Extreme heat poses an escalating threat to human health, yet current estimates of human heat tolerance across environmental conditions rely largely on shortcut or predictive approaches rather than direct evidence from prolonged experimental exposure. Here, we exposed 36 unacclimatized healthy young adults to four humid wet-bulb temperature (Tw) conditions (32°C, 33°C, 34°C and 35°C) for up to 8 h while performing seated light office work under unrestricted hydration. Core temperature (Tcore) responses were consistent across different dry-bulb temperature-humidity combinations at the same Tw, confirming the wet-bulb temperature as an effective integrated heat stress index in shaded extreme heat conditions without appreciable radiative sources. Conditions at Tw = 32-33°C were compensable, whereas Tw = 34-35°C were uncompensable, as evidenced by a clear progressive rise in Tcore over time. At Tw = 35°C, the widely cited upper survival limit, projected tolerance times to life-threatening hyperthermia were 7.1-7.7 h in males and 8.3-8.6 h in females, with females consistently exhibiting slower rates of Tcore increase. Collectively, these findings provide the first prolonged, stable-exposure experimental characterization of human heat tolerance across Tw = 32-35°C and enable psychrometric mapping of time-resolved, sex-specific tolerance limits. These empirical benchmarks constitute a critical empirical resource for refining climate-health projections of extreme humid heat and for validating biophysical models, while also complementing operational heat action plans and early-warning systems that focus on lower, sub-extreme heat exposures where the majority of heat-related morbidity occurs.

This study, based on an OVA-sensitized mouse model, systematically elucidates the molecular mechanisms through which 20 nm polystyrene nanoplastics (PS-NPs) exacerbate asthma. A series of assessments were conducted, including measurements of airway hyperresponsiveness (AHR), histopathological analysis of lung tissue using HE, PAS and Masson staining, immunohistochemical detection of phospholipase A2 (PLA2) and TRPV1 expression, quantification of serum immunoglobulins and tissue cytokines, as well as lung metabolomics and gut microbiota profiling. Exposure to PS-NPs activated PLA2 in lung tissue, leading to the accumulation of arachidonic acid metabolites such as prostaglandin E2 and leukotriene B4. This process increased TRPV1 channel expression and promoted the release of neuropeptides including substance P and calcitonin gene-related peptide. The resulting cascade activated the NF-κB signaling pathway, thereby enhancing Th2-type inflammatory responses characterized by elevated IL-4, IL-5 and IL-13, reduced IFN-γ, and increased oxidative stress markers such as 8-OHdG. PS-NPs also significantly altered the gut microbiota, increasing the abundance of Pseudomonadota, Actinomycetota and Verrucomicrobiota. Gram-negative bacteria released substantial amounts of hexa-acylated LPS, which activated the intestinal TLR4/NF-κB pathway and promoted pulmonary inflammation through the gut-lung axis. Furthermore, dysbiosis-induced reductions in short-chain fatty acid production and abnormalities in glycerophospholipid and amino acid metabolism further enhanced pulmonary PLA2 activity, forming a PLA2-TRPV1-neuroimmune positive feedback loop that aggravated airway hyperresponsiveness and lung tissue damage. Overall, this study suggests the central role of a metabolism-immune-neuroinflammatory network mediated by the gut-lung axis in asthma aggravated by PS-NPs, providing new insights into the respiratory toxicity of environmental nanoplastics.

When low supply momentum occurs, warm airflow from Diffuse Ceiling Ventilation (DCV) accumulates near the ceiling, leaving the occupied zone underheated. This study used ceiling fan to redistribute warm air above the occupied zone into the working area, thereby improving the indoor heating performance of DCV. DCV provided three vertical temperature differences (from 0.1 m to 2.2 m, 2 K, 4 K, and 6 K) during heating mode. Three ceiling fan speeds (forward and reverse at 45 rpm, 80 rpm, and 128 rpm) were chosen to force indoor heat transfer. The results showed that ceiling fans could significantly reduce vertical temperature differences. Forward rotation improved temperature distribution, whereas reverse rotation provided uniform airflow. Specific findings include: At 2 K, 4 K, and 6 K gradients, turning on the ceiling fan reduces the temperature difference in the room by up to 89 %, 91 %, and 88 %, respectively. After offsetting the cooling effect of air movement (CEAM), reverse rotation at 45 rpm can increase the T' (temperature variation and temperature corresponds to cooling effect of air movement superimposed) of the space below 1.1 m by 0.9 K, 1.6 K, 1.8 K and 1.7 m by 0.7 K, 1.2 K, 1.1 K. The lower reverse rotation 45 rpm of the ceiling fan resulted in less thermal dissatisfaction but less improvement in thermal stratification. Integrating ceiling fans could significantly improve DCV heating performance in the occupied zone while also providing a solution for airflow short circuits in winter.

