# Comparison of Thermal Response for RF Exposure in Human and Rat Models
## 4 Discussion
The increment of the effective SAR volume results in thermal diffusion, causing the delay in temperature elevation [23]. This result suggests that the effective SAR volume can be estimated as a metric in superficial tissues. Note that the SAR volume is not a good metric for 5 cm\({}^{3}\) or less, because it is larger than the heat diffusion length. The difference of the time constant in skin between rat and human models is caused by the difference of the effective SAR volumes (see Figure 8). The effective SAR is useful for estimating the thermal time constant of superficial tissue, which varies greatly depending on the shape and size of the head, type and position of antenna, and the frequency applied. In the inner tissue, such as the brain, the thermal time constant is also characterized by the heat conduction time, which is mainly attributable to the distance from the heat source to the target point. The heat conduction time (\(\tau_{D}\) in Equation (9)) observed in the human brain was approximately 40, 120, and 150 s at 3, 6 and 10 GHz, respectively. In contrast, \(\tau\) decreases as the frequency increases. Therefore, the thermal time constant (\(\tau+\tau_{D}\)) in the human brain is less sensitive to frequency. The difference of the time constant between rat and human brains is caused by the heat conduction time and differences in temperature elevations. The limitation of this study is in the modeling of the thermoregulatory response. We used the vasodilation models of the brain and skin that were based on our rat measurements for local RF exposure [25] and cutaneous veins of a dog with local warming [39], respectively. The thermoregulatory response model for blood perfusion was assumed to be the same for both rat and human because of the lack of specific data. In our earlier studies, the thermoregulatory models were based on measured values [7; 25], which allowed us to examine the temperature elevation for local RF exposure in rats [7; 25] and rabbit eyes [6; 55; 56] with good accuracy. However, extrapolation of these results from animals to humans is considered conservative [2]; lower temperature elevation may be expected in actual human tissues. The antenna position was chosen to be different from the how people actually use mobile phones, because we decided to set the antenna position for accurate comparison of the thermal response between human and rat models. In that situation, the antenna is likely to be present near pinna, cheek, and mouth. For a dipole antenna located at the side of the pinna, most of the power is absorbed in the pinna [57]. For a dipole antenna located close to the cheek or mouth, the temperature elevation in the brain obviously decreases because of distance. The thermal damage depends on tissue sensitivity, temperature, and exposure time [24]. The cumulative equivalent minutes at 43 \({}^{\circ}\)C are used as a model to calculate the thermal dose. In this evaluation, the thermal time constant is essential. Focusing on the brain, which is a highly heat-sensitive tissue, the thermal time constant in humans was more than twice that in rats. Thus, the exposure time required for thermal damage is correspondingly increased. Furthermore, because the characteristics of the temperature rise of deep tissues in rats and humans are different, extrapolation from small animals to humans in deep tissues needs further attention. However, these findings suggest that excessive temperature elevation is caused with increasing frequency in rat brains, but only marginally occurs in the human brain. These results exclude the possibility of thermally induced brain tissue damage for the limits stated in current international guidelines, especially at higher frequencies. To determine exactly how conservative these limits are when applied to humans, further experimental studies are necessary.