Thermal impact zoning of indoor overheating environments on the human body in hot and humid regions

With the intensification of climate change, heatwaves causing power outages and indoor overheating have become increasingly frequent. However, how such indoor overheating environments affect the human body over time remains insufficiently understood, and comprehensive evaluation methods are still lacking. This study explores the thermal effects of indoor overheating through a three-zone framework representing thermal comfort, thermal health, and thermal safety. A climate chamber experiment was conducted under typical overheating conditions in hot and humid regions. Objective measurements and subjective questionnaires were combined to track psychological responses, adverse symptoms and physiological parameters over the exposure period. Based on the experimental results, a three-zone system was proposed, comprising the “Unacceptable Without Symptoms Zone” (TUAZ), “Unacceptable With Symptoms Zone” (SZ), and “Multiple Symptoms With Danger Zone” (TDZ), representing progressively increasing thermal stress on the human body. The findings revealed that thermal acceptability declined rapidly in overheating environments, with symptoms such as drowsiness, fatigue, restlessness, and chest tightness emerging over time, and that prolonged exposure increased life-threatening risks. At a heat index (HI) of 47 °C, the time thresholds for TUAZ, SZ, and TDZ are 0 min, 30 min and 120 min, respectively. Skin temperature serves as a key physiological indicator, with thresholds of 34.6 °C and 35.8 °C for zoning TUAZ and SZ. This study highlights the cumulative thermal impacts of indoor overheating and establishes a thermal impact zoning framework to support building resilience design under extreme heat.