The cooling effect, caused by the sustainable radiative cooling, might be an adverse factor for a cause thermal management system during cold hours. To address this issue, a self-adaptive radiative cooler (SARC), integrating thermochromic microcapsule (TCM) and silica aerogel (SA) in porous polymer substrate, was designed and fabricated for dynamic thermal management system. SARC can adaptively change its solar reflectance according to the external temperature, thereby, achieving dynamic radiative cooling regulation. When the external temperature exceeding the transition temperature, SARC will experience an apparent color change (from black/red/yellow/blue to white), indicating its capacity (Delta R-sol similar to 30 %) in dynamic manipulation of solar reflection. The incorporation of SA significantly increases the infrared emittance. The prepared SARC exhibited excellent elective infrared emissivity (0.922 in the atmospheric window), promoting the heat dissipation via radiation. The field test shows that SARC exhibiting excellent solar heating effect under solar radiation when the ambient temperature lower than the transition temperature. SARC shows similar radiative cooling performance to the porous TPU when the temperature exceeds the transition temperature. This study provided a facile approach to prepared radiative cooler with temperature-responsive solar reflectance.

In order to explore reasonable prebedtime interventions to improve the sleep quality of the youth population, this experiment comprehensively investigated the effect of prebedtime footbath on the improvement of youth sleep quality. The experimental conditions of the experimental group were to take a 30‐min footbath at 40°C 1 h before bedtime, to compare the experiment with the control group that did not take footbath, and to strictly control other environmental parameters that may affect sleep quality. We recorded the sleep of 16 male subjects using a subjective sleep quality questionnaire and polysomnography (PSG) and recorded their distal skin temperature (DST) and proximal skin temperature (PST) during footbath and sleep using temperature records. The skin temperature data showed that footbath before bedtime helped to increase DST and accelerate heat dissipation from the terminal skin, which in turn increased the distal–proximal skin temperature gradient (DPG), and we found that the DPG of the experimental group was higher than that of the control group for 84.8% of the time during the whole night’s sleep. Both subjective questionnaire and PSG monitoring results showed that sleep quality and sleep calmness could be effectively improved by taking a 30‐min 40°C bedtime footbath 1 h before bedtime. The subjective sleep quality questionnaire score of the control group was only 84.1% of that of the experimental group. There were significant differences between the control and experimental groups in total sleep time (TST), sleep‐onset latency (SOL), wake after sleep onset (WASO), and arousal index (AI) (p < 0.05). Compared to the control group, the experimental group showed a 43.4‐min increase in TST, a 14.9‐min decrease in SOL, a 32‐min decrease in WASO, a 3.28 beats/hour decrease in AI, and a 9.0% increase in sleep efficiency by performing a prebedtime footbath. This study quantitatively describes the effect of prebedtime footbath on the improvement of sleep quality in young men and provides an effective reference for the rational improvement of sleep quality in young people.

