# Comparison of Thermal Response for RF Exposure in Human and Rat Models
## 1 Introduction  
The rapid development of wireless technology has changed the electromagnetic environment, leading to concerns regarding potential adverse health effects caused by electromagnetic field exposure. In radio frequency (RF) exposure (above 100 kHz), the dominant factor is the thermal effect. Therefore, induced temperature elevation in biological bodies is an important factor. There are two international guidelines/standards for human protection from electromagnetic fields, which have been mentioned by the World Health Organization (WHO) as international recommendations: the International Commission on Non-ionizing Radiation Protection (ICNIRP) guidelines [1] and IEEE standards [2]. In the guidelines/standards, the metric for human protection against RF fields is the specific absorption rate (SAR) averaged over 10 g of tissue for RF-localized fields from 100 kHz to 3 GHz [2] or to 10 GHz [1]. At frequencies higher than 3 or 10 GHz, surface heating becomes dominant; therefore, incident power density is used as a metric for human protection instead of SAR. The SAR limit is 10 W/kg for occupational exposure or in restricted areas, and a reduction factor of five is applied for the general public or in unrestricted areas. Wireless devices used in our daily life are designed to comply with the SAR limit for general public (2 W/kg). The rationale for the limit is based on studies of thermal damage in animals [3; 4], which were then extrapolated to humans (see the review in ICNIRP [5] and IEEE standards [2]). This was partly because of ethical reasons. To the best of our knowledge, only the authors' group has developed a computationalcode for temperature computation in animals, taking into account their thermoregulation [6; 7]. Even for humans, only a few groups have succeeded in this [8; 9; 10; 11]. Clarifying the difference of temperature elevation and its time evolution between humans and animals may contribute to the scientific rationale for establishing international guidelines/standards. In the guidelines/standards, the SAR should be averaged over a 6-min period for frequencies lower than 3 GHz [2] and 10 GHz [1]. The rationale behind the averaging time is not clearly mentioned in the guidelines/standards; according to [12], this time was originally \"0.1 h\", which implies lower precision than 6 min. Thermal damage, for example, occurs when rabbit eyes are exposed to high-intensity RF waves for \(\geq\)30 min [13; 14]. Moreover, many studies examined thermal damage in rats [15; 16; 17]. The WHO workshop in 2002 [18], ICNIRP thermal damage workshop [19], and United States Food and Drug Administration (FDA) thermal damage workshop concluded that thermal limits should consider both temperature and time of exposure whenever possible. [PERSON] et al. [12] derived an analytic formula to estimate the thermal time constant using a one-dimensional homogeneous model for intense brief exposures, especially at millimeter waves. Some studies using human anatomical models computed the thermal time constants for handset antennas without considering thermoregulation [20; 21; 22; 23]. However, no reports have compared thermal time constants between humans and animals, making direct extrapolation of findings in animals to humans difficult. Note that thermal damage depends on the time-temperature elevation relationship [24], which is not considered in the international guidelines/standards. This is because the limits for the SAR and incident power density are set so that the allowable temperature elevation is well below the point of thermal damage. We computed the time courses of temperature elevation, including the thermal time constants, in rat and human head models for several exposure scenarios. In our previous studies, an integrated computational technique, combining electromagnetics and thermodynamics, was developed by incorporating a thermoregulatory response model based on measured values [7; 25].