**5.1 Specific absorption rate (SAR)**

When the electromagnetic wave travel through the tissue, it will penetrate the tissue but part of the wave will be absorbed by the tissue and get dissipated as heat. The interaction between the electromagnetic wave and the tissue depends on the dielectric properties of the tissue and the operating frequency. The amount of power absorbed by the tissue during the interaction is called specific absorption rate (SAR), which can be expressed by the electric field (*E*) of the incident wave as follows:

$$SAR = \frac{\sigma}{2\rho} |E^2| \tag{11}$$

where *σ* and *ρ* are the tissue conductivity and volume density, respectively. When working at near field, the temperature of the tissue will be increased due to the dissipation of EM at the tissue interface. The rate of temperature change (ΔT) measured in (°C) depends mainly on SAR and can be given as:

$$\frac{\Delta T}{\Delta t} = \frac{(\text{SAR} + P\_m - P\_c - P\_b)}{h\_c} \tag{12}$$

where *Pm*, *Pc* and *Pb* are the metabolic heating rates; and *hc* is the tissue heat capacity.

There are two SAR standards enforced by IEEE to determine the maximum allowable power for safe interaction between the electromagnetic wave and the tissue without causing any damage or harmful interaction. These standards are IEEE C95.1-1999 standard (SAR1g≤ 1.6 W/kg) and IEEE C95.1-2005 standard (SAR10g≤ 2 W/kg).