Prolonged exposure to extreme humid heat can induce systemic inflammation, organ stress, and hormonal imbalance. While fluid replacement is commonly recommended, its mechanistic efficacy under humid heat stress remains unclear. This study investigated the impact of fluid intake on thermoregulation, inflammation, organ function, and stress signaling during 8 h of humid heat exposure (ambient temperature: 40 °C, relative humidity: 55%) in 32 healthy young adults (20 males and 12 females). Participants completed two randomized trials: limited fluid intake (LFI, 125 mL/h) and full fluid intake (FFI, 375 mL/h). Core temperature (Tcore), inflammatory cytokines (IL-6, IL-1β, IFN-γ, TNF-α), organ stress markers (ALT, BUN), oxidative stress indices (MDA, SOD), and cortisol were assessed pre- and post-exposure. FFI significantly reduced post-exposure Tcore (37.8 ± 0.3 °C vs. 38.1 ± 0.3 °C, p = 0.046), mitigated cytokine elevations, and decreased BUN (blood urea nitrogen), ALT (alanine aminotransferase), and cortisol levels. Western blot analysis of PBMCs revealed that LFI activated NF-κB p65, JNK2, p38, and STAT3α phosphorylation, whereas FFI suppressed these responses. These findings demonstrate that adequate hydration attenuates heat-induced systemic and molecular stress responses. Our results highlight hydration as a key modulator of inflammatory signaling pathways during prolonged heat stress, offering insights into preventive strategies for populations vulnerable to climate-induced extreme heat events.

Background Prolonged heat exposure disrupts immune homeostasis and can precipitate acute systemic inflammation. However, the core temperature threshold that triggers sex-specific leukocyte and neutrophil activation during passive heat stress remains undefined. Methods We studied 52 males and 58 females exposed to wet-bulb temperatures of 32–35 °C. Rectal temperature ( Trec ) was continuously monitored, and blood samples were collected at 0.5 °C increments up to 38.6 °C. Leukocyte and neutrophil counts were modeled using quadratic and segmented regression to identify inflection points of immune activation. Results Both leukocytes and neutrophils increased nonlinearly with rising Trec ( p < 0.05). A critical Trec of 38.1 °C—closely aligns with the 38.0 °C occupational core temperature limits—marked an inflection in leukocyte responses: below this point, females showed steeper increases; whereas above it, males showed accelerated activation. Neutrophils demonstrated consistently greater mobilization in males across the entire temperature range (36.9–38.6 °C). Conclusions A distinct core temperature threshold (∼38.1 °C) governs immune cell activation and reveals sex-dependent response patterns. This finding provides an immunological rationale for current occupational het limits and highlights the importance of integrating sex-specific considerations into protective guidelines under extreme heat. ![Figure][1] ### Competing Interest Statement The authors have declared no competing interest. National Excellent Young Scientist Program, 6119924022 [1]: pending:yes

Health agencies, including the Centers for Disease Control and Prevention (CDC) and World Health Organization (WHO), recommend different temperature thresholds for electric fan use during heat events (CDC: 32.2 degrees C, WHO: 40 degrees C). Nonetheless, these guidelines do not account for the fan's physiological effects on immune and inflammatory responses in males and females. This study evaluated the efficacy of electric fan use in mitigating immune function, inflammation, and organ function during an eight-hour simulated extreme heat event replicating conditions in Hangzhou, China, on the August 3, 2024 (semi-hourly fluctuations; average temperature: 39.9 degrees C [37.6-41.1 degrees C], RH: 47.1% [40-57%]). Thirty-two young adults (16 females) underwent three eight-hour trials: (1) no electric fan with limited fluid intake (500mL; Con), (2) fan use with limited fluid intake (500mL; Fan) and (3) fan use with sufficient fluid intake (3L; Fan+Fluid). Core temperature, cardiovascular responses, plasma electrolytes, stress hormones, inflammatory markers, and organ function biomarkers were assessed. Fan+Fluid significantly reduced core temperature, stress hormone levels, inflammatory responses, and organ function biomarkers in both sexes (all p<0.05). Notably, Fan+Fluid greatly reduced IL-6, IL-1 beta, ALT and BUN by 23.9%, 32.5%, 15.9% and 23.6%, respectively, compared to CON, despite the core temperature difference is marginal. Females exhibited consistently higher stress hormone levels, inflammatory responses, and organ function markers than males across all trials (all p<0.05). These findings highlight the benefits of electric fan use in mitigating the effects of prolonged extreme heat, and suggest that females may benefit from more intense cooling interventions due to their heightened inflammatory and organ stress responses.

