Development of New Heat Stress Index based on Human Energy Balance for Early Warming of Heat Disorder

Numerous thermal comfort and heat stress indices have been proposed, broadly classified into empirical indices, such as Wet-Bulb Globe Temperature (WBGT) and the Heat Index, and indices based on human heat balance models, such as the Universal Thermal Climate Index (UTCI) and the Standard Effective Temperature (SET*). Some of these indices are used for early warning of heat-related illnesses and assessing the health impacts of heatwaves. In Japan, a heat stroke alert system for community based on WBGT has been operational since 2021.

We will quantitatively demonstrate that while WBGT is widely used, it lacks accuracy in assessing heat stress, particularly when evaluating heat stroke risk. Furthermore, existing indices based on human heat balance models also have certain drawbacks when applied to heat stroke risk assessment; though the risk of heat stroke increases with Metabolic Equivalent of Task (METs) even in the same thermal condition, the lower values are yielded in these indices.

To address these limitations, we propose a new heat stress index that specifically considers the risk of heat stroke. Heat stroke occurs when excessive heat storage in the human body leads to an abnormal rise in core temperature. Therefore, our proposed index is designed to reflect the amount of heat stored in the body, providing a more physiologically relevant measure of heat stress. The effectiveness of this new index is evaluated and demonstrated, highlighting its potential as a more precise tool for heat stroke risk assessment.