Human body heat balance equation to consider core body temperature in assessment of heatstroke risk

Increased core body temperature and dehydration are critical thermal physiological responses to heat disorders. Dehydration is preventable through adequate water intake. However, a simple heat balance equation for assessing the heatstroke risk considering the core body temperature is currently lacking. Hence, this study proposes an equation for evaluating heatstroke risk. To derive the equation, the thermal regulation of the human body and heat interactions between the body core, skin, clothing, and indoor and outdoor environments were considered. The heat flow and heat capacity were represented as water flows caused by the water head difference and cross-sectional area of the tanks, respectively. Subsequently, the importance of considering the above elements as a unified system for designing a thermally comfortable environment and preventing heatstroke is discussed. Next, a heatstroke threshold equation is presented in the form of human body heat balance equation, which considers the core body temperature. This equation enabled us to evaluate the threshold for the combination of the environmental, clothing, and activity parameters beyond which the body temperature exceeds the acceptable limit for heatstroke. The equation was validated by comparing the thresholds with the reported upper limit of the prescriptive zone. The calculated thresholds for various parameters revealed that the parameters related to evaporative heat loss, such as clothing evaporative resistance, were the dominant parameters influencing the threshold. Although the equation can evaluate the influences of various types of activity and clothing, it can be applied only to acclimatized workers in good health with sufficient water and salt intake.