Acute uncompensable heat stress elevates core temperature and induces systemic inflammatory response syndrome (SIRS), a critical condition that can cause multi-organ failure. Electric fan use and fluid replacement are common strategies to reduce core temperature, but their efficacy in mitigating SIRS and acute liver and kidney injuries during prolonged humid heat remains unclear. This study investigated whether electric fan use with limited or with full fluid replacement could reduce SIRS and liver and kidney injuries after 8-h of extreme humid heat exposure (40 degrees C, 55 % RH). Twelve male participants underwent three 8-h trials: (1) no electric fan with limited fluid replacement (125 mL/h, Con), (2) electric fan use with limited fluid replacement (125 mL/h, Fan), and (3) electric fan use with fluid replacement (375 mL/h, Fan+Fluid). Body temperatures, inflammatory and organ-stress marker were measured. At the end of the exposure, both Fan and Fan+Fluid conditions resulted in lower core temperatures than Con at the end of the exposure (all P < 0.05), with Fan+Fluid showing the greatest reduction (P < 0.01). However, biomarkers including interlukin-6 and interlukin-1 beta and alanine transaminase (ALT), blood urea nitrogen (BUN) were not significantly different across conditions (all P > 0.07), although all increased from pre-exposure to the end of exposure (all P < 0.01). Furthermore, serum cortisol levels were all significantly elevated after the 8-h heat exposure (all P < 0.01), indicating increased psychological stress due to the prolonged heat exposure. Electric fan failed to prevent SIRS and acute liver and kidney injuries due to the inability to suppress cardiovascular and psychological strains.

Background: Increasing studies have associated physician-diagnosed otitis media (POM) with environmental elements. Nonetheless, the impacts of parental stresses and their interactions with interior environments and extraventricular air pollution on childhood POM remain unknown. Objectives: To investigate the impact of parental stresses and their interactions with interior and extraventricular environmental exposures on children's POM. Methods: We conducted a cross-sectional retrospective cohort study involving 8,689 children at Changsha in China. Health information, parental stress, and household environmental data for each infant were collected through questionnaires. Concurrently, individual exposure concentrations to pollutants and temperatures were computed using the Inverse Distance Weighted (IDW) technique. Multivariable logistic regression analyses were engaged to evaluate the effects of parental stresses and their interactions with interior environmental variables and ambient pollutants on childhood POM. Results: Children's POM were markedly associated with advantaged education and income among parents (ORs [95% CI] = 1.42 [1.12-1.80] and 1.45 [1.05-2.00]), while presence of psychological stress (fatigue, headache, and distractibility) enhanced POM risk (1.57 [1.29-1.91], 1.63 [1.39-1.91], and 1.30 [1.11-1.53]). Parents owning higher social stress (lower education) reported higher childhood POM risk of raising cats in previous year. Childhood POM risk from PM10, SO2 and NO2 exposure during previous year were higher in families with high economic stress (small housing size). POM risk from early life exposure to PM2.5 and SO2 during preconception, pregnancy, and first year was higher in parents with psychological stress. Conclusions: Parental stresses and their interactions with interior environmental factors and ambient air pollution play key roles in childhood POM.

Females have been found to react differently to temperature changes than males. In this study, we recruited 19 healthy young adults (9 males and 10 females) of comparable body size (height, body mass, skeletal muscle, and fat mass) to see if their thermophysiological and perceptual responses to stepwise cooling (from 24 °C to 20 °C and then to 16 °C, stepwise cooling: 4 °C/h) were similar. Perceptual responses (thermal sensation, thermal comfort, thermal pleasure, and thermal satisfaction) and physiological indicators (thyroid and sex hormone levels, metabolic rate, skin temperature, forearm-finger skin temperature gradient and core temperature) were recorded and compared. Results found no significant difference between males and females in thyroid hormone, metabolic rate, thermal comfort votes (TCVs), thermal sensation votes (TSVs), or mean skin temperature (all p > 0.05). Females demonstrated significantly higher thermal pleasure votes (TPVs), forearm-finger skin temperature gradient, core temperature, and chest and forehead skin temperatures compared to males (all p < 0.05). These findings indicate that males and females with matched body size/composition still have different thermal responses, which appears to be due to differences in cutaneous vasomotor tone between the sexes. Our findings lend support to the search for factors that influence differences in individual thermal preferences.

