# Comparison of Thermal Response for RF Exposure in Human and Rat Models
## 4 Discussion  
The thermal time constant has not been discussed until recently [23], except for a few studies on the exposure from handset antennas [20; 21; 22; 23]. Instead, thermal damage in animal studies has been commented on in the international guidelines/standards. For example, regarding the eye exposure in rabbits, temperature elevation was observed in the eye and the core, and then cataract formation was reported [13; 14]. To understand the phenomena as well as future design of experiments, the time course of the temperature elevation in the animal is important [13]. Thus, this study computed the temperature elevation in human and rat models for exposure at 3-10 GHz, taking into account the thermoregulation (vasodilatation). In addition to the temperature elevation, thermal time constants were evaluated and discussed. We demonstrated the difference between SAR and temperature distributions in the rat and human models. The ratios of local- and whole-body-averaged SAR were 18, 26, and 27, at 3, 6, and 10 GHz in the rat model, respectively. The SAR was distributed over the whole head at 3 GHz and in the rat brain, even at 10 GHz (Figure 2). The penetration depth of the skin is 18.8 mm at 3 GHz, 8.2 mm at 6 GHz, and 3.8 mm at 10 GHz [52]. The distance between the brain and the head surface was at most 2 mm in the rat. In the human model, the SAR was marginally distributed in the brain only at 3 GHz (Figure 3). The distance between the brain and the head surface of TARO was 13.5 mm, which was larger than the penetration depth above 6 GHz. As shown in Figure 4, the temperature elevation was distributed throughout the brain in the rat model but was contained locally in the human model. The temperature elevation above 0.1 \({}^{\circ}\)C was reached at approximately 2 cm in depth in the human model. We then compared the temperature elevation in rat and human models for different antenna distances and SARs. As shown in Figure 5, the peak temperature elevation in human skin was twice that of rat skin at the same head-antenna distance. This difference is attributable to the SAR distribution (volume of power absorption). The temperature elevation was characterized by the heat diffusion length in the volume (approximately 1 cm [50]). In addition, the SAR averaged over 10-g tissue in cubic shape was used as an index of comparison. Further, in the rat model, the skin is close to the brain, where blood perfusion is high, thus reducing skin temperature. In the brain, the peak temperature elevation in the human model was lower than in the rat model. Unlike the rat, the SAR was only slightly distributed in the human brain, and temperature elevation by heat conduction from external tissues became dominant. We compared the thermal time constants of rat and human models. As shown in Figure 7, the thermal time constant decreased at higher exposure intensity owing to the cooling effect of vasodilation  
Figure 8: (**a**) Effective SAR volume for each exposure scenario; and (**b**) relationship between the thermal time constant of skin and effective SAR volume in rat and human models. The output power of the antenna was adjusted so that the peak SAR averaged over 10-g tissue was 10 W/kg. or nonlinear thermoregulation. The thermal time constant also increased with the increase of the antenna-head distance, which can be attributed to a larger effective SAR volume. The effective SAR volume increased with the head-antenna distance, and may be related to the head size, which has been extensively discussed in [53; 54]. The thermal time constant in skin exhibited good correlation with the effective SAR volume, even if the head shapes were significantly different, such as in rats and humans. As shown in Figure 8, the thermal time constant increased as the effective SAR volume increased, but did so in nonlinear fashion. Focusing on each condition, the thermal time constant can be expected to saturate as the effective SAR volume increases.