.7 vs MIRA: 1.5±0.6; condition effect p =0.530), and thermal sensation (PLA: 3.5±1.1 vs MIRA: 3.4±1.1; condition effect p =0.363) did not differ between conditions. Conclusion: Our data indicate that acute ingestion of MIRA extended the duration of immersion during a progressive CWI challenge and mitigated reductions in body temperature. Further, MIRA ingestion augmented markers of BAT activity, suggesting that the pharmacological stimulation of BAT might improve resiliency during CWI. Funded by: Office of Naval Research N00014-21-1-2276 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Heat loss during cold exposure in adult and aged C57BL/6J mice.

To maintain heat balance during exercise, humans rely on skin blood flow and sweating to facilitate whole body dry and evaporative heat exchange. These responses are modulated by the rise in body temperature (thermal factors), as well as several nonthermal factors implicated in the cardiovascular response to exercise (i.e., central command, mechanoreceptors, and metaboreceptors). However, the way these nonthermal factors interact with thermal factors to maintain heat balance remains poorly understood. We therefore used direct calorimetry to quantify the effects of dose-dependent increases in the activation of these nonthermal stimuli on whole body dry and evaporative heat exchange during dynamic exercise. In a randomized crossover design, eight participants performed 45-min cycling at a fixed metabolic heat production (200 W/m2) in warm, dry conditions (30°C, 20% relative humidity) on four separate occasions, differing only in the level of lower-limb compression applied via bilateral thigh cuffs pressurized to 0, 30, 60, or 90 mmHg. This model provoked increments in nonthermal activation while ensuring the heat loss required to balance heat production was matched across trials. At end-exercise, dry heat loss was 2 W/m2 [1, 3] lower per 30-mmHg pressure increment (P = 0.006), whereas evaporative heat loss was elevated 5 W/m2 [3, 7] with each pressure increment (P < 0.001). Body heat storage and esophageal temperature did not differ across conditions (both P ≥ 0.600). Our findings indicate that the nonthermal factors engaged during exercise exert dose-dependent, opposing effects on whole body dry and evaporative heat exchange, which do not significantly alter heat balance.NEW & NOTEWORTHY To maintain heat balance during exercise, humans rely on skin blood flow and sweating to facilitate dry and evaporative heat exchange. These responses are modulated by body temperatures (thermal factors) and several nonthermal factors (e.g., central command, metaboreceptors), although the way thermal and nonthermal factors interact to regulate body temperature is poorly understood. We demonstrate that nonthermal factors exert dose-dependent, opposing effects on dry and evaporative heat loss, without altering heat storage during dynamic exercise.

Heat and mass transfer of preterm neonates nursed inside incubators - A review

The mid‐luteal phase of the menstrual cycle is characterized by an upward shift in basal body core temperature secondary to elevated circulating estradiol and progesterone levels as compared to the early‐follicular phase. This elevation in body core temperature, perhaps together with increased estradiol‐mediated cutaneous vasodilation, may increase convective and radiative (dry) heat loss by improving core‐to‐skin heat transfer. This increase in dry heat loss may, in turn, reduce the need for sweat secretion (evaporative heat loss) without necessarily altering the relative contribution of these avenues to total heat exchange. Thus, the purpose of this study was to evaluate the hypothesis that females in the mid‐luteal phase would exhibit a greater contribution of dry heat loss to total heat loss as compared to the early‐follicular phase during exercise in warm‐dry conditions.Seven young, recreationally active, ovulating females (3 using hormonal intrauterine devices, mean (SD), 24 (3) years, V̇O2peak 40.5 (3.4) mL kg‐1 min‐1) completed two 45‐min bouts of semi‐recumbent cycling at a low (175 W m‐2; ~40% V̇O2peak) and high (275 W m‐2; ~65% V̇O2peak) rate of metabolic heat production, interspersed by 15‐min rest, in warm‐dry conditions (30.0 (0.2) °C, (25 (9) % relative humidity) in the early‐follicular (cycle days 2‐6) and mid‐luteal phase (days 19‐23). Metabolic heat production and dry and evaporative heat loss were measured via indirect and direct calorimetry, respectively. The contribution of dry heat loss was expressed as a percentage of total heat loss. Body core temperature (esophageal, n=6) and mean skin temperature (8 sites) were measured continuously. Averages of the final 5‐min of exercise in each heat load were compared between phases using dependent, two‐tailed t‐tests.As anticipated, resting body core temperature in the mid‐luteal phase was 0.2 °C [95% CI: 0.1, 0.3] higher than in the early‐follicular phase (p&lt;0.001). Core temperature remained 0.2 °C [0.1, 0.5] higher at the low heat load (37.8 (0.2) vs. 37.6 (0.3) °C, p&lt;0.01), but initial differences in core temperature were not observed in the high heat load (38.4 (0.3) vs. 38.3 (0.4) °C, p=0.83). Skin temperature and core‐to‐skin gradient were similar between phases throughout (all p≥0.21). Total heat loss was not different between phases at the low (162 (8) vs. 162 (13) W m‐2, p=0.93) or high (255 (10) vs. 252 (7) W m‐2, p=0.39) heat loads. However, the contribution of dry heat loss to total heat loss was 3% [1, 4] greater in the mid‐luteal phase at the low heat load (20 (8) vs. 17 (9) %, p&lt;0.01). This difference was not observed at the high heat load (12 (8) vs. 10 (9) %, p=0.16).In this preliminary analysis, females in the mid‐luteal phase exhibited a higher body core temperature and greater contribution of dry heat loss to total heat loss at low exercise‐induced heat loads in warm‐dry conditions. However, these differences were not observed at higher heat loads. These data provide novel mechanistic insight into the shifts in the contribution of the main avenues of heat loss in ovulating females across the menstrual cycle.