This study assessed the validity of core temperature measurements obtained using the wearable CORE system, compared to the rectal temperature readings, during 8-hour passive exposures to a wet-bulb temperature exceeding 30 degrees C, with and without the use of an electric fan. Thirty-eight young participants (24 males and 14 females) underwent separate exposures in both a CON condition (40 degrees C & 57 % RH [i.e., wet-bulb temperature: 32.0 degrees C], air velocity 0.15+0.05 m/s) and a FAN condition (40 degrees C & 58 % RH, air velocity: 3.2 + 0.4 m/s). In the CON condition, CORE accurately measured core temperature in males from 210 min onwards, and in females from 270 min onwards. The limit of agreement (LoA) between the wearable core temperature (Twearable) and rectal temperature (Trec) was moderate for males (lower to upper LoA: -0.71 degrees C to +0.33 degrees C) and females (-0.44 degrees C to +0.56 degrees C), with 8.7 % and 19.4 % of data points outside of the LoA for males and females, respectively. However, in the FAN condition, CORE significantly underestimated male core temperature throughout the 8hour trials and female core temperature from 60 to 330 min. The LoA between Twearable and Trec was poor for males (-0.56 degrees C to -0.11 degrees C) and females (-0.46 degrees C to -0.08 degrees C), with 48.7 % of points outside of the LoA for males and 48.2 % for females. Collectively, the CORE device was valid for measuring core temperature after 5 h of heat exposure in the CON condition for both sexes. However, its validity was lower in males than in females under FAN conditions.

The Core Temperature Inflection Point (CTIP) method and biophysical modeling are widely used to determine critical environmental limits (CELs), yet their validity under prolonged heat exposure remains untested. This study evaluated their predictive accuracy by exposing 36 healthy young adults (20 males, 16 females; age: 20.9–22.4 yr) to five counterbalanced 8-hour heat trials in a controlled chamber (36 °C/74.5% RH, 40 °C/55.0% RH, 44 °C/29.2% RH, 47 °C/35.6% RH, 50 °C/24.5% RH). These conditions were selected based on prior CTIP and biophysical model predictions of CELs. Participants engaged in sedentary office tasks (1.29–1.67 METs), wore standardized summer clothing (0.39–0.40 clo), and had ad libitum access to an electrolyte drink, with a 500– kcal sandwich provided at midday. Rectal temperature (Trec) was continuously monitored. Contrary to CTIP and biophysical model predictions, all five conditions remained compensable (Trec rise rate ≤ 0.1 °C/h), with mean peak Trec staying well below heatstroke thresholds (38.2 ± 0.4 °C). At 44 °C/29.2% RH, females had significantly lower Trec than males (p < 0.05), but steady – state Trec responses were similar between sexes (all p > 0.10). Collectively, CTIP and biophysical models substantially underestimated CELs, leading to overpredicted heat risk across all trials. These findings challenge the reliability of current predictive methods, suggesting human tolerance may exceed existing estimates. Refining these models is essential for improving heat risk assessment during real–world heatwaves and informing public and occupational health guidelines in a warming climate. ### Competing Interest Statement The authors have declared no competing interest.

Extreme heat and traffic-related air pollution (TRAP) have been linked to worsening chronic health disorders, however, their combined effects on diabetic nephropathy (DN) are little understood. Type II diabetic mice were exposed to heat (40 °C) and NO2 (5 ppm) separately for 4 h per day over 6 weeks to investigate the synergistic effects on the progression of DN. We found that exposure to high temperature and NO2 elevated blood glucose levels and exacerbated histopathological changes. Additionally, there were increased oxidation indicators (ROS, MDA, 8-OHdG) and decreased antioxidant indicators (CAT, SOD, GSH-PX), along with elevated inflammation markers (TNF-α, IL-1β, IL-6). The expressions of transient receptor potential (TRP) ion channels (TRPV1, TRPV4, TRPA1, TRPM2) were also upregulated. Our findings suggest that simultaneous exposure to high temperature and NO2 impairs metabolic and autophagy pathways. Exposure to both high temperature and NO2 produces a synergistic effect, leading to more severe damage than exposure to either factor individually. This resulted in increased expression of APOA1, P62, and p-mTOR/mTOR while decreasing the expression of p-AMPKα/AMPKα and LC3-II/I. This disruption promoted the progression of DN. In contrast, capsazepine (CZP) reduced TRP expression, inflammatory markers, oxidative stress, metabolic and autophagy disorders, thereby mitigating renal damage and alleviating the progression of diabetic nephropathy. Our study provides some potential strategies for early prevention and effective reduction of DN.

