Computational estimation of body temperature and sweating in the aged during passive heat exposure

This study discusses the difference in temperature elevation and sweating between younger and older adults during ambient heat exposure. The bioheat equation is solved computationally in an anatomically based human body model to track the variation in the temperature and sweating in the time domain. Our computational code is improved by introducing different blood temperatures in different body regions and taking into account the maximum possible evaporative heat loss. The reduced thermal sensitivity of the hypothalamus in the aged adults (mean age of 73.9 years) is also estimated from literature data and taken into account in a revised formula for the thermoregulatory response. For ambient heat exposure (a temperature of 40 °C and relative humidity of 42%), our computational results are in good agreement with measurement data in the literature for both younger adults (mean age of 23.5 years) and the elderly (67.8 years old), suggesting the effectiveness of our improved bioheat modeling. The reduction in the thermal sensitivity of the hypothalamus is estimated as 0.6 ± 0.2 °C for the aged (mean age of 73.9 years), although it was not significant for the elderly (67.8 years). For an ambient temperature of 35 °C and relative humidity of 60%, the computed core temperature elevation in the model corresponding to the thermophysiological response of the aged is 0.92 °C, which is higher than those for the younger adults, 0.25 °C, and for the elderly, 0.45 °C. This difference in the core temperature elevation is attributable mainly to the decline in the thermal sensitivity of the hypothalamus. The total perspiration at ages of 67.8 years and 73.9 years was 904 g and 645 g, respectively, which is smaller than that of the younger adults, 1090 g.