Hyperthermia after epidural analgesia in obstetrics

Thermal insulation and clothing area factors of typical Arabian Gulf clothing ensembles for males and females: Measurements using thermal manikins

Keeping premature newborns warm is crucial for their survival. Their ability to prevent excessive heat loss to the environment and to control their body temperature is limited. The risk of hypothermia is particularly important for low-birth-weight newborns with a large body surface area in relation to their mass of heat-producing tissues. The present study was performed to assess the body heat loss difference between small and large body-size premature newborns using two anthropomorphic thermal manikins of premature newborns of 900 g and 1,800 g (respective body surface areas of 0.086 and 0.150 m2). The dry heat loss from the six body segments of the small manikin (S) was measured and compared with that of the large manikin (L). The two manikins were exposed to five different environmental temperatures ranging between 29 and 35 degrees C in a single-walled, air-heated closed incubator. The magnitudes of heat loss decreased significantly by 20.4% between the two manikins [small manikin 110.1 (44.3) W/m2 vs large manikin 87.6 (25.8) W/m2, mean values with one standard deviation]. The results obtained from the comparison of the heat loss measures from the two manikins confirm the fact that the heat loss increases with an increase in the ratio of the body surface area to body mass. The thermal manikin appears to provide an accurate method for the assessment of thermal conditions in neonatal care.

Metabolic aspects of continuous renal replacement therapies

We report on the perceived comfort data collected while ten female subjects exercised in the hot, humid environment ( 29.4°C, 75% RH) wearing garments made from the three experimental knit fabrics. These findings are related to the fabric thermophys iological comfort data reported in Part I, the mechanical and surface related comfort data in Part II, and the skin alteration data in Part III of this series. At four times during the wear protocol (after acclimation, after 10 minutes of wear, after 40 minutes of exercise, and after 20 minutes of rest following exercise), subjects were asked to indicate overall comfort and thermal, wetness, and contact sensations. There was no difference between the fabrics for wetness or thermal sensation, a result explainable in terms of the extremely small differences in water and heat transport data reported in Part I. The thermal insulation, permeability index, and comfort limit values we calculated predict that differences in perceived thermal and wetness sensation should be minimal. Skin temperature was a significant determinant of perceived thermal comfort in our regression model, but capillary blood flow was not. The regression model for wetness sensation showed that stratum corneum water content and evaporative water loss were statistically significant determinants. Use of wetness-related and contact sensation descriptors differed for the three experimental fabrics. Differences in the wetness-related descriptors appear related to the percent water uptake of the fabrics during exercise. Fiber denier and fabric mechanical and surface feature data were useful in explaining the difference in contact sensations. The fabrics differed in perceived overall comfort. In the regression analysis, capillary blood flow was the only physio logical factor with a statistically significant effect on overall comfort. We suspect a link between the mechanical and surface features and capillary blood flow.

Metabolic heat production (calculated from oxygen consumption), dry heat loss (measured in a calorimeter) and body temperature (measured by telemetry) were recorded simultaneously at 6 min intervals over five consecutive days in rats maintained in constant darkness. Robust circadian rhythmicity (confirmed by chi square periodogram analysis) was observed in all three variables. The rhythm of heat production was phase-advanced by about half an hour in relation to the body temperature rhythm, whereas the rhythm of heat loss was phase-delayed by about half an hour. The balance of heat production and heat loss exhibited a daily oscillation 180 deg out of phase with the oscillation in body temperature. Computations indicated that the amount of heat associated with the generation of the body temperature rhythm (1.6 kJ) corresponds to less than 1 % of the total daily energy budget (172 kJ) in this species. Because of the small magnitude of the fraction of heat balance associated with the body temperature rhythm, it is likely that the daily oscillation in heat balance has a very slow effect on body temperature, thus accounting for the 180 deg phase difference between the rhythms of heat balance and body temperature.

Total calorimetric measurements in the rat: Influences of the sleep-wakefulness cycle and of the environmental temperature

Heat production (M), dry heat loss (R+C), evaporative heat loss (E) and rectal temperature (Tre) were measured in a direct calorimeter in female mongrel dogs acclimatized to outdoor climate at Kanazawa (latitude; 36 degrees 35" N), Japan. M and total dry and evaporative heat losses (HL) were minimum at calorimeter wall temperatures (TW) of 26-29 degrees C in summer and 22-26 degrees C in winter (thermoneutral temperature; TNT). The seasonal shift of the lower critical temperature was confirmed. At TW below TNT, the values of M and HL were significantly higher in summer. At TW above TNT, these values increased. A TNT and above, M and HL were significantly higher in winter. (R+C) decreased linearly with increasing TW in both seasons. AT TW below 26 degrees C, (R+C) were significantly higher in summer. At TW above 26 degrees C, E increased greatly. The values of E were significantly higher in winter at TW 29-32 degrees C. Tre remained nearly constant at TNT and below, and increased at TW above TNT in both seasons. Mean body surface temperature (Tsf) decreased with decreasing TW. Body thermal conductance (K) was minimum at TW below 26 degrees C in summer and at TW below 22 degrees C in winter. At TW above these temperatures, K increased significantly. Whole body insulation (I) was significantly higher in winter, particularly at TW 18 degrees C. These results suggest that the dogs reared outdoors in winter acclimatized to cold in two ways; by increasing the insulating effect of the fur coat and by elevating resting heat production.

Theoretical Minimum Thermal Load in Buildings