Experimental investigation on human thermal responses to static and dynamic ramp temperatures in simulated hot-humid underground environments

• Thermal responses under static and dynamic temperatures were studied. • Subject experienced short-term heat acclimatization under static conditions. • Rising air temperature caused hysteresis effects in core temperature response. • Skin temperature threshold decreased by 0.82°C under dynamic conditions. Thermal conditions in underground environments are critical for occupants' safety, health, and productivity. While workers in these settings typically experience dynamic thermal variations, existing research has predominantly focused on static conditions. This study experimentally investigates human physiological and perceptual responses under both static (29, 32 °C) and dynamic ramp-change conditions (26–32/35/38 °C) in a climate chamber. The thermal sensation vote (TSV), thermal comfort vote (TCV), wetness sensation vote (WSV), core temperature (T c ), and mean skin temperature (T sk ) were assessed. Results indicate that under static conditions, these parameters stabilised after 20–30 min due to short-term thermal acclimatisation. Under dynamic conditions, they continued to evolve and exceeded static levels after approximately 40 min. As the ambient temperature increased from 26 to 38 °C, 7.39% of subjects experienced severe dyspnea, and heart rate and total fluid loss significantly increased by 10.51 bpm and 54.15 g compared with a static 32 °C condition. T sk , TSV, and WSV increased markedly, while TCV demonstrated a declining trend. A hysteresis phenomenon was observed, whereby the average core temperature under dynamic conditions was lower than that under static conditions, which diminished with larger temperature increases. A strong linear correlation was identified among TSV, TCV, T c, and T sk , indicating that the rate of T c acceleration increased when T sk exceeded 35.24°C. Notably, the core temperature threshold for mild discomfort (TCV>-0.5) under dynamic conditions is 37.19°C, which is 0.82°C lower than that under static conditions. The increase rate of T c accelerates significantly from 0.005°C/min to 0.012°C/min when T sk exceeds 35.24°C. Under 26–38°C condition, 7.39% of subjects experienced severe dyspnea, with heart rate and total fluid loss increasing by 10.51 bpm and 54.15 g compared to 32°C condition. These findings provide critical insights for improving workers' thermal comfort in hot-humid underground environments.