Several studies reported the difference in heat tolerance between younger and older adults, which may be attributable to the decline in the sweating rate. One of the studies suggested a hypothesis that the dominant factor causing the decline in sweating was the decline in thermal sensitivity due to a weaker signal from the periphery to the regulatory centres. However, no quantitative investigation of the skin temperature threshold for activating the sweating has been conducted in previous studies. In this study, we developed a computational code to simulate the time evolution of the temperature variation and sweating in realistic human models under heat exposure, in part by comparing the computational results with measured data from younger and older adults. Based on our computational results, the difference in the threshold temperatures for activating the thermophysiological response, especially for sweating, is examined between older and younger adults. The threshold for activating sweating in older individuals was found to be about 1.5 °C higher than that in younger individuals. However, our computation did not suggest that it was possible to evaluate the central alteration with ageing by comparing the computation with the measurements for passive heat exposure, since the sweating rate is marginally affected by core temperature elevation at least for the scenarios considered here. The computational technique developed herein is useful for understanding the thermophysiological response of older individuals from measured data.
Core temperature elevation and perspiration in younger and older adults is investigated for plane-wave exposure at whole-body averaged specific absorption rate of 0.4 W kg(-1). Numeric Japanese male model is considered together with a thermoregulatory response formula proposed in the authors' previous study. The frequencies considered were at 65 MHz and 2 GHz where the total power absorption in humans becomes maximal for the allowable power density prescribed in the international guidelines. From the computational results used here, the core temperature elevation in the older adult model was larger than that in the younger one at both frequencies. The reason for this difference is attributable to the difference of sweating, which is originated from the difference in the threshold activating the sweating and the decline in sweating in the legs.
The temperature elevation in the fetus is of concern for radio-frequency exposure. According to the IEC standard, the temperature elevation in the fetus should be lower than 0.5 °C for exposure in magnetic resonance equipment. However, no previous study succeeded to simulate the temperature difference between the fetus and mother in the thermoneutral condition. The present study proposes a new thermal modeling for the pregnant woman model. The thermal modeling is then applied to the temperature variation for plane wave exposure at 80 MHz. From computational results, the core temperature in the fetus at the thermoneutral condition was 37.5 °C, which is 0.35 °C larger than that of the mother and coincident with measured data. When the pregnant woman model is irradiated by the plane wave with the whole-body averaged specific absorption rate of 2 W/kg for 1 hour, the temperature elevation in the fetus is 0.50 °C, which is larger than that in the mother by 0.11 °C.
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