Previous research has linked stress and environmental factors to first-wave allergies (i.e., asthma, allergic rhinitis, and eczema), but their individual and combined effects on second-wave allergies (i.e., food allergies, including conditions such as egg, and milk allergies) remain unknown. We aim to investigate the effects of parental stress and its interaction with environmental factors on childhood physician-diagnosed food allergies (PFA). In Changsha (China), we performed a mixed cross-sectional and retrospective cohort investigation. we gather data on each preschooler's health status, parental stress, and living environment through questionnaires. Temperature and air pollutant exposures were calculated using inverse distance weighting methods. Multivariate logistic regression models were employed to examine relations pf PFA relates with parental stress, indoor and outdoor pollutants and allergens. We found that high/middle education and gross annual income were associated with total PFA (ORs [95% CI] = 1.89 [1.52-2.36] and 1.23 [1.01-1.50]) and other types of PFAs, while high psychological stress (fatigue, headache, and inattention) increased PFA risk (1.59 [1.34-1.90], 1.35 [1.18-1.55], and 1.41 [1.23-1.62]). Families experiencing higher economic stress reported higher PFA risk of blooming and non-blooming plants in first year. The PFA risk of mould and mildew stains in utero and the first year was higher in families with parental inattention. ORs of PFA due to PM2.5-10 and PM10 during the second trimester were higher in families with high parental social stress. PFA risk of PM2.5 in the late preconceptional and entire postnatal periods was higher in families with parental fatigue, headache, and inattention. We suggest parental stress independently and interplayed with indoor and outdoor environmental pollution and allergens, increases risk of childhood PFA.

With the intensification of global warming, extreme heat events such as heatwaves are becoming more frequent and severe, posing significant risks to the occupational safety and health of workers, particularly mental-demanding workers. Attentional decline during heat exposure is a major contributor to these risks. However, most prior studies on attention under sustained heat exposure have largely remained correlational, with limited use of quantitative physiological modelling. This study quantitatively evaluated different types of attentional performance under sustained heat exposures, and identified functional near-infrared spectroscopy (fNIRS) features that can predict attentional performance in real time. Fourteen right-handed participants were exposed to a control environment at 22 degrees C and three high-temperature environments (31 degrees C, 33 degrees C, 35 degrees C; relative humidity: 55 +/- 3 %) for 150 minutes while performing four different attentional tasks. fNIRS data from the left prefrontal cortex were continuously recorded. Eight time-domain and four frequency-domain features were extracted using a random forest algorithm. Results showed that the overall attentional performance index (OAPI) declined significantly at 33 degrees C and 35 degrees C compared to the control, with maximum reductions of 10.13 % and 8.87 %, respectively. Among all features, the time-domain metric Kurtosis exhibited a significant positive correlation with OAPI (p < 0.05). A quadratic regression model using Kurtosis explained 65 % of the variance in attentional performance (R-2=0.65, p < 0.05). These findings provide a quantitative framework for assessing cognitive function in hot environments and offer a potential tool for real-time monitoring to enhance the safety of mental workers posed by extreme heat events.

![Graphical abstract][1] Graphical abstract Highlights In brief Extreme humid heat, measured by wet-bulb temperature ( Tw ), is nearing the threshold of human survivability under climate change. In controlled trials with 36 adults, we provide the first empirical survival map across Tw =32–35°C, showing that hydration and behavioral adaptations extend endurance beyond the theoretical 6-hour limit, with women tolerating heat longer than men. These findings refine human heat tolerance thresholds and inform equitable climate adaptation and heat-health strategies. SCIENCE FOR SOCIETY As climate change accelerates, extreme humid heat events—measured by wet-bulb temperature ( Tw )—pose a growing threat to human survival. This study provides the first empirical benchmarks of human survivability across Tw =32–35°C, clarifying how long healthy adults can endure these conditions under full hydration and minimal activity. We find that while the theoretical 6-hour survival limit at Tw =35°C is partly supported, both men and women can withstand these conditions for 7–8 hours when hydrated, with women demonstrating greater tolerance. At Tw =34°C, projected heatstroke onset extends beyond 12–16 hours, while conditions at Tw =32–33°C remain largely compensable for more than 30 hours. These results challenge prevailing heat tolerance models, underscore the role of behavioral and physiological adaptations, and provide critical data for designing equitable heat-health policies and early warning systems that account for sex differences and vulnerable populations. Climate change-driven extreme heat events increasingly threaten human health. Here, we provide the first empirical map of human survivability across four extreme wet-bulb temperatures ( Tw =32– 35°C) in controlled trials. Thirty-six healthy young adults (20 males, 16 females) were exposed to twelve dry-bulb temperature and relative humidity combinations in shaded indoor settings under full hydration. Participants remained seated and performed light office tasks until core temperature ( Tcore ) reached 39°C or exposures lasted 8 hours. Clinical heatstroke threshold ( Tcore =40.5°C) were projected by extrapolating from our direct Tcore measurements during controlled exposure trials. At Tw =35°C, males and females reached heatstroke in approximately 7.1–7.7 and 8.3–8.6 hours, respectively—partly supporting the 6-hour theoretical limit while revealing enhanced short-duration resilience when hydrated. At Tw =34°C, projected times are 12.5–12.9 hours (males) and 15.5–16.2 hours (females). Conditions at Tw =32°C and 33°C remained (quasi-)compensable, with projected heatstroke onset after 33–35 hours. Females consistently tolerated heat stress longer than males. These physiological benchmarks are critical for improving heat-health models and embedding sex-specific vulnerability in equitable adaptation strategies amid accelerating global warming. ### Competing Interest Statement The authors have declared no competing interest. National Excellent Young Scientist Program, 6119924022 “Sanqin Scholars” Plan of Shaanxi Province, 2050225003 [1]: pending:yes

Background Humidity ramp protocols are widely used to determine human heat tolerance, yet it remains unclear how the rate and duration of environmental changes affect the apparent inflection in core temperature ( Tcr ). This study integrates theoretical modeling and empirical trials to examine how the temporal structure of humidity-ramp protocols affects the accuracy of estimated critical environmental limits (CELs). Methods A first-order model was developed to describe Tcr response to stepwise changes in equilibrium core temperature ( Tcr,eq ) determined by ambient humidity at a fixed dry-bulb temperature. Analytical solutions were derived for discrete humidity steps of duration Δ t , and sensitivity analyses were conducted across physiologically plausible time constants ( τ ). Fourteen healthy young males (23.5±1.8 yrs) completed two randomized trials in a 42 °C heat chamber: 1) Slow-ramp : 4-hour equilibration at 40% RH followed by +6% RH/hour for 2 h, then +3% RH/hour (40–61% RH); and 2) Aggressive-ramp : 30 min equilibration followed by +2% RH every 5 min (28–88% RH). Rectal and skin temperatures, heart rate, and perceptual ratings were recorded continuously. Results When Δt/τ ≪1, residual disequilibrium between Tcr,eq and Tcr accumulates, producing accelerated rises in Tcr and premature CELs. Longer dwell durations (≥ 1 hour per step) allowed near-equilibrium responses, yielding physiologically valid thresholds. Empirically, shorter ramp durations shifted apparent CELs downward by 3.4±1.9 °C. Conclusion Dynamic lags from short dwell intervals lead to the systematic underestimation of heat tolerance. Reliable determination of CELs requires either prolonged steady-state exposures or dynamic correction models validated against such conditions. Key points ### Competing Interest Statement The authors have declared no competing interest. National Excellent Young Scientist Program, 6119924022 “Sanqin Scholars” Plan Program